free space optics full report
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01-02-2010, 12:08 PM

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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 .

Communication, as it has always been relied and simply depended upon speed. The faster the means ! the more popular, the more effective the communication is !
Presently in the twenty-first centaury wireless networking is gaining because of speed and ease of deployment and relatively high network robustness. Modern era of optical communication originated with the invention of LASER in 1958 and fabrication of low-loss optical fiber in
When we hear of optical communications we all think of optical fibers, what I have for u today is AN OPTICAL COMMUNICATION SYSTEM WITHOUT FIBERS or in other words WIRE FREE OPTICS.
Free space optics or FSO “Although it only recently and rather suddenly sprang in to public awareness, free space optics is not a new idea. It has roots that 90 back over 30 years-to the era before fiber optic cable became the preferred transport medium for high speed communication. FSO technology has been revived to offer high band width last mile connectivity for today™s converged network requirements.
Free space optics or FSO, free space photonics or optical wireless, refers to the transmission of modulated visible or infrared beams through the atmosphere to obtain optical communication. FSO systems can function over distances of several kilometers.
FSO is a line-of-sight technology, which enables optical transmission up to 2.5 Gbps of data, voice and video communications, allowing optical connectivity without deploying fiber optic cable or securing spectrum licenses. Free space optics require light, which can be focused by using either light emitting diodes (LED) or LASERS(light amplification by stimulated emission of radiation). The use of lasers is a simple concept similar to optical transmissions using fiber-optic cables, the only difference being the medium.
As long as there is a clear line of sight between the source and the destination and enough transmitter power, communication is possible virtually at the speed of light. Because light travels through air faster than it does through glass, so it is fair to classify FSO as optical communications at the speed of light. FSO works on the same basic principle as infrared television remote controls, wireless keyboards or wireless palm devices.

Presently we are faced with a burgeoning demand for high bandwidth and differentiated data services. Network traffic doubles every 9-12 months forcing the bandwidth or data storing capacity to grow and keep pare with this increase. The right solution for the pressing demand is the untapped bandwidth potential of optical communications.
Optical communications are in the process of evolving Giga bits/sec to terabits/sec and eventually to pentabits/sec. The explosion of internet and internet based applications has fuelled the bandwidth requirements. Business applications have grown out of the physical boundaries of the enterprise and gone wide area linking remote vendors, suppliers, and customers in a new web of business applications. Hence companies are looking for high bandwidth last mile options. The high initial cost and vast time required for installation in case of OFC speaks for a wireless technology for high bandwidth last mile connectivity there FSO finds its place.
It is said that this mode of communication was first used in the 8th centaury by the Greeks. They used fire as the light source ,the atmosphere as the transmission medium and human eye as receiver.
FSO or optical wireless communication by Alexander Graham Bell in the late 19th centaury even before his telephone ! Bells FSO experiment converted voice sounds to telephone signals and transmitted them between receivers through free air space along a beam of light for a distance of some 600 feet, - this was later called PHOTOPHONE. Although Bells photo phone never became a commercial reality , it demonstrated the basic principle of optical communications.
Essentially all of the engineering of todayâ„¢s FSO or free space optical communication systems was done over the past 40 years or so mostly for defense applications.
The concept behind FSO is simple. FSO uses a directed beam of light radiation between two end points to transfer information (data, voice or even video). This is similar to OFC (optical fiber cable) networks, except that light pulses are sent through free air instead of OFC cores.
An FSO unit consists of an optical transceiver with a laser transmitter and a receiver to provide full duplex (bi-directional) capability. Each FSO unit uses a high power optical source ( laser ) plus a lens that transmits light through the atmosphere to another lens receiving information. The receiving lens connects to a high sensitivity receiver via optical fiber. Two FSO units can take the optical connectivity to a maximum of 4kms.
Auto tracking
The issue of maintaining accurate targeting of a Free-space Optics beam is something a lot of people have trouble with. But, think of it this way: you're standing outside in the dark with a laser pointer trying to hold it steady on a white building a block away. You "think" you're doing OK, but your helper next to the building says, "Hold it still, it's jumping all over the place."
The problem of coarse is that when you "wiggle" the pointer by a mere tenth of a degree (very little, approximately 2 mrad...mrad is the measure most commonly used to specify FSO beam divergence rates) you have exceeded the wiggle tolerance of 3/4ths of all non-active aligned FSO links.
"Well, just bolt the transceiver down tight, you say."
OK. But what if the building moves either through sway or more commonly as a result of thermal dimensional distortions that are solar diurnal events.
A Single-Beam Link Head
Simplified drawing of a single-beam LightPointe transceiver shows that the received laser beam [tan] is much wider than the transmitted beam [red]. That's why the receiver lens is so much larger than the transmitter lens. Both lenses, which share the same axis, are mounted behind a glass housing with an embedded defroster for cold environments.
Single vs. multiple beams
The more receive intensity available at each end of an FSO link, the longer the distance it can accommodate, and the greater its weather penetration.
Let's start with the advantages of multiple beams:
First, assuming that a given FSO link needs to be eye safe, there is a limit to the amount of allowable optical intensity exiting the aperture.
So, how do you increase the receive intensity, without increasing the "point intensity" of the beam(s) exiting the aperture
Solution: deploy multiple beams with adequate separation to prevent intensity aggregation in close proximity to the aperture. This is a means of increasing their intensity only as the beams converge down stream where dispersion has reduced the aggregated intensity to a safe level.
The benefits include not only increased received beam intensity, but this increase also means that the beam divergence rate can be "upped" increasing the link's "wiggle" or mis-targeting tolerance, a benefit for both mount rigidity issues as well as atmospheric beam steering (scintillation) issues.
There are some strikes against multiple beams though:
Cost, size, and optical complexity are all disadvantages of multiple beam systems. In spite of these, multiple beams has been the choice for most first generation products.
But the biggest issue is that active alignment solves the same problems as multiple beams, and can do it at less cost, smaller size, and with a simpler single beam optical design.
850nm vs. 1550nm
800-850nm is the common wavelength used in Free-Space Optics for the thru-the-air beam due to cost / performance optimization.
Some manufacturers market 1550nm devices based on the fact power levels can be increased 100 x's while remaining eye safe due to human eye aqueous fluid energy absorption science.
This is one of the times you can literally make the statistics support either side of the discussion, depending on which side you happen to be on. Although it is true you can hit the laser with 100 x power, it does not follow that the link's margin ends up anywhere near 100 x's as high.
It does not end up even 10 x's as high.
Our experience is that it is more like 10% more at best, which is lost due to high beam divergence rates and mis-pointing as the only links with 1550nm lasers in them currently do not offer active alignment.
And there is a fairly high price to be paid for going with 1550nm lasers. They require active heating and cooling to maintain intensity and/or their center frequency which is a costly addition on their system boards (about 2x $).
Also, night vision scopes, sometimes used in diagnosing alignment issues on non-active aligned links, cannot be used as they do not register 1550nm light.
Overall, 850nm is probably the winner.
Passive vs. Active
Going back ten or more years, some of the very early Free-Space Optics developmental work focused on passive FSO designs where the laser and all electronics were kept down in the data room, and the light was simply transmitted to and from the roof over special large diameter fiber.
MRV Passive GigE
In more recent years, various manufacturers have reintroduced the design promoting the savings from elimination of rooftop power requirements, and field-hardened electronics components in rooftop transceivers.
It has been our experience that the complications of working with proprietary, non-field repairable fiber; as well as unexpected high failure rates of the indoor units probably outweigh the proposed advantages.
LightPointe PoE
Also, passive designs typically carry shorter link distance capabilities, and are therefore in competition with all of the new 802.3af PoE (Power-Over-Ethernet) Free-Space Optics models that have hit the market recently at the same or lower price points.
In our opinion, passive loses out.
What does Class 1, 1M, 2, 2M, 3R, 3B and 4 mean
Laser products are classified to take account of the amount of laser beam you can get access to when the product is in normal use or during routine user maintenance. A laser product may contain a laser of a higher Class and this may be accessible during servicing. Labels on the product should provide guidance on the laser beam hazard. A brief description of each laser Class follows.
Class 1 lasers are products where the irradiance (measured in watts per metre square) of the accessible laser beam (the accessible emission) does not exceed the Maximum Permissible Exposure (MPE) value. Therefore, for Class 1 laser products the output power is below the level at which it is believed eye damage will occur. Exposure to the beam of a Class 1 laser will not result in eye injury and may therefore be considered safe. However, some Class 1 laser products may contain laser systems of a higher Class but there are adequate engineering control measures to ensure that access to the beam is not reasonably likely. Examples of such products include laser printers and compact disc players. Anyone who dismantles a Class 1 laser product that contains a higher Class laser system is potentially at risk of exposure to a hazardous laser beam.
Class 1M lasers are products which produce either a highly divergent beam or a large diameter beam. Therefore, only a small part of the whole laser beam can enter the eye. However, these laser products can be harmful to the eye if the beam is viewed using magnifying optical instruments. Some of the lasers used for fibre-optic communication systems are Class 1M laser products.
Class 2 lasers are limited to a maximum output power of 1 milliwatt (abbreviated to mW) or one thousandth of a watt and the beam must have a wavelength between 400 and 700 nm. A person receiving an eye exposure from a Class 2 laser beam, either accidentally or as a result of someone else's deliberate action (misuse) will be protected from injury by their own natural aversion response. This is a natural involuntary response that causes the individual to blink and avert their head thereby terminating the eye exposure. Repeated, deliberate exposure to the laser beam may not be safe. Some laser pointers and barcode scanners are Class 2 laser products.
Class 2M lasers are products which produce either a highly divergent beam or a large diameter beam. Therefore, only a small part of the whole laser beam can enter the eye and this is limited to 1 mW, similar to a Class 2 laser product. However, these products can be harmful to the eye if the beam is viewed using magnifying optical instruments or for long periods of time. Some lasers used for civil engineering applications, such as level and orientation instruments are Class 2M laser products.
Class 3R lasers are higher powered devices than Class 1 and Class 2 and may have a maximum output power of 5 mW or 5 times the Accessible Emission Limit (AEL) for a Class 1 product. The laser beams from these products exceed the maximum permissible exposure for accidental viewing and can potentially cause eye injuries, but the actual risk of injury following a short, accidental exposure, is still small.
Class 3B lasers may have an output power of up to 500 mW (half a watt). Class 3B lasers may have sufficient power to cause an eye injury, both from the direct beam and from reflections. The higher the output power of the device the greater the risk of injury. Class 3B lasers are therefore considered hazardous to the eye. However, the extent and severity of any eye injury arising from an exposure to the laser beam of a Class 3B laser will depend upon several factors including the radiant power entering the eye and the duration of the exposure. Examples of Class 3B products include lasers used for physiotherapy treatments and many research lasers.
Class 4 lasers have an output power greater than 500 mW (half a watt). There is no upper restriction on output power. Class 4 lasers are capable of causing injury to both the eye and skin and will also present a fire hazard if sufficiently high output powers are used
Competitive comparison of the major Free-Space Optics solutions. Vast array of models, features can be daunting.
Mbps Max
(Meters) Interface Special
Canon Canobeam DT-110 100/155 500 TP/MM/SM Active
DT-120 100/155 2000 TP/MM/SM Active
DT-130 1250 1000 MM/SM Active
Comments: 4th Generation real time Active Aligned models, IP Management standard all models. 1st year advance exchange warranty. $50 Billion Company.
LightPointe FL 100 100 1000 TP PoE
FL 100E 100 1500 TP PoE
FL155EW 100/155 900 MM/SM
FL 155E 100/155 1500 MM/SM
FL G 1250 800 MM/SM
FSA 155E 100/155 2200 MM/SM Active
FSA 155EW 100/155 2000 MM/SM Active
FSA 622 622 1600 MM/SM Active
FSA G 1250 1100 MM/SM Active
Comments: High quality, US built, great support, Active Alignment on FlightStrata models.
LaserBit PICO II 100 200 TP
PICO Plus 100 200 TP PoE
PINTO 100 500 MM/SM
PINTO Plus 100 500 TP PoE
PRONTO 100 1000 MM/SM
GigaPICO 1250 200 TP/MM/SM
GigaPINTO 1250 500 MM/SM
GigaPRONTO 1250 1200 MM/SM
Comments: Low price leader. Support and warranty a little slower, but simple installation, (especially PoE models) and a new 3-Year warranty.
fSona 155-E 100/155 970 MM/SM
1250-E 1250 800 MM/SM
155-S 100/155 1175 MM/SM
622-S 622 1150 MM/SM
1250-S 1250 1075 MM/SM
155-M 100/155 1600 MM/SM
622-M 622 1575 MM/SM
1250-M 1250 1475 MM/SM
Comments: High priced FSO line-up due in part to the use of 1550nm lasers which generally require heating/cooling.
MRV TS1A 100 240 GIPF Passive
TS1C 100 470 GIPF Passive
TS702/T1 T1/E1 570 CAT5
TS707/T1 T1/E1 850 CAT5
TS811/T1 T1/E1 1360 CAT5
TS940/T1 T1/E1 1650 CAT5
TS960/T1 T1/E1 1900 CAT5
TS702/4T1 4xT1/E1 450 CAT5
TS707/4T1 4xT1/E1 900 CAT5
TS940/4T1 4xT1/E1 1650 CAT5
TS702/ETH 10BaseT Eth. 280 CAT5
TS707/ETH 10BaseT Eth. 850 CAT5
TS811/ETH 10BaseT Eth. 1360 CAT5
TS940/ETH 10BaseT Eth. 1650 CAT5
TS700 100 500 Cat5 PoE
TS3303/SC 100/155 920 SM/MM
TS4000/SC 100/155 1850 SM/MM
TS5000/SC 100/155 2600 SM/MM
TS1000X 100/155 840 SM/MM
TS1000P 1250 260 GIPF Passive
TS700G 1250 500 SM/MM
TS5000Z 1250 2000 SM/MM

Comments: MRV was the early leader in FSO. 6000 installed links.
BridgeWave FE60 100 800 MM
FE60X 100 1130 MM
GE60 1250 570 MM
GE60X 1250 890 MM
AR60 1250 1060 MM
AR60X 1250 1990 MM
GE80 1250 5000 MM
AR80 1250 8000 MM
GE80X 1250 MM
AR80X 1250 MM
Comments: BridgeWave is not FSO, but rather 60GHz RF. Very similar to FSO.
Optical systems work in the infrared or near infrared region of light and the easiest way to visualize how the work is imagine, two points interconnected with fiber optic cable and then remove the cable. The infrared carrier used for transmitting the signal is generated either by a high power LED or a laser diode. Two parallel beams are used, one for transmission and one for reception, taking a standard data, voice or video signal, converting it to a digital format and transmitting it through free space .
Todayâ„¢s modern laser system provide network connectivity at speed of 622 Mega bits/sec and beyond with total reliability. The beams are kept very narrow to ensure that it does not interfere with other FSO beams. The receive detectors are either PIN diodes or avalanche photodiodes.
The FSO transmits invisible eye safe light beams from transmitter to the receiver using low power infrared lasers in the tera hertz spectrum. FSO can function over kilometers.
Currently available FSO hardware are of two types based on the operating wavelength “ 800 nm and 1550 nm. 1550 FSO systems are selected because of more eye safety, reduced solar background radiation and compatibility with existing technology infrastructure.

In the transmitting section, the data is given to the modulator for modulating signal and the driver is for activating the laser. In the receiver section the optical signal is detected and it is converted to electrical signal, preamplifier is used to amplify the signal and then given to demodulator for getting original signal. Tracking system which determines the path of the beam and there is special detector (CCD, CMOS) for detecting the signal and given to pre amplifier. The servo system is used for controlling system, the signal coming from the path to the processor and compares with the environmental condition, if there is any change in the signal then the servo system is used to correct the signal.

Optical communication systems are becoming more and more popular as the interest and requirement in high capacity and long distance space communications grow. FSO overcomes the last mile access bottleneck by sending high bitrate signals through the air using laser transmission.
Applications of FSO system is many and varied but a few can be listed.
1. Metro Area Network ( MAN ): FSO network can close the gap between the last mile customers, there by providing access to new customers to high speed MANâ„¢s resulting to Metro Network extension.
2. Last Mile Access : End users can be connected to high speed links using FSO. It can also be used to bypass local loop systems to provide business with high speed connections.
3. Enterprise connectivity : As FSO links can be installed with ease, they provide a natural method of interconnecting LAN segments that are housed in buildings separated by public streets or other right-of-way property.
4. Fiber backup : FSO can also be deployed in redundant links to backup fiber in place of a second fiber link.
5. Backhaul : FSO can be used to carry cellular telephone traffic from antenna towers back to facilities wired into the public switched telephone network.
Various approaches
There is more than one way of designing free-space optics equipment. Some companies include switches and routers in their products; others offer a physical-layer-only solution. AirFiber and Optical Access, both of San Diego, Calif., have focused on mesh-based asynchronous transfer mode (ATM) and Internet protocol (IP) models, respectively. Protocol-independent physical-layer equipment that can be used in any network topology is the focus of LightPointe, the authors' company, also in San Diego; Optical Crossing, in Glendale, Calif.; and fSONA, in Richmond, B.C., in Canada. Terabeam Corp., in Kirkland, Wash., is unusual in being a provider of both equipment and communications services. Outside North America, a handful of other free-space optics companies in Europe and the Middle East primarily serve enterprise and campus customers.
Similarly, there are different approaches to dealing with the various factors that can affect link performance and reduce link availability below the five nines (99.999 percent) figure, the Holy Grail of carrier-class technology. By definition, carrier-class service delivers only one bad bit out of every 10 billion it carries, and statistically is out of service no more than 5 minutes and 15 seconds a year. For the rest of the 12-month period (8759 hours, 54 minutes, and 45 seconds), it will be up and running.
For free-space optics, challenges to achieving this level of performance take the shape of environmental phenomena that vary widely from one micrometeorological area to another. Included here are scintillation, scattering, 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. In addition, it is highly effective in overcoming any scintillation that may occur near windows. In conjunction with a design that uses multiple and spatially separated large-aperture receive lenses, this multi-beam approach is even more effective.
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. Nevertheless, to ensure carrier-class availability for a single FSO link in most non-desert environments, the link length should be limited to 200-500 meters.
Free-space optics systems, when deployed through a network, can be engineered to provide the high availability desired by carriers. It's done by limiting link lengths in accordance with known local weather patterns. For example, LightPointe, which is in lab and field trials with more than a dozen carriers around the world, recently concluded a trial in Denver, Colo., in which its 1.25-Gb/s system obtained 99.997 percent availability over a three-month period at a challenging time of the year: November-February.
Other atmospheric disturbances, like snow and especially rain, are less of a problem for free-space optics than fog.
Swaying buildings
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. The techniques are effective, as evidenced earlier this year during an earthquake in Seattle, where Terabeam has its free-space optical service up and running. The company reported that only a few of its links lost connections, and those for only a short time.
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.
These closed-loop systems are also valuable for high-speed links that span long distances. In those applications, beam divergence is not a good approach. By its very nature, it reduces the beam power density just when receivers, being less sensitive at high data rates, need all the power they can get.
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.
Some case studies
In one free-space optics business case, a competitive local exchange carrier (CLEC) has an agreement with a large property management firm to provide all-optical 100-Mb/s Internet access capability to several buildings located in an office park. The carrier is building its network by leasing regional dark fiber rings and long-haul capacity from a wholesale fiber provider. It has identified a potential hub, or point-of-presence, less than a kilometer from the office park and within sight of one of its central offices. The CLEC currently has no fiber deployed to target customer buildings.
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.
Last-Mile Connectivity
Working via a hub building, free-space optics can connect each of the three buildings at the left to a competitive local exchange carrier's central office at 100-Mb/s. This office is a node on a metropolitan-area ring, which is connected to a regional ring by means of conventional fiber-optics
In planning communications networks, much money can be saved by building the network piecemeal, adding to it as warranted by customer demand. Free-space optical networks lend themselves to such a scalable model much better than fiber-based networks do. With fiber, the cost of digging a trench is so high that it makes sense to install as much fiber as possible while the trench is open. With FSO, only the equipment absolutely needed at any time needs to be deployed. As new customers are signed up, the equipment needed to support them is installed. This demand-based approach lowers the capital expenditure required to grow the customer base and allows the service provided to immediately begin recovering costs associated with the network equipment capital outlay. In this scenario, not only the service provider but also the customer wins, because he or she can be instantly online and start to benefit from the higher bandwidth network connection.
All the same, while free-space optics advantageously bypasses the need to dig up streets for fiber-optic lines, its exposure to weather variations will remain its No. 1 challenge. In an outdoor lab trial in 2000, XO Communications, Reston, Va. (the broadband voice and data service provider formerly known as Nextlink), used free-space optics equipment to protect some of its fiber systems from accidentally severed cables. In addition to providing redundancy for ground-based fiber, the FSO equipment was used to close off a Sonet ring and to connect additional buildings to the ring. In this scenario, free-space optics was complementary to fiber-optic cable.
The same applications have been demonstrated pairing free-space optics with local multipoint distribution systems (LMDS) radio communications networks. According to the XO Communications trial, the diversity (in both transport medium and traversed path) that comes from backing up fiber with FSO may provide better protection than backing fiber up with additional fiber.
Clearly, then, 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.
Where does Free-space Optics fit best
Unfortunately, there is no single "best" one-size-fits-all in the point-to-point wireless tool bag yet. Some day there will be, but not today.
Each alternative has a place based on its + and - characteristics as to Speed, Distance, Latency, Security, Availability, and Cost.
But, to help with a 30,000 foot high-altitude picture of the issues, the below chart may be a useful starting point:
Scale of 1-10 (worst to best)
Technology Speed Distance Latency Security Availability Cost

FSO 9 4 10 9 5 7
Telco 10 10 10 9 9 1
5.8 RF 2 9 5 1 6 10
24 GHz RF 5 6 6 2 7 5
60 GHz RF 9 3 9 3 7 6
So, as an example, if speed and security are your hot buttons, FSO with a 9 and a 9 respectively is probably something to look at early on.
If long distance and low cost are your main considerations, then 5.8RF with a 9 and a 10 respectively may be your ticket.
Where does Free-space Optics not fit
Free-space Optics is not a good fit for:
¢ Low throughput applications where a Telco T1 or low cost 5.8GHz RF link is sufficient unless high security is important.
Long link distances over 4-5km or so.
Non-rigid mount environments (static aligned links)
Highly unstable mount environments like tall guyed towers (active aligned links)
Very tight budgets of <$2-3k
Telecommunication 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 more recent emphasis on Metropolitan Area Networks ( MAN ). Meanwhile LAN giga bit Ethernet ports are being deployed with a comparable growth rate. Even then there is pressing demand for speed and high bandwidth.
The Ëœconnectivity bottleneckâ„¢ which refer the imbalance between the increasing demand for high bandwidth by end users and inability to reach them is still an unsolved puzzle. Of the several modes employed to combat this Ëœlast mile bottleneckâ„¢, the huge investment is trenching, and the non-redeployability of the fiber has made it uneconomical and non-satisfying.
Other alternatives like LMDS, a RF technology has its own limitations like higher initial investment, need for roof rights, frequencies, rainfall fading, complex set and high deployment time.
In the United States the telecommunication industries 5 percent of buildings are connected to OFC. Yet 75 percent are with in one mile of fiber. Thus FSO offers to the service providers, a compelling alternative for optical connectivity and a complement to fiber optics.
1. Free space optics offers a flexible networking solution that delivers on the promise of broadband.
2. Straight forward deployment-as it requires no licenses.
3. Rapid time of deployment.
4. Low initial investment.
5. Ease of installation even indoors in less than 30 minutes.
6. Security and freedom from irksome regulations like roof top rights and spectral licenses.
7. Re-deployability
Unlike radio and microwave systems FSO is an optical technology and no spectrum licensing or frequency co-ordination with other users is required. Interference from or to other system or equipment is not a concern and the point to point laser signal is extremely difficult to intercept and therefore secure. Data rate comparable to OFC can be obtained with very low error rate and the extremely narrow laser beam which enables unlimited number of separate FSO links to be installed in a given location.
The advantages of free space optics come without some cost. As the medium is air and the light pass through it, some environmental challenges are inevitable.
Fog substantially attenuates visible radiation, and it has a similar affect on the near-infrared wavelengths that are employed in FSO systems. Rain and snow have little affect on FSO. Fog being microns in diameter, it hinder the passage of light by absorption, scattering and reflection . Dealing with fog “ which is known as Mie scattering, is largely a matter of boosting the transmitted power. In areas of heavy fogs 1550nm lasers can be of more are. Fog can be countered by a network design with short FSO link distances. FSO installation in foggy cities like san Francisco have successfully achieved carrier-class reliability.
Flying birds can temporarily block a single beam, but this tends to cause only short interruptions and transmissions are easily and automatically re-assumed. Multi-beam systems are used for better performance.
Scintillation refers the variations in light intensity caused by atmospheric turbulence. Such turbulence may be caused by wind and temperature gradients which results in air pockets of varying diversity act as prisms or lenses with time varying properties.
This scintillation affects on FSO can be tackled by multi beam approach exploiting multiple regions of space- this approach is called spatial diversity.
This can be combated in two ways.
- The first is a long pass optical filter window used to block all wavelengths below 850nm from entering the system.
- The second is an optical narrow band filter proceeding the receive detector used to filter all but the wavelength actually used for intersystem communications.
Scattering is caused when the wavelength collides with the scatterer. The physical size of the scatterer determines the type of scattering.
- When the scatterer is smaller than the wavelength-Rayleigh scattering.
- When the scatterer is of comparable size to the wavelength -Mie scattering.
- When the scatterer is much larger than the wavelength -Non-selective scattering
In scattering there is no loss of energy, only a directional re-distribution of energy which may cause reduction in beam intensity for longer distance.
Absorption occurs when suspended water molecules in the terrestrial atmosphere extinguish photons. This causes a decrease in the power density of the FSO beam and directly affects the availability of a system. Absorption occurs more readily at some wavelengths than others.
However, the use of appropriate power, based on atmospheric conditions, and use of spatial diversity helps to maintain the required level of network availability.
One of the most common difficulties that arises when deploying FSO links on tall buildings or towers is sway due to wind or seismic activity Both storms and earthquakes can cause buildings to move enough to affect beam aiming. The problem can be dealt with in two complementary ways: through beam divergence, and active tracking
a. With beam divergence, the transmitted beam spread, forming optical cones which can take many perturbations.
b. Active tracking is based on movable mirrors that controls the direction in which beams are launched.
Infrared technology is as secure or cable applications and can be more reliable than wired technology as it obviates wear and tear on the connector hardware. In the future it is forecast that this technology will be implemented in copiers, fax machines, overhead project and implimentationors, bank ATMs, credit cards, game consoles and head sets. All these have local applications and it is really here where this technology is best suited, owing to the inherent difficulties in its technological process for interconnecting over distances.
Outdoors two its use is bound to grow as communications companies , broadcasters and end users discovers how crowded the radio spectrum has become. Once infraredâ„¢s image issue has been overcome and its profile raised, the medium will truly have a bright, if invisible, future !
We have discussed in detail how 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.
a. Vikrant kaulgnd , Free space optics Bridges the last mile™™ , Electronics for u , June 2003 pp . 38-40 .
b. Andy Emmerson , Fibreless Optics ™™ , Everyday practical electronics , April 2003 pp . 248 .
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