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Free Space Optics Seminar Report
Post: #1

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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, Free Space Optics (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 madium 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...

INTRODUCTION
Free Space Optics (FSO) communications, also called Free Space Photonics (FSP) or Optical Wireless, refers to the transmission of modulated visible or infrared (IR) beams through the atmosphere to obtain optical communications. Like fiber, Free Space Optics (FSO) uses lasers to transmit data, but instead of enclosing the data stream in a glass fiber, it is transmitted through the air. Free Space Optics (FSO) works on the same basic principle as Infrared television remote controls, wireless keyboards or wireless Palm® devices.
HISTORY OF FREE SPACE OPTICS (FSO)
The engineering maturity of Free Space Optics (FSO) is often underestimated, due to a misunderstanding of how long Free Space Optics (FSO) systems have been under development. Historically, Free Space Optics (FSO) or optical wireless communications was first demonstrated by Alexander Graham Bell in the late nineteenth century (prior to his demonstration of the telephone!). Bell™s Free Space Optics (FSO) experiment 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 photophone, Bell considered this optical technology “ and not the telephone “ his preeminent invention because it did not require wires for transmission.
Although Bellâ„¢s photophone never became a commercial reality, it demonstrated the basic principle of optical communications. Essentially all of the engineering of todayâ„¢s Free Space Optics (FSO) or free space optical communications systems was done over the past 40 years or so, mostly for defense applications. By addressing the principal engineering challenges of Free Space Optics (FSO), this aerospace/defense activity established a strong foundation upon which todayâ„¢s commercial laser-based Free Space Optics (FSO) systems are based.
HOW FREE SPACE OPTICS (FSO) WORKS
Free Space Optics (FSO) transmits invisible, eye-safe light beams from one "telescope" to another using low power infrared lasers in the teraHertz spectrum. The beams of light in Free Space Optics (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.
Free Space Optics (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, Free Space Optics (FSO) communication is possible.

FREE SPACE OPTICS (FSO) TECHNOLOGY
Lasers are one of the most significant inventions of the 20th century - they can be found in many modern products, from CD players to fiber-optic networks. The word laser is actually an acronym for Light Amplification by Stimulated Emiission of Radiation. Although stimulated emission was first predicted by Albert Einstein near the beginning of the 20th century, the first working laser was not demonstrated until 1960 when Theodore Maiman did so using a ruby. Maiman's laser was predated by the maser - another acronym, this time for Microwave Amplification by Stimulated Emission of Radiation. A maser is very similar to a laser except the photons generated by a maser are of a longer wavelength outside the visible and/or infrared spectrum.
A laser generates light, either visible or infrared, through a process known as stimulated emission. To understand stimulated emission, understanding two basic concepts is necessary. The first is absorption which occurs when an atom absorbs energy or photons. The second is emission which occurs when an atom emits photons. Emission occurs when an atom is in an excited or high energy state and returns to a stable or ground state “ when this occurs naturally it is called spontaneous emission because no outside trigger is required. Stimulated emission occurs when an already excited atom is bombarded by yet another photon causing it to release that photon along with the photon which previously excited it. Photons are particles, or more properly quanta, of light and a light beam is made up of what can be thought of as a stream of photons.

A basic laser uses a mirrored chamber or cavity to reflect light waves so they reinforce each other. An excitable substance “ gas, liquid, or solid like the original ruby laser “ is contained within the cavity and determines the wavelength of the resulting laser beam. Through a process called pumping, energy is introduced to the cavity exciting the atoms within and causing a population inversion. A population inversion is when there are more excited atoms than grounded atoms which then leads to stimulated emission. The released photons oscillate back and forth between the mirrors of the cavity, building energy and causing other atoms to release more photons. One of the mirrors allows some of the released photons to escape the cavity resulting in a laser beam emitting from one end of the cavity.


TERRESTRIAL LASER COMMUNICATIONS CHALLENGES
Fog
Fog substantially attenuates visible radiation, and it has a similar affect on the near-infrared wavelengths that are employed in laser communications. Similar to the case of rain attenuation with RF wireless, fog attenuation is not a show-stopper for optical wireless, 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. Laser communication systems can be enhanced to yield even greater availabilities by combining them with RF systems.
Physical Obstructions
Laser communications systems that employ multiple, spatially diverse transmitters and large receive optics will eliminate interference concerns from objects such as birds.
Pointing Stability
Pointing stability in commercial laser communications systems is achieved by one of two methods. The simpler, less costly method is to widen the beam divergence so that if either end of the link moves the receiver will still be within the beam. The second method is to employ a beam tracking system. While more costly, such systems allow for a tighter beam to be transmitted allowing for higher security and longer distance transmissions.
Scintillation
Performance of many laser communications systems is adversely affected by scintillation on bright sunny days. Through a large aperture receiver, widely spaced transmitters, finely tuned receive filtering, and automatic gain control, downtime due to scintillation can be avoided.
FSO: WIRELESS, AT THE SPEED OF LIGHT
Unlike radio and microwave systems, Free Space Optics (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 Free Space Optics (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 Free Space Optics (FSO) links that can be installed in a given location.
HOW FREE SPACE OPTICS (FSO) CAN HELP YOU
FSOâ„¢s freedom from licensing and regulation translates into ease, speed and low cost of deployment. Since Free Space Optics (FSO) transceivers can transmit and receive through windows, it is possible to mount Free Space Optics (FSO) systems inside buildings, reducing the need to compete for roof space, simplifying wiring and cabling, and permitting Free Space Optics (FSO) equipment to operate in a very favorable environment. The only essential requirement for Free Space Optics (FSO) or optical wireless transmission is line of sight between the two ends of the link.
For Metro Area Network (MAN) providers the last mile or even feet can be the most daunting. Free Space Optics (FSO) networks can close this gap and allow new customers access to high-speed MANâ„¢s. Providers also can take advantage of the reduced risk of installing an Free Space Optics (FSO) network which can later be redeployed.
THE MARKET. WHY FSO BREAKING THE BANDWIDTH BOTTLENECK
Why FSO The global telecommunications network has seen massive expansion over the last few years. First came the tremendous growth of the optical fiber long-haul, wide-area network (WAN), followed by a more recent emphasis on metropolitan area networks (MANs). Meanwhile, local area networks (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 usersâ„¢ 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% of office, commercial and industrial buildings are not connected to fiber, and transport prices are dropping dramatically.
Free Space Optics (FSO) systems represent one of the most promising approaches for addressing the emerging broadband access market and its last mile bottleneck. Free Space Optics (FSO) systems offer many features, principal among them being low start-up and operational costs, rapid deployment, and high fiber-like bandwidths due to the optical nature of the technology

BROADBAND BANDWIDTH ALTERNATIVES
Access technologies in general use today include telco-provisioned copper wire, wireless Internet access, broadband RF/microwave, coaxial cable and direct optical fiber connections (fiber to the building; fiber to the home). Telco/PTT telephone networks are still trapped in the old Time Division Multiplex (TDM) based network infrastructure that rations bandwidth to the customer in increments of 1.5 Mbps (T-1) or 2.024 Mbps (E-1). DSL penetration rates have been throttled by slow deployment and the pricing strategies of the PTTs. Cable modem access has had more success in residential markets, but suffers from security and capacity problems, and is generally conditional on the user subscribing to a package of cable TV channels. Wireless Internet access is still slow, and the tiny screen renders it of little appeal for web browsing.
Broadband RF/microwave systems have severe limitations and are losing favor. The radio spectrum is a scarce and expensive licensed commodity, sold or leased to the highest bidder, or on a first-come first-served basis, and all too often, simply unavailable due to congestion. As building owners have realized the value of their roof space, the price of roof rights has risen sharply. Furthermore, radio equipment is not inexpensive, the maximum data rates achievable with RF systems are low compared to optical fiber, and communications channels are insecure and subject to interference from and to other systems (a major constraint on the use of radio systems).
FREE SPACE OPTICS (FSO) ADVANTAGES
Free space optical (FSO) systems offers a flexible networking solution that delivers on the promise of broadband. Only free space optics or Free Space Optics (FSO) provides the essential combination of qualities required to bring the traffic to the optical fiber backbone “ virtually unlimited bandwidth, low cost, ease and speed of deployment. Freedom from licensing and regulation translates into ease, speed and low cost of deployment. Since Free Space Optics (FSO) optical wireless transceivers can transmit and receive through windows, it is possible to mount Free Space Optics (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 Free Space Optics (FSO) is line of sight between the two ends of the link.
Freedom from licensing and regulation leads to ease, speed and low cost of deployment.
Since FSO units can receive and transmit through windows it reduces the need to compete for roof space, simplifying wiring and cabling.
Only need is the line of sight between the two ends of the link.
Providers take advantage of the reduced risk in installing FSO equipment, which can even be re-deployed.
Zero chances of network failure.
Virtually unlimited bandwidth.
FREE SPACE OPTICS (FSO) SECURITY
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 SONAbeam„¢ 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 wireline connections. In fact, Free Space Optics (FSO) is far more secure than RF or other wireless-based transmission technologies for several reasons:
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
APPLICATIONS
Metro network extensions “ FSO is used to extend existing metropolitan area fiberings to connect new networks from outside.
Last mile access “ FSO can be used in high-speed links to connect end users with ISPs.
Enterprise connectivity - The ease in which FSO can be installed makes them a solution for interconnecting LAN segments, housed in buildings 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 networks.
FREE SPACE OPTICS (FSO) CHALLENGES
The advantages of free space optical wireless or Free Space Optics (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. When light is transmitted through the air, as with Free Space Optics (FSO) optical wireless systems, it must contend with a a complex and not always quantifiable subject - the atmosphere.
FOG AND FREE SPACE OPTICS (FSO)
Fog substantially attenuates visible radiation, and it has a similar affect on the near-infrared wavelengths that are employed in Free Space Optics (FSO) systems. Note that the effect of fog on Free Space Optics (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 with RF wireless, fog attenuation is not a show-stopper for Free Space Optics (FSO) optical wireless, 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. Free Space Optics (FSO) optical wireless-based communication systems can be enhanced to yield even greater availabilities.
PHYSICAL OBSTRUCTIONS AND FREE SPACE OPTICS (FSO)
Free Space Optics (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 an Free Space Optics (FSO) link to drop out. Thus birds are unlikely to have any impact on Free Space Optics (FSO) transmission.
FREE SPACE OPTICS (FSO) POINTING STABILITY “ BUILDING SWAY, TOWER MOVEMENT
Fied pointed Free Space Optics (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 pointed Free Space Optics (FSO) system suitable for most deployments. Fixed-pointed Free Space Optics (FSO) systems are generally preferred over actively-tracked Free Space Optics (FSO) systems due to their lower cost.
SCINTILLATION AND FREE SPACE OPTICS (FSO)
Performance of many Free Space Optics (FSO) optical wireless systems is adversely affected by scintillation on bright sunny days; the effects of which are typically reflected in BER statistics. Some optical wireless products have a unique combination of large aperture receiver, widely spaced transmitters, finely tuned receive filtering, and automatic gain control characteristics. In addition, certain optical wireless systems also apply a clock recovery phase-lock-loop time constant that all but eliminate the affects of atmospheric scintillation and jitter transference.
SOLAR INTERFERENCE AND FREE SPACE OPTICS (FSO)
Solar interference in Free Space Optics (FSO) free space optical systems operating at 1550 nm can be combatted 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 SONAbeam systems, allowing them to operate unaffected by solar interference that is more than 1.5 degrees off-axis.
FREE SPACE OPTICS (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 low 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.
The many sub-systems require a multi-faceted approach to system engineering that balances the variables to produce the optimum mix. A working knowledge of the issues faced by an optical wireless system engineer provides a foundation for understanding the differences between the various systems available. This paper aims to examine the many elements considered by the system engineer when designing a product so that the buyer can ask those same questions about the systems they are evaluating for purchase.
WHICH WAVELENGTH
Currently available Free Space Optics (FSO) hardware can be classified into two categories depending on the operating wavelength “ systems that operate near 800 nm and those that operate near 1550 nm. There are compelling reasons for selecting 1550 nm Free Space Optics (FSO) systems due to laser eye safety, reduced solar background radiation, and compatibility with existing technology infrastructure.
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 data rates.
ATMOSPHERIC ATTENUATION
Carrier-class Free Space Optics (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 that 1550 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.
RECEIVER
There are a number of factors to consider when examining the effectiveness of the receiver in an FSO system; these include the type of detector used, the sensitivity rating and size of the detector, the size and design of the receiver optics, and the operating wavelength itself. In order to correctly assess the efficiency of the overall system, one must also take into account the number and power of the lasers being used to generate the signal.

Types of optical detectors used in FSO equipment come in two flavors: PIN and APD. The PIN detector is a lower cost detector that has no internal gain, while the APD is a more expensive but also more sensitive detector with internal gain. The Benefits of using APD over PIN technology will vary, but real-world results indicate the benefits to be an improvement in sensitivity of approximately 4x that of a PIN detector. Although at first glance it would seem that systems using APD detectors should have a performance advantage; however, the performance of a system must also take into consideration the transmit characteristics. As an example, the SONAbeam155-M uses the lower-cost PIN detectors but because it produces 20-40 times the laser power of competing systems the SONAbeam155-M is still 5-10 times more effective than those systems utilizing APD based receivers. Thus, the SONAbeam is a much more powerful system, which allows it to outperform other products at the same distance, under the same weather conditions.
The size of the receiver optics is also important; a larger area receive optic contributes to reducing errors due to scintallation. Scintillation is atmospheric turbulence due to solar loading and natural convection, causing temporally and spatially varying refractive index changes in the air. As a laser beam propagates through the atmosphere, there is a time-varying intensity at the receiver due to this phenomenon; this is referred to as 'scintillation'. This is quite similar to the apparent twinkling of the stars or distant city lights, which is due to the same effect. The result is that an FSO communications receiver can experience error bursts due to surges and fades in the receive signal strength. One way to combat this scintillation effect, and thus improve the error-rate performance, is to use a large aperture receiver. A collecting aperture that is much larger than the spatial scale of the scintillation provides an averaging effect of the localized surges and fades, thus improving the error rate. This large-aperture approach is more effective for scintillation reduction than multiple smaller apertures, which perform less averaging at each lens. Another way to mitigate the effects of scintillation is to use multiple transmitters, each of which takes a slightly different path through the atmosphere, which also contributes an averaging effect. The net result is that a properly designed system can defeat scintillation impairments.
The operating wavelength of an FSO system also contributes to the performance of the receiver. It is generally true that high-quality photodiodes at both 800nm and 1550nm achieve comparable quantum efficiencies. However, longer wavelengths enjoy an advantage in the receiver due to their lower photon energies. Specifically, a 1550nm photon has half the energy of a 800nm photon. Consequently, for the same total energy (i.e. Watts of power), a beam of 1550nm light has twice the number of photons as a beam of 800nm light. This results in twice the photoelectrons (photocurrent) from the receiver photodiode. Since a certain minimum number of photoelectrons is required to detect an optical pulse, a pulse at 1550nm can be detected with ~ 3 dB less optical power. Hence, 1550nm has a fundamental 3 dB advantage over 800nm in receiver sensitivity.
PERFORMANCE “ TRANSMIT POWER & RECEIVER SENSITIVITY
Free Space Optics (FSO) products performance can be characterized by four main parameters (for a given data rate):
¢ Total transmitted power
¢ Transmitting beamwidth
¢ Receiving optics collecting area
¢ Receiver sensitivity
High transmitted power may be achieved by using erbium doped fiber amplifiers, or by non-coherently combining multiple lower cost semiconductor lasers. Narrow transmitting beamwidth (a.k.a. high antenna gain) can be achieved on a limited basis for fixed-pointed units, with the minimum beamwidth large enough to accommodate building sway and wind loading. Much narrower beams can be achieved with an actively pointed system, which includes an angle tracker and fast steering mirror (or gimbal). Ideally the angle tracker operates on the communication beam, so no separate tracking beacon is required. Larger receiving optics captures a larger fraction of the total transmitted power, up to terminal cost, volume and weight limitations. And high receiver sensitivity can be achieved by using small, low-capacitance photodetectors, circuitry which compensates for detector capacitance, or using detectors with internal gain mechanisms, such as APDs. APD receivers can provide 5-10 dB improvement over PIN detectors, albeit with increased parts cost and a more complex high voltage bias circuit. These four parameters allow links to travel over longer distance, penetrate lower visibility fog, or both.
In addition, Free Space Optics (FSO) receivers must be designed to be tolerant to scintillation, i.e. have rapid response to changing signal levels and high dynamic range in the front end, so that the fluctuations can be removed in the later stage limiting amplifier or AGC. Poorly designed Free Space Optics (FSO) receivers may have a constant background error rate due to scintillation, rather than perfect zero error performance.
FIXED-POINTING OR ACTIVE-POINTING
Another element of Free Space Optics (FSO) system design that must be considered by a prudent buyer is the challenge of maintaining sufficiently accurate pointing stability. A number of Free Space Optics (FSO) systems employ an active pointing-stabilization approach, which represents an effective approach for addressing this challenge. However, the cost, complexity, and reliability issues associated with active-pointing approach can be avoided in some applications (particularly for shorter ranges and lower data rates) by utilizing the fixed-pointed approach schematically shown in the figure.
According to this approach, the transmitted beam is broadened significantly beyond its near-perfect minimum beam divergence angle, and the receiver field of view is broadened to a comparable extent. The broadening of the transmitted beam and receiver field of view leads to large pointing/alignment tolerances and a very low probability of building motion being of sufficient magnitude to take the link down. Well engineered hardware exploits this approach of designing for loose alignment tolerances. Therefore, it is possible to perform initial alignment of the transceivers at opposite ends of the link during installation and then leave them unattended for many years of reliable service.
Note that this approach is facilitated for systems operating at wavelengths > 1400 nm, because the higher allowable eye-safe powers at such wavelengths allow the transmitted beam to be significantly broadened spatially while still maintaining an adequate intensity at the receiver. Of primary importance to prospective buyers will be selecting the right system for the situation.
RELIABILITY
Systems are designed, engineered and tested to ensure exceptional reliability. Building on their extensive experience in laser communications systems for military and space applications, our design engineers have ensured that critical sub-systems are manufactured using high-reliability components. Component reliability is further ensured by rigorous vendor qualification and incoming inspection procedures.
Our equipment reliability analysis is performed using the stringent Bellcore/Telcordia guidelines applicable to carrier equipment. This is further backed up by exhaustive qualification testing in our in-house test facilities, where subsystems are severely stressed and operational performance is validated at extremes ranging from -50°C to 75°C. The combination of active laser cooling, high-reliability components, sealed housings and rugged mechanical design enables us to offer carriers superior products with outstanding communications performance and a rated service life of 15 years.
Built for Dependability and Longevity
Depending on their bandwidth and operating range, NAbeam™ systems are designed with two-, four- or eight-fold redundancy of lasers, laser drivers, laser coolers and cooler controllers. SONAbeam's™ environmentally sealed cast-aluminum exterior housings, unique in the market, are impervious to water, sun and other environmental hazards. fSONA's rugged transceiver mounting structures maintain pointing accuracy through Class 1 hurricanes of 120 km/hr, and survive Class 2 hurricanes of 160 km/hr.
COST OF DEPLOYEMENT

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 $50,000.
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 world wide. 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.

REFERENCES
Websites:
1. http://lightpointe.com
2. http://spie.org
3. http://osa.org
Journals
1. IEEE Spectrum August 2001
2. IEEE Intelligent System May-June 2001


CONTENTS
1. INTRODUCTION 1
2. HISTORY OF FREE SPACE OPTICS (FSO) 1
3. HOW FREE SPACE OPTICS (FSO) WORKS 2
4. FREE SPACE OPTICS (FSO) TECHNOLOGY 3
5. TERRESTRIAL LASER
COMMUNICATIONS CHALLENGES 5
6. FSO: WIRELESS, AT THE SPEED OF LIGHT 6
7. THE MARKET. WHY FSO
BREAKING THE BANDWIDTH BOTTLENECK 7
8. FREE SPACE OPTICS (FSO) ADVANTAGES 10
9. FREE SPACE OPTICS (FSO) SECURITY 11
10. APPLICATIONS 12
11. FREE SPACE OPTICS (FSO) CHALLENGES 12
12. COST OF DEPLOYEMENT 22
13. CONCLUSION 23
14. REFERENCES 24

ACKNOWLEDGMENT

I express my sincere thanks to Prof. M.N Agnisarman Namboothiri (Head of the Department, Computer Science and Engineering, MESCE),
Mr. Sminesh (Staff incharge) for their kind co-operation for presenting the seminars.
I also extend my sincere thanks to all other members of the faculty of Computer Science and Engineering Department and my friends for their co-operation and encouragement.
Haris.K
Post: #2
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INTRODUCTION
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.
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 obataining government licenses for new fiber installation etc. are further problems that has 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.

WHAT IS FSO?

FSO technology is implemented using a laser device .These laser devices or terminals can be mounted on rooftops ,Corners of buidings or even inside offices behind windows. FSOdevices 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 fibre, 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. 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.
WHY FREE SPACE OPTICS?
Ultra 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.
RAPID DEPLOYMENT TIME:
Installing a FSO system can be done in a matter of days even faster if the gear cn be placed in offices behind windows instead of on rooftops. A fibre 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.
FSO ARCHITECTURES
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..

MESH ARCHITECTURE
Mesh architectures may offer redundancy and higher reliability with easy node addition but restrict distances more than the other options.


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.
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.

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.

ADVANTAGES OF FSO
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.
With FSO, there's also no capital overhang. FSO carriers can avoid heavy buildouts by deploying laser terminals after customers have signed on. No heavy capital investments for buildout are required. Low risk investment is another advantage of FSO.
Another plus is that an 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.
DISADVANTAGES OF FSO
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 wander.
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.
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 milliradians (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 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.
APPLICATIONS OF FSO
LAST MILE ACCESS:
FSO can be used in high-speed links that connect end-users with Internet service providers or other networks. It can also be used to bypass local-loop systems to provide businesses with high-speed connections.
.
ENTERPRISE CONNECTIVITY:
The ease with which FSO links can be installed makes them a natural for interconnecting local-area network segments that are housed in buildings separated by public streets or other right-of-way property
FIBER BACKUP:
FSO may also be deployed in redundant links to back up fiber in place of a second fiber link.
BACKHAUL:
FSO can be used to carry cellular telephone traffic from antenna towers back to facilities wired into the public switched telephone network.
SERVICE ACCELERATION:
FSO can be also used to provide instant service to fiber-optic customers while their fiber infrastructure is being laid.

CONCLUSION
Clearly, FSO is not the ideal choice for all communications applications. Equally clearly, it has important roles to play both as a primary access medium and as a backup technology. Key to its success will be a realistic analysis of historical weather patterns in combination with customers' needs for network availability. With proper planning, path blocks like window washers and rooftop maintenance workers can also be dealt with, and the technology will be able to realize its great potential.
Post: #3
[attachment=1947]

ABSTRACT
FREE SPACE OPTICS
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.
FSO uses lasers, or light pulses, to send packetized data in the terahertz (THz) spectrum range. Air. not 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; 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.
Known within the industry as free-space optics (FSO), this form of delivering communications services has compelling economic advantages. Despite its potential, FSO still has many hurdles to overcome before it will be deployed widely.
This Seminar consists of an outline of the technology behind FSO, the different architectures followed, some of its advantages and the hurdles it has to overcome before its wide implementation. This technology, with patches, will undoubtedly become one of the major leaps in communication industry.

l.INTRODUCTION
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.
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.
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 obataining government licenses for new fiber installation etc. are further problems that has turned FSO into the option of choice for short reach application.
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.
FREE SPACE OPTICS 2.WHAT IS FSO?
FSO technology is implemented using a laser device .These laser devices or terminals can be mounted on rooftops .Corners of bindings or even inside offices behind windows. FSOdevices 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.
3.HOW FREE SPACE OPTICS WORKS
Free space optics transmits invisible, eye-safe light beams from one telescope to another using low power infrared laser in the terahertz spectrum. The beam of light in Free space optics systems are transmitted by laser light focused on highly sensitive photon detector receivers. These receivers are telescopic lens able to collect the photon stream and transmit digital data containing a mix of internet messages, video images, radio signals or computer files.

A Free Space Optical system is a point-to-point. Infra red, wireless laser transmission designed for the interconnection of two points which have a direct line of sight. The systems operate by taking a standard data or telecommunications signal, converting it into a digital format and transmitting it through free space. The carrier used for the transmission of this signal is Infra red light and is generated by either high power LED or low power laser diode(s).

Free space optics 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, free space optics communication is possible,
Forward link

The FSO remains simple: a narrow beam of light is launched at a transmission station, transmitted through the atmosphere, and subsequently received at the receive station. In wireless optical system it uses the infrared of visual range frequencies to transmit data
Housing
TransmiRer
Rovers© link
Housing ^Receiver
Range 100m-5km
FSO transmitter and receiver
Free Space Optics (FSO) is a telecommunication technology that uses light propagating in free space to transmit data between two points. The technology is useful where the physical connection of the transmit and receive locations is difficult, for example in cities where the laying of fibre optic cables is expensive. Free Space Optics is also used to communicate between space-craft, since outside of the atmosphere there is little to distort the signal.

4.HOW FREE SPACE OPTICS CAN HELP YOU
FSO's freedom from licensing and regulation translates into ease, speed and low cost of deployment. Since Free Space Optics (FSO) transceivers can transmit and receive through windows, it is possible to mount Free Space Optics (FSO) systems inside buildings, reducing the need to compete for roof space, simplifying wiring and cabling, and permitting Free Space Optics (FSO) equipment to operate in a very favorable environment. The only essential requirement for Free Space Optics (FSO) or optical wireless transmission is line of sight between the two end of the link.
For Metro Area Network (MAN) providers the last mile or even feet can be the most daunting. Free Space Optics (FSO) networks can close this gap and allow new customers access to high-speed MAN's. Providers also can take advantage of the reduced risk of installing an Free Space Optics (FSO) network which can later be redeployed.

5. WHY FSO?
The increasing demand for high bandwidth in metro networks is relentless, and service providers' pursuit of a range of applications, including metro network extension, enterprise LAN-to-LAN connectivity, wireless backhaul and LMDS supplement has created an imbalance. This imbalance is often referred to as the "last mile bottleneck." Service providers are faced with the need to turn up services quickly and cost-effectively at a time when capital expenditures are constrained. But the last mile bottleneck is only part of a larger problem. Similar issues exist in other parts of the metro networks. "Connectivity bottleneck" better addresses the core dilemma. As any network planner will tell you, the connectivity bottleneck is everywhere in metro networks.
From a technology standpoint, there are several options to address this "connectivity bottleneck," but most don't make economic sense.
The first, most obvious choice is fiber-optic cable. Without a doubt, fiber is the most reliable means of providing optical communications. But the digging, delays and associated costs to lay fiber often make it economically prohibitive. Moreover, once fiber is deployed, it becomes a "sunk" cost and cannot be re-deployed if a customer relocates or switches to a competing service provider, making it extremely difficult to recover the investment in a reasonable timeframe.
Another option is radio frequency (RF) technology. RF is a mature technology that offers longer ranges distances than FSO, but RF-based networks require immense capital investments to acquire spectrum license. Yet, RF technologies cannot scale to optical capacities of 2.5 gigabits. The current RF bandwidth ceiling is 622 megabits. When compared to FSO, RF does not make economic sense for service providers looking to extend optical networks.

The third alternative is wire- and copper-based technologies, (i.e. cable modem, Tls or DSL). Although copper infrastructure is available almost everywhere and the percentage of buildings connected to copper is much higher than fiber, it is still not a viable alternative for solving the connectivity bottleneck. The biggest hurdle is bandwidth scalability. Copper technologies may ease some short-term pain, but the bandwidth limitations of 2 megabits to 3 megabits make them a marginal solution, even on a good day.
The fourth-and often most viable-alternative is FSO. The technology is an optimal solution, given its optical base, bandwidth scalability, speed of deployment (hours versus weeks or months), re-deployment and portability, and cost-effectiveness (on average, one-fifth the cost of installing fiber-optic cable).
Only 5 percent of the buildings in the United States are connected to fiber-optic infrastructure (backbone), yet 75 percent are within one mile of fiber. As bandwidth demands increase and businesses turn to high-speed LANs, it becomes more frustrating to be connected to the outside world through lower-speed connections such as DSL, cable modems or Tls. Most of the recent trenching to lay fiber has been to improve the metro core (backbone), while the metro access and edge have completely been ignored. Studies show that disconnects occurs in the metro network core, primarily due to cost constraints and the deployment of such non-scalable, non-optical technologies such as LMDS. Metro optical networks have not yet delivered on their promise. High capacity at affordable prices still eludes the ultimate end-user.
6. FSO ARCHITECTURES
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 lOGbits/sec at a distance of 2 kilometers (km) to 4km. "Access" claims it can deliver lOGbits/ sec. "Terabeam" can provide up to 2Gbits/sec now, while "AirFiber" and "Lightpointe" have promised Gigabit Ethernet capabilities sometime in 2001..

9b A

Sit


MESH ARCHITECTURE
Mesh architectures may offer redundancy and higher reliability with easy node addition but restrict distances more than the other options.

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.
POINT-.TO-MULT1POLNT 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.

In a point-to-multipoint arrangement, FSO can support the same speeds as the point-to-point arrangement -155Mbits/sec to lOGbits/sec-at 1km to 2km.
7.ADVANTAGES OF FSO
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.
Another plus is that an 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.
8.DISADVANTAGES OF FSO
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 wander.
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.

LAST MILE ACCESS:
FSO can be used in high-speed links that connect end-users with Internet service providers or other networks. It can also be used to bypass local-loop systems to provide businesses with high-speed connections.
ENTERPRISE CONNECTIVITY:
The ease with which FSO links can be installed makes them a natural for interconnecting local-area network segments that are housed in buildings separated by public streets or other right-of-way property
FIBER BACKUP:
FSO may also be deployed in redundant links to back up fiber in place of a second fiber link.
BACKHAUL:
FSO can be used to carry cellular telephone traffic from antenna towers back to facilities wired into the public switched telephone network.
SERVICE ACCELERATION:
FSO can be also used to provide instant service to fiber-optic customers while their fiber infrastructure is being laid.

10.CONCLUSION
Free space optics (FSO) provides a low cost, rapidly deployable method of gaining access to the fiber optic backbone. FSO technology not only delivers fiber-quality connections, it provides the lowest cost transmission capacity in the broadband industry.
As a truly protocol-independent broadband conduit, FSO systems complement legacy network investments and work in harmony with any protocol, saving substantial up-front capital investments.
A FSO link can be procured and installed for as little as one-tenth of the cost of laying fiber cable, and about half as much as comparable microwave/RF wireless systems. By transmitting data through the atmosphere, FSO systems dispense with the substantial costs of digging up sidewalks to install a fiber link. Unlike RF wireless technologies, FSO eliminates the need to obtain costly spectrum licenses or meet further regulatory requirements

ll.FUTUKE DEVELOPMENTS
The free space optical wireless or Free Space Optics (FSO) do not come without some cost. So FSO will reduce the costs.
Fog: The primary challenge to FSO-based communications is dense fog. Rain and snow have little effect on FSO technology, but fog is different. Fog is vapor composed of water droplets, which are only a few hundred microns in diameter but can modify light characteristics or completely hinder the passage of light through a combination of absorption, scattering, and reflection. The primary answer to counter fog when deploying FSO-based optical wireless products is through a network design that shortens FSO link distances and adds network redundancies. FSO installations in extremely foggy cities such as San Francisco have successfully achieved carrier-class reliability.
Absorption: Absorption occurs when suspended water molecules in the terrestrial atmosphere extinguish photons. This causes a decrease in the power density (attenuation) of the FSO beam and directly affects the availability of a system. Absorption occurs more readily at some wavelengths than others. However, the use of appropriate power, based on atmospheric conditions, and use of spatial diversity (multiple beams within an FSO-based unit) helps maintain the required level of network availability.
Currently researches are being conducted to overcome the disadvantages of FSO such as fog and absorption.

BIBLIOGRAPHY
freespaceoptics .org
networkmagazin.com
freespaceoptics.com
wikepidea.com
howstuff.com
CONTENTS
1.INTRODUCTION 1
2.WHAT IS FSO? 2
3 HOW FREE SPACE OPTICS WORKS? 3
4.HOW FSO CAN HELP YOU 5
5. WHY FSO? 6
6.FSO ARCHITECTURES 8
7 ADVANTAGES OF FSO 10
8.DISAD VANTAGES OF FSO 11
9.APPLICATION OF FSO 12
10.CONCLUSION 13
11 .FURTURE DEVELOPMENT 14
12.BIBLIOGRAPHY 15

FREE SPACE OPTICS
Post: #4
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Post: #5
FREE SPACE OPTICS
Free space optics (FSO) is a line-of-sight technology that currently enables optical transmission up to 2.5 Gbps of data, voice, and video communications through the air, allowing optical connectivity without deploying fiber optic cables or securing spectrum licenses. FSO system can carry full duplex data at giga bits per second rates over Metropolitan distances of a few city blocks of few kms. FSO, also known as optical wireless, overcomes this last-mile access bottleneck by sending high –bitrate signals through the air using laser transmission. FSO technology can be rapidly deployed to provide immediate service to the customers at a low initial investment, without any licensing hurdle making high speed, high bandwidth communication possible. Though not very popular in India at the moment, FSO has a tremendous scope for deployment companies like CISCO, LIGHT POIN few other have made huge investment to promote this technology in the market. It is only a matter of time before the customers realized, the benefits of FSO and the technology deployed in large scale.
Post: #6
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