The SILEX concept, based on 0.8-micron diodes, is summarized, and the key features of the mission and its technical definition are described. The SILEX project is at the beginning of its development phase. The two optical payloads will be ready for launch in 1994-1995 so as to allow a flight demonstration from 1996 on, depending on the satellite launch dates, which are to be decided as a function of the operation of SPOT 3 in orbit. The SILEX general schedule is given. One of the aims of the engineering phase has been to develop a complete system test bed which has made it possible to reinforce the overall selected concept and verify the expected performances. This test bed is to be refurbished to become an essential integration and verification tool during the hardware development and integration phase. Present technological perspectives on components confirm the selection of the wavelength range and its suitability to a longer-term application: optical power and reliability for the laser diodes, and very low noise performance for the detectors.

NASA/Goddard Space Flight Center (GSFC) has initiated the development of experimental optical communication system which will be installed on Space Station Freedom. This system is part of the Space Station Attached a Payloads Program and is currently scheduled for a 1997 launch. The system is being designed to carry out comprehensive set of tests to evaluate and demonstrate the capabilities of this relatively new technology. Communication tests at rates up to 1200 mbps will be conducted over the space-to-ground link using an existing tracking facility at the GSFC. GaAlAs semiconductor lasers will be intensity modulated using 4 slot pulse position modulation format. Direct detection receivers using silicon avalanche photodiodes will be utilized, and 1 microradian accuracy pointing will be achieved with 2 cascaded pointing stages. Successful completion of this in-orbit test program should demonstrate both the technical maturity and readiness of this technology for follow-up operational missions.

The Galileo Optical communications from an Earth-based Xmtr (GOPEX) demonstration is designed to exhibit deep-space optical communications using the Galileo spacecraft. The optical transmitter consists of a Nd:YAG laser coupled to a 24-in. telescope at the Table Mountain Observatory (TMO), and the receiver is the Solid-State Imaging camera on board Galileo. The objectives of the demonstration are to understand the issues involved in blind-pointing to a spacecraft in deep space, and to assess the quality of the optical uplink by comparing the experimental results with theoretical predictions. The demonstration is proposed for December 1992 during the second earth-flyby period of Galileo''s trajectory.

The SILEX system involves a large set of advanced techniques and technologies which need to be merged and confronted. A macroscopic approach makes it possible to consider an optical terminal, with reference to common space fields of activities as a combination of a communication payload, an attitude and orbit control system, an optical instrument, and an on-board data handling system. It requires great technical expertise in areas commonly mastered in these techniques, namely, highly sensitive detectors such as coupled charge detector matrices, avalanche photodiodes, accurate and/or high bandwidth pointing and steering mechanisms, high optical quality mirrors and optical coatings, and accurate thermal control. Basic system and engineering tasks are to be mastered to combine them in an optimal way. This calls, among other things, for requirements analysis to derive the main design drivers and specific constraints with respect to each technique involved, and sizing and configuration of the system.

Several advantages of coherent free-space optical communications are outlined. Theoretical analysis is formulated for an OPLL disturbed by shot noise, modulation noise, and frequency noise consisting of a white component, a 1/f component, and a 1/f-squared component. Each of the noise components is characterized by its associated power spectral density. It is shown that the effect of modulation depends only on the ratio of loop bandwidth and data rate, and is negligible for an OPLL with loop bandwidth smaller than one fourth the data rate. Total phase error variance as a function of loop bandwidth is displayed for several values of carrier signal to noise ratio. Optimal loop bandwidth is also calculated as a function of carrier signal to noise ratio. An OPLL experiment is performed, where it is shown that the measured phase error variance closely matches the theoretical predictions.

An optical communications system is described that supports bi-directional interconnections between Space Station Freedom (SSF) and a host of attached and co-orbiting platforms. These proximity-operations (Prox-Ops) platforms are categorized by their maximum distance from SSF, with several remaining inside 1-km range and several extending out to 37-km and 2000-km ranges in the initial and growth phases, respectively. Two distinct Prox-Ops optical terminals are described. A 1-cm-aperture system is used on the short-range platforms to reduce payload mass, and a 10-cm-aperture system is used on the long-range platforms and on SSF to support the optical link budgets. The system supports up to four simultaneous user links, by assigning wavelengths to the various platforms and by using separate SSF terminals for each link.

The application of optical space-to-ground links (SGLs) for high speed data distribution from geosynchronous and low earth orbiting satellites (e.g., sensor data from the planned Earth Observing System), for lunar and Mars links, and for links from interplanetary probes has been a topic of considerable recent interest. These optical SGLs could conceivably represent the system''s operational baseline, or could represent backup links in the event of a GEO relay terminal failure. In this paper the availability of optical SGLs for various system/orbit configurations is considered. Single CONUS sites are assessed for their probability of cloud free line of sight (PCFLOS), and cloud free field of view (PCFFOV). PCFLOS represents an availability metric for geosynchronous platforms, while PCFFOV is a relevant performance metric for non-geostationary platforms (e.g., low earth orbiting satellites). Additionally, the availability of multiple ground terminals utilized in a diversity configuration is considered. Availability statistics vs. the number of diversity sites are derived from climatological data bases for CONUS sites.

The performance of a direct detection optical communication receiver for pulse position modulation (PPM) signal formats which contains an avalanche photodiode photodetector is calculated as a function of the parameters which characterize a raised cosine filter used in the receiver. It is shown that the performance penalty of this receiver relative to that of an optimal receiver which contains an integrator and assumes a perfectly rectangular PPM pulse shape is less than 1 dB at a receiver bit error rate of 10 exp -6. The advantage of the raised cosine filter is that the electrical bandwidth required is no more than one-half that of the integrator, that is, 1/tau as opposed to 2/tau where tau is the PPM time slot duration.

The US Air Force has identified laser communications (lasercom) as a viable technology for several air-to-air communications applications, namely Strategic Data Exchange (SDE) between airborne command post (ABCP) aircraft and intraformation data links among fighter aircraft or low-flying, close-formation aircraft. Lasercom offers low probability of intercept (LPI), jam resistance, and high bandwidth capabilities due to its very short wavelength compared to radio frequency (RF) systems. This paper examines the requirements of lasercom systems to support these missions and presents the systems link analysis necessary to select preferred approaches. The state of key technologies such as lasers, detectors, filters, coding, acquisition, etc., form a basis to structure conceptual systems that meet mission requirements. There have been recent advances in these key technologies, which will significantly improve air-to-air lasercom system performance. The resultant systems are low risk and could be developed in minimum time. Adverse weather is a factor in lasercom capability, so several weather conditions are included in the analysis. The preferred systems are described and, finally, conclusions and recommendations are given that could lead to development of a prototype terminal for one of the above missions.

We describe a nonmechanical beam forming device which simultaneously provides the high precision beam deflection and beam spoiling required for narrow beamwidth laser communication systems. The device''s transmission is near unity and is composed of a liquid crystal optical phased array sandwiched between static phase plates. Device design and performance goals are described, along with the results of proof-of-concept experiments.

Transfering the principle of array antennas from the microwave regime into optics results in a system which permits non-mechanical steering of laser beams. The feasibility of such an optical array antenna strongly depends on the accuracy of realization because of the small wavelengths involved. Therefore we analyzed the sensitivity of optical phased array antennas to errors in subantenna phasing, positioning and aligning. Assuming independent Gaussian error distributions we determined the resulting antenna gain reductions by calculating the average reduction and upper probability limits for reductions which exceed a certain tolerable limit.

A proof-of-concept (POC) demonstration system has been developed which demonstrates acquisition, tracking and point-ahead angle sensing for a space optical communications terminal utilizing a single high speed area array detector. The detector is the 128 x 128 pixel Kodak HS-40 photodiode array. It has 64 parallel readout channels and can operate at frames rates up to 40,000 frames/sec with rms readout noise of 20 photoelectrons. A windowing scheme and special purpose digital signal processing electronics are employed to implement acquisition and tracking algorithms. The system operates at greater than 1 kHz sample (frame) rates. Acquisition can be performed in as little as 30 milliseconds with less than 1 picowatt of 0.85 micron beacon power on the detector. At the same power level, the rms tracking accuracy is approximately 1/16 pixel. Results of system analysis and measurements using the POC system are presented.

The overall architecture and the performances of the pointing, acquisition, and tracking (PAT) subsystem of the European SILEX program are examined. The mission constraints are described, and attention is given to the retained baseline built around a two-stage beam steering system, an open loop point-ahead control, and a full digital implementation. Performance prediction is based on signal processing, control algorithms, and detailed modeling of equipment and flexible structures. The way in which an accurate performance of the order of a microradian can be achieved with a correct allocation of the performance for each instrument of the system is shown. The rejection of the microvibrations coming from the platform when the terminal is operating is singled out as a key feature of the design. The PAT equipment and softwares - sensors, electromechanisms, and control units - are presented.

A ''free-space simulator for laser transmission'' is proposed for evaluating optical beam control subsystems for an optical intersatellite link. This method is based on the compact range approach which gives the far-field pattern (FFP) of the transmitted beam at a focal plane of the lens. The method allows the simulator to be placed in an in-house laboratory; the beam control systems can thus be evaluated easily, stably, and precisely. Experimental results show that the FFP of an optical beam emitted from an antenna is obtainable with accuracy on the order of microrads.

A low data rate laboratory heterodyne link demonstration operating at 1.06 micron using frequency stabilized Nd:YAG lasers was implemented. The data was modulated onto the optical carrier at 100 kbps using binary pulse position modulation. Phase coherent reception was achieved by closing the receiver phase tracking loop around the piezo-electric and temperature-tuned local oscillator laser. Initial frequency acquisition was accomplished by linearly scanning the LO frequency over the uncertainty range while a combined frequency and phase tracking loop searches for a lock indicator signal. The link performance was experimentally evaluated and compared to the theoretical predictions.

A 650 Mbps laser communications testbed under construction for the development of flight qualifiable hardware suitable for near-term operation on geosynchronous-to-geosynchronous crosslink missions is presented. The program''s primary purpose is to develop and optimize laser communications unique subsystems. Requirements for the testbed experiments are to optimize the acquisition processes, to fully simulate the long range (up to 21,000 km) and the fine tracking characteristics of two narrow-beam laser communications terminals, and to fully test communications performance which will include average and burst bit error rates, effects of laser diode coalignment, degradation due to internal and external stray light, and the impact of drifts in the optical components.

A preliminary study to define suitable optical terminals for intersatellite links is presented, and the main configuration tradeoffs are discussed. A modular design approach was pursued, and some innovative technology solutions were traded in order to satisfy the basic requirement of a limited terminal mass. A high-capacity (500 Mbps) return link channel is feasible and fast, and accurate acquisition and tracking operation are achieved.

A complex setup intended to fully and accurately measure the influence of the atmospheric turbulence on a near-IR laser beam propagating between two islands of the Spanish Canaries is presented. A measurement campaign was held on this site with the equipment presented. The main parameters under consideration are: far-field pattern, beam spreading, beam wandering, wavefront distortion, atmospheric transmission at the relevant wavelengths, scintillations, and angle of arrival fluctuations. Part of the acquired data is presented, with a preliminary analysis demonstrating agreement with previous data and theoretical predictions. The results confirm the transmission and turbulence levels expected over the propagation path. The use of a fast miniature CCD matrix for the acquisition of turbulence effects on beam propagation is demonstrated.

A novel angular discriminator for spatial tracking in free-space laser communication systems is proposed. The device''s operating principle is to transform the angular difference between two laser beams into the electrical phase difference between the IF component of two coherently detected photocurrents. The electrical phase difference is thus easily estimated by a post-detection electronic processor. The angle-to-phase conversion of the Fabry-Perot Etalon is verified experimentally. The advantage of the novel angular discriminator for spatial tracking in free-space laser communications is that the mean and standard deviation of the angular tracking errors are small. The angular span can be made larger than one beamwidth in order to minimize the probability of reacquisition. The spatial tracking sensors and the communication photodetectors are integrated into a single system. The discriminator can be used for spatial acquisition as well.

A laboratory model of an optical intersatellite link employing InGaAs DFB semiconductor lasers operating at a wavelength of 1.55 micron was designed and realized. Heterodyne sensing was used for both the spatial acquisition and the spatial tracking processes. The function of a quadrant detector was realized by splitting the superimposed beam at the top of a reflecting pyramid into four subbeams. The angular resolution achieved - without using a telescope - is less than 5 microrad at a detector field of view of 1 mrad. The transmitter laser can be moved within a transverse plane along circular tracks. A microcomputer controls the receiver operation. During the acquisition process spiral scanning of the area of uncertainty is performed. For each search position the local oscillator laser is swept until a beat signal at 700 MHz is detected. Acquisition times of typically less than 16 s for a 200-element uncertainty area and tracking accuracies better than +/- 50 microrad for any examined test condition were achieved.

The problem of acquiring and tracking a sun-lit earth, which poses a rather stringent demand on the design of the pointing acquisition and tracking subsystem for a deep-space optical communication package, is addressed. The acquisition subsystem must be capable of locating and tracking the receiver location based on a sun-lit earth image that is blurred by the point spread function of the imaging optics and spatially quantized by the detector pixels. Spatial tracking must be performed at bandwidths in excess of 1 kHz to effectively reject platform jitters. A simple solution of locating the earth limb shows a great potential for providing the necessary acquisition and tracking accuracy. By using separate acquisition and tracking detectors, the system can be implemented without extensive technology development. The slow bandwidth required for the acquisition process can be readily accomplished with an available CCD array, and an analog tracking loop can be implemented using a quadrant tracker and calibrated centroid tracking without the excessive computing overhead of the array detector-based spatial tracker.

Attention is given to the coarse pointing assembly (CPA) for the SILEX program, designed on the basis of 10 years of MATRA experience in very accurate drive mechanisms successfully conducted by the SPOT 1 and 2 flights as well as EURECA IOC. The basic key design feature of the mechanism is a 1200-step stepper motor driven in microstepping with harmonic defects compensation. This allows very low torque noise associated with a high accuracy (0.01 deg). The direct drive principle avoids backlash and permits a linear control of the output shaft of each drive. The only parts susceptible to possible wear are the ball bearings, which have a design margin of greater than 1000 for 10 yr of service life. In order to meet the dynamic performances required by SILEX, a closed loop active damping system is added to each drive unit. Two accelerometers used in a differential way sense the hinge microvibrations and an active damping loop reduces their Q factor down to a few dB. All CPA electrical parts (including motor, optical encoder, and accelerometer) are redundant to avoid single point of failure.

A description of the simulation model and the on-ground performances validation of the Acquisition and Tracking (PAT) subsystem and of the European SILEX program is presented. The SILEX scenario is composed of two terminals mounted on LEO orbit (SPOT 4) and geostationary orbit (ARTEMIS) spacecrafts. The PAT subsystem is in charge of the initialization of the link, of the tracking of the incoming beam and of the pointing of the emitted laser beam. A short description of the retained strategies (phases of the PAT mission and operating modes) and functional architecture (control laws and sampling frequencies) is presented. In order to validate the PAT performance assessment, a complete simulation software is developed. The model (equipment modeling, dynamical simulation of the terminal structure, and operating modes management) and the associated outputs are examined in detail. The chosen design and architecture of the PAT validation on a system testbed (STB) are described. The validation of a pointing system for optical intersatellite communication, thanks to the combination of simulations and STB measurements, is shown.

Freespace laser communication systems require an initial spatial acquisition process to coalign the cooperating terminal line of sites (typically to sub-microrad accuracy). The complexity and time required for this process can grow dramatically with the size of the host platform(s) initial uncertainty field-of-view. The feasibility of significantly refining the laser terminal attitude knowledge prior to the initiation of the spatial acquisition process via a preliminary star tracking operation is addressed. It is shown that currently planned laser terminal designs are capable of supporting this star tracking function with minimal hardware augmentation. An analysis of stellar spectral irradiance is presented which enables the computation of the tracking noise equivalent angle versus stellar class and visual magnitude.

A diode-laser transmitter designed for space-based coherent communications has been successfully space qualified. Environmental testing, which consisted of random vibration at levels up to 16.2g rms and thermal cycling over the range of -30 to 66 C, caused no significant degradation in the performance of the transmitter. Principal design issues and the qualification process of subassemblies and the complete transmitter are described.

A high-power three-stage laser suitable for use in a space communication system has been built. This laser uses three diode-pumped Nd:YAG oscillators coherently combined using the technique of injection chaining. All three oscillators are in one compact and permanently aligned package, and are actively frequency locked to provide CW single frequency output. The three stages provide the redundancy desirable for space communications.

An Nd:YAG laser transmitter was developed for free space optical communications with a projected output power of 600 mW. Suitable for data rates in excess of 1 Gbps, the transmitter employs a waveguide modulator to minimize overall power consumption. The transmitter''s feasibility is shown.

The procedures and preliminary results of accelerated life tests performed within the framework of an evaluation program under the ESA contract are described. In order to calculate the activation energy and median lifetime and to investigate the drift behavior of optical parameters, a conventional three-temperature aging test at 30, 50, and 70 C is performed on 80 laser diodes in total, split into two subgroups operating under quaternary pulse position modulation (QPPM) and nonreturn-to-zero (NRZ) modulation at 16 Mbit/s with a PN-code length of (2 exp 7)-1. Measurements before and upon completion of the aging tests consist of P0/I curves, V/I characteristics, photo diode tracking ratios, spectra, mode hopping behaviors, far-field patterns, wave-front errors and astigmatisms, and linear polarization ratios.

An optical intersatellite link transmitter which uses a high-power laser diode (LD) amplifier so as to simultaneously achieve a high output power, fast frequency response, and good beam profile, is presented. Calculated results indicate that a good beam profile could be achieved with a travelling-wave type broad area LD amplifier when an input beam is focused just in front of the input facet of the LD amplifier. It is also shown that optical output power is controllable as a parameter of the injection current to the LD amplifier.

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 of the amplifiers are coherently combined, modulation formats requiring coherent detection can be used with this type of transmitter. A one-dimensional phased array of semiconductor optical amplifiers is used to amplify a 200 Mbps PSK signal which is then detected using a self-homodyne technique. The communications performance when the phased array is used is not degraded compared to the performance when the array is not used. In addition, the array introduces no additional phase noise to cause performance degradation.

Optical and electrical performance of a high power semiconductor laser transmitter (HPST) and of the related driver electronics are presented. Stable single-lobed operation of the HPST is accomplished by retroreflection of one lobe of the dual-lobed far field using a GRIN lens and stripe mirror. The envisaged peak-power output power of 1 W is achieved and 120 Mbit/s data transmission applying QPPM is demonstrated. Preliminary results after 2400 h of a lifetest performed on a laser array without a stripe mirror yield a bias current increase of about 2.1 x 10 exp -5/h and an efficiency decrease of 1 x 10 exp -5 hr.

Adaptive optics correction of wavefront distortion induced b atmospheric turbulence is considered. Compensation 1 imitation after perfact phase correction is investigated. Compensation efficiency in the case of finite subaerture size is analysed.

The detailed component stability requirements for relays are derived via system stability specifications for an optical intersatellite link. The significance of the transmit/receive coalignment stability is discussed in relation to the optomechanical design of a key subassembly. The requirement for transmit/receive coalignment stability is shown to be particularly stringent in respect to short-term stability at the component level. The system level specification is achievable, but the ability to achieve still greater stabilities results in a conflict between short- and long-term stability.

Nematic liquid crystals (NLCs) have electronically tunable birefringence and can be used as voltage-controlled switches for channel or redundancy beam switching in a lasercom terminal. To assess the performance of such a switch, we have evaluated the operating voltage, transmission efficiency, polarization crosstalk, and long-term operational reliability of an NLC device at four laser diode wavelengths. Preliminary results are encouraging. Transmission is around 95 percent, and polarization rotation purity is comparable to that obtained with a quartz or polymer waveplate, making this solid-state, voltage-controlled waveplate competitive with a conventional implementation that uses a mechanically inserted or rotated half-wave plate.

An overview of prismatic anamorphic beam expander (ABE) design requirements for free space laser diode communications is presented. Methods of achieving both fixed and variable magnification devices are described, and design characteristics are compared. The design methods are shown to be valid for the particular requirements of optical intersatellite links. Both fixed and variable magnification designs are demonstrated to be capable of meeting the primary optical requirements. For fixed magnifications, the preferred solution in terms of CAD, reflection losses, and material tolerances has a beam deviation of 6.1 deg, usually considered unacceptable because system layout has already been fixed.

A laser diode current driver has been developed for free space laser communications. The driver provides 300 mA peak modulation current and exhibits an optical risetime of less than 400 ps. The current and optical pulses are well behaved and show minimal ringing. The driver is well suited for QPPM modulation at data rates up to 440 Mbit/s. Much previous work has championed current steering circuits; in contrast, the present driver is a single-ended on/off switch. This results in twice the power efficiency as a current steering driver. The driver electrical efficiency for QPPM data is 34 percent. The high speed switch is realized with a Ku-band GaAsFET transistor, with a suitable pre-drive circuit, on a hybrid microcircuit adjacent to the laser diode.

A resonant cavity electro-optic phase modulator has been designed and implemented to operate at a data rate of 10 Mbps. The modulator consists of an electro-optic crystal located in a highly resonant cavity. The cavity is electro-optically switched on and off resonance and the phase dispersion near the cavity resonance provides the output phase modulation. The performance of the modulator was measured by first heterodyne detecting the signal to an intermediate frequency and measuring the spectral characteristics using an RF spectrum analyzer. The measured phase shift is shown to be in good agreement with the theoretical predictions. Further theoretical analysis shows that the design of the modulator can be scaled to operate at 100 Mbps. 1.

The spatial pointing angle and far field beamwidth of a high-power semiconductor laser are characterized as a function of CW power and also as a function of temperature. The time-averaged spatial pointing angle and spatial lobe width were measured under intensity-modulated conditions. The measured pointing deviations are determined to be well within the pointing requirements of the NASA Laser Communications Transceiver (LCT) program. A computer-controlled Mach-Zehnder phase-shifter interferometer is used to characterize the wavefront quality of the laser. The rms phase error over the entire pupil was measured as a function of CW output power. Time-averaged measurements of the wavefront quality are also made under intensity-modulated conditions. The measured rms phase errors are determined to be well within the wavefront quality requirements of the LCT program.

Based on the behavior of magnetooptic modulator in resonators the output characteristics of intracavity magnetooptic modulation (TMOM) with a feedback device are analysed in detail in this paper. The results of multistable output versus input are obtained by numerical computation and the results show that the ouputs are also instability and chaos in some situations. Besides the relations between the output power and the controlling variables of IMOM are given and some results of IMOM filled with active mcdium are also analysed. 1 .

System and terminal level specifications for an inter-satellite Optical Multiple Access (OMA) communication system are presented, as well as the resulting hardware designs for both OMA relay and OMA user terminals. The OMA relay terminal design uses a mechanical innovation which moves multiple fiber optic pickups in the focal plane, thereby providing simultaneous links with multiple OMA user terminals via a single telescope. Thus, with such a terminal on a relay satellite, multiple access service can be provided with a minimum of impact on the relay satellite.

The ability of a system based on a high-speed area array detector to simultaneously track and receive data from multiple low-power optical communication beacons is demonstrated. Specifically, the MOCTR POC system has achieved tracking precision of order 0.1 pixel with 2.7 picowatts, the minimum beacon power for the assumed system architecture; a data rate of 1 kbps with BER of order 1 to 10 x 10 exp -5, readily correctable with forward error correction; tracking and communication with simulated maximum earth background in the detector FOV; and simultaneous tracking of asynchronously modulated beacons. Areas needed to be addressed before an operational system can be implemented include reduction in size, weight and power, the demonstration of independent acquisition of multiple beacons, and the implementation of a full tracking algorithm in firmware.

A compact optical communications transceiver breadboard was constructed. In order to keep the mass of the breadboard as low as possible the design was made very simple. The entire package weighs less than 5 kg. The optical module uses a 40 mW diode laser and a two-axis voice coil actuator for beam steering. The breadboard is capable of locking onto a laser beacon within its field of view and transmitting the diode laser beam back toward the beacon. Design of the breaboard, details of each component in the breadboard, and current experiments with the package are described.

A study of the effect of various atmospheric conditions on a free-space optical communications link are undertaken. A field study was conducted near the Lancaster County Airport, Lititz, Pa., between March and September 1989, and confirmed the theory presented. Two systems were installed. The first system, an Ethernet system, had an optical link distance of 0.98 km. During 76.67 days of testing the overall system error free availability was 96.979 percent. The limiting factor was visibility in fog with an availability of 93.287 percent. Rain had little effect with an availability of 99.882 percent. The second system, a T1/DS1 telecommunications system, had an optical link distance of 0.93 km. During 122.29 days of testing the overall system error free availability was 98.550 percent. The limiting factor was visibility in fog with an availability of 94.277 percent. Rain had little effect with an availability of 99.998 percent. With the material presented the availability of any such optical link can be predicted.

Quadrant avalanche photodiodes (QAPDs) cannot be made with a perfect transition region. We are pursuing development of several optical divider approaches to narrow the effective transition region of the RCA Juction-side divided QAPD, a superior device except that it has no response in the transition region. This divider will maximize the slope factor and minimize optical losses. Optical divider approaches include the total internal reflection prism divider, the etched silicon reflective divider, and the binary optics quadrant divider. The nature and progress of this work and the test results are reported.

Monte Carlo simulations are used to investigate and evaluate different design options for the SILEX acquisition beacon. A minimum specification for the quality of the optical coupling between the laser diodes and the beacon output for which the beacon meets its power specification with the required reliability is established. An improved reliability is demonstrated for beacon designs with an optimized thermal design including the control of the thermal wall temperature and the conductive coupling between the beacon emission unit and the terminal radiator. Such an optimized thermal design also offers the possibility of mass savings of the optical terminal, since only a very light thermal wall is needed for the beacon. The total power control of the laser diodes is demonstrated to be the best mode of operation.

The characteristics which contribute to the proper performance of narrowband optical interference filters are discussed. The appropriate manufacturing for stability is discussed. Some of the unique custom filters that have been used in military systems are described. The translation of systems'' needs into filter specifications via communication between the designer and the filter manufacturer is examined.

Experimental measurements of direct detection receiver sensitivity for state of the art radiation hardened silicon avalanche photodiodes are compared with simulation results and show excellent agreement, generally within less than 0.25 dB. It is shown that for low preamp thermal noise and zero source laser extinction ratio, the McIntyre gain statistics of the APD result in receiver sensitivity as much as 2 dB better than predicted by Gaussian statistics. However, for larger preamp noise and finite laser extinction ratios there is very little difference between Gaussian and McIntyre sensitivity predictions. A binary PPM receiver sensitivity of -52 dBm for 10 exp -6 bit error rate at 220 Mbps was measured with a laser extinction ratio of 13.2 percent, corresponding to 102 photoelectrons per bit.

The small signal modulation characteristics, large signal modulation characteristics and dc noise spectra on 70 micron-wide 20-element high-power phase-locked arrays of antiguides are reported. The relaxation resonance frequency at 1.5 times threshold is found to be 1.65 GHz. Large-signal pulse modulation produced no distortion to the far-field characteristics. The dc noise spectra showed a damped shot noise resonance at 1.32 GHz for a dc bias level of 1.18 times threshold.

Future manned missions to Mars, planned as part of NASA''s Space Exploration Initiative (SEI), will require large amounts of communications link capacity in order to return high resolution video and scientific data from the Martian vicinity back to earth. Optical communications technology has the potential of supporting these high return data rates while offering significant savings in size, mass, and power over microwave systems. In this paper several different optical systems based on Nd:YAG, GaAs, and CO2 laser sources and employing both direct and heterodyne detection are analyzed in order to assess their feasibility in providing high data rate (10-1000 Mbps) Mars-to-Earth communications. Telescope sizes which minimize the required transmit laser power in the presence of random (Rayleigh distributed) pointing and tracking errors are determined for each optical implementation. It is shown that the RMS pointing/tracking accuracy is a critical parameter in defining the Mars-Earth optical link. Results of the analysis indicate that unless extremely small RMS pointing and tracking errors can be achieved ((sigma)

1 x 10 exp -6, 60 Mb/s QPPMMacGregor, Andrew; Dion, Bruno; Noeldeke, Christoph; Duchmann, Olivier

Receiver structures are developed to implement homodyne detection operating near the quantum limit and optimal bandwidth efficiency using monolithic laser diodes. The homodyne receiver approaches examined used NRZ and RZ formats and AM and PM subcarriers. Signalling schemes using subcarriers or Manchester BPSK can achieve a factor of 20-27 relaxation in laser linewidth requirements by minimizing the spectral overlap between the data modulation and the phase tracking loop. It is shown that Manchester BPSK (with 0.5 dB carrier leakage) can achieve 2.5 dB better receiver sensitivity than heterodyne BPSK with better bandwidth efficiency and an overall IF linewidth requirement of 390 kHz for a Gbps data rate and 0.5 dB phase tracking penalty at BER of 10 exp -6. For 1 dB carrier leakage, the IF linewidth requirement relaxes to 830 kHz. The feasibility of homodyne BPSK receivers with monolithic laser diode sources is established.

Results of a 15,000-hr life test of Matsushita BTRS lasers operating at 70-mW peak output power under 50 percent duty cycle pulse modulations are presented. Twenty-two lasers were tested in three temperature groups to provide accelerated aging. Evidence of two different failure mechanisms affecting the lasers was found. Devices tested at 20 C exhibited an unexpectedly high failure rate compared to devices tested at 40 and 70 C, but still had a median lifetime in this test of 11,900 hr. All failed devices from the 20-C group showed facet damage, while no facet damage was seen on failed devices from the other two temperature groups. A median lifetime of 85,000 hr at a 20-C operating temperature is predicted.

A Q-switched Nd:YLF laser (model 110-02) and a Nd:YAG ring laser (model 120-04) from Lightwave Electronics were subjected to thermal and vibration tests similar to what can be expected during launch and flight on a spacecraft. Even though these lasers were not designed for space flight, environmental tests were performed to identify major design weaknesses. Laser performance (output power, energy, pulsewidth, lasing threshold etc.) were measured prior to and after thermal vibration tests. Average output power of the Q-switched laser degraded 15-20 percent after thermal tests and an additional 20-25 percent after vibration tests. Post diagnostic tests revealed that degradation of the Q-switched laser was due to misalignment of pump focusing optics and the laser cavity.

Results of a detailed design study for an advanced optical communication system based on diode-pumped Nd:YAG laser technology performed within the framework of an ESA contract are presented. Emphasis is placed on reaching a low mass/low power design with sufficient maturity to develop space-qualified systems by the middle of this decade. The systems employ coherent PSK homodyne Costas loop receiver technology on the high data rate links, while QPPM modulation and direct detection is foreseen on the 25-Mb/s link. For the intersatellite duplex link, the same communication laser line is used for both directions, thus allowing multiple connections within a given satellite network. With 15-cm aperture telescopes on both terminals, maximum transmitter power is 500 mW for the 650 Mb/s link. Overall communication terminal mass is in the 70-80 kg range, and typical power consumption is 120-160 W.