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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.