• Share
  • Email
  • Embed
  • Like
  • Save
  • Private Content
## Finalterm paper repport on fso#w245
 

## Finalterm paper repport on fso#w245

on

  • 706 views

 

Statistics

Views

Total Views
706
Views on SlideShare
706
Embed Views
0

Actions

Likes
1
Downloads
71
Comments
0

0 Embeds 0

No embeds

Accessibility

Categories

Upload Details

Uploaded via as Microsoft Word

Usage Rights

© All Rights Reserved

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
    Processing…
Post Comment
Edit your comment

    ## Finalterm paper repport on fso#w245 ## Finalterm paper repport on fso#w245 Document Transcript

    • 1ATERM PAPER REPORT ONFREE SPACE OPTICALLASER COMMUNICATIONSubmitted by:Priya HadaB.Tech (ECE)3rdSemesterUnder the Guidance ofMr.Sudhir MishraAmity School of Engineering &TechnologyAMITY UNIVERSITY RAJASTHANNOV, 2012
    • 2CERTIFICATEThis is to certify that Priya Hada, student of B.Tech. in Electronics andCommunication Engineering has carried out the work presented in the project of theTerm paper entitled “FREE SPACE OPTICAL LASER COMMUNICATION” as apart of Second Year programme of Bachelor of Technology in of B.Tech. in Electronicsand Communication Engineering from Amity School of Engineering and Technology,Amity University Rajasthan, under my supervision.STUDENT GUIDE(Priya Hada) (Sudhir Mishra)ASET (AUR)Date:
    • 3ACKNOWLEDGEMENTIt has come out to be a sort of great pleasure and experience for me to work on theproject Free Space Optical Laser Communication (FSO). I wish to express myindebtedness to those who helped us i.e. the faculty of our Institute Mr. Sudhir Mishraduring the preparation of the manual script of this text. This would not have been madesuccessful without his help and precious suggestions. Finally, I also warmly thanks toall our colleagues who encouraged us to an extent, which made the project successful.Priya Hada
    • 4TABLE OF CONTENTS1. INTRODUCTION………………………………………………………………………72. HISTORY......................................................................................................................... 93. FSO TECHNOLOGY .................................................................................................... 114. BASIC COMPONENT OF FSO.................................................................................... 124.1 TRANSMITTER.......................................................................................................... 134.1.1 OPTICAL SOURCES (LASER)............................................................................... 134.1.2 ELECTRO ABSORPTION MODULATOR (EAM)................................................ 164.1.3 DRIVER CIRCUIT ................................................................................................... 174.1.4 TRANSMITTER TELESCOPE................................................................................ 175. THE RECEIVER............................................................................................................ 186. THE ATMOSPHERIC CHANNEL............................................................................... 206.1 FREQUENCY MODULATION................................................................................. .217. FEATURES OF FSO ..................................................................................................... 237.1 FSO SECURITY .......................................................................................................... 237.2 EYE-SAFETY.............................................................................................................. 247.3 COST OF DEPLOYMENT.......................................................................................... 248. FSO-BREAKING THE BANDWIDTH BOTTLENECK ............................................ 259 .FSO ADVANTAGES AND CHALLENGES ............................................................... 269.1 ADVANTAGES........................................................................................................... 269.2 FSO CHALLENGES ................................................................................................... 2610. APPLICATIONS.......................................................................................................... 2911. CONCLUSION ............................................................................................................ 30REFERENCES................................................................................................................... 31
    • 5LIST OF FIGURES AND TABLEFigure 3.1 Basic overview of FSO systemFigure 4.1 Block diagram of FSO unitFigure 4.2 Laser Structure based on Fabry-Perot PrincipleFigure 4.3 A Simplified VSCEL LaserFigure 5.1 Block diagram of a Optical ReceiverFigure 6.1 FSO Beam through atmospheric turbulenceTable 4.1 Comparison between FB/DFB/VCSELTable 9.2 Losses in the FSO System
    • 6ABSTRACTFree Space Optics (FSO) or Optical Wireless, refers to the transmission ofmodulated visible or infrared (IR) beams through the air to obtain opticalcommunications. Like fiber, FSO uses lasers to transmit data, but instead of enclosingthe 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 deliveringcommunication has a lot of compelling advantages .Data rates comparable to fibertransmission can be carried with very low error rates, while the extremely narrow laserbeam widths ensure that it is possible to co-locate multiple tranceivers without risk ofmutual interference in a given location. FSO has roles to play as primary accessmedium and backup technology. It could also be the solution for high speed residentialaccess. Though this technology sprang into being, its applications are wide and many. Itindeed is the technology of the future...
    • 71. INTRODUCTIONFree Space Optics (FSO) communications, also called Free Space Photonics (FSP) refersto the transmission of modulated visible or infrared (IR) beams through the atmosphereto obtain optical communications. Like fiber FSO uses lasers to transmit data, butinstead of enclosing the data stream in a glass fiber, it is transmitted through the air. FSOworks on the same basic principle as Infrared television remote controls Wirelesskeyboards.It supports high bandwidth, with easy to install connections for the last-mile andcampus environments. Free space links behave similarly to fiber optic systems. Insteadof focusing the output of a semiconductor laser or Light Emitting Diode (LED) into astrand 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. Itrequire no licensing and only require frequency coordination.It also provide a line of sight link .FSO links are full duplex. Also it is unaffected byelectromagnetic interference and radio frequency interference, which increasingly plagueradio based communication systems. FSO systems are used in disaster recoveryapplications and for temporary connectivity while cabled networks are being deployed.The technology is useful where the physical connections are impractical due to high costsor other considerations.There has been an exponential increase in the use of FSO technology, mainly for “lastmile” applications, because FSO links provide the transmission capacity to overcomebandwidth bottlenecks.. Fiber optics has been traditionally used for transmission ofboth digital and analog signals.FSO has now emerged as a commercially viable alternative to radio frequency andmillimeter wave wireless systems for reliable and rapid deployment of data voicenetworks. The fact that FSO is transparent to traffic type and data protocol makes its
    • 8integration into the existing access network far more rapid, but also it has atmosphericchallenges 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 spectrumlicensing or frequency coordination with other users is required, interference from or toother systems or equipment is not a concern, and the point-to-point laser signal isextremely difficult to intercept, and therefore secure.Data rates comparable to optical fiber transmission can be carried by FSO systemswith very low error rates, while the extremely narrow laser beam widths ensure thatthere is almost no practical limit to the number of separate FSO links that can beinstalled in a given location.
    • 92. HISTORYOptical 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 tocommunicate with their recipientsFSO or optical wireless communications was first demonstrated by Alexander GrahamBell and his assistant Charles Sumner tainter in the late nineteenth century (prior to hisdemonstration of the telephone!). Bell’s FSO experiment on June 3,1880 at Bell’s newcreated Volta laboratory where they converted voice sounds into telephone signals andtransmitted them between receivers through free air space along a beam of light for adistance of some 600 feet. Calling his experimental device the “photo phone,” Bellconsidered this optical technology – and not the telephone – his pre eminent inventionbecause it did not require wires for transmission. Although Bell’s photo phone neverbecame a commercial reality, it demonstrated the basic principle of opticalcommunications.Carl Zeiss Jena developed the direct translation: light speaking device that the Germanarmy used in their World War II anti-aircraft defense units.The invention of lasers in the 1960s revolutionized free space optics. Militaryorganizations were particularly interested and boosted their development. However thetechnology lost market momentum when the installation of optical fiber networks forcivilian uses was at its peak. Many simple and inexpensive consumer remotecontrols use low-speed communication using Infrared (IR) light. This is known as IRconsumer technologyThe spectacular transmission of T.V signal over a 30 mile distance using GaAs LED byresearcher working in the MIT Lincolns Laboratory in 1962. The first laser link tohandle 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 andTamagawa, a distance of 14 km.
    • 10FSO has also been heavily researched for deep space application by NASA and ESAwith programmes such as the then Mars Laser Communication DemonstrationDemonstration (MLCD) and the Semiconductor- laser Inter-satellite Link Experiment(SILEX) respectively.In the past decade, near Earth FSO were successfully demonstrated in space betweensatellites at data rates of up to 10 Gbps.
    • 113. FSO TECHNOLOGYFSO transmits invisible, eye-safe light beams from one "telescope" to another usinglow power infrared laser in the Terahertz (1Trillion Hz) spectrum. The beams of light inFSO systems are transmitted by laser light focused on highly sensitive photon detectorreceivers. These receivers are telescopic lenses able to collect the photon stream andtransmit digital data containing a mix of Internet messages, video images, radio signalsor computer files .Commercially available systems offer capacities in the range of 100Mbps 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 aclear line of sight between the source and the destination, and enough transmitterpower, FSO communication is possible.(Courtesy: FSO communication Link, UCSI)Fig 3.1 Basic overview of FSO System
    • 124. BASIC COMPONENT OF FSO( Courtesy: Optical research group, NCR Lab)Fig 4.1 Block diagram of FSO Unit4.1 TRANSMITTERThis functional element has the primary duty of modulating the source data onto theoptical carrier which is then propagated through the atmospheric to the receiver.The most widely used modulation type is the intensity modulation (IM) in whichthe source data is modulated. This is achieved by varying the driving current of theoptical source directly in sympathy with the data to be transmitted or via an externalmodulator such as electro absorption modulator The use of an modulator guaranteesa higher data rates than what is obtainable with direct modulation but an externalmodulator has a non-linear response.
    • 13Other properties of the radiated optical field such as its phase, frequency and state ofpolarization can also be modulated with data/information through the use of an externalmodulator.The transmitters usually contain:1. Optical source (laser diode)2. Modulator (Electro Absorption)3. Driver Circuit4. Transmit Telescope4.1.1 OPTICAL SOURCES (LASER)The word laser is actually an acronym for Light Amplification by Stimulated Emissionof Radiation. A laser generates light, either visible or infrared, through a process knownas stimulated emission.MONOLITHIC FABRY-PEROT LASERSMonolithic semiconductor lasers with a resonance mechanism (or optical feedback)based on the Fabry-Perot principles,growing 3-D layers of crystals with controlledconsistency and doping.It form of a straight channel (p-type AIGaAs), which is both theactive region (for stimulated emission) and the optical waveguide (to guide photons inone directionFabry-Perot lasers can generate several longitudinal frequencies (modes) at once. Thesemiconductor laser material, the frequency spacing, and the Fabry-Perot laser lengthdetermine the range of frequencies. The bias current determines the thresholdfrequency.
    • 14( Courtesy: Optical Component 2nd(Chapter 6, Light sources))4.2 Laser structure based on the Fabry-Perot principle.Distributed-Feedback LaserDistributed-feedback (DFB) lasers are monolithic devices that have an internalstructure based on InGaAsP waveguide technology and an internal grating. DFBs are anextension of the Electro absorption-modulated lasers and take their name from theirstructure. 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 fewhundred 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 around1310 nm, and also in the 1520-1565 nm range; the latter makes them compatible witherbium-doped fiber amplifiers and excellent sources in dense wavelength divisionmultiplexing (DWDM) applications.
    • 15Vertical Cavity Surface-Emitting LaserFabry-Perot devices, DFBs, and DBRs typically require substantial amounts of currentto operate, in the order of tens of mill amperes. Moreover, their output beam has anelliptical cross section, typically an aspect ratio of 3:1, which does not match thecylindrical cross section of the fiber core. Thus, a non cylindrical beam may requireadditional optics. A structure that produces a cylindrical beam is known as verticalcavity, surface-emitting.(Courtesy: Optical Component 2nd(Chapter 6, Light sources))Fig 4.3 A Simplified VCSEL LASER
    • 16TABLE 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 alaser beam via an electric voltage. Its principle of operation is based on, i.e., a change inthe absorption spectrum caused by an applied electric field, which changes the bandgap energy (thus the photon energy of an absorption edge) but usually does not involvethe excitation of carriers by the electric field. For modulators in telecommunicationssmall size and modulation voltages are desired. The EAM is candidate for use inexternal modulation links in telecommunications.
    • 174.1.3 DRIVER CIRCUITIn electronics, a driver is electrical circuit an or other used to control electroniccomponent another circuit or other component, such as a high-power transistor. Theyare usually used to regulate current flowing through a circuit or is used to control theother factors such as other components, some devices in the circuit. The term is oftenused, for example, for a specialized integrated circuits that controls high-powerswitches in switched-mode power converter An Amplifier can also be considered adriver for loudspeaker, or a constant voltage circuit that keeps an attached componentoperating within a broad range of input voltages.For example in a transistor power amplifier, typically the driver circuit requires currentgain, often the ability to discharge the following transistor bases rapidly, and low outputimpedance to avoid or minimise distortion.4.1.4 TRANSMITTER TELESCOPEThe transmitter telescope collects, collimates and direct the optical radiation toward thereceiver telescope at the other end of the channel.
    • 185. THE RECEIVERThe receiver helps recover the transmitted data from the incident optical field. Thereceiver is composed of:RECEIVER TELESCOPE: It collects and focuses the incoming optical radiation on tothe Photodetector. It should be noted that a large receiver telescope aperture is desirableas it collects multiple uncorrelated radiation and focuses their average on thePhotodetector.This is referred to as aperture averaging but a wide aperture also means morebackground radiation/noise.AN OPTICAL BAND: It contains the pass filter to reduce the amount of backgroundradiation.A PHOTODETECTOR: It operates by converting light signal that hits the junction to avoltage or current.Photodiode- It is commonly used Photodetector. A photodiode is based on a junction ofopposite doped region (pn junction) in a sample of semiconductor. This creates a regiondepleted of charge carriers that results in high impedance. The high impedance allowthe construction of detectors using silicon and germanium to operate with highsensitivity 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. Itmust be reverse bias due to high resistivity of the intrinsic layer The PIN has a layerdepletion region which allows more electron-hole pair to develop a lower capacitances.
    • 19Avalanche Photodiode: It is operated at reverse bias close to the breakdown, whichcauses photo excited change carrier to accelerate in the depletion region and produceadditional carrier by avalanching They are good for fiber optic system that require lowlight levels with quantum efficiency larger than 100 percent.POST-DETECTION PROCESSOR (decision circuit): It is the circuit where thenecessary amplification, Filtering and signal processing necessary to guarantee a highfidelity data recovery are carried out.(Courtesy: FSO Communication Link, UCSI)Figure 6.1 Block Diagram of a optical receiver
    • 206. THE ATMOSPHERIC CHANNELIn the optical system SNR s proportional to A (A is the receiver detector area) thisimplies that for a given transmit power; a high SNR can be attained by using an largearea detector. However as A increases so does its capacitance, which has a limitedeffect on the receiver bandwidth.1. POWER LOSSFor an optical radiation traversing the atmosphere ,some of the photons areextinguished (absorbed) by the molecular constitutes(water vapour, Carbondioxide,ozone etc) and their energy converted into heat while other experience no loss ofenergy but their initial direction of propagation changed (scattering).a. ATMOSPHERIC CHANNEL LOSS: The atmospheric channel attenuates the fieldtraversing it as a result of atmosphere and scattering processes. The concentration ofmatter in the atmosphere, which result in the signal attenuation vary spatially andtemporarily and will depend on the current local weather condition.b. BEAM DIVERGENCE LAW: One of the advantage of FSO system is the ability totransmit the a very narrow optical beam, thus , offering advanced security. But dueto diffraction, the beam spreads out. This results in a situation in which the receiveaperture 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 andother optical elements used in design of both transmitter and receiver. It accountsfor the reflection, absorption, scattering due to lenses in system.d. POINTING LOSS: It occurs due to imperfect alignment of the transmitter and thereceiver.
    • 21(Fiber Courtesy: Corning Optical,Peter Rouo)Figure 5.1 FSO beam propagation through atmospheric turbulence6.1 FREQUENCY MODULATIONA 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 transmissionbandwidth that is larger than the signal’s information bandwidth, thus enabling anenhancement of the SNR and hence the effective information rate per unit transmitterpower. Because of the atmospheric conditions, any optical free-space communicationsystem, contemplated at a terrestrial level, must operate at mid-infrared wavelengthsin the range λ = 2.5-2.8 μm. Development of rapidly tunable single-frequency lasersin this wavelength range is quite feasible, based on the current experience withtunable telecom lasers at 1.5 μm. Nevertheless, there is no currently available opticalFM system. The main difficulty is associated not so much with the tunable opticalsources, as with the of a wavelength-discriminating receiver system that would takeadvantage of the enhanced SNR. In our view, the key enabling solution is opticalsuper heterodyne with a local oscillator implemented as a tunable mid-infrared lasersimilar to that at the source. The intermediate frequency can be tuned to lie either in a
    • 22frequency range directly accessible to electronic limiting amplifier and frequencydiscriminator.CONCEPTWideband frequency-modulation (FM) systems offer a trade of the bandwidth excessfor SNR, thus relaxing the transmitter power requirement as compared to AMtransmission. Energy efficiency is essential for satellite communications, sensornetworks and mobile platforms. The FM advantage is proportional to the squaredratio (∆F /fS)2of the range of frequency excursion ΔF to the signal bandwidth fS ,Thus, current direct broadcast satellite systems are made possible by using amicrowave.To preserve the FM advantage, the signal bandwidth is limited by the inequality,fS<<∆F<<fOThis should not be a serious limitation for optical FM in any wavelength range, sinceOptical frequencies are far larger than any conceivable signal bandwidth. A moreStringent condition limits the spectral width Δf0 of the laser emission. Line width isnot an issue in radio systems. Compared to such systems, any laser is a high-Qresonator in the sense of ΔfO << fO However, as we shall argue below, the onlypractical receiving system that can be contemplated for optical FM should be basedon 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. Thisis certainly quite feasible with single-mode semiconductor lasers.
    • 237. FEATURES OF FSO1. FSO transmission links can be deployed quicker, and in some instances moreeconomically, than optical fiber links.2. When compared with wireless rf links, FSO requires no licensing and providesbetter link security and much higher immunity from electromagnetic interferenceEMI.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 dishantennas.5. FSO provides a viable transmission channel for transporting IS-95 CDMA signalsto base stations from macro- and microcell sites and can decrease the setup costs oftemporary microcells deployed for particular events, e.g., sporting events, byeliminating the need for installing directional microwave or connecting cable.6. FSO introduces a viable transmission medium for the deployment of cabletelevision _CATV_ links in metropolitan areas where installing new fiberinfrastructure can be relatively expensive.7. Analog FSO can reduce the cost of transmission equipment as compared to adigital implementation.7.1 FSO SECURITYSecurity is an important element of data transmission, irrespective of the networktopology. It is especially important for military and corporate applications security.FSO is far more secure than RF or other wireless-based transmission technologies forseveral reasons:1. FSO laser beams cannot be detected with spectrum analyzers or RF meters.
    • 242. FSO laser transmissions are optical and travel along a line of sight path that cannotbe intercepted easily. It requires matching.3. FSO transceiver carefully aligned to complete the transmission. Interception is verydifficult and extremely unlikely.4. The laser beams generated by FSO systems are Narrow and invisible, making themharder to find and even harder to Intercept and crack5. Data can be transmitted over an encrypted connection adding to the Degree ofsecurity available in FSO network Transmissions .7.2 EYE-SAFETYLaser beams with wavelengths in the range of 400 to 1400 nm emit light that passesthrough the cornea and lens and is focused onto a tiny spot on the retina whilewavelengths above 1400 nm are absorbed by the cornea and lens, and do not focus ontothe retina, as illustrated in Figure 1. It is possible to design eye-safe laser transmitters atboth the 800 nm and 1550 nm wavelengths but the allowable safe laser power is aboutfifty times higher at 1550 nm. This factor of fifty is important as it provides up to 17 dBadditional margin, allowing the system to propagate over longer distances, throughheavier attenuation and to support higher rates7.3 COST OF DEPLOYMENTHigher performances with little extra cost penalty, provides the best value. The keyfactor that affects the cost are system design, minimization of manual labour and bulkmanufacturing. An 850 nm laser can cost up to $5000 while a 1550 nm laser can go upto $50000.
    • 258. FSO-BREAKING THE BANDWIDTH BOTTLENECKThe global telecommunications network has seen massive expansion over the last fewyears. First came the tremendous growth of the optical fiber long-haul, WAN followedby a more recent emphasis on MANs. Meanwhile, LANs and gigabit Ethernet ports arebeing deployed with a comparable growth rate. In order for this tremendous networkcapacity to be exploited, and for the users to be able to utilize the broad array of newservices 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 aconsequence, 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 Timesarticle reported that more than 100 million miles of optical fiber was laid around theworld in the last two years, as carriers reacted to the Internet phenomenon and endusers’ insatiable demand for bandwidth. The sheer scale of connecting wholecommunities, cities and regions to that fiber optic cable or “backbone” is something notmany players understood well. Despite the huge investment in trenching and opticalcable, most of the fiber remains unlit, 80 to 90 percent of office, commercial andindustrial buildings are not connected to fiber, and transport prices are droppingdramatically.FSO systems represent one of the most promising approaches foraddressing the emerging broadband access market and its “last mile” bottleneck. FSOsystems offer many features, principal among them have being less start-up andoperational costs, rapid deployment, and high fiber-like bandwidths due to the opticalnature of the technology.
    • 269 .FSO ADVANTAGES AND CHALLENGES9.1 ADVANTAGESAn FSO system offers a flexible networking solution that delivers on the promise ofbroadband. Since FSO optical wireless transceivers can transmit and receive throughwindows, it is possible to mount FSO systems inside buildings, reducing the need tocompete for roof space, simplifying wiring and cabling, and permitting the equipmentto operate in a very favorable environment. The only essential for FS is line of sightbetween 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 failure9.2 FSO CHALLENGESThe advantages of free space optical wireless or FSO do not come without some cost.When light is transmitted through optical fiber, transmission integrity is quitepredictable – barring unforseen events such as backhoes or animal interference.FOGFog 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 onFSO 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
    • 27with RF wireless, fog attenuation is not a “show-stopper” for FSO, because the opticallink can be engineered such that, for a large fraction of the time, an acceptable powerwill be received even in the presence of heavy fogPHYSICAL OBSTRUCTIONSFSO products which have widely spaced redundant transmitters and large receive opticswill 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 causea FSO link to drop out. Thus birds are unlikely to have any impact on FSO transmissionPOINTING STABILITY-BUILDING SWAYFixed pointed FSO systems are designed to be capable of handling the vast majority ofmovement found in deployments on buildings. The combination of effective beamdivergence and a well matched receive Field-of-View (FOV) provide for an extremelyrobust fixed FSO system suitable for most deployments. Fixed-pointed FSO systemsare generally preferred over actively-tracked FSO systems due to their lower cost.SCINTILLATIONScintillation is one of the effects related to turbulence. Turbulence is caused whentemperature differentials change the air particle density. Cells or hot pockets of air arecreated that move randomly in space and time thus also changing the refractive index ofthe 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 inthe very early morning hours scintillation effects can be best observed.
    • 28SOLAR INTERFERENCESolar 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 wavelengthsbelow 850 nm from entering the system; the second is an optical narrowband filterproceeding the receive detector used to filter all but the wavelength actually used forintersystem communications. To handle off-axis solar energy, two spatial filters havebeen implemented in systems, allowing them to operate unaffected by solar interferencethat is more than 1.5 degrees off-axis.ATMOSPHERIC ATTENUATIONCarrier-class FSO systems must be designed to accommodate heavy atmosphericattenuation, particularly by fog. Although longer wavelengths are favored in haze andlight fog, under conditions of very low visibility this long-wavelength advantage doesnot apply. However, the fact that1550 nm-based systems are allowed to transmit up to50 times more eye-safe power will translate into superior penetration of fog or anyother atmospheric attenuatorTABLE 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/
    • 2910. APPLICATIONSMETRO NETWOK EXTENSIONS – FSO is used to extend existing metropolitan areafibers to connect new networks from outsideLAST MILE ACCESS – FSO can be used in high speed links to connect the end userswith ISPs.ENTERPRISE CONNECTIVITY - The ease in which FSO can be installed Make thema solution for interconnecting LAN segments, housed in building separated by publicstreets.FIBER BACKUP - FSO may be deployed in redundant links to backup fiber in place ofa second fiber link.BACKHAUL – Used to carry cellular telephone traffic from antenna towers back tofacilities into the public switched telephone network.FSO COMPARISONSFree space optical communications is now established as a viable approach foraddressing the emerging broadband access market and its “last mile” bottleneck..Theserobust systems, which establish communication links by transmitting laser beamsdirectly through the atmosphere, have matured to the point that mass- produced modelsare now available. Optical wireless systems offer many features, principal among thembeing slow start-up and operational costs, rapid deployment, and high fiber-likebandwidths. These systems are compatible with a wide range of applications andmarkets, and they are sufficiently flexible as to be easily implemented using a variety ofdifferent architectures. Because of these features, market projections indicate healthygrowth for optical wireless sales. Although simple to deploy, optical wirelesstransceivers are sophisticated devices.
    • 3011. CONCLUSIONFSO 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 bythe need for high speed local loop connectivity and the cost and the difficulties ofdeploying fiber, the interest in FSO has certainly picked up dramatically among serviceproviders worldwide. Instead of fiber coaxial systems, fiber laser systems may turn outto be the best way to deliver high data rates to your home. FSO continues to acceleratethe vision of all optical networks cost effectively, reliably and quickly with freedomand flexibility of deployment.
    • 31REFERENCES[1]. Harry J. R. Dutton (1999), Understanding Optical Communications .[2]. Dettmer, R. "A ray of light" IEEE Review, Volume: 47 Issue:2, March 2001 Page(s): 32 -33[3]. H.A. Willebrand and B.S. Ghuman, “Fiber optics without fiber”, IEEE Spectrum,Aug 01, p.40[4]. Dr. Michael Connelly (1999), Optical Fibre: Communications Highway for the 21stCentury.[5]. A campora, A.S. and Krishnamurthy, S.V. “A broadband wireless access networkbased on mesh-connected free space optical links” IEEE Personal Communications [seealso IEEE Wireless Communications], Volume: 6 Issue: 5, Oct 1999 Page(s): 62 –65[6]. Chinlon Lin, Kung-Li Deng and Chun-Kit Chan “Broadband optical accessnetworks”, Lasers and ElectroOptics, 2001. The 4th Pacific Rim Conference on,Volume: 2, 2001 Page(s): II-576 -II-577 vol.2[7]. H. Willebrand and B. S. Ghuman, Free Space Optics: Enabling OpticalConnectivity in Today’s Networks, Sams Publishing, 2002.[8]. Christopher C. Davis, Igor I. Smolyaninov, and Stuart D. Milner, “FlexibleOptical Wireless Links and Networks,” Magazine, March 2003.[9]. Andy Dorman, (2004), Wireless Optics: Fiber Is Cheap, But Space Is Free,Network Magazine (September 2004).
    • 32