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 (GEO) spacecraft. The link allows the transmission of 50 Mbps between LEO and GEO in an experimental and pre-operational 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. Other concepts for optical space communication links using Nd:YAG lasers and heterodyne receive systems are outlined.

An account is given of current and prospective activities in Japan in the field of optical space communications (OSC) and its associated optical technologies. These activities are conducted by NASDA, the Advanced Telecommunications Research Institute, and the Ministry of Posts and Telecommunications. Attention is given to such initiatives as high-precision tracking, bidirectional OSC, laser beam propagation, optical ground stations, and a free-space simulator for laser transmission.

A development history, current status, and prospective development account are given for deep-space optical communications, giving attention to overall systems-related activities and to the generic technology developments required for their implementation. An account is given of the progression envisioned from laboratory subsystems demonstrations to the full-scale flight experiment for the Mars Communications Relay Orbiter missions.

Germany's Solid State Laser Communications in Space, or 'SOLACOS' program has undertaken the terrestrial verification of coherent laser communications systems based on Nd:YAG lasers, giving attention to the evaluation and breadboarding of critical components and subsystems. These components encompass the pointing/acquisition/tracking subsystem breadboard, an optical Costas-loop receiver, and advanced Nd:YAG transmitter technology. Results are presented for subsystem components developed to date.

The present optical terminal design for a high data rate interorbit communications link with 500 Mbit/sec (or higher) transmission rate is based on high-power laser diode transmitters with 1 W peak power at 800 nm, in conjunction with a direct-detection receiver; two-channel wavelength-division multiplexing, together with the high transmission capability of the off-axis telescope used, contribute to the achievement of the requisite data rates. In order to minimize system mass, the only rotatable device is the telescope.

The SILEX experimental program is concerned with demonstrating the technologies of an optical communications link between two satellites; in order to expand system capabilities to the high data rates required for future LEO-GEO interorbit links, a detailed design study has been conducted for a system predicated on diode-pumped Nd:YAG laser technology. Even with telescopes whose apertures are less than 10 cm on the LEO satellite, and transmitter powers of less than 1 W, system transmission performance is greater than 1 Gbit/sec.

GEO and LEO terminals for optical Interorbit Links (IOL), based on diode—pumped Nd:Host laser technology, are under definition. Assuming a future european scenario with a few LEO users candidate for laser communications, our design choice is oriented towards a compact and lightweight LEO terminal and a GEO one capable of relaying in single—access mode also other channels, implemented at different wavelengths. System and design issues relevant to the overall terminal and the Communication, PAT (Pointing, Acquisition and Tracking) and Optics subsystems are presented. The ternIinal configurations are shown with respect to their possible layout and to their impact on the host spacecraft in terms of mass and power consumption.

Free-space laser communication systems have been envisioned for satellite-to-satellite links for the past 30 years. To date no demonstration hardware has been placed in orbh, although present programs have this as a near term goal. Operational requirements frequently can be satisfied by either laser or rf based systems. In a comparison between the two base technologies it is frequently held that laser communications offers the more costly approach. This paper reviews the economic factors and addresses the cost drivers behind laser communication systems. Examples of such drivers are optics quality, aperture size, transmitter power, pointing error, and system weight. Requirements are presented to form baseline specifications for a geo-geo crosslink application. Analysis of cost drivers and recommendations are presented for terminal development.

A fast optical fiber based tracking detector has been developed, tested and characterized as a cmponent for a coherent Nd:YAG laser space communication system. A freely suspended and conductively coated single-mode fiber end is excited to a fast small amplitude conical motion when electrostatically deflected close to its mechanical resonances by the capacitive action of one or two electrodes. The induced intensity modulation of the laser light coupled into the fiber is phase-sensitively detected (optionally after coherent amplification) and used for spatial input coupling optimization in a servo loop steering the laser beam. Modulation frequencies higher than 10 kHz are feasible in a very compact, rugged device at low power consumption. The device can also be modified for scanning of a transmitter beam.

An engineering model of an electrooptic intensity modulator for Nd:YAG laser wavelength (1.06 micron) was designed, manufactured, and tested. Two lithium niobate crystals were cascaded to constitute a traveling wave electrooptic bulk modulator. The crystals were embedded in a beryllia holding structure to guarantee mechanical alignment and good heat transfer. Electronic bias voltage control was implemented to compensate thermal disturbances and aging effects.

Proton-exchanged LiNbO3 electrooptic waveguide devices are applicable to communications components employing phase and intensity modulation, in virtue of their (1) high optical damage threshold, (2) extremely high polarization maintenance, and (3) flexibility as to waveguide parameters, due to different annealing procedures. Attention is presently given to two examples of such devices, an LiNbO3 phase modulator and an LiNbO3 Mach-Zehnder interferometer modulator operating at 1300 nm.

An optical multiple-aperture antenna whose subantenna fields are phased should permit nonmechanical steering of the outgoing laser beam in analogy to the microwave regime. Corresponding theoretical analysis showed promising results. To our knowledge, however, little experimental evidence of optical phasing exists, prompting a beam steering experiment having 'proof-of-concept' character. Our design foresees a small optical antenna array consisting of three subapertures fed by a diode-pumped Nd:YAG ring laser via single-mode fibers. A measurement and control unit generates the necessary phase adjustment signals applied to piezoelectric fiber stretchers acting as phasing actuators. To this end, a 'pilot-beam' concept was developed. The individual subantenna beams are phased with respect to the pilot beam, which defines the direction of the antenna axis.

An optical intersatellite communication link terminal should be capable of simultaneously transmitting and receiving laser signals. Therefore backreflection of the transmitted laser light from the TxIRx telescope components into the receiving channel is one of the topics in Tx/Rx optics design. This paper summarizes the backreflection analysis and measurements performed on the SILEX telescope breadboard model.

The eccentric three-mirror telescope presently proposed for free space optical communications exhibits diffraction-limited image performance for a relatively large field of view, allowing the design of a simpler coarse-pointing mechanism and a wider range for a fine-pointing mechanism without associated losses of antenna gain. Light baffling in this telescope design is excellent, and allows a reduction of stray light to a minimum; its system parameters are less sensitive than those of two-mirror telescopes, allowing looser tolerances and simplifying structural design.

The Chernoff upper and lower boundaries are presently noted to be applicable to free space optical communications receiver design, as can be demonstrated in the case of optimum filters and optimum thresholds for MPAM communications. Optimum internal gain and level spacing are also obtainable by these means. Attention is given to M-ary receiver structures, the receiver model used, and receiver structures with either paired filters of M filters.

Various image processing algorithms for acquiring and tracking an inter-satellite laser beam are assessed. These algorithms aim at detecting a faint point target of a few picowatts embedded in a background by means of acquisition systems based on CCD sensors. Simple algorithms for real-time operation have been designed. A model which takes into account the background, its spatial and temporal fluctuations as well as the opto-electrical parameters of the CCD has been developed. Fluctuation of the background and CCD noise appear to be the main perturbations that hinder obtaining the target's spatial coordinates. Two types of image processing techniques have been analyzed: single frame processing using spatial filtering and multiple frame processing using frame subtraction. The performed analyses show that in systems where fixed spatial fluctuations are the main constraints, the frame subtraction method is the most efficient in terms of probability of acquisition. Whereas the spatial filtering method is much simpler to implement, it can offer a significant improvement if the spectral energy of the fixed pattern parasitic background is low within the passband of the chosen spatial filter. The performances of these techniques are compared in terms of successful target acquisition.

This paper discusses the coarse pointing assembly (CPA) requirements for the Optical Multiple Access intersatellite link system. Mechanical and optical design aspects of a periscopic CPA with azimuth and elevation angular ranges of 360 and +/- 120 deg respectively, clear aperture of 7 cm, and a step size of 0.09 deg are described. Mass budget estimates for a flight unit are presented. The total mass and maximum power consumption are 3.8 kg and 12 W respectively.

Future laser intersatellite communication links are planned which will have beaxnwidths in the arc second range and below. The performance, and even the feasibility of such links, depends heavily upon the mechanical vibrations induced into the optical terminals by the host satellite(s). In particular, it is important to know the level of mnicrovibrations on a spacecraft for the design of the tracking control loop of an optical communication payload. An attempt to optimize the tracking con,trol loop of an optical communication payload, taking into account the different sources of pointing error, such as detector noise and mnicrovibrations, will be presented.

The monolithic, micromachined devices presently discussed can be used in future satellite payloads to achieve low mass, low volume, and low power consumption requirements associated with pointing-acquisition-tracking purposes in optical free-space communications systems. Such performance parameters as resonance frequency and deflection characteristics can be adjusted through simple variation of the lithographic masks and their processing parameters. The technical feasibility of such monolithic scanners has been demonstrated.

We designed, realized, and tested a laboratory model of an optical intersatellite link employing InGaAS DFB semiconductor lasers operating at a wavelength of 1.55km. Heterodyne sensing was used for both the spatial acquisition and the spatial tracking processes. During the acquisition process spiral scanning of the transmitter area of uncertainty is performed. A microcomputer controls the receiver operation. For each spatial search position the local oscillator laser is swept through a predefined frequency uncertainty range until the beat signal appears at the desired intermediate frequency (IF=700MHz). A heterodyne quadrant detector was realized by using a four-sided reflecting pyramid to split the superimposed beam. During tracking, the microcomputer reads the tracking sensor output signals, calculates the required antenna correction, and feeds the appropriate signals to the beam steering unit. Thus we realized a digital tracking loop, whereas the intermediate frequency is stabilized by means of an analog control loop. The tracking sensor angular resolution -withoutusing a telescope - is 5rad, limited by the digital signal processing, at a detector field of view of lmrad. The typical tracking error measured for various system conditions amounts to less than 1/20 of the FOV, i.e. less than

The optical transmiter component of a free space optical communication system is critical, in that it impacts on the mechanical configuration, power requirements, mass, reliability, and transmission bit-rate of the entire system. Attention is presently given to the transmitter output power and beam quality, as well as its electrical-to-optical power conversion efficiency, in view of state-of-the-art high power transmitters for intensity modulation/direct detection and semiconductor laser transmitter systems.

The present diode laser-pumped Nd:YAG 1.06-micron wavelength ring laser for free-space communications is able to operate in single-frequency TEM 00 mode through the use of the 'twisted-mode cavity' (TMC) technique. Attention is given to passive stabilization methods that make possible the generation of a laser linewidth of less than 10 kHz. The absolute stabilization of the laser on a molecular absorption line via TMC is noted to be especially advantageous for intersatellite communications system applications.

A design concept suitable for space applications is proposed on the basis of a diode-pumped Nd:YAG laser transmitter laboratory breadboard; this transmitter must not only furnish the data carrier but also serve as a tracking reference and (in conjunction with a boost amplifier) as a beacon for receiver-station acquisition. The diffraction-limited beam is generated with 6.3-percent electrical-to-optical conversion efficiency. The proposed space communications system will incorporate astigmatically-focused stacked-laser diodes for continuous reliable operation over the course of seven years.

The very high detection sensitivities and selectivities associated with coherent lightwave techniques furnish a basis for the exploitation of the great potential bandwidth of single-mode fibers in point-to-point and point-to-multipoint transmission. Attention is presently given to the theoretical advantages and limitations of the modulation and detection formats envisioned. An evaluation is made of the development status of prospective implementations of these techniques.

We describe the realization of an optical 565 Mbit/s DPSK heterodyne breadboard communication system at 1064 nm for experiments concerning future space applications. The optical transmitter is based on a diode-pumped Nd:YAG single mode TMC laser with 620 mW which has been offset-locked to a stable low power ring laser. Phase modulation is obtained by a LiNbO3 traveling wave modulator. The receiver frontend consists of two InGaAs PIN Photodiodes in a balanced configuration and a transimpedance amplifier. The IF of 1130 MHz (only twice the data rate) is stabilized by an AFC which is able to follow frequency deviations of more than 10 GHz. The DPSK encoder and the clock/data recovery unit is based on GaAs technology. The whole system works with a single 12 V power supply useful when performing outdoor experiments. To obtain a bit error rate of 10 exp -10, a received light power of less than -48 dBm is required.

An investigation of the most promising coherent receiver configurations for application in Nd:YAG laser based optical transmission links is presented. A BPSK homodyne receiver based on a Costas phase locked loop was realized for a data rate of 600 Mbit/s, using a commercially available diode pumped ring laser at 1064 nm. Furthermore, a DPSK heterodyne receiver with the same data rate using an intermediate frequency of 1.2 GHz was realized. Both have been tested successfully. An optical 90-deg hybrid, built of bulk optical components, was used for the investigation of the receivers. Design and performance of this bulk hybrid are described.

The power losses and noise of the present 90-deg hybrid optical coherent receiver are minimized through the use of a six-port configuration in which channel balancing is furnished by half-wave plates. The 90-deg shift is introduced between orthogonally polarized beam components by a quarter-wave plate, and the output ports are phase-shifted by 180 deg through the sole use of the half-wave plates and polarizing beam splitters.

The design and performance of a large diode-pumped multi-stage Nd:YAG laser system for space and airborne applications will be described. The laser operates at a repetition rate of 40 Hz and produces an output either at 1.064 micron or 532 nm with an average power in the Q-switched mode of 30 W at the fundamental and 20 W at the second harmonic wavelength. The output beam is diffraction limited (TEM 00 mode) and can optionally also be operated in a single longitudinal mode. The output energy ranges from 1.25 Joule/pulse in the free lasing mode, 0.75 Joule in a 17 nsec Q-switched pulse, to 0.5 Joules/pulse at 532 nm. The overall electrical efficiency for the Q-switched second harmonic output is 4.