Lab Report Fiber Optics

Laboratory Activity: Fiber Optics and Optical Power

Measurements J. A. D. Bautista

A. A. M. Castillo

Abstract— This laboratory report will discuss the characteristics

of optical fibers, specifically, the single-mode fiber (SMF) and the

multi-mode fiber (MMF). The report will go into the power

measurements of both types of fibers and will also observe the

power outputs for the mechanical splicing combinations using the two fibers. Furthermore, the characteristics of a 50/50 fiber

coupler is also observed and discussed.

I. CONCEPT AND THEORY

In 1854 a British physicist by the name of John Tyndall

discovered that light could be bent around a corner through a

curved spout of running water. In this experiment he permitted

water to spout from a tube, the light on reaching the limit ing

surface of air and water was totally reflected and seemed to be

washed downwards by the descending liquid [1] . Tyndall

discovered the idea of total internal reflection (TIR) and it is

from this concept where optical fiber communication is built

on.

Like any other fo rm of communication, fiber optic

communicat ion is composed of three elements, a light source

which acts as the sender of informat ion, a fiber media which

acts as the transmission medium, and a light detector for the

receiving end [2]. Most light sources emit light with

wavelengths of 1300nm and 1550nm since these are the points

when the least attenuation is experienced, as will be discussed

in depth later.

For this activity, the focus is on the transmission medium

known as the optical fiber.

Optical Fibers

Optical fibers are the actual media that guides the ligh t [2].

The fibers can either be made of glass or plastic, but glass

fibers are more preferred because they exh ibit less attenuation.

The typical fiber structure is usually made up of a core center

where the light actually propagates in; a cladding of lower

index of refract ion that allows the light to undergo TIR and

propagate down the fiber; and the buffer coating which serves

as protection for the other parts of the fiber. A typical

structure for an optical fiber is shown in Fig. 1.

Fig. 1. Optical Fiber Structure

There are basically two types of fibers: stepped index and

graded index. Graded index fibers has a h igh index of

refract ion at the center of the fiber and exh ibits a gradual

decrease of the index as one moves away from the center. On

the other hand, step-index fibers have an abrupt and distinct

difference between the fiber core and cladding. The graded

index fiber and the step index fiber are illustrated in Fig. 2 and

Fig. 3, respectively.

Fig. 2. Multi-mode Graded index fiber.

Fig 3. Multi-mode Stepped index fiber.

The stepped index fiber is further classified into two types:

the single mode and the multi-mode fiber. The multi-mode

stepped index fiber has, mult iple paths for the light to travel,

as shown in Fig. 2 and Fig. 3 while the single mode fiber only

allows a single light ray to propagate as shown in Fig. 4 [2].

Fig. 4. Single Mode Fiber

Refractive Index and Total Internal Reflection

Optical fiber communication relies on the concept of Total

Internal Reflection (TIR) for light to properly propagate down

the media to its destination. TIR is achieved when light goes

from a medium of higher refractive index to a lower refractive

index and the angle of the reflected beam exceeds 90 degrees

from the normal of the interfaces. This property is governed

by Snell’s law given below, and Fig. 5. illustrates the concept

of TIR.

where n1 and n2 are refractive indexes of material 1 and

material 2, while θ1 and θ2 are angles of the incident ray and

the reflected ray, respectively, with respect to the normal of

the interface.

Fig. 5. Total Internal Reflection inside the Optical Fiber.

Optical Power in Watts and dBM

In optical communicat ion, optical power measures the rate

at which photons arrive at a detector, it is a measure of energy

transfer per time and has a unit of Watts [5]. The power level

is too wide to be expressed on a linear scale. Thus, the

logarithmic scale known as decibel (dB) is used to express in

optical communicat ions [4]. The decibel does not give a

magnitude of power, but it is a rat io of the output power to the

input power, both in Watts, as expressed by,

dB = 10log(Pout/Pin) (1)

The power level related to 1mW is noted as dBm and the

power level related to 1µW is noted as dBµ. The dBm and

dBµ equations are given as follows,

dBm = 10log(P/1mW)

(2)

dBµ = 10log(P/1µW) (3)

Attenuation

The material most used in optical fibers is silica (SiO2) [3].

Silica fiber exh ibit d ifferent attenuation rates given different

wavelengths for the source input. A graph of the spectral

attenuation of silica fiber is shown in Fig. 7. As shown in the

graph, three “windows” are identified as ideal wavelengths for

light sources. Nowadays, the 1300nm and 1550nm windows

are commonly in use. These are the points where the

attenuation of silica is at a local minima [3]. The most

significant factors contributing to the attenuation are Rayleigh

scattering and material absorption.

Material absorption occurs as a result of the imperfect ion

and impurities in the fiber. The most common impurity is the

hydroxyl (OH-) molecule, which remains as a residue from

manufacturing of the fiber [4]. The absorbed light particles are

lost to the impurities thus causing a loss in power.

Rayleigh scattering is the result of elastic collisions between

the light wave and the silica molecules in the fiber [4].When

th elastic collisions occer, the light scattered in all directions.

If the scattered light continues to propagate down the fiber, no

attenuation occurs but there is also the chance that the

scattered light is unable to continue down the fiber.

Splicing

Two optical fiber splicing methods are available for

permanent jo ining of two optical fibers. The optic cable fusion

splicing with an insertion loss of less than 0.1db is

implemented using a special equipment called fusion splicer.

The other type is mechanical splicing with an insertion loss of

less than 0.5dB. Mechanical splicing uses a small mechanical

splice, that precisely aligns two bare fibers and secures them

mechanically [7]. Mechanical splicing is the splicing method

mentioned in this activity.

Fiber coupler

A fiber coupler is an optical fiber device with one or more

input fibers and one or several output fibers. Light from an

input fiber can appear at one or more outputs, with the power

distribution potentially depending on the Wavelength and

polarization [6].

II. METHODOLOGY

The activity calls for following safety guides for eye safety

as well as proper handling of fiber optic cable. A paraphrased

list of the guidelines is given below.

Eye Safety

Do not directly shine visib le and infrared radiation

into your eyes.

Turn off power source during manipulat ion and

concatenation of optical fiber.

No bare fiber will be handled in this lab to

eliminate danger of serious eye injury due to

microscopic glass particles.

Proper fiber handling

maintain optical quality and cleanliness of the

fiber endfaces and instrument connector interfaces.

Wash hands in soap before the activity.

Use a lint-free tissue and residue free isopropanol

for cleaning optical surfaces .

Allow 15 seconds for surfaces to dry before

mating.

Always cap fiber end, bulkheads and mating

sleeves to percent contamination of optically clean

surfaces.

List of Materials and Equipment

single mode

fiber

multimode fiber

optical power

meter

optical source

FC connector

FC bulkhead

infrared sensor

fiber mating

sleeves

2x2 fier coupler

semiconductor

grade isopropyl

alcohol

lint-free tissue

bulkhead caps

fiber connector

caps

To observe the different optical power behavior with the

SM and MM fiber several measurements are taken. In Part I of

the activity, the SM fiber is coupled with a 1.3 micron light,

then measurements of the optical power at the opposite end of

the fiber are taken. Next, the SM fiber is coupled with a

mat ing sleeve into a MM fiber, then optical power is

measured at the end of the MM fiber. A similar procedure is

done in Part II of the activity for the MM fiber, except that

this time, it is the SM fiber that is coupled with the MM fiber,

and the power output is measured from the end of the SM

fiber.

For the Part III of the activity, a 50/50 also called a 3 dB

fiber coupler is used and output power is measured from the

remain ing three ends. The characteristics and parameters of

the fiber coupler is then analyzed based on the observed data. .

III. RESULTS AND DISCUSSION

TABLE I VALUES FOR PART I OF THE ACTIVITY

Fiber Type Power in dBm Power (mW)

Single Mode -7.94 0.160mW

SMF-MMF splice -8.75 0.133mW

For the first part of the activity, the value of the optical

output power measured at the opposite end of the SM optical

fiber is read in dBm and it is converted to mW by isolating P

in (2). The equation for dBm to mW is given by,

(4)

Computations from dBm to mW in Table I is as follows,

= 0.160mW

= 0.133mW

(5)

A comparison of the values of the output power of the SMF

alone to when it was coupled with the MMF via the mating

sleeve, gives the observation that the addition of the MMF

also added further attenuation or loss in power. The loss in

power can be computed by simply, subtracting the dBm value

of the SMF alone from the power of the SM-MMF power loss,

given by,

( )

(6)

This additional loss could have been brought about by

connector losses (caused by the mating sleeve). But the more

probable cause would be the fact that because there is a longer

fiber, the light travels a longer d istance, thus, being more

prone to Rayleigh scattering or absorption losses.

TABLE III

VALUES FOR PART II OF THE ACTIVITY

Fiber Type Power in dBm Power in mW

Multi-mode -7.69

0.170mW

MMF-SMF splice -10.16 0.096mW

Part II of the activ ity is essentially similar to the procedures

done for Part I, except that this time the MMF was used and a

MMF-SMF splice was created. Measure power values for Part

II of the activity is shown in Table II. Computations using (4)

for converting dBm to mW for Part II are as follows,

= 0.170mW

= 0.096mW

For the additional loss of the MMF-SMF splice, the loss can

be calculated using (6) as in Part I,

( )

The additional 2.47dB loss can again be attributed to loss

caused by the mating sleeve or the Rayleigh scattering and

absorption because of the added length of the fiber,

TABLE IIIII VALUES FOR PART III OF THE ACTIVITY

Port Power in

dBm

Power in

mW

Power in

µW

Port 2 -23.26 0.004 4

Port 3 -9.97 0.101 101

Port 4 -11.48 0.071 71

For the values in Part III, the measure power are converted

into mw and µW using a version (2) and (3). Computations

are as follows,

Port 2

= 0.004mW

= 4µW

Port 3

= 0.101mW

= 101µW

Port 4

= 0.071mW

= 71µW

Based on the values of the power from Port 3 and Port 4, it

can be seen that the theoretical coupling ratio of 50/50 is not

followed, instead, the experimental coupling rat io is computed

by,

( )

( )

Thus the experimental ratio is 46/54 (Port 3/ Port 4),

instead of 50/50.

IV. ANSWERS TO QUESTIONS

1. What is the core diameter of SMF-28 optical fiber?

SMF-28 is manufactured to the most demanding

specifications in the industry and is widely used in the

transmission of voice, data and/or video. It has a core

diameter o f 8.2um a numerical aperture of 0.14 and a

refractive difference of 0.36% [9].

2. What is the conventional color of singlemode fiber?

The fiber's jacket color is at times used to differentiate

multi-mode fibers (orange) from single-mode (yellow)

fibers [10].

3. Assuming 100% coupling efficiency of power into the

optical power meter, how much optical power is lost in

the SMF-MMF mechanical splice?

As shown in (6), a loss of 0.81 dB is added when the

SMF-MMF splice was made. This translates to an

additional 18% loss.

4. Assuming 100% coupling efficiency of power intothe

optical power meter, how much optical power is lost in

the MMF-SMF mechanical splice?

Similar to question 3, the additional loss of the MMF-

SMF splice was already computed in the discussion

and the results are about -2.47 dB which translates to

an additional 43% loss.

5. The measured output powers at 3 and 4 are consistent

with what launched input power(at port 1)?

No, they do not add up, the sum of Port 3 and Port o f

are less than the input power. This is because of the

loss incurred by the ray as it p ropagated down the fiber

coupler.

6. Given your data, what is the coupling ratio of the

device?

As computed in the discussion, the experimental

coupling ratio is 46/54 (Port 3/ Port 4), instead of

50/50.

7. Assuming a 4% reflection off the glass-air interface at

port 3 and 4, estimate the power that should be

measured at port 2 (state in both dBm and mW or µW).

Explain why how this is consistent with your measured

value, and if there is no discrepancy, hypothesize

reasons for such by identifying possible other sources

of loss in path.

The computation for the experimental power at port 2

given a 4% reflection is given by,

Power at Port 2 = (Power at Port 3(µW) + Power at

Port 4(µW)) x 0.04

From the formula the experimental power for Port 2 is

6.88 µW or .006mW or -22.22 dB, which are values

greater than the experimental value meaning loss is

also experienced by the reflected beam that enters port

2.

V. CONCLUSION

Optical fibers are essential for optical communicat ion. It is

important to understand the characteristics of the fiber

especially with how power is los t as light propagates down the

fiber. With an understanding of the attenuation characteristics

of the fiber, an efficient communication system can be

realized.

VI. REFERENCES

[1] Allan, W. B., Fiber Optics: Theory and Practice,

(Plenum Press, NewYork, 1973).

[2] http://www.openoptogenetics.org/images/f/fb/Funda

mentals_of_Fiber_Optics.pdf.

[3] http://lib.tkk.fi/Diss/2006/isbn9512282658/ isbn9512

282658.pdf.

[4] http://books.google.com/books?id=5LMp7yxfeDAC

&pg=PA53&dq=optical+power&hl=en&ei=uweCTe

SDNc3Ccdb

IAD&sa=X&oi=book_result&ct=result&resnum=2&

ved=0

DUQ6AEwAQ#v=onepage&q=optical%20power&f

=false

[5] http://books.google.com/books?id=hw1PFAr2L0s C&

pg=PA237&dq=optical+power+definition&hl=en&ei

=7weCTd3xCIO3cL65xaMD&sa=X&oi=book_resul

t&ct=result&resnum=2&ved=0CDYQ6AEwAQ#v=

onepage&q=optical%20power%20definition&f=false

[6] http://www.timbercon.com/Fiber-Opt ic-Coupler.html

[7] http://www.fiberoptics4sale.com/Merchant2/fiber-

optic-splicing-tutorial.php

[8] http://www.scribd.com/doc/3942245/Optical-fiber-

Structures

[9] http://www.photonics.byu.edu/FiberOpticConnectors

.parts/images/smf28.pdf

[10] www.tech-faq.com/multi-mode-fiber.htm