A double-layered constellation for a future global satellite communications network connected by optical intersatellite links has been proposed. The constellation consists of lower layer satellites for mobile and personal satellite communications, and upper layer satellites for large- capacity fixed satellite communications and feeder links. Optical inter-satellite links, which can perform high-capacity communications with small terminals, are used for all intersatellite data transmission. Although a polar orbit constellation offers the merit of simpleness in network configurations, the inclined orbit constellation offers the potential for reducing the required number of satellites, improving link properties, and enhancing the coverage in middle and low latitudes, by selecting the most adequate inclination of the orbits. The optical inter- satellite link properties, coverage properties, and required number of satellites are evaluated for constellations using inclined orbits, and then compared with those of a polar orbit constellation. Three constellation types in each layer achieving continuous double coverage are assumed. For each constellation, the relations between these properties and the inclination of the orbits are examined. The basic parameters of optical inter-satellite links on satellite constellations using inclined orbits are also shown.

The Laser Communications Demonstration System (LCDS) Phase A/B was initiated by NASA-Headquarters through JPL to show that improvements in both technology and multi- discipline system engineering expertise since the initiation of previous programs enable the practical demonstration of space laser communication terminals that exhibit the anticipated benefits relative to rf communications. This paper presents an overview of the Laser Communication Demonstration System as developed by the Ball team.

In some applications signals originate in analog form and are to be delivered to a remote destination as analog signals. In such cases, it is sometimes more economical or technically feasible to transmit these signals in their original analog form than to convert to a digital format, particularly when the signal bandwidth and/or dynamic range is large enough to stress the capabilities of current digital-to-analog and analog-to-digital converters. This paper discusses analog signal transmission at optical wavelengths in the context of free space links. The technology of optical modulators, amplifiers, and receivers is discussed and an example of an analog crosslink design is given.

Communications Research Laboratory (CRL) developed laser communication equipment (LCE) onboard the engineering test satellite VI(ETS-VI) and a ground system for establishment of basic technologies in optical intersatellite communications. The experiments using a ground-to-space laser link started on December 1994. In the paper, preliminary evaluation for the performance of LCE is presented based on a part of experimental data. Included in the paper are a brief description of operation and data acquisition system, acquisition, tracking and pointing subsystem performance, and communication subsystem performance.

Laser communications between high flying aircraft such as high altitude unmanned aerial vehicles and between the aircraft and the ground (weather permitting) offers the potential to transfer extremely high amounts of information faster and with a smaller package than is currently possible with a radio frequency and microwave technologies. Over the last few years, BMDO has funded a number of technology efforts through the U.S. Army Space and Strategic Defense Command reducing the risks associated with laser communications. One of these efforts, at ThermoTrex Corporation in San Diego, California, is now being carried forward towards an advanced technology demonstration. The program will lead to the demonstration of high data rate communications of 274 Mbit/s to 1.2 Gbit/s between high altitude aircraft and between a satellite and the ground. To identify and further reduce the risks associated with operating the lasercom system on a high flying aircraft, a demonstration of a long range link in the presence of simulated aircraft motion was performed. Link acquisition utilizes an interface to an inertial navigation unit for initial pointing and atomic line filter technology for background light rejection. In addition, data was taken on the effects of atmospheric turbulence on the intensity of received laser light across the 150 km path. We present the development of the tracking system and results of the experiment performed at Mauna Loa, Hawaii and Haleakala, Maui in May and September 1995.

Experimental results of laser beam transmission from a ground station to the engineering test satellite VI (ETS-VI) are presented. The ETS-VI that was launched in August last year has onboard laser communication equipment (LCE). A very narrow laser beam with less than 60 (mu) rad divergence was continuously transmitted to the LCE from a ground station, using a highly accurate satellite optical tracking technique. The optical power fluctuation detected at the LCE was very large. Its log-amplitude variance was about 0.5 and the maximum duration for the LCE to be able to receive an optical power more than the LCE communication detector sensitivity was about 50 milli-seconds. Theoretical analysis of laser beam fluctuation at a satellite is described and compared with the experimental results.

The optical communication experiment a the Communications Research Laboratory (CRL) in Japan has been performed using the engineering test satellite VI (ETS-VI) which was launched into a high elliptical orbit. The laser communication equipment (LCE) installed on the ETS-VI satellite can measure the radiation power from various areas on the earth like the sea, land and clouds. The LCE charge coupled device (CCD) sensor and quadrant detector (QD) sensor are used to obtain the spectral radiant emittance at a wavelength lambda equals 0.5 micrometers from the earth surface. This paper shows the results of the experiment to measure the background radiation.

In this paper, we describe the architecture and performance of a unidirectional, terrestrial laser communication link designed to operate over a range of 100 m to 1 km. The optical carrier is provided by a 1.3 micrometer laser which is modulated using an electro-optic modulator. Quadrature amplitude modulation (QAM) is used to modulate 150 Mbps input binary data at a subcarrier frequency of 140 MHz. At the receive side of the link, the optical signal is detected with a InGaAs p-i-n photodetector. Because the pointing of our low-cost link is passive, atmospheric fluctuations will induce amplitude fading of the optical carrier. The bit error rate performance of the QAM format is particularly sensitive to amplitude fading. Therefore, the performance of our link is sensitive to the effects of atmospheric turbulence. We discuss how these effects can be minimized by an appropriate choice of the beam diameter with respect to the receiver diameter and by incorporating an automatic gain control circuit. We present experimental results which demonstrate the operation of the link with 16-QAM and 256-QAM over a 100 m path with an atmospheric structure constant (C_n²) of 3 multiplied by 10^-13 m^-2/3.

The laser utilizing communications equipment (LUCE) installed on the optical inter-orbit communications engineering test satellite (OICETS) has been developed by NASDA (National Space Development Agency of Japan) from 1993 in order to perform the inter-satellite optical link experiment with ARTEMIS (advanced relay and technology mission satellite) of ESA (European Space Agency). The engineering model (EM) of LUCE has been manufactured and the development test has been prepared for verification of the design. The design of LUCE EM and its test plan are reported in this paper.

Rf and optical communication conceptual terminal designs; size, weight, and power requirements; and development costs are compared for three representative 1-Gbps links. Year 2000 technology is assumed for the designs of LEO-to-GEO, LEO-to-LEO, and LEO-to- Ground terminals based on 32-GHz rf, 60-GHz rf, AlGaAs optical, and InGaAsP/EDFA optical technologies. The optical technologies exhibit advantages in the areas of antenna size, terminal weight, and recurring cost at the 1-Gbps data rate.

We have performed a study on telecommunication systems for a hypothetical mission to Mars. The objective of the study was to evaluate and compare the benefits that microwave (X-band and K_a-band) and optical communications technologies afford to future missions. The telecommunication systems were required to return data after launch and in-orbit at 2.7 AU with daily data volumes of 0.1, 1, or 10 Gbits. Space-borne terminals capable of delivering each of the three data rates were proposed and characterized in terms of mass, power consumption, size, and cost. The estimated parameters for X-band, Ka-band, and optical frequencies are compared and presented here. For data volumes of 0.1 and 1 Giga-bit per day, the X-band downlink system has a mass 1.5 times that of Ka-band, and 2.5 times that of optical system. Ka-band offered about 20% power saving at 10 Gbit/day over X-band. For all data volumes, the optical communication terminals were lower in mass than the rf terminals. For data volumes of 1 and 10 Gb/day, the space-borne optical terminal also had a lower required dc power. In all three cases, optical communications had a slightly higher development cost for the space terminal.

The Optical Communications Group of the German Aerospace Research Establishment (DLR) has investigated the feasibility of a fiberless receiver telescope for high sensitive coherent optical space communication, resulting in an elegant pointing, acquisition and tracking (PAT) concept. To demonstrate the feasibility of this new concept, an optical receiver terminal that coherently obtains both the spatial error signal for tracking and the data signal with only one set of detectors has been built. The result is a very simple and compact setup with few optical surfaces. It does not require fibers for superpositioning and is capable to compensate for microaccelerations up to about one kilohertz.

Free space laser communications offers a distinct advantage over radio frequency technologies in situations where extremely high data rates are required with minimal size, weight and power. Experiments using state of the art high speed laser diodes and high sensitivity avalanche photodiodes have demonstrated the capabilities of lasercom for long range high data rate applications. Over the last two years, the Ballistic Missile Defense Organization (BMDO), contracting through the U. S. Army Space and Strategic Defense Command (SSDC), has been funding the development of a high data rate laser communication terminal projected to be flown on the second Space Technology Research Vehicle (STRV-2) in 1998. The objective of this experiment is to demonstrate a laser uplink and downlink from a low earth orbit (LEO) satellite. The success of this experiment will validate the readiness of laser communication technology for satellite cross links and LEO up and down links. The design of the steering system is based on a novel gimbal design using direct drive hollow core brushless dc motors, high accuracy inductive encoders and a high frame rate CCD camera beacon tracking system. This paper discusses the details of the control and electronic design of the tracking system.

Presented in this paper is an introduction of a fiber-type optical phased array antenna for free- space optical communications. Optical phased array antenna is a phased array antenna operated at optical frequency. The high speed beam steering, spatial power combining, and flexible beam forming characteristics of them will drastically improve the performance of future free-space optical communication systems. The system design of a fiber-type optical phased array which can steer an optical beam in 2-dimension is reviewed. Fundamental characteristics of the integrated optical beam forming network (BFN) are shown. The wide bandwidth response up to 924 MHz of optical phase shifters, which enables high speed beam steering, is highlighted. Radiation patterns demonstrate coherent spacial power combining.

The simulation and optimization of a high sensitive coherent detection and tracking receiver for free-space laser communications is described. The data and the spatial tracking error signals are obtained with the same set of photodetectors, hereby avoiding systematic losses. Simulation models for the tracking tilt mirror transfer function, the diffraction limited data and tracking receiver gain functions and the performance of the digital signal processing algorithms are derived. The optimization of the telescope's Fresnel-number and of the local laser's Gaussian beamwidth for the photodetectors used in our laboratory system is described. The intermediate frequency current and signal-to-noise ratio of the data signal are compared with theoretical limits.

For extremely high data rate communications between satellites, aircraft, and ground sites in situations where atmospheric interference is minimal, laser communications offers significant advantages over current radio frequency technologies in the areas of achievable data rate, size, weight, and power. Over the last two years, the Ballistic Missile Defense Organization (BMDO), contracting through the U.S. Army Space and Strategic Defense Command, has been funding the development of a laser communications terminal to be flown on the second Space Technology Research Vehicle (STRV-2) which is projected to launch in 1998. It is hoped that a successful satellite demonstration will validate the capability and readiness of lasercom for inter-satellite crosslinks and low Earth orbit (LEO) satellite downlinks to the ground. The design of the terminal is based on direct modulation of semiconductor lasers, direct detection using avalanche photodiodes, separate acquisition/tracking and communications wavelengths, atomic line filter (ALF) technology for background light rejection, separate transmit and receiver apertures, and a hemispherical field-of-regard gimbal based upon a novel design. This paper discuses details of the optomechanical design of the terminal as presented at the program's critical design review.

In this study, we clarify the performance of a Nd-doped fiber amplifier (NDFA) at 1.06 micrometer as a booster amplifier and a preamplifier. As a booster amplifier, we propose a Nd-doped double-clad fiber (DCF) amplifier. The DCF configuration could be the most promising technique for obtaining high output power from the NDFA, because of its potentially low amplifying threshold characteristics in an ideal four-state system, and the easy pump lightwave coupling to a broader first cladding area in the DCF. The proposed DCF is spliced with a normal single mode fiber at one end of the DCF as a signal input port, to avoid multi-mode propagation of the input signal lightwave through the first cladding area. Design parameters, such as the length and Nd concentration of the DCF, will be optimized by further study. The theoretical result predicts a 2-watt signal output power by a 10-watt pump power coupling to the DCF. For comparison, a booster amplifier employing four-LDs pump scheme is also demonstrated. As a preamplifier, we clarify the NDFA's high-sensitivity performance in giga-bit transmission. The NDFA will provide high-power, low-noise, robust, multigigabit optical transmitters and receivers for free-space laser communications systems.

We report on the operation of a direct-detection 980 nm receiver based upon a semiconductor traveling wave optical preamplifier. An InGaAs/GaAs/AlGaAs single-quantum-well ridge- waveguide angled-facet optical amplifier was fabricated with an internal gain of over 30 dB and a noise figure of 3.8 dB. When used as an optical preamplifier, an improvement of 13 dB in receiver signal-to-noise sensitivity was measured relative to the PIN receiver without optical preamplifier. The signal-to-noise performance of a 250 MHz APD receiver was also measured and was observed to be 8 dB more sensitive than the optically preamplified receiver. Calculations based on these results indicate that with a narrower optical filter bandwidth (0.1 nm) and improved amplifier input coupling efficiency (50%), the optically preamplified receiver can simultaneously achieve high-sensitivity (5 dB better than 250 MHz bandwidth APD) and high-bandwidth (30 GHz).

The model of non-stationary multiplication process in Avalanche semiconductor structure with Negative Feedback (ANF) is presented. Simulated dependencies of multiplication probability distribution, noisefactor and character avalanche rise time illustrate the main features ofANF process. Some experimental resuits, obtained on a laboratory Si-based ANF device samples, are found to be in good agreement with the model predictions, and demonstrate high threshold characteristics of such devices. A unique combination of high gain, low noise, fast response and high quantum efficiency is shown to allow an analog detection of few-photon short light pulses -thetask, unsolved up to present time. Opportunities of creation of matrix devices and devices, based on non-silicon semiconductors and intended for other spectral ranges are also considered as well as the prospects of their use in free space laser communication.

The concept of a new coding scheme is proposed, which combines the benefits of a spectral shaping line code and the error correcting capabilities of a convolutional code with Viterbi decoder. A new semi-analytic line code is introduced, which allows partly soft decision outputs from the line decoder. An additional reliability information provided from the line decoder is proposed to enhance the Viterbi decoder performance. Because the coded signal is binary, this transmission scheme is especially well suited for the binary modulation formats commonly used in free space optical communications such as OOK (on-off-keying) or PSK (phase shift keying).

Radiation-induced loss in erbium-doped fiber and fiber-based components used in optical amplifiers at 1.5 micrometer has been studied under electron and gamma irradiation up to total doses of 100 krad(Si). Radiation sources used to simulate the natural space environment were a Van de Graaff generator for 1.5 MeV electrons, and Cobalt-60 for 1.17 and 1.33 MeV photons. Induced-loss and 60-hour anneal were measured in Er-doped fiber while simultaneously illuminated with 0.7 W of 980-nm pump, and 1 mW of 1550-nm signal light in an operating amplifier configuration. Initial induced loss from electron irradiation was found to decrease under intense photoillumination to 0.010 dB/m-krad at 980 nm and .007 dB/m- krad at 1550 nm, versus 0.017 dB/m-krad and 0.008 dB/m-krad respectively when unilluminated, and showed a more rapid anneal in some tested fibers. Individual components of an optical isolator were tested for gamma-irradiation sensitivity. A 1 degree change in polarization rotation angle was found in a garnet-film Faraday rotator after exposure, and the GRIN lens showed an initial induced loss of 0.5 dB that annealed to 0.1 dB after 2 weeks under conditions of 1 mW, 1550-nm photoillumination.

A database of component and subsystem technologies for free-space laser communications has been compiled. This document discusses technology assessment for free-space lasercomm components, and contains a collection of characteristics of commercially available and one-of- a-kind components that were made for laser communication and other relevant applications. The document also includes quantitative data on laser communication systems that were constructed in the past, along with plots of development trends for specific component technologies as a function of time. We intend to continually update this information with the assistance of lasercomm community and industries involved. First draft of the document will be distributed by mid 1996 for comments and corrections. It is expected that this collection of data will serve as a handbook to lasercomm system engineers and designers.

Atmospheric turbulence causes intensity and phase disturbances on the wavefront of electromagnetic waves propagation through it that can seriously degrade the reliability of free- space optical communication links. This paper deals with the estimation of the statistics for power fades resulting from the combined effects of distortion of the receiving system instantaneous point-spread function and from the fluctuations of the collected power arising from wavefront intensity fluctuations. Fractal techniques are employed to simulate the turbulence-induced point-spread function distortions, while a log-normal model is assumed for the collected-power fluctuations. The reduction in the cumulative probability of losses due to these two effects through spatial diversity using a multiaperture receiver configuration is assessed.

Satellites in free space suffer from periods of high displacement amplitude vibrations (for example: during the operation of the thruster, the antenna pointing mechanism, or the solar array drive mechanism). In order to utilize the advantages of optical communication in space, very narrow divergence transmitted beams are used. The high amplitude vibrations of the satellite cause decrease of received signal power in the receiver satellite due to mispointing of the transmitted beam. One way to overcome this problem is to develop very complicated stabilization systems. The disadvantages of this solution are: complexity, reliability, and cost. Most of the time, the amplitude of the vibrations is low and does not affect the communication performance. Considering these facts, we derive a model of a communication system that adapts the communication system parameters to changes in received signal caused by changes in vibration amplitude. The purpose of this model is to keep the bit error rate (BER) low and constant by adapting the system bandwidth and the receiver parameters to the vibration amplitude. The duration amplitude and occurrence of the high amplitude vibrations are assumed to be known so the adaptation of the communication system parameters is simple. The adaptive model is derived for the on off keying (OOK) modulation method. An example for practical optical space communication systems based on the adaptive model is given. Comparison and analysis of the performance of standard and adaptive models of communication systems for variable amplitudes of vibration amplitude are presented.

The beacon is a powerful non-coherent cw infra-red laser source which will provide a high divergence beam used during the first tracking acquisition sequence of the Spot 4/Artemis optical communication link. The beacon uses high efficiency anamorphic couplers to deliver output from 19 laser diodes into a single multi-mode mixing fiber, the exit of which is integrated at the focal plane of a collimator. Beacon output is maintained at the required level (nominally 8 KW/Sr) during unit life using an optical monitoring system and a beacon output tele-command. Following successful environmental testing, the proto-flight model (PFM) has recently been delivered ready for integration onto the SILEX terminal. This paper describes the overall performance of the PFM beacon with respect to SILEX requirements. An improved beacon using 1.2 W laser diodes which will be capable of delivering up to 17 KW/Sr is currently under construction. An analysis of the design aspects to be considered when using high power laser diodes in this type of application is presented. Finally, a brief summary is given of SPACEBEL activities associated with high power and more compact optical communication units for future missions.

For inter satellite cross links the use of optical communications technology has the potential of reducing the size, weight and power consumption of current rf communication terminals. Since the electromechanical hardware for pointing, acquisition and tracking i.e. mirrors, motors and actuators are counter productive to these goals, several technologies for non- mechanical beam steering and beam diverging have been developed as an alternative. This paper describes a nematic liquid crystal phased array which is capable of simultaneously steering and diverging a laser beam. Characteristics such as insertion loss, steer range, steering speed are considered for the application of optical satellite communication. Since liquid crystals respond very slowly to a change in drive voltage, methods for increasing the speed were investigated. A series of measurements was performed to evaluate an optimized drive algorithm. As a result of these measurements ideas for an improved design are presented.

The laser transmitter systems have been our latest and largest topic in optical inter-orbit communications (OIC). We believe that laser subsystem using erbium doped fiber amplifier (EDFA) or neodymium doped fiber amplifier (NDFA) are eminently suitable for high power and wide bandwidth transmitters. Thus, we have been testing commercial EDFA and have obtained good results. In addition, results of FFP measurement of transmitted laser power from single mode silica fiber are also discussed. It is also feasible to apply transmitter subsystems using EDFA or NDFA to future OIC systems, but there are some optical features that have to be considered when designing OIC equipment.

The Ground/Orbiter Lasercomm Demonstration (GOLD) is an optical communications demonstration between the Japanese engineering test satellite (ETS-VI) and an optical ground transmitting and receiving station at the Table Mountain Facility in Wrightwood, California. Laser transmissions to the satellite are performed approximately four hours every third night when the satellite is at apogee above Table Mountain. The experiment required the coordination of resources at CRL, JPL, NASDA's Tsukuba tracking station and NASA's Deep Space Network at Goldstone, Calif. to generate and transmit real-time commands and receive telemetry from the ETS-VI. Transmissions to the ETS-VI began in November 1995 and are scheduled to last into the middle of January 1996 when the satellite is expected to be eclipsed by the Earth's shadow for a major part of its orbit. The eclipse is expected to last for about two months, and during this period there will be limited electrical power available on board the satellite. NASDA plans to restrict experiments with the ETS-VI satellite during this period, and no laser transmissions are planned. Post-eclipse experiments are currently being negotiated. GOLD is a joint NASA-CRL (Communications Research Laboratory) experiment that is being conducted by JPL in coordination with CRL and NASDA.

Historical applications for space-based, free-space laser communications have almost exclusively been borne out of government programs. While governments will remain the primary customer for research and exploratory applications, commercial demand is sure to drive the future of laser communications. It will either drive the industry to prosperity or out of business. This paper characterizes and postulates an end to lasercom's 'developmental era.' A paradigm shift is described which may lead the industry into its 'operational era.' Some future customers and their requirements, which will likely demand industry evolution, are identified. Characteristics of requisite change are descried. And finally, the implications of such change are postulated.

Originating out of a SDIO-funded, Phase 1 study effort, two ground systems and an orbiting EO payload/spacecraft were the primary equipment for the RME. The RME was originally conceived to be a shuttle deployed experiment. Shortly after program start, the Challenger disaster occurred, with the promise of extensive delays. A completely new space segment was to be designed incorporating a free-flying spacecraft. During the midphase of the program, a variety of launch vehicles were envisioned to replace the shuttle, requiring the BASD team to design accommodations for Delta, Atlas, and Titan, with a Delta launch being the final solution. The ground systems tracked the spacecraft and illuminated it with green and blue beacon lasers. The Payload Experiment Package (PEP) housed the bisection tracker, a key innovation central to the experiment. The bisection tracker acquired both beacons and controlled steerable mirrors to accomplish fine tracking of the two cooperative beacons. In the process, the relay mirror was precisely positioned to enable a successful relay of a third infrared laser between the two ground sites via the orbiting spacecraft. Many of the key technologies employed in the PEP were originally developed for Ball laser communications research and development programs and other laser pointing efforts. The WAVE sensor package, built by ATA and integrated by Ball, measured the vibrations of the optical base structure on which it was mounted. These spacecraft vibration data are critical to the accurate pointing of space laser communication terminals.

A laboratory development model (LDM) to demonstrate pointing, acquisition, and tracking (PAT) as applied to laser communications would provide valuable data as to efficiencies of different communication scenarios and network concepts. Such a system was designed, constructed, and tested to perform the PAT functions and to measure the effects on an actual laser communication link. Three PAT concepts were investigated for simulation. The first is an open-loop, one-way system in which a single beam director points in a serial manner to a number of remote stations. Acquisition is accomplished by assuring that the transmitted beam is sufficiently broad to cover the region of uncertainty of the receiver and that the field of view of the staring receiver is sufficiently broad to cover the region of uncertainty of the transmitter. The second concept is a two-way link, also employing a single beam director. The concept is similar to the traditional point-to-point lasercom link, requiring mutual acquisition and tracking. The third concept involves two beam directors, allowing slewing and acquisition of a new station while simultaneously communicating with a previous station. When the slew is complete, a beam to the switch redirects the communication beam to the second beam director. The process may be repeated in a 'leapfrog' manner until all stations have been communicated. These concepts were demonstrated by LDM hardware through generation of the necessary computer code. Other networks can easily be simulated by changes in the software.