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Novel Applications of Fiber Optic Sensor Technology for Diagnostics of Underground Cables Technical Report
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Novel Applications of Fiber Optic SensorTechnology for Diagnostics of Underground Cables

Technical Report

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EPRI Project Managers B. Damsky W. Zenger

EPRI • 3412 Hillview Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 94303 • USA 800.313.3774 • 650.855.2121 • [email protected] • www.epri.com

Novel Applications of Fiber Optic Sensor Technology for Diagnostics of Underground Power Cables

1008712

Final Report, October 2004

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DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT.

ORGANIZATION(S) THAT PREPARED THIS DOCUMENT

Power Delivery Consultants, Inc.

Thomas J. Rodenbaugh

ORDERING INFORMATION

Requests for copies of this report should be directed to EPRI Orders and Conferences, 1355 Willow Way, Suite 278, Concord, CA 94520, (800) 313-3774, press 2 or internally x5379, (925) 609-9169, (925) 609-1310 (fax).

Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric Power Research Institute, Inc.

Copyright © 2004 Electric Power Research Institute, Inc. All rights reserved.

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CITATIONS

This report was prepared by

Power Delivery Consultants, Inc. 28 Lundy Lane Suite 102 Ballston Lake, NY 12019

Principal Investigator E. Bascom, III

Thomas J. Rodenbaugh Consultant Ione, CA

This report describes research sponsored by EPRI.

The report is a corporate document that should be cited in the literature in the following manner:

Novel Applications of Fiber Optic Sensor Technology for Diagnostics of Underground Power Cables, EPRI, Palo Alto, CA: 2004. 1008712.

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REPORT SUMMARY

This report provides an overview of the state of the art in the use of fiber optics in measurement applications with particular regard to applications suitable for measuring or monitoring significant parameters for underground power cables. The report summarizes the physics of fiber-based measurement systems and discusses what measurement systems can be implemented today with commercial or near-commercial equipment.

Background Various fiber optic measurement systems commercially available or actively being researched could be applied to buried cable systems. Fiber-based temperature measurements—commonly known as Distributed Temperature Sensing (DTS) or Distributed Fiber Optic Temperature Sensing (DFOTS)—have been used for 10-15 years by many cable using utilities. While cable users have somewhat embraced temperature measurements using fiber, many other fiber-based measurements could potentially be used to gather information on the operation and condition of underground power cables.

Objectives • To identify the measurements that can be made using fiber optics and to describe the physical

basis of these measurement systems.

• To discuss application of fiber-based measurement methods to monitoring and diagnostic functions on underground cable systems.

Approach The project team performed a literature search on optics and related subjects and conducted discussions with various researchers in the fields of optics and fiber-based measurement. The team also reviewed developments at universities and at military and industrial organizations. The team synthesized the information they gathered from these sources into a high-level technical summary that includes discussion of several systems suitable for monitoring and diagnostic applications on underground cable systems.

Results Fiber-based sensing technologies can measure temperature, pressure, mechanical forces, chemical interaction, electric fields, and magnetic fields. However, although many fiber-based technologies are available to measure various parameters besides temperature, significant research will be required to extend these applications to power cable systems. Several of the technologies would probably require integrating a sensing fiber or fibers into the cable at the time of manufacture. There are fewer opportunities to retrofit sensors to existing systems.

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EPRI Perspective The primary function of this report is to help EPRI and its utility advisors formulate future development projects in the promising area of fiber optic sensors. This report will also be useful for utility engineers who want to understand more of the technical background of fiber-based sensing or to assess the alternatives when applying fiber-based sensing technology. The list of sources and the detailed glossary contained in the appendices of the report should be helpful to anyone researching fiber optic sensing for underground cable applications.

Keywords Diagnostics Transmission Underground Cables Fiber Optic Sensors DTS (Distributed Temperature Sensing) DFOTS (Distributed Fiber Optic Temperature Sensing)

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ABSTRACT

Technologies based on optical fibers are becoming more widely available and have applications in various technical fields. This report summarizes many of the available fiber-based sensing technologies for measurement of various parameters including temperature, pressure, mechanical forces, chemical interaction, electric fields, and magnetic fields. The report provides some detail on the physics of how each fiber measurement technology works and on which parameters each technology can measure. Finally, the report discusses fiber-based applications to underground cable systems. Fiber-based temperature measurement has been used for the last 10-15 years, but other fiber-based measurements might be employed to improve the reliability and general operation of cable systems. The appendix to the report includes a detailed glossary of fiber-related terms.

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CONTENTS

1 INTRODUCTION....................................................................................................... 1-1

1.0 Introduction ....................................................................................................... 1-1

2 SENSOR SYSTEMS BACKGROUND...................................................................... 2-1

2.0 Types of Sensors .............................................................................................. 2-1

2.1 Distributed Measuring on Fibers ....................................................................... 2-2

Temperature and Strain....................................................................................... 2-2

2.2 How Point or Extrinsic Sensors Function .......................................................... 2-6

2.3 Use of Gratings on Fibers for Making Measurements....................................... 2-8

3 TEMPERATURE MEASURING SYSTEMS .............................................................. 3-1

3.0 Types of Temperature Measurement Systems ................................................. 3-1

3.1 Distributed Raman Systems.............................................................................. 3-1

3.1.1 Typical Setup............................................................................................. 3-3

3.1.2 Mode of operation...................................................................................... 3-4

3.1.3 System Specifications................................................................................ 3-4

3.1.4 Current Applications .................................................................................. 3-5

3.2 Distributed Brillouin Systems ............................................................................ 3-7

3.2.1 Typical Setup............................................................................................. 3-7

3.2.2 Mode of Operation ..................................................................................... 3-8

3.2.3 System Specifications................................................................................ 3-9

3.2.4 Current Application .................................................................................... 3-9

3.3 Point Sensor Systems..................................................................................... 3-10

3.3.1 Typical Setup........................................................................................... 3-11

3.3.2 Modes of Operation ................................................................................. 3-12

3.3.3 Specifications........................................................................................... 3-12

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3.3.4 Applications ............................................................................................. 3-13

3.4 Fiber Bragg Gratings (FBGs) .......................................................................... 3-14

3.4.1 Setups ..................................................................................................... 3-15

3.4.2 Modes of Operation ................................................................................. 3-15

3.4.3 Specifications........................................................................................... 3-16

3.4.4 Applications ............................................................................................. 3-16

4 PRESSURE MEASUREMENTS ............................................................................... 4-1

4.0 Introduction ....................................................................................................... 4-1

4.1 Intrinsic Sensors ............................................................................................... 4-1

4.2 Extrinsic Sensors .............................................................................................. 4-2

4.3 Fiber Bragg Sensors ......................................................................................... 4-5

4.4 Application of Pressure Sensors ....................................................................... 4-6

5 CHEMICAL MEASUREMENT WITH FIBER-OPTIC SENSORS.............................. 5-1

5.0 Introduction ....................................................................................................... 5-1

5.1 Distributed Non-Intrinsic Chemical Sensing ...................................................... 5-2

5.2 Extrinsic Point Chemical Sensors ..................................................................... 5-4

5.3 Fiber Bragg Grating Chemical Sensing............................................................. 5-6

5.4 Evanescent Wave Chemical Sensing ............................................................... 5-6

6 ELECTRIC FIELD MEASUREMENT........................................................................ 6-1

6.0 Introduction ....................................................................................................... 6-1

6.1 Distributed Sensors........................................................................................... 6-2

6.2 Extrinsic Sensors .............................................................................................. 6-3

6.3 Fiber Bragg Grating Sensors ............................................................................ 6-5

7 MEASURING MAGNETIC FIELDS WITH FIBER-OPTICS ...................................... 7-1

7.0 Introduction ....................................................................................................... 7-1

7.1 Intrinsic and Distributed Sensors ...................................................................... 7-3

7.2 Extrinsic Point Sensing ..................................................................................... 7-4

7.3 Fiber Bragg Gratings......................................................................................... 7-5

7.4 Other Items of Interest for Measurement of Magnetic Fields ............................ 7-7

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7.4.1 Magneto-strictive Elastomers..................................................................... 7-7

7.4.2 Ferromagnetic Shape Memory Alloys, “FSMA”.......................................... 7-8

8 CONCLUSIONS AND RECOMMENDATIONS......................................................... 8-1

8.0 Summary........................................................................................................... 8-1

9 REFERENCES.......................................................................................................... 9-1

A BACKGROUND INFORMATION ............................................................................ A-1

A.1 Companies, Contacts, Sources and Manufacturers .........................................A-1

A.1.1 Companies / Manufacturers ......................................................................A-1

A.1.2 University / Government Sources..............................................................A-3

A.1.3 Other Information Sources ........................................................................A-5

B GLOSSARY OF OPTICS TERMS........................................................................... B-1

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LIST OF FIGURES

Figure 1-1 Light traveling through an optical fiber by multiple internal reflections .................... 1-1 Figure 2-1 Fabry-Perot setup .................................................................................................. 2-2 Figure 2-2 Mach-Zehnder setup.............................................................................................. 2-3 Figure 2-3 An example wave spectrum showing the backscattered signals for both the

Stokes and Rayleigh lines ............................................................................................... 2-3 Figure 2-4 Distributed fiber optic temperature sensing equipment........................................... 2-4 Figure 2-5 Typical setup for making temperature and strain measurements ........................... 2-5 Figure 2-6 Flowchart showing the various mechanisms and the measurands for intrinsic

sensors. The flowchart starts with the Rayleigh OTDR which is the spatial locator for intrinsic fiber sensors.................................................................................................. 2-6

Figure 2-7 There are hundreds of variations for point or extrinsic sensors. The chart depicts the operational mechanism, i.e., total reflection, and then the application, i.e., liquid level................................................................................................................. 2-7

Figure 2-8 An input of broad wavelengths into an FBG results in a coherent single wavelength backscatter. Each grating acts like a tiny mirror that reflects only a preferred wavelength....................................................................................................... 2-9

Figure 2-9 Schematic of a grating written onto the fiber core. The transmission signature is on the right side. .........................................................................................................2-10

Figure 3-1 Illustration showing Raman spectroscopy .............................................................. 3-2 Figure 3-2 Schematic of typical distributed temperature sensing equipment ........................... 3-4 Figure 3-3 View of internal components in SensorTran’s distributed temperature sensing

equipment ....................................................................................................................... 3-6 Figure 3-4 Typical Brillouin setup for measurements of temperature or strain ......................... 3-8 Figure 3-5 Illustration showing one of the first optical-based temperature measurement

systems..........................................................................................................................3-10 Figure 3-6 Light patterns in phosphorescent temperature sensor...........................................3-10 Figure 3-7 Schematic of extrinsic measurement system ........................................................3-11 Figure 3-8 Example of a fiber with multiple discrete Bragg gratings on the fiber.....................3-14 Figure 3-9 Figure showing a typical multiplexed grating arrangement. ...................................3-15 Figure 4-1 Example micro-resonator in a silicon substrate ...................................................... 4-3 Figure 4-2 Example of a small gap extrinsic pressure sensor.................................................. 4-4 Figure 4-3 Wavelength response to pressure applied to three sizes of diaphragms with

two polarizations.............................................................................................................. 4-5

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Figure 5-1 A schematic of a geo-synthetic monitoring system................................................. 5-3 Figure 5-2 Figure showing a robust sensor head arrangement for making chemical

measurements. The dye is the fluorescent material shown............................................. 5-5 Figure 5-3 Evanescent wave sensor concept for detection of chemical species...................... 5-7 Figure 6-1 A typical experimental setup that produces a rotation in polarization plan

caused by an applied field to the cell ............................................................................... 6-1 Figure 6-2 Pockels cell setup for measurement of large electric fields showing a change

in the incident light polarization........................................................................................ 6-2 Figure 6-3 Figure showing an alternating field modulated through an electro-optic crystal ...... 6-2 Figure 6-4 Example electric field measuring point sensor ....................................................... 6-4 Figure 7-1 Illustration of indirect magnetic field sensing .......................................................... 7-1 Figure 7-2 Figure showing plane polarized light turned into a Faraday rotation (refracted

polarized rotation) after passing through a magnetic material.......................................... 7-2 Figure 7-3 Illustration of of the magneto-optic Kerr effect ........................................................ 7-5 Figure 7-4 Figure showing a magneto-strictive element covering a region containing a

Bragg grating................................................................................................................... 7-6 Figure 7-5 A photograph is shown of a small magneto strictive sensor element...................... 7-6 Figure 7-6 Illustration of indirect magnetic field sensing .......................................................... 7-7 Figure 7-7 Example shape memory alloy ................................................................................ 7-8 Figure 7-8 Shape memory alloy with and without applied magnetic field ................................. 7-9

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LIST OF TABLES

Table 3-1 Comparison of Point and Distributed Temperature Sensors.................................... 3-1 Table 4-1 Pressure Measurement Characteristics with a Fiber ............................................... 4-4 Table 5-1 Commercial Chemical Sensor Companies and Applications ................................... 5-5 Table 6-1 IPMC Force, Reaction Speed and Voltage Responses for Actuator Usage............. 6-3 Table 6-2 Electrostriction Constants for Polymer and Ceramic Materials ................................ 6-6

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1 INTRODUCTION

1.0 Introduction

Fiber optic sensors have been used for many years, principally in the area of communication. Fiber optic technology relies on “total internal reflection” when light moves from a medium having a high index of refraction (glass fiber) to one of a lower index of refraction (air or a “buffer” coating on the outside of a fiber), with a relatively low loss in the reflected light waves even when the fiber is flexible as shown in the following illustration.

Figure 1-1 Light traveling through an optical fiber by multiple internal reflections

There are many physiological characteristics of fibers and the way light interacts within the fiber and the fiber coating or environment around the fiber. These characteristics serve as the basis for various sensor technologies that will be discussed in the balance of this report.

Sensors and networks of sensors for controlling or for diagnostics have been very important in almost every facet of industry. Most sensors still in use today use semiconductors as the main “ingredient”. These types of devices are subject to failures caused by external influences.

Fiber-optic sensors have a great advantage over discrete semiconductor sensors for the following reasons:

• They are not subject to EMF

• They are small, using photons instead of electrons

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• They can be configured to function as a distributed sensor, a point sensor, or both

• They can operate at high electrical potential because they are an insulator

Fiber-optic sensors have become prominent due to the marriage of two technological areas; communications and optoelectronics1. The latter area has shown that lasers and photons can be made to do a variety of functions and measurements. Compact disc players, CD-ROM drives, DVDs, and laser printers are a few consumer items. The medical area also uses optoelectronic devices for measuring blood gases, insulin level and oxygen uptake.

The principal advantages that fiber optic sensors possess over conventional sensors include:

• Small size

• Very wide frequency bandwidth response

• Low weight

• Simultaneous sensing of more than one parameter

• Robust

• Very wide operating temperature range

• Low unit cost

• High tensile strength

• High sensitivity

• High fatigue life

• High spatial resolution

• Fast response times

• Corrosion resistance

• Immunity to electromagnetic interference (EMI)

• Non-conductive

• Plus numerous systems-related advantages

Optical sensing of the external environment can be cross-categorized by the influence on the fiber and by the type of fiber sensor. Sensors are either extrinsic or intrinsic to begin with. Extrinsic means that the glass fiber wire and the interrogation means (usually a laser or diode) are used as the transportation system for the measurement. In other words, the fiber attaches to an item or black box, which makes the measurement and the laser interrogates the item for information to be transported back over the fiber. Most extrinsic sensors are considered as “point” or discrete sensors. An intrinsic sensor is one where the glass fiber itself is influenced by an external event and the laser interrogates the glass fiber for answers to the measurement. The 1 There is a comprehensive glossary of fiber optic-related terms included in Appendix B. The reader is encouraged to review these terms when reading this report.

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external item, such as temperature, changes the glass indices that can be measured by infrared spectroscopy and the proper interferometer. This type of sensor is usually considered as a distributed sensor.

There is still one more type of sensor that uses a glass fiber. It has been the subject of thousands of research projects in many different technical areas. These are referred to as Fiber Bragg Gating Sensors. These are considered combination sensors since the fiber is etched at various locations along its length, effectively creating a “sensor” at each etching. These etchings are the lines that make up the grating.

Bragg gratings have been used for 30 years along with photonics. In the 1970’s, these were called surface acoustic wave devices (or SAWs). SAWs are used to modulate light with sound, thus allowing for transport of sound over a light beam. Now, Bragg gratings that are incorporated into fibers themselves can be used to modulate light with respect to a change in wavelength of the grating caused by vibration, sound, pressure, etc.

Industries have found commercial uses for both intrinsic and extrinsic types of sensors. For instance, applications use extrinsic sensors to measure temperature, pressure, liquid level, and flow in process control and to monitor linear and angular position in aircraft fly-by-light operations. In other applications, intrinsic sensors make rotation, acceleration, strain, acoustic, pressure, and vibration measurements. Multimode fibers also make temperature measurements.

There are advantages and disadvantages to using either type of sensor. Extrinsic sensors tend to be less sensitive but are more easily multiplexed, and easier to use. They also require more connections, which can cause problems (into and out of the light modulator regions) from signal loss and optical interference (“cross-talk”) through the multiplexed connections. Generally, there is much greater signal attenuation when extrinsic sensors are placed into a matrix material such as a smart skin structure. Intrinsic sensors are more sensitive and more difficult to shield from unwanted external influences. Their “all-fiber” design reduces or eliminates most connection problems experienced with extrinsic sensors, but they usually require more elaborate signal processing via interferometry and spectrometer analysis. From a commercial standpoint, intrinsic sensors tend to be significantly more expensive than extrinsic sensors but are more versatile for installation and are more robust.

In this report there will be an attempt to cover the various types of sensors under each measurand, i.e. what is being measured. Some measured parameters may only be supported using one type or another. In these cases, a statement will appear letting the reader know that to date only one type might be able to be used.

Measured parameters that might have the greatest importance to the electric industry, and specifically the underground transmission community, are:

• Temperature

• Electric Field

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• Magnetic Field

• Chemicals / Hydrocarbons

• Chemicals /XLPE by-products

• Acoustics and or Vibration

• Strain (splices)

• Proximity / Perimeter Security

A more complete listing of possible measured parameters is presented below:

• Strain

• Displacement

• Damage

• Residual strain

• Acceleration

• Cracking

• Vibration

• Deformation

• Wear

• Frequency

• Impact

• Corrosion

• Acoustic emission

• Liquid levels

• pH levels

• Pressure

• Index of refraction

• Temperature

• Load

• Angular velocity

• Linear velocity

• Chemical composition

• Chemical reactions

• Electric fields

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The above list shows the many measurements that can be gleaned from the interrogation of either type of fiber optic sensors. As is usual in most sensor applications, the measured parameter is measured by proportion or other relationship to some optical characteristic that is measured; direct measurement is generally not used for most of these sensors.

When appropriate, each measurement style that is useful for other power applications, i.e. transitions from overhead to underground, substations and power station get-a-ways, will be discussed as well. It is also important to note that there may be some redundancy in the report due to the fact that some of the measured parameters utilize the same intrinsic property or the same point sensing element. As an example, acoustic, pressure, vibration and intrusion detection all use common mechanisms.

A practical consideration for many of the fiber sensor technologies described in this report is access to the appropriate type of fiber in a location where measurements are meaningful. While insulated cables date back to the late 1800s, optic fibers have only been around for 40years (initially in laboratories such as Hughes Aircraft Corporation) and somewhat routinely combined with power cables in the last 10 or so years. While many of the fiber technologies described in this report show great merit, there may be some difficulty in introducing a fiber to an existing cable system. However, new cable systems may incorporate sensors based on these technologies much in the way power transformers now include fiber-based extrinsic sensors to measure hot spots and other transformer parameters. This installation or incorporation of the necessary fiber to use these sensor technologies is not the subject of this report but is a practical matter the reader may face when applying some of these methodologies.

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2 SENSOR SYSTEMS BACKGROUND

2.0 Types of Sensors

The technology and applications of optical fibers have progressed very rapidly in recent years. Optical fiber is subjected to various forces on it at all times and therefore can have size, shape and or property changes. In communications, external factors affecting the fiber are minimized as best possible since signal quality is paramount to data transfer. This is in contrast to sensor applications where it is desirable to enhance an external factor to improve the measurement quality. Responses to external influences are deliberately enhanced so that the resulting change in optical radiation can be used effectively as a measure of an external influence. Data and communication fibers use signal modulation to affect the data transfer rate, but in sensing, the fiber itself acts as a modulator in response to influences. As a result, fibers for communications and for sensing generally are on separate dedicated fibers, although they may cohabitate in the same fiber optic cable. The sensing fiber also serves as a transducer and converts measurands like temperature, stress, strain, rotation or electric and magnetic currents into a corresponding change in the optical radiation.

The advantages of fiber optic sensors are freedom from electro-magnetic interference (EMI), wide bandwidth, compactness, geometric versatility and economy. In general, fiber sensors exhibit fairly high sensitivity when compared to other types of sensors. Specially prepared fibers can withstand high temperature and other harsh environments. In telemetry and remote sensing applications, it is possible to use a segment of the fiber as a sensor gauge while a long length of the same or another fiber can convey the sensed information to a remote station. This arrangement can be taken one step further and become a distributed sensor, by multiplexing the various point sensors along the length.

Deployment of distributed and array sensors covering extensive structures and geographical locations are also feasible, though retrofitting on an existing cable system may be difficult. Many signal processing devices (splitter, combiner, multiplexer, filter, delay line etc.) can be made of fiber elements thus enabling the realization of an all-fiber measuring system.

There are a variety of fiber optic sensors. These can be classified as follows.[2]

A) Based on the modulation and demodulation process, a sensor can be called an intensity (amplitude), a phase, a frequency, or a polarization sensor. Since detection of phase or frequency in optics calls for interferometric techniques, the latter are also termed as interferometric sensors. From a detection point of view, the interferometeric technique

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implies heterodyne detection/coherent detection where the backscatter is of a differing wavelength from the incident wavelength using an interferometric means to determine the maxima and minima. On the other hand intensity sensors are basically incoherent in nature. Intensity or incoherent sensors are simple in construction, while coherent detection (interferometric) sensors are more complex in design but offer better sensitivity and resolution.

B) Fiber optic sensors can also be classified on the basis of their application: physical sensors (e.g., measurement of temperature, stress, etc.); chemical sensors (e.g., measurement of pH content, gas analysis, spectroscopic studies, etc.); bio-medical sensors (inserted via catheters or endoscopes which measure blood flow, glucose content, etc.). Both the intensity types and the interferometric types of sensors can be considered in any of the above applications.

C) Extrinsic sensors and intrinsic sensors are other classification schemes. In the former, sensing takes place in a region outside of the fiber and the fiber essentially serves as a conduit for the to-and-fro transmission of light to the sensing region efficiently and in a desired form. In an intrinsic sensor, one or more of the physical properties of the fiber undergo a change as mentioned in A) above.

2.1 Distributed Measuring on Fibers

Temperature and Strain

For distributed sensor applications, there are three important types of scattering that must be measured; Rayleigh, Brillouin and Raman. There are other mechanisms that use one of these scattering approaches in spectroscopy and or interferometry, such as Sagnac Interferometry used in fiber optic gyroscopes. Two basic concepts of fiber optic interferometers are known: Mach-Zehnder and Fabry-Perot interferometers. In fiber optic Fabry-Perot interferometer, the interference occurs at the partially reflecting end face surface of the fiber and an external mirror.

Figure 2-1 Fabry-Perot setup

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Figure 2-2 Mach-Zehnder setup

Rayleigh scattering is elastic meaning that the incident wavelength of light is the same as the scattered wavelength. This type of scattering in the glass fiber accounts for the majority of backscattered light. Brillouin and Raman scattering are inelastic types; in Brillouin scattering, phonons scatter the incident light in a process that changes its wavelengths while in Raman scattering, the wavelength of the incident light is shifted by electronic energy level shifts in molecules of the material. These both have their incident light affected by acoustic and optical phonons. As one may remember from solid state physics or a good materials book, phonons are vibrations in the crystal structure and the gap between the optical and acoustic phonons is the “band gap”. Raman scattering relies on the optical band of phonons whereas Brillouin relies on the acoustic ones. Because these are inelastic, the power of the backscattered light is very much weaker than elastic or Rayleigh (these will be discussed in more detail in Section 3).

Figure 2-3 An example wave spectrum showing the backscattered signals for both the Stokes and Rayleigh lines

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As can be seen, the Rayleigh line is much stronger since it represents an elastic scatter. The Stokes and Anti-Stokes lines are much smaller and are shifted symmetrically around the central Rayleigh line. The temperature information gleaned from Raman backscatter is the ratio of the Stokes and Anti-Stokes intensities; the ratio is proportional to temperature.

Raman backscatter in silica or glass fiber is responsive (a small component) to variations in temperature. This is why the initial commercial systems use this type of approach. Because of the Brillouin frequency shift property in glass fiber, more recent applications of this sensor technology have observed that this shift is affected by both a temperature change and also strain in the fiber itself. For this reason, more commercial systems are now being marketed as a distributed temperature and strain sensor combination.

Figure 2-4 Distributed fiber optic temperature sensing equipment

Because of the strain measurement capability, the communications industry have been using the Brillouin approach for making cable measurements in both buried underground and submarine types of cable. Since these cables are pulled into conduits (similar to power cables), the fibers are measured for strain before the installation and after the pulling to make sure there are no kinks in the cable or excessive strain regions. The fibers are also used to sense pressure caused by frozen water in the conduit. If the frozen water applies too much pressure, permanent damage may occur. This technique will indicate the location for further testing.

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Figure 2-5 Typical setup for making temperature and strain measurements

In the figure, “DFB” is “distributed feedback”, “EOM” is “electro-optic modulator”, and “EDF” is “erbium doped fiber”.

There have been many attempts to turn a single fiber into a multi-use sensor for measurement of temperature, pressure, strain, acoustic emissions and many more. It becomes fairly obvious that after several years and hundreds of R&D teams attempts, that the best way to a multi-performing wire is to use the Bragg grating approach or a multiplexed point sensor approach.

For certain, distributed fiber-optic sensing is a much more complicated setup due to the fact that one must not only use the basic backscatter property to find the measurand value but also use optical time domain reflectometry (OTDR) to locate each section being measured along the path length. Figure 2-6 illustrates the many items that need OTDR to locate the location. Each technique requires a certain length of fiber to actually get a significant response. In the case of the first category, birefringence-dependent measurands, one must have an effected length of fiber long enough for the Kerr electro-optic effect to yield a change in rotation of the incident and then back reflected light. The Kerr effect is for electro and magneto-optic measuring, whereas the stress /strain would use the Faraday rotation of the glass. In each case, approximately one meter of length – usually referred to as the “spatial resolution” – would be necessary to obtain a sufficient signal to correlate to the OTDR location measurement.

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Figure 2-6 Flowchart showing the various mechanisms and the measurands for intrinsic sensors. The flowchart starts with the Rayleigh OTDR which is the spatial locator for intrinsic fiber sensors.

2.2 How Point or Extrinsic Sensors Function

Just as in distributed systems, discrete extrinsic fiber-optic point sensors were first used to measure temperature as well.

Initially fiber optic wire was merely the transport medium for the light to pass down to an ampoule filled with a phosphorescent material that would change the incident light’s wavelength with temperature.

EPRI had funded such research in the early 1970’s with the application of transformer temperature measurement. While a good idea, this technology still required a person to carry out the proper spectrometer to measure the wavelength of the reflecting light and interpret the measured temperatures. Some companies started then to develop an all-in-one approach making it easier to perform measurements. Luxtron was one of the first to develop commercially viable systems.

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Extrinsic or point sensors have progressed significantly in the last 35 years. The most up to date list of techniques that are have either commercial backing or have undergone extensive R&D are represented in the chart below.

Figure 2-7 There are hundreds of variations for point or extrinsic sensors. The chart depicts the operational mechanism, i.e., total reflection, and then the application, i.e., liquid level.

As can be seen, there are nine other technologies involved in extrinsic sensing beyond that of fluorescence.

Many of the types of sensor modes – encoder, reflection/transmission, gratings etc. – all work off of similar principles in that the fiber makes a contact or the light coming out of the fiber impinges onto a substrate, mirror, or Fabry-Perot etalon (simple Fabry-Perot interferometer placed at the end of a fiber where there are at least two reflecting parallel surfaces). The spacing of the two reflecting surfaces provides for the interference of the incident and reflected light back at the source. The main difference between an etalon and interferometer is that the etalon has a fixed spacing between the plates for a specific measurement only (wavelength and resulting

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pattern). A standard interferometer can be adjusted providing a variety of interference patterns for many wavelength separations.

An interesting sensing type is the evanescent wave mode of operation where the light in one fiber is somewhat affected by another fiber not in direct contact. The evanescent wave mode uses the electromagnetic radiation properties of light (magnetic vector , B, and electric vector, E). The light’s power flow density in the direction of travel, also known as the Poynting Vector, defined by:

BEP ×=0

[Watts / meter2]

The perpendicular component of the light’s electric field from the light’s direction of travel can interact with the environment, causing a change in the Poynting Vector’s magnitude that may be detected. This has been used successfully in the medical industry to detect blood oxygen content or glucose levels.

2.3 Use of Gratings on Fibers for Making Measurements

Surface acoustic wave devices have been around for over 30 years. When used with light, these devices sparked a fury of research involved with acousto-optical systems. The surface acoustic wave or SAW, used Bragg gratings. Now Bragg gratings can be etched into glass fibers to allow for measuring many different items that affect the wavelength spacing of the grating. Heat or temperature affects the glass and or cladding which then changes the grating and thus the effective wavelength. Pressure and flexure on or of the fiber also has an effect on the grating and

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thus the wavelength spacing. These Bragg gratings when placed onto a fiber optic wire as part of a sensor “suit” are termed Fiber Bragg Gratings (FBGs).

Most of the research being done today focuses on making the grating on the fiber in a continous and lasting process, rather than on making measurements themselves. Claddings that are internally optically sensitive to certain laser wavelengths have now been shown to retain the grating lines when the proper laser interference pattern is “burned” into the cladding. Another way of doing this is to directly affect the fiber core material.

To make the fiber Bragg gratings as sensor elements, the gratings or lines are etched similar to semiconductor chips by using photo-writing using intense ultra-violet laser beams. These types of sensor elements, when written onto long lengths and then multiplexed, have the potential for the measurement of strain/deformation and temperature together. Current applications include, monitoring of highways, bridges, aerospace components, naval ship and submarine monitoring and diagnostics and also as chemical and biological sensors. In Great Britain and the United States, the expansion caused by many R&D efforts has now played a role in the development of a fiber Bragg grating (FBG) measuring system for monitoring and recording the actual seismic responses of underground structures, rock mass and bridges etc. These are also used along fault lines for measuring pressures and shifts.

The basic principle of a fiber Bragg grating (FBG)-based sensor system lies in the monitoring of the wavelength shift of the returned Bragg-signal, as a function of the external influence or measurand (e.g. strain, temperature and force); the measurand will change the Bragg grating spacing resulting in a shift of the reflected wavelength. The Bragg wavelength is related to the refractive index of the material and the grating pitch or Bragg angle. Sensor systems usually work by injecting light from a broadband spectral light source into the fiber, with the result that the grating reflects a narrow spectral component of the incident light at the Bragg wavelength. In the transmission through the fiber length, the Bragg component is missing from the observed spectrum.

Figure 2-8 An input of broad wavelengths into an FBG results in a coherent single wavelength backscatter. Each grating acts like a tiny mirror that reflects only a preferred wavelength.

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The grating itself acts like little mirrors whose reflection is only at an angle known as the Bragg angle. This is a preferred wavelength only. As such, when a broad spectrum is sent into the fiber, a narrow wave spectrum is reflected back. This is the same reflection that occurs when a person looks at a glass window at the Brewster angle. That is the angle of total reflection. The Bragg grating acts in the same manner, only at a preferred wavelength instead.

Figure 2-9 Schematic of a grating written onto the fiber core. The transmission signature is on the right side.

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3 TEMPERATURE MEASURING SYSTEMS

3.0 Types of Temperature Measurement Systems

Measuring temperatures with glass fibers was first used in the point sensor arrangement back in the late 1970’s. These point sensors, which used phosphorescent material at the end of the fiber, quickly became commercialized. Many of the companies were start-ups that still exist today. Distributed sensors took a while longer to emerge. Raman spectroscopy initialized the work in glass fibers that resulted in the Raman backscattering approach to the measurement of temperature along the length of fiber. As more applications developed, other interferometric means were commercialized. Typical sensitivities for temperature measuring are below.

Table 3-1 Comparison of Point and Distributed Temperature Sensors

Parameter Point Sensing Distributed Sensing

Range (°C) -50°C to 1,100°C -10°C to 500°C

Accuracy (°C) +/-1°C +/-1°C

Resolution (°C) +/- 0.1°C +/-0.1°C

Length Resolution (meters) n.a. 1-5m

The range of sensitivities and / or specifications for both point (extrinsic) and distributed (intrinsic) sensors are given above. These are general sensitivities since there are many newer sensors in the laboratory environment and near commercial that take advantage of various multiplexing schemes to improve items like spatial resolution down to 1 centimeter.

3.1 Distributed Raman Systems

Raman spectroscopy is based upon the Raman Effect, which is described as the scattering of light from a gas, liquid or solid with a shift in wavelength from that of the usually monochromatic incident radiation.

When a transparent medium is irradiated with an intense source of monochromatic light, such as a laser, and the scattered light is examined with a spectrometer, not only is the incident light, ν0 , observed in the scattering, i.e. same frequency- Rayleigh scattering, but also some weaker “bands” of a shifted frequency from the incident light are detected. The shifted bands are of lower frequency ν0 - νi, and some are at higher frequency, ν0 + νi. These bands above and below

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the center frequency are known as the anti-Stokes and Stokes bands respectively. The Stokes and anti-Stokes bands are equally displaced about the Rayleigh band; however, the intensity of the anti-Stokes bands is much weaker than the Stokes bands and they are seldom observed. It is these bands that exhibit the temperature dependence for Raman distributed temperature measuring fibers.

Figure 3-1 Illustration showing Raman spectroscopy

The phenomenon of Raman spectroscopy from material science follows the general backscatter wave number or frequency shifts shown above for the Stokes / Anti-Stokes lines. The laser interacts with optical phonons in an inelastic manner to produce this backscatter. The Rayleigh line in the center is an elastic scatter and thus much bigger intensity.

A molecule changes its polarization due to its rotation or vibration modes. These modes scatter incident light back elastically which is the Rayleigh scattering in glass. The Raman scattering is based on the molecules rotation or vibration. These are temperature dependent and are represented as the molecule’s frequency νI, which are the shifts away from the incident radiation of frequency ν0

Sensors that use a multi-mode fiber or single mode2 fiber and the Raman backscattering phenomena for measurement of temperature are thusly of the intrinsic type. The temperature effect is a material property of the glass caused by the glass molecular vibration. Temperatures are measured along the entire length of the fiber by using a time domain reflectometer system or OTDR. The only requirement is that there is enough of an active length of fiber to produce a significant backscatter response. In multi-mode fibers, this is approximately one meter of length

2 Single mode fibers are much less lossy, which makes the photon interaction with optical phonons more difficult. When there are multiple modes of propagation, some are more preferential to scattering by the phonon lattice that produces the phase shifts and the backscatter. Fibers with only one mode of propagation produce less backscatter so a much longer time and distance (lower spatial resolution) are required to produce a detectable reaction.

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while in single-mode fibers, this is 4-10m. The resulting measurement effectively averages the temperature over the 1m (for multi-mode or 4-10m for single mode) fiber length. This is where the system resolution is set.

In Raman-based multi-mode fiber optic systems, there is a length limitation that is based on the fact that light does attenuate per unit length faster in these types of fibers. Attenuation also occurs at junctions and or splices. The typical system, be it from Sumitomo, Hitachi, LG, Sensa, or SensorTran has a multi-mode limit of around 10 km. Some of the aforementioned companies now have single mode fiber systems that can go 3-4 times farther for a single system.

For the Raman-based systems to work, information about the fiber must be known ahead of time to apply single-ended measurements. Specifically, the losses per unit length and the propagation velocity must be known or else calibration of the fiber must be done using a known temperature. Some instrumentation uses “doubled-ended” measurements that are self calibrating, but this effectively reduces the test length by half (down to 5-6km instead of 10-12km).

3.1.1 Typical Setup

As mentioned above, there are several commercial systems that are widely sold throughout the world. All use pretty much the same style of setup in the field. All of these systems are portable, but typically are not installed on a permanent basis. Most of the Raman-based equipment has a fairly narrow operating range (10-35°C) for the instrumentation, which limits its long-term installation in outdoor locations (e.g., substations, etc.). Technicians will truck the unit out to the field and gain access to the already installed fibers at opportune locations. Several utilities, including ComEd/Exelon and BC Hydro have dedicated Raman type units permanently installed in a substation for monitoring cable temperatures.

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Figure 3-2 Schematic of typical distributed temperature sensing equipment

Above is a schematic of a typical distributed temperature measuring instrument based on the separation of the Stokes and Anti-Stokes frequency shifts for measuring temperature.

3.1.2 Mode of operation

Multi-mode or single-mode fibers are used to make the temperature measurements. The fibers’ molecules increasingly vibrate as the local temperature along the length increases. The backscattered light is dependent on this. In multi-mode Raman systems, about one meter of fiber length is needed to give a significant backscatter signal; 4-10 meters is needed for the less lossy single-mode fiber. These requirements designate the spatial resolution.

3.1.3 System Specifications

There are several manufacturers of temperature profiling systems, but all of them must use essentially the same specifications limits. The specification limits are set by the physics of the fiber and the interferometric limits.

The typical multi-mode Raman system has a measurement limitation of approximately 10 km. This length limit changes to shorter lengths when more connectors are use, increasing the signal

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loss. The connectors have more losses – 0.1-0.2 dB for fusion spliced fibers, 0.5 dB for mechanical splices – than the fiber itself.

As mentioned above; the spatial resolution is one meter over long length fiber systems. The signal that is received from each segment of fiber is equated to approximately 1 degree Celsius in temperature resolution.

EPRI and other organizations have done research using Raman systems for transformer end winding monitoring. Those systems are not long in length and are compact. As a result, temperature resolution over short lengths is 1/10th degree Celsius, and the spatial resolution is 1cm. The entire distributed sensor is only a few meters long, allowing for very small Stokes/Anti-Stokes signals to be received from relative short (1cm) sections of fiber. The resolution and sensitivity is good because signal attenuation over the few-meter length is virtually non-existent (as compared to conventional distributed temperature sensing equipment that might be looking over several kilometers).

3.1.4 Current Applications

Temperature monitoring and profiling using Raman backscatter received its start in the drilling industry for oil and gas. Down hole monitoring during drilling is important. These intrinsic or distributed systems have found hundreds of applications over the last ten years.

EPRI began using this technology for underground cable systems in the mid-1990s with a York DTS-80 (in 2003, the equipment was updated to a Sensa DTS-800) system for profiling several underground cable circuits. Others such as Con Edison, ComEd/Exelon, NSTAR, Southern California Edison and BC Hydro have done the same. End windings were mentioned for generators as another application using the same principles.

The following industries either use deployed systems or have embedded systems into their application infrastructure:

1. Aerospace for use as aircraft wing monitoring on Shuttle flights and in the military aircraft arena. NASA has used fiber-based distributed temperature sensing to monitor cryogenic fuel tank temperatures.

2. Environmental industry uses distributed systems for temperature profiling ocean and fresh water.

3. The building industry, more specifically high rise buildings, use these for temperature control zones for HVAC.

4. Power industry can apply this technology for monitoring buried cable temperatures, transformer tank temperatures or overhead line conductor temperatures (as is being done currently by SouthWire and ComEd/Exelon over a 3-span test length).

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There are literally hundreds of uses for this type of short-range temperature sensing. The biggest hurdle towards the Raman interferometric system is the cost. The black box that contains the signal separation optics and the software to produce a temperature equivalent is very expensive (US$60-140k at the time this report was prepared). In actuality, the optics portion of the instrument is relatively inexpensive, as is the multimode fiber, but the fiber must be connected to the cable, and there are typically a limited number of underground cable circuits going into a central location -- especially at transmission voltages – for a single unit to monitor multiple fiber loops without transporting the electronics.

There are some companies that are taking these measurement systems and making them stand alone field mounted devices. SensorTran is one company that has auto calibration and field mounted boxes. They have taken the laboratory type of box, made it smaller, and multiplexed it to make temperature measurements on multiple fibers.

Figure 3-3 View of internal components in SensorTran’s distributed temperature sensing equipment

SensorTran, which is an affiliate company of SPEC, is improving the use of Raman backscatter DTS systems for utility applications by providing rugged, self calibrating systems that can be installed outside in a substation environment. These units, shown open above, can measure many fibers through the use of a multiplex system. In addition to the optics and a reference coil of fiber (with a known temperature), the equipment includes an industrial computer, hard disk for

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storing data, monitor and keyboard. It’s fair to say that the Raman-based temperature measurement equipment is the most mature of the fiber-based instrumentation.

3.2 Distributed Brillouin Systems

Brillouin light scattering is generally referred to as inelastic scattering of an incident optical light by thermally excited elastic waves in a sample.

In most Brillouin experiments, the Fabry-Perrot interferometer has been the instrument of choice. In the case of a transparent solid, most of the scattered light emanates from the refracted beam in a region well away from the surface, and the conditions relating wave vector and frequency shift of the light to bulk acoustic wave scattering (acoustic phonons). The scattering, in this case, is caused by a strain field. This brings about fluctuations in the dielectric constant and this also affects the index of refraction. As a result, the fluctuations are the cause of inelastic scattering. The phonons present inside a solid move in thermal equilibrium with very small amplitudes creating fluctuations in the dielectric constant, which is viewed as a moving diffraction grating by an incident light wave. Therefore Brillouin scattering can be explained by the two concepts of Bragg reflection and Doppler shifts. The Doppler Effect comes into play because of the frequency response of the acoustic phonons; moving away gives a lower frequency and moving towards gives a higher one. The acoustic phonon band then becomes a moving Bragg grating in effect.

Brillouin distributed temperature systems use single mode fiber as the measurement waveguide. Because single mode has much lower attenuation rates per unit length, longer distances can be monitored over one fiber. Many of the companies that supply Raman systems also have commercialized these as well.

Raman systems use the intrinsic property of optical phonons and Brillouin systems acoustic phonons. Interferometers vary by company. The interferometer is used exclusively to “convert” the backscattered phase shifts (frequency) to an amplitude that can be related to the temperature. Optical time domain reflectometers are fundamental of both systems. These OTDR’s are the location measurer.

3.2.1 Typical Setup

A Brillouin type of system looks identical to that of a Raman based system. Pictured below is a typical setup for making measurements of both temperature and strain. With the advent of “stimulated” Brillouin scattering, many more measurands can be measured, while providing improved signal response and longer measurement distances.

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Figure 3-4 Typical Brillouin setup for measurements of temperature or strain

In the figure, “DFB” is “distributed feedback”, “EOM” is “electro-optic modulator”, and “EDF” is “erbium doped fiber”.

3.2.2 Mode of Operation

As mentioned before, Brillouin scattering is caused by the interaction of photons with acoustic phonons in the glass fiber. These phonons represent certain vibration rotation states of the material. These lattice vibrations cause the photon source to be scattered. This is still an inelastic scattering process so the signal levels are low as in Raman systems.

Most distributed sensing systems based on “stimulated” Brillouin scattering have to use a pulsed light source – a sharply pulsed laser is also used for “regular” (non-stimulated) Brillouin systems. The problem is that the spatial resolution of the sensor is no better than several meters. This resolution is not sufficient for many underground cable applications. A continuous-wave (CW) source excites the acoustic phonons to a higher energy states as compared to the non-continuous source. By doing this, they provide better back-scattered responses to the pulsed light (as compared to the non-continuous source) for measurements. This coherent sensing approach can improve the spatial resolution down to centimeter lengths. This means that for measuring temperature and strain together, Brillouin systems have the same resolutions as Fiber Bragg Gratings, but do it in a distributed manner as opposed to discrete grating locations.

Both temperature and strain measurements are almost always done together in these systems, by nature of the measurement itself. When making strain measurements caused by the strain field set up by the acoustic phonons, temperature effects must be correlated and filtered out. The same is true for responses to temperature; the strain field must be factored out.

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3.2.3 System Specifications

Systems specification limits for distributed Brillouin systems are a little different than for the previous technique. First of all, acoustic phonons reside in a lower energy state in glass as well as in semiconductors. Single mode (with only one mode of light propagation) fiber has less signal attenuation. All of this translates to a spatially active segment that is longer than in the multi-mode / Raman case. Brillouin systems need approximately 5-10 meters of length for a significant signal return. Temperature resolution over this length is the same. Where this system shines is that much longer lengths – 30-40km – can be monitored from a single site. In the new area of Stimulated Brillouin Scattering, spatial resolution is improved dramatically however range might be degraded. By stimulated emission of Brillouin backscattering, the range is degraded due to higher attenuation and pulse broadening; the range comes down to 2-4km. However, the spatial resolution is dramatically improved because of the acoustic phonon excitations.

3.2.4 Current Application

Most of the cable / power industry applications of this technology have switched over to Brillouin systems because of the distance advantage. Cable temperatures vary little over their length unless caused by “hot spots” external to the cable. The 10 meter spatial resolution is deemed okay for cable monitoring as a result, particularly for pipe-type systems where there is a large cable mass and some thermal smoothing from dielectric fluid migration during load cycling (if no active fluid circulation is in place).

Other industries still need smaller spatial resolutions. 5-10 meters is too large a limit for aerospace applications; however the Navy uses some systems in ships.

For smart structures or skins, stimulated scattering systems can be used, however these are less mature systems to date.

Undersea communications organizations have now employed the system to keep aware of temperature and strain at once. The system is in essence a condition monitor that will alert the company of a break or a problem area.

Down hole well monitoring has been one of the bigger driving forces for multiple sensing distributed fiber. The original York DTS systems were designed for well monitoring.

Another potential advantage is that Brillouin-based systems provide better temperature accuracy over single-mode Raman-based systems using single-mode fiber. There is more often an existing infrastructure of single-mode fiber installed for communications that could be used for temperature measurement. It’s much less common to see multi-mode fiber installed for anything but temperature monitoring because it is so much more lossy. Raman backscatter is limited by photon and optical phonon scattering, while Brillouin scattering in single mode fibers is caused by acoustic phonons. Lattic vibrations are acoustic phonons at the lower frequencies of a

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thermal field. Although the signal intensity with acoustic phonons is less than with optical phonons, a highly coherent light source is used to enhance the signal.

3.3 Point Sensor Systems

Fiber optic wire coupled to point type or extrinsic sensor elements were the first to measure temperatures in various harsh environments. Typically the fiber wire was used to transmit a light pulse down to the element where either a reflection based on temperature or a change of wavelength in the light is sent back on the wire due to a fluorescent material that changes with temperature.

Figure 3-5 Illustration showing one of the first optical-based temperature measurement systems

One of the first optical-based temperature measurement systems (see Figure 3-6) used the principle of fluorescence to change the incident light to a reflected one with a different wavelength that is based on the temperature of the phosphor.

Figure 3-6 Light patterns in phosphorescent temperature sensor

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Figure 3-6 shows the transmitted light pulse (shown as an LED in the figure) traveling down the optical cable to the phosphorescent tip. This tip is the extrinsic sensor that returns a response in wavelength and intensity that is shifted from that of the LED. Three temperature responses are shown. This arrangement can have multiple sensors tied into one optical cable.

Because glass fibers do not conduct electricity, many of the harsh environments where point sensors excelled were in the electric, automotive and aerospace industries. The utility industry immediately saw opportunities for use in transformer cores, SF6 equipment and even gas bus.

Point sensing has the advantage of simplicity of use as well as the ability to measure much higher temperatures. Point or extrinsic sensors can be multiplexed for measuring many points at once. The setup would be similar to that of having isolated Bragg gratings as well, or somewhat like application of thermocouples where all “leads” (copper or optical) are brought back to a central location.

There are many commercial companies that make excellent sensors, with many gaining experience supplying sensor arrays to the increased demand in the automotive industry.

3.3.1 Typical Setup

Figure 3-7 Schematic of extrinsic measurement system

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As the figure above indicates, extrinsic measurement systems are by far less complicated and as a result much less expensive that the intrinsic distributed systems.

3.3.2 Modes of Operation

Extrinsic temperature sensors can use any external means of measuring temperature where the light transmitted is affected by an external stimulus. Photoluminescent materials respond to the incident light but the temperature of the material gives a change in reflected wavelength. Note that photoluminescence is the long afterglow light re-emitted after excitation (e.g., as is sometimes apparent when turning off a television but a glow remains on the screen for a period of time). Basically a material at the end of a fiber absorbs the energy and emits it later (few seconds) at a wavelength dependent on the temperature. Sometimes used interchangeably, fluorescence is the absorption of light at one wavelength, which is changed to another wavelength by the material vibrations. Stated differently, the absorption and subsequently re-radiation of light is the process of photoluminescence. If the light emission persists for up to a few seconds after the incident light is withdrawn, this is phosphorescence. Fluorescence, however only continues during the absorption of the incident light. The time interval between absorption of incident light and emission of re-radiated light in fluorescence is very short duration, usually less than a millionth of a second based on molecular vibration times. The mode of operation in most extrinsic sensors involves the use of fluorescent materials that are usually dyes. A portion of the fiber is doped with dye, and different dyes span different temperature ranges.

3.3.3 Specifications

Temperature is most often measured by the fluorescent emission decay times from rare-earth-doped tips and or doped phosphors. Neodymium-doped glass(which is also a major type of laser material) shows good performance over the range -50°C to 300°C . Chromium (LiSAF) crystal has sensitivity from 0°C to 100°C and is used for biomedical sensing. Yttrium oxide and yttrium orthovanadate, activated with europium (Eu), are suitable only for measurements in the 500°C to 1000°C range. Recently, Cr:YAG (another lasing material) has been shown to operate over the range -25°C to 500°C.

By using a digital signal processing scheme for the decay measurement, resolution of 0.1°C over these entire ranges is possible. Companies using and selling these instruments using this technique include Nortech Fibronics of Canada, whose products operate in the range -40°C to 250°C; Optrand, Inc., which sells devices for high-temperature engine control applications; and Takaoka Electric, which has taken over the activities of ASEA.

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3.3.4 Applications

Point Sensors have been around the longest. They also utilize proven simple technologies for making temperature measurements. Depending on the environment, temperature measuring range is custom made.

Point sensors are used in the automotive industry, as well as in just about all industrial complexes. They are suited for being placed in extreme high heat environments for process control monitoring. They are also used to monitor chemicals for temperature of reactions.

• Temperature

• Pressure

• Flow

• Liquid level

• Displacement (position)

• Vibration

• Rotation

• Magnetic fields

• Acceleration

• Chemical species

• Force

• Radiation

• pH

• Humidity

• Strain

• Velocity

• Electric fields

• Magnetic fields

Extrinsic point sensors can measure just about any entity when properly configured. The list above is just a short representation of the many measurands. As stated before, these instruments are used in hundreds of applications. Many of the applications use the fiber as the transmission medium to connect to a MEMS device. MEMS is the acronym for micro-electro-mechanical systems.

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3.4 Fiber Bragg Gratings (FBGs)

This type of sensing mechanism can be deployed on a fiber, either discretely at multiple points or in a continuous manner. The grating is etched onto the fiber’s glass, or is done on the cladding. The grating can be made by a variety of methods. One of the more common methods of fabrication is to use two very strong UV lasers to setup interference fringe patterns of the needed spacing and wavelength. This effectively creates a grating onto the specially doped glass fiber. The result is a long lasting continuous FBG.

The production of gratings at discrete locations is done by lesser means. These grating locations can be mechanically etched onto the fiber. In this way, the sensing location is predetermined and pre-set before installation. Wavelength Domain Multiplexing, WDM is then employed by the “black box” to make sense of the signals since many different grid spacings might be etched onto one fiber.

Figure 3-8 Example of a fiber with multiple discrete Bragg gratings on the fiber

FBG’s are very versatile and can be used for many other measurands as well.

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3.4.1 Setups

Figure 3-9 Figure showing a typical multiplexed grating arrangement

In Figure 3-9, a multiplexed systems using a single fiber with multiple discrete gratings is shown. This arrangement is simple to use since the grating locations along the fiber are already known. The signals simply need to be time gated in order to determine the location with the incoming returned wavelength.

FBG’s are not limited to only the measurement of temperature. If one is to employ the above setup on a utility system, it would be prudent to configure the grating areas with other capability. The cladding in those areas could be made to measure acoustics through acousto-optic materials. The clad material can also be magneto-strictive or electro-strictive for magnetic and electric field measurements.

3.4.2 Modes of Operation

The grating on the fiber measures the temperature adjacent to the fiber in an indirect fashion. The grating, when interrogated with a broadband light or white light source, has a certain wavelength response based on its grating spacing. The grating area returns a specific wavelength of light based on the temperature. Each grating line at the same location does the same. This produces a pulse of light which returns to the instrument that has one wavelength and is coherent. The spacing varies because of the temperatures affect on the fiber’s expansion, or in the case of cold, contraction. The spacing then changes the wavelength response, which is then converted into a temperature measurement back at the box.

Strain, pressure and sound can all exert or affect a change in the grating that will also result in a specific wavelength response to those influences as well. This can cause some confusion with the temperature responses if not filtered effectively.

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3.4.3 Specifications

FBGs can be very accurate because of the lack of attenuation effects since wavelength (not intensity) is the main response to be measured. The location is of course known in the case of discrete gratings, and an optical time domain reflectometer is used to determine signal origin when using a continuous grating of the fiber. Resolution of the temperature spatially along the length is about one centimeter.

3.4.4 Applications

FBGs have been used in many different industries. Because the grating is a selection of tiny mirrors that have a preferential reflection, when these “mirrors,” so to speak, are changed, the reflection is also affected. The two things that affect the grating response are temperature and strain. Any application that produces a pressure or strain on the fiber can be measured. Bonding various other materials to the grating location on a fiber can also be use to affect a strain onto the fiber.

An example is to bond a palladium plate to the grating for measuring the gas hydrogen. Another example would be to use a magneto-strictive material to make magnetic field measurements.

The civil engineering area has been the most prolific user of FBGs to monitor structures such as highways, bridges and buildings. Overhead towers for transmission lines can be monitored to determine their response to wind and ice loading in the field. The temperature aspects of the monitoring process have not been as prevalent as the strain and pressure monitoring.

Fiberglass and carbon fiber composite materials are found more often than metal in the aerospace and aeronautics areas. Composites are very lightweight and strong, but they also can have internal crazing and de-laminations that may not be picked up from surface measurements or inspection. Optical fibers that have Bragg gratings can be spun and molded with the composite to become part of the structure itself. This is a built in internal monitoring capability. The concept of “smart structures” and smart skins thus has come about.

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4 PRESSURE MEASUREMENTS

4.0 Introduction

Strain sensors, vibration sensors and acoustic sensors all rely on the application of a pressure to the sensor head or grating in order to register an effect on the transmitted light. Pressure monitoring is extremely important in the oil and gas industries, drilling industries and chemical plants.

Distributed pressure sensing is not yet commercial although there are strain sensors that use Brillouin scattering in a single mode fiber. On a distributed basis there are micro-bend sensors and those that are interferometric. Strain seems to be the major measurand since composite material cracking and fatigue are important to monitor. Hydrostatic pressure monitoring tends to be at discrete points in most systems. Because hydrostatic pressure on a fiber is fairly well understood, some sensors are of the fiber Bragg grating variety. The two largest categories that are discussed below are extrinsic and FBGs sensors. Distributed sensors or intrinsic designs are discussed briefly below for continuity, however these are not as effective for pressure measurement.

4.1 Intrinsic Sensors

Micro-bending sensors are those that rely on a pressure induced localized loss of radiative signature in the fiber that correlates to the extent of the applied pressure (indicated by extreme attenuation in the monitoring equipment due to the power loss where the pressure is applied). This is the simplest way of detecting pressure at a point along a continuous fiber. Rayleigh backscatter is employed in the form of an OTDR to detect those regions of signal loss due to the bending action of the fiber. This mode of sensing is in essence a radiation attenuation measurement. A quasi-continuous microbending loss can be introduced in a fiber by stringing it along a zigzag path attached to a structure. This provides for multiple measurement points because of the point bends. Another way to get a somewhat continuous effect along the length is to use a special cladding material that can introduce micro-bending on the fiber when subjected to outside pressure. One such concept is the use of a spiral jacket material over the fiber that will produce a localized bend loss when a lateral pressure is applied to the jacket.

A more distributed intrinsic sensing approach uses the internal birefringence property of glass fiber. Specifically, single mode fiber has the best bi-refringent property. Because this intrinsic

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detection scheme relies on polarization of the backscattered light and not radiation loss (amplitude or intensity), a polarization OTDR is required.

Polarization optical time-domain reflectometry (PO-OTDR) is a variant of OTDR, which was the first to be proposed for use in sensing applications. Here the polarization of the Rayleigh backscattered light in a single mode fiber is detected as a function of time. The birefringence parameters of single mode fiber are very sensitive to a number of measurands, with pressure being one of them. Other measurands that can be measured if there is enough of a change in polarization are strain and electric and magnetic fields. The state of polarization (SOP) of the backscattered light varies with distance along the fiber. This variation of the standard OTDR technique is proposed for the distributed monitoring of electric (via the electro-optic Kerr Effect) and magnetic fields (via the magneto-optic Faraday Effect) as well. These effects on the polarization of light will be discussed later in the report.

Since the concept of a polarization OTDR is introduced here, it is worth noting the various other OTDR strategies in use and their respective acronyms.

• Coherent OTDR (CO-OTDR) - The weak returned backscattered signal is mixed with a strong coherent local oscillator optical signal to provide coherent amplification

• Correlation OTDR (COR-OTDR)

• Low correlation OTDR (LC-OTDR)

• Photon-Counting OTDR (PC-OTDR)

• Optical Frequency-Domain Reflectometry (OFDR)

– OFDR with the frequency scanning (OFDR-FS)

– OFDR with the synthesized coherence function (OFDR-SCF)

• Polarization OTDR (PO-OTDR)

4.2 Extrinsic Sensors

Reflection and transmission mechanisms, total reflection, gratings and photo elastic effects are all extrinsic systems that can measure pressure. Some sensor heads use small diaphragms that deflect. The light is then used to measure the degree of deflection and correlated to the external pressure applied. Micro-machined balance beams and ceramics are other sensor head designs.

Fiber optic sensors that use a micro-mechanical resonator as the sensing element can be used for measurement of many physical parameters, such as force, temperature, pressure and acceleration. The operating principle is that the measurand changes the micro-resonator’s natural frequency. The flexural vibrations of the micro-resonator are excited and detected by light. This optical approach of using fibers for light transmission offers immunity to electrical problems. The output of the actual sensor head is a frequency. Because of this output, the line length is really not a factor. The use of metallic glasses (such as the now famous “Metglas” used for transformers) as a micro-resonator material opens the possibility for sensing the changes of a

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measured magnetic field and still perform as a pressure sensor as well. The metallic glasses used in transformers offer magnetic field point measurements. The micro resonant system is an extrinsic type of sensor.

Figure 4-1 Example micro-resonator in a silicon substrate

The figure above shows an example of a very small micro-resonator built into a silicon substrate. Shown is #1, the silicon microstructure surrounding frame, #2 is a support beam, #3 is a movable mass area, and #4 are the micro-resonators.

Other extrinsic pressure sensors that are not as exotic as the micro-resonator use diaphragms and small beams to relay pressure information over a fiber. An effective way of making a quasi-distributed sensor system and measuring several pressure locations is to “pigtail sensors onto a distributed temperature system. In this way each point sensor can be made very accurate due to knowing the temperature in order to self-calibrate the pressure measurement. This arrangement uses extremely small ceramic sensor heads. The head has a micron-sized gap in it that responds to pressure applied to it externally. Light entering the gap is then caused to setup interference fringes. These interference fringes are then responsible for measuring the pressure through their shifting. The ceramic heads of these sensors are around ½ inch outside diameter (12 mm.). They exhibit good response over a wide range as shown below

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Table 4-1 Pressure Measurement Characteristics with a Fiber

Functional ranges 0-18,000psi

Individual pressure sensor range 20003

Accuracy + / - 1.5psi

Resolution < 0.05 psi

Stability < 5 psi / year

Range 12,000 meters

The above table reflects the accuracy for making pressure measurements using a distributed temperature fiber and also as a standalone point sensor connected to a short fiber length. There is no degradation of performance, although there is a slight signal loss and time delay as the distance to the point sensor increases.

Figure 4-2 Example of a small gap extrinsic pressure sensor

The figure above shows a small gap extrinsic sensor that could be very useful for utility point sensing applications in pipe-type cable joints.

3 Pressure range can be extended to 6,000 psi if so required.

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4.3 Fiber Bragg Sensors

FBGs offer a variety of ways to measure pressures. External enhancement tools such as diaphragms can be attached to the sensing region of the grating. The diaphragm pushes on the grating, which changes the grating spacing slightly. This in turn changes the coherent backscattered wavelength. The black box instrumentation then needs to have a small program in it to determine wavelength with pressure.

A diaphragm places both stress and strain on the fiber it is attached to. This in turn can cause a birefringence on the backscattered light. The birefringence is made up of a parallel component and a perpendicular component. These two polarizations need to be taken into account when measuring pressure since each one will yield a slightly different pressure range for the same wavelength range. The figure below illustrates the two polarizations and the pressure ranges of diaphragms the have differing radii to thickness ratios. Each ratio corresponds to a different pressure range.

Figure 4-3 Wavelength response to pressure applied to three sizes of diaphragms with two polarizations

FBGs have been looked at as acoustic sensors for the detection of very small pressures. These sensors are, in essence, acoustic transducers that are responding to sound pressures. The grating’s response is due to the axial and radial strain caused by the application of hydrostatic pressure. These sensors are extremely sensitive and can be a considerable an asset to the cable engineer possibly for detecting insulation partial discharge, cracking or crazing of plastic cable jackets, detection of singular events such as dig-ins, and other events such as joint movement.

The FBGs sensitivity is on the order of 34 psi for every 0.001 nm of wavelength shift in the Bragg Grating backscatter. To increase sensitivitity to below the 1 psi range, two coated FBGs are arranged to form an interferometer. By doing so, acoustic emissions could be detected. This

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is important in the monitoring of composites for crazing (minute crack propagation) in laminated composite materials. Pressures down to 0.002 psi were detected for an EPRI experiment.

4.4 Application of Pressure Sensors

There are a myriad of industries that already use point or extrinsic sensors. The automobile, oil, gas, industrial chemicals and processing plants all use these.

Extrinsic sensors for the utility industry could be a valuable monitoring item for the following:

• Transformer Fluid Monitoring

• High Pressure Fluid Filled Cable

• Pot Head or Termination Gas Pressure Sensing

• SF6 Gas Bus and Interruptors

The FBG style of extrinsic sensors can be used in a “pigtail” (e.g., a fiber section with the FBG is connected on an as-needed basis to the end of a distributed temperature sensing fiber) fashion and be coupled to a distributed temperature sensor for simultaneous pressure and temperature monitoring at joints and in the joint casings for high pressure fluid filled cable systems. By using this approach, a utility could take the concept one step further and monitor for gas chemicals in the manholes by “pigtailing” in chemical sensors to the distributed temperature sensing fiber.

This is mentioned here and in the next chapter on chemical sensing because fiber Bragg Gratings can be used as discrete chemical sensors by using sensitive cladding that reacts to certain chemical species and returns a pressure measuring from the fiber grating by causing a wavelength shift due to strain.

As for the intrinsic style of fiber sensor; a Brillouin temperature and strain sensor could be pulled into a pipe-type cable for continuous monitoring over the entire length. Enhancement points can be made to the cladding material to affect strain changes caused by pressure variations. In this way pressure drops can be monitored. The temperature and pressure information could be inputted into hydraulic calculation programs to determine size and location of leakage areas along the pipe length. In extruded cables, the fiber might be able to detect capacitor discharge (“thumping”) during fault location. While there is no commercial system available for the utility use, there are systems provided by the former York Sensors Co. (now called Sensa) for the oil industry and the undersea cable communications industry.

Taking all of the sensor types into account, the most versatile and easy to employ are the extrinsic sensors although “pigtails” must be brought back to a central monitoring location. There are many commercial entities that can supply these, and the instrumentation is less complicated the less expensive than distributed sensor types.

A versatile sensor configuration when more than one measurand (other than pressure) is being obtained involves a fiber Bragg grating. Instrumentation is more expensive, but still is easy to

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operate. Bragg gratings at discrete points can be pigtailed into other fibers used for distributed sensing as well as for communications. Bragg gratings can be used to monitor stresses on overhead line towers and relay the information over the optical fiber ground wire (OPGW).

One last application of any of these sensor types is intrusion detection. Substations and generating stations can use pressure and or strain sensing fibers for the detection of intruders. Security systems can be tied into the fiber optic system to provide for security that is immune to electric and magnetic interference (e.g., Intelligent Fiber-Optic Sensors Systems, Inc. See Appendix A).

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5 CHEMICAL MEASUREMENT WITH FIBER-OPTIC SENSORS

5.0 Introduction

Measurement of chemicals and or component species of chemicals is usually done in a laboratory environment. The tools of the trade are mass spectrometers, gas chromatographs, liquid chromatographs, IR and NIR spectroscopy, NMR and many more.

By no means will fiber optic sensing of chemicals take the place of laboratory analyses. However, these sensing elements and or fibers can make for less expensive and less complex systems that are field portable.

In the case of hydrocarbons, fiber optic wire is being used to monitor buried tanks, lines and storage structures for leakage into the surrounding soil. Optosci Ltd. is one company that sells a distributed hydrocarbon sensor. The sensor comprises a length of proprietary fiber optic cable addressed by an optical time domain reflectometer (OTDR). The OTDR measures the distribution of optical power loss with distance along the entire length of the cable up to a maximum of 2.5km. Hydrocarbons in contact with the cable induce a local power loss, which is detected and located by the OTDR to a precision of ±2.5m. Cables can be designed for the detection of almost any petroleum derivative plus many synthetic organic liquids such as solvents.

There are many other systems that use fibers to make chemical measurements. Most of the distributed systems rely on a change in refractive index of the fiber, or an effect on a special cladding. The more high technology sensors use evanescent waves between two sections of similar fiber to pick up chemical species. An example of this type of measurement is in the medical field for making oxygen concentration measurement in a patient’s blood.

Fiber-optic hydrocarbon and other chemical sensors have been used for a few years for monitoring large chemical storage facilities for leaks. The fiber sensor can be “tuned”, so to speak, for various chemical species. The application for the power industry turns out to be the easiest chemical to detect; aromatic hydrocarbon fluids.

Point sensing is usually accomplished by passing the light spectra, reflected back for a certain chemical species, through an optical spectrometer. Ocean Optics out of San Diego, California makes complete optical systems for making ocean measurements. These are compact rugged systems that lend themselves to being towed behind boats. Such a setup would most likely detect

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hydrocarbon fluids emanating from submarine pipe type or self-contained fluid-filled cable circuits.

Point sensors that use a fluorescent tip to monitor temperature are also used in the chemical industry. Because optical fiber can withstand harsh chemicals, chemistry labs use fiber-optic probes to monitor the temperature of electrochemical syntheses and oxidation reactions.

The concept of optical chemical sensing uses the following principles or mechanisms, in conjunction with the glass fiber:

• Absorption

• Change of index of refraction

• Polarization

• Fluorescence.

5.1 Distributed Non-Intrinsic Chemical Sensing

Distributed sensing of chemicals is usually a non-intrinsic phenomenon. What is meant is that even though the sensor spans a great distance, the sensing itself is not a property of the glass fiber. The sensing is more likely a change in property of the cladding material.

Some commercial systems either employ one or two fibers. In the case of the single fiber system, the cladding material is made up of a composition such that a large attenuation of light occurs in an area where the cladding reacted to the chemical or chemicals of interest.

Since the only real application for chemical sensing external to underground cable is to sense for hydrocarbon leaks, the distributed hydrocarbon sensor is very applicable. The only drawbacks are that it must be installed in the trench when the cable is placed. Also, with fiber leak detection that uses a swellable cladding, the fiber section exposed to the dielectric liquid must be replaced when the leak is repaired. Other systems using an evanescent method do not detect the nearby dielectric liquid through a cladding change, so the leak detection system would have lower maintenance and repair requirements.

A particular commercial system for hydrocarbon detection has two types or styles of sensor:

• Type I sensors are cut on contact with the contaminant because of swelling and degradation of the polymer cladding material. An optical time domain reflectometer sends a light pulse along the cable and measures the amount of time it takes to reflect back to determine the distance to the cut, and therefore the location of the leak. These cables can monitor distances of up to 50 miles.

• In type II sensors, contaminants near the fiber effectively change the refraction of light pulses traveling through the cable. A light emitting diode and a photo detector are used to detect the leak. This type of sensor is reversible and can be used again after cleanup. This type of cable can only monitor distances of up to 100 feet because the cables are made of fibers with high

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optical attenuation. The good news is that a low attenuation fiber can be used as the transmission path over longer lengths where the 100 ft sections of “active sensor” are pigtailed in.

Type II sensors are also used to monitor storage tanks that contain other types of chemical. The fibers and the refractive changes can be geared or tailored to the specific application.

As mentioned in the introduction, Optosci, is another commercial company that uses a proprietary glass fiber to obtain a true distributed chemical sensing and location system. Their system extends the detection/location range to 2.5 km (1.56 miles).

Since the underground application is to detect and locate hydrocarbon fluid leaks, another optical fiber(s) system shows some promise since it is the underground equivalent of a “smart skin” structure. Fiber optic sensors made in a variety of manners are woven into a geo-synthetic fabric to be laid in a trench; this is called the “smart geo-membrane concept”. The fabric is then connected to a long length of standard non-sensor fiber to take the data back to a central processing area.

Geo-synthetic materials are used in some countries to surround XLPE cables with a backfill material to enhance heat transfer away from the buried circuit. The same fabric material could be use in a trench for HPFF cable as well. The only difference is the fabric would have optical sensors distributed throughout its length along the cable. Bragg or FBGs and evanescent wave sensors can be incorporated. Point sensors could also be used, multiplexed and connected to the transmission fiber. The figure below is a schematic.

Figure 5-1 A schematic of a geo-synthetic monitoring system

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The use of a fabric membrane brings up other possibilities as well. The use of a surrounding membrane that contains optical hydrophones or micro-bend sensors could be used to detect excessive strain over the top of an XLPE cable circuit and possibly the detection of partial discharge acoustics. The acoustics signature would be enhanced in this particular concept by the fact that the cable is wrapped in the smart membrane.

5.2 Extrinsic Point Chemical Sensors

Point sensors used for the detection of certain chemical species, sometimes referred to as fiber optic chemical sensors (FOCS), offer several advantages over traditional sensors. These extrinsic type sensors are lightweight and small size, and fiber optic sensors are robust and have a strong immunity to electromagnetic interference. Since the fiber sensors are made of glass, they are environmentally rugged and can tolerate high temperatures, vibration, shock and they can operate in extremely harsh conditions.

Extrinsic sensors consist of three main parts: light source, optrode and detector. The main part of the sensor, the extrinsic head section is referred as an optrode, The optrode or sensor head contains an appropriate indicator which changes its optical properties in dependence on the chemical species. In these types of sensors, it is necessary to use a sensing head indicator because the measurand chemical most likely cannot directly affect the optical properties of the glass fiber itself. The sensor head indicator can change, for example, absorbance or fluorescence intensity. The light source is matched to the so-called analytical wavelength of the head. In this way, the best sensitivity of the sensor can be obtained per chemical species.

The extrinsic sensor head can use a variety of properties to make a chemical concentration measurement. They can use absorption, phosphorescence, fluorescence, diaphragms and membranes sensitive to chemicals and hydrocarbons and a newer method that is based on color changes.

The color-changing sensor is a reversible one which makes it very useful for the detection of fluid leaks from HPFF cable circuits. The color changes occur when special dyes called solvatochromic (SV) dyes experience polarity changes, in a similar manner as liquid crystals (displays, etc.) change dipole polarity in response to an applied electric field. Effectively, the SV material “senses” the hydrocarbon by the electric field of the incoming light interacting with both the hydrocarbon and SV dye causing a change in the dipole orientation and the color that flouresces. The sensor head itself is then made by immobilizing an SV dye in a polymer film that is coated and then attached to the end of an optical fiber. These SV’s can also be placed along the length (short length) on the sides of an unclad optical fiber for monitoring a larger area. The figure below shows such a setup.

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Figure 5-2 Figure showing a robust sensor head arrangement for making chemical measurements. The dye is the fluorescent material shown.

Environmental engineering groups for monitoring contaminants in water and soils use extrinsic sensors. These sensors can be configured to measure chemicals and items such as moisture, PH and airborne gases.

Table 5-1 Commercial Chemical Sensor Companies and Applications

Company Measurement Technique Application

ADC Ltd. Absorption/Interference Multiple Gas sensing

AVL Photronics Fluorescence CO2/Blood Gas Analysis

Biomedical Sensors Fluorescence PH, CO, CO2, O2, Blood Gas Analysis

CDI/3M Healthcare Fluorescence PH, CO, CO2, O2, Blood Gas Analysis

Ciencia Inc. Fluorescence Aromatic Monitoring

Fiberchem Inc. Refractive Index/Cladding Hydrocarbon Detection

Lightsense Corporation Fluorescence Medical and Industrial CO2 and O2 monitoring

Rosemount Analytical Gas Correlation Spectrometry Flue Gases - CO

Seres Spectrometer THC and Nitrate Monitoring in Water

Servomex NIR Spectrophotometer CO, CO2, CH4, NOX, SOX

Synectics Medical Dual Wavelength Absorption Bile Detection

UOP/Guided Wave NIR Spectrophotometer Petroleum Octane and Total Aromatics

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Fiber Bragg Gratings are also extremely effective in making measurements as well. This is discussed in the next section.

5.3 Fiber Bragg Grating Chemical Sensing

Because Bragg gratings respond to strain by returning a coherent reflected wavelength of light, almost any item that can exhibit a strain field onto the grating can in fact become a sensor. Claddings that are susceptible to chemical species can be used to place strain onto the grating and thus become a point sensor using FBGs.

There are a few ways to measure chemicals and more importantly hydrocarbon fluids, using these gratings:

• The first approach (mentioned above) is done by bonding materials to the fiber grating that will change shape and exhibit a force upon absorption of a particular chemical. An example is the use of a very thin palladium wire bonded to the fiber grating for the detection of hydrogen. Parts per million can be detected like this.

• A second way of detection using gratings is to use an evanescent field approach. This field will be discussed below in more detail since it is not necessary to use a grating, but sensitivities are improved when the two are used together. The use of the grating in this situation does require a special fiber that is “D” shaped. The evanescent field is a component of the electric vector of light that responds to chemicals by changing the index of refraction. This in turn changes the grating and produces a wavelength shift from the Bragg wavelength.

• A third sensor uses an optrode, or sensor head that is fluorescent. The fiber is “tapped” so as to allow the light to exit and enter the fiber after passing through a fluorescent material that responds to external chemicals. The grating itself is used to measure temperature to help compensate for the temperature response of the fluorescent dyes. This then becomes a self-compensating system of point sensors along a fiber length that are all queried via OTDR.

In general, the Bragg sensor has several advantages over all other fiber optic sensors; it is constructed from a single fiber and its response is linear to the measurand. The Bragg sensor also has the greater advantage of temperature compensation by overlaying two gratings where one is for temperature compensation.

5.4 Evanescent Wave Chemical Sensing

Another fiber optic sensing technique that has been gaining popularity is evanescent wave spectroscopy (EWS). EWS have been extensively applied for studying the chemical reactions associated with the polymer curing.

Evanescent wave sensors function by passing a light beam through the length of the fiber such that internal reflections occur at the fiber core-cladding interface. At each reflection, a small amount of the electric field associated with the light beam (called evanescent wave) actually goes

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beyond the fiber and penetrates the cladding surrounding the fiber. The strength of this evanescent electric field depends on the refractive indices of the core and cladding, wavelength of the light beam, and the angle of incidence of the light beam at the core-cladding interface. These fibers use transparent cladding in most cases. The evanescent wave is set up internally to the fiber and cladding by “total” internal reflection, so, if the cladding is thin enough or a portion of the cladding is removed, the evanescent wave can be used to sense the medium in which the fiber is embedded. If the vibration frequency of the molecules in the surrounding medium matches with the frequency of the light wave penetrating the medium, the wave is absorbed. When infrared (IR) radiation is used as the light beam, the absorption becomes more along the lines of an infrared spectrometer by using absorption bands in the IR spectrum. As a result, the fiber uses its internal reflections to only allow the e-field of the light to penetrate. This becomes a total (e.g., infrared, near-infrared and far-infrared) infrared spectrometer with which one can analyze many types of chemicals that have vibrations tuned to certain IR wavelengths. The concept in sensing makes for a very powerful chemical analyzing system.

Figure 5-3 Evanescent wave sensor concept for detection of chemical species

The style of evanescent wave sensor shown in the figure uses a thin section in the cladding and is an absorption technique for determining the chemical through IR spectroscopy.

A very unique means of providing distributed environmental chemistry data and instrumentation data employs a D-shaped optical fiber. A D-shaped optical fiber consists of a normally circular single mode fiber with a portion of its cladding removed on one side parallel to the optical axis. This asymmetrical or half cylinder shape resembles the alphabetical letter "D" when viewed at the fiber's face. Within this fiber type, a single optical mode propagates and has its IR energy extending into the cladding as the evanescent wave and subsequent mode field diameter. Normally this field is a lot closer to the core surface and / or deep within the cladding thickness. However, because of the D-shaped geometry, the evanescent wave propagates precariously close to and, in the proximity of, the flat surfaced side. This continuous feature allows for the ambient conditions and / or chemicals in the vicinity of the "D", to affect the propagating field. The affects along this fiber type can be measured with the appropriate “black box”, known as an optical-to-electrical converter.

This “D” shaped evanescent wave system can measure such items as air quality, pollutants, surface contamination and chemical and biological corrosion on metal pipelines and structures. Other affects can be measured as well, including pH, blood gases, water chemistry and magnetic

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fields. This sensor system has been developed and used to monitor the presence of pollutants on high-voltage insulators helping to predict a flashover potential. Researchers at the University of New Orleans, who have had great success in developing the “D” shaped system, apply this fiber optic sensing to damage control monitoring and shipboard corrosion monitoring.

Another application on ships is with the U.S. Navy. The Navy has been moving towards the use of double hull warships. This is similar to oil tanker double hulls. The hulls are separated with closed cells of expanded metal. The area between the two hulls is thusly, not accessible after the hulls are built. Since these areas are inaccessible, the Navy is employing corrosion monitoring inside these hulls, with the sensor systems being inserted when the ship is first manufactured.

Virtually every one of the sensors and types of sensors for measuring chemicals could be deployed and used by the utility industry. The sensors, whether Bragg type or evanescent or point, would of course have to be precisely “tuned” to give the right response to the chemical need for detection and location. All of the technologies discussed in this chapter can be used for detection and location of hydrocarbon leakage from pipe-type cable systems. Evanescent fibers under the jacket of XLPE cable could monitor the curing process of the insulation. Likewise, the sensor could also be tuned to measure moisture ingress under the jacket.

Extrinsic point sensors could be used in manholes for fire and explosion monitoring. If explosive gas starts to build up in the vault, the point sensor could relay that particular information back to the operator over an embedded fiber under the jacket as well.

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6 ELECTRIC FIELD MEASUREMENT

6.0 Introduction

Measuring electric fields used to be very difficult since the measuring devices of the past would also affect the fringe fields around them. By using non-conductive measurement devices, the fields are less skewed. Many point sensors use a crystal that has a strong Kerr coefficient for measuring the electric component of electromagnetic fields. The Kerr effect is where certain materials change the polarization of light entering the medium when the medium is placed in an electric field. The change in light polarization is dependent on field strength.

Figure 6-1 A typical experimental setup that produces a rotation in polarization plan caused by an applied field to the cell

Another way of using crystals to measure electric fields is to use a piezo-electric crystal. Mechanically striking a crystal produces an impulse field, and the reverse is possible – applying an electric field will produce a mechanical vibration in the crystal.

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Figure 6-2 Pockels cell setup for measurement of large electric fields showing a change in the incident light polarization

An electro-optic crystal can have a wide bandwidth for beam modulation. The figure below shows an alternating field that modulates the beam passing through. This modulation can go from a few Hertz to tera-Hertz levels.

Figure 6-3 Figure showing an alternating field modulated through an electro-optic crystal

There are other materials and polymers that will respond to an applied field directly. These would be useful as cladding materials on distributed and Bragg fiber sensor types.

6.1 Distributed Sensors

Intrinsic distributed fiber optic sensing needs to rely on an external cladding to produce a strain field in the glass. A Brillouin strain / temperature sensor could be configured to measure electric field strength along the length.

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Some of the cladding materials that could be used would employ an electrostriction effect, a dielectric effect or a change in the index of refraction. Since this report examines novel approaches to the problem; a concept used for making artificial muscles seems to be very appropriate for use as a very sensitive cladding material for either a single distributed sensor and or a Bragg Grating.

Called ionic polymer metal composites, IPMC’s, these materials are biometric sensors used for artificial muscles and activation. Strips of these composites can undergo large bending and flapping displacement if an electric field is imposed across their thickness. Thus, in this sense they are large motion actuators. Conversely by bending the composite strip either quasi-statically or dynamically, a voltage is produced across the thickness of the strip. These materials can be manufactured and cut in any size and shape.

Sometimes referred to as ionic chemo-mechanical deformation of polyelectrolytes, polyacrylic acid (PAA) and polyvinyl chloride (PVC) transduction systems became the first of many composites used for this purpose. Some of the properties of these IPMC’s are compared below to other electric field-reacting materials used as muscle actuators.

Table 6-1 IPMC Force, Reaction Speed and Voltage Responses for Actuator Usage

Property Ionic polymer-Metal Composites (IPMC)

Shape Memory Alloys (SMA)

Electro active Ceramics (EAC)

Actuation displacement >10% <8% short fatigue life 0.1 - 0.3 %

Force (MPa) 10 - 30 About 700 30-40

Reaction speed Micro-sec to sec Sec to min Micro-sec to sec

Density 1- 2.5 g/cc 5 - 6 g/cc 6-8 g/cc

Drive voltage 4 - 7 V NA 50 - 800 V

Power consumption Watts Watts Watts

Fracture toughness Resilient, elastic Elastic Fragile

6.2 Extrinsic Sensors

Most point sensors or extrinsic sensors make use of either the Kerr effect or an electrostriction effect of some crystals. Perhaps the most accurate is to use the optics as a Fabry-Perot cavity. Typical applications for Fabry-Perot cavities to measure field are mainly in the high voltage and equipment areas:

• Remote measurement of electric fields.

• Monitoring of electrical equipment.

• Measurement of high-voltage outputs from transmission lines.

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The following figure shows an electric field-measuring point sensor that uses a thin membrane on a silicon substrate to form a Fabry-Perot cavity. This affects the reflected light source coming out of a fiber optic wire.

Figure 6-4 Example electric field measuring point sensor

Such a sensor can be used to safely measure high voltages and to monitor the performance of electrical equipment without having to make a hard electrical contact. The sensing mechanism is based on a Fabry-Perot cavity that consists of two closely spaced, partially transmitting mirrors, as shown in the above figure. The light reflected from these mirrors is dependent on the spacing between them. When they are placed in an electric field, a surface charge develops and causes the spacing to change. The sensor can be calibrated to relate the intensity of the reflected light to the electric field.

A fiber-optic cable that monitors the light reflected from the cavity interrogates the sensor. Initially the sensor has demonstrated the ability to measure large fields. It was used to measure fields of 40 kV/cm and higher. New advances are now continuing to increase the sensitivity of this type of sensor by making the silicon membrane more compliant. This can be done by decreasing its thickness, reducing its stress, or using material like silicon nitride.

At the time this report was prepared, SRICO's Inc. of Ohio has been working with NASA on a fiber optic voltage sensor that could offer improved accuracy in voltage and high voltage measurements.

A NASA prototype device has also been developed commercially for terrestrial applications. Among the many commercial uses are;

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• Measurement of electric field and voltage in electric power systems

• Hazardous environments

• Lightning detection in avionics

• Mining

• Fiber optic communications systems

• Non-contact probing of high-speed integrated circuits

• Biomedical engineering and instrumentation

• Charge measurement in photocopiers and ion neutralization systems

This type of integrated optic voltage sensor employs reverse poling technology that permits the use of simple electrode structures for high voltage sensing without the need for voltage division. This design eliminates electrical isolation problems between the high voltage system and the control system. The sensor is immune to electromagnetic interference, thus yielding accurate measurements over a wide dynamic range.

6.3 Fiber Bragg Grating Sensors

FBG’s or Fiber Bragg Gratings, rely on a change in the strain along the grating area in order to receive the reflected change in wavelength. The measurement of an electric field then has to use an outside entity to produce a change in strain. An electrostriction approach yields the desired result of affecting a strain change.

Some examples of electro-strictive ceramics include, lead magnesium niobate, lead titanate and lead lanthanum zirconate titanate. These exhibit strain deformation in the presence of an electric field, along with a hysteresis on the order of 2-3%. They can also achieve positional feedback of around 10nm and work at voltage frequencies as high as 40kHz. The only disadvantage to these materials is they are strongly temperature dependent. For example, a material designed to operate at room temperature may experience a deterioration of up to 50% at 50°C. If used as the mechanism for FBG’s, then a second FBG would be need to measure the localized temperature in order to correct for the degradation of the field measurement. Still by using this tandem approach of measuring the strain field caused by the electrostriction material and the temperature all at once, a very accurate multiplexed system could be deployed along a power line or cable system.

The table below shows the electrostriction constants for some polymer and ceramic materials that could be used as cladding for FBG’s. PMMA is Plexiglas, a common amorphous polymer. PMMA also exhibits a strong Kerr effect as well.

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Table 6-2 Electrostriction Constants for Polymer and Ceramic Materials

ES Material ES constant m2/V2 Polymer/Ceramic

α-PMMA -3.7 •10-16 Polymer

s-PMMA -7.7 •10-15 Polymer

i-PMMA -1.5 •10-14 Polymer

PMN-PT 6.0 •10-16 Ceramics

PLZT 6.0 •10-16 Ceramics

While not exactly a distributed sensing system in the purest form, continuous FBG’s can use doped fibers or special cladding materials that have a response to the electric field along the length of the fiber. Either the glass is made to have an enhanced Kerr response by doping, or the cladding is made to have a similar enhancement.

Cladding materials are important components for FBG’s as well as for distributed systems. As mentioned before, cladding materials made from IPMC’s could be useful for the grating to see an electric field dependent strain field. Another cladding is a piezoelectric polymer, polyvinylidine fluoride or PVF2. This material responds directly to an external modulated field. The piezoelectric effect is small in low field situations, however, given the high voltage environment the utility industry lives with, the field levels are substantial enough to be accurately measured along the length.

Some of these materials may be suited for measurement of partial discharges directly by sensing the high frequency field. Many materials mentioned in this chapter have very high frequency responses. This holds for all of the sensor technologies; extrinsic, intrinsic and gratings.

In the table and discussion above, PMN is “lead magnesium niobate” and PLZT is “lead ziconate titanate”, a polycrystalline composite of lead zirconate (PZ) and lead titanate (PT).

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7 MEASURING MAGNETIC FIELDS WITH FIBER-OPTICS

7.0 Introduction

Measuring magnetic fields with an optical sensor is very different than using sensors to measure pressure, temperature, strain and other characteristics. This is because the magnetic sensor has to look far a change in the field (differential) as opposed to say a pressure sensor being able to correlate directly to a specific pressure (absolute measurement). Magnetic sensors (fiber optic or otherwise) must look for disturbances in magnetic fields that have been created or modified and from them derive information on properties such as direction, presence, rotation, angle, or electrical currents. The figure below illustrates the sensing strategy.

Figure 7-1 Illustration of indirect magnetic field sensing

There are several techniques for magnetic field measurement. Depending on the application, an acoustic sensing system may work better than a magneto-optic or magneto- striction approach.

There have been two sensor technologies studied in the past that offer different advantages:

• Faraday effect sensors offer spatial resolution on the order of a few cubic millimeters with a sensitivity4 in the low nT/Hz0.5.

• Magneto-strictive sensors offer sensitivity4 below 1pT/Hz0.5 (pic-Tesla / square root of rotational frequency in Hertz) at frequencies over 500Hz but with larger spatial resolutions on the order of cubic centimeters.

4 The sensitivity of the magnetic field is measured in nano-Tesla or pico-Tesla but must consider the Faraday rotation (frequency that the polarization rotates in Hertz). So the Faraday Effect is measured in magnetic field strength per square root of rotational frequency in Hertz.

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Magneto-optical effects or devices that use either the Faraday Effect or the Kerr Effect have been used in the past to make current transformers for measurements on power systems. The Kerr Effect was described in Section 6 of this report. The Faraday Effect is where there is a change in the circular polarization of the incident light to that of the opposite polarization. Materials that exhibit a strong magneto-optic effect are often more effectively used as extrinsic, point type sensors.

Figure 7-2 Figure showing plane polarized light turned into a Faraday rotation (refracted polarized rotation) after passing through a magnetic material

For power applications, the need for spatial resolutions less than a few centimeters is not really needed. Magneto-strictive sensors then would become the more plausible approach for making field measurements along power cables. Certainly 1pT (pico-Tesla) is enough sensitivity to even detect magnetic field energy from partial discharges.

Magneto-strictive materials are the magnetic analogues of piezoelectric materials and electrostriction materials that were discussed in Section 6. As such, they change dimensions when magnetized. These materials are intrinsically ferromagnetic, i.e. their ratio of magnetic permeability “µ”, divided by the permeability of free space “µ0” is much greater than 1. There are also paramagnetic and diamagnetic materials, but these do not produce the necessary changing dimension with field strength.

In a magneto-strictive material, large magnetic fields (the saturation flux density is typically around 1T or 10,000 Gauss) will distort the shape of the electron orbit in the material to the extent that a dimensional change will occur. This change or distortion of the material can be directly correlated to the intensity of the magnetic field. The only drawback is that it is independent of the field polarity.

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Materials are said to have positive magneto-striction when the material expands in the direction of the magnetic field. This results in a transverse constriction. In other words, if a cylinder of materials lies in the direction of the field lines then the constriction would be radially inward. The converse applies to negative magneto-strictive materials.

The induced strain has relatively low hysteresis (typically 2-3%), and magneto-strictive elements can exert high forces (e.g. a 10mm rod operated at 0.1% strain requires a clamping force of around 4kN (900 lbf. to produce zero displacement). A 0.1% strain is typically about the maximum strain that can be achieved. Response times to field application are about 1ms, which is good enough for operation at 60Hz.

Of interest is that these types of materials can be used to convert electrical energy into sound energy and vice versa. Magneto-striction is the cause of the audible hum heard around transformer cores in substations and in distribution transformers.

The only disadvantage to these materials is the high cost of production. Raw materials such as terbium are among the largest contributors to this. These materials are also very brittle and must be installed carefully.

The best known magneto-strictive material is Terfenol which is a compound consisting of Terbium (Te) and iron (Fe), and these were first researched at the Naval Ordinance Laboratory, for ship and submarine detection. Nickel, cobalt and iron are also known to exhibit magneto-striction, as do some rare earth elements. Nickel and cobalt are negatively magneto-strictive, whilst iron is positively magneto-strictive in the presence of a weak magnetic field and negatively magneto-strictive when subjected to stronger magnetic fields.

Magneto-strictive materials are most likely the best for grating type sensors.

7.1 Intrinsic and Distributed Sensors

The measurement of a magnetic field along the length of a distributed sensor has not been effective to date. The advent of a Brillouin temperature and strain sensor in one fiber leads to some intriguing possibilities for future development.

The intrinsic nature of the glass fiber is to respond to strain, which causes losses in the fiber. The strain is caused be bending of the fiber. Work is being done on fabric materials that incorporate shape memory and magneto-strictive materials within a polymer woven matrix. As discussed before, Bragg gratings and point sensors had already been incorporated into a geo-synthetic material. The possibility of a magneto-strictive polymer could become a useful cladding material for a Brillouin sensor.

Based on a review of available research at the time this report was prepared, there currently is no true distributed measurement of magnetic field. Multiplexed gratings stretched out along the

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fiber length can make for a quasi-distributed system of sensors; however, this is still not true intrinsic behavior.

7.2 Extrinsic Point Sensing

The coupling of external sensing probes or sensor heads to optical fiber, for measuring magnetic field, is very easily accomplished with a wide variety of materials. Magnetic sensing can be accomplished through use of magnetically affected diaphragms, garnet crystals and even magneto-rheologic fluids. The following discussion will focus on garnets only.

Iron garnets, such as yttrium iron garnet (YIG), are attractive materials for high-sensitivity current sensors because their magneto-optic effects are several orders of magnitude larger than those of glasses. The Faraday Effect is more complicated in these ferrimagnetic materials. Ferrimagnetic materials are in essence the combining of two ferromagnetic lattice structures. This produces what is known as twin boundaries. Faraday rotation is linear with applied field for fields well below a material’s saturation field. Polarization rotation depends not only on material composition and wavelength, but also on the size and shape of the sensor material.

For an example: a 1mm diameter garnet cylinder, (YIG), has a magneto-optic sensitivity at a 1.3µm wavelength of, 5 x 10-4 Å /(A/m), 833 Å/T to 8x10 –2 Å /(A/m) , (133x10 3 Å/T) at 5 mm long. This is a variation of 160 times. With proper design, high sensitivity, wide bandwidth, and high dynamic range sensors can be built.

Many of the same optical materials used as solid state lasers are (or can be) used as extrinsic sensing elements for magnetic fields due to their strong magneto-optic coefficients. A magnetic sensing strip of material can be used to look at the changes in light polarization. In the case of using the Faraday Effect, a linear response is had at low field levels. At higher levels the Kerr magneto-optic effect comes into play by using a reflection instead of transmission through a material. A point sensor that would use this technique is illustrated below.

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Figure 7-3 Illustration of of the magneto-optic Kerr effect

The magneto-optic Kerr Effect is most useful in the extrinsic type sensor due to the fact the it changes the polarization of the incident light in the reflection mode off of a magneto-optic strip of material.

Macroscopically, an electron in the material feels a Lorentz force in the magnetic field being measured. This changes the magnetic strip’s dielectric constant that in turn changes the surface reflectivity. So if incident light is linear polarized, reflected light is elliptically polarized. The rotation of polarization is known to be proportional to the magnetization of the sample. This can then be correlated to the field to be measured.

7.3 Fiber Bragg Gratings

As has been discussed in this report, the use of bonded materials to a Bragg grating that can cause a strain field to be set up is appropriate for use as a FBG sensor. Magneto-strictive materials can perform this function. Terfenol “D” is probably the more widely known and used, however, more common items can be useful for an FBG sensor as well.

Nickel is a magneto-strictive material that can be deposited onto the fiber. Amorphous steel (or the trade name Metglas) is another material useful for sensing. Magnetic field sensors made from these could be used as excellent methods of measuring current on high voltage transmission lines.

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Figure 7-4 Figure showing a magneto-strictive element covering a region containing a Bragg grating

If a cyclinder is placed over the grating, a radial compression occurs, causing a strain in the region of the magneto-strictive element.

A sensor that utilizes a magneto-strictive induced strain onto a FBG looks very similar to the schematic above. The material that constricts around the grating location is visible in the photo below. In this case the grating location along the fiber and the magneto-strictive material are mounted onto a non-conducting polymer bracket.

Figure 7-5 A photograph is shown of a small magneto strictive sensor element

The U.S. Navy’s Naval Research Laboratory has developed a family of fiber optic sensors for measuring magnetic fields. Their development uses an interferometric method coupled with

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magneto-strictive materials. Fiber optic Fabry-Perot Interferometers are employed as the sensing device along with the constriction of a Bragg grating. The sensors can detect magnetic signals over a wide range of frequencies and amplitudes. They are robust, easily manufactured and can be incorporated with other fiber optic sensors to form multiple sensor arrays. The advantages of using this type of sensor arrangement are:

• Wide range of measurement frequencies and amplitudes

• Easy formation into sensor arrays

• Compatibility with a variety of fiber-optic sensors

• Compatibility with fiber optic telemetry systems

• All-weather detection and tracking of vehicles

• Can be applied to identification of vehicles

• Can monitor ambient magnetic conditions in hard to reach locations

• Has application to traffic control in streets harbors and airport runways

7.4 Other Items of Interest for Measurement of Magnetic Fields

7.4.1 Magneto-strictive Elastomers

Midé, a materials spin-off company from MIT, has developed a magneto-strictive active rubber material to improve the passive and active isolation performance of rubber, the preferred engineering solution for vibration isolation. Magneto-striction, as mentioned above, is the phenomena characterized by a material changing its length when placed in an electro-magnetic field. Essentially, the material generates an electro-magnetic field when it is deformed by external forces (this is a reversible phenomenon). With a Magneto-strictive Active Rubber Isolation Mount (MARIM), the passive isolation provided by a rubber mount is augmented by the ability to actively compensate for static load, reject narrow-band disturbances through notching, adapt control for varying environments, etc. Selectively adding magneto-strictive properties to rubber is an attractive and innovative approach to enhance both the passive and active isolation characteristics of rubber.

Figure 7-6 Illustration of indirect magnetic field sensing

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7-8

Magneto-strictive rubber materials for engine mount isolation are shown above. If successful, a rubber coating or cladding could be produced for fiber optic strands.

7.4.2 Ferromagnetic Shape Memory Alloys, “FSMA”

Shape memory effects can be found in more than fifty alloy systems. Today, however, only the Ni-Ti based shape memory alloys are important for medical applications, and the ternary alloys are pertinent to thermo-activated actuators and magnetic actuators caused by magneto-striction. The basic mechanics governing the properties of SMA is a change in crystal structure: a martensitic structure transforms at a predetermined temperature. Likewise, because of a twin boundary in ternary alloys, a magnetic field can move the boundary and cause a mechanical deformation.

Ferromagnetic shape memory alloys (FSMA’s) are an attractive new class of active materials that have shown very large magnetic-field-induced strains (6%) in single-crystal form. FSMA's have been observed with blocking stresses ranging from 1 to 10 MPa. The mechanism by which these materials strain is field-induced twin-boundary motion as opposed to magnetization rotation relative to the crystallographic axes in the case of Terfenol D.

Shape-memory alloys are characterized by significant dimensional changes. They compete with piezoelectric and magneto-strictive materials in many smart-material and actuator applications. Shape memory materials can exhibit large strains but are typically activated by a temperature change. This mode of actuation is inefficient and has strict frequency limitations.

Figure 7-7 Example shape memory alloy

Shape memory alloys based on nickel have been investigated for approximately 30 years. Magneto-striction was primarily researched for use as thermal recovery and heat engines, but research in the last few years has shown promise for magneto-striction. These alloys (as shown in the figure above) have twin boundaries, which react, to an applied field.

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Figure 7-8 Shape memory alloy with and without applied magnetic field

Figure 7-8 shows the effect of an applied magnetic field. In the left part of the figure, a 26 mm long crystal of Ni-Mn-Ga alloy at room temperature with no applied magnetic field. In the right part of the figure, the same sample after application of a field of order 0.4T by a permanent magnet. The metallic sample exhibits a 5° kink at the twin boundary.

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8 CONCLUSIONS AND RECOMMENDATIONS

8.0 Summary

There is no one system to meet all of the sensing requirements for power cable applications. However, with the many commercial systems available today, a utility could piece together a combination of intrinsic and extrinsic devices that could become a cable condition monitoring system.

With hundreds of available sensor and sensor types available today, it should be fairly easy to put together an adequate monitoring and diagnostic system for cable circuits. Many of the distributed and FBG sensors appear to have direct application to cross-linked cable. Temperature, electric field, pressure/vibration and chemical are all elements that would be worth monitoring.

Temperature monitoring is an obvious application and the first to be used widely on power cable systems. Electric fields could be important to monitor in XLPE because of partial discharge activity. The same is true with the vibration measurands since this can be correlated to acoustic pressure. The two together (electric and vibration) provide for redundancy in measurement that may be fairly accurate. The measurement of chemistry for XLPE could give information as to contaminant ingress into the insulation shield. Water and or cross-linking byproducts may provide important degradation information.

FBG’s can be “pig-tailed” to the fiber optic wire under the shield of an XLPE cable. In this way, gratings can monitor the ambient surroundings to provide important information as to volatile gas build up in manholes, as well as, to measurement of temperature of the cable inside the duct.

For self-contained and high pressure fluid filled circuits, the sensing of and the location of hydrocarbon leaks is important. The hydrocarbon sensing technology employed with fiber optics is a winning combination. The combination provides for alarm, detection and very accurate location. The only drawback is a means of retrofitting existing installations. New installations could be instrumented with sensing fibers at the time of construction and installation.

A complete diagnostics and monitoring system that can span substation, over head line and underground cable areas would be the most logical approach. The sensors are either fiber wire or some phosphor material. It is the interpretation / interrogation system that runs that is the greatest cost for these systems. If one or a few “black boxes” can be tied into a multiplexed sensor array

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Conclusions and Recommendations

8-2

spanning a transmission system, then the larger cost components can be centralized to distribute the sunk cost over a large area.

Specifically related to magnetic field sensing, magnetic field measurement is plausible using point sensing and or FBG’s, but there doesn’t appear to be any good sources of intrinsic distributed sensors that can do the job.

In this report, all of the measurands identified in the previous chapters were either measured directly or correlated through an intermediate process for all sensor types. Magnetic fields are the hold out.

In the power industry there are many manufacturers of optical current sensors. These tend to be large bulky boxes compared to the miniature sensors presented in this report. As a result, these current sensors are not useful for underground cable application.

The measurement of magnetic fields seems to be the most recent research in the area of optical sensing. A good example is shape memory alloys. These were first observed in the 1930’s. In 1961 nickel was studied for its magneto-striction properties. It wasn’t until Nitinol, NiTi, which is nickel and titanium, was discovered in the early 1980’s, that shape memory started to take off as a commodity worth further investigation.

With continued research into the Ni-alloys, it wasn’t until the 1990’s that the relation of nickel’s magneto-striction and shape memory alloys were tied together.

With so many variants of alloy, it would be worth a further look at materials aspects and composites in the future that may be much more directly applicable to underground transmission and power engineering.

Regarding ongoing research, the most useful technologies for distributed fiber sensing are:

• Temperature measurement – Raman-based temperature measurement systems are commercially available using either single or multi-mode fiber with fairly good range of up to 10km. Bragg gratings or other fiber techniques could be applied to provide better spatial resolution for sensitive portions of underground circuits to improve temperature details at critical locations. Any new technology would have to either improve on the maximum range of temperature sensing (maintaining meter or better spatial resolution). A Bragg grating combined with single-mode fiber would give selecting accuracy with a range – 30km – more typical of single-mode Raman-based systems.

• Partial Discharge Detection – Detecting partial discharges in extruded cables might offer commercial interest for some applications. If power cable manufacturing could routinely incorporate fibers for partial discharge detection, this would be possible. The fiber would have to located close to the cable core – probably under the metallic sheath – which would mean the use of the technology would have to initiate from the time of cable design.

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• Leak Detection – Fiber coatings that react to fluorocarbons would be invaluable to leak detection systems on pipe-type and self-contained cables, particularly those methods utilizing an evanescent method that essentially makes the leak detection fiber “self healing” after the dielectric liquid is cleaned following a leak repair.

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9-1

9 REFERENCES

The following references were used for this report:

Chapter 1

1. Jasenek, J., “The Theory and Applications of Fiber Optic Sensors with Spread Parameters”,Slovak University of Technology, Bratislava, Slovakia, 2003

Chapter 2

2. Dakin and B.Culshaw (Eds.), "Optical Fiber Sensors, Principles and Components", Vol. I & II , Artech House, Boston 1988.

3. D.B.Keck, “Optical Sensors and Specialty Fibers, Chapter 6 of “Optoelectronics in Japan and the United States”, National Science Foundation & Japanese technology Evaluation Center, 1996

4. J.Dakin, B. Culshaw, “Optical Fiber Sensors, Applications, Analysis and Future Trends”, Vol. 4, Artech House, Boston 1997.

5. ChenYang- “Sensors, Magnetics and Measurements”, German Research Center for Earth Sciences, Potsdam

6. unknown, “Distributed Measurement of Temperature, Moisture and Strain for Civil Engineering Application Using a Fiber-Optic Sensor”, Lehigh University & Drexel University, Sponsor NSF, 2002

7. Jozef Jasenek, The Theory and Application of Fiber Optic Sensors with Spread Parameters”, “A brief overview and classification of the fiber optic sensors”, Bratislava, EAEEIE Organization, Slovakia

8. Chen Yang, “Fiber Optic Bragg-Grating Sensors”, German Research Center for Earth Sciences, Potsdam

9. A. Krispin, “Fiber Bragg Grating Summary, EE558, April 2000

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References

9-2

Chapter 3

10. G. Pitt. 1990. “Optical Fiber Sensors,” Fiber Optics Handbook, McGraw-Hill, New York.

11. ebrochure from Sensor Tran, Division of SPEC, Austin, TX

12. Kazuo Hotate , Masato Tanaka, “Correlation-based continuous-wave techniques provide high spatial resolution for distributed fiber-optic sensing”, OE Magazine, The International Society for Optical Engineering, Nov.2001

13. J. Stokes, G. Palmer, “A Fiber-Optic Temperature Sensor”, Sensor Technology and Design, Sensors Online, August 2002

14. point sensor specs

15. D.J.F. Cooper, P.Smith, Time Division Multiplexing of a Serial Fiber Bragg Grating Strain Sensor Array, University of Toronto

Chapter 4

16. Sensa Organization, “Multipoint Pressure Sensing”, Sensa Systems Reference 1000, 09/25/2000

17. Physics@ nad.ru, Siltec Fiber-optic Systems Components, Russia

18. Luna Sensors, Luna Innovations, Blacksburg, Virginia

19. W.Morey, “Development of Fiber Bragg Grating Sensors for Utility Applications”, EPRI Report # TR-105190, September 1995

Chapter 5

20. Borns, D. J.“Geo-membrane with incorporated optical fiber sensors for geo-technical and environmental applications.” Proceedings of the International Containment Technology , 1997

21. Optical Chemical Sensor, Lawrence Livermore Laboratories National Lab

22. J.Dakin, B. Culshaw, S. Crossly, “Optical Fiber Sensors, Applications, Analysis and Future Trends”, “Fiber-Optic Sensors: Commercial Presence”, Vol. 4, Artech House, Boston 1997.

23. V. Kapila, L. Kjerengtroen, W. M. Cross et. al. , “Strain Monitoring by Evanescent Wave Spectroscopy”, National Science Foundation under Grant #CMS-9453467, South Dakota School of Mines and Technology. 2000

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References

9-3

24. unknown, “Science of Fiber Optic Sensors”, 1998

25. Omni Technology, Inc., University of New Orleans, GCRMTC Concept. For Ship Board Monitoring.

Chapter 6

26. R. Besancon, “The Encyclopedia of Physics”, Van Nostrand Reinhold, 1974

27. M. Megumu, et. al., “Electric Fields Near Triggered Lightning Channels Measured with Pockels Sensors”, Journal of Geophysical Research, Vol.107, No. D16, 2002

28. Modulated Pockels, Technical Article, Opteos, Inc.,1340 Eisenhower Place,Ann Arbor, Michigan 48108, 2003

29. M. Shahinpoor, Y. Bar-Cohen, J.O. Simpson and J. Smith, “Ionic Polymer-Metal Composites (IPMC) As Biomimetic Sensors, Actuators & Artificial Muscles- A Review”,

30. Sang Sheem, “Remote Electric Field Sensor”, Applications “Lawrence Livermore National Laboratory – STR14, October 1995

31. SRICO, Inc., ”High Voltage Sensor for NASA Applications, SRICO, Inc. Powell, Ohio

32. Y. Bar Cohen, “Low Mass, Compact, Muscle-Actuators Using Electrostrictive Polymers(ESP)”, Jet Propulsion Laboratiry, The TeleRobotic Intercenter Working Group (TRIWG), March 1-3, 1995

Chapter 7

33. M.J. Caruso,T. Bratland, “A New Perspective On Magnetic Field Sensing”, Sensors Online, December 1998.

34. Author Unknown, “Magneto-Strictors – An Overview”, Azom, Institute of Materials

35. G.W.Day, K.B. Rochford, A,H. Rose, “Fundamentals and Problems of Fiber Current Sesnors”, NanoNews, October 1996.

36. Technical Release on “Fiber-optic Sensor Developments”, Naval Research Laboratory , Technology Transfer Office, Code 1004, 4555 Overlook Avenue, SW • Washington, DC 20375-5320, CY 2001

37. Mide, 200 Boston Ave, Suite 1000 Medford, MA 02155 U.S.A.

38. Jizhao, Ma, “Ferromagnetic Shape Memory Alloys”, Office of Naval Research Multi-University Research Initiative, MIT Magnetic Materials Group, December 2003]

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A-1

A BACKGROUND INFORMATION

A.1 Companies, Contacts, Sources and Manufacturers

A.1.1 Companies / Manufacturers

Airak, Inc. 9058 Euclid Ave. Manassas Virginia, 20110

Astro Technology, Inc Ellington Field, Houston, TX.

Boeing 100 N. Riverside Chicago, IL. 60606

Extrema-USA, 2500 North Loop Dr Zes, IA 50010

Fibercore Limited University Parkway, Chilworth Science Park Southhampton, Hampshire, 50167QQ United Kingdom

Future Fiber Technologies, Inc. 1311 Londontown Blvd, Suite121, Unit 110 Eldersburg, MD 21784 and 10 Harnett Close Mulgrave, Victoria, 3170 Australia

Fysel / Norwegian University of Science and Technology Trondheim, Norway

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A-2

Intelligent Fiber-Optic Sensor Systems, Inc Division of Perimax GmbH Lothar Guttstein Am Hang 8 24238 Selent

Mide, Corporation 200 Boston Ave, Suite 1000 Medford, MA 02155 U.S.A

NXT Phase, Inc 3040 East Broadway Vancouver BC V5M 1Z4 and 2075 W. Pinnacle Peak Road, Suite 100 Phoenix, AZ 85027-1215

Northropt-Grumman Woodland Hills California

Opteos, Inc 1340 Eisenhower Place Ann Arbor, Michigan 48108

Optisense Private Road 1400 Bridgeport, TX 76426

Opto-Electronics Magazine – SPIE, The International Society for Optical Engineering 17 Old Nashua Rd., Suite 25 Amherst, NH 03031

Optosci Ltd. 141 St. James Road Glasgow G4 0LT Scotland

Optrand Plymouth, Michigan 48170, USA

Sabeus Sensor Systems 20630 Nordhoff St Chatsworth, CA 91311 USA

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Background Information

A-3

Safibra Polických v z 1233 251 01 í any

Sensa Organization Gamma House, Chilworth Science Park, Southampton, Hampshire, SO 16 7NS and 7030 Ardmore Street Houston Texas 77054

Sensors Magazine 1 Phoenix Mill Lane Peterborough, NH 03458

Siltec, LTD. Russia

SPA Systems Planning and Analysis, Inc. 2000 North Beauregard St., Suite 400 Alexandria, Virginia 22311

SRICO, Inc 2724 Sawbury Blvd Columbus, Ohio, 43235

Technology Enhancement Corporation 399 Pearl Street Woodbridge, New Jersey 07095

Wikipedia Organization – An Internet Encyclopedia

A.1.2 University / Government Sources

Azom, Institute of Materials

California State University at San Jose San Jose, Ca

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Background Information

A-4

CIRL; Canadian Instrumentation Research Ltd E8-1155 Appleby Line,Burlington, Ontario Canada L7L 5H9

Defense Advanced Research Projects Agency (DARPA) Joint Program Steering Group, Arlington, Virginia

Drexel University 3141 Chestnut Street Philadelphia, PA 19104

EAEEIE: Electrical and Information EngineeRing in Europe, Slovak University of Technology in Bratislava, Slovakia

Federal Highway Administration

Jet Propulsion Laboratory Pasadena. CA

Lawrence Livermore National Laboratory Livermore, CA

Lehigh University 27 Memorial Drive West Bethlehem, Pa. 18015

Massachusetts Institute Of Technology 77 Massachusetts Avenue Cambridge, MA 02139-4307

National Aeronautics and Space Administration Langley Research Center Langley, Virginia

Naval Research Laboratory 4555 Overlook Avenue, S.W Washington DC 20375-5326

Sandia National Laboratory Albuquerque, NM

South Dakota School of Mines and Technology 501 East Saint Joseph Street Rapid City, SD 57701

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Background Information

A-5

U.S. Government Office of Science and Technology Office of Science and Technology Policy Executive Office of the President Washington, DC 20502

United States Navy Naval Command, Control and Ocean Surveillance Center, In ServiceEngineering -East (NISE East) P.O. Box 190022 North Charleston, South Carolina 29419-902

University of New Orleans 2000 Lakeshore Drive New Orleans, LA 70148

University of Technology, Bratislava, Slovakia Vazovova 5 812 43 Bratislava 1 Slovakia

University of Toronto Toronto, Ontario, Canada, M5S 1A1

Virginia Tech Fiber & Electro-Optics Research Center, Virginia Tech 106 Plantation Road MC 0356 Virginia Tech Blacksburg VA 24061

A.1.3 Other Information Sources

Electric Power Research Institute

Institute of Electrical and Electronic Engineers-Lasers & Electro-optics Society

Association of State Dam Safety Officials

Fiber Optics Handbook, McGraw-Hill, NY

Journal of Applied Physics / Applied Physics Letters

Optical Fiber Sensors, Vol. 4, Artech House, Boston

Handbook of Physical Calculations, McGraw-Hill, NY

Solid State Physics, W.B. Saunders Company, Philadelphia

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Background Information

A-6

Chemical and Biochemical Sensing With Optical Fibers and Waveguides, Artech House, Boston

Photonics Technology Letters, Lasers & Electro-optics Society 2000, 2002, 2002 Databases.

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B-1

B GLOSSARY OF OPTICS TERMS

A

Absorption: That portion of optical attenuation in optical fiber resulting from the conversion of optical power to heat, and is caused by impurities in the fiber such as hydroxyl ions.

Acceptance Angle: The half-angle of the cone within which incident light is totally internally reflected by the fiber core. It is equal to sin-1(NA).

AM: See amplitude modulation.

Amplifier: A device that boosts the strength of an electronic signal. In a cable system, amplifiers are spaced at regular intervals throughout the system to keep signals picture-perfect no matter where you live.

Amplitude Modulation (AM): A transmission technique in which the amplitude of the carrier is varied in accordance with the signal.

Angstrom (Å): A unit of length in optical measurements where:

1Å =10-10 meters, =10-4 micrometers, =10-1 nanometers.

The angstrom has been used historically in the field of optics, but it is not an SI (International System) unit. Rarely used in fiber optics; nanometers is preferred.

Angular Misalignment: Loss at a connector due to fiber end face angles being misaligned.

Anti-Stokes Band: See Stokes Band

APC: Angle polished connector. An 5°-15° angle on the connector tip for the minimum possible backreflection.

AR Coating: Antireflection coating. A thin, dielectric or metallic film applied to an optical surface to reduce its reflectance and thereby increase its transmittance.

Asynchronous: Data that is transmitted without an associated clock signal.

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B-2

Asynchronous Transfer Mode (ATM): A digital transmission switching format, with cells containing 5 bytes of header information followed by 48 data bytes. Part of the B-ISDN standard.

Attenuation: The decrease in signal strength along a fiber optic waveguide caused by absorption and scattering. Attenuation is usually expressed in dB/km.

Attenuation Constant: For a particular propagation mode in an optical fiber, the real part of the axial propagation constant.

Attenuation-Limited Operation: The condition in a fiber optic link when operation is limited by the power of the received signal (rather than by bandwidth or distortion).

Attenuator: 1) In electrical systems, a usually passive network for reducing the amplitude of a signal without appreciably distorting the waveform. 2) In optical systems, a passive device for reducing the amplitude of a signal without appreciably distorting the waveform.

Avalanche Photodiode (APD): A photodiode that exhibits internal amplification of photocurrent through avalanche multiplication of carriers in the junction region.

Average Power: The average level of power in a signal that varies with time.

Axial Propagation Constant: For an optical fiber, the propagation constant evaluated along the axis of a fiber in the direction of transmission.

Axis: The center of an optical fiber.

B

Backscattering: The return of a portion of scattered light to the input end of a fiber; the scattering of light in the direction opposite to its original propagation.

Bandwidth: The range of frequencies within which a fiber optic waveguide or terminal device can transmit data or information.

Bandwidth-Limited Operation: The condition in a fiber optic link when bandwidth, rather than received optical power, limits performance. This condition is reached when the signal becomes distorted, principally by dispersion, beyond specified limits.

Baseband: A method of communication in which a signal is transmitted at its original frequency without being impressed on a carrier.

Baud: A unit of signaling speed equal to the number of signal symbols per second, which may or may not be equal to the data rate in bits per second.

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Beamsplitter: An optical device, such as a partially reflecting mirror, that splits a beam of light into two or more beams. Used in fiber optics for directional couplers.

Bel (B): The logarithm to the base 10 of a power ratio, expressed as B = log10(P1/P2), where P1

and P2 are distinct powers. The decibel, equal to one-tenth bel, is a more commonly used unit.

Bending Loss: Attenuation caused by high-order modes radiating from the outside of a fiber optic waveguide which occur when the fiber is bent around a small radius. See also macrobending, microbending.

Bend Radius: The smallest radius an optical fiber or fiber cable can bend before increased attenuation or breakage occurs.

Bidirectional: Operating in both directions. Bidirectional couplers operate the same way regardless of the direction light passes through them. Bidirectional transmission sends signals in both directions, sometimes through the same fiber.

Birefringent: Having a refractive index that differs for light of different polarizations.

Bit: The smallest unit of information upon which digital communications are based; also an electrical or optical pulse that carries this information.

Bit Error Rate (BER): The fraction of bits transmitted that are received incorrectly.

BNC: Popular coax bayonet style connector. Often used for baseband video.

Broadband: A method of communication where the signal is transmitted by being impressed on a high-frequency carrier.

Buffer: 1) In optical fiber, a protective coating applied directly to the fiber. 2) A routine or storage used to compensate for a difference in rate of flow of data, or time of occurrence of events, when transferring data from one device to another.

Butt Splice: A joining of two fibers without optical connectors arranged end-to-end by means of a coupling. Fusion splicing is an example.

BW: See bandwidth.

Byte: A unit of eight bits.

C

Cable: One or more optical fibers enclosed within protective covering(s) and strength members.

Cable Assembly: A cable that is connector terminated and ready for installation.

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Carrier-to-Noise Ratio: The ratio, in decibels, of the level of the carrier to that of the noise in a receiver's IF bandwidth before any nonlinear process such as amplitude limiting and detection takes place.

Center Wavelength: In a laser, the nominal value central operating wavelength. It is the wavelength defined by a peak mode measurement where the effective optical power resides. In an LED, the average of the two wavelengths measured at the half amplitude points of the power spectrum.

Chirp: In laser diodes, the shift of the laser's central wavelength during single pulse durations due to laser instability.

Chromatic Dispersion: All fiber has the property that the speed an optical pulse travels depends on its wavelength. This is caused by several factors including material dispersion, waveguide dispersion and profile dispersion. The net effect is that if an optical pulse contains multiple wavelengths (colors), then the different colors will travel at different speeds and arrive at different times, smearing the received optical signal.

Cladding: Material that surrounds the core of an optical fiber. Its lower index of refraction, compared to that of the core, causes the transmitted light to travel down the core.

Cladding Mode: A mode confined to the cladding; a light ray that propagates in the cladding.

Cleave: The process of separating an optical fiber by a controlled fracture of the glass, for the purpose of obtaining a fiber end, which is flat, smooth, and perpendicular to the fiber axis.

CMOS: Complementary metal oxide semiconductor. A family of IC's. Particularly useful for low-speed or low-power applications.

CNR: See carrier-to-noise ratio.

Coating: The material surrounding the cladding of a fiber. Generally a soft plastic material that protects the fiber from damage.

Coherent Communications: In fiber optics, a communication system where the output of a local laser oscillator is mixed optically with a received signal, and the difference frequency is detected and amplified.

Concatenation: The process of connecting pieces of fiber together.

Concentrator: A multiport repeater.

Concentricity: The measurement of how well-centered the core is within the cladding.

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Connector: A mechanical or optical device that provides a demountable connection between two fibers or a fiber and a source or detector.

Connector Plug: A device used to terminate an optical conductor cable.

Connector Receptacle: The fixed or stationary half of a connection that is mounted on a panel/bulkhead. Receptacles mate with plugs.

Connector Variation: The maximum value in dB of the difference in insertion loss between mating optical connectors (e.g., with remating, temperature cycling, etc.). Also called optical connector variation.

Core: The light-conducting central portion of an optical fiber, composed of material with a higher index of refraction than the cladding. The portion of the fiber that transmits light.

Counter-Rotating: An arrangement whereby two signal paths, one in each direction, exist in a ring topology.

Coupler: An optical device that combines or splits power from optical fibers.

Coupling Ratio/Loss (CR,CL): The ratio/loss of optical power from one output port to the total output power, expressed as a percent. For a 1 x 2 WDM or coupler with output powers O1 and O2, and Oi representing both output powers:

CR(%) = (Oi/(O1 + O2)) x 100% and CR(%) = -10•log10 (Oi/(O1 + O2)).

Critical Angle: In geometric optics, at a refractive boundary, the smallest angle of incidence at which total internal reflection occurs.

Cutback Method: A technique of measuring optical fiber attenuation by measuring the optical power at two points at different distances from the test source.

Cutoff Wavelength: In single-mode fiber, the wavelength below which the fiber ceases to be single-mode.

CW: Abbreviation for continuous wave. Usually refers to the constant optical output from an optical source when it is biased (i.e., turned on) but not modulated with a signal.

D

dB: Decibel.

dBc: Decibel relative to a carrier level.

dBµ: Decibels relative to microwatt.

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dBm: Decibels relative to milliwatt.

Decibel (dB): A unit of measurement indicating relative optic power on a logarithmic scale. Often expressed in reference to a fixed value, such as dBm (1 milliwatt) or dBµ (1 microwatt).

dB = 10•log10 (P1/P2)

Demultiplexer: A module that separates two or more signals previously combined by compatible multiplexing equipment.

Detector: An opto-electric transducer used in fiber optics to convert optical power to electrical current. Usually referred to as a photodiode.

DFB: See distributed feedback laser.

DG: See differential gain.

Diameter-Mismatch Loss: The loss of power at a joint that occurs when the transmitting fiber has a diameter greater than the diameter of the receiving fiber. The loss occurs when coupling light from a source to fiber, from fiber to fiber, or from fiber to detector.

Dichroic Filter: An optical filter that transmits light according to wavelength. Dichroic filters reflect light that they do not transmit.

Dielectric: Any substance in which an electric field may be maintained with zero or near-zero power dissipation. This term usually refers to non-metallic materials.

Differential Gain: A type of distortion in a video signal that causes the brightness information to be distorted.

Differential Phase: A type of distortion in a video signal that causes the color information to be distorted.

Diffraction Grating: An array of fine, parallel, equally spaced reflecting or transmitting lines that mutually enhance the effects of diffraction to concentrate the diffracted light in a few directions determined by the spacing of the lines and by the wavelength of the light.

Diode: An electronic device that lets current flow in only one direction. Semiconductor diodes used in fiber optics contain a junction between regions of different doping. They include light emitters (LED's and laser diodes) and detectors (photodiodes).

Diode Laser: Synonymous with injection laser diode.

DIP: Dual in-line package.

Diplexer: A device that combines two or more types of signals into a single output.

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Directional Coupler: A coupling device for separately sampling (through a known coupling loss) either the forward (incident) or the backward (reflected) wave in a transmission line.

Directivity: See near-end crosstalk.

Dispersion: The temporal spreading of a light signal in an optical waveguide caused by light signals traveling at different speeds through a fiber either due to modal or chromatic effects.

Dispersion-Shifted Fiber: Standard single-mode fibers exhibit optimum attenuation performance at 1550 nm and optimum bandwidth at 1300 nm. Dispersion-shifted fibers are made so that both attenuation and bandwidth are optimum at 1550 nm.

Distortion: Nonlinearities in a unit that cause harmonics and beat products to be generated.

Distortion-Limited Operation: Generally synonymous with bandwidth-limited operation.

Distributed Feedback Laser (DFB): An injection laser diode which has a Bragg reflection grating in the active region in order to suppress multiple longitudinal modes and enhance a single (clean) longitudinal mode.

Dominant Mode: The mode in an optical device spectrum with the most power.

Double-Window Fiber: This term is used two ways. For multimode fibers, the term means that the fiber is optimized for 850 nm and 1300 nm operation. For single-mode fibers, the term means that the fiber is optimized for 1300 nm and 1550 nm operation.

DP: See differential phase.

Duplex Cable: A two-fiber cable suitable for duplex transmission.

Duplex Transmission: Transmission in both directions, either one direction at a time (half-duplex) or both directions simultaneously (full-duplex).

E

EDFA: See Erbium-doped fiber amplifier.

Edge-Emitting Diode: An LED that emits light from its edge, producing more directional output than surface-emitting LED's that emit from their top surface.

Electromagnetic Interference (EMI): Any electrical or electromagnetic interference that causes undesirable response, degradation, or failure in electronic equipment. Optical fibers neither emit nor receive EMI.

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Electromagnetic Radiation (EMR): Radiation made up of oscillating electric and magnetic fields and propagated with the speed of light. Includes gamma radiation, X-rays, ultraviolet, visible, and infrared radiation, and radar and radio waves.

Electromagnetic Spectrum: The range of frequencies of electromagnetic radiation from zero to infinity.

ELED: See edge-emitting diode.

Ellipticity: Describes the fact that the core or cladding may be elliptical rather than circular.

EMD: See equilibrium mode distribution.

EMI: Electromagnetic interference.

EMP: Electromagnetic pulse.

EMR: Electromagnetic radiation.

E/O: Abbreviation for electrical-to-optical converter.

EOM: Electro-Optic Modulator

Equilibrium Mode Distribution (EMD): The steady modal state of a multimode fiber in which the relative power distribution among modes is independent of fiber length.

Erbium-Doped Fiber (EDF) Amplifier: Optical fibers doped with the rare earth element erbium, which can amplify light in the 1550 nm region when pumped by an external light source.

Evanescent Wave: Light guided in the inner part of an optical fiber's cladding rather than in the core.

Excess Loss: In a fiber optic coupler, the optical loss from that portion of light that does not emerge from the nominal operation ports of the device.

External Modulation: Modulation of a light source by an external device that acts like an electronic shutter.

Extinction Ratio: The ratio of the low, or OFF optical power level (PL) to the high, or ON optical power level (PH).

Extinction Ration (%) = (PL/PH) x 100

Extrinsic Loss: In a fiber interconnection, that portion of loss not intrinsic to the fiber but related to imperfect joining of a connector or splice.

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F

Fall Time: Also called turn-off time. The time required for the trailing edge of a pulse to fall from 90% to 10% of its amplitude; the time required for a component to produce such a result. Typically measured between the 80% and 20% points or alternately the 90% and 10% points.

Faraday Effect: A phenomenon that causes some materials to rotate the polarization of light in the presence of a magnetic field parallel to the direction of propagation. Also called magneto-optic effect.

Far-End Crosstalk: See wavelength isolation.

FC: A threaded optical connector that originated in Japan. Good for single-mode or multimode fiber and applications requiring low backreflection.

FC/PC: See FC. A special curved polish on the connector for very low backreflection.

FDM: See frequency-division multiplexing.

Ferrule: A rigid tube that confines or holds a fiber as part of a connector assembly.

FET: Field-effect transistor.

Fiber Grating: An optical fiber in which the refractive index of the core varies periodically along its length, scattering light in a way similar to a diffraction grating, and transmitting or reflecting certain wavelengths selectively.

Fiber Optic Attenuator: A component installed in a fiber optic transmission system that reduces the power in the optical signal. It is often used to limit the optical power received by the photodetector to within the limits of the optical receiver.

Fiber Optic Cable: A cable containing one or more optical fibers.

Fiber Optic Gyroscope: A coil of optical fiber that can detect rotation about its axis.

Fiber Optic Link: A transmitter, receiver, and cable assembly that can transmit information between two points.

Fiber Optic Span: An optical fiber/cable terminated at both ends which may include devices that add, subtract, or attenuate optical signals.

Fiber Optic Subsystem: A functional entity with defined bounds and interfaces which is part of a system. It contains solid state and/or other components and is specified as a subsystem for the purpose of trade and commerce.

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Filter: A device which transmits only part of the incident energy and may thereby change the spectral distribution of energy.

FIT Rate: Number of device failures in one billion device hours.

Fluoride Glasses: Materials that have the amorphous structure of glass but are made of fluoride compounds (e.g., zirconium fluoride) rather than oxide compounds (e.g., silica). Suitable for very long wavelength transmission.

FM: See frequency modulation.

FOG-M: Fiber optic guided missile.

FOTP: See fiber optic test procedure.

FP: Fabry-Perot. Generally refers to a type of laser.

Frequency-Division Multiplexing (FDM): A method of deriving two or more simultaneous, continuous channels from a transmission medium by assigning separate portions of the available frequency spectrum to each of the individual channels. In optical communications, one also encounters wavelength-division multiplexing (WDM) involving the use of several distinct optical sources (lasers), each having a distinct center wavelength.

Frequency Modulation (FM): A method of transmission in which the carrier frequency varies in accordance with the signal.

Fresnel Reflection Loss: Reflection losses at the ends of fibers caused by differences in the refractive index between glass and air. The maximum reflection caused by a perpendicular air-glass interface is about 4% or about -14 dB.

FSK: Frequency shift keying. A method of encoding data by means of two or more tones.

Full-Duplex: Simultaneous bidirectional transfer of data.

Fused Coupler: A method of making a multimode or single-mode coupler by wrapping fibers together, heating them, and pulling them to form a central unified mass so that light on any input fiber is coupled to all output fibers.

Fused Fiber: A bundle of fibers fused together so they maintain a fixed alignment with respect to each other in a rigid rod.

Fusion Splicer: An instrument that permanently bonds two fibers together by heating and fusing them.

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FWHM: Full width half maximum. Used to describe the width of a spectral emission at the 50% amplitude points.

FWHP: Full width half power. Also known as FWHM.

G

GaAlAs: Gallium aluminum arsenide. Generally used for short wavelength light emitters.

GaAs: Gallium arsenide. Used in light emitters.

GaInAsP: Gallium indium arsenide phosphide. Generally used for long wavelength light emitters.

Gap Loss: Loss resulting from the end separation of two axially aligned fibers.

Gate: 1) A device having one output channel and one or more input channels, such that the output channel state is completely determined by the input channel states, except during switching transients. 2) One of the many types of combinational logic elements having at least two inputs.

Gaussian Beam: A beam pattern used to approximate the distribution of energy in a fiber core. It can also be used to describe emission patterns from surface-emitting LED's. Most people would recognize it as the bell curve.

GBaud: One billion bits of data per second or 109 bits.

Gb/s: See GBaud.

Ge: Germanium. Generally used in detectors. Good for most wavelengths (e.g., 800-1600 nm).

GHz: Gigahertz. One billion Hertz (cycles per second) or 109 Hertz.

Graded-Index Fiber: Optical fiber in which the refractive index of the core is in the form of a parabolic curve, decreasing toward the cladding.

GRIN: Gradient index. Generally refers to the SELFOC lens often used in fiber optics.

Ground Loop Noise: Noise that results when equipment is grounded at points having different potentials thereby creating an unintended current path. The dielectric properties of optical fiber provide electrical isolation that eliminates ground loops.

Group Index: Also called group refractive index. In fiber optics, for a given mode propagating in a medium of refractive index (n), the group index (N), is the velocity of light in a vacuum (c), divided by the group velocity of the mode.

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Group Velocity: 1) The velocity of propagation of an envelope produced when an electromagnetic wave is modulated by, or mixed with, other waves of different frequencies. 2) For a particular mode, the reciprocal of the rate of change of the phase constant with respect to angular frequency. 3) The velocity of the modulated optical power.

H

Half-Duplex: A bidirectional link that is limited to one-way transfer of data, i.e., data can't be sent both ways at the same time.

Hard-Clad Silica Fiber: An optical fiber having a silica core and a hard polymeric plastic cladding intimately bounded to the core.

Hertz: One cycle per second.

Heterodyne Detection: To take one frequency or wavelength and use it to mix with a second to produce one of two outcomes; a constructive interference mode and / or a destructive interference mode.

HIPPI: High performance parallel interface as defined by ANSI X3T9.3 document.

Hydrogen Losses: Increases in fiber attenuation that occur when hydrogen diffuses into the glass matrix and absorbs some light.

Hz: See Hertz.

I

IDP: See integrated detector/preamplifier.

Index-Matching Fluid: A fluid whose index of refraction nearly equals that of the fiber's core. Used to reduce Fresnel reflection at fiber ends. See also index-matching gel.

Index-Matching Gel: A gel whose index of refraction nearly equals that of the fiber's core. Used to reduce Fresnel reflection at fiber ends. See also index-matching fluid.

Index of Refraction: Also refractive index. The ratio of the velocity of light in free space to the velocity of light in a fiber material. Symbolized by n. Always greater than or equal to one.

Infrared (IR): The region of the electromagnetic spectrum bounded by the long-wavelength extreme of the visible spectrum (about 0.7 mm) and the shortest microwaves (about 0.1 mm).

Infrared Fiber: Colloquially, optical fibers with best transmission at wavelengths of 2 mm or longer, made of materials other than silica glass. See also fluoride glasses.

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InGaAs: Indium gallium arsenide. Generally used to make high-performance long-wavelength detectors.

InGaAsP: Indium gallium arsenide phosphide. Generally used for long-wavelength light emitters.

Injection Laser Diode (ILD): A laser employing a forward-biased semiconductor junction as the active medium. Stimulated emission of coherent light occurs at a pn junction where electrons and holes are driven into the junction.

Insertion Loss: The loss of power that results from inserting a component, such as a connector or splice, into a previously continuous path.

Integrated Detector/Preamplifier (IDP): A detector package containing a PIN photodiode and transimpedance amplifier.

Intensity: The square of the electric field strength of an electromagnetic wave. Intensity is proportional to irradiance and may get used in place of the term "irradiance" when only relative values are important.

Interchannel Isolation: The ability to prevent undesired optical energy from appearing in one signal path as a result of coupling from another signal path. Also called crosstalk.

Interferometric Sensors: Fiber optic sensors that rely on interferometric detection.

Interferometer Detection: see Heterdyne Detection using lines of maxima and minima.

Intrinsic Losses: Splice losses arising from differences in the fibers being spliced.

IR: See infrared.

Irradiance: Power per unit area.

Isolation: See near-end crosstalk.

J

Jacket: The outer, protective covering of the cable.

Jitter: Small and rapid variations in the timing of a waveform due to noise, changes in component characteristics, supply voltages, imperfect synchronizing circuits, etc.

Jitter, Data Dependent (DDJ): Also called data dependent distortion. Jitter related to the transmitted symbol sequence. DDJ is caused by the limited bandwidth characteristics, non-ideal individual pulse responses, and imperfections in the optical channel components.

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Jitter, Duty Cycle Distortion (DCD): Distortion usually caused by propagation delay differences between low-to-high and high-to-low transitions. DCD is manifested as a pulse width distortion of the nominal baud time.

Jitter, Random (RJ): Random jitter is due to thermal noise and may be modeled as a Gaussian process. The peak-to-peak value of RJ is of a probabilistic nature, and thus any specific value requires an associated probability.

Jumper: A short fiber optic cable with connectors on both ends.

K

KL: Kelvin. Measure of temperature where water freezes at 273° and boils at 373°.

kBaud: One thousand bits of data per second.

kb/s: See kBaud.

Kevlar®: A very strong, very light, synthetic compound developed by DuPont which is used to strengthen optical cables.

kHz: One thousand cycles per second.

L

Lambertian Emitter: An emitter that radiates according to Lambert's cosine law. This law states that the radiance of certain idealized surfaces is dependent upon the angle from which the surface is viewed. The radiant intensity of such a surface is maximum normal to the surface and decreases in proportion to the cosine of the angle from the normal. Given by:

N = N0cosA

Where:

N= The radiant intensity N0= The radiance normal (perpendicular) to an emitting surface. A= The angle between the viewing direction and the normal to the surface.

Large Core Fiber: Usually, a fiber with a core of 200 µm or more.

Laser: Acronym for light amplification by stimulated emission of radiation A light source that produces, through stimulated emission, coherent, near monochromatic light. Lasers in fiber optics are usually solid-state semiconductor types.

Laser Diode: A semiconductor that emits coherent light when forward biased.

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Lateral Displacement Loss: The loss of power that results from lateral displacement of optimum alignment between two fibers or between a fiber and an active device.

Launch Fiber: An optical fiber used to couple and condition light from an optical source into an optical fiber. Often the launch fiber is used to create an equilibrium mode distribution in multimode fiber. Also called launching fiber.

LD: See laser diode.

LED (Light-Emitting Diode): A semiconductor that emits incoherent light when forward biased.

L-I Curve: The plot of optical output (L) as a function of current (I) which characterizes an electrical to optical converter.

Light: In a strict sense, the region of the electromagnetic spectrum that can be perceived by human vision, designated the visible spectrum and nominally covering the wavelength range of 0.4 µm to 0.7 µm. In the laser and optical communication fields, custom and practice have extended usage of the term to include the much broader portion of the electromagnetic spectrum that can be handled by the basic optical techniques used for the visible spectrum. This region has not been clearly defined, but, as employed by most workers in the field, may be considered to extend from the near-ultraviolet region of approximately 0.3 µm, through the visible region, and into the mid-infrared region to 30 µm.

Light Piping: Use of optical fibers to illuminate.

Lightguide: Synonym for optical fiber.

Lightwave: The path of a point on a wavefront. The direction of the lightwave is generally normal to the wavefront.

Longitudinal Mode: An optical waveguide mode with boundary condition determined along the length of the optical cavity.

Loose-Tube: A type of fiber optic cable construction where the fiber is contained within a loose tube in the cable jacket.

Loss: The amount of a signal's power, expressed in dB, that is lost in connectors, splices, or fiber defects.

Loss Budget: An accounting of overall attenuation in a system.

M

mA: Milliampere. One thousandth of an Amp or 10-3 Amps.

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Macrobending: In a fiber, all macroscopic deviations of the fiber's axis from a straight line.

Magnetostriction: is a property of ferromagnetic materials to undergo a change of their physical dimensions when subjected to a magnetic field. This effect was first identified in 1842 by James Joule when observing a sample of nickel.

This property, which allow magnetostrictive materials to convert magnetic energy into mechanical energy and conversely, is used for the building of both actuation and sensing devices. It is often quantified by the magnetostrictive coefficient, L, which is the fractional change in length as the magnetization of the material increases from zero to the saturation value.

The reciprocal effect, the change of the magnetization of a material when subjected to a mechanical stress, is called the Villari effect. Two other effects are related to magnetostriction: the Matteuci effect is the creation of a helical magnetic field by a magnetostrictive material when subjected to a torque and the Wiedemann effect is the twisting of these materials when an helical magnetic filed is applied to them.

Margin: Allowance for attenuation in addition to that explicitly accounted for in system design.

Mass Splicing: Simultaneous splicing of many fibers in a cable.

Material Dispersion: Dispersion resulting from the different velocities of each wavelength in a material.

MBaud: One million bits of information per second. Also referred to as Mbps or Mb/s.

Mb/s: See MBaud.

Mean Launched Power: The average power for a continuous valid symbol sequence coupled into a fiber.

Mechanical Splice: An optical fiber splice accomplished by fixtures or materials, rather than by thermal fusion.

MFD: See mode field diameter.

MHz: MegaHertz. One million Hertz (cycles per second).

Microbending: Mechanical stress on a fiber may introduce local discontinuities called microbending. This results in light leaking from the core to the cladding by a process called mode coupling.

Micrometer: One millionth of a meter or 10-6 meters. Abbreviated mm.

Microsecond: One millionth of a second or 10-6 seconds. Abbreviated µs.

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Microwatt: One millionth of a Watt or 10-6 Watts. Abbreviated µW.

MIL-SPEC: Military specification.

MIL-STD: Military standard.

Misalignment Loss: The loss of power resulting from angular misalignment, lateral displacement, and end separation.

mm: Millimeter. One thousandth of a meter or 10-3 meters.

MM: Abbreviation for multimode.

Modal Dispersion: See multimode dispersion.

Modal Noise: Modal noise occurs whenever the optical power propagates through mode-selective devices. It is usually only a factor with laser light sources.

Mode: A single electromagnetic wave traveling in a fiber.

Mode Coupling: The transfer of energy between modes. In a fiber, mode coupling occurs until equilibrium mode distribution (EMD) is reached.

Mode Evolution: The dynamic process a multilongitudinal laser undergoes whereby the changing distribution of power among the modes creates a continuously changing envelope of the laser's spectrum.

Mode Field Diameter (MFD): A measure of distribution of optical power intensity across the end face of a single-mode fiber.

Mode Filter: A device that removes higher-order modes to simulate equilibrium mode distribution.

Mode Scrambler: A device that mixes modes to uniform power distribution.

Mode Stripper: A device that removes cladding modes.

Modulation: The process by which the characteristic of one wave (the carrier) is modified by another wave (the signal). Examples include amplitude modulation (AM), frequency modulation (FM), and pulse-coded modulation (PCM).

Modulation Index: In an intensity-based system, the modulation index is a measure of how much the modulation signal affects the light output. It is defined as follows:

m = (highlevel - lowlevel) / (highlevel + lowlevel)

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ms: Milliseconds. One thousandth of a second or 10-3 seconds.

Multilongitudinal Mode Laser (MLM): An injection laser diode which has a number of longitudinal modes.

Multimode Dispersion: Dispersion resulting from the different transit lengths of different propagating modes in a multimode optical fiber. Also called modal dispersion.

Multimode Fiber: An optical fiber that has a core large enough to propagate more than one mode of light The typical diameter is 62.5 micrometers.

Multimode Laser Diode (MMLD): Synonym for Multilongitudinal mode laser.

Multiple Reflection Noise (MRN): The fiber optic receiver noise resulting from the interference of delayed signals from two or more reflection points in a fiber optic span. Also known as multipath interference.

Multiplexer: A device that combines two or more signals into a single output.

Multiplexing: The process by which two or more signals are transmitted over a single communications channel. Examples include time-division multiplexing and wavelength-division multiplexing.

MUSE: Multiple sub-nyquist encoder. A high-definition standard developed in Europe that delivers 1125 lines at 60 frames per second.

mV: Millivolt. One thousandth of a Volt or 10-3 Volts.

mW: Milliwatt. One thousandth of a Watt or 10-3 Watts.

N

N: Newtons. Measure of force generally used to specify fiber optic cable strength.

nA: Nanoamp. One billionth of an Amp or 10-9 Amps.

NA: See numerical aperture.

NA Mismatch Loss: The loss of power at a joint that occurs when the transmitting half has a numerical aperture greater than the NA of the receiving half. The loss occurs when coupling light from a source to fiber, from fiber to fiber, or from fiber to detector.

Near-End Crosstalk (NEXT, RN): The optical power reflected from one or more input ports, back to another input port. Also known as isolation directivity.

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Near Infrared: The part of the infrared near the visible spectrum, typically 700 nm to 1500 nm or 2000 nm; it is not rigidly defined.

NEP: See noise equivalent power.

nm: Nanometer. One billionth of a meter or 10-9 meters.

Noise Equivalent Power (NEP): The noise of optical receivers, or of an entire transmission system, is often expressed in terms of noise equivalent optical power.

NRZ: Nonreturn to zero. A common means of encoding data that has two states termed "zero" and "one" and no neutral or rest position.

ns: Nanosecond. One billionth of a second or 10-9 seconds.

Numerical Aperture (NA): The light-gathering ability of a fiber; the maximum angle to the fiber axis at which light will be accepted and propagated through the fiber. The measure of the light-acceptance angle of an optical fiber. NA = sin a, where a is the acceptance angle. NA is also used to describe the angular spread of light from a central axis, as in exiting a fiber, emitting from a source, or entering a detector.

nW: Nanowatt. One billionth of a Watt or 10-9 Watts.

O

O/E: Optical-to-electrical converter.

OEIC: Opto-electronic integrated circuit.

1U: One "U". "U" = 1.75 inches.

Optical Amplifier: A device that amplifies an input optical signal without converting it into electrical form. The best developed are optical fibers doped with the rare earth element, erbium.

Optical Bandpass: The range of optical wavelengths which can be transmitted through a component.

Optical Channel: An optical wavelength band for WDM optical communications.

Optical Channel Spacing: The wavelength separation between adjacent WDM channels.

Optical Channel Width: The optical wavelength range of a channel.

Optical Continuous Wave Reflectometer (OCWR): An instrument used to characterize a fiber optic link wherein an unmodulated signal is transmitted through the link, and the resulting light

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scattered and reflected back to the input is measured. Useful in estimating component reflectance and link optical return loss.

Optical Directional Coupler (ODC): A component used to combine and separate optical power.

Optical Fall Time: The time interval for the falling edge of an optical pulse to transition from 90% to 10% of the pulse amplitude. Alternatively, values of 80% and 20% may be used.

Optical Fiber: A glass or plastic fiber that has the ability to guide light along its axis.

Optical Isolator: A component used to block out reflected and unwanted light. Used in laser modules, for example. Also called an isolator.

Optical Link Loss Budget: The range of optical loss over which a fiber optic link will operate and meet all specifications. The loss is relative to the transmitter output power.

Optical Loss Test Set (OLTS): A source and power meter combined to measure attenuation.

Optical Path Power Penalty: The additional loss budget required to account for degradations due to reflections, and the combined effects of dispersion resulting from intersymbol interference, mode-partition noise, and laser chirp.

Optical Power Meter: An instrument that measures the amount of optical power present at the end of a fiber or cable.

Optical Return Loss (ORL): The ratio (expressed in units of dB) of optical power reflected by a component or an assembly to the optical power incident on a component port when that component or assembly is introduced into a link or system.

Optical Rise Time: The time interval for the rising edge of an optical pulse to transition from 10% to 90% of the pulse amplitude. Alternatively, values of 20% and 80% may be used.

Optical Time Domain Reflectometer (OTDR): An instrument that locates faults in optical fibers or infers attenuation by backscattered light measurements.

Optical Waveguide: Another name for optical fiber.

OTDR: Optical time domain reflectometer.

P

pA: Picoamp. One trillionth of an Amp or 10-12 Amps.

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Passive Branching Device: A device which divides an optical input into two or more optical outputs.

PC: Physical contact. Refers to an optical connector that allows the fiber ends to physically touch. Used to minimize backreflection and insertion loss.

PCM: See pulse-code modulation.

PCS Fiber: See plastic clad silica.

Peak Power Output: The output power averaged over that cycle of an electromagnetic wave having the maximum peak value that can occur under any combination of signals transmitted.

PFM: Pulse-frequency modulation. Also referred to as square wave FM.

Phase Constant: The imaginary part of the axial propagation constant for a particular mode, usually expressed in radians per unit length. See also attenuation.

Phase Noise: Rapid, short-term, random fluctuations in the phase of a wave caused by time-domain instabilities in an oscillator.

Photoconductive: Losing an electrical charge on exposure to light.

Photodetector: An optoelectronic transducer such as a PIN photodiode or avalanche photodiode.

Photodiode: A semiconductor device that converts light to electrical current.

Photon: A quantum of electromagnetic energy. A particle of light.

Photonic: A term coined for devices that work using photons, analogous to "electronic" for devices working with electrons.

Photovoltaic: Providing an electric current under the influence of light or similar radiation.

Pigtail: A short optical fiber length that is permanently attached to a source, detector, or other fiber optic device usually by some sort of connector.

PINFET: PIN detector plus a FET amplifier. Offers superior performance over a PIN alone.

PIN Photodiode: See photodiode.

Planer Waveguide: A waveguide fabricated in a flat material such as thin film.

Plastic Clad Silica (PCS): Also called hard clad silica (HCS). A step-index fiber with a glass core and plastic or polymer cladding instead of glass.

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Plastic Fiber: An optical fiber having a plastic core and plastic cladding.

Polarization: The direction of the electric field in the lightwave.

Polarization Maintaining Fiber: Fiber that maintains the polarization of light that enters it.

Polarization Mode Dispersion (PMD): Polarization mode dispersion is an inherent property of all optical media. It is caused by the difference in the propagation velocities of light in the orthogonal principal polarization states of the transmission medium. The net effect is that if an optical pulse contains both polarization components, then the different polarization components will travel at different speeds and arrive at different times, smearing the received optical signal.

Port: Hardware entity at each end of the link.

p-p: Peak-to-peak. A peak-to-peak value is the algebraic difference between extreme values of a varying quantity.

PPM: Pulse-position modulation. A method of encoding data.

Preform: The glass rod from which optical fiber is drawn.

Profile Dispersion: Dispersion attributed to the variation of refractive index contrast with wavelength.

ps: Picosecond. One trillionth of a second or 10-12 seconds.

Pulse: A current or voltage which changes abruptly from one value to another and back to the original value in a finite length of time. Used to describe one particular variation in a series of wave motions.

Pulse Dispersion: The spreading out of pulses as they travel along an optical fiber.

Pulse Spreading: The dispersion of an optical signal as it propagates through an optical fiber.

pW: Picowatt. One trillionth of a Watt or 10-12 Watts.

PZT: Lead Zirconate Titanate: a crystal that exhibits a broad response to the piezo-electric effect, used in microphones and hydrophones as well as many other applications.

Q

Quantum Efficiency: In a photodiode, the ratio of primary carriers (electron-hole pairs) created to incident photons. A quantum efficiency of 70% means seven out of ten incident photons create a carrier.

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Quaternary Signal: A digital signal having four significant conditions.

R

Radiation-Hardened Fiber: An optical fiber made with core and cladding materials that are designed to recover their intrinsic value of attenuation coefficient, within an acceptable time period, after exposure to a radiation pulse.

Radiometer: An instrument, distinct from a photometer, to measure power (Watts) of electromagnetic radiation.

Radiometry: The science of radiation measurement.

Rayleigh Scattering: The scattering of light that results from small inhomogeneities of material density or composition.

Rays: Lines that represent the path taken by light.

Receiver: A terminal device that includes a detector and signal processing electronics. It functions as an optical-to-electrical converter.

Receiver Overload: The maximum acceptable value of average received power for an acceptable BER or performance.

Receiver Sensitivity: The minimum acceptable value of received power needed to achieve an acceptable BER or performance. It takes into account power penalties caused by use of a transmitter with worst-case values of extinction ratio, jitter, pulse rise and fall times, optical return loss, receiver connector degradations, and measurement tolerances. The receiver sensitivity does not include power penalties associated with dispersion, jitter, or reflections from the optical path; these effects are specified separately in the allocation of maximum optical path penalty. Sensitivity usually takes into account worst-case operating and end-of-life (EOL) conditions.

Recombination: Combination of an electron and a hole in a semiconductor that releases energy, sometimes leading to light emission.

Refraction: The changing of direction of a wavefront in passing through a boundary between two dissimilar media, or in a graded-index medium where refractive index is a continuous function of position.

Refractive Index: A property of optical materials that relates to the speed of light in the material.

Refractive Index Gradient: The change in refractive index with distance from the axis of an optical fiber.

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Refractive Index Profile: The description of the value of the refractive index as a function of distance from the optical axis along an optical fiber diameter.

Residual Loss: The loss of the attenuator at the minimum setting of the attenuator.

Responsivity: The ratio of a photodetector's electrical output to its optical input in Amperes/Watt (A/W).

Return Loss: See optical return loss.

RFI: Radio frequency interference. Synonym of electromagnetic interference.

Ribbon Cables: Cables in which many fibers are embedded in a plastic material in parallel, forming a flat ribbon-like structure.

RIN: Relative intensity noise. Often used to quantify the noise characteristics of a laser.

Rise Time: The time taken to make a transition from one state to another, usually measured between the 10% and 90% completion points of the transition. Alternatively the rise time may be specified at the 20% and 80% amplitudes. Shorter or faster rise times require more bandwidth in a transmission channel.

S

Scattering: The change of direction of light rays or photons after striking small particles. It may also be regarded as the diffusion of a light beam caused by the inhomogeneity of the transmitting material.

Selfoc Lens: A trade name used by the Nippon Sheet Glass Company for a graded-index fiber lens; a segment of graded-index fiber made to serve as a lens.

Sensitivity: See receiver sensitivity.

Sheath: An outer protective layer of a fiber optic cable.

Shot Noise: Noise caused by current fluctuations arising from the discrete nature of electrons.

Si: Silicon. Generally used in detectors. Good for short wavelengths only (e.g., < 1000 nm).

Sideband: Frequencies distributed above and below the carrier that contain energy resulting from amplitude modulation. The frequencies above the carrier are called upper sidebands, and the frequencies below the carrier are called lower sidebands.

Silica Glass: Glass made mostly of silicon dioxide, SiO2, used in conventional optical fibers.

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Signal-to-Noise Ratio (SNR): The ratio of the total signal to the total noise which shows how much higher the signal level is than the level of the noise. A measure of signal quality.

Simplex: Single element (e.g., a simplex connector is a single-fiber connector).

Simplex Cable: A term sometimes used for a single-fiber cable.

Simplex Transmission: Transmission in one direction only.

Single-Line Laser: Synonym for single-longitudinal mode laser.

Single-Longitudinal Mode Laser (SLM): An injection laser diode which has a single dominant longitudinal mode. A single-mode laser with a side mode suppression ratio (SMSR)< 25 dB.

Single-mode (SM) Fiber: A small-core optical fiber through which only one mode will propagate. The typical diameter is 8-9 microns.

Single-mode Laser Diode (SMLD): Synonym for single-longitudinal mode laser.

Single-mode Optical Loss Test Set (SMOLTS): An optical loss test set for use with single-mode fiber.

SLED: See surface-emitting diode.

SM: Abbreviation for single-mode.

SMA: A threaded type of optical connector. One of the earliest optical connectors to be widely used. Offers poor repeatability and performance.

Smart Structures: Also smart skins. Materials containing sensors (fiber optic or other types) to measure their properties during fabrication and use.

SMD: Surface-mount device.

SMT: Surface-mount technology.

S/N: See signal-to-noise ratio.

SNR: See signal-to-noise ratio.

Soliton Pulse: An optical pulse having a shape, spectral content, and power level designed to take advantage of nonlinear effects in an optical fiber waveguide, for the purpose of essentially negating dispersion over long distances.

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Source: In fiber optics, a transmitting LED or laser diode, or an instrument that injects test signals into fibers.

Spectral Width: A measure of the extent of a spectrum. For a source, the width of wavelengths contained in the output at one half of the wavelength of peak power. Typical spectral widths are 50 to 160 nm for an LED and 0.1-5 nm for a laser diode.

Spectral Width, Full Width, Half Maximum (FWHM): The absolute difference between the wavelengths at which the spectral radiant intensity is 50 percent of the maximum power.

Splice: A permanent connection of two optical fibers through fusion or mechanical means.

Splitting Ratio: The ratio of power emerging from two output ports of a coupler.

ST: Straight tip connector. Popular fiber optic connector originally developed by AT&T.

Stabilized Light Source: An LED or laser diode that emits light with a controlled and constant spectral width, central wavelength, and peak power with respect to time and temperature.

Star Coupler: A coupler in which power at any input port is distributed to all output ports.

Star Network: A network in which all terminals are connected through a single point, such as a star coupler or concentrator.

Step-Index Fiber: Fiber that has a uniform index of refraction throughout the core.

Stokes Band: When a transparent medium is irradiated with an intense source of monochromatic light, such as a laser, and the scattered light is examined with a spectrometer, not only is the incident light, ν~ , observed in the scattering, i.e. same frequency- Rayleigh scattering, but also some weaker “bands” of a shifted frequency from the incident light are detected. The shifted bands are of lower frequency ν~ - ∆ν, and some are at higher frequency, ν + ∆ν. These bands above and below the center frequency are known as the anti-Stokes and Stokes bands respectively.

Strength Member: The part of a fiber optic cable composed of aramid yarn, steel strands, or fiberglass filaments that increase the tensile strength of the cable.

Submarine Cable: A cable designed to be laid underwater.

Surface-Emitting Diode: An LED that emits light from its flat surface rather than its side. Simple and inexpensive, with emission spread over a wide angle.

Synchronous: A data signal that is sent along with a clock signal.

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T

Tap Loss: In a fiber optic coupler, the ratio of power at the tap port to the power at the input port.

Tap Port: In a coupler where the splitting ratio between output ports is not equal, the output port containing the lesser power.

TAXI: Transparent asynchronous transmitter-receiver interface. A chip used to transmit parallel data over a serial interface.

TDM: See time-division multiplexing.

TEC: Abbreviation for thermoelectric cooler.

Tee Coupler: A three-port optical coupler.

Ternary: A semiconductor compound made of three elements (e.g., GaAlAs).

Thermal Noise: Noise resulting from thermally induced random fluctuation in current in the receiver's load resistance.

Throughput Loss: In a fiber optic coupler, the ratio of power at the throughput port to the power at the input port.

Throughput Port: In a coupler where the splitting ratio between output ports is not equal, the output port containing the greater power.

Tight-Buffer: A material tightly surrounding a fiber in a cable, holding it rigidly in place.

Time-Division Multiplexing (TDM): A transmission technique whereby several low-speed channels are multiplexed into a high-speed channel for transmission. Each low-speed channel is allocated a specific position based on time.

Total Internal Reflection: The reflection that occurs when light strikes an interface at an angle of incidence (with respect to the normal) greater than the critical angle.

Transceiver: A device that performs, within one chassis, both telecommunication transmitting and receiving functions.

Transducer: A device for converting energy from one form to another, such as optical energy to electrical energy.

Transmitter: A device that includes a source and driving electronics. It functions as an electrical-to-optical converter.

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U

Unidirectional: Operating in one direction only.

UV: Ultraviolet.

V

V: Volt.

VAC: Volts, AC.

VDC: Volts, DC.

Vestigial-Sideband (VSB) Transmission: A modified double-sideband transmission in which one sideband, the carrier, and only a portion of the other sideband are transmitted. See also sideband.

Visible Light: Electromagnetic radiation visible to the human eye; wavelengths of 400-700 nm.

W

W: See Watt.

Watt: Linear measurement of optical power, usually expressed in milliwatts, microwatts, and nanowatts.

Waveguide: A material medium that confines and guides a propagating electromagnetic wave. In the microwave regime, a waveguide normally consists of a hollow metallic conductor, generally rectangular, elliptical, or circular in cross-section. This type of waveguide may, under certain conditions, contain a solid or gaseous dielectric material. In the optical regime, a waveguide used as a long transmission line consists of a solid dielectric filament (optical fiber), usually circular in cross-section. In integrated optical circuits an optical waveguide may consist of a thin dielectric film. In the RF regime, ionized layers of the stratosphere and the refractive surfaces of the troposphere may also serve as a waveguide.

Waveguide Couplers: A coupler in which light is transferred between planar waveguides.

Waveguide Dispersion: The part of chromatic dispersion arising from the different speeds light travels in the core and cladding of a single-mode fiber (i.e., from the fiber's waveguide structure).

Wavelength: The distance between points of corresponding phase of two consecutive cycles of a wave. The wavelength, is related to the propagation velocity, and the frequency, by:

Wavelength = Propogation Velocity / Frequency

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Wavelength-Division Multiplexing (WDM): Sending several signals through one fiber with different wavelengths of light.

Wavelength Isolation: A WDM's isolation of a light signal in the desired optical channel from the unwanted optical channels. Also called far-end crosstalk.

WDM: See wavelength-division multiplexing.

Wideband: Possessing large bandwidth.

X

XT: Abbreviation for crosstalk.

Y

Y Coupler: A variation on the tee coupler in which input light is split between two channels (typically planar waveguide) that branch out like a Y from the input.

Z

Zero-Dispersion Wavelength (λλλλ0): In a single-mode optical fiber, the wavelength at which material dispersion and waveguide dispersion cancel one another. The wavelength of maximum bandwidth in the fiber.

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