Since about 1977, the European Space Agency ESA is pursuing activities in the field of optical communications for space applications. With the worldwide recognition which these technologies experienced over the last few years, optical communication payloads are now seriously considered for some of ESA's forthcoming satellite projects. Parallel to this project-oriented work, ESA continues with research and development activities on optical communication systems and components.

The technology development, spacecraft systems impact, design examples and overall development plan for optical deep space communications are described. Design examples include moderate distance links like ones from Mars to Earth, out through a potential mission to a distance of 1000 A.U. The technology development plan, which includes both ground-based as well as Earth orbit-based reception considerations, spans the period from 1985 to the year 2003. Past technology developments in high efficiency lasers, optical modulation and coding, and high power efficiency communications technqiues at multiple bits of information per detected photon are also discussed.

Index-guided channeled-substrate-planar (CSP) AlGaAs diode lasers are being developed for reliable, high-power operation for use as sources in spaceborne optical communications sytems. Although most work on this AlGaAs structure has been aimed at optimizing preformance at output wavelengths less than 8400 Å, emission in the 8700 Å regime is also of interest. In particular, such wavelengths are required for use in the Direct Detection Laser Transceiver (DDLT), to be incorporated into NASA's Advanced Communications Technology Satellite (ACTS), in order to avoid absorption of the light by the atmosphere when communicating with ground-based terminals. Lowest order spatial mode and substantially single longitudinal mode output has been ob-served in 0.87 pm CSP devices in excess of 50 mW cw and 100 mW 50% duty-cycle, with rms phase-front aberrations measured to be = λ/40.

Single quantum well (SQW) lasers have recently been shown to offer superior performance compared with conventional double heterostructures. These devices have demonstrated very low threshold current densities, high efficiency, and high output powers. The application of SQW laser diodes to free-space laser communication systems is expected to have significant impact on total system performance and increase the utility and viability of many system concepts.

Dichroic combination of multiple beams is demonstrated to be an efficient and reliable method of increasing the radiance of diode laser transmitters for free space laser communication.

Optical communications is an alternative to the microwave solution for high data rate interorbit (IOL) or intersatellite link (ISL), typically 500 Mbit/s. The optical system, using laser diode technology, only requires small size antennae, provided that multiplexing or power combining techniques are used. This paper presents a Wavelength Division Multiplexing/Demultiplexing System involving orthogonal polarizations for adjacent channels. This solution is preferred to the power combining technique to satisfy some specific system and interfaces constraints and to avoid a very accurate control of laser wavelength. The proposed design complies with system constraints like redundancy, minimum back-light on the laser junction, interchannel interferences, depolarization in the optical system, compacity, ... Its simplicity -the required components are only λ/4 plates, filters under normal incidence and polarizer beamsplitters- is attractive and the concept, descri-bed here for 4 wavelengths in the 0.817-0.84311μ range, can be applied to an unlimited number of wavelengths. The computed performances in transmission and isolation taking into account all parasitic reflections are given. A breadboard model is currently under development at MATRA to be tested in 1987. Preliminary results of sub-assemblies tests are presented.

A compact device for spectrally combining many laser-diode beams into a single multi-wavelength beam has been developed for use in NASA's intersatellite communications programs. The prototype device combines seven 30 milliwatt beams into a single beam with 70 percent efficiency producing an output of approximately 150 milliwatts. All beams are coaxial and can be collimated with a single transmitter optical system. The combining technique is relatively insensitive to drifts in the laser-diode wavelength and provides both increased power output and laser-diode source redundancy. Combination of more than 100 laser-diodes producing an output greater than 5 watts appears feasible with this technique.

The theoretical and measured performance characteris-tics of a direct detection optical communication system are reported. The system consisted of a semiconductor diode laser, silicon avalanche photodiode detector, and used Q=4 optical pulse position modulation signaling to transmit binary source data at a rate of 25 megabits/sec. Operation at a receiver bit error probability of 10^-6 was demonstrated at received signal energies corresponding to an average of 160 detected photons per PPM symbol, or 80 photons per source bit.

Optical intersatellite links use a beacon beam sent from a remote terminal and tracked by a home terminal to keep an information bearing beam sent by home pointed directly at the remote. Noise and mechanical disturbances at home induce pointing errors in the track loop, causing a loss of optical signal seen at the remote, adversely affecting data transmission reliability. This paper describes an experiment which measures the statistical and time-varying properties of this pointing error. A tracking loop was assembled from a high-speed fine-steer-ing mirror (FSM), lens, and an avalanche quadrant cell (AQC). A collimated IR diode laser provides illumination, and a second quadrant cell monitors pointing error. Actual intersatellite link (ISL) criteria was used in the design and implementation of the system. Measured pointing error distributions (PED) tend to look Gaussian as theory suggests. Disturbances in orthogonal axes are statistically independent, resulting in Rayleigh radial distributions, except at frequencies where structural modes are present in the FSM. Anomalies in the PED are attributed to seismic and acoustical mechanical disturbances.

Turbulence effects on the performance of M'ary PPM format optical communication channel using an APD-based receiver are investigated. The bit error probability vs. received signal has been computed for no turbulence, mild turbulence ( σ =0.2), and strong turbulence (σ =0.4). Calculations were done for a typical APD characteristics, background power of 0.25 nW, integrating time of 2 nsec, and signal power in the range of 5-50 nW. throughout the calculation, Gaussian statistics were assumed. A set of curves, describing the bit error probability vs. received power were generated. From these curves, the turbulence induced loss factors were estimated. The values of these factors are approximately 1.3 for mild and 2-2.5 for strong turbulence. for a given signal power, the increase in the bit error probability due to turbulence may amount to three orders of magnitude.

Coding is routinely used in microwave links to improve bit error rate (BER) performance and has frequently been suggested as a method to improve the performance of an optical ISL. We report here on investigations of an optical ISL using rate 7/8 BCH coding (128, 112). When the noise equivalent angle (NEA) is above about 0.4 microradians for the links studied, the numerical calculations showed that the gain due to coding would be much less than calculations based on the average BER. For larger NEAs, coding could result in a degradation of the link.

Results of computer modelling of (1) the interaction of the pointing subsystems of two optical ISL terminals and (2) the ISL performance versus lens diameter are presented. The interaction between two ISL terminals, each with their own noise equivalent angle (NEA) was iteratively calculated (the NEA of one terminal reduces the average received beacon signal of the other, increasing its NEA, etc.). For the specific system modelled, it was found that the final NEA was very close to the initial NEA when the initial NEA was small (below 0.4 microradians). When the initial NEA was large (greater than 0.7 microradians) the iterations diverged indicating the terminals could not lock onto each other. Next, rather than assume a fixed NEA when the lens diameter was varied, the NEA was calculated for each diameter, assuming a fixed percentage of the communications laser beam was used for tracking. This substantially changes the form of the BER versus lens diameter graphs. Assuming 10% of the energy of a 120 Mbit/s link was used for tracking, the NEA for realistic links was usually less than 0.2 microradians.

Optical Space Communications is one area where the presence of Error Correction can fundamentally change the trade-offs facing the system designer, as we will attempt to point out. However, there are some immediate questions to be answered: i) What is Forward Error Correction? ii) How should it be regarded? iii) What can it achieve in the optical communications arena?

The communications performances of laser intersatellite links for a future NASA data-relay system are analyzed and block diagrams of necessary optical terminals generated based on technology projected to be available by 1992. Direct-detection optical communications subsystems are selected to interconnect two geosynchronous (GEO) satellites separated by 160° along the GEO arc and to interconnect a low-earth-orbit (LEO) satellite and either of the GEO satellites. The terminal designs are based on power budget calculations performed for the various links with and without an optical background in either the receiver or transmitter fields of view (F0Vs). Optical antennas with 18-cm and 30-cm primary mirror diameters on the LEO and GEO satellites can achieve a 10^-6 average bit error rate during normal operations in each of the forward and return optical links with beginning-of-life (BOL) power margins of approximately 7 dB. With an optical background in the receiver FOV, communications in the GEO-GEO return link are interrupted, the LEO-GEO return link is undegraded, and the GEO-GEO and LEO-GEO forward links remain functional with reduced BOL power margins. With an optical background in the transmitter FOV, the GEO-GEO forward and return links are noticeably degraded, the LEO-GEO return link is only slightly degraded, and the LEO-GEO forward link remains undegraded.

The use of optical communications for the high data-rate links of the future European Data Relay Satellite System has been receiving growing attention for the past few years. This paper presents a system design based upon GaALAs laser diodes transmitters and direct-detection receivers. A Wavelength Division Multiplexing technique is used to accomodate data-rates up to 500 Mb/s on the return link with moderate antenna diameters. Si CCD matrices are used as acquisition and tracking sensors, and angular error informations are extracted directly from the communication signal, thus avoiding the need for a dedicated tracking beacon. Expected performances are given ; The system is shown to be capable of supporting variable user requirements. The architecture of the terminals, including adequate redundancies, is described. Mass estimates are given.

The majority of today's optical communication systems employ non-coherent detection or what is commonly referred to as direct detection. However, recent advances in technology have made the concept of coherent optical detection feasible. A properly designed coherent system can offer a significant improvement in receiver sensitivity. In addition a coherent system based on diode pumped Nd:YAG technology offers the additional advantages of high source power and high reliability. This paper describes the system performance of a coherent Nd:YAG system'in terms of receiver sensitivity and compares that performance to that of a typical direct detection system. A comparison is also made between Nd:YAG and semiconductor laser diodes when used in coherent systems.

In free space optical communication systems, precise control of optical polarization state is essential. This is especially true where diplexing of transmit and receive paths depends on separation of orthogonal polarization states. It is also true since changes in polarization state in transmitter and receiver optical paths can lead to link losses, therefore influencing link margin and link bit error probability. We discuss the design, implementation and application of very high reflectance controlled retardance coatings for use at oblique incidence angles. These coatings allow the maintenance of highly circular or linear polarization states in a link. Coating design trade-offs are reviewed. Spectral bandwidth, angular sensitivity and thermal behavior are also discussed. Coatings were designed and produced having better than 2 degrees residual p state - s state retardance for 45 degree incidence. Coated components exhibited less than 0.3 degree retardance variation over their surfaces. Contamination sensitivity of these coatings was evaluated using a design code based approach. Contamination that produced only small changes in reflectivity was found to produce relatively large changes in relative retardance. Link performance can be optimized through use of properly designed and carefully executed controlled retardance coatings.

An analytical cost-performance model for a ground-based optical communication receiving telescope is presented. The model considers costs of existing telescopes as a function of diameter and field-of-view. This, coupled with communication performance as a function of receiver diameter and field-of-view yields the appropriate telescope cost versus communication performance curve.

This paper describes wide bandwidth, high frequency diode lasers and photodetectors which make possible the direct transmission of microwave signals over optical fiber to frequencies as high as 20 GHz. The processing, packaging, and optoelectronic properties of these devices will be described, and estimates of maximum bandwidth capabilities will be given.

For many spacecraft and avionic applications, weight, size and layout flexibility are critical parameters for transmission media. This paper describes the advantages of using RF fiber optic links as replacements for waveguides and coaxial cable, and describes the measured performance of a 2.85 to 3.15 GHz link and a widehand 2 to 12 GHz link.

The integration of monolithic microwave and millimeter-wave integrated circuits with optoelectronic components is crucial to the implementation of compact, light weight, multigigabit signal transmission and processing systems for future airborne and space borne applications. Recently, a number of authors have reported the integration of detector/amplifier combinations onto common semi-insulating GaAs substrates. Applications of these receiver arrays in linear and two dimensional integrated topologies are discussed.

The advantages for the use of fiber optic components in communication electronics have led to their increased use in control and signal distribution applications. Recent theoretical and experimental work has surfaced some interesting possibilities for fiber-compatible aperture adjustment and signal feeding for phased array antennas. This paper presents the results of experimental investigations into direct microwave phase shift by optical excitation. The subject phase control method applies novel, light-sensitive PIN diodes in a microstrip phase shifter circuit sized for X-band operation. This phase shifter is made part of a two-element phased array breadboard to show the viability of this approach to array beamforming. Other areas presented include signal distribution methods suitable for large apertures consisting of many (e.g. thousands) radiating elements. Trade-off issues relevant to this subject area are highlighted.

The persistent trend to use millimeter-wave frequencies for satellite communications presents the challenge to design large-aperture phased arrays for space applications. These arrays, which comprise 100-10,000 elements, are now possible due to the advent of lightwave technology and the availability of monolithic microwave integrated circuits. In this paper, system aspects of optically-controlled array design are studied. In particular, two architectures for a 40 GHz array are outlined, and the main system-related issues are examined : power budget, synchronization in frequency and phase, and stochastic effects.

Fiber optic networks are considered as a viable alternative to the conventional coaxial distribution systems for Space Station. The multiple access communication subsystem provides 450-750MHz data signal on offset quadrature phase shift keying (OQPSK) format to antenna mounted electronics. For coherent communication 100MHz frequency reference signal is transmitted to each module to phase lock a dielectric resonator oscillator at 14.15GHz. Experimental results of phase locked oscillator's spectral purity, synchronized via two commercial fiber-optic links at 1.3tim and 0.8gm, are reported. These two fiber-optic links were compared in terms of phase noise degradation. Furthermore, experimental results of fiber-optic link linearity, third order intermodulation distortion, dynamic range and effect of mixing between data and the frequency reference are reported.