Top Banner
CHAPTER 1 INTRODUCTION 1.1 INTRODUCTION Communication, as it has always been relied and simply depended upon speed. The faster the means! The more popular, the more effective the communication is! Presently in the twenty-first century wireless networking is gaining because of speed and ease of deployment and relatively high network robustness. Modern era of optical communication originated with the invention of LASER in 1958 and fabrication of low- loss optical fiber in 1970. When we hear of optical communications we all think of optical fibers, what I have for u today is AN OPTICAL COMMUNICATION SYSTEM WITHOUT FIBERS or in other words WIRE FREE OPTICS. Free space optics or FSO –Although it only recently and rather suddenly sprang in to public awareness, free space optics is not a new idea. It has roots that 90 back over 30 years-to the era before fiber optic cable became the preferred transport medium for high speed communication. FSO technology has been revived to offer high band width last mile connectivity for today’s converged network requirements.
71
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Page 1: Free Space Optics

CHAPTER 1

INTRODUCTION

1.1 INTRODUCTION

Communication, as it has always been relied and simply depended upon speed. The

faster the means! The more popular, the more effective the communication is! Presently in

the twenty-first century wireless networking is gaining because of speed and ease of

deployment and relatively high network robustness. Modern era of optical communication

originated with the invention of LASER in 1958 and fabrication of low-loss optical fiber in

1970.

When we hear of optical communications we all think of optical fibers, what I have

for u today is AN OPTICAL COMMUNICATION SYSTEM WITHOUT FIBERS or in

other words WIRE FREE OPTICS. Free space optics or FSO –Although it only recently

and rather suddenly sprang in to public awareness, free space optics is not a new idea. It has

roots that 90 back over 30 years-to the era before fiber optic cable became the preferred

transport medium for high speed communication. FSO technology has been revived to offer

high band width last mile connectivity for today’s converged network requirements.

Mention optical communication and most people think of fiber optics. But light

travels through air for a lot less money. So it is hardly a surprise that clever entrepreneurs

and technologists are borrowing many of the devices and techniques developed for fiber-

optic systems and applying them to what some call fiber-free optical communication.

Although it only recently, and rather suddenly, sprang into public awareness, free-space

optics is not a new idea. It has roots that go back over 30 years--to the era before fiber-optic

cable became the preferred transport medium for high-speed communication. In those days,

the notion that FSO systems could provide high-speed connectivity over short distances

seemed futuristic, to say the least. But research done at that time has made possible today's

free-space optical systems, which can carry full-duplex (simultaneous bidirectional) data at

gigabit-per-second rates over metropolitan distances of a few city blocks to a few

kilometers.

Page 2: Free Space Optics

FSO first appeared in the 60's, for military applications. At the end of 80's, it

appeared as a commercial option but technological restrictions prevented it from success.

Low reach transmission, low capacity, severe alignment problems as well as vulnerability to

weather interferences were the major drawbacks at that time. The optical communication

without wire, however, evolved! Today, FSO systems guarantee 2.5 Gb/s taxes with carrier

class availability. Metropolitan, access and LAN networks are reaping the benefits. FSO

success can be measured by its market numbers: forecasts predict it will reach a USS 2.5

billion market by 2006.

The use of free space optics is particularly interesting when we perceive that the

majority of customers does not possess access to fibers as well as fiber installation is

expensive and demands long time. Moreover, right-of-way costs, difficulties in obtaining

government licenses for new fiber installation etc. are further problems that have turned

FSO into the option of choice for short reach applications.

FSO uses lasers, or light pulses, to send packetized data in the terahertz (THz)

spectrum range. Air, ot fiber, is the transport medium. This means that urban businesses

needing fast data and Internet access have a significantly lower-cost option.

An FSO system for local loop access comprises several laser terminals, each one

residing at a network node to create a single, point-to-point link; an optical mesh

architecture; or a star topology, which is usually point-to-multipoint. These laser terminals,

or nodes, are installed on top of customers' rooftops or inside a window to complete the last-

mile connection. Signals are beamed to and from hubs or central nodes throughout a city or

urban area. Each node requires a Line-Of-Sight (LOS) view of the hub.

Free space optics (FSO) has been used for more than a decade as a short/medium

distance point-to-point (building-to-building) connectivity solution in campus enterprise

LAN markets. The license free nature of this technology combined with its high-speed

bandwidth capabilities, comparable to optical fiber, allow network administrators to

interconnect LAN segments at real networking speeds (e.g. 100 Mbps or 1000 Mbps)

without the hastle of digging to install optical fiber. Since digging to install fiber is typically

a very expensive and time-consuming process, the value proposition of using FSO can be

very appealing. Only recently has the carrier market started to look into FSO technology as

Page 3: Free Space Optics

an alternative network connectivity solution. However, when considering the carrier market,

the requirements in terms of component reliability and overall weather related system

availability is much more stringent than system requirements in the enterprise market. This

paper addresses some of the issues that are most important in the design of an overall carrier

system architecture. Briefly described are the basic physics of transmission at various short

and long infrared wavelengths and their overall impact on the system design. This is

followed by an overview of basic transmitter and detector technologies. When selecting

suitable components, reliability and commercial availability of those components should

play an important factor. Eye safety is another factor that has to be taken into consideration

in a carrier class system design. Finally, the link budget will determine the overall system

availability under various weather conditions. This aspect is discussed near the close of this

document.

1.2 FSO - FREE SPACE OPTICS

Free space optics or FSO, free space photonics or optical wireless, refers to the

transmission of modulated visible or infrared beams through the atmosphere to obtain

optical communication. FSO systems can function over distances of several kilometers.

FSO is a line-of-sight technology, which enables optical transmission up to 2.5 Gbps

of data, voice and video communications, allowing optical connectivity without deploying

fiber optic cable or securing spectrum licenses. Free space optics require light, which can be

focused by using either light emitting diodes (LED) or LASERS(light amplification by

stimulated emission of radiation). The use of lasers is a simple concept similar to optical

transmissions using fiber-optic cables, the only difference being the medium.

As long as there is a clear line of sight between the source and the destination and

enough transmitter power, communication is possible virtually at the speed of light.

Because light travels through air faster than it does through glass, so it is fair to classify

FSO as optical communications at the speed of light. FSO works on the same basic

principle as infrared television remote controls, wireless keyboards or wireless palm

devices.

Page 4: Free Space Optics

FSO technology is implemented using a laser device .These laser devices or

terminals can be mounted on rooftops, Corners of buildings or even inside offices behind

windows. FSO devices look like security video cameras.

Low-power infrared beams, which do not harm the eyes, are the means by which

free-space optics technology transmits data through the air between transceivers, or link

heads, mounted on rooftops or behind windows. It works over distances of several hundred

meters to a few kilometers, depending upon atmospheric conditions.

Commercially available free-space optics equipment provides data rates much

higher than digital subscriber lines or coaxial cables can ever hope to offer. And systems

even faster than the present range of 10 Mb/s to 1.25 Gb/s have been announced, though not

yet delivered.

Generally the equipment works at one of two wavelengths: 850 nm or 1550 nm.

Lasers for 850 nm are much less expensive (around $30 versus more than $1000) and are

therefore favored for applications over moderate distances. But a 1550 nm lasers are also

used. The main reasons revolve around power, distance, and eye safety. Infrared radiation

at 1550 nm tends not to reach the retina of the eye, being mostly absorbed by the cornea.

Regulations accordingly allow these longer-wavelength beams to operate at higher power

than the 850-nm beams, by about two orders of magnitude. That power increase can boost

link lengths by a factor of at least five while maintaining adequate signal strength for proper

link operation. Alternatively, it can boost data rate considerably over the same length of

link. So for high data rates, long distances, poor propagation conditions (like fog), or

combinations of those conditions, 1550 nm can become quite attractive.

As the differences in laser prices suggest, such systems are quite a bit more

expensive than 850-nm links. An 850-nm transceiver can cost as little as $5000 (for a 10-

100-Mb/s unit spanning a few hundred meters), while a 1550-nm unit can go for $50 000

(for gigabit-per-second setups encompassing a kilometer or two).

Air fiber, a major FSO vendor, says it can get a link up and running within two to

three days at one-third to one-tenth the cost of fiber (about $20,000 per building). FSO is

not only cost-effective and easy to deploy but also fast. The technology is not for everyone.

Page 5: Free Space Optics

A major reason companies might not adopt FSO is its confinement to urban areas. FSO

deployments must be located relatively close to big hubs, which means only customers in

major cities will be eligible-at least initially. Businesses in more remote locations are out of

luck, unless a provider sets up hubs in their area, wh ich seems like a distant reality right

now.

When fiber was compared with free-space optics, deployment costs for service to

the three buildings worked out to $396 500 versus $59 000, respectively. The fiber cost was

calculated on a need for 1220 meters: 530 meters of trunk fiber from the CLEC's central

office to its hub in the office park plus an average of 230 meters of feeder fiber for each of

the runs from the hub to a target building, all at $325 per meter. Free-space optics is

calculated as $18 000 for free-space optics equipment per building and $5000 for

installation. Supposing a 15 percent annual revenue increase for future sales and customer

acquisition, the internal rate of return for fiber over five years is 22 percent versus 196

percent for free-space optics.

1.3 RELEVANCE OF FSO IN PRESENT DAY COMMUNICATION

Presently we are facing with a burgeoning demand for high bandwidth and

differentiated data services. Network traffic doubles every 9-12 months forcing the

bandwidth or data storing capacity to grow and keep pace with this increase. The right

solution for the pressing demand is the untapped bandwidth potential of optical

communications.

Optical communications are in the process of evolving Giga bits/sec to terabits/sec

and eventually to pentabits/sec. The explosion of internet and internet based applications

has fuelled the bandwidth requirements. Business applications have grown out of the

physical boundaries of the enterprise and gone wide area linking remote vendors, suppliers,

and customers in a new web of business applications. Hence companies are looking for high

bandwidth last mile options. The high initial cost and vast time required for installation in

case of OFC speaks for a wireless technology for high bandwidth last mile connectivity

there FSO finds its place.

Page 6: Free Space Optics

1.3.1Ultra high bandwidth:

The laser systems operate in the terahertz frequency spectrum and usually

operate in the 194 THz or 375 THz range. Their performance is comparable to the best fibre

optic system available, giving speeds between 622 Mbps and 1.25 Gbps. This technology

uses devices and techniques developed for fibre optic systems.

1.4 RAPID DEPLOYMENT TIME:

Installing a FSO system can be done in a matter of days even faster if the gear

can be placed in offices behind windows instead of on rooftops. A fiber based competitor

has to seek municipal approval to dig up a street to lay its cable. Unlike most of the lower

frequency portion of the electromagnetic spectrum, the part above 300 GHz is unlicensed

worldwide. So no extra time is needed to obtain right-of-way permits or trench up the

streets or to obtain FCC frequency licenses.

1.5 ORIGIN OF FSO

It is said that this mode of communication was first used in the 8 th century by the

Greeks. They used fire as the light source, the atmosphere as the transmission medium and

human eye as receiver.

FSO or optical wireless communication by Alexander Graham Bell in the late 19 th

centaury even before his telephone! Bells FSO experiment converted voice sounds to

telephone signals and transmitted them between receivers through free air space along a

beam of light for a distance of some 600 feet, this was later called PHOTOPHONE.

Although Bells photo phone never became a commercial reality, it demonstrated the basic

principle of optical communications.

Essentially all of the engineering of today’s FSO or free space optical

communication systems was done over the past 40 years or so mostly for defense

applications.

Page 7: Free Space Optics

1.6 THE TECHNOLOGY OF FSO

The concept behind FSO is simple. FSO uses a directed beam of light radiation

between two end points to transfer information (data, voice or even video). This is similar to

OFC (optical fiber cable) networks, except that light pulses are sent through free air instead

of OFC cores.

An FSO unit consists of an optical transceiver with a laser transmitter and a receiver

to provide full duplex (bi-directional) capability. Each FSO unit uses a high power optical

source (laser) plus a lens that transmits light through the atmosphere to another lens

receiving information. The receiving lens connects to a high sensitivity receiver via optical

fiber. Two FSO units can take the optical connectivity to a maximum of 4kms.

CHAPTER 2

Page 8: Free Space Optics

WORKING OF FSO SYSTEM

2.1 INTRODUCTION

Optical systems work in the infrared or near infrared region of light and the easiest

way to visualize how the work is imagine, two points interconnected with fiber optic cable

and then remove the cable. The infrared carrier used for transmitting the signal is generated

either by a high power LED or a laser diode. Two parallel beams are used, one for

transmission and one for reception, taking a standard data, voice or video signal, converting

it to a digital format and transmitting it through free space.

Today’s modern laser system provide network connectivity at speed of 622 Mega

bits/sec and beyond with total reliability. The beams are kept very narrow to ensure that it

does not interfere with other FSO beams. The receive detectors are either PIN diodes or

avalanche photodiodes.

The FSO transmits invisible eye safe light beams from transmitter to the receiver

using low power infrared lasers in the tera hertz spectrum. FSO can function over

kilometers.

2.1.1 WAVELENGTH:-

Currently available FSO hardware is of two types based on the operating

wavelength – 800 nm and 1550 nm. 1550 FSO systems are selected because of more

eye safety, reduced solar background radiation and compatibility with existing

technology infrastructure.

2.1.2 SUBSYSTEM:-

Modulator Driver Laser Transmit

optic

Data in

De-Modulator

preamplifier detector Receive opticData out

Page 9: Free Space Optics

Fig 2.1.1 Subsystem Of Fso

In the transmitting section, the data is given to the modulator for modulating signal and the

driver is for activating the laser. In the receiver section the optical signal is detected and it is

converted to electrical signal, preamplifier is used to amplify the signal and then given to

demodulator for getting original signal. Tracking system which determines the path of the

beam and there is special detector (CCD, CMOS) for detecting the signal and given to pre

amplifier. The servo system is used for controlling system, the signal coming from the path

to the processor and compares with the environmental condition, if there is any change in

the signal then the servo system is used to correct the signal.

2.2 FSO TRANSMITTER:-

To ensure the highest performance of an FSO system, it is

important to choose a transmission wavelength within one of the two atmospheric windows

that coincide with one of the fiber optics transmission windows. Within these two

wavelength windows, namely 850 nm and 1550 nm, a suitable transmitter for a

telecommunication grade FSO system must have the following characteristics:

• Operation at higher power levels (Important for longer distance FSO systems)

• Favorable high-speed modulation characteristics (Important for high speed FSO systems)

Components small in footprint and low in power consumption (Important for the overall

system design and system maintenance)

• Capability to operate over a wide temperature range without showing major performance

decay or degradation (Important for outdoor system installation)

• Mean time between failure (MTBF) operation exceeding 10 years

preamplifier Special detector

Tracking

optic

ProcessorServo systems

Page 10: Free Space Optics

For these reasons manufacturers offering carrier-class FSO equipment generally use

Vertical Cavity Surface Emitting Lasers (VCSELs) in the shorter infrared wavelength

range and Fabry Perot (FP) or Distributed Feed Back (DFB) lasers for operation in the

longer infrared wavelength range.

Fig 2.1.2 Different Types Of Fso Transmitter

Aside from general availability of high-quality components and efficient transmission

window, there are several laser types that, for a variety of reasons, are not very well

suited to FSO systems. At the current stage of development of these sources, solid-

state lasers (e.g. YdYag lasers operating at 1060 nm) or any form of gas-based lasers

fall within this category. Indeed, the majority of high power lasers operating in the

near infrared spectral range cannot fulfill the MTBF requirements of carrier-grade

systems. For example, high power GaAlAs lasers operating slightly beyond 800 nm or

slightly above 900 nm, though generally available from many vendors and at very low

cost, do not normally qualify as telecommunication grade due to insufficient MTBF

values.

2.3 FSO RECEIVER

At the receiver detectors are present. Detectors are used to convert the light

signal into electrical signal.

Detector choices are much more limited when compared to the variety of

wavelength options available. This is due to vast amounts of different semiconductor laser

compound structures. The two most common material systems used to detect light in the

near infrared spectral range are either silicon (Si) or indium gallium arsenide (InGaAs).

Detectors are based either on PIN or APD technology. A thorough discussion of these

technologies is not within the scope of this paper. More detailed information on PIN and

Page 11: Free Space Optics

APD technology and their use in FSO systems can be found in the book “Free Space Optics:

Enabling Optical Connectivity in Today’s Networks” authored by H. Willebrand and B.

Ghuman and published by Sams Publishing.

Fig 2.2 Fso Receiver

Silicon is the most commonly used detector material in the visible and near infrared

wavelength range. Silicon technology is very mature and silicon receivers can detect

extremely low levels of light. Detectors based on silicon typically have a sensitivity

maximum or spectral response around 850 nm. Therefore silicon detectors are ideal

candidates for light detection in conjunction with short wavelength 850 nm VCSEL

laser. Silicon drastically loses sensitivity toward the longer infrared wavelength

spectrum; for wavelengths beyond 1 micrometer. 1100 nanometers defines the cutoff

wavelength for potential light detection and therefore silicon cannot be used as

detector material beyond this range. Silicon detectors can operate at very high

bandwidth. Recently, operation at 10 Gbps has been commercialized for use in short

wavelength 850 nm 10 GbE systems. Lower bandwidth (1 Gbps) silicon PIN and APD

detectors are widely available in a variety of mechanical packages such as TO-46

cans. Very common are also Si-PIN detectors with integrated trans-impedance

amplifiers (TIA). The sensitivity of which is a function of the signal modulation

bandwidth – decreasing as bandwidth increases. Typical sensitivity values for a Si-

Page 12: Free Space Optics

PIN diode are around –34 decibel mill watts (dBm) at 155 megabits per second

(Mbps). Si-APDs are far more sensitive due to an internal amplification (avalanche)

process. Therefore Si-APD detectors are very useful for low light level detection in

free space optics systems. Sensitivity values for higher bandwidth applications can be

as low as –50dBm at 10 Mbps, -45dBm at 155 Mbps, or –38 dBm at 622 Mbps.

Silicon detectors can be quite large in size (e.g. 0.2 x 0.2 mm) and still operate at

higher bandwidths. This feature minimizes loss when light is focused on the detector

by using either a larger diameter lens or a reflective parabolic mirror. For longer

wavelength radiation, InGaAs is the most commonly used detector material. The

performance of InGaAs detectors has been constantly improved in terms of sensitivity

and bandwidth capabilities as well as the development of 1550 nm fiber optic

technology. The vast majority of longer wavelength fiber optics systems use InGaAs

as detector material. Commercially InGaAs detectors are either optimized for

operation at 1310 or 1550 nm. Due to the drastic decrease in sensitivity towards the

shorter wavelength range, InGaAs detectors are typically not used in the 850 nm

wavelength range. The main benefit of InGaAs detectors is higher bandwidth

capability. The majority of InGaAs receivers are based on PIN or PIN-TIA

technology. Typical sensitivity values for InGaAs Pin diodes are similar to those of

Si-PIN diodes (e.g. –33dBm at 155 Mbps). InGaAs diodes operating at higher speed

are typically smaller in size than Si-PIN diodes. This is because most high-speed Inga

As receivers are designed for fiber optic transmission in conjunction with 9-

micrometer core diameter, single mode (SM) fiber, and the small SM core diameter

doesn’t require a large detection surface. This makes the light coupling process a more

challenging task and overall losses that occur when the light is coupled from free-

space onto the detector surface are higher, thus impacting the link budget of the

systems. The conclusion is that both Si and InGaAs detectors are capable of fulfilling

the stringent service provider systems standard requirement since both detector

technologies are already used in carrier class fiber optic communication systems.

2.4 A SIMPLE PROPAGATION MODEL – LINK EQUATION

When taking a closer look at an FSO performance, it is important to take

several system parameters into consideration. In general, these parameters can be divided

into two different categories: internal parameters and external parameters (see Fig. 6).

Page 13: Free Space Optics

Internal parameters are related to the design of a specific FSO system and can be impacted

by the system designer or engineer. Examples are: optical power, transmission bandwidth or

divergence angle on the transmitter side and receiver sensitivity, receive lens diameter or

receiver field-of-view on the receive side. Other important parameters that determine

system performance are related to external or non-system specific parameters and all of

them are related to the climate under which the system has to operate. Typical examples are

the deployment distance and visibility.

Fig2.3 Schematic explanation of internal and external FSO system design parameters.

It is important to understand that many of these parameters are linked and not

independent of each other. Two examples: 1) System availability is not only a function of

the deployment distance but also a function of the inherent atmospheric attenuation

coefficient and 2) Increasing the modulation bandwidth on the transmitter side will impact

the sensitivity figure and the BER performance of the receiver side. In general, the focus on

the improvement of one system parameter (e.g. increase of transmission power) does not

lead to an overall improved system performance. The next section demonstrates that the

ability to launch a high amount of power is certainly beneficial within the overall link

budget calculation. However, it becomes obvious that simply launching higher power levels

will not automatically result in a better performing FSO system. Many other factors have to

be considered. These factors can actually outperform the advantage of being able to launch

higher power levels. A professional FSO system designer must balance all of these

parameters. Under the assumption that the transmission source can be seen as a point

source, the simple link equation (1) below shows the impact of various system parameters

Page 14: Free Space Optics

on the power received at the receiving station. The climate/weather impact on FSO system

availability is solely contained in the last part of the equation. In particular, and as can be

easily seen from this equation, the value of the atmospheric extinction coefficient’ ‘is

extremely important due to the exponential dependency on the receive power level.

2.5 ALIGNMENT IN FSO:-

In free-space optical communication using narrow laser beams, it is required to maintain the

optical alignment between the stations in spite of their relative motion. This relative motion

is caused by the mobile nature of the stations, mechanical vibration, or accidental shocks. In

order to establish and maintain a free-space optical link, a two-phase optical alignment

mechanism is required. In the first phase, a coarse alignment is achieved through the open-

loop operation of spatial acquisition. Following the coarse alignment phase, data

transmission is established and simultaneously a closed-loop fine alignment operation is

performed to precisely compensate for the persistent relative motion of the stations. A

possible scheme to achieve this fine alignment is cooperative (reciprocal) optical beam

tracking. A cooperative optical beam tracking system consists of two stations in such a

manner that each station points its optical beam toward the other one. The receiving station

continuously measures the arrival direction of its incident optical beam in order to employ it

as a guide to precisely point its own beam toward the other station. In short range

applications with negligible light propagation delay, this direction is approximately along

the line-of-sight of the stations, thus the stations transmit their optical beams along this

measured direction. In applications with a large propagation delay, the optical beams must

be transmitted within a certain angle with respect to the arrival direction in order to

compensate for the variation of the line-of-sight during the travel time of the transmitted

beams. This requires the transmitter to predict the future location of the receiver and point

its optical beam toward the predicted location. To implement the alignment scheme above,

the stations are equipped with a position-sensitive photo detector (e.g., quadrant detector)

Page 15: Free Space Optics

and a focusing lens (or an arrangement of curved mirrors) to measure the azimuth and

elevation components of the beam arrival direction. In addition, each station employs an

electromechanical pointing assembly to adjust the direction of its optical devices according

to the control signals provided by a closed-loop controller. The controller incorporates the

output of the position-sensitive photo detector and generates proper azimuth and elevation

control signals. As an alternative (or complement) to adjusting the transceiver direction, the

incoming and outgoing optical fields can be directed using an arrangement of steerable flat

mirrors.

The goal of this chapter is to develop a mathematical model for a cooperative optical beam

tracking system, which includes the nonlinear effects, major disturbance sources, and light

propagation delay. For analyzing the optical alignment between two fast maneuvering

stations (e.g. aircrafts), the nonlinearity of the dynamical equations is essential; however, in

applications such as intersatellite communication in which the relative motion consists of a

predetermined large component and an unknown small component, we can linearize the

nonlinear dynamics around a nominal state trajectory.

Fig 2.4 Schematic Diagram Of A Simple Optical Receiver

In the last section, we shall describe the relative motion of the stations by means of a set of

stochastic differential equations. This stochastic model will be used for a stochastic analysis

of the system, as an alternative to the deterministic approach of System Architecture. In this

Page 16: Free Space Optics

section, we first consider the structure and components of an optical transceiver and then

describe the operation of a cooperative optical beam tracking system

which employs two transceivers of this type.

2.5.1 TRANSCEIVER STRUCTURE:-

A schematic diagram of a simple transceiver used in short range free-space

optical links is illustrated in Fig. This transceiver comprises a lens, a position-sensitive

photo detector, and a narrow laser source, all installed on a rigid platform. The photo

detector surface is perpendicular to the lens axis and its center is placed at the focus of the

lens. The axes of the lens and the laser source are parallel to transceiver axis. The azimuth

and elevation of the transceiver axis can be controlled by means of an electromechanical

pointing assembly, which is mounted on the station body. The optical beam generated by

the laser source is used for two purposes: as a carrier of information and as a beacon

assisting the opposite station in its tracking and pointing operations. For the purpose of

communication, the instantaneous laser power is modulated by the information-bearing

signal, usually with a digital form of on-off-keying. The position-sensitive photo detector is

a photoelectron converter whose surface is partitioned into small regions. The output of

each region counts the number of converted electrons regardless of their location on the

region. The photoelectron conversion rate depends on the instantaneous optical power

absorbed by the region. The image of the received optical field on the surface of the photo

detector is a spot of light with a bell-shaped intensity pattern whose location depends on the

angle of arrival of the optical field with respect to the transceiver axis. Hence, using the

position-sensitive photo detector, this angle can be tracked by measuring the location of the

spot of light. Many practical optical beam tracking systems employ a quadrant detector1 as

their optical sensing device, while the low spatial resolution of a quadrant detector can be

improved using a finer partition. For instance, the authors of describe a beam tracking

system which employs a photo detector with 512 × 512 pixels.

The pointing assembly is usually a two-axes gimballed system with two independent motor

which control the azimuth and elevation of the transceiver. Gimballed pointing systems

generally suffer from low bandwidth (in order of 10 Hz) and low slew rate, while being able

to cover a large solid angle. Also, they have the disadvantage of being singular at certain

points, which limits their coverage region. To resolve this difficulty, Omni-Wrist III is an

alternative antenna pointer with double universal joints and linear actuators, which has 2π

Page 17: Free Space Optics

steradian range of motion without singularity. A more sophisticated transceiver design, used

for intersatellite communication, is illustrated in Figure.

This design employs a position-sensitive photo detector, a pointing assembly,

and a laser source; however, instead of a lens, it employs a reflecting telescope. The

telescope which is shared between the receiving and the transmitting optics, consists of a

primary and a secondary curved mirror with one of the several common designs. The most

popular design, Cassegrainian telescope, employs a parabolic primary mirror and a

hyperbolic secondary mirror which share the same focus. In addition to the telescope, an

arrangement of lenses might be used for extra magnification. In design of the transceiver,

the incoming and outgoing optical fields must be isolated as much as possible, since the

backscattered photons caused by the outgoing light emerge as a source of noise for the

photo detector. This can be achieved by a combination of spectral isolation, spatial

separation, and polarization isolation.

Fig 2.5 optical transreceiver for inter satellite communication.

In the situations that these techniques

cannot provide enough isolation, two separated telescopes are required for the incoming and

the outgoing optical beams while this dual telescope approach leaves the tracking function

of the transceiver unchanged. The tracking mirror in Figure is intended to control the

direction of the incoming light toward the position-sensitive photo detector and the outgoing

Page 18: Free Space Optics

light toward the target. This steerable flat mirror, which is equipped with miniature

actuators, provides a complementary (or alternative) means for the pointing assembly. The

steering machinery consists of a support plate with a single pivot and three or four

piezoelectric linear actuators (fast steering mirror). Although, the scanning region of a

steerable mirror is small (less than 5 degrees in each direction), its small mass and fast

actuators result in a high bandwidth (up to 1 kHz) and high slew rate. This provides

considerable assistance to the pointing assembly in suppressing the high bandwidth

disturbance. The point-ahead mirror is another steerable flat mirror with the purpose of

compensating for the displacement of the receiver during light propagation time. This

mirror provides an additional degree of freedom in controlling the pointing direction of the

outgoing light.

2.6 FINE ANGULAR POINTING, ACQUISITION, AND TRACKING SYSTEMS:-

The goal of the FPAT system is to complete the link, which implies that

the alignment procedure must take the received power into consideration. Also, the FSO

system, incorporated with the FPAT system, must be compact enough to be carried by the

actuator of the CPAT system. These two conditions make the spatial scan method the best

candidate, because this method (1) determines the orientation of the targets from the same

light ray that carries the information bits and (2) can be easily incorporated into a traditional

transceiver.

Pointing, acquisition, and tracking systems have been successfully

implemented in many applications ranging from short-distance cases such as human motion

track-ing to long distance applications such as missile guidance systems. Different

applications may adapt different principles of operation into the PAT system design.

Rolland et.al reviewed these techniques and further classified them into seven categories

including time of light (TOF), spatial scan, inertial sensing, mechanical linkages, phase-

difference sensing, direct-field sensing, and hybrid methods. Among these techniques, the

spatial scan method, which is based on analyzing the incoming light ray to determine the

orientation of a target, is the best match to the capabilities of an FSO system. The sensor of

the spatial scan method is usually a combination of a front-end optical system and a position

sensing diode(PSD), including coupled charge detectors (CCD), quadrant detectors (QD),

and lateral effect detectors (LEP). The CCD-based sensor can simultaneously measure the

Page 19: Free Space Optics

incident angles for multiple rays, whereas the QD-based and LEP-based sensor can only

measure the angle of one ray.

2.6.1 ENHANCED FSO TRANSCEIVERS:-

An FSO transceiver consists of a transmitter and a receiver to achieve duplex

transmission. The data in the transmitter is ¯rst modulated onto an optical carrier, typically a

laser, then the laser beam is collimated through an optical system, and finally transmitted as

an optical field into the atmospheric channel. In order to comply with the PAT requirement,

beam steering capability must be incorporated into the design, which converts a simple

transmitter into a beam steerer. Gibson categorized the fine laser beam steering systems into

(1) mechanical and (2) non-mechanical. Mechanical Beam steerers have advantages in their

large steering range and inexpensive design. Non-mechanical beam steerers are useful for

eliminating potentially bulky mechanical components and can have a high pointing

accuracy. At the receiver, the arriving optical is collected through an optical front-end and

projected onto a photodiode for signal detection. For a high-speed FSO application, the

power collected from the front-end optics may not be focused onto the photodiode because

of pointing errors resulting from turbulence or misalignment. A better strategy is to utilize a

PSD to first determine the location of the focused spot and then move the photodiode to

optimize the received power using feedback control. An FSO receiver capable of estimating

azimuthal and elevation angles is defined as an angular resolver (AR).The combination of

the beam steerer and AR enhances the FSO transceiver with fundamental pointing and

tracking capability. If the beam steerer and AR are combined such that their optical axes are

identical, the resulting transceiver is denoted as mono-static as in figure otherwise it is

denoted as bi-static as in

figure. Generally, mono-static transceivers suffer from strong interference resulting from

strong energy coupled from self-reflection between the forward and backward links. Most

mono-static transceivers require additional power-isolation devices (e.g. a polarizing beam

splitter) to prevent this effect, called narcissus.

2.6.2 INTRODUCTION TO TRANSCEIVER ALIGNMENT:-

An FSO link is established if the optical axes of the local/remote beam steerer

and the remote/local AR are aligned to the vector connecting between the local/remote

beam steerer and remote/local AR, respectively. Since aligning a vector to the other vector

Page 20: Free Space Optics

in general takes 2 rotations (one in azimuth and the other in elevation), it requires 4

rotations to complete a link and 8 rotations to develop a duplex channel (2 from each beam

steerer and AR). In general, the image position in the local AR is capable of providing only

the rotation angles for the local AR which optimizes the received power but not the rotation

angles which leads the local beam steerer to the remote AR. Since each link must be aligned

individually, we therefore de¯ne this alignment problem as the single alignment problem.

The details are depicted in figure If the transceivers are mono-static, and since the optical

axes for the local beam steerer and AR are identical, the alignment takes only 4 rotations.

Most importantly, the image position in the local AR

Fig 2.6 Schematic diagram of an FSO Transreceiver (a) monostatic (b) bi-static

is sufficient to determine the rotation angles for both the AR and beam steerer, which

implies that once either one of the two links is built, the other link can be automatically

aligned. Such an alignment problem is defined as a coupled alignment problem because the

Page 21: Free Space Optics

two links are geometrically related. The details are shown in figure In this work, we propose

a scenario where the alignment can still be treated as a coupled alignment problem even

though the transceivers are not mono-static.

Fig 2.7 Different alignment problems for a pair of FSO transceivers: (a) Single

alignment (between two bi-static transceivers with a short link length), (b) Coupled

alignment (between two mono-static transceivers), and (c) Coupled alignment (be-

tween two bi-static transceivers with a long link length).

In this scenario, once either one of the two links is built, the other link can be

formed since the two links are related by a linear mapping, which can be calibrated in

advance. Such a scenario takes place if the following inequality is satisfied

where is the distance from the local AR to the remote beam steerer, is the

displacement between the local beam steerer and AR, is the beam divergence of the

beam steerer, and is the angle between the vector T and L. For example,

let us consider a duplex communication channel formed by a into this particular scenario.

Page 22: Free Space Optics

Therefore, we can assume that most FSO transceiver alignments are

coupled alignment problems that can always be solved in three steps:

1. Apply a scanning process and point the local beam steerer to the remote AR by trial-and-

error.

2. Compute the rotation angles for the remote AR and beam steerer according to the focused

spot on the remote AR. Point the remote beam steerer back to the local AR.

3. Compute the rotation angles for the local AR and beam steerer according to the focused

spot in the local AR. Point the local beam steerer back to the remote AR.

Page 23: Free Space Optics

CHAPTER 3

FSO ARCHITECTURES

3.1 POINT-TO-POINT ARCHITECTURE

Point-to-point architecture is a dedicated connection that offers higher bandwidth

but is less scalable .In a point-to-point configuration, FSO can support speeds between

155Mbits/sec and 10Gbits/sec at a distance of 2 kilometers (km) to 4km. “Access” claims it

can deliver 10Gbits/ sec. “Terabeam” can provide up to 2Gbits/sec now, while “AirFiber”

and “Lightpointe” have promised Gigabit Ethernet capabilities sometime in 2001..

Fig 3.1

point to point architecture

3.2 MESH ARCHITECTURE

Mesh architectures may offer redundancy and higher reliability with easy node

addition but restrict distances more than the other options.

Fig 3.2 Mesh Architecture

Page 24: Free Space Optics

A meshed configuration can support 622Mbits/sec at a distance of 200 meters (m) to

450m. TeraBeam claims to have successfully tested 160Gbit/sec speeds in its lab, but such

speeds in the real world are surely a year or two off.

3.3 POINT-.TO-MULTIPOINT ARCHITECTURE

Point-to-multipoint architecture offers cheaper connections and facilitates node

addition but at the expense of lower bandwidth than the point-to-point option.

Fig 3.3 Point-.To-Multipoint Architecture

In a point-to-multipoint arrangement, FSO can support the same speeds as the point-

to-point arrangement -155Mbits/sec to 10Gbits/sec-at 1km to 2km.

Page 25: Free Space Optics

CHAPTER 4

FREE SPACE OPTICS (FSO) SECURITY

4.1 INTRODUCTION

Security is an important element of data transmission, irrespective of the network

topology. It is especially important for military and corporate applications. Building a

network on the SONA beam platform is one of the best ways to ensure that data

transmission between any two points is completely secure. Its focused transmission beam

foils jammers and eavesdroppers and enhances security. Moreover, fSONA systems can use

any signal-scrambling technology that optical fiber can use.

The common perception of wireless is that it offers less security than wire line

connections. In fact, Free Space Optics (FSO) is far more secure than RF or other

wireless-based transmission technologies for several reasons:

4.2 INFORMATION SECURITY

Free Space Optics (FSO) laser beams cannot be detected with spectrum

analyzers or RF meters.

Free Space Optics (FSO) laser transmissions are optical and travel along a

line of sight path that cannot be intercepted easily. It requires a matching

Free Space Optics (FSO) transceiver carefully aligned to complete the

transmission. Interception is very difficult and extremely unlikely.

The laser beams generated by Free Space Optics (FSO) systems are narrow

and invisible, making them harder to find and even harder to intercept and

crack.

Data can be transmitted over an encrypted connection adding to the degree

of security available in Free Space Optics (FSO) network transmissions.

Page 26: Free Space Optics

CHAPTER 5

TOPOLOGIES USED IN FSO

5.1 TOPOLOGIES:-In free space optics communication system the data transmission is

done with the help of different topologies used in computer networking. we can easily

communicate two network through free space optics .In networking the physical layer

of OSI model is used for communicating between two network. the topologies used in

free space optics are as follows :-

1. Mesh Topology

2. Ring Topology

3. Bus Topology

4. Star Topology

Fig 5.1 Mesh Topology, Star Topology, Bus Topology, Ring

Topology in FSO

Page 27: Free Space Optics

CHAPTER 6

APPLICATIONS OF

FSO

6.1 INTRODUCTION

Optical communication systems are becoming more and more popular as the

interest and requirement in high capacity and long distance space communications grow.

FSO overcomes the last mile access bottleneck by sending high bit rate signals through the

air using laser transmission.

6.2 APPLICATIONS

Applications of FSO system are many and varied but a few can be listed.

1. Metro Area Network (MAN): FSO network can close the gap between the last

mile customers, there by providing access to new customers to high speed MAN’s

resulting to Metro Network extension.

2. Last Mile Access: End users can be connected to high speed links using FSO. It

can also be used to bypass local loop systems to provide business with high speed

connections.

3. Enterprise connectivity: As FSO links can be installed with ease, they provide a

natural method of interconnecting LAN segments that are housed in buildings

separated by public streets or other right-of-way property.

4. Fiber backup: FSO can also be deployed in redundant links to backup fiber in

place of a second fiber link.

5. Backhaul: FSO can be used to carry cellular telephone traffic from antenna towers

back to facilities wired into the public switched telephone network.

6. Service acceleration: Instant services to the customers before fiber being laid.

7. Satellite Laser Communication:- Fso Is Widely Used In Satellite

Communication. It provides Space-to-Ground Lasercom Link.Link distance of

communication is approx 2000 km.Its Data Transmission Rate is 1 Gbps.

Page 28: Free Space Optics

Fig 6.1 satellite communication using FSO

8. Military Application of FSO :- FSO is very useful in communication between

aircraft to aircraft . Its potential for low electromagnetic emanation when

transferring sensitive data. Secure communication with submerged submarines.It

also very useful in Navigation also.

Fig 6.2 FSO used in Aircraft and Navigation

Page 29: Free Space Optics

CHAPTER 7

. MARKET

Telecommunication has seen massive expansion over the last few years. First was

the tremendous growth of the optical fiber. Long-haul Wide Area Network (WAN)

followed by more recent emphasis on Metropolitan Area Networks (MAN). Meanwhile

LAN giga bit Ethernet ports are being deployed with a comparable growth rate. Even then

there is pressing demand for speed and high bandwidth.

The ‘connectivity bottleneck’ which refers the imbalance between the increasing

demand for high bandwidth by end users and inability to reach them is still an unsolved

puzzle. Of the several modes employed to combat this ‘last mile bottleneck’, the huge

investment is trenching, and the non- redeploy ability of the fiber has made it uneconomical

and non-satisfying.

Other alternatives like LMDS, a RF technology has its own limitations like higher

initial investment, need for roof rights, frequencies, rainfall fading, complex set and high

deployment time.

In the United States the telecommunication industries 5 percent of buildings are

connected to OFC. Yet 75 percent are with in one mile of fiber. Thus FSO offers to the

service providers, a compelling alternative for optical connectivity and a complement to

fiber optics.

Page 30: Free Space Optics

CHAPTER 8

MERITS OF FSO

8.1 INTRODUCTION

Known within the industry as free-space optics (FSO), this form of delivering

communications services has compelling economic advantages.

Free-space systems require less than a fifth the capital outlay of comparable ground-

based fiber-optic technologies. Moreover, they can be up and running much more quickly.

Installing an FSO system can be done in a matter of days--even faster if the gear can be

placed in offices behind windows instead of on rooftops. Using FSO, a service provider can

be generating revenue while a fiber-based competitor is still seeking municipal approval to

dig up a street to lay its cable. Street trenching and digging are not only expensive, they

cause traffic jams (which increase air pollution), displace trees, and sometimes destroy

historical areas. For such reasons, some cities, such as Washington, D.C., are considering a

moratorium on fiber trenching. Others, like San Francisco, are hoping to limit disruptions

by encouraging competing carriers to lay fiber within the same trench at the same time.

FSO works in a completely unregulated frequency spectrum (THz), unlike LMDS or

MMDS. Because there's little or no traffic currently in this range, the FCC hasn't required

licenses above 600GHz. This means FSO isn't likely to interfere with other transmissions.

Regulation could come about, however, when and if FSO carriers start to fill up the

spectrum. License free frequency band is an advantage of FSO.

Cost is one of the major advantage of this technology. Airfiber has prepared a cost

model based on deploying an FSO mesh in Boston. According to its analysis, deployment

would cost about $20,000 per building, with an average link length of 55 meters and a

maximum length of 200 meters. The mesh would also provide full redundancy. A

comparable fiber network would run between $50,000 to $200,000 per building.

Page 31: Free Space Optics

With FSO, there's also no capital overhang. FSO carriers can avoid heavy build outs

by deploying laser terminals after customers have signed on. No heavy capital investments

for build out are required. Low risk investment is another advantage of FSO.

Another plus is that FSO network architecture needn't be changed when other nodes

(buildings) are added; customer capacity can be easily increased by changing the node

numbers and configurations.

High transmission capacity is an advantage of this technology.

8.2 MERITS OF FSO

1. Free space optics offers a flexible networking solution that delivers on the

promise of broadband.

2. Straight forward deployment-as it requires no licenses.

3. Rapid time of deployment.

4. Low initial investment.

5. Ease of installation even indoors in less than 30 minutes.

6. Security and freedom from irksome regulations like roof top rights and spectral

licenses.

7. Re-deploy ability.

Unlike radio and microwave systems FSO is an optical technology and no

spectrum licensing or frequency co-ordination with other users is required. Interference

from or to other system or equipment is not a concern and the point to point laser signal is

extremely difficult to intercept and therefore secure. Data rate comparable to OFC can be

obtained with very low error rate and the extremely narrow laser beam which enables

unlimited number of separate FSO links to be installed in a given location.

Page 32: Free Space Optics

CHAPTER9

FSO CHALLENGES

9.1 INTRODUCTION

The advantages of free space optics come without some cost. As the medium is air

and the light pass through it, some environmental challenges are inevitable.

Despite its potential, FSO has many hurdles to overcome before it will be deployed

widely.

FSO is an LOS technology, which means nodes must have an unobstructed path to

the hub antenna. This, of course, means that interference of any kind can pose problems.

Inclement weather is the main threat. Although rain and snow can distort a signal,

fog does the most damage to transmission. Fog is composed of extremely small moisture

particles that act like prisms upon the light beam, scattering and breaking up the signal.

Most vendors know they have to prove reliability in bad weather cities in order to gain

carrier confidence, especially if those carriers want to carry voice. So these vendors try to

distinguish themselves by running trials in foggy cities. TeraBeam, for example, ran trials in

Seattle, figuring if it could make it there, it could make it anywhere.

The technology is affected badly by the environmental phenomena that vary widely

from one meteorological area to another. Some of them are scattering, scintillations, beam

spread and beam wanders.

Scintillation is best defined as the temporal and spatial variations in light intensity

caused by atmospheric turbulence. Such turbulence is caused by wind and temperature

gradients that create pockets of air with rapidly varying densities and therefore fast-

changing indices of optical refraction. These air pockets act like prisms and lenses with

time-varying properties. Their action is readily observed in the twinkling of stars in the

night sky and the shimmering of the horizon on a hot day.

Page 33: Free Space Optics

FSO communications systems deal with scintillation by sending the same

information from several separate laser transmitters. These are mounted in the same

housing, or link head, but separated from one another by distances of about 200 mm. It is

unlikely that in traveling to the receiver, all the parallel beams will encounter the same

pocket of turbulence since the scintillation pockets are usually quite small. Most probably,

at least one of the beams will arrive at the target node with adequate strength to be properly

received. This approach is called spatial diversity, because it exploits multiple regions of

space.

Dealing with fog, more formally known as Mie scattering, is largely a matter of

boosting the transmitted power, although spatial diversity also helps to some extent. In areas

with frequent heavy fogs, it is often necessary to choose 1550-nm lasers because of the

higher power permitted at that wavelength. Also, there seems to be some evidence that Mie

scattering is slightly lower at 1550 nm than at 850 nm. However, this assumption has

recently been challenged, with some studies implying that scattering is independent of the

wavelength under heavy fog conditions.

One of the more common difficulties that arises when deploying free-space optics

links on tall buildings or towers is sway due to wind or seismic activity. Both storms and

earthquakes can cause buildings to move enough to affect beam aiming. The problem can

be dealt with in two complementary ways: through beam divergence and active tracking.

With beam divergence, the transmitted beam is purposely allowed to diverge, or

spread, so that by the time it arrives at the receiving link head, it forms a fairly large optical

cone. Depending on product design, the typical free-space optics light beam subtends an

angle of 3-6 mill radians (10-20 minutes of arc) and will have a diameter of 3-6 meters after

traveling 1 km. If the receiver is initially positioned at the center of the beam, divergence

alone can deal with many perturbations. This inexpensive approach to maintaining system

alignment has been used quite successfully by FSO vendors like LightPointe for several

years now.

If, however, the link heads are mounted on the tops of extremely tall buildings or

towers, an active tracking system may be called for. More sophisticated and costly than

Page 34: Free Space Optics

beam divergence, active tracking is based on movable mirrors that control the direction in

which the beams are launched. A feedback mechanism continuously adjusts the mirrors so

that the beams stay on target.

Beam wander arises when turbulent eddies bigger than the beam diameter cause

slow, but large, displacements of the transmitted beam. It occurs not so much in cities as

over deserts over long distances. When it does occur, however, the wandering beam can

completely miss its target receiver. Like building sway, beam wander is readily handled by

active tracking.

9.2 CHALLENGES OF FSO

(a). FOG

Fog substantially attenuates visible radiation, and it has a similar affect on the

near-infrared wavelengths that are employed in FSO systems. Rain and snow have little

effect on FSO. Fog being microns in diameter, it hinder the passage of light by absorption,

scattering and reflection. Dealing with fog – which is known as Mie scattering, is largely a

matter of boosting the transmitted power. Fog can be countered by a network design with

short FSO link distances. FSO installation in foggy cities like San Francisco has

successfully achieved carrier-class reliability.

Fig 9.1 Interruption Of Fog In FSO

(b). PHYSICAL OBSTRUCTIONS

Page 35: Free Space Optics

Flying birds can temporarily block a single beam, but this tends to cause only

short interruptions and transmissions are easily and automatically re-assumed. Multi-beam

systems are used for better performance.

(c). SCINTILLATION

Scintillation refers the variations in light intensity caused by atmospheric

turbulence. Such turbulence may be caused by wind and temperature gradients which

results in air pockets of varying diversity act as prisms or lenses with time varying

properties. This scintillation affects on FSO can be tackled by multi beam approach

exploiting multiple regions of space- this approach is called spatial diversity.

Scintillation is one of the effects related to turbulence. Scintillation cannot be characterized

using visibility. Turbulence is caused when temperature differentials change the air particle

density. Cells or hot pockets of air are created that move randomly in space and time thus

also changing the refractive index of the air media. Turbulence affects laser beams

propagating through the atmosphere in three different ways. First, beam wander occurs

when the refractive index changes and acts like a lens, deflecting the beam from its given

path. Second, turbulence results in a beam spread greater than diffraction theory predicts.

Third, scintillation or intensity variations (peaks and troughs across the face of the beam)

can occur that consequently change the amplitude of the beam at the receiver side.

Scintillation mainly causes a sudden increase in BER during very short time

intervals (typically less than a second). During hot summer days and around midday and/or

in the very early morning hours scintillation effects can be best observed. Depending on the

specific system configuration, the variation in the signal strength both in time and across the

cross section of the beam can reach levels in signal variation beyond 10 dB. Scintillation

can act in both ways: Troughs can cause the signal to disappear, while peaks in amplitude

can saturate the detector. Scintillation is distance dependent andin general the system

designer has to reserve more link margin for scintillation effects over longer distances.

Research has revealed that there are several very successful geometric solutions that can

decrease the effect of scintillation significantly. One of these strategies involves the use of

multiple transmission beams that are sufficiently separated in space when they leave the

transmission aperture plane. In this way they pass through different air (refractive index)

cells, experiencing different intensity variations. The variations are averaged out when the

Page 36: Free Space Optics

signals are added together at the receiving terminal where they overlap in space. By

separating multiple transmitters and by making the receiver optics sufficiently large (or

sufficiently separating smaller receiving lenses), different parts of the receiver lenses are

illuminated when the beam propagates through different air cells. As a statistical result as

this approach signal amplitude variations are averaged out at the receiver. Even though

scintillation is not physically correlated with visibility, scintillation under low visibility

conditions, usually involving wet, cooler weather, can be neglected. For high visibility

conditions that typically occur on hot and sunny days, one has to reserve the maximum loss

for scintillation in the link budget analysis.

(d). SOLAR INTERFERENCE

This can be combated in two ways:

The first is a long pass optical filter window used to block all wavelengths below

850nm from entering the system.

The second is an optical narrow band filter proceeding the receive detector used to

filter all but the wavelength actually used for intersystem communications.

(e). SCATTERING

Scattering is caused when the wavelength collides with the scatterer. The

physical size of the scatterer determines the type of scattering.

When the scatterer is smaller than the wavelength-Rayleigh scattering.

When the scatterer is of comparable size to the wavelength -Mie scattering.

When the scatterer is much larger than the wavelength-Non-selective scattering

In scattering there is no loss of energy, only a directional re-distribution of energy

which may cause reduction in beam intensity for longer distance.

(f). ABSORPTION

Absorption occurs when suspended water molecules in the terrestrial

atmosphere extinguish photons. This causes a decrease in the power density of the FSO

beam and directly affects the availability of a system. Absorption occurs more readily at

some wavelengths than others.

Page 37: Free Space Optics

However, the use of appropriate power, based on atmospheric conditions, and use of spatial

diversity helps to maintain the required level of network availability.

(g). BUILDING SWAY / SEISMIC ACTIVITY

One of the most common difficulties that arises when deploying FSO links

on tall buildings or towers is sway due to wind or seismic activity Both storms and

earthquakes can cause buildings to move enough to affect beam aiming. The problem

can be dealt with in two complementary ways: through beam divergence and active

tracking

With beam divergence, the transmitted beam spread, forming optical cones

which can take many perturbations.

Active tracking is based on movable mirrors that control the direction in which

beams are launched.

Page 38: Free Space Optics

CHAPTER 10

RAPIDLY ADVANCING FSO TECHNOLOGY

10.1 LIGHT POINTE:-

Light Pointe’s FSO products utilize a multi-beam sending process, which

overcomes atmospheric degradations and temporary beam obstructions by

overlapping redundant infrared beams.

Light Pointe was founded in 1998 and has become the

global market leader for high capacity wireless outdoor bridges with over 5000 systems

deployed in over 60 countries worldwide and in vertical markets such as Health Care,

Education, Military & Government networks, large and small campus enterprise

networks, Wire line and Wireless Service Provider networks. Over the last 10 years the

company has established a unique diversified product portfolio based on high capacity

Free Space Optics (FSO) and Millimeter Wave (MMW) technology. With more than 10

patents granted in the FSO, RF/MMW and in the hybrid bridging solution space Light

Pointe has established a strong IP and patent portfolio position manifesting the

company’s technology leadership position.

Light Pointe has a long list of global customers including but

not limited to Wal-Mart, DHL, Sturm Foods, Siemens, Sprint, AOL, FedEx, BMW,

Lockheed Martin, Dain Rauscher, Barclays, Nokia, Deutsche Bank, IBM, Corning,

Cisco, Hawaii just to mentioned a few.

Page 39: Free Space Optics

Fig 10.1 Light Pointe In Free Space Optics

The addition of the licensed-free 60 GHz Airebeam G60 product

complements the LightPointe comprehensive product portfolio of high capacity wireless

bridges. By offering both , outdoor wireless bridges based on Free Space Optics (FSO) and

millimeter-wave technology, we can fulfill any customer's high capacity transport

requirements as far as bandwidth, distance and pricing is concerned." said Heinz

Willebrand, LightPointe CEO and President. The 60 GHz band is license-free in the USA,

Canada and soon other select countries including Europe.

About Light Pointe Communications, In Light Pointe

designs, manufactures and distributes ultra high-speed wireless point to point network

bridging solutions based on patented free-space optics (FSO) and millimeter wave (MMW)

technology. The products are used in fixed wireless last mile access for campus or

enterprise building-to-building connectivity, and in infrastructure applications such as

broadband cellular networks and wireless backhaul for WiMAX or WiFi networks. Light

Pointe installation base of high capacity wireless bridges consists of more than 5000

systems deployed in more than 60 countries. The company is recognized worldwide for the

highest standards of quality and service.

Page 40: Free Space Optics

10.2. VCSEL LASERS:-

Over the last decade, VCSEL structures have gained a massive amount of popularity in the

communications industry. In addition, laser lifetime, transmission power performance and

modulation characteristics have shown dramatic improvements in the shorter 850 nm and

980 nm wavelength range. VCSELs clearly established a milestone and revolutionized the

transmission component market due to the exceptional and dramatic cost/performance

advantage over previously available technology. The success of VCSEL technology has

been so tremendous that many VCSEL laser manufactures can produce shorter wavelength

850 nm laser structures with direct modulation speeds beyond 3 Gbps at power levels in

excess of 10 mW. Direct electrical modulation of VCSEL lasers beyond 10 Gbps have been

demonstrated and commercialized for OC-48 (STM-16) and 10 gigabit Ethernet (GbE)

operations. VCSEL lasers can operate at very low threshold currents (a few mill amperes)

and the electro-optic conversion efficiency of these special semiconductor laser cavity

structures is extremely high. Power dissipation is not typically an issue and active cooling

of the VCSEL structure is not required. In addition, VCSELs emit light in the form of a

circular beam instead of an elliptical beam shape found in hetero-junction DFB lasers. The

round shape of the beam pattern perfectly matches the round core of an optical fiber strand.

Therefore, the coupling process is far easier and coupling efficiency is much higher when

compared to a standard DFB laser. Nonetheless, the most remarkable success in VCSEL

technology is certainly related to MTBF: Some tests have measured and extrapolated failure

rates below 1 FIT (1 failure in 1 billion hours) at 35 degrees Celsius junction temperature

for the first 4000 years. This corresponds to a MTTF value of more than 4*107 hours! Even

in environments that are exposed to high ambient temperature (such as outdoor FSO

equipment) where the junction temperature can reach 90°C for extended periods of time, a

MTTF value of 3.9*105 hours or 44 years was estimated. An example of short wavelength

VCSEL laser lifetime improvement since 1995 is shown in Fig. 5. Initial VCSEL laser

production showed lifetime cycles around 50,000 hours. Through constant improvements in

the fabrication process this value has been pushed beyond 5,000,000 hours for the

Honeywell VCSEL product line.

Page 41: Free Space Optics

Fig 10.2 Vcsel Lasers In Fso

choosing the right transmitter is an important component of a free space optics system,

critical to satisfying telecommunications equipment requirements. Besides the transmitter,

the receiver is another important electronic component that has to be picked carefully. The

following section focuses on suitable receivers for high performance FSO systems.

10.3. TERA BEAM: -

Terabeam's FSO products have advanced beam-steering features that update beam direction

up to 300 times per second.

Terabeam is the latest in a line of high-bandwidth,

carrier-grade systems developed by Terabeam Wireless. TeraBeam is unique among our

solutions in that it operates via free-space optics and thus does not use radio frequency. The

system is a cost-effective solution for high-bandwidth connectivity at ranges less than one

kilometer, and is ideal for deployments such as mobile wireless backhaul, single customer

access, multi-tenant building access, enterprise E1/T1, Fast Ethernet extension and LAN-to-

LAN or campus connectivity.

Page 42: Free Space Optics

Fig 10.3 Tera Beam In Fso

10.3.1 FEATURES OF TERA BEAM:-

Designed for outdoor installations and provides bandwidths of 125 megabits per

second (Mbps), independent of transport protocol

Ideal for dense metro deployments in the range of 20 meters to 1 kilometer

Operates at a wavelength of 850 nm and is completely eye-safe, with a Class 1

IEC/CDRH rating, which means no warning labels or access restrictions are required

License-free operation worldwide eliminates the need for spectrum licensing or

frequency planning

Provides reliable performance using high-performance lasers with a mean time

between failures (MTBF) of one million (1,000,000) hours

Lightweight, advanced industrial design includes an integrated optical scope for ease

of alignment and high/low power settings for optimized performance

Design includes advanced laser delivery technology which maximizes availability

while maintaining eye safety

Includes built-in management functionality using Simple Network Management

Protocol (SNMP) version 1

Page 43: Free Space Optics

10.4. AIRFIBER: -

AirFiber's products combine FSO with 60 GHz millimeter-wave radio, makes

wireless communication possible in any weather.

The Airfiber provides low-latency, full-duplex, wireless point-to- point Gigabit

Ethernet (GbE) connectivity and combines low cost-per-bit transport and high transmission

security, all within a compact, easy to install, fully outdoor-rated unit.

The AireFiber comes equipped with a hot swappable, GbE SFP optical fiber

port as well as a GigE RJ-45 port for connecting to the network. The AireBeam G60's true

flexibility can be found in the use of these two data ports, the secondary of which can be

used as an integrated backup solution or as an add/drop port. At a total power consumption

of less than 20 W the system can be powered by either a Cat5/6 Power-over-Ethernet (PoE)

connection or by using a low voltage 48 Vdc power feed. The system supports an Ethernet

based management system for SNMP v1/2c support and comes with an integrated Web

browser agent. The system offers advanced features like a signal strength bar graph LED

and flexible mounting options to allow for easy system installation and alignment.

Fig 10.4 Air Fiber in FSO

With bandwidth and latency similar to fiber optic cable, the AirFiber G60 targets a very

rapidly increasing number of short to medium distance outdoor wireless networking

applications that require Gigabit Ethernet bandwidth. Many of these applications are in the

Page 44: Free Space Optics

high capacity enterprise campus building-to-building and Metro Ethernet connectivity

market where the challenge is to interconnect buildings that have no fiber access and/or

where laying fiber simply takes too long and is cost prohibitive. The AirFiber G60 design

allows for alternative data flow when used with the LightPointe FSO, AirFiber G70 MMW

system or leased fiber for load sharing or back-up for ultimate uptime network performance.

Page 45: Free Space Optics

CHAPTER 11

FSO AS A FUTURE TECHNOLOGY

Infrared technology is as secure or cable applications and can be more reliable

than wired technology as it obviates wear and tear on the connector hardware. In the future

it is forecast that this technology will be implemented in copiers, fax machines, overhead

projectors, bank ATMs, credit cards, game consoles and head sets. All these have local

applications and it is really here where this technology is best suited, owing to the inherent

difficulties in its technological process for interconnecting over distances.

Outdoors two its use is bound to grow as communications companies,

broadcasters and end users discovers how crowded the radio spectrum has become. Once

infrared’s image issue has been overcome and its profile raised, the medium will truly have

a bright, if invisible, future!

Page 46: Free Space Optics

CONCLUSION

FSO enables optical transmission of voice video and data through air at very high

rates. It has key roles to play as primary access medium and backup technology. Driven by

the need for high speed local loop connectivity and the cost and the difficulties of deploying

fiber, the interest in FSO has certainly picked up dramatically among service providers

worldwide. Instead of fiber coaxial systems, fiber laser systems may turn out to be the best

way to deliver high data rates to your home. FSO continues to accelerate the vision of all

optical networks cost effectively, reliably and quickly with freedom and flexibility of

deployment.

Page 47: Free Space Optics

BIBLIOGRAPHY

1) www.fsona.com

2) www.freespaceoptics.com

3) www.freespaceoptic.com

4) www.fsocentral.com

5) www.lightpointe.com

6) www.proxim.com

7) www.wikipedia.com

8). www.fsonews.com

9) www.cablefreesolutions.com

10) www.thefoa.org

11) www.opticsreport.com

12) www.free-space-optics.org