The European Space Agency is going to conduct an inter orbit link experiment which will connect a low Earth orbiting satellite and a Geostationary satellite via optical terminals. This experiment has been called SILEX (Semiconductor Inter satellite Link Experiment). Two payloads have been built. One called PASTEL (PASsager de TELecommunication) has been embarked on the French Earth observation satellite SPOT4 which has been launched successfully in March 1998. The future European experimental data relay satellite ARTEMIS (Advanced Relay and TEchnology MISsion), which will route the data to ground, will carry the OPALE terminal (Optical Payload Experiment). The European Space Agency is responsible for the operation of both terminals. Due to the complexity and experimental character of this new optical technology, the development, preparation and validation of the ground segment control facilities required a long series of technical and operational qualification tests. This paper is presenting the operations concept and the early results of the PASTEL in orbit operations.

The results from a 13.8 km horizontal laser communication (lasercom) link between the Space Technology Research Vehicle 2 (STRV-2) ground terminals are presented. These terrestrial tests are a precursor to the STRV-2 satellite-to-ground lasercom experiment scheduled to commence shortly after launch in June of 1999. Testing ground terminal performance through long terrestrial horizontal ranges (greater than 10 km) mimics the atmospheric effects of an uplink slant path to the low earth orbit (LEO) satellite. Bit error rates averaging 10^-6 to 10^-7 were achieved with only one of the three transmit telescopes operating. The terrestrial path also passed very close to a ridge, which most likely added additional ground scintillation. We expect much improved link performance once all three transmit telescopes are operational. These long-range horizontal tests will assist in the development of a long-range commercial terrestrial lasercom product, which will be baselined on the ground terminal design.

The Optical Communications Demonstrator (OCD) is a laboratory based lasercom terminal that was developed to validate several key technologies such as precision beam pointing, high bandwidth beacon tracking and beacon acquisition. The unique architecture of OCD uses a single focal plane array (FPA) and a single fine steering mirror (FSM) for beacon acquisition, beacon tracking and point-ahead compensation. A fiber-coupled laser transmitter further reduces the complexity of the terminal. Over the last year, system level integration, test and characterization of the terminal were conducted. Here we present results from the integration and test (I&T) of the terminal with the Lasercom Terminal Evaluation Station (LTES), with particular emphasis on the fine tracking performance. Furthermore, we will describe lessons learnt from the implementation and testing of OCD that are relevant to the design of future flight optical communication terminals. The completed OCD is now being used in a series of ground-ground experiments to understand atmospheric effects and to gain experience operating the OCD.

The Semi conductor Inter satellite Link EXperiment, SILEX, consists of two terminals, one terminal embarked on the French LEO observation satellite SPOT4 and one terminal embarked on the ESA GEO telecommunication satellite ARTEMIS. The objective of SILEX is first to perform optical communication experiments in orbit and then on an operational basis transmit SPOT4 earth observation data to ARTEMIS, which will relay the data to ground via its Ka band feeder link. SPOT4 with the SILEX terminal was successfully launched on 22nd March 1998. While waiting for the counter terminal on ARTEMIS, a test program has been undertaken to characterize the performances without a counter terminal. The test program involves CCD calibrations, laser diode calibrations, emit/transmit co- alignment calibrations, measurement of point ahead mechanism accuracy, star acquisitions and tracking, sensitivity to sunlight, and characterization of platform/terminal dynamic interaction. The paper reports on test results of the in orbit testing, with comparison to similar ground testing and predictions. The conclusion of the test program is that the first optical communication terminal in orbit is in very good health and that the demonstrated performances are stable and considerably better than the expected.

Mountain-top to mountain-top optical link experiments have been initiated at JPL, in order to perform a systems level evaluation of optical communications. Progress made so far is reported. The NASA, JPL developed optical communications demonstrator (OCD) is used to transmit a laser signal from Strawberry Peak (SP), located in the San Bernadino mountains of California. This laser beam is received by a 0.6 m aperture telescope at JPL's Table Mountain Facility (TMF), located in Wrightwood, California. The optical link is bi-directional with the TMF telescope transmitting a continuous 4-wave (cw) 780 nm beacon and the OCD sending back an 840 nm, 100 - 500 Mbps pseudo noise (PN) modulated, laser beam. The optical link path is at an average altitude of 2 Km above sea level, covers a range of 46.8 Km and provides an atmospheric channel equivalent to approximately 4 air masses. Average received power measured at either end fall well within the uncertainties predicted by link analysis. The reduction in normalized intensity variance ((sigma) _I²) for the 4- beam beacon, compared to each individual beam, at SP, was from approximately 0.68 to 0.22. With some allowance for intra-beam mis-alignment, this is consistent with incoherent averaging. The (sigma) _I² measured at TMF approximately 0.43 plus or minus 0.22 exceeded the expected aperture averaged value of less than 0.1, probably because of beam wander. The focused spot sizes of approximately 162 plus or minus 6 micrometer at the TMF Coude and approximately 64 plus or minus 3 micrometer on the OCD compare to the predicted size range of 52 - 172 micrometer and 57 - 93 micrometer, respectively. This is consistent with 4 - 5 arcsec of atmospheric 'seeing.' The preliminary evaluation of OCD's fine tracking indicates that the uncompensated tracking error is approximately 3.3 (mu) rad compared to approximately 1.7 (mu) rad observed in the laboratory. Fine tracking performance was intermittent, primarily due to beacon fades on the OCD tracking sensor. The best bit error rates observed while tracking worked were 1E - 5 to 1E - 6.

There is an upper bound on the effective size of receiving aperture for both the improvement in carrier-to-noise ratio and the reduction in signal fading of a conventional coherent detection system due to the atmospheric turbulence-induced finite transverse coherence length of the received laser field for laser radar and communications. But a coherent array detection system, which uses multiple independent apertures/receivers whose IF's are electro-optically co-phased in real time and then added, can overcome the effective aperture limitation and mitigate the signal fading. This paper presents experimental comparison of the performance of an eight-aperture coherent array detection system and the conventional coherent detection system. The field tests were conducted over two-km range (round-trip). The carrier-to-noise ratio and the fade statistics of the IF signals were investigated for both systems. The results show a gain factor of up to six on the mean carrier-to-noise ratio of the IF signal from the eight-aperture coherent array detection system compared to the conventional coherent detection system even though the two systems collected the same power of laser signal. Also shown is a factor of 100,000 reduction in probability of fade of the IF signal.

SILEX (Semi-Conductor Inter-satellite Link EXperiment) consists of one optical terminal on-board the French LEO observation satellite SPOT 4, and another on-board the European GEO telecommunication satellite ARTEMIS. While the first part of the SILEX verification plan had been oriented towards verification at equipment and subsystem levels, the final stages have mainly been devoted to terminal and system (terminals coupling effects) verification. During this final stage, a thermal vacuum test was conducted in a class 100- cleanliness environment with optical ground support equipment of outstanding performances. The obtained tests results, used to determine software compensations and verify optical and static pointing performances, have been entered into overall system simulation models to finalize flight performances budgets. In addition, systems tests were performed on each terminal with respective partner simulator to validate system simulation models and assess link performances and robustness and to verify communication bit error rate.

Trade-off between Intensity Modulation Direct Detection (IM- DD) and coherent system oriented to the inter-orbit lightwave communication link is performed. Improvement in sensitivity of IM-DD system adopting high power optical fiber booster/pre- amplifiers is discussed preciously. In this paper, IM-DD system with booster/pre-amplifier and five kinds of coherent system (ASK-heterodyne, FSK-heterodyne, PSK-heterodyne, DPSK- heterodyne, and PSK-homodyne) are compared for high-data rate communication. Numerical analysis shows the superior characteristics of coherent communication system in low BER transmission under severe ASE noise.

Although much of today's space lasercom development is concentrated in digital communications, there still remain applications where the analog lasercom link is preferred. An analog lasercom link transmits RF or IF carriers instead of decoded bits. The most important application is in commercial bent pipe satellite constellations, where modulated carriers are to be relayed between satellites, avoiding the use of costly and risky onboard demod - remod digital processors. In this paper some basic tradeoffs are reported which aid in scoping the performance, limitations, and shortfalls of an analog crosslink. The analysis assumes fiber based technology at either 1.55 or 1.3 microns for the crosslink hardware, using optical intensity modulation (IM) and direct photodetection. When multiple carriers are used to IM the laser, the inherent nonlinearity of the modulation causes power suppression that limits the available bandwidth (number of carriers) that can be relayed using as single optical wavelength. With single wavelength bandwidth limited, the crosslink capacity can only be improved by the use of multiple wavelengths. This tradeoff of suppression and wavelengths is considered here.

The secure distribution of the secret random bit sequences known as 'key' material, is an essential precursor to their use for the encryption and decryption of confidential communications. Quantum cryptography is a new technique for secure key distribution with single-photon transmissions: Heisenberg's uncertainty principle ensures that an adversary can neither successfully tap the key transmissions, nor evade detection (eavesdropping raises the key error rate above a threshold value). We have developed experimental quantum cryptography systems based on the transmission of non- orthogonal photon polarization states to generate shared key material over line-of-sight optical links. Key material is built up using the transmission of a single-photon per bit of an initial secret random sequence. A quantum-mechanically random subset of this sequence is identified, becoming the key material after a data reconciliation stage with the sender. We have developed and tested a free-space quantum key distribution (QKD) system over an outdoor optical path of approximately 1 km at Los Alamos National Laboratory under nighttime conditions. Results show that free-space QKD can provide secure real-time key distribution between parties who have a need to communicate secretly. Finally, we examine the feasibility of surface to satellite QKD.

A theory for the signal-to-noise ratio of optical direct detection receivers employing return-to-zero coding (and possibly optical preamplification) is developed. The results are valid for both signal-independent noise limited and signal-dependent noise limited receivers, as well as for arbitrary optical pulse shapes and receive filter characteristics, taking into account intersymbol interference. Even if the same receiver bandwidth is used, return-to-zero coding is seen to yield higher signal-to-noise ratio than nonreturn-to-zero coding. Asymptotic expressions for the signal-to-noise ratio for very high and very low receiver bandwidths show that the full sensitivity enhancement potential of return-to-zero coding is exhausted at fairly moderate duty cycles. A realistic example taking into account inter-symbol interference shows that a receiver sensitivity gain (compared to nonreturn-to-zero coding) of e.g. 3.2 dB can be obtained in a signal-independent noise limited receiver with a bandwidth of 80% of the data rate, using a duty cycle of 3. For the signal-independent noise limited case, the sensitivity enhancement potential depends on the receive filter characteristics; we provide a design rule for filters with high sensitivity enhancement potential. Further, we investigate the role of rare-earth doped booster amplifiers in impulsively coded communication links: It is shown that, due to the average power limitation of these devices, a less powerful booster amplifier as with nonreturn-to-zero coding can be employed if certain conditions regarding the data rate and the return-to-zero duty cycle are met.

M-ary pulse position modulation (PPM) has received considerable attention for direct-detection photon communications over unguided channels. The analysis generally assumes that the signaling set is orthogonal. However the orthogonality of the signaling set will be destroyed by the finite area and bandwidth of optical detectors, resulting in severe intersymbol interference. This paper presents the analysis of a trellis-based pulse position modulation (T-PPM) scheme for photon communications with non-rectangular pulses. The novelty of the scheme includes the use of a set partitioning methodology to increase the minimum distance using a simple convolutional encoder. The Viterbi algorithm is used at the receiver to separate the signaling set as part of the demodulation process. It has been shown that T-PPM will restore performance losses due to reduced peak intensity during the detection process. Furthermore, for a large range of background radiation levels and when an APD detector is used, the average number of photons per information bit for T- PPM is smaller than that of the regular PPM. Numerical examples show that for a symbol error rate of 10^-3 when the received pulses extend over 4 PPM slots, the average laser energy per symbol for 256-ary T-PPM could be reduced by as much as 2 dB.

We investigate the role of amplified spontaneous emission (ASE) produced by an optical booster amplifier at the transmitter of free-space optical communication links. In a communication terminal with a single telescope for both transmission and reception, this ASE power has to be taken into account in connection with transmit-to-receive channel isolation, especially since it partly occupies the same state of polarization and the same frequency band as the receive signal. We show that the booster ASE intercepted by the receiver can represent a non-negligible source of background radiation: In a typical optical intersatellite link scenario, the ASE power spectral density generated by the booster amplifier at the transmitter and coupled to the receiver will be on the order of 10^-20 W/Hz, which equals the background radiation of the sun. Exploiting these findings for pointing, acquisition, and tracking (PAT) purposes, we describe a patent-pending PAT system doing without beacon lasers and without the need for diverting a part of the data signal for PAT. Utilizing the transmit booster ASE over a bandwidth of e.g. 20 nm at the receiver, a total power of about -46 dBm is available for PAT purposes without extra power consumption at the transmitter and without the need for beacon lAser alignment.

The functional requirements and design drivers for an Optical Communications subsystem are assessed based on the system requirements imposed by a proposed Europa Orbiter mission. Unlike near-Earth optical communications systems, deep space missions impose a unique set of requirements that drives the subsystem design. Significant challenges on laser efficiency, thermal control, pointing and tracking, stray/scatter light control, and subsystem mass/power need to be addressed for a successful subsystem implementation. The baseline design concept for a lasercom subsystem for the Europa orbiter mission employs a 30-cm diameter, diffraction-limited telescope, and a diode pumped solid state laser operating at 1.06 micrometer to support downlink communications. The baseline pointing and tracking approach is to perform Earth Image Tracking with occasional calibration using the Earth- moon or Earth-star images. At high phase angles when the Earth image does not provide sufficient brightness for high rate tracking, inertial sensors (accelerometers) measurements are used to propagate the knowledge of the optical boresight at a higher rate in between celestial reference updates. Additionally, uplink beacon tracking will be used to support pointing at short range and near solar opposition when Earth image alone does not provide sufficient signal power for tracking.

One of the NASA technology development programs at the Jet Propulsion Laboratory aims to increase the information return capability while reducing the size of the spacecraft via laser communications. The deep space optical transceiver developed under this program employs pulse position modulation (PPM) for both uplink and downlink transmissions. An integral part of the transceiver is the development of signal acquisition and tracking subsystem. This paper describes the baseline design of the electronic assembly within the transceiver and modifications that are necessary for deep space communications. A two phase breadboard activity will be described to reduce technological risks associated with the development.

Experiments were conducted to evaluate the viability of commercial-off-the-shelf (COTS) technology in the design of high-performance optical intersatellite communications links. The optical links were operated at 155, 622 and 2488 Megabits per second (Mbps) and at 1550 nm wavelength. Communications components were evaluated against performance parameters for on-off keyed (OOK) systems. Transmitters, receivers, optical amplifiers, and filters, used to minimize the impact of amplified spontaneous emission (ASE), were characterized individually and within system configurations. Bit error rate (BER) as a function of photons per bit was characterized for several system configurations. Extinction ratio analysis experiments were conducted to determine the limiting factors on the systems performance. Widely varying optical powers in space (for low earth orbit) imply a robust receiver dynamic range requirement. Dynamic range of COTS receivers was examined and resulted in sufficient performance. In addition, gamma radiation tests on fiber amplifiers were also evaluated in a system context.

A high data rate laser transmitter assembly (LTA) has been designed as the source for an optical free-space communication link between the International Space Station and the 1-meter Optical Communication Telescope Laboratory (OCTL) to be built at the Table Mountain Facility (TMF, Wrightwood, CA). the transmitter design concept uses a fiber-based master oscillator power amplifier (MOPA) configuration with an average output power of 200 mW at a 1550 nm transmit wavelength. This transmitter source is also designed to provide a signal at 980 nm to the silicon-based focal plane array for the point-ahead beam control function. This novel integration of a 980 nm boresight signal allows the use of silicon based imagers for the acquisition/tracking and point- ahead functions, yet permits the transmit signal to be at any wavelength outside the silicon sensitivity. The LTA, a sub- system of the Flight Terminal, has been designed to have a selectable data rate from 155 - 2500 Mbps in discrete steps. It also incorporates a 2.5 Gbps pseudo-random bit sequence (PRBS) generator for complete link testing and diagnostics. The design emphasizes using commercial off the shelf components (COTS).

This paper presents an overview of the preliminary design of both the flight and ground systems of the Optical Communication Demonstration and High-rate Link Facility which will demonstrate optical communication from the International Space Station to ground after its deployment in October 2002. The overview of the preliminary design of the Flight System proceeds by contrasting it with the design of the laboratory- model unit, emphasizing key changes and the rationale behind the design choices. After presenting the preliminary design of the Ground System, the timetable for the construction and deployment of the flight and ground systems is outlined.

The Next Generation Space Telescope (NGST), part of NASA's Origins program, is a follow on to the Hubble Space Telescope expected to provide timely new science along with answering fundamental questions. NGST is a large diameter, infrared optimized telescope with imaging and spectrographic detectors which will be used to help study the origin of galaxies. Due to the large data NGST will collect, Goddard Space Flight Center has considered the use of optical communications for data downlink. The Optical Communications Group at the Jet Propulsion Laboratory has performed a study on optical communications systems for NGST. The objective of the study was to evaluate the benefits gained through the use of optical communication technologies. Studies were performed for each of four proposed NGST orbits. The orbits considered were an elliptical orbit about the semi stable second Lagrangian point, a 1 by 3 AU elliptic orbit around the sun, a 1 AU drift orbit, and a 1 AU drift orbit at a 15 degree incline to the ecliptic plane. An appropriate optical communications system was determined for each orbit. Systems were evaluated in terms of mass, power consumption, size, and cost for each of the four proposed orbits.

The deployment of advanced hyperspectral imaging and other Earth sensing instruments on board Earth observing satellites is driving the demand for high-data-rate communications. Optical communications meet the required data rates with small, low-mass, and low-power communications packages. JPL, as NASA's lead center in optical communications, plans to construct a 1-m Optical Communications Telescope Laboratory (OCTL) at its Table Mountain Facility (TMF) complex in the San Gabriel Mountains of Southern California. The design of the building has been completed, and the construction contractor has been selected. Ground breaking is expected to start at the beginning of the 1999 TMF construction season. A request for proposal (RFP) has been issued for the procurement of the telescope system. Prior to letting the RFP we conducted a request for information with industry for the telescope system. Several vendors responded favorably and provided information on key elements of the proposed design. These inputs were considered in developing the final requirements in the RFP.

Preliminary optical design and mechanical conceptual design for a 30 cm aperture transceiver are described. A common aperture is used for both transmit and receive. Special attention was given to off-axis and scattered light rejection and isolation of the receive channel from the transmit channel. Requirements, details of the design and preliminary performance analysis of the transceiver are provided.

This paper describes an extended-source spatial acquisition and tracking scheme for planetary optical communications. This scheme uses the Sun-lit Earth image as the beacon signal, which can be computed according to the current Sun-Earth-Probe angle from a pre-stored Earth image or a received snapshot taken by other Earth-orbiting satellite. Onboard the spacecraft, the reference image is correlated in the transform domain with the received image obtained from a detector array, which is assumed to have each of its pixels corrupted by an independent additive white Gaussian noise. The coordinate of the ground station is acquired and tracked, respectively, by an open-loop acquisition algorithm and a closed-loop tracking algorithm derived from the maximum likelihood criterion. As shown in the paper, the optimal spatial acquisition requires solving two nonlinear equations, or iteratively solving their linearized variants, to estimate the coordinate when translation in the relative positions of onboard and ground transceivers is considered. Similar assumption of linearization leads to the closed-loop spatial tracking algorithm in which the loop feedback signals can be derived from the weighted transform-domain correlation. Numerical results using a sample Sun-lit Earth image demonstrate that sub-pixel resolutions can be achieved by this scheme in a high disturbance environment.

A wide and fine pointing mechanism (WFPM) composed of electromagnetic actuators and flexible supports has been developed for optical inter-satellite communications. It covers the whole of the scan area when it searches for the target satellite. The electromagnetic actuators, which consist of four moving-coil-type motors with long strokes, ensure a wide scan range. The flexible supports, which consist of a metal pivot and four thin springs, have vertical stiffness and rotational flexibility. Balancing the elasticity between the pivot and the thin springs fixes the center of rotation on the mirror surface to keep the beam path length constant. A digital control system is designed to drive the WFPM with gap sensor feedback. It has a two-degrees-of-freedom robust controller that incorporates a disturbance cancellation algorithm that cuts off low-frequency vibrations from the satellite body to achieve highly precise tracking. Experiments show that the WFPM has both a wide scanning range of over 8 degrees and a highly precise pointing capability that is accurate to within 30 microradians, for the case when only gap sensor feedback is used without quadrant detector feedback. These characteristics are suitable for high-speed switching over target satellites.

This paper summarizes part of a FY1998 effort on the design and development of an optical communications (Opcomm) subsystem for the Advanced Deep Space System Development (ADSSD) Project. This study was funded by the JPL X2000 program to develop an optical communications (Opcomm) subsystem for use in future planetary missions. The goal of this development effort was aimed at providing prototype hardware with the capability of performing uplink, downlink, and ranging functions from deep space distances. Such a system was envisioned to support future deep space missions in the Outer Planets/Solar Probe (OPSP) mission set such as the Pluto express and Europa orbiter by providing a significant enhancement of data return capability. A study effort was initiated to develop a flyable engineering model optical terminal to support the proposed Europa Orbiter mission -- as either the prime telecom subsystem or for mission augmentation. The design concept was to extend the prototype lasercom terminal development effort currently conducted by JPL's Optical Communications Group. The subsystem would track the sun illuminated Earth at Europa and farther distances for pointing reference. During the course of the study, a number of challenging issues were found. These included thermo- mechanical distortion, straylight control, and pointing. This paper focuses on the pointing aspects required to locate and direct a laser beam from a spacecraft (S/C) near Jupiter to a receiving station on Earth.

The Optical Communications Demonstrator (OCD) is a laboratory- based lasercom demonstration terminal designed to validate several key technologies, including beacon acquisition, high bandwidth tracking, precision beam pointing, and point-ahead compensation functions. It has been under active development over the past few years. The instrument uses a CCD array detector for both spatial acquisition and high-bandwidth tracking, and a fiber coupled laser transmitter. The array detector tracking concept provides wide field-of-view acquisition and permits effective platform jitter compensation and point-ahead control using only one steering mirror. This paper describes the detailed design and characterization of the digital control loop system which includes the Fast Steering Mirror (FSM), the CCD image tracker, and the associated electronics. The objective of this work is to improve the overall system performance using laboratory measured data. The design of the digital control loop is based on a linear time invariant open loop model. The closed loop performance is predicted using the theoretical model. With the digital filter programmed into the OCD control software, data is collected to verify the predictions. This paper presents the results of the system modeling and performance analysis. It has been shown that measurement data closely matches theoretical predictions. An important part of the laser communication experiment is the ability of FSM to track the laser beacon within the required tolerances. The pointing must be maintained to an accuracy that is much smaller than the transmit signal beamwidth. For an earth orbit distance, the system must be able to track the receiving station to within a few microradians. The failure to do so will result in a severely degraded system performance.

Super compact, 3W power, 1060 nm polarization maintaining, diode-pumped ytterbium fiber amplifier with extinction ratio greater than 17 dB and wall-plug efficiency greater than 14% have been developed. Performance of the device fits requirements of free-space and satellite optical communications applications.

Master-oscillator power-amplifier (MOPA) laser systems, for future lasercom terminals to be flown in space are being developed and characterized. These lasers fall into groups for applications in deep space and near earth (including GEO) orbiting satellites. For deep space applications a diode pumped Q-switched Nd:YVO₄ oscillator and a multi-pass Nd:YAG amplifier laser with an average output power of 1.5 W and capable of supporting repetition rates of 100 KHz was studied. This lasers emits at 1064 nm and was tested with pulse position modulation at data rates of 400 Kbps. The laser pulse width (approximately 2 ns FWHM) and timing jitter (plus or minus 8 ns) are reported with relevant discussions pertaining to deep space performance. For near earth applications, a two stage diode pumped fiber amplifier system providing 1072 nm emission with 4 W of average power and capable of supporting data rates of 2.5 Gbps is briefly described. A second semiconductor flared amplifier system emitting at 980 nm has also been developed and is awaiting characterization.

For high data rate (greater than 1 Gbps) Optical Inter- Satellite Link (OISL), a compact laser transmitter with high power and good efficiency is required. A trade-off analysis between the technologies such as the mature 840 nm laser diodes, 1064 nm diode-pumped solid state laser and the more recent 1550 nm Erbium Doped Fiber Amplifier (EDFA) is used to find the optical solution. The Si-APDs are preferred for their large detector areas and good noise figures which reduce the tracking requirements and simplify optical design of the receiver. Because of significant amount of power needed to close the link distance up to 7000 km (LEO-LEO), use of 840 nm diodes is limited. In this paper, we present an alternative system based on a system concept denoted as the SLYB (Semiconductor Laser Ytterbium Booster). The SLYB uses a polarization maintaining double-clad ytterbium fiber as a power amplifier. The device houses two semiconductor diodes that are designed to meet telecom reliability: a broad-area 917 nm pump diode and a directly modulated FP laser for signal generation. The output signal is in a linearly polarized state with an extinction ratio of 20 dB. The complete module (15 X 12 X 4.3 cm³) weighs less than 0.9 kg and delivers up to 27 dBm average output power at 985 nm. Designed primarily for direct detection using Si APDs, the transmitter offers a modulation data rate of at least 1.5 Gb/s with a modulation extinction ratio better than 13 dB. Total power consumption is expected to be lower than 8 W by using an uncooled pump laser. Preliminary radiation testing of the fiber indicates output power penalty of 1.5 dB at the end of 10 years in operation. We are presently investigating the fabrication of an improved radiation-hardened Yb-fiber for the final prototype to reduce this penalty. For higher data rate the design can be extended to a Wavelength Division Multiplexing (WDM) scheme adding multiple channels.

Terrestrial fiber optic communication systems handle most of the inter-continental communication systems today. However recent studies indicate that these links, in spite of their huge bandwidth will be saturated in the near future. Hence attempts are being made is augment and may be even replace these by inter satellite links (ISLs). Though high power laser diodes have been found to be suitable for ISLs, they are unfortunately inadequate for satellite-ground links (SGL and GSL) as they are not powerful enough. So we have to look for more powerful lasers for SGLs & GSLs. One possible candidate is the CO₂ laser. It is a gas laser. It provides a number of advantages over other sources. They include high life time, high efficiency and stability. Besides it can generate a high power continuous wave and requires only radiative cooling. CO₂ can provide a high bit rate and long range transmission with low bit error rate. Also CO₂ laser is in near infra-red and hence the turbulence effects due to clouds is minimum. All these make CO2 laser a very economical choice. The use of optical communication in GSLs provides many advantages over radio links. Laser being a high energy source provides the advantage of greater bandwidth, smaller beam divergence angles, smaller antennae, greater security and a new spectrum. Lesser power consumption and smaller size make it more suitable for use in a satellite. The present paper deals with a case study of a CO₂ laser based free space optical communication link by making the link budget analysis.

A coherent array detection system can improve the heterodyne mixing efficiency compared to a conventional coherent detection system which is limited by the finite transverse coherence length of received laser field due to atmospheric turbulence for laser radar and communications. This implies a gain factor in the mean power of the IF signal from the coherent array detection system compared to the conventional coherent detection system even though the two systems receive the same power of laser signal. This gain factor is actually the mean carrier-to-noise ratio (CNR) gain factor of the coherent array detection over the conventional coherent detection if the two systems have the same local oscillator and are shot noise-limited of the local oscillator. But the mean CNR gain factor depends on the conditions of atmospheric turbulence. This paper describes the theoretical analyses of the mean CNR gain factor for the coherent array detection system under different conditions of atmospheric turbulence which include weak, strong, and saturation conditions, corresponding to the lognormal, the K, the I-K, the negative exponential (Rayleigh for amplitude), and the gamma (Nakagami for amplitude) distributions for the laser intensity fluctuations.

The Atmospheric Visibility Monitoring (AVM) project gathers data on the transmission of light through the atmosphere. These data are measured and collected at autonomous observatories using stellar photometric techniques. The information gathered is used to build statistical models that assess the quality of future space-to-ground optical communication links. The first of the three currently running AVM observatories became operational in 1994. Jet Propulsion Laboratory (JPL) is upgrading all three observatories. The upgrade includes new Charge Coupled Device (CCD) imagers, Windows NT-based computers and new control software. The new CCDs improve the following: near IR performance, daylight photometry, pixel field-of-view, intensity resolution and signal-to-noise ratios. Employing these new CCDs requires upgrading the CPU, operating system and control software. Together these changes improve the quality of future data obtained and processed by the AVM system.

The design of any optical system operating in the atmosphere requires previous investigations of atmospheric turbulence conditions at the system's location. In addition, if outdoor optical measurements are to be useful in assessing the performance of the optical system, or in checking propagation theory, they must be supported by simultaneous direct measurements of the atmospheric turbulence. Stellar observations provide a useful and convenient means for these purposes in the case of ground-satellite optical communications. We first review the refractive index structure parameter profiling techniques based on stellar observations. For implementation we have selected the technique based on spatiotemporal analysis of captured speckle patterns. There is ample evidence that turbulence in the free atmosphere confined to thin, horizontal layers separated by nonturbulent regions. The lifetime of such layers are of the order of several hours. The technique allows us to detect these layers, giving simultaneously the altitude, horizontal mean wind velocity and integrated refractive index value for each 1 km interval. We briefly describe the experimental setup and the measurement technique. We then present some sample results. Initial results show that the current state of the experimental setup and the processing algorithm can detect only the most prominent layers, and altitude accuracy is less than expected. We are currently looking for ways to improve the technique to allow detection of more layers.

A coherent array detection system, which uses multiple independent apertures/receivers whose IFs are electro- optically co-phased in real time and then added together, can mitigate the laser signal fluctuations and fading due to atmospheric turbulence for laser communications and radar. Thus the system performance greatly depends on the electo- optic phase-locked loops (EOPLL) used for the real time co- phase of signals from the independent aperture/receivers. But if the fast phase fluctuations due to atmospheric turbulence or a high Doppler frequency shift due to the target (satellite) movement are induced to the laser signal which is beyond the response speed of the EOPLL, the system can not mitigate the signal fluctuations and fading anymore. This paper presents a new scheme which can make the coherent array detection system get rid of the EOPLL. This scheme uses a reference laser beam as well as laser signal with different polarization as the transmitted beam. By separating the received laser reference beam and signal due to their polarization and through two stage mixing, the IF signals from the multiple independent aperture/receivers will be co-phased automatically.

Commercial high speed fiber optic transmitters and receivers were tested in a high energy proton environment at the Crocker Nuclear Laboratory to determine the transient impact of the space proton environment on a simulated communications link. The link was designed to simulate free-space communication between satellites. The transmit/receive bit error ratio was used as the evaluation metric. Individual electronic components on each transmitter and receiver module were tested using a variety of proton energies, angles of incidence, and optical link configurations. No change was found in the bit error ratio for any of the transmitters tested. Significant increases in bit error ratio were found on two of the receivers when several individual receiver components were irradiated. Small increases in bit error ratio were found on several receivers when exposed to 63 MeV proton fluxes greater than 1 X 10⁷ p/cm²-s. The remaining receivers were found to be immune to the influence of the protons. This testing demonstrates the feasibility of operating commercial transmitters and receivers in a free-space optical link when exposed to the space proton environment.

Intermediate infrared Raman laser at wavelength 9.18 micrometer is generated by using Nd:YAG laser at wavelength 1.064-micrometer as a pumping source. Raman cell is filled with compressed hydrogen with an optical waveguide in it. The second order Stokes light is measured with germanium detector and its quantum conversion efficiency is 1.12%.

Given are the results of experimental study on the quasi real time holographic correction for the lens distortions in the passive observational telescope in the visible range of spectrum, using the liquid crystal optically addressed spatial light modulator.

Proposed is the novel method of dynamic nonlinear-optical correction for distortions in wide spectral band. The method is based on combining of the negative optical feedback correction and dynamic holography correction in the system, using optically addressed phase modulators. State-of-the-art of key technologies is evaluated.

Large numerical aperture telescope with nonlinear optical correction for distortions, designed for the remote self- luminous object imaging, was realized in experimental and investigated. Dynamic hologram, recorded in optically addressed liquid crystal spatial light modulator, was used as the corrector. Nearly diffraction limited performance of the system was demonstrated.