We report our research results of system trial fabrication and tests as the first step of Optical Intersatellite Communication research. Second, the results of the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) feasibility study are reported. Last, we describe the draft results of a conceptual study of a future optical communications system that will be needed as space infrastructure.

Recent accomplishments and programmatic changes in direction of the NASA- funded optical communications project at JPL are discussed. The applications for the technology are first described. The systems and technologies for both the spacecraft optical communications terminals and the supporting optical reception infrastructure are covered. Plans for a comprehensive set of systems-level demonstrations are presented. These include early aircraft and ground demonstrations, a proposed mission enhancement experiment in space, and a planned solicitation for the first (mission definition) phase for a flight hardware development and demonstration of the technology in space contract.

Space based optical communications offer several advantages over traditional RF systems. They include: smaller beam divergence, smaller antennas, higher data rates, low probability of intercept, reduced EMI, and low probability of jamming. Additionally, the potentials for light weight, small volume and low power terminals make laser communications a consideration for several potential DOD programs. There have been may proposed configurations for both the laser communication terminal and the satellite network but architectures have remained fairly fluid. Despite these changes, there are several enabling technologies that must be fostered to meet program requirements. Efforts at Rome Lab are currently directed to the development of higher powered laser transmitters; rapid and accurate pointing, acquisition, and tracking systems; multiple channel operation; and sensitive low noise receivers. This paper will provide an overview of these efforts.

Parametric tradeoffs were performed for candidate technologies to support an orbiting relay satellite for communication with spacecraft in deep-space in the early 21st century. Both RF and optical system architectures were examined with a methodology which equitably compared the performance and attractiveness for the deep-space communication mission. Two candidate system architectures based on optical technologies were recommended for more detailed design-a system using coherent detection with an astronomical quality 4 meter telescope, and a system using direct detection with a 10 meter photon bucket telescope. Both of these systems appear capable of providing more than an order of magnitude improvement in communication performance over the DSN after it is upgraded to Ka Band.

A transmission experiment through a free-space laser transmission simulator for an optical intersatellite link is proposed and demonstrated. For multigigabit optical intensity modulation, adoption of both a direct current modulation of the laser diode and external optical intensity modulator are examined. Bit error rate (BER) characteristics, including the far-field pattern (FFP) of the optical antenna and beam coupling loss, as well as beam pointing error fluctuation are clarified in assuming intersatellite distances. Design directions of the optical beam tracking system and high capacity optical transmission in the 0.83 micrometers wavelength region, between optical intersatellite links, are clarified in this experiment.

Adaptive optics can mitigate the turbulence-induced wavefront distortions that limit the minimum practical beam divergence in a ground-to-space optical link, and enable high intensity laser beam propagation through the atmosphere. The CEMERLL experiment will use laser guide star adaptive optics to transmit a near-diffraction-limited laser beam from the Starfire Optical Range to the Apollo lunar retroreflectors. The experiment will validate theoretical models that predict the effect of atmospheric turbulence on uncompensated and compensated laser beam propagation, and explore strategies to compensate for atmosphere-induced wavefront tilt not corrected for by laser guide star adaptive optics.

Acquisition and tracking performance of a space-based optical communications system engineering model has been quantitatively measured using a dedicated optical test set. The test set includes a beacon with simulated angle jitter for tracking, a heterodyne optical receiver, and an accurate, calibrated line- of-sight monitor. The system has the capability of measuring each component of the pointing and tracking budget with angles down to approximately 1% of a diffraction-limited bandwidth. Acquisition and handover-to-tracking probabilities exceeding 99.9% are achieved at power levels 9-dB below expected on-orbit levels, and tracking rejection with jitter amplitudes comparable to expected spacecraft levels has been measured optically at -70 dB. Point- ahead angle repeatability over a POM 15 bandwidth range is found to be better than 0.01 beamwidths.

The Optical Communications Demonstrator is a laboratory-based lasercom demonstration instrument designed to validate effective beacon acquisition, high bandwidth tracking, precision beam pointing, and point ahead compensation functions. The instrument is designed using an array detector for both spatial acquisition and high bandwidth tracking, and a fiber coupled laser transmitter. The array detector tracking concept provides wide field of view acquisition as well as high update rate using a single output channel device. At the same time, it permits effective platform jitter compensation and point ahead control using only one steering mirror. The use of fiber coupled transmitter further modularizes transmitter design and decouples the thermal management problem. The reduction in design complexity can lead to a reduced system cost and an improved system reliability. Furthermore, it can permit the implementation of a new generation of lasercom instruments capable of realizing the inherent advantages of optical frequency communication systems.

Laser communications between satellites, high flying aircraft (such as JSTARS), and the ground offer the potential to transfer extremely high amounts of information faster and with a much smaller package than is possible using current radio frequency and microwave technologies. This can be especially important in downlinking time sensitive satellite reconnaissance information because the satellite stays within range of a ground station or aircraft for only a few minutes. A capability to downlink from a satellite to an aircraft can provide all weather performance, and multiple data transfers for every satellite orbit. Over the last few years, SDIO (now BMDO) has funded a number of technology efforts through the US Army Space and Strategic Defense Command reducing the risks associated with laser communications. This paper describes one of these efforts which is now being carried forward to an Advanced Technology Demonstration at ThermoTrex Corporation. The program will lead to the demonstration of high data rate communications of 270 MBPS (Mega Bits Per Second) to 1.08 GBPS (Giga Bits Per Second) between high altitude aircraft and possibly between a satellite and the ground. The Laser Communications Terminals incorporate Atomic Line Filter technology for background light rejection during acquisition, reactionless Roto-Lok offset cable drive gimbals for fast slewing and high accuracy pointing, and direct digital modulation of semiconductor diode lasers detected with low noise avalanche photodiodes. We present designs and preliminary performance results for both a simplified terminal appropriate for a near term satellite-to-ground data transfer experiment, and a full capability terminal appropriate for ground, aircraft, or satellite operations.

A 4km line-of-sight optical link, operating at 1.5 micrometers , has been established over central London. The link is being used to investigate the effects of atmospheric attenuation and turbulence on the transmission of high speed data. The power margin of the link has been increased to greater than 20dB by the use of an erbium fiber amplifier and a bootstrapped optical receiver. Results indicate that it is viable to transmit 155Mbit/s OOK NRZ data across the link in a variety of weather conditions with this margin.

The Small Optical User Terminal (SOUl) is an optical communications package for interconnecting a LEO satellite to future optical data relay terminals and is the subject of an ESA development programme led by British Aerospace. The baseline terminal has a data rate of 2 Mbps over the return inter-orbit link and a mass of around 25 kg. Special features include a periscopic coarse pointing assembly, refractive telescope, passive anti-vibration mount, combined acquisition and tracking sensor, and fibre coupled lasers and receivers. The flight configuration which allows these features to be combined in a compact unit is described in this paper. Details are given of the pointing, acquisition and tracking, optical, and thermal and structural subsystems.

This paper describes an earth orbiting Deep Space Relay Satellite System (DSRSS) based on optical direct detection communication for the user spacecraft to DSRS link. The optical direct detection DSRSS is considered as a possible augmentation to the Deep Space Network (DSN), after the 70 meter antennas are upgraded to Ka Band near the turn of the century. While development is required, the extrapolation from current technology appears relatively straightforward, and the direct detection system appears capable of exceeding an order of magnitude improvement over the upgraded Ka Band DSN. For example, with a 75 cm aperture on the user spacecraft, the optical direct detection system provides a capability of 1.23 Mbps at Pluto, a 13 dB advantage over the upgraded DSN. The direct detection system can also provide 0.83 Mbps with a 60 cm user aperture, an 11 dB improvement.

This article describes optical subnets of ground based receiving stations for earth-space optical communications. The optical subnet concepts presented here provide full line-of-sight coverage of the ecliptic, 24 hours a day, with high weather availability. The technical characteristics of the optical station and the user terminal are presented as well as the effects of cloud cover, transmittance through the atmosphere, and impact of background noise for day or night time operation upon the communication link. In addition, candidate geographic sites are identified, and a link design for a hypothetical Pluto mission in 2015 is included.

There is a growing interest in applying the resources of the Tracking and Data Relay Satellite System (TDRSS) as the primary support capability for future small satellite users. This interest is based on a variety of benefits offered by the TDRSS, and not available with globally-distributed space-ground links. An architecture based on an optical augmentation to the current TDRSS space network is discussed, including a candidate design for the user and relay terminals.

In order to simplify system architectures and make efficient use of laser power, space lasercom system designers often try to consolidate the receiver subsystems. In this paper, we present a receiver which uses a single subsystem for both spatial tracking error sensing and communication signal reception. It makes use of an electro-optic crystal as a conical scanner for tracking error measurement, couples the scanned light into a single mode fiber, and uses standard fiber-based heterodyne techniques to derive an intermediate frequency signal. This signal is processed to retrieve both the binary FSK signal and the tracking error signal, as well as an estimate of signal power for use in normalizing the tracking error. The fiber-coupled receiver makes possible a modular architecture, whereby the transmitter, receiver, and telescope subsystems can reside in different parts of the spacecraft. Such an architecture is known to have a number of desirable properties. We present a discussion of the frequency plan, data demodulation, frequency tracking, spatial tracking, and gain control subsystems. Design considerations and experimental results are presented.

An optical terminal is described that supports bidirectional communications among a constellation of low-Earth-orbit satellites. The concept uses AlGaAs semiconductor diode laser transmitters, a silicon charge-coupled device (CCD) acquisition array, and a silicon-avalanche-quadrant detector for fine tracking and 10-Mbps communications over a 5000-km range. The 13-lb laser terminal consumes 13 watts of peak spacecraft power. Order-of-magnitude data rate increases can be supported with little terminal impact by incorporating diode- laser master oscillator/power amplifier (MOPA) transmitter enhancements into the design.

This paper describes an earth orbiting Deep Space Relay Satellite System (DSRSS) based on optical coherent detection communication for the user spacecraft to DSRS link. The optical coherent detection DSRSS is considered as a possible augmentation to the Deep Space Network (DSN), after the 70 meter antennas are upgraded to Ka Band near the turn of the century. While significant development is required and technical complexity issues remain, the coherent optical system appears capable of achieving the desired order of magnitude improvement relative to the Ka Band DSN. For example, the coherent optical system provides a capability of 670 kbps at Pluto (40 A.U.), a 10.4 dB improvement over the upgraded DSN.

The SDL-5400 series commercial AlGaAs laser diode was characterized and screened for potential use as communication laser in the SILEX program. The lasers were initially tested through environmental extremes and for vacuum reliability to obtain preliminary indications that the laser was suitable to the requirement. Potential flight lasers were then built and screened for use. Endurance testing of samples from the potential flight lot has been completed.

Satellite laser communications offers the potential for lightweight, high speed data transfer. One of the critical aspects of such a system are small, lightweight, high power laser sources. We will evaluate the applicability of SDL's MOPA laser diode for laser communications. Methods for achieving high data rates will be discussed. Output power and beam quality measurements will be made, as well as characterization under high speed modulation (>1GHz).

Optimization of a laser communication system that uses pulse position modulation (PPM) of a cavity-dumped Nd:YAG laser was investigated analytically. Net communication efficiency, which is a product of PPM efficiency and laser efficiency, was shown to optimize at M-ary pulse formats in the range of 4 bits/pulse when communicating at rates of 4 to 12 Mb/s. The small number of bits/pulse optimizes the efficiency of a cavity-dumped laser by maintaining its pulse rate constant within a narrow range.

A compact, broad beam diode based laser transmitter has been developed for moderate range and data rate free space laser communications. The laser transmitter spatially combines high average power AlGaAs laser diodes in the near field and overlaps several beams into a uniform beam in the far field. Individual high brightness laser diodes are lensed with a cylindrical lens along the length of the emitting aperture and projected with a short focal length lens into a broad beam of several milliradian divergence in the far field. The engineering prototype laser transmitter consists of six precision aligned diode assemblies, two fold mirrors and a six lenslet macrooptic assembly all mounted on a beryllium baseplate. Data will be presented showing that the laser transmitter is highly efficient by reserving the inherent high brightness of the individual diodes in the optical design, by the development of a pulsed switcher electrical circuit based on recent lightweight dc power converter designs for spacecraft applications and the removal of excess diode heat via the beryllium baseplate.

A modulated fiber-coupled diode laser assembly has been developed. This module will serve as the laser transmitter assembly for the Optical Communication Demonstrator (OCD) system. The laser is capable of greater than 20 mW of output at 100 Mbps Q-PPM modulation. It consists of a high-speed driver, a controller to maintain output amplitude under varying duty cycles, and a temperature controller to control output wavelength. The driver provides over 280 mA of peak modulation current with an optical pulse risetime of approximately 1 ns. The switching speed is limited by the inductance of the laser mount and the cable connecting the driver to the mount. The laser is coupled to a single-mode polarization preserving fiber that can feed directly into the telescope assembly of OCD.

A CCD-based spatial acquisition and tracking subsystem has been developed to perform both spatial acquisition and tracking functions for a lasercom instrument. By operating the CCD in the 'windowed' read mode, the detector can achieve both wide field of view required for spatial acquisition and the high update rate needed for effective platform jitter compensation. Furthermore, spatial tracking subsystem based on the CCD tracker requires only one steering mirror to perform both line-of-sight stabilization and point ahead functions, and provides means to optically close the point ahead control loop without additional sensors. When incorporated into the lasercom system designs, the array tracking concept can lead to reduced system complexity and hence a lower system cost.

We have developed a high-speed tracking target simulator and pointing direction measurement system to evaluate a tracking/pointing subsystem of an optical intersatellite link (ISL) transceiver. Developing high-precision beam tracking/pointing technology requires a more precise performance measurement system. We have been studying a tracking target simulator as a signal generator and pointing direction measurement method to evaluate a transfer function of a fine tracking/pointing subsystem. In this paper, we will describe a wide-diameter collimated beam intensified uniformly as a tracking target, and how we control the direction of it with 1 µrad in amplitude and 100 Hz in frequency.

An experimental examination of the suitability of using a CCD imaging sensor for space lasercom applications was conducted. A specialized CCD camera capable of operating in high rate partial frame mode for tracking as well as normal rate full frame mode for acquisition was built. CCD pixel rate is 7.5 MHz. A flash A/D digitizes the pixel signals and inputs them to a 30 MIPS digital signal processor which processes the input data between pixels. The processor inputs the pixel magnitude, compares it to the previous brightest pixel in the frame, saves it to memory, remembers its location and magnitude if it is brighter than the previous brightest pixel, and then inputs the next pixel.

A tracking control loop was designed for precision beam pointing based on a high frame rate CCD array and a fine steering mirror. The effect of CCD frame rate and readout delay on performance of control loop was analyzed. A digital compensation filter was designed to achieve high bandwidth tracking with a CCD frame rate of 2 KHz.

In the area of free space optical communications a significant amount of attention has been given to the overall problems of the acquisition and tracking of satellites. Accomplishing these functions with minimal investment in hardware size, weight, and power is essential to the successful evolution of space laser communications (LASERCOM). This paper studies and compares two different types of nonmechanical laser beam steering/diverging devices. A nematic liquid crystal phased array has been tested and compared to an acousto-optic Bragg cell. Both electro-optic devices are capable of simultaneous laser beam spoiling and steering which will provide an alternative to the use of electromechanical hardware for acquisition, fine tracking and point ahead in LASERCOM terminals. Characteristics such as optical efficiency, response time, beam steer range and divergence and power consumption have been measured. Device design and performance parameters are described.

A novel coherent detection spatial tracking system based on a highly sensitive angular discriminator was designed. The incoming signal beam and the local oscillator (LO) beam was split into two paths in order to estimate the azimuth and elevation angles of the incoming signal beam. The optics design in each path comprised of an electrooptically tuned Fabry-Perot Interferometer (EO- FPI) and two photodetectors. The only difference in the two paths was the orientation of the EO-FPIs. In this paper, the azimuth and elevation angles of the incoming signal beam are calculated given that the incident angles of the signal beam at the two EO-FPIs are measured by the FPIs. It will be shown that the solution is obtained at the intersection of two ellipses centered on the principal axes of the reference coordinate system. Each of the ellipses is the locus of all possible angular direction of the signal beam with respect to the normal of the EO-FPI. This exact solution can be approximated by the intersection point of two straight lines. Each straight line is tangent at the point of the ellipse intersecting the principal axis. When the angular bias of the EO-FPIs deviates from its nominal value, the location of the ellipses will be rotated. This will cause the estimated signal beam direction to be angular biased from its true direction. The effect of the deviation of the EO-FPI's angular bias must be corrected in order for the coherent detection spatial tracking system to work properly.

Satellite jitter adversely affects pointing, acquisition and tracking (PAT) functions of an intersatellite laser communication system. Reliable aircraft- based testing of PAT systems requires that the detrimental effects of aircraft jitter be controlled and a realistic satellite jitter environment be emulated. A novel jitter rejection technique, the self-tuning feedforward compensation scheme, is developed to minimize effects of aircraft vibration on the PAT terminal. The self-tuning results in the implicit characterization of the mechanical jitter propagation path thus facilitating the injection of prerecorded satellite jitter in the control circuitry of steering mirrors.

A novel design is presented for an electromagnetic device which provides linear translation of a focal plane detector array, for optical systems where a re-imaging plane is not available to use conventional angular beam steering mirrors. High bandwidth, high reliability without lubrication, and extremely low friction are achieved using two-axis magnetic motors which pivot a flexure mounted post. Near linear translation of the post-mounted detector array without defocus is provided by designing the optical system with a spherical focal plane matched to the radius of the post.

Before a communication link can be established between free-space coherent optical communication terminals, the frequency of the optical signal and spatial direction of the transmitter or receiver must be acquired. During the communication period, the operations of frequency tracking and spatial tracking must be maintained. The frequency of the optical signal will deviate from its nominal value due to Doppler shift and laser frequency instability. The frequency tracking system must be able to track out the frequency variation to maintain the detected signal within the passband of the communication receiver. A coherent detection spatial tracking system based on a Fabry-Perot Interferometer (FPI) had been designed. Electro-optic material was used for the cavity of the FPI. In this paper, a scheme is presented to use the same electro-optic FPI (EO-FPI) to track the frequency of the optical signal. The method is to apply a small sinusoidal modulation voltage to the electro-optical cavity of the FPI. In doing so, the coherently detected signal has the amplitude and phase modulated at the same frequency. The amplitude of the coherently detected signal is used to generate the frequency error signal to minimize the frequency difference between the signal and local oscillator (LO) optical field. While the phase of the coherently detected signal is low- passed filtered to generate the angular error signal. The angular error signal is proportional to the angular difference between the signal and LO beams. The length of the cavity and the angular bias of the EO-FPI is chosen to optimize the frequency and spatial tracking performance of the system.

An active jitter compensation scheme intended for free space intersatellite laser communication, utilizing self-tuning feedforward compensation, is developed. It is implemented via computer-controlled analog circuitry. The theory, design and implementation of the laboratory prototype are discussed. A performance evaluation, comparing the feedforward compensator to existing closed loop control, is presented with the feedforward technique demonstrating significant jitter reduction.

Performance of deep space optical communications is impacted by atmospheric outages. By monitoring intensities of known stellar objects with calibrated telescopes, atmospheric attenuation statistics can be recorded. This paper describes a program to develop an atmospheric transmission model for optical communications by making autonomous visibility measurements of known stellar objects from three locations in the southwest United States. The model will be used in link margin analyses for optical communications channels. The statistics will be updated on a quarterly basis and calibrated using image data taken over a long period of time. This calibration is expected to allow extrapolation to other ground-based locations through the use of global weather data bases.

The objectives of the CEMERLL experiment are to measure the signal enhancement obtained in a two way laser propagation link using laser guidestar adaptive optics from the Earth to the Moon using the Apollo retroreflector arrays, and to predict and verify the resulting signal strength and variability. A theory is presented for the probability density functions of the laser link by combining multiple effects of the: 1) compensated laser uplink through turbulence, 2) reflection from the lunar retroreflector array, 3) passage through turbulence on the downlink, aperture averaging by the receiving telescope, and 4) signal detection with a photovoltaic detector. The most important element in the chain is the uplink propagation, all other effects propagation effects modify only the mean number of photons of this two way link, and do not significantly change the probability density functions of the uplink laser beam. The resulting probability density functions are defined by parameters that include the effective number of scatterers, the average intensities in the specular and diffuse portions of the beam, and the beam jittering effect of using a laser guidestar. Using intensity moments derived from the far field propagation, performance data on the laser guidestar adaptive optics system, and approximations for higher order moments, the parameters of these distributions can be numerically evaluated from experimental conditions. These show a widely diffuse speckle pattern for the uncompensated beam, and a similar shaped but long tailed distribution for the compensated beam. Uncorrected tilt effects cause the well compensated beam to randomly jitter and results in an intensity distribution where there are some 'hits' of high intensity light, but more frequently there is a portion of the beam side lobes which illuminate the corner cube array. A separate tip-tilt correction using either an illuminated lunar feature or the return pulses themselves would mitigate this effect.

Applications such as high resolution image transmission and the aggregation of multiple bit streams have increased the interest in very high speed (Gbit/s) data communications for space applications. The ability to perform error correction on a data link can yield significant improvements in channel efficiency and can mitigate the effects of various bit error rate (BER) impairments afflicting many communications systems. A codec integrated circuit (IC) has been constructed which is capable of supporting up to 2 Gbit/s of data throughput. The device has been demonstrated in two optical communications systems, a 1 Gbit/s binary optically preamplified OOK system at 1.5 µm and an 800 Mbit/s FSK MOPA system at 0.98 µm. At a BER of 10^-9, this coding provided a 3.7 dB sensitivity improvement for the OOK system, and a 6.2 dB improvement with the FSK system. The measured sensitivity of the coded OOK system was 37 photons/bit, which is better than could be realized with an ideal uncoded system. The viability of applying error correcting coding to higher data rate systems is discussed and a method of utilizing these ICs to provide four Gbit/s operation is shown.

In this paper, effects of atmospheric turbulence on the Phase Shift Keying (PSK) modulation schemes of an electromagnetic wave carrier operating at optical frequencies are investigated. Tatarski's model for the atmospheric turbulence has been employed in these investigations. It is noted that the amplitude/intensity fluctuations of the optical carrier caused by the turbulent propagation medium will have an effect on this modulation scheme. The turbulent atmosphere through which the optical beam is propagating can also directly produce phase fluctuations which enhances the Bit Error Rate (BER) of the optical communication link.

An exact derivation is presented for the M-ary detection probability using the conventional decision rule. The analysis assumes a shot-noise-limited case, since in practice the thermal and background noises are negligible. Poisson statistics are employed for the detectors and this takes care of the shot noise. An error in detection occurs when the count corresponding to the signal is less than the count of any of the other detectors. Results indicate that the system performance cannot be improved beyond a certain level, since a component of signal acts as a source of noise to the other detectors. To combat this drawback, an alternate decision rule is suggested so as to improve the system performance. The results obtained for the BER in the two cases indicate that the alternate decision rule leads to a marked improvement especially at higher SNR.

In freespace laser communication systems, optical background noise rejection is a very important issue. We have designed and built a building to building direct detection optical communication system that uses a Faraday Anomalous Dispersion Optical Filter (FADOF) in the receiver. The FADOF is a narrow bandpass optical filter, which can provide a background noise rejection of 10⁵ approximately 10⁶, while transmitting the signal with up to 80% efficiency. The FADOF also has a signal bandwidth that is variable between 0.5 GHz and 5 GHz, a field-of-view that is flat over +/- 20 degree(s). FADOFs offer new capabilities to freespace laser communication by effectively reducing the solar background radiation that reaches the photodetector. Using the FADOF receiver, we have demonstrated that 27nW of received signal power gives a bit error rate of 10^-6 (limited by the photoreceiver electronic noise) independent of solar noise up to 0.15Watt. We also repeated these measurements under the same operation conditions after replacing the FADOF with an interference filter. The experiments showed three orders higher background noise rejection capability for the FADOF receiver.

The Stark anomalous dispersion optical filter is a wide-frequency-tunable ultra-narrow bandwidth optical filter. The first theoretical investigation of this filter, which matched wavelength with doubled Nd:YLF lasers for deepspace laser communications, is reported. The results show that the filter may provide about 86% transmission, 1.6 GHz bandwidth, 3 GHz noise equivalent bandwidth, and wide frequency tuning range.

Laser communication and optical remote sensing systems often require a highly sensitive detection scheme to measure extremely weak optical signals. This is particularly true for a receiver designed for a deep space optical communication link. The conventional direct detection scheme is sensitive to receiver noise and detector quantum efficiency while heterodyne detection requires complex wavefront matching and a highly stable local oscillator. This paper considers a direct detection scheme where the optical signal is amplified by an optical parametric amplifier prior to photodetection. It is shown through analysis and simulation that such a scheme can outperform the direct detection receiver when the gain of the amplifier is large and the detector quantum efficiency falls below a certain value. A method to calculate the value of quantum efficiency below which the preamplifier is useful is presented. Performance comparisons are carried out between direct detection schemes with and without preamplifier. Conditions under which the optical parametric amplifier results in an improvement of the performance of the receiver are pointed out.

Optical frequency offset-locking is demonstrated for application to Doppler shift compensation for a spaceborne optical communication system using a laser diode transmitter and a narrow band-pass filter with a small tuning range at the receiver. The system performs closed loop computer control of a laser diode output frequency and allows tuning of this frequency with respect to a reference. A tuning range of 21 GHz and a tuning speed of 44 THz/sec is demonstrated while maintaining a transmitting stability of <80 MHz. The theory modeling the system is shown to have agreement with the experimental results to within 5%.

The feasibility of phased telescope arrays for coherent optical space communications is demonstrated by a proof-of-concept laboratory experiment. The incident optical power is collected by four subtelescopes and coherently combined into a single monomode output fiber. The implemented optical receive array antenna is self-phasing, i.e. the optical subfield pistons are automatically adapted with respect to the direction of the incident wavefront. The telescope array is completely independent of any subsequent receiver and of the data modulation format employed. Our experimental setup operates at a wavelength of 1064nm. With an optical input power of 1nW per subaperture, the system efficiently combines the optical input subwaves and responds to a step- shaped change of the input wavefront direction within 1ms.

Modifications to an existing radiofrequency telescope are described which would enable it to operate as an R&D optical receiver terminal for deep-space communications. The low overall cost of the telescope is due to the unique process of fabricating the 10.4 meter primary mirror and its support structure, the lightweight of the primary (< 15 kg/m²), and the requirement that the telescope act only as a photon bucket, which lowers the cost of optically figuring and polishing the mirror. The entire optical ground terminal facility is estimated to cost approximately $DLR10 M to construct.

This paper describes the optical system for the Optical Communication Demonstrator (OCD) instrument. With an aperture of only 4 inches, the OCD instrument is designed to demonstrate the capability of communicating from space to a ground station with a small instrument using optical wavelengths.

A mechanical design has been developed for the Telescope Optical Assembly (TOA) of the Optical Communications Demonstrator (OCD). The TOA is the portion of the OCD instrument that integrates all the optical elements of the system with the exception of the Laser Transmitter Assembly (LXA) which is fiber coupled to the TOA. The TOA structure is composed primarily of aluminum components with some use of steel and invar. The assembly is contained within a 16 cm MUL 20 cm X 33 cm envelope and has an estimated mass of 5.5 kg. The mechanical design was developed using Computervision's CADDS 5 computer aided design software. Code V optical design data was used as a primary input and was efficiently and accurately transferred form the optical designer to the mechanical designer through the use of IGES files. In addition to enabling rapid transfer of the initial optical design as well as subsequent optical design refinements, the IGES transfer process was also used to expedite preliminary thermal and dynamic analyses.

Based on unparaxial diffraction theory, the imaging properties of spheric optical array with large size and large radius of curvature are analyzed in this paper. This theoretical model develops the paraxial theoretical model mainly in two aspects. First, it shows that not all, but only those kinds of optical arrays which satisfy certain conditions have non-Gaussian images, however, the paraxial shows that all kinds of optical arrays have non-Gaussian images. Secondly, it shows that various kinds of optical arrays have different imaging properties, but paraxial theoretical models show that all kinds of optical arrays have the same imaging properties. Moreover, by this theoretical model some experiments can be explained which cannot be explained by the paraxial. However, when the paraxial approximation is employed, the results from these two theoretical models will be the same. Furthermore, we numerically simulate the process of beam propagation in the optical array system without distorted media, and obtain the 3D intensity distribution on each receiving plane. The numerical results in this paper agree with our theoretical analysis.

Under nonparaxial approximation, the pseudo-phase conjugation properties of spherical optical arrays with large size and large radius of curvature are analyzed in this paper. A theoretical model of optical arrays for compensation of static phase distortion has also been obtained. This theoretical model illustrates the types and degree of phase distortion that optical arrays can eliminate and the condition that optical arrays should satisfy. This paper also illustrates the difference of compensation properties between nonlinear phase conjugator and optical array. The numerical results agree with our theory model.

Three 10-W fiber-coupled diode lasers were used to pump a single Nd:YAG laser crystal. Average output powers exceeding 11 W of continuous-wave 1064 nm, and 3.5 W of 532-nm at 50 kHz pulse repetition frequency were achieved. An L- shaped cavity which compensated for thermal lensing in the laser crystal was utilized. The Nd:YAG rod and an acousto-optical Q-switcher were located in one arm of the cavity while a frequency-doubler was in the other arm. The 532 output beam quality (M²) was 1.5.

A processing system was designed and implemented to support beacon acquisition, precision beam pointing, and command/control functions for the Optical Communications Demonstrator. The central processor operates as a command controlled embedded system, but by blending real-time and linear design techniques, a single digital signal processor handles all spaceborne functions. Real-time functions include operation of a CCD array detector, calculation of centroids, computation of point-ahead error, and execution of a steering mirror servo loop at 2 kHz update rate. Linear functions include command interpretation, diagnostic control of all functions, storage and interpolation of data for acquisition and point-ahead calculation, downlinking of status information, hardware control, transmit laser control, and image capture.