This conference on Optical Technologies for Communication Satellite Applications is the first conference devoted to explore ways that future space communications systems can benefit from the rapid advances in optical communications technologies. We will hear 37 contributed and invited papers covering a variety of topics on advanced satellite systems, technologies for optical intersatellite links, fiber optics distribution networks for on-board use, optical technologies for microwave systems, on-board processing technology and reliability aspects of optical components.

NASA's return to laser communications technology development was heralded by three timely events, the development of reliable, high performance semiconductor lasers by a number of research groups in the early 1980's, the TDAS study by Stanford Telecommunication Inc. which summarized NASA's future requirements for high data rate crosslink and leo-to-geo communications, and the advent of the Advanced Communications Technology Satellite program (ACTS). As a result of these factors NASA, with the GSFC acting as lead center, has reestablished a vigorous program in laser intersatellite communications, through a strong Proof-of-Concept (POC) program funded by the Office of Space Sciences and Applications (OSSA) and an equally strong technology development program funded by the Office of Advanced Science and Technology (OAST). With this technology base it is now appropriate to initiate meaningful space experiments aimed at testing and demonstrating the maturity of industry as well as government developed laser communication technology and its ability to support NASA's communications requirements in the 1990's and beyond. This presentation will outline NASA's POC program, its component development program and outline and discuss in detail its ACTS Laser Intersatellite Communications Experiment scheduled for the 1990-1992 time frame.

Many advanced microwave system concepts such as active aperture phased array antennas use distributed topologies in which lightwave circuits are being proposed to interconnect both the analog and digital modules of the system. Lightwave components designed to implement these interconnects are reviewed and their performance analyzed. The impact of trends in component development are discussed.

Multiple high bandwidth signals can be time division multiplexed to give continuous data transmission at the optical source bandwidth, e.g. 1016 Hz. The individual components need not operate anywhere near this speed.

This paper describes a laser communication (lasercom) transceiver platform to be used for laboratory experiments. The platform features a directly modulated semiconductor laser, avalanche photodiode receiver, and microprocessor controlled acquisition and tracking system. The platform provides a test-bed, enabling study in vital areas of lasercom hardware such as system performance versus link distance, optical power, tracking accuracy, and sub-system and component characterization for future system specifications, including critical areas limiting present system performance.

Six selected optical technologies are evaluated for use in a full-duplex communications link between geostationary satellites. The six systems are based upon the carbon dioxide gas laser, the neodymium-doped yttrium aluminum garnet (Nd:YAG) solid-state laser, the indium gallium arsenide phosphide (InGaAsP) semiconductor diode laser, and the gallium aluminum arsenide (GaAlAs) semiconductor diode laser. The salient features of the various technologies are described and their communications performance analyzed. The antenna diameter requirements for various modulation formats are determined by the analysis. The GaAlAs systems, with their small size, high electrical-to-optical conversion efficiency, direct modulation capability, wavelength selectability, and high reliability are found to he promising technologies for the full-duplex geostationary intersatellite link.

The ESA (Europe Space Agency) Ministerial Conference on the European Long Term Programme (Rome - 30/31 January 1985) has recognized the Data Relay System necessary in order to satisfy Europe's future operational needs : - Columbus Space Station - Free Flyers (Eureca,...) - Future European stations - Hermes (starting mid 90's, 4/6 missions per year towards the end of the 90's) - Ariane 5 (6 to 10 launches per year from the mid 90's).

This paper describes the baseline design of an optical intersatellite communication system using CO₂ laser technology and reports about the present status of the relevant technologies and the various hardware development activities going on. These activities are primarly aimed towards laboratory breadboarding and full operational testing of the system, with the potential of space qualifying all hardware components for reasons of ex-perimental inflight testing in the early 1990's.

As part of NASA's planetary exploration augmented missions set, the Mars Sample Return mission will attempt to land a roving vehicle on the Martian surface, collect samples from various sites, and return these samples to Earth. An optical communications system is being considered for the prime Earth to Rover communications link. Such a system, as currently envisioned, would provide 30 kbs telemetry from a 10 cm transmitter aperature utilizing approximately 1 watt of laser power to a 3 m non-diffraction limited receive aperature located in low Earth orbit. Channel effects such as Martian atmospheric dust, wind, and Earth outages were studied and their effects upon the package design were assessed.

After discussing the sensitivity limit of optical PSK homodyne receivers we describe the realisation and properties of a mechanically cooled photodiode/preamplifier module used in our breadboard model of a 10 _pm receiver. Several receiver concepts are described which have in common frequency and phase synchronization between input signal and local laser oscillator signal with a phase-locked loop (PLL) control circuit. Means to avoid DC-coupling of the PLL are discussed. The results of bit error measurements with an ordinary, DC-coupled homodyne receiver operated at 140 Mbit/s are presented.

A GaAlAs laser was developed which dichroically combines six single-mode 830 nm diodes in a 100 Angstrom total bandpass. The laser is designed to produce 250 mW peak power at a pulse rate of greater than SOO Mpps. Single mode operation of each diode is necessary since diode-to-diode channel spacing is only 20 Angstroms. Use of an etalon for each diode maintains single mode operation in the presence of wideband modulation. The diode mount, a universal header, accommodates diodes from various manufacturers and includes an impedance matching network for the diode as well as a thermistor and heater for diode closed-loop temperature control. Diffraction-limited optics are used to maximize the combined beam far field irradiance for free-space applications. Since the diodes are strongly linearly polarized, two six-diode lasers may be added using polarization combining to produce 500 mw peak power in a 100 Angstrom total bandpass.

A Wideband Lasercom Transmitter based on dichroic combination of directly modulated GaAlAs laser beams was built, and its performance with regard to various requirements was evaluated; these include rise and fall times, extinction ratio, modulation bandwidth, wave-length stability, power, polarization, and transverse mode structure.

The alternative detection strategies for optical communication with pulse position modulation (PPM) are analyzed and compared both for systems with and without error correcting codes. If no error encoding is used, the detection alternatives include the threshold and maximum count M-ary strategies, as well as a method of bit decoding on parallel binary channels. M-ary maximum count offers the best performance as it is a maximum-a-posteriori (MAP) strategy, but threshold may be much easier to implement at high data rates. For the particular choice of metric analyzed, the binary scheme is competitive only for low values of PPM word size. For PPM communication with error correcting codes, the decoder has the additional option for hard or soft decisioning. Each of the unencoded detection schemes can interface directly with a hard decision decoder. The hard decision coding performance can be characterized in terms of input/output bit error probability for a given code. Coding gain is defined to reflect the optical power savings by incorporating the code, and example hard decision coding gains are evaluated for M-ary Reed-Solomon and binary BCH codes with an APD receiver. Soft decision decoder performance is dependent on both the input error and erasure probabilities from the channel. Soft decisions strategies offer additional coding gain, and the performance of rate 1/2 Reed-Solomon codes is compared for A-Max and threshold soft decision strategies with an APD receiver.

A space-based laser communication system must be capable of high-speed, high-accuracy pointing for reliable communications. Results of recent studies and developments indicate that a pointing, acquisition, and tracking subsystem with these capabilities can be accomplished with current technology. This paper discusses the development of a nested-servo control system and a series of fine-steering mirrors for high-accuracy pointing control. It also presents the results of performance modeling on acquisition and tracking detectors for use with the nested-servo control system.

A model is described for analyzing the effect of tracking errors on the performance of optical intersatellite links (ISL). For a representative link configuration, it was concluded that state-of-the-art diode laser and tracking subsystem technologies are adequate to limit link impairments by burst errors to acceptable levels. In the future systems with transmitter power levels of -1W and noise equivalent angles of -0.4 I./rad, the effects of burst errors are ex-pected to be negligible.

The influence of burst errors on a satellite-borne, free space optical communication systan is evaluated. Burst errors occur when the instantaneous mispoint loss exceeds the allocation in the link budget. The single axis mispoint probability distribution is described as a gaussian function and used to define the probability of burst error. It is shown that the probability is related to the ratio of the pointing error and the rms standard deviation of the pointing error distribution. It is also shown that an optimum ratio of the beam-width to the pointing error may be deduced from the range ecuation. A relationship between the rms standard deviation of the pointing error distribution, the probability of burst error and the Airy far field beamwidth has been derived. The limitations caused by mispointing and the probability of burst error on the systan beamwidth and the antenna diameter are discussed. It is shown that a satellite-borne optical camiunication system must include both the probability of bit error and the probability of burst error for system requirements to be adequately specified.

The possibility of using laser systems for space-based communications is attracting considerable attention in Europe in connection with the future European data relay satellite. The choice of a semiconductor laser as transmitter is only workable if a high-accuracy control system can be developed for precision beam pointing. The accuracy requirement is sub-microradian. Another major challenge is the acquisition of the line of sight (between the two spacecraft) over a relatively large cone of uncertainty (apex angle of 0.4 °). This paper gives details of a proposed definition for an acquisition and tracking subsystem using semiconductor technology. For a link between a spacecraft in low-Earth orbit and a geostationary relay satellite, the separation is taken as being 45,000 km. Two different versions of the acquisition and tracking subsystem are considered, depending on the mission specifications: - version 1 uses a pulsed diode laser and is suitable for applications where no significant volume of data is transmitted over the forward link (GEO -- LEO direction according to the established terminology); - version 2 uses a continuous-wave diode laser and can accommodate a 25 Mbps forward link. In both cases, the return link is rated to carry 400 Mbps. A different optical subsystem is proposed for each version. To demonstrate the feasibility of the pulsed version, a breadboard model of the LEO acquisition and tracking subsystem has been built. Acquisition time of around 4 s have been measured for an initial beam pointing error of 0.25 prod. Aspects of the GEO optical beacon have also been investigated using an optical fiber bundle. This permits the illumination of the entire GEO cone of uncertainty while maintaining speckle modulation at an acceptable level.

The results of an investigation of the performance of three basic precision pointing and tracking control subsystems considered for application to satellite to satellite optical communication missions are discussed. The three-control subsystems include: 1) gyro-stabilized, 2) mass-stabilized and 3) complementary filter. The sources of error included in the analysis included: 1) sensor noise from the optical detector, 2) host satellite baseframe vibrational noise and 3) frictional and bearing noise. The measured vibrational and disturbance data fran the LANDSAT satellite was used to generate the power spectral density parameter needed to model the baseframe noise environments of the two satellites used for the evaluation. The results of the study indicate that the 1 microradian rms pointing and tracking accuracy may be achieved with either the gyro-stabilized or the complementary filter approach.

In free-space optical communications systems, a number of mechanisms can produce a temporally fluctuating signal-to-noise ratio (SNR), giving rise to fluctuations in the bit error probability that have time constants which are extremely long in comparison to the bit length. Among these mechanisms are pointing errors and atmospheric turbulence. This paper presents simple methods of properly accounting for the fades and surges in optical communication system design through the concepts of burst error and burst error probability. Examples are given for the cases of atmospheric turbulence and pointing errors.

Two techniques are proposed for using laser links to measure the relative radial velocity of two spacecraft. The first technique determines the relative radial velocity from a measurement of the two-way doppler shift on a transponded radio-frequency subcarrier. The subcarrier intensity-modulates reciprocating laser beams. The second technique determines the relative radial velocity from a measurement of the two-way doppler shift on an optical frequency carrier which is transponded between spacecraft using optical Costas loops. The first technique might be used in conjunction with noncoherent optical communications, while the second technique is compatible with coherent optical communications. The first technique simultaneously exploits the diffraction advantage of laser beams and the maturity of radio-frequency phase-locked loop technology. The second technique exploits both the diffraction advantage of laser beams and the large doppler effect at optical frequencies. The second technique has the potential for greater accuracy; unfortunately, it is more difficult to implement since it involves optical Costas loops.

A great deal of advancements were made previously in planar guided-wave acousto-optic devices. These advancements include analytical treatment of complex interaction geometry, design and fabrication of wideband Bragg modulators and deflectors (Bragg cells), and demonstration of applications in spectral analysis and correlation of wideband RF signals. The continuing progress in the fabrication and performance of other components including new types of optical waveguides and waveguide lenses, diode laser sources and photodetector arrays, and their integration has significantly advanced the prospects for realization of integrated AO device modules and circuits. In this paper, a new planar-channel AO device module as well as a spherical waveguide AO device module in LiNb03 that are being developed at the author's institution are reported. The results obtained thus far have shown that such AO device modules will take up small substrate dimensions along the optical path and are also inherently of high modularity and versatility. Consequently, such integrated AO device modules should find novel applications in wideband multichannel integrated- and fiber-optic communication, signal processing, and computing systems. Some of these potential applications are also discussed.

The modulation and/or Q switching of laser radiation has often been accomplished by using the electrooptic properties of crystals such as lithium niobate. Until recently the photorefractive effect, an internal electric field due to the photoionization of iron and other transition metal impurities, has drastically limited the usable intensity of the laser beam. It has now been found that magnesium-doped lithium niobate has a much higher photoconductivity than the undoped crystal, greatly reducing the photorefractive effect and permitting the modulation of beams of higher intensity. The advantages and drawbacks accompanying the use of magnesium-doped crystals will be discussed.

The lasing wavelength shift and mode jumping of a GaAlAs laser diode have been observed by a optical feedback as low as -61dB or even lower (-66dB). The lasing frequency shift and mode jumping occur periodically when the phase of the reflected light changes 22(. This effect already applied into adjusting the diode lasing frequency to the definite line of the atom by simply putting a mirror away from the facet of the diode at a sizable distance nevertheless this atom resonance line is located at the lasing mode hopping region in the case of diode free running.

The application of fiber-optic gyros for space use is becoming very attractive due to the significant progress made in recent years in the McDonnell Douglas development of producible fiber-optic modules which can be environmentalized to withstand a wide range of vibration, shock, and temperature effects. This modular approach has improved the reliability of the fiber-optic gyro and reduced its complexity and fabrication time. Currently, rate control devices are being developed. Attitude control systems should be available in the next few years. To provide a benchmark for evaluation of the applica-tions for the fiber-optic gyro, performance and technology comparisons of both the fiber-optic analog and digital gyro with the ring laser gyro is presented.

Two years of effort have gone into the development and build of a high-speed fiber optic local area network suitable for space applications. In tailoring this system (Star*Bus ) to space applications, particular attention has been paid to system redundancy capabilities, radiation susceptibility, protocol versatility, network application flexibility, size, power, and weight. The result is a 100 Mb/s, autonomously redundant network incorporating a dual star topology and an extremely robust protocol that, with the incorporation of state-of-the-art VLSI technology, can provide a small, lightweight, low-power solution to multiple high-rate data transfers in a variety of applications. The network achieves both versatility and robustness through the use of a dual mode protocol that incorporates the advantages of both contention (Ethernet) and deterministic (token passing) types of protocols. In this manner, it achieves the short message delays under light network loading possible with a contention protocol while retaining the efficiency and deterministic characteristics of a token passing or timeslot protocol at high network loads. The network employs LED and PIN photodiodes to minimize power and to provide high reliability. It uses radiation-hardened optical fiber and passive star couplers. The initial network performance evaluation has been completed using an eight-node system and redundant, 20x20 optical couplers. Network delays correlated well with predicted values. Throughput has been measured at greater than 95 Mb/s and redundancy operation is as expected.

With recent developments in fiber optic communications and optical distribution networks, short haul optical communications becomes an economical alternative to conventional cable TV systems. This paper presents a system design for a direct broadcast satellite receiver system with a fiber optic distribution network based on the reception of Ku-band signals from ANIK C2, a Canadian direct broadcast satellite. Such a system is proposed for the first time and can address small communities in remote areas. Theoretical power budget calculations predict that 37 subscribers can access 128 television channels using a 3ft. reflector dish antenna. To implement such a design, a number of components that are not commercially available are custom designed.

The applicability of a fiber-optic telemetry and command (T&C) system for a large communications satellite system is analyzed. This system is characterized by low bit rates (below 1 Mbit/s) and relatively short transmission distances (below 20 meters). Various LAN (local area network) architectures and required components are examined for this application. The most suitable system is found to be a transmissive star network using LEDs, PIN detectors, and large size multi-mode optical fibers. This system is competitive in power and mass for certain T&C applications with conventional wire systems but offer a much higher immunity to electromagnetic interference. A new optical rotary joint, required for spin-stabilized satellite applications, is discussed.

Future communication satellites and space stations will carry large switching arrays which will allow rapid interconnectivity between many up-link and down-link channels. Key considerations in any complex space-based system include optimization of the size, weight, bandwidth and crosstalk of the components and sub-systems employed in the design . In satellites signal distribution scheme is often of prime concern. It is here where an optoelectronic system is an alternative to and has potential advantages over competing systems using coaxial cables. The advantages of a fiber optic link include small size and low weight, broad bandwidth and excellent crosstalk isolation. The overall system under consideration for this study is a satellite or space station equipped with a number of microwave receivers and transmitters operating in the microwave frequencies (10 to 40GHz). The task is to design a distribution network which is capable of routing the communication signals between the receivers and transmitters. Present design include down and upconverters at the receivers and tranmitters and an IF fiberoptic link connecting the microwave components. The paper will describe the construction and performance of a 750 MHz bandwith optical IF link. The characterization of the link includes the frequency response, intermodulation distortion, noise and power budget.

The rapidly expanding field of optoelectronics includes a wide variety of both military and non-military applications in which the systems must meet radiation exposure requirements. Herein, we review the work on radiation effects on sources for such optoelectronic systems. It will be shown that the principal problem is permanent damage-induced light output degradation.

In this talk, we will survey recent research in the area of radiation hardened optical detectors. We have studied conventional silicon photodiode structures, special radiation hardened silicon photodiodes, and special double heterojunction AlGaAs/GaAs photodiodes in neutron, gamma, pulsed x-ray and charged particle environments. We will present results of our work and summarize other research in this area. Our studies have shown at detectors can be made to function acceptably after exposures to neutron fluences of 10¹⁵ n/cm², total dose gamma exposures of 10⁸ rad (Si), and flash x-ray environments of 10⁸ rad/sec (Si). We will describe detector structures that can operate through these conditions, pre-rad and post-rad operational characteristics, and experimental conditions that produced these results.

This paper reviews the procedures we developed to select lasers for deployment in high reliability systems. The problem of screening out short-lived lasers with the aging controlled by low thermal activation energy failure mechanisms is analyzed. The rationale for and development of a purge (a set of laser-specific over-stresses) is described. Reliability results using real time device hours and more than 2 years of field use show that these selection procedures work well.

This paper presents the results of an extensive trade study of various techniques for combining multiple laser diode beams. Each of the candidate concepts was tested against a stringent set of criteria. As a result of this study a Dichroic Reflection Combiner was chosen for further development into flight qualifiable hardware.

Issues regarding combining of laser diodes for application to free-space optical communication systems are examined. It is shown that to deliver a greater number of photons to a distant collector the combining technique must use wavelength and/or polarization. Optical elements for combining include multilayer dielectric interference filters (bandpass, dichroic, and polarizing), diffraction gratings, and thin film evaporative metal polarizers. Design considerations for a combiner include high throughput efficiency, diffraction-limited angular divergence, and reasonable volume constraints. It is found that combiners require a generalized concept of Strehl Ratio which includes relative source misalignment, and that diffraction grating combiners may have a limited number of laser sources which can meet the spectral requirements. Next, given an optical system for laser diode combining, methods to incorporate the combiner into the communication system are compared. The communication system alternatives include power combining, multiplexing parallel independent channels, or a generalized matrix modulation scheme. Power combining may be limited by background radiation, and matrix modulation may be limited by the source peak-to-average power ratio. Parallel channels does not have these limitations but requires the most hardware to implement. It is concluded that power combining is the best trade-off of performance and complexity for all systems except those severely limited by background radiation or component bandwidth.

Optical fibers are being considered for many space applications because of their high data rate capability, reduced weight over conventional transmission media and lower operating power requirements. In utilizing optical fibers in space environments, the effects of ionizing radiation on performance must be characterized. This characterization entails understanding the relationship between fiber parameters, the environment and system dependent parameters. Data highlighting some of these relationships is presented for many contemporary fibers.