A

TERM PAPER REPORT ON

FREE SPACE OPTICAL

LASER COMMUNICATIONSubmitted by:

Priya Hada

B.Tech (ECE)

3rd Semester

Under the Guidance ofMr.Sudhir Mishra

Amity School of Engineering & Technology

AMITY UNIVERSITY RAJASTHAN

NOV, 2012

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CERTIFICATE

This is to certify that Priya Hada, student of B.Tech. in Electronics and

Communication Engineering has carried out the work presented in the project of the

Term paper entitled “FREE SPACE OPTICAL LASER COMMUNICATION” as a

part of Second Year programme of Bachelor of Technology in of B.Tech. in Electronics

and Communication Engineering from Amity School of Engineering and Technology,

Amity University Rajasthan, under my supervision.

STUDENT GUIDE

(Priya Hada) (Sudhir Mishra)

ASET (AUR)

Date:

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ACKNOWLEDGEMENT

It has come out to be a sort of great pleasure and experience for me to work on the

project Free Space Optical Laser Communication (FSO). I wish to express my

indebtedness to those who helped us i.e. the faculty of our Institute Mr. Sudhir Mishra

during the preparation of the manual script of this text. This would not have been made

successful without his help and precious suggestions. Finally, I also warmly thanks to

all our colleagues who encouraged us to an extent, which made the project successful.

Priya Hada

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TABLE OF CONTENTS

1. INTRODUCTION………………………………………………………………………7

2. HISTORY..........................................................................................................................9

3. FSO TECHNOLOGY.....................................................................................................11

4. BASIC COMPONENT OF FSO.....................................................................................12

4.1 TRANSMITTER...........................................................................................................13

4.1.1 OPTICAL SOURCES (LASER)................................................................................13

4.1.2 ELECTRO ABSORPTION MODULATOR (EAM).................................................16

4.1.3 DRIVER CIRCUIT....................................................................................................17

4.1.4 TRANSMITTER TELESCOPE.................................................................................17

5. THE RECEIVER.............................................................................................................18

6. THE ATMOSPHERIC CHANNEL................................................................................20

6.1 FREQUENCY MODULATION..................................................................................21

7. FEATURES OF FSO......................................................................................................23

7.1 FSO SECURITY...........................................................................................................23

7.2 EYE-SAFETY...............................................................................................................24

7.3 COST OF DEPLOYMENT..........................................................................................24

8. FSO-BREAKING THE BANDWIDTH BOTTLENECK.............................................25

9 .FSO ADVANTAGES AND CHALLENGES................................................................26

9.1 ADVANTAGES............................................................................................................26

9.2 FSO CHALLENGES....................................................................................................26

10. APPLICATIONS..........................................................................................................29

11. CONCLUSION.............................................................................................................30

REFERENCES....................................................................................................................31

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LIST OF FIGURES AND TABLE

Figure 3.1 Basic overview of FSO system

Figure 4.1 Block diagram of FSO unit

Figure 4.2 Laser Structure based on Fabry-Perot Principle

Figure 4.3 A Simplified VSCEL Laser

Figure 5.1 Block diagram of a Optical Receiver

Figure 6.1 FSO Beam through atmospheric turbulence

Table 4.1 Comparison between FB/DFB/VCSEL

Table 9.2 Losses in the FSO System

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ABSTRACT

Free Space Optics (FSO) or Optical Wireless, refers to the transmission of

modulated visible or infrared (IR) beams through the air to obtain optical

communications. Like fiber, FSO uses lasers to transmit data, but instead of enclosing

the data stream in a glass fiber, it is transmitted through the air. It is a secure, cost-

effective alternative to other wireless connectivity options. This form of delivering

communication has a lot of compelling advantages .Data rates comparable to fiber

transmission can be carried with very low error rates, while the extremely narrow laser

beam widths ensure that it is possible to co-locate multiple tranceivers without risk of

mutual interference in a given location. FSO has roles to play as primary access

medium and backup technology. It could also be the solution for high speed residential

access. Though this technology sprang into being, its applications are wide and many. It

indeed is the technology of the future...

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1. INTRODUCTION

Free Space Optics (FSO) communications, also called Free Space Photonics (FSP) refers

to the transmission of modulated visible or infrared (IR) beams through the atmosphere

to obtain optical communications. Like fiber FSO uses lasers to transmit data, but

instead of enclosing the data stream in a glass fiber, it is transmitted through the air. FSO

works on the same basic principle as Infrared television remote controls Wireless

keyboards.

It supports high bandwidth, with easy to install connections for the last-mile and

campus environments. Free space links behave similarly to fiber optic systems. Instead

of focusing the output of a semiconductor laser or Light Emitting Diode (LED) into a

strand of optical fiber, the output is broadcast in a thin beam across the sky at a 1600nm

.It is basically used to transmit data for telecommunication or computer networking. It

require no licensing and only require frequency coordination.

It also provide a line of sight link .FSO links are full duplex. Also it is unaffected by

electromagnetic interference and radio frequency interference, which increasingly plague

radio based communication systems. FSO systems are used in disaster recovery

applications and for temporary connectivity while cabled networks are being deployed.

The technology is useful where the physical connections are impractical due to high costs

or other considerations.

There has been an exponential increase in the use of FSO technology,

mainly for “last mile” applications, because FSO links provide the

transmission capacity to overcome bandwidth bottlenecks.. Fiber

optics has been traditionally used for transmission of both digital and

analog signals.

FSO has now emerged as a commercially viable alternative to radio

frequency and millimeter wave wireless systems for reliable and

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rapid deployment of data voice networks. The fact that FSO is

transparent to traffic type and data protocol makes its integration

into the existing access network far more rapid, but also it has

atmospheric challenges like thick fog, smoke and turbulences to

attain a long range terrestrials FSO.

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

licensing or frequency coordination with other users is required, interference from or to

other systems or equipment is not a concern, and the point-to-point laser signal is

extremely difficult to intercept, and therefore secure.

Data rates comparable to optical fiber transmission can be carried by FSO systems

with very low error rates, while the extremely narrow laser beam widths ensure that

there is almost no practical limit to the number of separate FSO links that can be

installed in a given location.

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2. HISTORY

Optical communications, in various forms have been used for thousands of years.

The Ancient Greeks polished their shield to send signals during battle. In the modern era,

wireless solar telegraphs called heliograph were developed, using coded signals to

communicate with their recipients

FSO or optical wireless communications was first demonstrated by Alexander Graham

Bell and his assistant Charles Sumner tainter in the late nineteenth century (prior to his

demonstration of the telephone!). Bell’s FSO experiment on June 3,1880 at Bell’s new

created Volta laboratory where they converted voice sounds into telephone signals and

transmitted them between receivers through free air space along a beam of light for a

distance of some 600 feet. Calling his experimental device the “photo phone,” Bell

considered this optical technology – and not the telephone – his pre eminent invention

because it did not require wires for transmission. Although Bell’s photo phone never

became a commercial reality, it demonstrated the basic principle of optical

communications.

Carl Zeiss Jena developed the direct translation: light speaking device that the German

army used in their World War II anti-aircraft defense units.

The invention of lasers in the 1960s revolutionized free space optics. Military

organizations were particularly interested and boosted their development. However the

technology lost market momentum when the installation of optical fiber networks for

civilian uses was at its peak. Many simple and inexpensive consumer remote

controls use low-speed communication using Infrared (IR) light. This is known as IR

consumer technology

The spectacular transmission of T.V signal over a 30 mile distance using GaAs LED by

researcher working in the MIT Lincolns Laboratory in 1962. The first laser link to

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handle commercial traffic was built in Japan by Nippon electric company (NEC)

around 1970. The link was a full duplex He-Ne laser FSO between Yakohama and

Tamagawa, a distance of 14 km.

FSO has also been heavily researched for deep space application by NASA and ESA

with programmes such as the then Mars Laser Communication Demonstration

Demonstration (MLCD) and the Semiconductor- laser Inter-satellite Link Experiment

(SILEX) respectively.

In the past decade, near Earth FSO were successfully demonstrated in space between

satellites at data rates of up to 10 Gbps.

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3. FSO TECHNOLOGY

FSO transmits invisible, eye-safe light beams from one "telescope" to another using

low power infrared laser in the Terahertz (1Trillion Hz) spectrum. The beams of light in

FSO systems are transmitted by laser light focused on highly sensitive photon detector

receivers. These receivers are telescopic lenses able to collect the photon stream and

transmit digital data containing a mix of Internet messages, video images, radio signals

or computer files .Commercially available systems offer capacities in the range of 100

Mbps to 2.5 Gbps, and demonstration systems report data rates as high as 160 Gbps.

FSO systems can function over distances of several kilometers. As long as there is a

clear line of sight between the source and the destination, and enough transmitter

power, FSO communication is possible.

(Courtesy: FSO communication Link, UCSI)

Fig 3.1 Basic overview of FSO System

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4. BASIC COMPONENT OF FSO

( Courtesy: Optical research group, NCR Lab)

Fig 4.1 Block diagram of FSO Unit

4.1 TRANSMITTER

This functional element has the primary duty of modulating the source data onto the

optical carrier which is then propagated through the atmospheric to the receiver.

The most widely used modulation type is the intensity modulation (IM) in which

the source data is modulated. This is achieved by varying the driving current of the

optical source directly in sympathy with the data to be transmitted or via an external

modulator such as electro absorption modulator The use of an modulator guarantees

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a higher data rates than what is obtainable with direct modulation but an external

modulator has a non-linear response.

Other properties of the radiated optical field such as its phase, frequency and state of

polarization can also be modulated with data/information through the use of an external

modulator.

The transmitters usually contain:

1. Optical source (laser diode)

2. Modulator (Electro Absorption)

3. Driver Circuit

4. Transmit Telescope

4.1.1 OPTICAL SOURCES (LASER)

The word laser is actually an acronym for Light Amplification by Stimulated Emission

of Radiation. A laser generates light, either visible or infrared, through a process known

as stimulated emission.

MONOLITHIC FABRY-PEROT LASERS

Monolithic semiconductor lasers with a resonance mechanism (or optical feedback)

based on the Fabry-Perot principles,growing 3-D layers of crystals with controlled

consistency and doping.It form of a straight channel (p-type AIGaAs), which is both the

active region (for stimulated emission) and the optical waveguide (to guide photons in

one direction

Fabry-Perot lasers can generate several longitudinal frequencies (modes) at once. The

semiconductor laser material, the frequency spacing, and the Fabry-Perot laser length

determine the range of frequencies. The bias current determines the threshold

frequency.

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( Courtesy: Optical Component 2nd (Chapter 6, Light sources))

4.2 Laser structure based on the Fabry-Perot principle

.

Distributed-Feedback Laser

Distributed-feedback (DFB) lasers are monolithic devices that have an internal

structure based on InGaAsP waveguide technology and an internal grating. DFBs are an

extension of the Electro absorption-modulated lasers and take their name from their

structure. The DFB structure may be combined with multiple quantum well (MQW)

structures to improve the line width of the produced laser light (as narrow as few

hundred kilohertz). The resonant cavity may be of the Mach-Zehnder or the Fabry-

Perot type.DFB lasers are reliable sources with center frequencies in the region around

1310 nm, and also in the 1520-1565 nm range; the latter makes them compatible with

erbium-doped fiber amplifiers and excellent sources in dense wavelength division

multiplexing (DWDM) applications.

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Vertical Cavity Surface-Emitting Laser

Fabry-Perot devices, DFBs, and DBRs typically require substantial amounts of current

to operate, in the order of tens of mill amperes. Moreover, their output beam has an

elliptical cross section, typically an aspect ratio of 3:1, which does not match the

cylindrical cross section of the fiber core. Thus, a non cylindrical beam may require

additional optics. A structure that produces a cylindrical beam is known as vertical

cavity, surface-emitting.

(Courtesy: Optical Component 2nd (Chapter 6, Light sources))

Fig 4.3 A Simplified VCSEL LASER

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TABLE 4.1 COMPARISONS BETWEEN LED/ FB/DFB/VCSEL

(Courtesy: Optical Component 2nd (Chapter 6, Light sources))

4.1.2 ELECTRO ABSORPTION MODULATOR (EAM)

EAM is a semiconductor device which can be used for modulating the intensity of a

laser beam via an electric voltage. Its principle of operation is based on, i.e., a change in

the absorption spectrum caused by an applied electric field, which changes the band

gap energy (thus the photon energy of an absorption edge) but usually does not involve

the excitation of carriers by the electric field. For modulators in telecommunications

small size and modulation voltages are desired. The EAM is candidate for use in

external modulation links in telecommunications.

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4.1.3 DRIVER CIRCUIT

In electronics, a driver is electrical circuit an  or other  used to control electronic

component another circuit or other component, such as a high-power transistor. They

are usually used to regulate current flowing through a circuit or is used to control the

other factors such as other components, some devices in the circuit. The term is often

used, for example, for a specialized integrated circuits that controls high-power

switches  in switched-mode power converter  An Amplifier  can also be considered a

driver for loudspeaker, or a constant voltage circuit that keeps an attached component

operating within a broad range of input voltages.

For example in a transistor power amplifier, typically the driver circuit requires current

gain, often the ability to discharge the following transistor bases rapidly, and low output

impedance to avoid or minimise distortion.

4.1.4 TRANSMITTER TELESCOPE

The transmitter telescope collects, collimates and direct the optical radiation toward the

receiver telescope at the other end of the channel.

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5. THE RECEIVER

The receiver helps recover the transmitted data from the incident optical field. The

receiver is composed of:

RECEIVER TELESCOPE: It collects and focuses the incoming optical radiation on to

the Photodetector. It should be noted that a large receiver telescope aperture is desirable

as it collects multiple uncorrelated radiation and focuses their average on the

Photodetector.

This is referred to as aperture averaging but a wide aperture also means more

background radiation/noise.

AN OPTICAL BAND: It contains the pass filter to reduce the amount of background

radiation.

A PHOTODETECTOR: It operates by converting light signal that hits the junction to a

voltage or current.

Photodiode- It is commonly used Photodetector. A photodiode is based on a junction of

opposite doped region (pn junction) in a sample of semiconductor. This creates a region

depleted of charge carriers that results in high impedance. The high impedance allow

the construction of detectors using silicon and germanium to operate with high

sensitivity at low impedance.

Since the light is used as an input, the diode is operated under reverse bias condition.

Photodiodes are usually made of GaAs.

PIN Photodiode: It includes an intrinsic layer in between the P and N type material. It

must be reverse bias due to high resistivity of the intrinsic layer The PIN has a layer

depletion region which allows more electron-hole pair to develop a lower capacitances.

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Avalanche Photodiode: It is operated at reverse bias close to the breakdown, which

causes photo excited change carrier to accelerate in the depletion region and produce

additional carrier by avalanching They are good for fiber optic system that require low

light levels with quantum efficiency larger than 100 percent.

POST-DETECTION PROCESSOR (decision circuit): It is the circuit where the

necessary amplification, Filtering and signal processing necessary to guarantee a high

fidelity data recovery are carried out.

(Courtesy: FSO Communication Link, UCSI)

Figure 6.1 Block Diagram of a optical receiver

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6. THE ATMOSPHERIC CHANNEL

In the optical system SNR s proportional to A (A is the receiver detector area) this

implies that for a given transmit power; a high SNR can be attained by using an large

area detector. However as A increases so does its capacitance, which has a limited

effect on the receiver bandwidth.

1. POWER LOSS

For an optical radiation traversing the atmosphere ,some of the photons are

extinguished (absorbed) by the molecular constitutes(water vapour, Carbondioxide,

ozone etc) and their energy converted into heat while other experience no loss of

energy but their initial direction of propagation changed (scattering).

a. ATMOSPHERIC CHANNEL LOSS: The atmospheric channel attenuates the field

traversing it as a result of atmosphere and scattering processes. The concentration of

matter in the atmosphere, which result in the signal attenuation vary spatially and

temporarily and will depend on the current local weather condition.

b. BEAM DIVERGENCE LAW: One of the advantage of FSO system is the ability to

transmit the a very narrow optical beam, thus , offering advanced security. But due

to diffraction, the beam spreads out. This results in a situation in which the receive

aperture is only able to collect a fraction of the beam, hence beam divergence loss.

c. OPTICAL AND WINDOW LOSS: It includes losses due to imperfect lenses and

other optical elements used in design of both transmitter and receiver. It accounts

for the reflection, absorption, scattering due to lenses in system.

d. POINTING LOSS: It occurs due to imperfect alignment of the transmitter and the

receiver.

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(Fiber Courtesy: Corning Optical,Peter Rouo)

Figure 5.1 FSO beam propagation through atmospheric turbulence

6.1 FREQUENCY MODULATION

A FSO system is based on optical FM, where the information is encoded by a time-

variable wavelength. As is well known, broadband FM systems use a transmission

bandwidth that is larger than the signal’s information bandwidth, thus enabling an

enhancement of the SNR and hence the effective information rate per unit transmitter

power. Because of the atmospheric conditions, any optical free-space communication

system, contemplated at a terrestrial level, must operate at mid-infrared wavelengths

in the range λ = 2.5-2.8 μm. Development of rapidly tunable single-frequency lasers

in this wavelength range is quite feasible, based on the current experience with

tunable telecom lasers at 1.5 μm. Nevertheless, there is no currently available optical

FM system. The main difficulty is associated not so much with the tunable optical

sources, as with the of a wavelength-discriminating receiver system that would take

advantage of the enhanced SNR. In our view, the key enabling solution is optical

super heterodyne with a local oscillator implemented as a tunable mid-infrared laser

similar to that at the source. The intermediate frequency can be tuned to lie either in a

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frequency range directly accessible to electronic limiting amplifier and frequency

discriminator.

CONCEPT

Wideband frequency-modulation (FM) systems offer a trade of the bandwidth excess

for SNR, thus relaxing the transmitter power requirement as compared to AM

transmission. Energy efficiency is essential for satellite communications, sensor

networks and mobile platforms. The FM advantage is proportional to the squared

ratio (∆F /fS)2 of the range of frequency excursion ΔF to the signal bandwidth fS ,

Thus, current direct broadcast satellite systems are made possible by using a

microwave.

To preserve the FM advantage, the signal bandwidth is limited by the inequality,

fS<<∆F<<fO

This should not be a serious limitation for optical FM in any wavelength range, since

Optical frequencies are far larger than any conceivable signal bandwidth. A more

Stringent condition limits the spectral width Δf0 of the laser emission. Line width is

not an issue in radio systems. Compared to such systems, any laser is a high-Q

resonator in the sense of ΔfO << fO However, as we shall argue below, the only

practical receiving system that can be contemplated for optical FM should be based

on optical heterodyne and since the line width is “inherited” in heterodyne detection,

one must ensure it stays well below the tuning range, viz.

Condition (2) can be viewed as an optical analog of the so-called FM threshold. This

is certainly quite feasible with single-mode semiconductor lasers.

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7. FEATURES OF FSO

1. FSO transmission links can be deployed quicker, and in some instances more

economically, than optical fiber links.

2. When compared with wireless rf links, FSO requires no licensing and provides

better link security and much higher immunity from electromagnetic interference

EMI.

3. FSO is highly invulnerable to interference from other sources of laser radiation.

4. FSO can be implemented for portable applications, e.g., movable radar dish

antennas.

5. FSO provides a viable transmission channel for transporting IS-95 CDMA signals

to base stations from macro- and microcell sites and can decrease the setup costs of

temporary microcells deployed for particular events, e.g., sporting events, by

eliminating the need for installing directional microwave or connecting cable.

6. FSO introduces a viable transmission medium for the deployment of cable

television _CATV_ links in metropolitan areas where installing new fiber

infrastructure can be relatively expensive.

7. Analog FSO can reduce the cost of transmission equipment as compared to a

digital implementation.

7.1 FSO SECURITY

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

topology. It is especially important for military and corporate applications security.

FSO is far more secure than RF or other wireless-based transmission technologies for

several reasons:

1. FSO laser beams cannot be detected with spectrum analyzers or RF

meters.

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2. FSO laser transmissions are optical and travel along a line of sight path that

cannot be intercepted easily. It requires matching.

3. FSO transceiver carefully aligned to complete the transmission.

Interception is very difficult and extremely unlikely.

4. The laser beams generated by FSO systems are Narrow and invisible,

making them harder to find and even harder to Intercept and crack

5. Data can be transmitted over an encrypted connection adding to the Degree

of security available in FSO network Transmissions .

7.2 EYE-SAFETY

Laser beams with wavelengths in the range of 400 to 1400 nm emit light that passes

through the cornea and lens and is focused onto a tiny spot on the retina while

wavelengths above 1400 nm are absorbed by the cornea and lens, and do not focus onto

the retina, as illustrated in Figure 1. It is possible to design eye-safe laser transmitters at

both the 800 nm and 1550 nm wavelengths but the allowable safe laser power is about

fifty times higher at 1550 nm. This factor of fifty is important as it provides up to 17 dB

additional margin, allowing the system to propagate over longer distances, through

heavier attenuation and to support higher rates

7.3 COST OF DEPLOYMENT

Higher performances with little extra cost penalty, provides the best value. The key

factor that affects the cost are system design, minimization of manual labour and bulk

manufacturing. An 850 nm laser can cost up to $5000 while a 1550 nm laser can go up

to $50000.

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8. FSO-BREAKING THE BANDWIDTH BOTTLENECK

The global telecommunications network has seen massive expansion over the last few

years. First came the tremendous growth of the optical fiber long-haul, WAN followed

by a more recent emphasis on MANs. Meanwhile, LANs and gigabit Ethernet ports are

being deployed with a comparable growth rate. In order for this tremendous network

capacity to be exploited, and for the users to be able to utilize the broad array of new

services becoming available, network designers must provide a flexible and cost-

effective means for the users to access the telecommunications network. Presently,

however, most local loop network connections are limited to 1.5 Mbps (a T1 line). As a

consequence, there is a strong need for a high-bandwidth bridge (the “last mile” or

“first mile”) between the LANs and the MANs or WANs. A recent New York Times

article reported that more than 100 million miles of optical fiber was laid around the

world in the last two years, as carriers reacted to the Internet phenomenon and end

users’ insatiable demand for bandwidth. The sheer scale of connecting whole

communities, cities and regions to that fiber optic cable or “backbone” is something not

many players understood well. Despite the huge investment in trenching and optical

cable, most of the fiber remains unlit, 80 to 90 percent of office, commercial and

industrial buildings are not connected to fiber, and transport prices are dropping

dramatically.FSO systems represent one of the most promising approaches for

addressing the emerging broadband access market and its “last mile” bottleneck. FSO

systems offer many features, principal among them have being less start-up and

operational costs, rapid deployment, and high fiber-like bandwidths due to the optical

nature of the technology.

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9 .FSO ADVANTAGES AND CHALLENGES

9.1 ADVANTAGES

An FSO system offers a flexible networking solution that delivers on the promise of

broadband. Since FSO optical wireless transceivers can transmit and receive through

windows, it is possible to mount FSO systems inside buildings, reducing the need to

compete for roof space, simplifying wiring and cabling, and permitting the equipment

to operate in a very favorable environment. The only essential for FS is line of sight

between the two ends of the link.

Freedom from licensing and regulation .

Ease, high speed and low cost of deployment.

It reduces the need to compete for roof space, simplifying wiring

Only need is the line of sight between two links

Zero chances of network failure

9.2 FSO CHALLENGES

The advantages of free space optical wireless or FSO do not come without some cost.

When light is transmitted through optical fiber, transmission integrity is quite

predictable – barring unforseen events such as backhoes or animal interference.

FOG

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

infrared wavelengths that are employed in FSO systems. Note that the effect of fog on

FSO optical wireless radiation is entirely analogous to the attenuation – and fades –

suffered by RF wireless systems due to rainfall. Similar to the case of rain attenuation

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with RF wireless, fog attenuation is not a “show-stopper” for FSO, because the optical

link can be engineered such that, for a large fraction of the time, an acceptable power

will be received even in the presence of heavy fog

PHYSICAL OBSTRUCTIONS

FSO products which have widely spaced redundant transmitters and large receive optics

will all but eliminate interference concerns from objects such as birds. On a typical day,

an object covering 98% of the receive aperture and all but 1 transmitter; will not cause

a FSO link to drop out. Thus birds are unlikely to have any impact on FSO transmission

POINTING STABILITY-BUILDING SWAY

Fixed pointed FSO systems are designed to be capable of handling the vast majority of

movement found in deployments on buildings. The combination of effective beam

divergence and a well matched receive Field-of-View (FOV) provide for an extremely

robust fixed FSO system suitable for most deployments. Fixed-pointed FSO systems

are generally preferred over actively-tracked FSO systems due to their lower cost.

SCINTILLATION

Scintillation is one of the effects related to turbulence. 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.

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.

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SOLAR INTERFERENCE

Solar interference in FSO system operating at 1550 nm can be combated in two ways.

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

below 850 nm from entering the system; the second is an optical narrowband filter

proceeding the receive detector used to filter all but the wavelength actually used for

intersystem communications. To handle off-axis solar energy, two spatial filters have

been implemented in systems, allowing them to operate unaffected by solar interference

that is more than 1.5 degrees off-axis.

ATMOSPHERIC ATTENUATION

Carrier-class FSO systems must be designed to accommodate heavy atmospheric

attenuation, particularly by fog. Although longer wavelengths are favored in haze and

light fog, under conditions of very low visibility this long-wavelength advantage does

not apply. However, the fact that1550 nm-based systems are allowed to transmit up to

50 times more eye-safe power will translate into superior penetration of fog or any

other atmospheric attenuator

TABLE 9.1 Rough Estimate of Power losses in the system Infrared light (765 nm) :

Clear, still air -1 dB/km -5 dB/km

Scintillation 0 to -3 dB/km

Birds or foliage Impenetrable 0 to -20 dB/km

Window (double-glazed) -3 dB/km -1 dB /km

Light mist (visibility 400m) -25 dB/km -1 dB/km

Medium fog (visibility 100m) -120 dB/km -1 dB/km

Light rain (25mm/hour) -10 dB/km -10 dB/

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10. APPLICATIONS

METRO NETWOK EXTENSIONS – FSO is used to extend existing metropolitan area

fibers to connect new networks from outside

LAST MILE ACCESS – FSO can be used in high speed links to connect the end users

with ISPs.

ENTERPRISE CONNECTIVITY - The ease in which FSO can be installed Make them

a solution for interconnecting LAN segments, housed in building separated by public

streets.

FIBER BACKUP - FSO may be deployed in redundant links to backup fiber in place of

a second fiber link.

BACKHAUL – Used to carry cellular telephone traffic from antenna towers back to

facilities into the public switched telephone network.

FSO COMPARISONS

Free space optical communications is now established as a viable approach for

addressing the emerging broadband access market and its “last mile” bottleneck..These

robust systems, which establish communication links by transmitting laser beams

directly through the atmosphere, have matured to the point that mass- produced models

are now available. Optical wireless systems offer many features, principal among them

being slow start-up and operational costs, rapid deployment, and high fiber-like

bandwidths. These systems are compatible with a wide range of applications and

markets, and they are sufficiently flexible as to be easily implemented using a variety of

different architectures. Because of these features, market projections indicate healthy

growth for optical wireless sales. Although simple to deploy, optical wireless

transceivers are sophisticated devices.

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11. 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.

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REFERENCES

[1]. Harry J. R. Dutton (1999), Understanding Optical Communications .

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