An optical satellite communication system is being developed for performing basic optical communications experiments aboard the Japanese ETS-VI satellite to be launched in 1993. This paper describes the experimental plan and the present status of the communication system. The optical communication payload is described, including basic data and block diagrams.

The main areas of research being conducted at NASA Goddard Space Flight Center are reviewed. Research on transmitter source technology is addressed, emphasizing the development of AlGaAs semiconductor laser diodes. Research on receiver technology is examined, and progress being made in the development of the Pointing, Acquisition, and Tracking System (PATS) is reviewed. Plans for an in-space technology demonstration are briefly discussed.

The European Space Agency (ESA) is developing an optical interorbit communication system enabling a link between a low earth orbiting (LEO) and a geostationary (GEO) spacecraft. The link allows the transmission of up to 65 Mbps between LEO and GEO in an experimental and preoperational mode. The system uses laser diodes of typically 100 mW optical power at a wavelength of 830 nanometer. Direct intensity modulation is applied. Telescopes of 25 cm diameter are used on both terminals. The breadboard phase has been completed and the launch of both terminals is scheduled for 1994.

Mass, prime power, and volume requirements are estimated for an optical intersatellite link (ISL) communication payload that supports a full-duplex interconnection between two geostationary spacecraft, while supplying a 0.9 system reliability over a 10-year lifetime. Communication subsystems based on CO2, Nd:YAG, InGaAsP, and GaAlAs laser sources aare assessed, and comparison of the results is found to support previous recommendations of GaAlAs diode laser systems for optical ISLs in a global telecommunications network.

A novel design for a optical intersatellite link terminal is presented. The terminal has a mass of 22 kg, uses approximately 25 W of power, and is extremely compact. Moving mass is also minimized, making it ideal for applications on microgravity platforms. The terminal can support a data rate of 1 Mbps over a distance of 45,000 km, using available 800-nm semiconductor laser diodes. The primary mission is expected to be data relay. However, with projected advances in laser technology, the design will also have application to telecommunications links between geostationary spacecraft and be able to support higher data rates over the baseline mission.

The generic system level requirements for lasercom satellite crosslinks are considered. The system and subsystem design approaches and technologies that can be pursued to achieve a lasercom terminal with minimum size, weight, and cost are examined. A preliminary lasercom terminal design is described which represents a major reduction in size and weight, resulting from a minimum cost design philosophy.

The use of single mode fiber optics to simplify the design of space-based optical communication systems is explored. Design considerations concerning the issues of acquisition, pointing, tracking, communication performance, efficient source-to-fiber coupling, isolation, and transmitter power limitations are presented. Preliminary experimental results of a breadboard fiber-based coherent optical communication system are also presented.

A heterodyne 840-nm laser diode system operating up to 1 Gbit/s has been studied in detail with respect of its potential use for intersatellite link (ISL) communication. The breadboard uses only commercially available components and is designed to operate with single filter FSK or CPFSK demodulation scheme. The system sensitivity has been measured for 1 Gbit/s pseudorandom patterns and has been compared with direct detection results from the same test bed. The overall breadboard performances are discussed with respect to ISL communication requirements.

The configuration of the acquisition and tracking system of the laser communication system aboard the Japanese Engineering Test Satellite VI (ETS-VI) is described, and the control loop design of the coarse and fine loops are examined. Simulations performed to evaluate the dynamic error for each loop, which showed that the dynamic error satisfies the error budget, are described. The tracking accuracy of each loop was measured with a breadboard model and the resulting data were found to meet the error budget.

This paper briefly describes conventional divider implementations including the transconductance cell, the log-antilog, and the inverted multiplier. It considers the limitations of each configuration, particularly in the space environment. The paper also outlines a new analog divider configuration based on pulse-modulation techniques. This circuit is able to achieve excellent performance in space due to its reliance on switching elements for the numerator action. The paper discusses the theory of operation of this approach and the error sources that can be expected.

A new type of the fine pointing mechanism (FPM) was examined as part of the optical ISL studies conducted by NASDA. Compact piezo-electric mirror deflecting mechanism (MDM) , having a very high resolution (less than 3 irad) was fabricated, and the performance of the MDM was evaluated aiming for application in the ISL optical FPM. The fundamental performance of the fine pointing assembly (FPA) in optical ISL with the multi-layered piezo—electric MDM was confirmed sufficient for optical ISL having pointing error by disturbance (± 0.75 irad) and pointing angle (± 720 Mrad). It was confirmed that the examined FPM is suitable for the fine pointing of optical ISL, by combining appropriately designed optics and coarse pointing assembly (CPA) , through a preliminary system design.

Two tracking sensor breadbroads have been developed for the European SILEX program. Critical requirements to be met by these sensors include the determination of the 1-sigma position of the focused laser spot on the detector surface to accuracies of less than 100 nm, with update frequencies as high as 8 kHz, and the minimum optical power level of 110 pW. The design is based on the use of a 14 x 14 pixel CCD array, where the spot position is determined to small subpixel accuracies over the central 2 x 2 pixel area via a centroiding algorithm executed in a microprocessor. Coarse position measurements are also made over the full area at rates up to 2 kHz.

Combining double—phase conjugation with various coherent amplifiers leads to an amplified beam (forward or conjugated) derived from a local laser, but bearing the transversal phase pattern of a received signal beam from a remote laser. We suggest several uses in optical communication through a distorting atmosphere.

The design concept of an optical transmit-receive antenna telescope developed in the framework of the ESA SILEX program is presented. SILEX involves optical communication between satellites in GEO, using semiconductor laser diodes operating at 825 nm as the light source. The telescope requirements include entrance diameter 250 mm, exit pupil 8 mm, acquisition FOV 8500 microrad, communication FOV 2000 microrad, angular magnification -31.25, retroreflection 3 microW/sq m nm or less, stray light 1.05 microW/sq m nm or less, and alignment stability 10 years with no refocusing in orbit. The present compact two-mirror configuration employs the glass-ceramic Zerodur for all of the major components (primary mirror/baseplate, secondary mirror, tube, front ring, and ocular) for a total mass of only 5760 g. The prototype manufacturing process gave surface errors of 25 nm rms-WF for the primary and 15 nm rms-WF for the secondary.

A design concept for an optical-antenna telescope to be used in intersatellite communication (in the framework of the ESA SILEX project) is presented. The main technical requirements for a SILEX transmit-receive antennna telescope are reviewed, and a Cassegrain configuration based on a parabolic primary mirror, a hyperbolic secondary mirror, and a five-lense collimator (with baffles to limit stray light) is shown in diagrams and discussed in detail, with particular attention to local angular distortion and transmission-antenna gain. Results from tests on a breadboard version are presented in tables and graphs, including transmission of 92.6 percent at 820 nm, rms wavefront error less than lambda/28, optical gain 114.47 dB, backscattered energy 1.7 x 10 to the -6th at 838 nm, and stray-light intensity slightly above specification at sun aspect angles less than 4.5 deg.

In optical communication links between GEO satellites, all optical beam control functions required for the transceiver may be realized without using external gimbals. Configuration of such a transceiver internal optical system can be designed considering the satellite dynamics in orbit. This paper presents a design of the transceiver internal optical systems, taking into account the accuracy of satellite station-keeping and attitude control as well as an orbital separation. An experimental optics was constructed on the basis of the results for a bidirectional link between satellites separated by about 60 deg in orbit. The attitude control accuracy of + or - 0.1 deg and the satellite station-keeping of + or - 0.1 deg were assumed. It can be utilized to develop a variety of beam-controlling schemes.

Recent applications of the very light Al/SiC metal-matrix composite SXA in the construction of telescopes for use as receiver antennas in optical intersatellite communication systems are reviewed and illustrated with drawings and diagrams. Data on the mechanical properties (specific stiffness, fundamental frequency, dynamic response, and fracture toughness) and the thermal expansion, distortion, and diffusivity of SXA are compared with those for Al 6016-T6, Be I-70A, SiC, and Zerodur in tables, and the advantages of SXA structural foams of density 250-500 kg/cu m are indicated. The criteria evaluated for optimization of the mirror shape and the overall telescope design are discussed, and four prototype Cassegrain telescopes (with Meinel or Dall truss structures) are described in detail.

The basic principle and implementation of a near-field technique developed at ESTEC for measuring stray and backscattered light in optical telescopes are summarized. The measurement problems posed by transmit-receive antenna telescopes for optical intersatellite communication systems are described; the inadequacies of conventional methods for ground-based testing of such telescopes are explained; and the theoretical basis, instrumentation, and calibration of the present method are discussed. In this approach, a 40-nsec-pulsed laser-diode source is employed, and the intensity of the backscattered and stray light is measured by a microchannel-plate photomultiplier during the time of flight of the pulse from the optics assembly to a light trap.

A technique developed at ESTEC for the measurement and theoretical estimation of optical polarization in the antenna telescopes of space optical communication systems (e.g., the ESA SILEX intersatellite system using 825-nm semiconductor lasers) is described. The design methods used to achieve high channel separation ratios in the SILEX telescopes are recalled; the failure of straightforward measurement techniques in practical ground-based testing is explained; and a solution based on the mathematical relationship between far-field and near-field intensity distributions is outlined. The primary instrument used in this method is a scanning ellipsometer.

A transmitter laser breadboard suitable for a coherent space communication system is under development. Major design goals are 1W CW output power, 100 kHz linewidth at low drift, 8 percent electrical to optical conversion efficiency, and 3 dB power degradation over 7 years operation. An oscillation output of 600 mW is amplified by two double-pass stages to 1 W.

Laser-diode pumping of monolithic ND:YAG rings can lead to output powers of hundreds of milliwatts from a single laser. We have built several lasers with diffraction-limited single-mode output of 380 mW. These lasers can be injection—locked in a chain configuration to sum their power, while maintaining diffraction-limited, single-frequency operation. We demonstrated this chaining technique with two lasers, with a total output of 340 mW, and expect that it is practical for up to about ten lasers. Thus with lasers of 380 mW, output of 3 Watts is possible. The chaining technique, if properly engineered, results in redundancy. Monolithic, diode-pumped Nd:YAG ring lasers can provide narrow-linewidth, tunable output which is adequate for use as a local oscillator in a coherent conununication system. We build a conunercial laser which has a linewidth of about 2 kHz, a power of 5 inilliwatts, and which is tunable over a range of 30 MHz in a few microseconds. We phase—locked one of these lasers to a second, similar laser. This demonstrates that the powerful technique of heterodyne detection is possible with a diode-pumped laser used as a local oscillator. A key to the usefulness of these lasers in space communication is modulation. We have experimented with a technique for converting the cw, infrared output of these lasers into randomly pulsed, green output useful for Pulse—Position Modulation. Energy storage in an external resonant cavity permits peak green output to be much more powerful than cw infrared input. We have also begun experimentation with techniques for phase and amplitude modulating the output of these lasers. We are taking advantage of the coherence of the output in order to reduce the voltage required for conventional electro—optic modulation.

A tunable Wavelength Division Multiplexing [WDM] scheme, suitable for Optical InterSatellite Link [OISLJ communications, has been designed. The scheme can combine [and separate] four or more beams from laser diode devices in the 800—900nm wavebands and incorporates a number of tunable narrow-band holographic reflection filters. By choosing the correct point source construction geometry the 'tuning' along one direction of the filter can be made to vary approximately linearly with position on the substrate. The advantages of such filters with regard to achieving the tight tuning tolerances [typically < O.5nm] required for space communications is discussed. In the DEMUX arrangement a 'double bounce' method is employed to minimise the effect of holographic sidebands on the received noise levels. The overall transmission for the scheme has been predicted for each channel.

The design of this equipment based on the use of nineteen 500 mW laser diodes is presented. The Beacon is a powerful and collimated optical source based upon semi-conductor lasers. This equipment is implemented in the SILEX GEO2 terminal and its function is to provide a continuous wave light beam towards the LEO or GEO1 satellites during the acquisition phases. The design was experimentally validated with a functional breadboard, and test results demonstrated the feasibility of every performance required by the SILEX system.

A laser diode beam combiner employing diffraction gratings has been fabricated and tested. The Grating Laser Beam Combiner (GLBC) uses two holographic diffraction gratings to incoherently combine the first order diffraction components of four 35 mW AlGaAs lasers. The grating rhomb design minimizes the transmitter sensitivity to the inherent frequency instability of laser diodes. The overall throughput of the combiner is 74 percent. Each laser was temperature controlled to 0.1 C and modulated with 110 Mbps QPPM data. Two lasers under modulation were coaligned to within 76 microrad with a combined average power of 45 mW.

Single frequency oscillation with a near diffraction-limited beam pattern is demonstrated from an otherwise multimode and multi-lobe high power GaAlAs laser array using a self-injection locking technique. A single mode output power of 500 mW with a single lobe far field beam of 0.5 deg divergence angle was obtained. The single frequency laser array also shows high frequency modulation response with high modulation depth and low distortion that make this laser array a suitable candidate for optical communication applications.

A semiconductor laser phased-array transmitter is described that operates by coherently combining the output of many semiconductor laser amplifiers to form a near-diffraction-limited beam in the far field with sufficient power for most free space optical communications applications. Because the outputs are coherently combined, modulation formats requiring coherent detection can be used with this transmitter. Results are presented on the operation of the phased array and on preliminary communications experiments conducted with this transmitter.

This paper discusses the characteristics of a new concept involving high-power laser diode (HPLD) arrays which are able to emit up to 1000 mW of optical output power but which suffer from poor far-field pattern and poor optical quality, related to the gain guiding mechanism used in such devices. Methods are proposed for improving the optical quality of HPLDs. These include the use of apertured AM scheme for HPLDs and the use of a graded-index lens and a stripe mirror to stabilize the HPLD. A HPLD optical transmitter is designed which will have the capability of operating the HPLD in the 180 deg phase alternating mode under modulation.

The characteristics of commercially available 1.5-micron distributed feedback (DFB) laser diodes obtained from seven manufacturers were investigated, and the results are correlated with the lasers' structural parameters. It was found that buried heterostructures with first-order gratings showed consistently good results (as opposed to buried heterostructures with second-order gratings). A ridge waveguide laser was near the bottom of most performance charts.

Experimental results on the frequency modulation (FM) and linewidth characteristics of laser diodes in external cavities are presented. The laser is an uncoated 825 nm AlGaAs semiconductor laser diode. The external cavities consist of a 10X, 0.3 NA microscope objective and a diffraction grating. Measurements were made of the FM coefficient and spectral linewidth as a function of external cavity length.

This paper discusses the use of forward error correction (FEC) in a 300 to 1000 Mbit/s free-space optical communications link. It also considers the tradeoffs involved in applying block codes or convolutional codes, emphasizing the peak and average power limitations of GaAlAs diode laser sources. Direct-detection optical receivers are assumed throughout. The application of FEC technology to a high-data-rate optical communications system is discussed, including available coding gain, correction for both random errors and mispointing-induced burst errors, and electronic implementation difficulties. This is followed by a discussion of the major system benefits derivable from FEC. Consideration is given to using the available coding gain for reducing diode laser source power, aperture size, or fine tracking accuracy. Regarding optical system design, it is most favorable to apply the coding gain toward reducing diode laser power requirements.

The configuration and specifications of an optical IM/DD modulator/demodulator (modem) developed for optical intersatellite communications are described. The modem's performance was verified for the case of a commercially available high-power laser diode (LD). It was found that the modulator could modulate the high-power LD whose average output was 100 mW, at the bit rate of 360 Mbit/sec. Less than 1.0 dB degradation was measured to obtain the bit error rate of 10 to the -6th at this bit rate.

Using the Gaussian approximation, a method is found for determining the peak optical power required to allow laser communication at a specified bit error rate and communication data rate for the bipolar raised cosine pulses modulation format. The method is particularly applicable to laser diode communication systems, since the problem is solved subject to a constraint on the extinction ratio, which is the ratio of the peak optical power (Pmax) to the minimum (usually the lasing threshold) optical power (Pmin) available from a given laser diode. The receiver threshold may be chosen as an equal distance between Pmax and Pmin, or by implementing either an equal probability of error or minimum probability of error receiver.

The use of raised cosine shaped symbols has been proposed for optical digital communications. The raised cosine pulse shape reduces the receiver signal bandwidth to that of the data rate while simultaneously minimizing inter-symbol interference. This has the effect of increasing receiver sensitivity due to the noise density characteristics of transimpedance amplifiers when bandwidth is reduced. Receiver sensitivity is an essential criteria for free space laser communication. An optical communications experiment was conducted consisting of a laser transmitter modulated by a 20 megabit per second bipolar coded raised cosine bit stream and a matched filter receiver utilizing an APD detector and a transimpedance amplifier. Pulse shaping and matched filter circuits were implemented using a tapped delay line filter type topology. The bit error rate of the receiver as a function of incident optical power was measured and is compared to predicted performance. Implementation losses are accounted for in terms of matched filter mismatch loss, inter-symbol interference, sample time uncertainties, and system nonlinearities.

A 50 Mbps direct detection optical communication system for use in an intersatellite link was constructed with an AlGaAs laser diode transmitter and a silicon avalanche photodiode photodetector. The system used a Q = 4 PPM format. The receiver consisted of a maximum likelihood PPM detector and a timing recovery subsystem. The PPM slot clock was recovered at the receiver by using a transition detector followed by a PLL. The PPM word clock was recovered by using a second PLL whose input was derived from the presence of back-to-back PPM pulses contained in the received random PPM pulse sequences. The system achieved a bit error rate of 0.000001 at less than 50 detected signal photons/information bit. The receiver was capable of acquiring and maintaining slot and word synchronization for received signal levels greater than 20 photons/information bit, at which the receiver bit error rate was about 0.01.

A complete system test bed (STB) built for demonstrating optical space communication feasibility is described. The STB uses functional breadboards of all key equipment required for an optical terminal. Results of experiments carried out to evaluate the communication and pointing functions of the STB, both at subsystem and at terminal levels, are presented and compared to theoretical calculations.

An experimental coherent communication link has been realized to investigate various aspects of a diode-pumped Nd:YAG laser space communication system. The breadband 500 Mb/s link is based on low power monolithic ring lasers and a balanced receiver PSK homodyne detection scheme. Fiberoptic components are being investigated and a novel fiber-nutator device for coherent tracking has been developed.

Results are presented on laser-diode (LD) based intermountain tests carried out as a support activity to ESA's Free-space Optical Communications program to evaluate the Canary Islands (Spain) as an optical communicaton test range. The model predictions of low atmospheric attentuation and turbulence-induced effects were confirmed by a simple link test between observatories, showing that the Canary Islands are a proper site for performing terrestrial experiments of free-space optical communication systems.

The use of hand held laser communications devices for airborne applications is described. The usefulness of a laser communicator versus the more common flashing light system is discussed as is the ability to hold the laser spot on target. Potential sources for detecting the laser communications signal have been examined and evaluated. System improvements with regard to the concept of deployment is also discussed. The Air Force performs airborne laser communications experiments to examine the potential usefulness of this technology to existing and future aircraft. Lasers can be used, for example, to replace flashing light signals which are commonly employed on a variety of missions. Laser communications have a high degree of security and resistance to jamming and detection which increases the survivability of the users against hostile threats.

This paper describes the design of the Laser Communication Transceiver (LCT) system which was planned to be flight tested as an attached payload on Space Station Freedom. The objective in building and flight-testing the LCT is to perform a broad class of tests addressing the critical aspects of space-based optical communications systems, providing a base of experience for applying laser communications technology toward future communications needs. The LCT's functional and performance requirements and capabilities with respect to acquisition, spatial tracking and pointing, communications, and attitude determination are discussed.

Four areas of research in optical communications in support of future manned space missions being carried out at Johnson Space Center are discussed. These are the Space Station Freedom proximity operations, direct LEO-to-ground communications, IR voice communications inside manned spacecraft, and deep space and lunar satellite operations. The background, requirements, and scenario for each of these areas of research are briefly described.

Technological developments leading toward the use of optical communications on deep-space missions are reviewed. Developments in the characterization of atmospheric visibility and the development of advanced composite materials for use in optical communications systems are discussed. System demonstrations involving the Galileo Optical Experiment and a very lightweight optical communications package intended for the Cassini mission to Saturn are addressed. Proposed flight experiments and a deep-space optical communication long-range plan are discussed.

An integrated testbed consisting of five nodes has been proposed for proof of concept of a space-based network using laser communications. The acquisition of signals by image-preserving atomic line filter is discussed along with multichannel modulation and overall design considerations for space-based laser-communications networking. A demonstration of the testbed is reported.

Atmospheric turbulence effects on multigigabit/s data rate, pulse-position modulation (PPM) laser downlinks are examined for M = 4 or less using laser transmitter power of 100 mW to 1 W. From the impulse response curves, it is shown that the pulse spreading can be about 5 ps for strong turbulence and about 0.05 ps for weak turbulence. Thus in clear air turbulence without clouds and fog, the contribution from the stretching of a short pulse of 50 ps width will be negligible in both strong and weak turbulence. The high-data rate communication system performance is analyzed in terms of a BER using a thermalized equation derived here. It is concluded that, for the weak turbulence case, the required laser transmitter power to achieve a BER of 10 to the -8th is about 700 mW of power. For the strong turbulence case, a peak laser transmitter power of 1 kW is required to achieve a BER of 10 to the -5th.

A 'many points in one point' multiaccess laser system has been examined, and results are reported which indicating the feasibility of optical communications multiaccess communications is feasible and that the resultant terminal size and with savings are very substantial compared to the dedicated optical terminals or RF multiaccess approaches. A preliminary design has been accomplished of a package that can weigh less than 150 pounds and which uses a 10-in. effective diameter fixed telescope and requires less than 100 W of regulated power and less than 8 cu ft of space.

The Block-Oriented System Simulator (BOSS), an operating system that provides an interactive environment for simulation-based analysis and design of radiofrequency and optical communication systems, has been developed. BOSS facilitates a hierarchical block diagram approach to the system by first defining a detailed component module which is used in turn to define subsystem and larger system models. The simulation can be executed in Monte Carlo fashion or in some cases using a processing-efficient semianalytic model. Simulation results can be displayed as time waveforms, frequency domain spectra, eye diagrams, or BER plots. A consistent user interface utilizing interactive graphics allows the modules to be easily reconfigured for analyzing design alternatives or defining a new system.

Two different designs for laser diode drivers with bit rates up to 120 Mbit/s are presented. One driver, designed for single-strip medium power, uses a low-impedance coaxial line to connect the laser diode, while the other is designed for high-power single stripe or multiple-stripe diodes. The requirements, design and technology, and performance of the drivers using the designs are reviewed.

The concept of double-beam modulation (DBM) has been demonstrate using SWAOM, SWSAWM, and DMB frequency-locked lasers (DBM-FLL). The concept and characteristics of DBM are reviewed, and some initial experiments to demonstrate the concept of DMB-FLL are described. DBM technologies for space communications are briefly addressed.

Optical detectors and processing electronics are key elements of electrooptical systems. It is nearly always the goal of the designer to develop a combination that works with less light than previously achieved. The results of a development effort that focuses on a heated avalanche protodiode and hybrid transimpedance preamp are discussed. Evaluations are performed to identify the operating parameters that maximize the threshold detection of pulses. With the package case held at 20 C, the best sensitivity of less than 2.0 nW was found for a responsivity of 1.2 to 2.2 MV/W depending on the temperature. Sensitivity remained approximately the same from a photodiode temperature of 20 to 50 C.

An offset feedback-type point-ahead method is described and its application to the Engineering Test Satellite VI (ETS-VI) laser communication equipment (LCE) is examined. The role in the method of a high-resolution mirror-deflection mechanism driven by a multilayered piezoelectric actuator is pointed out. The pointing resolution using the method is estimated to be less than 1 microrad at the output of the transmitting optics, which is sufficient of the LCE experiment.

The design of an inertially stabilized 2-axis gimbal for use with space laser communications systems is presented along with test results obtained for this gimbal during operation within static and induced vibration disturbance environments. When operated in a seismically quiet background environment, the gimbal has demonstrated a closed loop line-of-sight (LOS) angular stability of 38 nanoradians, 2-axis, 1 sigma, RMS. When exposed to an input vibration disturbance force of 1 milli-G, RMS, (simulating a worst case disturbance environment anticipated for space operational use) the LOS stability remains better than 100 aRad over a frequency range of 0-1 KHz. The experimental apparatus, test methodology, probable error sources and error magnitudes are described.

The concept of beam control in Multiple-Aperture Transmitting Systems (MATS) is presented. Results of computer simulations for beam pointing, tracking, and combining are demonstrated in three-dimensional graphics. The results show that these operations can be fulfilled in the far field by adjusting and controlling the wavefront phases transmitted by each subaperture in MATS.

Space based optical communications systems that communicate with several satellites need a method to simultaneously track multiple platforms and receive low rate data from each platform. The tracking and data link are required to manage the communications network and to facilitate rapid switching of a high data rate optical link. We will describe a proof-of-concept (POC) system that utilizes a high speed area array detector to simultaneously track and receive data from multiple optical communications beacons. The tracking has sufficient precision to permit rapid acquisition and tracking of any of the beacons by a high speed optical link. The sensitivity of the system is sufficient to permit open loop pointing of divergent beacons. The beacon can be modulated at kilobit/sec rates and the tracking system is capable of synchronizing and extracting the data from all the beacons being tracked.

A laser communication system concept for intersatellite link applications, based on diode-pumped Nd laser technology and coherent detection, is analyzed. As a result of modulation format comparison, parametric link budget and trade-off analysis, a DPSK heterodyne receiver-based communication system is proposed. The coherent-detection full duplex 1.064-1.047-micron Nd-laser link offers the advantages of high link capacity and quality, reduced telescope aperture and relaxed pointing accuracy requirements.

The capabilities of the Nd-YAG laser for optical intersatellite links have been experimentally demonstrated. The experimental setup is described and the heterodyne ASK receiver sensitivity is reported. The receiver exhibited a degradation of -5.6 dB compared to the shot noise limit.

The design, assembly and testing of the laboratory breadboard model of a CO2 laser free-space transceiver terminal with full scale optics have been successfully developed at comparatively low cost. The transceiver has a 140 Mbit/s communication subsystem and a spatial pointing, acquisition, and tracking subsystem. The most important test results are presented and their relevance for other free-space communication systems using different laser technologies is pointed out. A comparison to specifications for intersatellite and interorbit link applications is used to show the basic feasibility of optical space communications and to point out areas which need technological improvements.

We designed and built a full duplex atmospheric optical communication link to convey video and audio signals at ranges up to 1 mile. We used an active tracking system to point each terminal at the other. This paper describes elements of the design and discusses test results.

On ESA's large communication satellite OLYMPUS a set of three orthogonally arranged microaccelerometers is installed. The aim of this experiment is to characterize and observe the behaviour of the different mechanisms in space and to get measurements of the vibration levels which are of relevance for the design of optical communication payloads. During the commissioning phase of the spacecraft in October 1989 a lot of data recordings were made at ESTEC. The translation of the measured acceleration spectra into a base motion spectra shows a higher amount of vibration in the frequency region above 100 Hz, when compared with previously reported data from Landsat spacecraft. The main disturbing vibration sources, solar array drive and thruster firing, can not be characterized precisely enough before the launch. This confirms that measurements in space are mandatory before flying an optical payload. Therefore ESA plans to install this kind of equipment on several other spacecrafts which will be launched before launch of ESA's optical communication payload SILEX. In this paper the equipment on board OLYMPUS will be described followed by presentation of typical measured data. A specification for the micro-vibration environment with which an optical payload has to cope is then derived.

For the ESA Semiconductor Intersatellite Link Experiment (SILEX), elements of the communication chain have been breadboarded. The electrooptical converter, called the laser diode transmitter package (LDTP), is described here. The requirements on the LDTP optical quality are deduced from the overall system requirements. The tolerable wavefront errors (WFE) and the stability of beam direction are most critical. Four breadboards have been assembled and tested. The very stringent requirements on WFE were surpassed, with a resulting rms value of 1/40 waves. In order to achieve this wavefront quality, the typical astigmatism of index-guided laser diodes (1-10 microns) had to be compensated by adjustable cylindrical lenses.

The transcript proceedings of the panel discussion has been included in this volume to provide a permanent record of the opinions and views of the panel members and the audience participants. It also provides a snap-shot of the status of free-space laser communications and the projections for the future. There were a few difficulties encountered in the compilation of this transcript. It was not possible to include all of the view graphs presented by the speakers. Additionally the quality of the audio recording was less than desirable. Every effort was made to accurately interpret the more garbled sections. lt is hoped that these omissions will not detract form the utility of the text. The panel members are acknowledged for their thoughtful presentations and ensightful discussions. Members of the audience asking questions, who could be identified by the conference chairs, have also been included in the transcript.