freespace optics full report
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When we talk about optical communication, most people think about optical-fiber. But optical communication is also possible without optical-fiber. We know that light travels through air for a lot less money. This makes possible the optical communication without optical-fiber. Optical communication without fiber is known as Free Space Optics. It is used due to economic advantages. Since the introduction of internet the backbone traffic is increasing at the rate greater than 100%, hence the owner of the backbone infrastructure (which is entirely based on fiber optics) are eagerly embracing technologies that add of the capacity of the fiber optics without adding mountains of optical cables.
FSO is not a new idea. 30-years back optical-fiber cables are used for high-speed communication. In those days FSO are used for high-speed connectivity over short distances. Todayâ„¢s FSO can carry full-duplex data at gigabit-per-second rates over metropolitan distances.
What is Free Space Optics (FSO)
Free Space Optics (FSO) is a line-of-sight technology that uses lasers to provide optical bandwidth connections. Currently, Free Space Optics are capable of up to 2.5 Gbps of data, voice and video communications through the air, allowing optical connectivity without requiring fiber-optic cable or securing spectrum licenses. Free Space Optics require light, which can be focused by using either light emitting diodes (LEDs) 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 is the medium. Light travels through air faster than it does through glass, so it is fair to classify Free Space Optics as optical communications at the speed of light.
Free Space Optics (FSO) technology is relatively simple. It's based on connectivity between FSO units, each consisting 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 (i.e. laser), plus a lens that transmits light through the atmosphere to another lens receiving the information. The receiving lens connects to a
high-sensitivity receiver via optical fiber. FSO technology requires no spectrum licensing. FSO is easily upgradeable, and its open interfaces support equipment from a variety of vendors, which helps service providers protect their investment in embedded telecommunications infrastructures.
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
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.
Light Beam Used for FSO System
Generally equipment works at one of the two wavelengths: 850 nm or 1550 nm. Laser for 850 nm are much less expensive (around $30 versus more than $1000) and are favored for applications over moderate distances. One question arises that why we use 1550 nm wavelength. The
main reason revolves 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. 1550 nm beams operate at higher power than 850 nm, by about two orders of magnitude. That power can boost link lengths by a factor of at least five while maintaining adequate strength for proper link operation. So for high data rates, long distances, poor propagation conditions (like fog), or combinations of those conditions, 1550 nm can become quite attractive.
Why FSO Now
Substantial investments by carriers to augment the capacity of their core fiber backbones have facilitated dramatic improvements in both price and performance, and they have also increased the capacity of these large backbone networks. However, to generate the communications traffic and revenue needed to fully utilize and pay for these backbone upgrades, higher bandwidth connections must reach the end customers. This requires substantial bandwidth upgrades at the network edge. Essentially, to fully leverage their backbone investments, service providers will also need to expand and extend the reach of their metropolitan optical network to the
edge. FSO presents an opportunity that allows carriers to achieve that goal for one-fifth the cost when compared to fiber (if even available) and at a fraction of the time.
Increased competition: Regulation changes and significant investments by various funds have increased the competitive climate in these metro networks. Each of the existing or new entrants is racing to gain an advantage over their competition. FSO is one of the evolutionary technologies that allows a carrier to acquire and retain new customers quickly and cost-effectively, thereby gaining an entry point over competition. Metro optical networks are expected to see $57.3 billion invested by 2005.
International growth: Due to the growing number of Internet-based applications, most countries are experiencing tremendous growth in bandwidth needs. In growing economies like Latin America and Chinaâ€where the ability to have high-bandwidth connectivity outweighs standards for reliabilityâ€the lack of infrastructure and rising bandwidth demands offers a unique opportunity for FSO.
Changing traffic patterns and protocol standards: Multiple traffic types characterize metro networks. Where voice was once the dominant traffic type, data has emerged as the winner. Moreover, these networks are also a mixture of multiple protocols ranging from Ethernet, SONET, IP, ESCON, FICON, etc. As a Layer One technology, FSO is protocol agnostic.
Wireless world: With the rapid adoption and slow deployment of wireless technologies such as LMDS and MMDS in response to high bandwidth communication needs in the metro area, many service providers still find themselves short of bandwidth to satisfy their needs. To better understand this growing need for FSO, it is important to understand the key drivers for FSO.
Applications of FSO
The applications of free-space-optics are many. Some of them are as follows â€œ
1:- Metro Network Extensions
Carriers can deploy FSO to extend existing metropolitan-area fiber rings, to connect new networks, and, in their core infrastructure, to complete Sonet rings.
2:- 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 business with high-speed connections.
3:- 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.
4:- Fiber Backup
FSO may also be deployed in redundant links to backup fiber in place of a second fiber link.
FSO can be used to carry cellular telephone traffic from antenna towers back to facilities wired into the public switched telephone network.
6:- Service Acceleration
FSO can be also used to provide instant service to fiber-optic customers while their fiber infrastructure is being laid.
FSO: Optical or Wireless
FSO is clearly an optical technology and not a wireless technology for two primary reasons. One, FSO enables optical transmission at speeds of up to 2.5 Gbps and in the future 10 Gbps using WDM. This is not possible using any fixed wireless/RF technology existing today. Two, FSO obviates the need to buy expensive spectrum (it requires no FCC or municipal license approvals), which distinguishes it clearly from fixed wireless technologies. Thus, FSO should not be classified as a wireless technology. Its similarity to conventional optical solutions will enable a seamless integration of access networks with optical core networks and help to realize the vision of an all-optical network.
Free-Space Optics (FSO) Security
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
Challenges To Free-Space Optics
Fiber-optic cable and FSO share many similarities. However, there is a difference in how each technology transmits information. While fiber uses a relatively predictable medium that is subject to outside disturbances from wayward construction backhoes, gnawing rodents and even sharks when deployed under sea, FSO uses an open medium (the atmosphere) that is subject to its own potential outside disturbances. Networks with FSO must be designed to counter the atmosphere, which can affect an FSO system's capacity. FSO is also a line-of-sight technology and interconnecting points must be free from physical obstruction and able to "see" each other.
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 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, 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.
It is the scattering of beam due to fog. It is largely a matter of boosting the transmitted power. Spatial diversity also helps to deal with scattering. 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. But some studies shows that scattering is independent of the wavelength under heavy fog conditions. Other atmospheric disturbances, like snow and especially rain, are less of a problem for free-space optics than fog.
3:- 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 activities. Both storms and earthquakes can cause buildings to move enough to affect beam aiming.
The problem of swaying buildings can be dealt with in two ways.
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 kilometer. If the receiver is initially positioned at the center of the beam, divergence alone can deal with many perturbations.
This method is used when the link heads are mounted on the top of extremely tall buildings or towers.
Active tracking is based on movable mirrors that control the direction in which the beams are launched.
A feedback mechanism continuously adjust the mirrors so that the beams stay on target. It is more sophisticated and costly than beam divergence method.
6:- Physical Obstructions
Flying birds can temporarily block a single beam, but this tends to cause only short interruptions, and transmissions are easily and automatically resumed. LightPointe uses multi-beam systems (spatial diversity) to address this issue, as well as other atmospheric conditions, to provide for greater availability.
To those unfamiliar with FSO, safety is often a concern because the technology uses lasers for transmission. This concern, however, is based on perception more than reality. The proper use and safety of lasers have been discussed since FSO devices first appeared in laboratories more than two decades ago. The two major concerns involve human exposure to laser beams (which present
much more danger to the eyes than any other part of the human body) and high voltages within the laser systems and their power supplies. Standards have been set for laser safety and performance and FSO systems comply with these standards.
Advantages Of Free-Space Optics
The FSO system requires less than one fifth of the capital outlay of comparable ground-based fiber-optic technologies. Optical-fibers are too costly. Connecting the buildings with optical-fiber cost US $100000 - $200000/km in metropolitan areas, 85 percent of the total figure tied to trenching and installation. To install fiber you have to dig the road. Street trenching and digging are not only expensive, they cause traffic jams (which increase air pollution), displace trees, and sometimes destroy historical areas. 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.
It is flexible, offers freedom, and is fast (speeds from 20 Mbps to 2.5 Gbps and beyond)
Demand for bandwidth is increasing and has been increasing exponentially for the past few years. Service providers have been struggling to keep up with such demand. Service providers must extend the reach of metro optical networks, and FSO offers service providers the opportunity to accomplish this objective.
The primary advantages of FSO are high throughput, solid security, and low cost.
The entire face of the Free-Space Optics community is about to change radically as driven by the need for high-speed local loop connectivity and the costs and difficulties of deploying fibers. FSO can be the ultimate solution for high-speed access. Instead of hybrid fiber-coax system, hybrid fiber-laser system may turn out to be the best way to deliver the high capacity last-mile access. FSO provide higher security, and throughput. FSO is capable to fulfill the increasing demand of bandwidth.
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04-02-2010, 10:29 AM
FREE SPACE LASER COMMUNICATIONS.pdf (Size: 178.99 KB / Downloads: 709)
FREE SPACE LASER COMMUNICATIONS
Laser communications offer a viable alternative to RF communications for inter satellite links and other applications where high-performance links are necessity. High data rate, small antenna size, narrow beam divergence, and a narrow field of view are characteristics of laser communication that offer a number of potential advantages for system design.
1. INTRODUCTION Lasers have been considered for space communications since their realization in 1960. However, it was soon recognized that, although the laser had potential for the transfer of data at extremely high rates, specific advancements were needed in component performance and systems engineering, particularly for space-qualified hardware. Advances in system architecture, data formatting, and component technology over the past three decades have made laser communications in space not only a viable but also a attractive approach to inter satellite link applications. The high data rate and large information throughput available with laser communications are many times greater than in radio frequency (RF) systems. The small antenna size requires only a small increase in the weight and volume of host vehicle. In addition, this feature substantially reduces blockage of fields of view of the most desirable areas on satellites. The smaller antennas, with diameters typically less than 30cm, create less momentum disturbance to any sensitive satellite sensors. Fewer onboard consumables are required over the long lifetime because there are fewer disturbances to the satellite compared with larger and heavier RF systems. The narrow beam divergence of affords interference-free and secure operation. 2. Transmit laser source Transmit data electronics Terminal control electronics Optics Detector and signal processing Power Structure Thermal regulator
FEATURES OF LASER COMMUNICATIONS SYSTEM A block diagram of typical terminal is illustrated in Fig 1. Information, typically in the form of digital data, is input to data electronics that modulates the transmitting laser source. Direct or indirect modulation techniques may be employed depending on the type of laser employed. The source output passes through an optical system into the channel. The optical system typically includes transfer, beam shaping, and telescope optics. The receiver beam comes in through the optical system and is passed along to detectors and signal processing electronics. There are also terminal control electronics that must control the gimbals and other steering mechanisms, and servos, to keep the acquisition and tracking system operating in the designed modes of operation. Transmit beam Receive beam To/from host data system From host Power To equipment system groups Figure 1. A block diagram of a typical laser communication terminal. 3. The extremely high antenna gain made possible by the narrow beams enables small telescope apertures to be used. Plots of aperture diameter Vs. data rate for millimeter and optical waves are shown in Fig 2. A laser communications system operating at 1 Gb/s requires an aperture of approximately 30 cm. In contrast, a 1 Gb/s millimeter wave system requires a significantly larger aperture, 2-2.75 m. The laser beam width can be made as narrow as the diffraction limit of the optics allows. This is given by the beam width equal to 1.22 times the wavelength of the light, divided by the radius of the output beam aperture. This antenna gain is proportional to the reciprocal of the beam width squared. The most important point here is that to achieve the potential diffraction-limited beam width given by the telescope diameter, a single-mode high-beam-quality laser source is 4. Required, together with very high-quality optical components throughout the transmitting subsystem. The beam quality cannot be better than the worst element in the optical chain, so the possible antenna gain will be restricted not only by the laser source itself, but also by any of the optical elements, including the final mirror or telescope primary. Because of the requirement for both high efficiency and high beam quality, many lasers that are suitable in other applications are unsuitable for long distance free-space communication. In order to communicate, adequate power must be received by the detector to distinguish signal from noise. Laser power, transmitter optical system losses, pointing system imperfections, transmitter and receiver antenna gains, receiver losses, and receiver tracking losses are all factors in establishing receiver power. The required optical power is determined by data rate, detector sensitivity, modulation formats, noise, and detection methods. 5. When the receiver is detecting signals, it is actually making decisions as to the nature of the signal (when digital signal are being sent it distinguishes between ones and zeros). Fig 3. shows the probability of detection vs. measured photocurrent in a decision time. There are two distributions: one when a signal is present (including the amount of photocurrent due to background and dark current in the detector), and one when there is no signal present (including only the nonsignal current sources). A threshold must be set that maximizes the success rate and minimizes the error rate. One can see that different types of errors will occur. Even when there is no signal present, the fluctuation of the nonsignal sources will periodically cause the threshold to be exceeded. This is the error of stating that a signal is present when there is no signal present. The signal distribution may also fall on the other side of the threshold, so errors stating that no signal is present will occur even when a signal is present. For laser communication systems in general, one wants to equalize these two error types. In the acquisition mode, however, no attempt is made to equalize these errors since this would increase acquisition time.
6. OPERATION Free space laser communications systems are wireless connections through the atmosphere. They work similar to fiber optic cable systems except the beam is transmitted through open space. The carrier used for the transmission of this signal is generated by either a high power LED or a laser diode. The laser systems operate in the near infrared region of the spectrum. The laser light across the link is at a wavelength of between 780 â€œ 920 nm. Two parallel beams are used, one for transmission and one for reception. Figure 4: MAGNUM 45 High-Speed Laser-Communication System (Source: LSA Photonics)
7. ACQUISITION AND TRACKING There are three basic steps to laser communication: acquisition, tracking, and communications. Of the three, acquisition is generally the most difficult; angular tracking is usually the easiest. Communications depends on bandwidth or data rate, but is generally easier than acquisition unless very high data rates are required. Acquisition is the most difficult because laser beams are typically much smaller than the area of uncertainty. Satellites do not know exactly where they are or where the other platform is located, and since everything moves with some degree of uncertainty, they cannot take very long to search or the reference is lost. Instability of the platforms also causes uncertainty in time. In the ideal acquisition method, shown in Figure 4, the beamwidth of the source is greater than the angle of uncertainty in the location of receiver. The receiver field of includes the location uncertainty of the transmitter. Unfortunately, this ideal method requires a significant amount of laser power. 8. It is possible to operate a number of laser types at high peak power and low duty cycle to make acquisition easier. This is because a lower pulse rate is needed for acquisition than for tracking and communications. High peak power pulses more easily overcome the receiver set threshold and keep the false alarm rate low. A low duty cycle transmitter gives high peak power, yet requires less average power, and is thus a suitable source for acquisition. As the uncertainty area becomes less, it becomes more feasible to use a continues source of acquisition, especially if the acquisition time does not have to be short.
9. OPTICAL NOISE Noise characteristics play an important role in laser communication systems. At optical frequencies noise characteristics are significantly different than those at radio frequencies. In the RF domain, quantum noise is quite low, while thermal noise predominates and does not vary with frequency in the microwave region. However, as the wavelength gets shorter, quantum noise increases linearly, and in the laser regime thermal noise drops off very rapidly, becoming insignificant at optical wavelengths. Because there is so little energy in a photon at radio frequencies, it takes many problems to equal the thermal noise. The quantum noise is actually the statistical fluctuations of the photons, which is the limiting sensitivity at optical frequencies. However, in optical receivers employing direct detection and avalanche photodiodes, the detection process does not approach the quantum limit performance. For this type of optical receiver, the thermal noise due to the preamplifier is usually a significant contributor to the total noise power. Free space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. Atmospheric turbulence can degrade the performance of free-space optical links, particularly over ranges of the order of 1 km or longer. Inhomogeneities in the temperature and pressure of the atmosphere lead to variations of the refractive index along the transmission path. These index inhomogeneities can deteriorate the quality of the received image and can cause fluctuations in both the intensity and the phase of the received signal. 10. These fluctuations can lead to an increase in the link error probability, limiting the performance of communication systems. Aerosol scattering effects caused by rain, snow and fog can also degrade the performance of free-space optical communication systems. The primary background noise is the sun. The solar spectral radiance extends from the ultraviolet to the infrared, with the peak in the visible portion of the spectrum. Atmospheric scattered sunlight, sunlit clouds, the planets, the moon, and the Earth background have similar radiances; the sunâ„¢s radiance is much higher and a star fieldâ„¢s much lower. A star field is an area of the sky that includes a number of stars. If one were able to look only at an individual star, one would find a brightness similar to that of the sun; but a star field as a whole is composed of small point sources of light, the stars in the field, against a dark area having no background level. The background is reduced by making both the field of view and the spectral width as narrow as possible. For direct detection systems, narrow field of view spectral filters on the order of 20A*(2 nm) are typical. Heterodyne systems will enable further reduction, but with a increase in terminal complexity. However, some systems can be signal-quantum-noiselimited, rather than background-limited, without having to resort to heterodyne detection.
11. SYSTEM CHARACTERISTICS AND DESCRIPTON Here we discuss specific key system characteristics which, which when quantified, together give a detailed description of a typical laser communication system. Key system characteristics are identified and subsequently quantified for a particular application. In the first part of this section we identify the key parameters that make up a link table listing. In the second part, we will describe how a link analysis is used to provide a description of a laser communications cross-link operating at 10 Mb /s. This low data rate is only used as an example and gives a point of reference for RF systems of similar performance. Key system characteristics or parameters must be identified and quantified to fully describe the system. Critical parameters can be grouped in to five major categories: link, transmitter, channel, receiver, and detector parameters. Free-space laser communications is a very flexible means to connect end users to a high-bandwidth data network via ground-based terminals on top of buildings or to bring a variety of data services to remote locations via satellite terminals in space. External influences on the optical link due to atmospheric turbulence and vibrations in the transmitter's environment require some method of beam control to stabilize the optical link and maintain a high transmission rate. Liquid crystal (LC) optics can provide a compact and low-power solution to beam control in laser communications systems.
12. LINK PARAMETERS The link parameters are the type of laser, wavelength, type of link, and required signal criteria. Although virtually every laser type has been considered at one time of another, today the lasers typically used in free space laser communications system are either semiconductor laser diodes, solid state lasers, or fiber amplifiers/lasers. Laser sources are typically described as operating in either single or multiple longitudinal modes. In single longitudinal mode operation the laser emits radiation at a single frequency, while in multiple longitudinal mode operation multiple frequencies are emitted. Single-mode sources are required in coherent detection systems and typically have spectral widths of the order of 10 kHz- 10MHz. Multimode sources are employed in direct detection systems and typically have spectral widths from 1.5 to 10 nm. Semiconductor lasers have been in development for the three decades and have only recently (within last five years) demonstrated the levels of performance needed for reliable operation as direct sources. Typically operating in the 800-900 nm range (gallium arsenide/gallium aluminum arsenide, GaAs/GaAlAs, material system), their inherently high efficiency (approaching 50%) and small size made this technology attractive. However key issues have been the lifetimes, asymmetric beam shape, and output power. Research into integrated phased arrays proved to be more challenging than first anticipated, forcing the use of single emitters and output powers in the 100-150mW range. Inherent beam combiners employing wavelength-division multiplex or other techniques were employed for those application requiring greater power. 13. Solid state lasers have offered higher power levels and the ability to operate in high peak power modes for acquisition. When diode lasers are used to optically pump the lasing media graceful degradation and higher overall reliability (compared to lamp pumped systems) is achieved. A variety of materials have been proposed for laser transmitters; however, neodymium doped yttrium aluminum garnet (Nd:YAG) is the most widely developed. Operating at 1064nm, these lasers require an external modulator, leading to a slight increase in complexity and reliability. The modulator must be capable of operating at required pulse rates as well as handling the power levels from the laser. With the rapid development of terrestrial fiber communications, a wide array of components are available for potential application in space. These include detectors, lasers, multiplexers, amplifiers, drive electronics, optical preamplifiers, and others. Operating at 1500 nm, erbium doped fiber amplifiers (EDFA) have been developed for commercial optical fiber communications that offer levels of performance consistent with many free-space laser communications applications (500mW range). Issues here revolved around the space qualification of terrestrial components and the desire to achieve as much performance (i.e., laser power) as possible to keep telescope apertures small. There are three basic link types: acquisition, tracking, and communications. The major differences between the link types are reflected in the required signal criteria for each. For acquisition, the criteria are typically the acquisition time, false alarm rate, probability of detection, and, if a multiple detection scheme is used, how many detections m (of the total number possible, n) are required. For the tracking link, the key consideration is the amount of angle error induced by the receiver circuitry. 14. This angle error is commonly referred to as noise effective length (NEA), and depends on the signal-to-noise ratio (SNR), the angular sensitivity of the tracking detector, and the characteristics of the tracking control loops. For the communications link, the key considerations are the required data and bit error rates. Also of prime importance, once a laser type is selected, is the modulation format used to impress information on the laser carrier. A brief description of the required signal calculations for each of the three major link types is given laser in this section. Figure 6. Photo of 1.55-_m high power diode laser FSO system by Terabeam.
15. TRANSMITTER PARAMETERS The transmission parameters consist of certain key laser characteristics, losses incurred in the transmit optical path, transmit antenna gain, and transmit pointing loss. The key laser characteristics include peak and average optical power, pulse rate, and pulse width. In a pulsed configuration the peak laser power and duty cycles are specified, while in continues-wave applications the average power is specified. In a pulsed application the pulse rate and width describe the laserâ„¢s temporal performance. In continues-wave applications, such as coherent communication employing frequency shift keying (FSK) or phase shift keying (PSK), the pulse rate and width describe the symbol rate and symbol duration of the data impressed on the laser carrier. Transmit optical path loss is made up of optical transmission losses and loss due to the wave-front quality of the transmitting optics, degrading the theoretical far-field on-axis gain. The wave front error loss is analogous to the surface roughness loss associated with RF antennas. The optical transmit antenna gain is exactly analogous to the antenna gain in RF systems, and describes the on-axis gain relative to an isotropic radiator with the distribution of the transmitted laser radiation defining the transmit antenna gain. 16. The laser sources suitable to the free-space laser communications trend to exhibit a Gaussian intensity distribution in the main lobe. The reduction in the far-field signal strength due to transmitter mispointing is the transmitter pointing loss. For each link in a laser system, a pointing budget must be determined. The pointing budget is typically composed of bias (slowly varying) and random (more rapidly varying) components. The bias components are the alignment and detector gain mismatch errors; the random components are the NEA and residual error due to base motion disturbances. For a system employing a Gaussian beam, where the pointing loss is predominantly a bias, the on-axis transmitted gain-pointing loss product is maximized when the1/e2 beamwidth is set equal to approximately 2.8 times the pointing error. Increasing antenna diameter further (decreasing the 1/e beamwidth) will degrade performance. When pointing error is a combination of bias and random terms, a somewhat more complex expression must be evaluated. The point to stress here is that once the pointing error is determined, the system beamwidth must be sized appropriately.
17. CHANNEL PARAMETERS The channel parameters for an optical intersatellite link (ISI) consist of the range and associated loss, background spectral radiance, and spectral irradiance. Since this article deals with ISLs, losses due to the atmosphere are not considered. These losses can be quite large and mitigation of the effects complex. The range loss is simply RL = (l/(4pR))2, where R is the separation between the two platforms in meters, and l is the wavelength. The background level depends on the relative altitudes of the platforms, the time of the year, and the wavelength selected.
18. RECEIVER PARAMETERS The receiver parameters are the receiver antenna gain, the receiver optical path loss, the optical filter bandwidth and the receiver field of view. The receiver antenna gain is given by GR = (pDR/l)2 where D is the effective receiver diameter diameters in meters. The receiver optical path loss is simply the optical transmission loss for systems employing direct detection techniques. However, for laser systems employing coherent optical detection (either homodyne or heterodyne) there is an additional loss due to wavefront error. The preservation of the wavefront quality is essential for optical mixing of the received signal and local oscillator fields on the detector surface. To first order, the loss expression is the same as that previously defined for the transmit wavefront error. The optical filter bandwidth specifies the spectral width of the narrow-bandpass filter employed in optical intersatellite links. Optical filter reduce the amount of unwanted background entering the system. The optical width of the filter must be compatible with the spectral width of the laser source. In addition to source considerations, the minimum width also be determined by the acceptable transmission level of the filter; typically the transmission of the filter decreases with spectral width. 19. The final receiver parameter to be discussed is the angular field of view (FOV), in radians, which limits the background power of an extended source incident on the detector. To maximize background rejection, the FOV should be as small as possible, since for the typically small angles considered (< 1 mrad) the background power incident on the detector is proportional to FOV. However, the minimum FOV is limited by optical design constraints and the receiver pointing capability. Optical design constraints require the FOV to satisfy the expression FOV = DD/FL, where DD is the detector diameter in meters and FL is the system focal length in meters (both DD and FL is limited by the practical considerations). The receiver FOV must be greater than the receiver pointing capability so that the received signal spot falls onto the detector surface.
20. DETECTOR PARAMETERS The detector parameters are the type of detector, gain of the detector (if any), quantum efficiency, heterodyne mixing efficiency (for coherent detection only), noise due to the detector, noise due to the following preamplifier, and (for track links) angular sensitivity or slope factor of the detector. For optical ISLs based on semiconductor laser diodes or Nd: YAG lasers, the detector of choice is a p-type-intrinsic-n-type (PIN) or an avalanche photodiode (APD). A PIN photodiode can be operated in the photovoltaic or photoconductive mode, and has no internal gain mechanism. An APD is always operated in the photoconductive mode and has internal gain by virtue of the avalanche multiplication process. At shorter wavelengths (810-900 nm) PINs and APDs made of silicon show the best response, but at longer wavelengths (1300- 1550 nm) InGaAs and Ge APDs have significantly more excess noise than comparable silicon devices. For application requiring gain and operating at Nd: YAG wavelengths, a silicon APD is typically preferred because of its internal gain. However, if gain is not required an InGaAs PIN would be preferred because of the higher quantum efficiency. The quantum efficiency, h, of the detector is the efficiency with which the detector converts incident photons to electrons. 21. The mean output current for both PINs and APDs is proportional to the quantum efficiency. By definition, quantum efficiencies are always less than unity. For silicon detectors operating at GaAlAs wavelengths, h = 0.85-0.9, while at the Nd: YAG wavelength h may be only 0.4. For InGaAs detectors, operated at InGaAsP and Nd: YAG wavelengths, h is about 0.8. Another detector parameter to consider is the noise due to the detector alone. Typically, in detector there is a DC current even in the absence of signal or background. This DC dark current, as it is commonly called, produces a shot-noise current just as the signal and background currents do. In an APD there are two contributors to the total dark current: an unmultiplied current and a multiplied current. The multiplication is provided by the avalanche gain mechanism and, as expected, for typical operating gains (>50) the multiplied term is dominant. In a PIN photodiode there is only the unmultiplied term. The output of the detector is input to a preamplifier that converts the detector signal current into a voltage and amplifies it to a workable level for further processing. Being the first element past the detector, the noise due to the preamplifier have a significant effect on the systemâ„¢s sensitivity. The selection of preamplifier design (transimpedance or high impedance), internal transistor design (bipolar or FET), and device material (GaAs or silicon) depends on a number of factors. Transimpedance designs have greater dynamic range, but are nominally less sensitive than high-impedance designs. Silicon bipolar transistors may come from a more mature technology, but GaAs FETs have a higher bandwidth capability and are inherently radiation resistant.
22. AN EXAMPLE Here we give a simple example of hoe the parameters just described are used in link analysis to design a laser communications system capable supporting a full duplex 10 Mb/s geosynchronous orbit crosslink. The detailed link analysis is not covered in this article but employs all of the element described above. To size the system, however, a link analysis for the communications function was performed. The source peak power requirement, 3 dB of the system margin, was determined to be 0.6 W. A semiconductor laser diode beam combiner is assumed for the transmitter source employing four lasers at 150 mW each. A 5 in aperture was determined to produce a beamwidth compatible with the fine-track pointing budget of 4.0 mrad. The pointing budget was determined by assuming a tracking system employing both fine-steering mirrors and a gimballed telescope. The transmit and receive optics efficiencies are representatives of nominal values achievable totally in similar systems. The peak received signal power was determined to be 1.64 nW from the assumed parameter values given. 23. The diode laser source is modulated directly in a Manchester modulation format by changing the drive current to the diodes. The link employs a rate Ã‚Â½, constraint length 7 convolutional code with Viterbi decoding and hard decisions. This permits the link to operate at a higher channel symbol error rate (0.014), but still produce a decoded bit error rate of 10-6 . the code employed yields approximately 2 dB of coding gain for direct detection laser communications link. A quadrant APD was selected as the detector because of its compactness, high reliability, and high sensitivity (compared to a PIN photodiode). The desired communications signal was obtained by summing the four quadrants. It is assumed that 0.6 W of laser power is adequate to support the acquisition and track functions. This example is representative of a typical laser communications system for satellite applications.
24. APPLICATIONS Depending on the climatic zone where the free space laser communications systems are used, they can span distances up to 15 km at low bitrates or provide bitrates up to 622 Mbps at shorter distances. The systems are protocol transparent allowing transmission of digital computer data (LAN interconnect), video, voice over IP, multiplexed data, or ATM. They are suitable for temporary connectivity needs such as at conventions, sporting events, corporate and university campuses, disaster scenes or military operations.
25. ADVANTAGES AND DISADVANTAGES Free space laser communications links eliminate the need for securing right of ways, and buried cable installations. As the equipments operate within the near infrared spectrum, they are not subject to government licensing and no spectrum fees have to be paid (according to Art. 7 in  requires only the use of the frequency spectrum below 3â„¢000 GHz a licence). Additionally, since no radio interference studies are necessary, the systems are quickly deployable. The narrow laser beamwidth precludes interference with other communication systems of this type. Free space laser communications systems provide only interconnection between points that have direct line-of-sight. They can transmit through glass, however, for each glass surface the light intensity is reduced, due to a mixture of absorption and refraction, thus reducing the operational distance of a sys-tem. Occasionally, short interruptions or unavailability events lasting from some hours up to a few days can occur.
26. CONCLUSIONS The system and component technology necessary for successful intersatellite laser communication link exist today. The growing requirements for efficient and secure communications has led to increased interest in the operational deployment of laser crosslinks for commercial and military satellite systems in both low earth and geosynchronous orbits. With the dramatic increase in the data handling requirement for satellite communication services, laser intersatellite links offer an attractive alternative to RF with virtually unlimited growth potential and an unregulated spectrum. The demonstration programs underway in the United States, Europe, and Japan will show the way for future large-scale applications of laser communications to satellite cross-links.
27. REFERENCES 1. IEEE communications Magazine. August 2000, free space laser communications :Laser cross-link systems and technology by: David L. Begley, Ball Aerospace & technologies corporation 2. Chaotic Free-Space Laser Communication over a Turbulent Channel By: N. F. Rulkov,1 M. A. Vorontsov, and L. Illing institute for Nonlinear Science, University of California, San Diego, La Jolla, California 92093 Army Research Laboratory, Adelphi, Maryland 20783 3. Free Space Optics or Laser Communication through the Air BY: Dennis Killinger Optics & Photonics News Ã‚Â¦ October 2002 4. High data-rate laser transmitters for free-space laser Communications. BY:A. Biswas, H. Hemmati and J. R. Lesh Optical Communications Group Jet Propulsion Laboratory, California Institute of Technology 28.
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please read publik.tuwien.ac.at/files/pub-et_4235.pdf for more of freespace optics
please read topicideashow-to-freespace-optics-full-report and topicideashow-to-optical-satellite-communication-seminar and presentation-report and topicideashow-to-future-satellite-communication--5509 and topicideashow-to-optical-satellite-communication-download-full-report-and-abstract for getting Laser Communication Systems or Free Space Laser Communications or future satellite communication seminar and presentation informations
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NEXT GENERATION WIRELESS COMMUNICATION- FREE SPACE OPTICS (FSO)
Shyam Dikshit 2/4 B.Tech B. Neha Singh 2/4 B.Tech
AURORAâ„¢S ENGINEERING COLLEGE,BHUVANGIRI,NALAGONDA DIST.
This paper deals with communication through optics using one of the latest technologies called the free space optics (FSO).
FSO may sound new and experimental but in fact it predates optical fiber and has its roots in wartime efforts to develop secure communication systems that did not require cable and could withstand radio jamming.
As a commercial communications technology, FSO has been around for more than a decade, but it is only recently that interest in this technology has started to grow. It was only recently developed for use in metropolitan area networks. The technology has its roots in military applications that reach back as far as the 1940s.
It was not until the 1960s, however, that the first significant FSO technology advancements began to occur in the United States, Europe and Middle East, where military researchers, engineers and technicians applied the use of infrared lasers in communications devices with the aim of providing secure data and voice transmission that would not be susceptible to jamming of radio frequency-based communications systems.
These early FSO systems were capable of transmitting merely a handful of kilobits over the air, but the advent of the Internet and its impact on telecommunications was decades away. In fact, European researchers of FSO systems in the 1960s experimented with ways to send FSO signals through both underground and underwater pipes, seeking to bend the invisible light beams with mirrors where a straight line-of-site could not be established.
FUNDAMENTALS OF FREE SPACE OPTICS
FSO is an optical wireless, point-to-point, line-of-sight broadband solution.
a) Lasers Through Free Space
FSO is an optical technology and simple concept involving the transmission of voice, video and data through the air using lasers. It is not a disruptive technology; it is more of an enabling technology that promises to deliver that ever-eluding high-speed optical bandwidth to the ultimate end users. FSO offers many advantages when compared to fiber. It is a zero sunk-costs solution. The principle advantages of free space optics (FSO) are:
1. Significantly lower cost on average than the build out of a new fiber optical solution, or leased lines
2. FSO can be deployed in days to weeks vs. months to years
3. Bandwidth can easily be scaled (10 Mbs to 1.25 Gbps) per link
As opposed to fiber, FSO can be redeployed if the customer moves or cancels service. It is also a fraction of the cost and time, allowing carriers to generate revenue, while also taking advantage of the high capacity of optical transmissions. FSO allows service providers to accelerate their deployment of metro optical networks as well as extend the reach of such optical capacity to anyone who needs it.
b) FSO: Optical or Wireless
FSO systems share several characteristics with fiber optics. FSO can use the same optical transmission wavelengths as fiber optics, namely 850nm and 1550nm and they use the same components such as lasers, receivers and amplifiers. Some systems already include fiber connections inside the transmission link heads, to separate electronics and optics. Similar to fiber optics, FSO systems also target the high-bandwidth market. However, while fiber optics can be used over longer distances, FSO targets shorter distances due to the variability of the terrestrial atmosphere as a transmission medium.
One common feature of FSO equipment commercially available today is that most of these systems perform optical to electrical back to optical (O-E-O) conversion steps in the process of sending and receiving information through the air and connecting back to the attached networking interface fiber. This feature does not automatically constitute a performance limitation, but O-E-O conversion can impact the ability to scale an FSO system easily to ultra-high bandwidth capabilities. The fiber optic communications industry realized from the start the importance of an all-optical system approach, as higher backbone capacity â€ along with wavelength division multiplex technology. An important breakthrough to reach this goal occurred when fiber systems with erbium doped fiber amplifier (EDFA) became commercially available. It was then, that the concept carrying multiple wavelengths over a single piece of optical fiber achieved commercial attention. The invention of EDFA amplifier technology paved the way for optical transmission at multiple wavelengths over longer distances without the need to perform expensive O-E-O conversion and separate electrical amplification of each specific wavelength at every repeater station.
c) Bandwidth Drivers/Trends
The push to build more high-speed networks was spurred by unprecedented growth in bandwidth usage. Telecommunications carriers will implement multiple technologies in their networks and will use the best access technology for the particular situation. The chart below shows how these technologies address different market segments based on technology, technical capabilities (reach, bandwidth), and economic realities.
A number of compelling factors are influencing this bandwidth surge:
HOW IT WORKS
FSO technology is surprisingly simple. It's based on connectivity between FSO-based optical wireless units, each consisting of an optical transceiver with a transmitter and a receiver to provide full-duplex (bi-directional) capability. Each optical wireless unit uses an optical source, plus a lens or telescope that transmits light through the atmosphere to another lens receiving the information. At this point, the receiving lens or telescope connects to a high-sensitivity receiver via
FSO SYSTEM DESIGN ISSUES
a) Free-Space Optics Subsystems
Fig.2.FSO Major Subsystems
Figure2 illustrates the major subsystems in a complete carrier-grade free-space optics communications system. The optical apertures on a free-space system can have an almost infinite variety of forms and some variety of function. They can be refractive, reflective, diffractive, or combinations of these. In figure2, the transmit, receive, and tracking telescopes are illustrated as separate optical apertures; there are several other configurations possible where, for example, a single optic performs all three functions thereby saving cost, weight, and size. On the transmit side, the important aspects of the optical system are size and quality of the system. Size determines the maximum eye-safe laser flux permitted out of the aperture and may also prevent blockages due to birds. Quality, along with the f-number and wavelength, determine the minimum divergence obtainable with the system. On the receive side, the most important aspects are the aperture size and the f-number. The aperture size determines the amount of light collected on the receiver and the f-number determines the detector's field of view. The tracking system opticsâ„¢ field of view must be wide enough to acquire and maintain link integrity for a given detector and tracking control system.
Several means are available for coupling the laser to the output aperture. If a discrete
diode is used; the diode is usually micro-lensed to clean up the astigmatism of the output beam and then is free-space coupled to the output aperture by placing the laser at the focus of the output aperture optical system. The distance from the laser aperture to the output aperture must be maintained such that the system divergence remains in specification over the temperature ranges encountered in an outdoor rooftop environment. This can be accomplished with special materials and/or thermal control.
Diode lasers are driven with a DC bias current to put the devices above threshold, and then, on top of that, are modulated with an AC current to provide, for example, on/off keying (OOK) for data transmission. For lasers with output powers below approximately 50 mW, off-the-shelf current bias and drive chips are available; for higher power lasers, custom circuits or RF amplifiers are generally used. The receive detector is coupled to the receive aperture through either free-space or fiber. Depending on the data rate and optical design alignment, tolerances can be extremely restrictive. For example, for data rates to 1.25 Gbit/s, detectors with relatively large active areas (500-micron diameter) can be used, making alignment to the receive aperture fairly straightforward. For fiber-optic coupling into multimode fibers, the correct size is about 63 microns in diameter, which makes alignment much tougher.
Detectors are generally either PIN diodes or avalanche photodiodes (APD). For carrier class free-space optics systems, an APD is always advantageous since atmospheric induced losses can reduce received signals to very low levels where electronics noise dominates the signal-to-noise (SNR) ratio. Of course the APD must be capable of meeting the system bandwidth requirements. Usually a trans-impedance amplifier is used after the detector because in most cases they provide the highest gain at the fastest speed.
b) Bit Error Rate, Data Rates, and Range
In figure3, which depicts a set of buildings in Denver, Colorado, the effects of fog on visibility range are illustrated. The tall building in the foreground is about 300 m from the photographer. The left photo shows clear air, at 6.5 dB/km (2000 m visibility range), as measured with a nephelometer mounted at the photographer's site. The distant mountain ranges are easily visible at many miles distance. During a fog which measured about 150 dB/km (visibility range of about 113 m), as shown in the middle photo, the building is still visible at 300 m, but the scenery is washed out beyond this range. As shown in the right photo, at 225 dB/km (visibility range of about 75 m) the building is completely obscured.
Fig.3. Denver, Colorado Fog/Snowstorm Conditions
Table1: Environmental Attenuation
FSO SYSTEMS AND NETWORK SECURITY
FSO systems operating in the near infrared wavelength range do not require licenses worldwide for operation. FSO system installations are very simple, and the equipment
requires little maintenance. Because FSO systems send and receive data through the airâ€or the free space between remote networking locationsâ€network operators and administrators are concerned about the security aspect of this technology.
Such concerns are not valid for FSO systems. FSO systems operate in the near infrared wavelength range slightly above the visible spectrum. Therefore, the human eye cannot visibly see the transmission beam. The wavelength range around 1 micrometer that is used in FSO transmission systems is actually the same wavelength range used in fiber-optic transmission systems. The wavelength range around 1 micrometer translates into frequencies of several hundred terahertz (THz) which is higher than that used in commercially available microwave communications systems operating around 40 GHz. FSO systems use very narrow beams that are typically much less than 0.5 degrees. E.g., a radial beam pattern of 10 degrees roughly corresponds to a beam diameter of 175 meters at a distance of 1 kilometer from the originating source, whereas a beam of 0.3 degrees divergence angle typically used in FSO systems corresponds to a beam diameter of 5 meters at the same distance1. This wide spreading of the beam in microwave systems, combined with the fact that microwave antennas launch very high power level is the main reason for security concern. To overcome security concerns, the microwave industry uses wireless encryption protocols (WEP) to protect the transmission path from being intercepted.
Fig.4. Example of beam spot diameters at various distances for a beam divergence angle of 4 mrad.
The interception of FSO systems operating with narrow beams in the infrared spectral wavelength range is by far more difficult. The small diameter of the beam of typically only a few meters in diameter at the target location is one of the reasons why it is extremely difficult to intercept the communication path of an FSO system
The intruder must know the exact origination or target location of the (invisible) infrared beam and can only intercept the beam within the very narrow angle of beam propagation. Fig.4. shows an actual example of a 4 m rad beam project and implimentationed onto the target location where the opposite terminal is located. At a distance of 300 meters the beam diameter is about 1.3 meters, while at a distance of 1 kilometer the beam expands to 4 meters.
The direct interception of an FSO beam between the two remote networking locations is basically impossible because the beam typically passes through the air at an elevation well above ground level. Due to the fact that the transmission beam is invisible and that any attempts to block the beam would occur near the FSO equipment terminus points, the transmission process imposes another obstacle.
Picking up the signal from a location that is not directly located within the light path by using light photons scattered from aerosol, fog, or rain particles that might be present in the atmosphere is virtually impossible because of the extremely low infrared power levels used during the FSO transmission process.
The main reason for excluding this possibility of intrusion is the fact that light is scattered isotropic ally and statistically in different directions from the original propagation path. This specific scattering mechanism keeps the total number of photons or the amount of radiation that can potentially be collected onto a detector that is not directly placed into the beam path well beyond the detector noise level. Fig 5 illustrates the physics of this scattering mechanism.
Fig .5 . Illustration of the physics of the light scattering mechanism while the light beam travels from the originating laser sources (left) to the receiver at the opposite communication location.
The key drivers for FSO: market, economic, service, business and environment are as shown
a) MARKET DRIVERS
Increasing Number of Internet Users/Subscribers
Increasing E-Commerce Activities
Deployment of 3G and 4G
b) ECONOMIC DRIVERS
Faster Service Activation
Ultra-scalability and Bandwidth Allows for Lower Inventory Costs
Multiple Applications/Services Support
c) SERVICE DRIVERS
Increasing Demand for High-Speed Access Interfaces
Need to Eliminate the Metro Gap
Need for Real Time Provisioning
FSO CORE APPLICATIONS
Common applications of FSO include:
Metro Network Extensions
FSO can be deployed to extend an existing metro ring or to connect new networks.
These links generally do not reach the ultimate end user, but are more an application for the core of the network.
The flexibility of FSO allows it to be deployed in many enterprise applications, including LAN-to-LAN connectivity, storage area networking and intra-campus connections. FSO can be deployed in point-to-point, point-to-multipoint, ring or mesh connections.
FSO may also be deployed as a redundant link to back-up fiber. Most operators deploying fiber for business applications connect two fibers to secure a reliable service plus backup in the event of outage. Instead of deploying two fiber links, operators can deploy an FSO system as the redundant link.
With the integration of WDM and FSO systems, independent players aiming to build their own fiber rings may use FSO to complete part of the ring. Such a solution could save rental payment to Incumbent Local Exchange Carriers (ILECs), which are likely to take advantage of this situation.
FSO performance can be affected by some conditions:
Weather severity at which FSO signal attenuation can be impacted
Rain at 6 inches per hour, Wet snow rate of 4 inches per hour, Dry snow rate of
2 inches per hour, Fog with visibility of < 6% of the transmission distance
Birds can temporarily block the beam, but this tends to cause only short interruptions and transmissions are easily resumed.
Building sway/seismic activity:
The movement of buildings can upset receiver and transmitter alignment. Light Pointeâ„¢s FSO-based optical wireless offerings use a divergent beam to maintain connectivity. When combined with tracking, multiple beam FSO-based systems provide even greater performance and enhanced installation simplicity.
Heated air rising from the ground creates temperature variations among different air pockets. This can cause fluctuations in signal amplitude which lead to image dancing at the receiver end.
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.
The safety of FSO is often a concern, since it uses lasers for transmission. This challenge has more to do with perception than reality.
The two major concerns typically expressed involve questions about human exposure to laser beams and high voltages within the laser systems and their power supplies. Several standards have been developed covering the performance of laser equipment and the safe use of lasers.
Safety of the lasers does not depend on its frequency, but rather on the classification of the laser. There are two primary classification bodies, the CDRH and the IEC. Commercial systems on the market today are compliant with both standards.
Beam wander is caused by turbulent eddies that are larger than the beam.
Beam spreading â€ long-term and short-term â€ is the spread of an optical beam as it propagates through the atmosphere.
FSO equipment currently is being deployed for a variety of applications, such as last-mile connections to buildings, which may provide the greatest opportunity since FSO provides the high-speed links that customers need without the costs of laying fiber to the end user. In 2005, last-mile access will represent over two-thirds of the total FSO equipment market.
FSO allows them to provide this optical connectivity cost effectively, quickly and reliably. Such flexibility makes FSO systems an extremely attractive method for service providers to truly solve the connectivity bottleneck. Free-Space Optics communication systems are among the most secure networking transmission technologies.
To intercept this invisible light beam, the intruder must be able to obtain direct access to the light beam. Carriers, like Allied Riser and XO Communications, may use FSO in conjunction with other technologies to expand their current networks while others, such as Terabeam, see the technology as a means to break into the broadband market."
1. Mendelson, James S. and Dorrier, Charles R. Free Space Optics: Fixed Wireless
2. White, Chad. How to Squeeze More Data Over Whatâ„¢s Already There
3. Smith, Brad. "Going the Last Mile" Wireless Week
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FREE SPACE OPTICS.ppt (Size: 3.02 MB / Downloads: 335)
This article is presented by:
IV yr (VIII sem)
AN OPTICAL COMMUNICATION SYSTEM WITHOUT FIBERS or in other words ‘’WIRE FREE OPTICS’’.
FSO is a line-of-sight technology that uses invisible beams of light to provide optical bandwidth connections that can send and receive voice, video, and data information.
Infrared ( IR) beams , laser beams, light-emitting diodes, IR-emitting diodes (IREDs) is used as optical source for transmission.
In the 1880, Alexander Bell expanded his "photo-phone" communication which modulated by sunlight.
BIRTH OF LASER FSO COMMUNICATIONS
In the mid-1960's NASA initiated experiments to utilize the laser as a means of communication between the ground and space.
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FREE SPACE OPTICS.doc (Size: 1.58 MB / Downloads: 160)
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 century 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 1970.
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.
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 obtaining government licenses for new fiber installation etc. are further problems that have 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.
Free space optics (FSO) has been used for more than a decade as a short/medium distance point-to-point (building-to-building) connectivity solution in campus enterprise LAN markets. The license free nature of this technology combined with its high-speed bandwidth capabilities, comparable to optical fiber, allow network administrators to interconnect LAN segments at real networking speeds (e.g. 100 Mbps or 1000 Mbps) without the hastle of digging to install optical fiber. Since digging to install fiber is typically a very expensive and time-consuming process, the value proposition of using FSO can be very appealing. Only recently has the carrier market started to look into FSO technology as an alternative network connectivity solution. However, when considering the carrier market, the requirements in terms of component reliability and overall weather related system availability are much more stringent than system requirements in the enterprise market. This paper addresses some of the issues that are most important in the design of an overall carrier system architecture. Briefly described are the basic physics of transmission at various short and long infrared wavelengths and their overall impact on the system design. This is followed by an overview of basic transmitter and detector technologies. When selecting suitable components, reliability and commercial availability of those components should play an important factor. Eye safety is another factor that has to be taken into consideration in a carrier class system design. Finally, the link budget will determine the overall system availability under various weather conditions. This aspect is discussed near the close of this document.
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FREE_SPACE_OPTICS.DOC (Size: 187 KB / Downloads: 48)
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.
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Free Space Optics (FSO) or Optical Wireless
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.
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Free Space Optics (FSO)??
FSO is a line of sight technology that uses devices such as lasers to establish connectivity for video and voice communication
Currently, It can allow upto 2.5 Gbps of data rate but can be increased to 10 Gbps using WDM
FSO is based on connectivity between two stations consisting of optical transceiver to achieve full duplex communication
Technology: How it works?
FSO is a pretty old and relatively simple technology which has been used for more than 30 years. FSO enables similar bandwidth transmission abilities as fiber optics, using similar optical transmitters and receivers
It involves two FSO units (similar to BTS of wireless technology) each consisting of high power laser transmitters and receivers, a telescope is used in conjunction to guide the light and capture it at the receiving end. It is then typically interface with network switches, hub, bridge or router via multimode fiber
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01-11-2010, 12:28 PM
fso communication.ppt (Size: 1.84 MB / Downloads: 113)
Free Space Optics (FSO)
WHAT IS FSO ???
FSO is communication technique in which line-of-sight technology is used. It uses invisible beams of light to provide optical
It is like enabling fiber-optic connectivity without requiring physical fiber-optic cable. It enables optical communications at the speed of light.
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10-03-2011, 11:07 AM
FOS.ppt (Size: 410.5 KB / Downloads: 116)
Free Space Optics (FOS)
WHAT IS FREE SPACE OPTICS (FSO)?
->The Technology at the Heart of Optical Wireless
Imagine a technology that offers full-duplex Gigabit Ethernet throughput.
A technology that can be installed license-free worldwide, can be installed in less than a day. A technology that offers a fast, high ROI.That technology is free-space optics (FSO).
How it Works
FSO technology is surprisingly simple. It's based on connectivity between FSO-based optical wireless units, each consisting of an optical transceiver with a transmitter and a receiver to provide full-duplex (bi-directional) capability. Each optical wireless unit uses an optical source, plus a lens or telescope that transmits light through the atmosphere to another lens receiving the information. At this point, the receiving lens or telescope connects to a high-sensitivity receiver viaoptical fiber.
This FSO technology approach has a number of advantages:
Requires no RF spectrum licensing.
Is easily upgradeable, and its open interfaces support equipment from a variety of vendors, which helps enterprises and service providers protect their investment in embedded telecommunications infrastructures.
Requires no security software upgrades.Is immune to radio frequency interference or saturation.
Can be deployed behind windows, eliminating the need for costly rooftop rights.
FSO: Optical or Wireless?
Speed of fiber — flexibility of wireless.
Optical wireless, based on FSO-technology, is an outdoor wireless product category that provides the speed of fiber, with the flexibility of wireless. It enables optical transmission at speeds of up to 1.25 Gbps and, in the future, is capable of speeds of 10 Gbps using WDM. This is not possible with any fixed wireless or RF technology. Optical wireless also eliminates the need to buy expensive spectrum (it requires no FCC or municipal license approvals worldwide), which further distinguishes it from fixed wireless technologies. Moreover, FSO technology’s narrow beam transmission is typically two meters versus 20 meters and more for traditional, even newer radio-based technologies such as millimeter-wave radio. Optical wireless products' similarities with conventional wired optical solutions enable the seamless integration of access networks with optical core networks and helps to realize the vision of an
While fiber-optic cable and FSO technology share many of the same attributes, they face different challenges due to the way they transmit information. While fiber is subject to outside disturbances from wayward construction backhoes, gnawing rodents, and even sharks when deployed under sea, FSO technology is subject to its own potential outside disturbances. Optical wireless networks based on FSO technology must be designed to combat changes in the atmosphere, which can affect FSO system performance capacity. And because FSO is a line-of-sight technology, the interconnecting points must be free from physical obstruction and able to "see" each other.
All potential disturbances can be addressed through thorough and appropriate network design and planning. Among the issues to be considered when deploying FSO-based optical wireless systems:
4. Physical obstructions
5. Building sway/seismic activity
Typically scenarios for use are:
* LAN-to-LAN connections on [[campus]]es at [[Fast Ethernet]] or [[Gigabit Ethernet]] speeds.
* LAN-to-LAN connections in a [[city]]. ''example, [[Metropolitan area network]]''.
* To cross a public road or other barriers which the sender and receiver do not own.
* Speedy service delivery of high-bandwidth access to [[optical 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]].
3rd Workshop on Chip Multiprocessor Memory Systems and Interconnects held in conjunction with
the 36th International Symposium on Computer Architecture, June 2009.chip communication.
The light beam can be very narrow, which makes FSO hard to intercept, improving security. In any case, it is comparatively easy to encrypt any data traveling across the FSO connection for additional security. FSO provides vastly improved (Electromagnetic interference|EMI) behavior using light instead of microwaves.
== Advantages ==
* Ease of deployment
*( Broadcast license|License)-free long-range operation (in contrast with radio communication)
* High bit rates
* Low bit error rates
* Immunity to electromagnetic interference
* Full Duplex (telecommunications)duplex operation
* Communications protocol transparency
* Very Computer security due to the high directionality and narrowness of the beam(s)
* No Fresnel zone necessary
For terrestrial applications, the principal limiting factors are:
-> Beam dispersion
-> Atmospheric absorption
-> Fog (10..~100 dB/km attenuation)
-> Background light
-> 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.
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15-03-2011, 02:06 PM
FREE SPACE OPTICS COMMUNICATION
Free Space Optics communication is also called as Free Space Photonics or Optical Wireless Communication. FSO uses LASER to transmit data , but instead of enclosing the data stream in a glass fiber , it is transmitted through air.
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 ends of the link.
he near infrared region of the spectrum. The laser light across the link is at a wavelength of between 780 - 920 nm. Two parallel beams are used, one for transmission and one for reception.
Working of FSO
Free Space Optics transmits invisible eye safe light beams from a transmitter part (source) to receiver part (destination) using low power infrared LASERs in the terahertz spectrum. The beams of light in FSO systems are transmitted by LASER light focused on highly sensitive photon detector receivers. Commercially available systems offer capacitances in the range of 100Mbps to 2.5Gbps and demonstration systems report data rates as high as 160Gbps. FSO systems can function over distances of several kilometers. As long as there is a clear line of sight between the source and the destination and enough transmitter power, FSO communication is possible.
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15-03-2011, 04:22 PM
This is the basic about it. 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. This is the main advantage of it.
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29-03-2011, 09:17 AM
Our topic for the seminar and presentation in FSO ie Free Space Optics. This is a new technology that is expanding in recent times in the field of digital communication.
Under this Topic we shall take you through the following sections
• Intro to optical comm. their advantages & disadvantages
• Intro to FSO
• Relevance of FSO in present day communication
• Origin of FSO
• The technology of FSO
• Working of FSO system
• Application of FSO
• Merits and demerits of FSO
• FSO ! As a future technology
Communication as we all know is a process of sending information from source to sink via a channel. Optical comm. Uses optical fibre made of glass as the comm. channel. Optical fiber works on the principle of total internal reflection..
The transmitters in optical fiber links are generally light-emitting diodes (LEDs) or laser diodes. Infrared light, rather than visible light is used more commonly, because optical fibers transmit infrared wavelengths with less attenuation and dispersion. They find wide application in Telephone system, cable TV system, and also internet fiber optic lines along with commercial application.
Their Advantages over electrical comm. May include:
• Higher bandwidth (extremely high data transfer rate).
• Less signal degradation.
• Optical fibers are resistive to corrosion
• Lighter and thinner then copper wire.
• Immunity to tapping information
• Immunity to cross talk
• High information carrying capacity
• High investment cost
• Need for more expensive optical transmitters and receivers
• More difficult and expensive to splice than wires
• At higher optical powers, is susceptible to "fiber fuse"
• Affected by chemicals - The glass can be affected by various chemicals including hydrogen gas (a problem in underwater cables.)
• Less flexible
• Point to point comm
Therefore we opt for a better solution ie FSO “The Technology at the Heart of Optical Wireless”
Free Space Optics communications, also called Free Space Photonics (FSP) or Optical Wireless, refers to the transmission of visible and infrared (IR) beams through the atmosphere to obtain optical communications. Like fiber, Free Space Optics uses lasers to
transmit data, but instead of enclosing the data stream in a glass fiber, it is transmitted through the air.
• FSO is a line-of-sight technology in which fiber is replaced by free space
• FSO can transmit data, voice or video with speed capable of reaching 2.5Gbytes which is far superior to fiber comm.
• It is bidirectional or full duplex comm. In which info can pass from both as well as sink
• FSO systems use invisible infrared laser light wavelengths in the 750nm to 1550nm range.
• 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.
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05-04-2011, 12:51 PM
please send the report on free space laser communication