Lasers are now over thirty years old. In the years immediately following their first demonstration many potential uses were postulated and communications was at or near the top of most lists. Indeed, simple communication demonstrations using lasers were the stock-in- trade of many laboratory open houses during the '60s and '70s. However, as is often the case with new discoveries, much ancillary development must occur before the potential can be usefully exploited in practical systems--and this has certainly been the case with communications via laser beams. I personally believe that all of the pieces exist for practical space laser communication systems. However, funding constraints and the existence of a mature microwave (RF) technology base have contributed to a slow acceptance of laser crosslinks in space.

The European Space Agency ESA is developing an optical inter-orbit communication system enabling a link between a low-Earth orbiting (LEO) and a geostationary spacecraft (GEO). This so-called SILEX system allows a transmission of 50 Mbps from LEO to GEO in an experimental and pre-operational mode. The system uses GaAlAs laser diodes operating at 60 mW average power and direct detection. Both terminals are in phase C/D. The terminals will be launched on board the ESA satellite ARTEMIS and the French satellite SPOT 4 with nominal launch dates in 1996 and 1995, respectively. A fully operational data relay satellite DRS 1 will be launched in 1999 to complement the European data relay infrastructure. Apart from the SILEX project the Agency has investigated advanced system concepts to optimize the terminals and to fulfill the data rate requirements of future Earth observation instruments. Results of system studies and ongoing R&D activities in the areas of high-power Nd:YAG lasers and coherent receivers are presented.

Communications Research Laboratory (CRL) plans to perform basic optical communication experiments using the Japanese Engineering Test Satellite-VI (ETS-VI) to be launched in 1994. The optical payload for the ETS-VI, named Laser Communication Equipment (LCE), was manufactured in 1992. The integrated satellite system is now in a series of proto-flight tests. The optical communication experiment will be done between LCE and ground stations. The main ground optical system is located at the Space Optical Communication Research Center of CRL. It has a transmitting telescope (20 cm diameter) with a fine pointing mechanism, a main 1.5 m diameter telescope with an image intensifier tube camera and a communication signal detector (APD:avalanche photo diode), and optics to modulate an argon laser beam intensity. We have also other additional optical systems such as a 50 cm diameter cassegrainian telescope. To control and monitor the condition of LCE, we are developing a telemetry/command terminal, taking into account of realization of an easy and safe operation system for the planned experiments.

For over ten years, the Optical Communications Group at M.I.T. Lincoln Laboratory has been investigating and developing the technologies required to make high to very high data rate optical intersatellite crosslinks a reality. Based on the design for the Laser Intersatellite Transmission Experiment (LITE) of the mid-1980's, an Engineering Model program has developed flight-qualified hardware and an integrated optical terminal. In parallel, technology extension work has concentrated on achieving higher data rate and higher power terminals in smaller, lighter packages.

Progress in the NASA-funded optical communications program at the Jet Propulsion Laboratory (JPL) is described. This description includes a system-level breadboard for an optical communications flight package, the planning for the Earth-reception facilities, and the results of a recent optical communications experiment to deep space with the Galileo spacecraft.

A geosynchronous free space optical communications crosslink system is described. System analysis and technology development for a direct detection baseband digital optical crosslink at 650 Mbps is presented.

A space-based optical communications system requires the development of high-precision yet rugged electro-optical hardware. Lincoln Laboratory has built and assembled an engineering model of a high-data-rate heterodyne laser communication satellite crosslink to demonstrate the fabrication, integration, and test methods required of an actual flight system. A description of the hardware that has been developed and the testing program to which it has been subjected is presented.

The communications, laser-control, beam-steering, boresighting, spatial acquisition, tracking, point-ahead, and control functions of the LITE Engineering Model are described. The hardware is described in a companion article.

We measured far-field patterns of on-board laser communication equipment (LCE) using a free-space laser transmission simulator. The LCE was developed by Communications Research Laboratories (CRL) for basic optical communications experiments using the Japan's Engineering Test Satellite-VI (ETS-VI), while the free-space simulator is being developed by ATR as an on-ground test system for laser communication terminals. Far-field patterns of an on-board laser communication terminal have measured experimentally by an on-ground test system for the first time. The LCE emitted a laser beam whose peak directive gain was 104.6 dB and whose beamwidth was 31 X 19 (mu) rad in full-width at half-maximum. It was confirmed that the transmitted beam of the LCE met the experimental requirements. Through the measurement, the free-space laser transmission simulator demonstrated its effectiveness in an on-ground measurement of the beam characteristics of a laser communication terminal.

An atmospheric lasercom link over a 4 km path has been implemented between The MITRE Corporation and Lahey Clinic, Bedford, MA. This testbed employs emerging 1550 nm laser and receiver technology for greatly enhanced covertness and eye safety compared to current 820 nm technology. Supporting testbed equipment provides automated monitoring of link performance, correlated with automated data acquisition of the local visibility and precipitation. This report describes the laser link and supporting testbed equipment under IBM PS/2 computer control in which continuous monitoring of the atmospheric conditions, link bit error rate, pointing jitter, and beam wander are accomplished. The bit error rate drives an automated adaptive data rate modem at one of three data rates, depending on weather conditions. Criteria for operation at 1550 nm, such as trade-offs with regard to atmospheric absorption, covertness and eye safety are presented. Characteristics of our custom designed laser transmitter (laser diode, beam parameters, modulation, etc.), and receiver (optics, detector electronics, bit synchronizer) are described. System alignment procedures and transmission performance as a function of temperature, visibility, and precipitation are presented.

For very high data rates, optical communications holds a potential performance edge over other technologies, especially for space applications where size, weight, and power are of prime importance. We report demonstrations of several Gigabit-per-second (Gbps) class all- semiconductor optical communications systems which have been developed for free-space satellite crosslink applications. These systems are based on the master-oscillator-power- amplifier (MOPA) transmitter architecture which resolves the conflicting requirements of high speed and high power on a single-laser coherent transmitter. A 1 Gbps, 1 Watt system operating at 973 nm with a frequency-shift-keyed (FSK) modulation format is the highest power coherent optical communications system using all semiconductor lasers reported to date. A 3 Gbps differential-phase-shift-keyed (DPSK) system uses a 2-stage injection-locked diode array as a power amplifier at 830 nm. At a wavelength of 1.5 micrometers , an optically- preamplified direct-detection on-off-keyed (OOK) receiver was demonstrated at both 3 and 10 Gbps. A 3 Gbps optically-preamplified direct-detection DPSK receiver was also demonstrated and represents, to our knowledge, the highest sensitivity DPSK receiver reported to date for data rates above 2 Gbps.

A next generation space lasercom system terminal design is presented, capable of 3 Gbps full duplex data transmission. The terminal incorporates an eight inch telescope, with all optical elements mounted in the payload of the gimbal, and is expected to require less than 100 lbs., 100 W. The modulation technique can transmit digital data, or analog signals in 'bent-pipe' transponder mode. The system design utilizes a TRW patent for direct phase modulation of laser diodes, and incorporates previous research on laser linewidth-tolerant coherent modulation techniques, control system architectures for CCD tracking, diffraction-limited collimating optics for high aspect ratio laser diode arrays, and a magnetically-suspended fast steering mirror. The terminal uses modified designs with existing technology, and is suitable for near term operational status. The overall terminal weight/power is just under 80 lbs., 80 W, which is estimated to be less than one-third the weight and power of a comparable 60 GHz system.

This paper describes an optical direct-detection multiple access communications system for free-space satellite networks utilizing code-division multiple-access (CDMA) and forward error correction (FEC) coding. System performance is characterized by how many simultaneous users operating at data rate R can be accommodated in a signaling bandwidth W. The performance of two CDMA schemes, optical orthogonal codes (OOC) with FEC and orthogonal convolutional codes (OCC), is calculated and compared to information-theoretic capacity bounds. The calculations include the effects of background and detector noise as well as nonzero transmitter extinction ratio and power imbalance among users. A system design for 10 kbps multiple-access communications between low-earth orbit satellites is given. With near- term receiver technology and representative system losses, a 15 W peak-power transmitter provides 10^-6 BER performance with seven interfering users and full moon background in the receiver FOV. The receiver employs an array of discrete wide-area avalanche photodiodes (APD) for wide field of view coverage. Issues of user acquisition and synchronization, implementation technology, and system scalability are also discussed.

One of the activities being undertaken in the Free-Space Optical Communications Program of the European Space Agency, is a detailed description of the Link Budget for Laser Communications through the atmosphere. A link budget model definition, for a system using a tracking system with a given probability of failure shall be presented here, as well as an application to a ground station to/from the geostationary ARTEMIS satellite optical link.

With ever increasing improvements in the technologies which support direct detection laser crosslinks, projections for even smaller, lighter, and lower power crosslinks are realistic. The rapid increase in diffraction-limited laser power is one of the main drivers for the trend toward smaller, more efficient terminals. Most particularly, the power summing of multiple high power laser diodes and the parallel trend toward higher power single diode sources is yielding projected powers in excess of several watts. This rapid increase in diode power coupled with long life enables a reduction in crosslink aperture with resultant simplicity in acquisition, pointing, and tracking requirements as well as a reduction in terminal size, weight, and power. Based on projected improvements in laser power and efficiency, detector sensitivity, electronics size and efficiency, and terminal simplicity, the physical characteristics of advanced terminals are projected. A fifty pound, direct detection terminal operating over 84,000 km synch to synch range at 10 Mbps is projected as being realistic by the end of this decade. In addition, higher data rate and longer range terminal characteristics are projected. The rationale for the projections is discussed.

Trade studies and close examination of direct detection laser crosslinks for a number of applications have shown that the development risk is comparable to or lower than that of RF systems. Since laser crosslinks offer significant advantages over RF crosslinks for the same applications, the specification of laser crosslinks is a good choice. The reason that the risk is low is examined in this paper which details the technologies and components which comprise current, second generation design laser crosslinks. The specifics which are examined include diode power summing, diode life, data rates of a Gbps and beyond, detector sensitivity/radiation effects, telescopes, gimbals and optics, alignment stability, acquisition, pointing and tracking, electronics and the space qualification of assemblies and systems. Differences between first and current (second generation) system designs and technologies are examined. An assessment of the maturity of the current direct detection technology is made and conclusions formulated.

A new modulation technique called combinatorial pulse position modulation (CPPM) is presented as a power-efficient alternating to quaternary pulse position modulation (QPPM) for direct-detection, free-space laser communications. The special case of ₁₆C₄PPM is compared to QPPM in terms of data throughput and bit error rate (BER) performance for similar laser power and pulse duty cycle requirements. The increased throughput from CPPM enables the use of forward error corrective (FEC) encoding for a net decrease in the amount of laser power required for a given data throughput compared to uncoded QPPM. A specific, practical case of coded CPPM is shown to reduce the amount of power required to transmit and receive a given data sequence by at least 4.7 dB. Novel hardware techniques for maximum likelihood detection and symbol timing recovery are presented.

This paper summarizes the design for a laboratory testbed currently under development to evaluate the performance of intersatellite laser crosslinks that directly impose a modulated waveform as an analog signal on the laser carrier. In our analog scheme we use a classical 'bent pipe' satellite implementation, where the RF uplink signal spectrum is frequency translated to directly modulate the laser as an analog waveform, and the received signal at the other end of the optical crosslink is frequency translated to directly modulate the RF downlink. This system design and hardware proof-of-concept addresses the optimum approach to such an analog optical crosslink to facilitate evaluation of its performance.

The paper describes the investigations carried out for the communication of analogue video with digital data signals over a free space optical laser link for the remote control of a mobile robot. A number of digital data with analogue video modulation schemes are investigated. The aim is to make effective use of the bandwidth whilst maintaining an adequate duplex data link and a simplex video link. The rate of data transmission required to implement the control loop and the error rate are measured. Picture and data quality tests are described with degradation of the signal. A communication protocol is recommended for the particular application.

The design concept of simplified gimbals for the purpose of geostationary satellite tracking from low earth orbits is discussed and the preliminary results of a trial fabrication of a tracking system with a 19-cm-diameter telescope and an acquisition/tracking/pointing control system are shown. A new method is introduced for producing light-weight and small equipment by a self-alignment mechanism and its implementation into the optics design is described.

By combining an aspherical reflector and a thick lens at its center, it is possible to produce an ultra wide-angle optic called the GEM. The device was originally conceived as means for broadcasting optical signals over nearly a full angular sphere. Since the fabrication and testing of several GEMs, wide angle receiving and imaging functions have also been evaluated.

A novel coherent detection spatial tracking with angular span larger than one beamwidth was designed. The coherent detection spatial tracking system had the advantages that it is not susceptible to background radiation and thermal noise, and the azimuth and elevation tracking errors are independent with each other. It was shown that the optics design of the coherent detection spatial tracking system was based on a highly sensitive angular discriminator which was implemented using a Fabry-Perot Etalon. However, it was also shown that the Fabry- Perot Etalon must be rotated with respect to the optical axis of the optical receiver at a certain angle. This angular bias must be maintained at all times. In this paper, the advantages of using electro-optic materials between the two partially reflecting parallel surfaces of the Fabry-Perot Etalon will be investigated. When electrical voltage is applied to the electro-optic material, the optical phase of the electric field of the laser beam will be shifted by an amount proportional to the applied electrical voltage. This optical phase change will alter the nominal angular operating point of the Fabry-Perot Etalon at which the gain of the angular discriminator is maximum. It will be shown that the angular operating point of the Etalon can be easily locked in using a simple feedback loop.

A compact, light-weight, two-axis beam steerer (TABS) with a 38 mm (1.5') Beryllium clear aperture was designed into a 1000 Hz servo loop for precision tracking of a remote laser source. The design is composed of a quadrant photodiode, a track-tone demodulator, a linear ratiometric processor with sum and difference circuits, and a proportional/integral controller with a phase lead compensator. The demodulator is a phase-lock-loop used for locking onto and then synchronously detecting the envelope of a modulated tracking beacon. This approach provides background rejection, star discrimination (or any other CW source), receiver AC coupling, and 1/f noise reduction. The linear ratiometric processor provides x and y error signals to the controller which are independent of input signal levels. The far-field performance goal is < 0.5 microradians rms from a Landsat-type of base motion disturbance.

Application of very fast CCD detectors to both fine pointing and tracking and to low data rate reception for free space optical communications has been demonstrated in several prior programs. CCD detector based wide field multi-channel receivers have been considered for several new applications. We are conducting a program whose goal is evaluation of radiation induced degradation in the properties of CCD devices for use in such systems. In this paper we present initial results of this ongoing effort. We present a brief overview of systems design and detector design for multi-channel low rate data receivers. Following this we present results from a comprehensive experimental determination of CCD performance degradation due to ionizing radiation. Ionizing radiation testing was accomplished using a Co60 source at the Salk Institute. Dose levels up to 1 Mrad were utilized in this test activity. We will also briefly discuss aspects of displacement damage testing activity at the University of Rochester Tandem VandeGraaff accelerator. Primary emphasis of this effort will be on evaluation of charge transfer efficiency changes due to displacement damage. Special attention is given to charge trapping phenomena and their impact on CTE. A review of test results for the ionizing radiation test portion of our program is given.

The diode combiner, or multiple diode laser (MDL), as it is currently known, was conceived at McDonnell Douglas and development started in 1984. Since that time some five generations have been developed and tested. Currently the multiple diode laser is baselined on a military crosslink. As single mode diode sources increase in power with demonstrated reliability, the usefulness of the multiple diode laser seems assured. Even where new high power semiconductor technology emerges, such as the Master Oscillator Power Amplifier (MOPA), the combining technique still offers advantages to the laser crosslink designer. The development and current status of this laser is examined along with some of the options available to designers of future laser communication crosslinks.

The Atlantic Laser Ground Station (ALGS) is an optical transceiver developed to provide initial checkout and calibration of Laser Crosslink Subsystem (LCS) equipped satellites. The ALGS consists of laser, optics and electronics integrated into the Phillips Laboratory Malabar Test Facility near Malabar, Florida. The laser transmitter consists of three separate laser systems, one of which is a modified Quantronix Model 117 lamp pumped Nd:YAG used as a beacon for the LCS to acquire and track. The receiver consists of a package of relay optics and avalanche photodiodes mounted on a facility's 1.2 meter diameter telescope. Results of optical wrap-around tests, tests reflecting off of passive satellites, and tests involving active transceiving with the laser radar package on the SDIO DM 43 satellite are presented.

Laser satellite networking is a key element of effective communications operations to support both strategic and tactical missions. Lasercom offers a number of important advantages over conventional RF satellite communications. The shorter wavelength available using lasers provides higher data rates at less power and smaller apertures, both resulting in lower weight requirements. On the other hand, lasercom entails more difficulty in acquisition and tracking because of the narrow beams used. Technology problems to be overcome before intersatellite laser communications can reliably outperform RF communications include acquisition in the presence of significant background light from the earth, tracking to resolutions of a few microradians, high speed modulation of semiconductor lasers with close to one watt of power in a diffraction limited beam, high bandwidth low noise detector response, and demonstrated long term performance. We have developed critical technologies to solve some of these problems, and demonstrated them in a laboratory testbed which also supports development and testing of network protocols and algorithms. Our hardware provides new capability in background light rejection by using innovative atomic line filter technology, improved tracking accuracy by using innovative zero backlash Roto-Lok drive gimbal telescopes, and increased communications bandwidth by incorporating multi-link networking protocols.

A breadboard sync satellite terminal has been developed incorporating a unique concept of a gimballess multi-access transceiver, which is capable of simultaneously communicating with six independent, asynchronous LEO satellites. The developmental hardware illustrates that low power, weight, and volume is achievable compared to multiple independent gimballed transceivers, while also putting a lesser burden of power, size, and weight on the LEO satellite transceivers that communicate with the multiple-access transceiver. Evaluation tests demonstrate the feasibility of the concept.

In the Galileo Optical Experiment (GOPEX), optical transmissions were beamed to the Galileo spacecraft by Earth-based transmitters at Table Mountain Observatory (TMO), California, and Starfire Optical Range (SOR), New Mexico. The demonstration took place over an eight-day period (December 9 through December 16) as Galileo receded from Earth on its way to Jupiter. At 6 million kilometers (15 times the Earth-Moon distance), the laser beam sent from Table Mountain Observatory eight days after Earth flyby covered the longest known range for laser transmission and detection.