Free Space Optics

1. INTRODUCTION.

A fiber optic communication link uses light sources and detectors to send and receive information through a fiber optic cable. Similarly, FSO uses light sources and detectors to send and receive information, but through the atmosphere instead of a cable. The motivation for FSO is to eliminate the cost, time, and effort of installing fiber optic cable, yet retain the benefit of high data rates (up to 1 Gb/s and beyond) for transmission of voice, data, images, and video. However, swapping light propagation through a precisely manufactured dielectric waveguide for propagation through the atmosphere imposes significant penalties on performance. Specifically, the effective distance of FSO links is limited; depending on atmospheric conditions the maximum range is 2-3 km, but 200-500 meters is typical to meet telco grades of availability. Thus, at present, FSO systems are used primarily in last mile applications to connect end users to a broadband network backbone as shown in Figure 1. Although FSO equipment is undergoing continuous development, the emphasis is on improving its application to local area networks (LAN) and, in some cases, MANs (e.g., to close a short gap in a ring network), but not to long-haul relay systems. The design goal of a long-haul transmission system is to maximize the separation of relays in spanning distances between cities and countries. For that purpose, FSO is uneconomical compared to fiber optic or microwave radio systems .
Figure 1. Example of End-user Access to Backbone Network using FSO System

2. HISTORY

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 Bell’s photophone never became a commercial reality, it demonstrated the basic principle of optical communications. By addressing the principal engineering challenges of Free Space Optics (FSO), the aerospace/defense activity established a strong foundation upon which today’s commercial laser-based Free Space Optics (FSO) systems are based.

3. OVERVIEW

Optical wireless communication has emerged as a viable technology for next generation indoor and outdoor broadband wireless applications. Applications range from short-range wireless communication links providing network access to portable computers, to last- mile links bridging gaps between end users and existing fiber optic communications backbones, and even laser communications in outer-space links. Indoor optical wireless communication is also called wireless infrared communication, while outdoor optical wireless communication is commonly known as free space optical (FSO) communication. In applying wireless infrared communication, non-directed links, which do not require precise alignment between transmitter and receiver, are desirable. They can be categorized as either line-of-sight (LOS) or diffuse links. LOS links require an unobstructed path for reliable communication, whereas diffuse links rely on multiple optical paths from surface reflections. On the other hand, FSO communication usually involves directed LOS and point-to-point laser links from transmitter to receiver through the atmosphere. FSO communication over few kilometer distances has been demonstrated at multi- Gbps data rates. FSO technology offers the potential of broadband communication capacity using unlicensed optical wavelengths. However, in- homogeneities in the temperature and pressure of the atmosphere lead to refractive index variations along the transmission path. These refractive index variations lead to spatial and temporal variations in optical intensity incident on a receiver, resulted in fading. In FSO communication, faded links caused by such atmospheric effects can cause performance degradation manifested by increased bit error rate (BER) and transmission delays. FSO technology has also emerged as a key technology for the development of rapidly deployable, secure, communication and surveillance systems, which can cooperate with other technologies to provide a robust, advanced sensor communication network. However, the LOS requirement for optical links reduces flexibility in forming FSO communication networks. Compared with broadcast radio frequency (RF) networks, FSO networks do not have an obvious simple ability to distribute data and control information within the network. The objective of the research work presented here is to answer the following questions regarding: 1) how to improve the performance of FSO links for long-range FSO communications, where atmospheric turbulence effects can be severe; and 2) how to accommodate the broadcast requirements for short-range FSO sensor networking applications. These challenging problems are addressed by two different approaches, yet there is a possibility that these two techniques can be combined and realized in one general purpose FSO communication system.

3.1 Comparison of Free Space Optical and Radio Frequency
Technologies

Traditionally, wireless technology is almost always associated with radio transmission, although transmission by carriers other than RF waves, such as optical waves, might be more advantageous for certain applications. The principal advantage of FSO technology is very high bandwidth availability, which could provide broadband wireless extensions to Internet backbones providing service to end-users. This could enable the prospect of delay-free web browsing and data library access, electronic commerce, streaming audio and video, video-on-demand, video teleconferencing, real-time medical imaging transfer, enterprise networking and work-sharing capabilities, which could require as much as a 100 Mbps data rate on a sustained basis. In addition, FSO permits the use of narrow divergence, directional laser beams, which if deployed appropriately, offer essentially very secure channels with low probability of interception or detection (LPI/LPD). Narrow FSO beams also have considerable obscuration penetrating capability. For example, penetration of dense fog over a kilometer distance is quite feasible at Gbps data rates with beam divergence of 0.1 mrad. The tight antenna patterns of FSO links allow considerable spatial re-use, and wireless networks using such connectivity are highly scalable, in marked contrast to ad- hoc RF networks, which are intrinsically non-scalable. However, FSO has some drawbacks as well. Since a LOS path is required from transmitter to receiver, narrow beam point-to-point FSO links are subject to atmospheric turbulence and obscuration from clouds, fog, rain, and snow causing performance degradation and possible loss of connectivity. In addition, FSO links can have a relatively short range, because the noise from ambient light is high, and also because the square- law nature of direct detection receiver doubles the effective path loss (in dB) when compared to a linear detector. Obviously, FSO communication will not replace RF communication, rather they will co-exist. Hybrid FSO/RF networks combine the advantages and avoid the disadvantages of FSO or RF alone. Even if the FSO connectivity cannot be provided all the time, the aggregate data rate in such networks is markedly greater than if RF links were used alone. RF alone does not have the band width for the transfer of certain types of data, for example high-definition video quality full-spectrum motion imagery. Hybrid wireless networks will provide maximum availability and capacity.





3.2 Free Space Optical Networking

The individual FSO link between transmitter and receiver can be naturally extended to an FSO network topology. FSO networks could serve in a metropolitan area to form a backbone of base stations providing service to both fixed and mobile users. For some other application where no base station is present, FSO transceivers may need to be able to communicate with one another to form ad-hoc sensor networking. In both cases, a scalable, robust, and controlled network topology is required.
A major drawback of directed LOS systems is their inability to deal with broadcast communication modes. In networking, broadcasting capability is frequently required in order to establish communication among multiple nodes. With this capability, networking data and control messages can easily be flooded over the whole network. This problem can be eliminated by using non-directed LOS optical links, which can be described as omnidirectional links. There have been a number of systems based on non-directed communications . They provide reliable performance mainly in short-range applications, i.e. several to tens of meters. For longer range networking applications, such as in a military context with a network of high-altitude aircraft or in battlefield scenarios, only directional FSO links can provide high data rate capability and channel security. For directional FSO links, pointing, acquisition, and tracking (PAT) schemes are necessary in order to establish and allow the flow of information within the networks. PAT involves a beamsteering device, which may be mechanical, such as galvo- mirror or gimbal; or nonmechanical, using an acousto-optic crystal or piezo-electric actuator. In typical directional FSO systems, there is a trade-off between the requirements of maximizing received optical power, yet minimizing the PAT sensitivity of the system.
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 keyboard or wireless Palm® devices.

4. HOW 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.







4.1 Technology description.

4.1.1 General Framework.

Communication system design is concerned with tradeoffs between channel length, bit rate, and error performance. The generalized schema of a single-link communication system in Figure 2 provides the necessary framework to compare fiber optic and FSO technologies. Under each block are characteristics that transform its signal input to the different physical form of the signal output. The superscript N for each block transform represents noise contributed to the signal. For example, the “channel” block degrades the transmitter output signal due to processes listed under the block for fiber optic cable or FSO. Although both are optical communication systems, the fundamental difference between fiber optic and FSO systems is their propogation channels: dielectric waveguide versus the atmosphere. As a consequence, signal propagation, equipment design, and system planning are different for each type of system. The main thesis of the following discussion is that, because of their different propagation channels, the performance of FSO cannot be expected to match that of advanced fiber optic systems; therefore FSO applications will be more limited.

Figure 2. Single-link Communication System
4.2 FSO Characteristics.


Figure 3. Block Diagram, FSO Communication System

A generalized FSO system is shown in Figure 3. The baseband transmission bit stream is an input to the modulator, turning the direct current bias current on and off to modulate the laser diode (LD) or light emitting diode (LED) light source. The modulated beam then passes through a collimating lens that forms the beam into a parallel ray propagating through the atmosphere. A fundamental physical constraint, the diffraction limit, comes into play at this point. It says that the beam of an intensity modulated (non-coherent) light source cannot be focused to an area smaller than that at its source. Apart from the effects of atmospheric processes, even in vacuum, a light beam propagating through free space undergoes divergence or spreading. Recalling the single-link communication system in Figure 2, the transmitted FSO beam is transformed by several physical processes inherent to the atmosphere: frequency-selective (line) absorption, scattering, turbulence, and sporadic misalignment of transmitter and receiver due to displacement (twist and sway) of buildings or structures upon which the FSO equipment is mounted. These processes are non- stationary, which means that their influence on a link changes unpredictably with time and position. At the distant end, a telescope collects and focuses a fraction of the light beam onto a photo-detector that converts the optical signal to an electrical signal. The detected signal is then amplified and passes to processing, switching, and distribution stages. The. Figure 5 is an illustration of a simplified single-beam FSO transceiver that shows how the major functional blocks of the equipment are arranged and integrated.




Figure 5. Single-beam FSO Transceiver.

5. FACTORS AFFECTING THE PERFORMANCE OF THE FSO SYSTEM

The non-stationary atmospheric processes absorption, scattering, refractive turbulence, and displacement, are the factors that most limit the performance of FSO systems. A brief description of each is given in the following paragraphs. Divergence determines how much useful signal energy will be collected at the receive end of a communication link. It also determines how sensitive a link will be to displacement disturbances (see below). Of the processes that cause attenuation, divergence is the only one that is independent of the transmission medium; it will occur in vacuo just as much as in a stratified atmosphere. Laser light can be characterized as partially coherent, quasimonochromatic electromagnetic waves passing a point in a wave field. At the transmitter, beam divergence is caused by diffraction around the circular aperture at the end of the telescope. In practice, an FSO transmit beam is defocused from the diffraction limit enough to be larger than the diameter of the telescope at the receive end, and thus maintain alignment with the receiver in the face of random displacement disturbances.

5.1 Absorption.

Molecules of some gases in the atmosphere absorb laser light energy; primarily water vapor, Carbon Dioxide (CO2), and Methane, Natural Gas (CH4). The presence of these gases along a path changes unpredictably with the weather over time. Thus their effect on the availability of the link is also unpredictable. Another way of stating this is that different spectrum windows of transmission open up at different times, but to take advantage of these, the transmitter would have to be able to switch (or retune) to different wavelengths in a sort of wavelength diversity technique.

5.2 Scattering.

Another cause of light wave attenuation in the atmosphere is scattering from aerosols and particles. The actual mechanism is known as Mie scatter in which aerosols and particles comprising fog, clouds, and dust, roughly the same size as the light’s wavelength, deflect the light from its original direction. Some scattered wavelets travel a longer path to the receiver, arriving out of phase with the direct (unscattered) ray. Thus destructive interference may occur which causes attenuation. Note how attenuation is much more pronounced for the spectrum in 6(b) for transmission through fog.


Figure 6. Transmission Spectra for Light Traveling through (a) Clear
Air, and (b) Moderate Fog.

5.3 Refractive turbulence.

The photograph in Figure 7 shows the change from a smooth laminar structure of the atmosphere to turbulence. In the laminar region light refraction is predictable and constant, whereas in the turbulent region it changes from point to point, and from instant to instant. Small temperature fluctuations in regions of turbulence along a path cause changes in the index of refraction. One effect of the varying refraction is scintillation, the twinkling or shimmer of objects on a horizon, which is caused by random fluctuations in the amplitude of the light. Another effect is random fluctuations in the phases of the light’s constituent wavelengths, which reduces the resolution of an image.

figure 7

Refractive turbulence is common on rooftops where heating of the surface during daylight hours leads to heat radiation throughout the day. Also, rooftop air conditioning units are a source of refractive turbulence. These items must be considered when installing FSO transceivers to minimize signal fluctuations and beam shifts over time.

5.4 Displacement.

For an FSO link, alignment is necessary to ensure that the transmit beam divergence angle matches up with the field of view of the receive telescope. However, since FSO beams are quite narrow, misalignment due to building twist and sway as well as refractive turbulence can interrupt the communication link. One method of combating displacement is to defocus the beam so that a certain amount of displacement is possible without breaking the link. Another method is to design the FSO head with a spatial array of multiple beams so that at least one is received when the others are displaced. The latter technique circumvents the problem of displacement without sacrificing the intensity of the beam. FSO is technologically very similar to communication using fiber optic cables. Both use laser light to carry the 1s and 0s of digital data. But while traditional fiber optics transmits the laser light through a strand of glass, FSO sends the laser light through the air (“free space”). Since the two technologies are so similar, they share the same advantages of high data rate capacity and protocol independence. Both technologies are also very secure.
Some principle attributes of FSO communication:

1. Directional transmission with an extremely narrow transmit beam for point-to-point (line of sight) connectivity
2. The absence of “side lobe” signals
3. Complete, uninterrupted links required for successful communication
4. Protocol transparent transmission
5. Physical Layer operation
6. “Plug and Play” devices
These key features allow for very secure transmission over an FSO channel. To understand why this is the case, we first need to consider what must take place to successfully steal a communication signal. Two criteria must be satisfied for an individual to overcome the security in a network:
(1) they must intercept enough of the signal to reconstruct data packets and
(2) they must be able to decode that information.
If these two primary requirements cannot be met, the security of the network will remain intact. Given these two conditions, we will now examine how the above attributes of FSO transmission can be used to maintain a secure data link.



6. NETWORKING CONSIDERATIONS.
6.1 Characteristics of Transmission Control Protocol (TCP).

Because TCP does not differentiate between packet loss due to link errors and packet delay due to network congestion, FSO networking can be seriously crippled by packet loss due to signal attenuation (such as that caused by heat, fog, sand, or dirt). The effect of attenuation-induced packet loss is to invoke TCP’s congestion control algorithms, seriously reducing throughput on any particular link.

6.2 Routing Protocol Issues.

To maintain link and path availability, multiple routes from each node must be maintained due to the easily disrupted nature of FSO networking. Because FSO links are easily disrupted due to occlusion and other factors both on a very short time scale (millisecond to minute) as well as on a longer scale (minutes or more), normal routing protocols are not adequate. Normal routing protocols do not deal well with the very short time scale disruptions and, by design, are intended to deal with longer disruptions only (minutes or more). Three normal routing protocols, Routing Information Protocol (RIP), Open Shortest Path First (OSPF), and Enhanced Interior Gateway Routing Protocol (EIGRP), can take 10 to 90 seconds to discover a wireless link failure and re-route the traffic accordingly; during which time, data will be lost as the network will continue to attempt to use the failed link. To reacquire or reestablish a link that went down for perhaps a second or less at an inopportune time in the route status discovery cycle could take just as long. Mobile Ad Hoc Network (MANET) Protocols are being developed to be more responsive to topology dynamics, but are better suited to bandwidth constrained links as they trade routing performance for a reduction in network overhead. The best option that we have seen to date to overcome the routing problem is to exploit the ability of OSPF and EIGRP to respond to a loss of carrier at the physical interface. One study has shown that, after linking this to the existing re-route triggering mechanism in EIGRP, that re-routing can occur after 10 milliseconds as opposed to an average of 12 seconds.

6.3 Serial Networking Considerations.

Technical control facilities (TCF) are currently based on multiplexing data serially. The majority of the information processed through a TCF is serial data and voice. The usual multiplexing technique is Time Division Multiplexing (TDM) where each user is assigned to one (or more) ports of a multiplexer. All of the ports are then aggregated into one data stream. The current infrastructure allows transmission from point to point by many different means including radio transmission, wire, and fiber. FSO is able to transmit and receive this data seamlessly. User networks and the networks in the TCFs have started migrating to Internet Protocol (IP) based systems and will continue to do so. FSO is able to handle the transmission requirements for this migration.

7. MATURITY OF THE TECHNOLOGY.

As noted earlier, the free space propagation channel is essentially uncontrollable, so that FSO is more akin to microwave radio than to fiber optics. The opportunities for advancing the FSO art fall into two areas: equipment enhancements at the physical layer and system enhancements at the network layer. The physical layer enhancements would mitigate atmospheric and displacement disturbances, whereas the network layer would implement decision logic to buffer, retransmit, or reroute traffic in the event of an impassable link.Changeable atmospheric conditions along a path favor different wavelengths at different times; no single wavelength is optimal under all conditions. This raises the question whether FSO link performance can be improved by adaptively changing the source wavelength to match the conditions. Quantum cascade lasers (QCL), for example, can be tuned over a wide range of long-infrared (IR) wavelengths that includes the known atmospheric low absorption windows. Adaptive retuning to an optimal transmission wavelength, in response to dynamic conditions, might be done using either a single laser or an array of fixed wavelength lasers. In any case, one study indicates that adaptive retuning may result in only marginal improvements to link performance. At the receive end of a link, it turns out that the thermal noise from an array of small photo detectors is less than the noise from a single large detector with an equivalent field of view. Thus a significant improvement in the noise performance of FSO receivers is possible using the photo detector array. Scattering through fog and dust causes pulse spreading that leads to inter-symbol interference. A decision feedback adaptive equalizer has been proposed to combat this effect, but the authors caution that it would be effective only for relatively low data rates. Furthermore, adaptive optics could use wavefront sensors, and deformable mirrors and lenses to reduce FSO wavefront distortion from refractive turbulence. One author claims that, under certain circumstances, adaptive optics could provide several orders of magnitude improvement in BER against scintillation caused by turbulence. Several commercial FSO products use pointing and tracking control systems to compensate for displacement induced alignment errors. Existing systems employ electromechanical two-axis gimbal designs, therefore they are relatively expensive to adjust and maintain. As a non-mechanical alternative, optical phased arrays (OPA) are under development in which the phase difference of an array of lasers is controlled to form a desired beamwidth and orientation. Such arrays would be part of both the transmitter and receiver assemblies so as to achieve the maximum alignment over a path. The algorithms for such control systems are also an active research area in which the goal is replace simple proportional-integral-derivative (PID) loops with adaptive neural-network-based algorithms that enable more accurate estimates of the stochastic processes of particular FSO links.
At the network level buffering and retransmitting data are conventional communication protocol strategies, but they are less than optimal for networks bearing real-time services such as voice and video in addition to computer data. The concept of topology control has been proposed as a method of dealing with link degradation or outages without interrupting services. The idea is to establish a mesh of stations over a desired coverage area that would adaptively reroute traffic in response to link interruptions. This scheme requires either a proliferation of point-to-point transceivers for the network or an advanced pointing and tracking control system to accomplish the rerouting. Sophisticated software would also be required to monitor and control the route switching.

8. BENEFITS OF THE TECHNOLOGY.

The attraction of FSO is its high data transmission rate and its exemption from spectrum regulation. The latter is especially significant for military ground forces setting up camps and forward operating bases overseas. Whereas application for frequency assignments in the United States is a ponderous process, in a foreign country it is all the more so, and fraught with some uncertainty; the request may be denied, or services may be impaired by interferers due to poor frequency planning or intentional jamming. At the very least it is time consuming. To be able to circumvent the spectrum management bureaucracy is a huge advantage given urgent communication requirements. Since light beams do not interfere with each other as long as they are not coaxial, commanders need not be concerned with electromagnetic compatibility problems. FSO is as ready a resource as a light bulb in a socket, and installation of FSO equipment is quick and inexpensive. FSO’s drawbacks in the commercial world are perhaps not as serious in the military context. Using short FSO repeater spacings for camp communications may still be more economical than installing fiber optic cable and it allows more flexibility for re-routing lines of communication as the camp grows. In the Southwest Asia Theater for example, FSO could free up tactical equipment that has been used as a stopgap for camp communications, and eliminate runs of loose field wire. FSO would carry all communication services, not just voice or data separately. In the future the layout of new camps should perhaps plan for lanes for the paths of an FSO network. The transceivers should be placed low to the ground to employ short rigid mounts, but not so low as to be adversely affected by the bottom atmospheric layer disturbed by radiative heat energy from the ground surface.

9. CHALLENGES OF THE TECHNOLOGY.
9.1 Laser eye safety.
It is important to keep in mind, especially if FSO is to gain widespread use for camp communications, that lasers must be operated within certain levels of irradiance [w/m2] for eye safety. The harmful level of exposure is a function of wavelength and is tabulated in American National Standards Institute (ANSI) Standard Z136.1.

9.2 Disruption by weather.

Although FSO may at times be capable of greater range, its greater susceptibility to degradation from incidents of heavy fog or dust will drive down its attainable availability figures. This will depend on which region of the world FSO is planned for. For example, frequent dust storms of such severity as to result in black out conditions often occur in tactical desert conditions. Furthermore, the summer heat in the desert and along coastlines induces extreme refractive turbulence that would cause optical defocusing and beam wander.

10. ADVANTAGES

The transmission medium selection is based on many differing engineering requirements with cost and schedule being major considerations. FSO in serial transmission may be advantageous when requirements call for short transmission paths requiring quick installations. FSO devices have advantages to radio and fiber based systems if speed of installation is the dominating concern when providing the last mile connectivity. The setup of these systems is quick and as long as the distance requirements are within their scope of operation these devices may be considered as a viable option.
Main advantages are:
Quick link setup
License-free operation
High transmission security
High bit rates
Low bit error rate
No Fresnel zone necessary
Low snow and rain impact
Full duplex transmission
Protocol transparency
No interference
Great EMI behavior
In some devices, the beam can be visible, facilitating aiming and detection of failures.


11. DISADVANTAGES
As the serial data nature of TCFs change into IP based infrastructures point-to-point applications will decrease in favor of network centric infrastructures. This will reduce point-to-point applications in general. The limited link distance provided by FSO equipment limits the consideration of transmission applications to last mile applications. Path selection must be engineered to ensure that there are no obstacles that would impair signal quality.Technology disadvantages and behavior When used in a vacuum, for example for inter-space craft communication, FSO may provide similar performance to that of fibre-optic systems. However, for terrestrial applications, the principle limiting factors are:

•Atmospheric absorption
•Rain (lower attenuation)
•Fog (10..~100dB/km attenuation)
•Snow (lower attenuation)
•Scintillation (lower attenuation)
•Background light
•Shadowing
•Pointing stability in wind
•Pollution / smog
•If the sun goes exactly behind the transmitter, it can swamp the signal.
These factors cause an attenuated receiver signal and lead to higher bit error ratio (BER). To overcome these issues, vendors found some solutions, like multi-beam or multi-path architectures, which use more than one sender and more than one receiver. Some state-of-the-art devices also have larger fade margin (extra power, reserved for rain, smog, fog). To keep an eye-safe environment, good FSO systems have a limited laser power density and support laser classes 1 or 1M. Atmospheric and fog attenuation, which are exponential in nature, limit practical range of FSO devices to several kilometres.

12. PRODUCTS.
12.1 Current Products.

Current FSO technology is still developing. The number of manufacturers and types of systems are growing. In traditional FSO technology a single light source transmits to a single receiver. These systems typically have a throughput of 1 Gb/s. The distance transmitted is very limited from 200 to 1000 meters (typical systems operate up to 500 meters). Reliability of these devices is typically 99.9 percent in clear conditions, varying greatly depending on distance and weather conditions. The current cost of these systems is from $2500 - $3000 per unit (twice that per link). These traditional types of FSO products were evaluated by USAISEC’s engineering and evaluation facility, the Technology Integration Center (TIC) at Fort Huachuca, Arizona. The evaluations were to determine if an FSO solution could provide extensions to, a back up for, or an alternative to wired link technology in support of the Installation Information Infrastructure Modernization Program (I3MP). Recommendations for use were made for LightPointe Flight Spectrum 1.25G (TR. No. AMSEL-IE-TI-03067, July 2003), MRV TS3000G (TR. No. AMSEL-IE-TI-03070, July 2003), and Alcatel SONAbeam (TR. No. AMSEL-IE-TI- 03081, September 2003). The Terabeam Elliptica (TR. No. AMSEL-IE-TI-03068, July 2003)) was recommended as a backup link only due to bandwidth limitations (TR No. AMSEL-IE-04009, November 2003). Another product, AirFiber 5800 (TR No. AMSEL-IE-TI-03059, July 2003) was not recommended, because the manufacturer is no longer in business. Field testing was scheduled (TR No. AMSEL-IE-TI-05003) in Germany to test FSO technology over time and varying weather conditions. The preliminary field tests indicated that weather was a significant factor in link performance. In another military field application at the Pentagon, the SONAbeam S-Series FSO configuration performed with no link outages except when the line of sight path was blocked by helicopter air traffic. This was a point-to-point link and the loss of line of site path caused link outages. The link between the Pentagon and the Navy Annex covered approximately 500 meters. This loss of line-of-sight issue was significant at the Pentagon due to repeated path blockage by the air traffic eventually leading to the link being discontinued after 1 year of service. Industry has recognized the weather anomaly as a significant issue. SonaBeam and WaveBridge systems have four redundant lasers transmitting to a receiver. This provides physical diversity, increases link performance, and allows for a limited extended range increase over single source FSO products. The range increase provides an additional 1000 meters extending the total link distance to 2000 plus meters. Several manufacturers such as Pulse’s Omni-Node use active pointing and tracking control systems. FSO Mesh Network systems have also been developed. Omni-Node by Pulse provides three transceivers per device with an active tracking system. Also included in this product offering is redundant link fail-over. Hybrid systems using FSO and millimeter microwave technology are also available. Such systems are available from AirFiber and LightPointe. Hybrid systems approach carrier class reliability of 99.999 percent over 1 km at 1.25 GBs. These systems reduce the vulnerability of FSO during heavy fog conditions by using the millimeter microwave path and conversely reduce the vulnerability of millimeter microwave during heavy rain by using the FSO system. The two weather conditions rarely are simultaneous. Distance limitations are still less than 2 kms.

12.2 Near Future Products.

Crinis Networks has introduced an FSO product that competes with Ethernet and Fast Ethernet LAN connectivity for indoor applications. Crinis uses the terminology “indoor Free Space Optics (iFSO)” to describe this application. The Federal Communications Commission (FCC) issued license guidance for "E-Band" in October 2003. E-Band is an upper-millimeter wave band that operates over 71-76 Gigahertz (GHz), 81-86 GHz, and 92-95 GHz bands. It is licensed by the link, which can be done on line in a matter of days. It is meant to allow industry to use as a last mile solution for broadband applications. This technology should be a competitor with FSO and/or as part of the Hybrid system. Bandwidth of these devices is 1.25 Gb/s. Range is up to 2 kms. Manufacturers include Loea and ElvaLink. Costs are approximately $20K per link.

13. POTENTIAL APPLICATIONS

The current reliability of FSO systems with varying weather conditions severely limit the wide spread military application of these devices. Under conditions of rapid deployment requiring interconnected network nodes, these products provide a good temporary solution. This is especially true in urban areas. Due to the possibility of link interference due to obstruction and weather instability, the systems should be replaced with a cable infrastructure when possible. Mesh systems and multiple transmitter systems are an upgrade to the original FSO concept but have similar issues of reliability. Hybrid systems offer higher reliability and performance approaching carrier class reliability. Hybrid systems offer the most likely solution for military systems, but need further testing in varying conditions to confirm reliability in the deployed environment.
Typically scenarios for use are:
•LAN-to-LAN connections on campuses at Fast Ethernet or Gigabit Ethernet speeds.
•LAN-to-LAN connections in a city. Example, Metropolitan area network.
•To cross a road or other barriers.
•Speedy service delivery of high bandwidth access to fiber networks.
•Converged Voice-Data-Connection.
•Temporary network installation (for events or other purposes).
•Reestablish high-speed connection quickly (disaster recovery).
•As an alternative or upgrade add-on to existing wireless technologies.
•As a safety add-on for important fiber connections (redundancy).
•For communications between spacecraft, including elements of a satellite constellation.
•For interstellar communication.
The lightbeam can be very narrow, which makes FSO hard to intercept, improving security. FSO provides vastly improved EMI behavior using light instead of microwaves.








14. CONCLUSION.

While it is obviously an up and coming technology, it could also easily be described as only mature enough in its current state to use in limited applications. The applications that FSO technology seems most suited to are clear weather, short distance link establishment, such as last-mile connections to broadband network backbones, and backbone links between buildings in a MAN or CAN environment. There is also significant potential for use of this technology in temporary networks, where the advantages of being able to establish a CAN quickly or be able to relocate the network in the relatively short time frame outweigh the network unreliability issues. It should be noted that tactical implementations of this technology, or any highly-mobile implementation, are possible, but in its current state FSO has challenges providing adequate enough reliability to be considered a solution for the mobile Warfighter without resorting to a hybrid solution of FSO paired with another transmission technology (typically Millimeter Wave). Finally, past and current implementations and tests indicate that any future implementations of FSO technology should be carefully evaluated to ensure that no potential link interruptions are a factor before making the decision to actually implement an FSO link.









15. REFERENCES

1.http://www.freespaceoptic.com/
2.http://freespaceoptics.org/
3.http://www.free-space-optic.org/
4.http://www.wikipedia.org/