Lincoln Laboratory has been pursuing the development of Space Laser communication technology for the U.S. Air Force for the past 14 years. Starting around 1980, the Laboratory, already a pioneer in advanced RF Satcom technology, embarked on an effort to systematically develop, demonstrate and space qualify the necessary components and subsystems to enable efficient space lasercom systems.

We review the current applications base for the use of free-space optical communications technology. We then describe the NASA programs in the optical communications area addressing those applications areas. In particular, we describe the Laser Communications Demonstration System, an industry-driven program for the space-demonstration of this technology at data rates of at least 750 Mbps. We then describe NASA technology development and study activities in support of these applications. Activities described include the Optical Communications Demonstrator, a study of an air-to-ground system demonstration, a mission application study for the TOPSAT radar mapping mission, and a program to acquire detailed statistics on atmospheric transmission for space-to-ground links.

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) in order to perform the inter-satellite optical link experiment with ARTEMIS (Advanced Relay and Technology Mission Satellite) of ESA (European Space Agency). To acquire the acquisition, tracking and pointing technology of an optical link is aimed by this in-orbit experiment. The preliminary design of OICETS including LUCE was finished and its result is reported in this paper.

Within the frame of the Date Relay and Technology Mission Program the European Space Agency (ESA) is currently developing optical terminals for high data rate free space laser communication via inter-satellite and inter-orbit links. As a first application such an optical terminal will be embarked on the French low-Earth orbiting satellite SPOT 4. This terminal will communicate with a similar optical terminal on-board ESA's geostationary spacecraft ARTEMIS. It is planned to implement similar optical communication payloads also on ESA's Data Relay Satellite and other spacecraft in geostationary or low-earth orbits. For the purpose of in-orbit check-out of such payloads ESA is establishing an Optical Ground Station (OGS) at the Observatorio del Teide at Izana, Tenerife, Spain, in cooperation with the Instituto de Astrofisica de Canaries, La Laguna, and supported by the German Space Agency (Deutsche Agentur fur Raumfahrtangelegenheiten, Bonn. The selected OGS site provides excellent seeing conditions combined with high atmospheric transmission, which are important prerequisites for successful ground station operations. This paper describes the design of the OGS, highlights some design criteria and discusses the expected performance.

High data rate communications systems will soon be needed for space applications. Technology and applications which support high data rates are already in place for ground- based telecommunications, or will be in the future. The advantages of an optical system over a traditional RF link for free space communications are particularly compelling for high data rates. We have been developing the necessary technology to demonstrate the feasibility of high rate free-space optical communications technology at 1.5 micrometers . The existence of a large, mature technology base at 1.5 micrometers developed for the telecommunications industry has allowed us to focus our development effort on two key technologies needed for space applications that have not been developed for the ground: a 1 Watt class optical power amplifier and a near quantum limited receiver. This paper will describe the overall system design for a high data rate optical communications system and present experimental results demonstrating < 50 photons/bit sensitivity at 10 Gbps with 1 Watt of optical power. The existence of a feasibility demonstration at this data rate enables downward scalability to data rates of 1 Gbps or less with small, inexpensive terminals.

As a new constellation for global satellite communication networks applying optical inter- satellite links, a double-layered low earth orbit satellite constellation with two different altitudes is proposed. In the lower layer, several hundred satellites operating as links for user terminals are arranged. The upper layer contains several tens of satellites that relay data from the lower layer satellites to other satellites or earth-based gateway stations. The satellites deployed in the lower layer can be simplified and miniaturized, because their only function is to link small earth-based terminals and they have only three inter-satellite link terminals. The satellites in the upper layer are complex and heavy, but only a small number of them required. Orbital parameters and inter-satellite link parameters are evaluated in the constellation. Also, special features of this constellation compared with usual constellations with a single altitude are shown.

Laser communications offers the potential for transferring information at data rates higher than 1 Gigabit per second (Gb/s) between satellites, aircraft, and ground sites using terminals with weights between 15 and 30 pounds, and volumes less than 1 cubic foot. The Ballistic Missile Defense Organization (BMDO), contracting through the U.S. Army Space and Strategic Command, has been funding the development of laser communications technology and demonstration hardware. Applications of near term interest include inter-satellite crosslinks, Low Earth Orbit satellite downlinks to the ground, and Aircraft-to-Aircraft crosslinks. To demonstrate the capability and readiness of laser communications for these applications, we are building a space capable laser communications terminal to be flown on BMDO's STRV-2 experiment with a projected launch date in 1997. The design of that terminal is based on direct modulation of semiconductor lasers, direct detection using avalanche photodiodes, separate acquisition/tracking and communications wavelengths, separate transmit and receive apertures, and a simple optical transceiver package mounted on a hemispherical field of regard gimbal.

An optical terminal is described that supports bi-directional communications among a constellation of low-Earth-orbit satellites. The concept uses AlGaAs semiconductor diode master oscillator/power amplifier laser transmitters, a silicon charge-coupled device array for acquisition and fine tracking, and a silicon avalanche photodiode for 1-Gbps communications over a 4000-km range. The 19-lb laser terminal consumes 31 watts of peak spacecraft power.

We summarize a compact lasercom terminal implementation based on a previously described system concept, and report on measurements made on a prototype optical system built on internal MITRE funds. This paper discusses the fabrication and test of an innovative hardware proof-of-concept for an advanced satellite lasercom terminal with a ten-fold size, weight, and production cost reduction over current practice. We have built and tested a proof-of-concept of the optics portion of a full duplex `monolithic glass block' (MGB) lasercom terminal. The complete MGB optical system is only 6' X 4' X 0.5' and weighs less than a pound. A complete terminal which includes a 3 - 4 inch telescope and gimbal could be implemented for as little as 15 - 30 pounds. The optical system test results are reported. Our approach uses emerging technologies and a highly integrated system design, based on representative system requirements for satellite crosslinks. Technology evaluation and system trades led to a novel optics design for a lasercom terminal, based on thin film coatings and half-inch glass cubes. The emerging photonic technologies employed include liquid crystals for solid state switching, automatic gain control, and microradian alignment; multi-layer dielectric films for optical bandpass filters, dichroic separation, and polarization control; semiconductor lasers with microlens optics; and an original design where all the optics are realized in planar thin films incorporated on small glass substrates, nominally one-half inch in size. These glass cubes are permanently bonded together to form a monolithic ensemble. Hence, we have coined this implementation the monolithic glass block (MGB) approach. Fused silica is used throughout for reasons of superb radiation resistance and thermal stability. The thin film filters, switches, and polarizers perform all the necessary functions in collimated light. This approach is feasible because the optical paths have been dramatically reduced to eliminate the need for relay optics, and the design has been refined such that the entire optical assembly is implemented with a single lens for each laser transmitter for beamforming, and a single lens for each receiver for imaging. The collimated optics design and MGB approach results in greatly relaxed fabrication, alignment, and assembly requirements, which are dominant schedule and cost drivers in the implementation of current lasercom terminal designs.

Influence of the atmosphere on airborne coherent optical communications is analyzed and compared to space-borne transmission. An experiment that was performed to evaluate important system parameters for coherent atmospheric communication is described. Measurement results are presented which show that in this experiment the spectral densities of angle-of-arrival fluctuations and received power were similar to fluctuations observed in a satellite system due to mechanical vibration. In addition, the possibility to use such a set-up to test and optimize PAT subsystems as well as receiver electronics and optics under far-field conditions is discussed.

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 much smaller package than is possible using current radio frequency and microwave technologies. This can be especially important in transferring time sensitive reconnaissance information because the value of the data can deteriorate rapidly with time. 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, BMDO has funded a number of technology efforts through the US 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 communication of 270 MBPS (Mega Bits Per Second) to 1.2 GBPS (Giga Bits Per Second) between high altitude aircraft and 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 results of a 42 km, 1.2 GBPS laser communications demonstration performed at NASA/JPL Table Mountain facility in Wrightwood, CA. Also presented are designs for a space qualifiable satellite terminal presently under construction.

The testing and performance of a prototype modem developed at NASA Lewis Research Center for high-speed free-space direct detection optical communications is described. The testing was performed under laboratory conditions using computer control with specially developed test equipment that simulates free-space link conditions. The modem employs quaternary pulse position modulation at 325 Megabits per second (Mbps) on two optical channels, which are multiplexed to transmit a single 650 Mbps data stream. The measured results indicate that the receiver's automatic gain control (AGC), phased-locked-loop slot clock recovery, digital symbol clock recovery, matched filtering, and maximum likelihood data recovery circuits were found to have only 1.5 dB combined implementation loss during bit-error-rate (BER) performance measurements. Pseudo random bit sequences and real-time high quality video sources were used to supply 650 Mbps and 325 Mbps data streams to the modem. Additional testing revealed that Doppler frequency shifting can be easily tracked by the receiver, that simulated pointing errors are readily compensated for by the AGC circuits, and that channel timing skew affects the BER performance in an expected manner. Overall, the needed technologies for a high-speed laser communications modem were demonstrated.

In planned intersatellite optical communication systems, the optical payload on board the geostationary satellite will be periodically pointed towards an Optical Ground Station. When the satellite-ground link is established, the turbulence-induced disturbances must be taken into account. The subject of this paper is to assess the statistics for the power fadings that result from the instantaneous point-spread function distortion. Results predicted by an approximate technique which considers the instantaneous point-spread function as a gaussian intensity distribution displaced from the focus due to the angle-of-arrival tilt are compared against results obtained from wavefront simulations produced by fractal generation techniques. The reduction in the cumulative probability of losses that can be obtained by spatial averaging using a multiaperture receiver is also assessed.

Atmospheric turbulence corrupts both the amplitude and phase of an optical field propagating from space to an earth-based receiver. While aperture averaging can mitigate amplitude scintillation effects, the performance of single spatial-mode receiver systems such as coherent detection or preamplified direction detection can be significantly degraded by the corrupted phase when the ratio of aperture diameter D to atmospheric coherence length r₀ exceeds unity. Although adaptive optics may be employed to correct the wavefront, in practice the correction is imperfect and the residual phase errors induce a communications performance loss. That loss is quantified here by Monte Carlo simulation techniques. Single-mode-receiver fade statistics for imperfect phase correction are calculated in terms of the atmospheric Greenwood frequency f_g, the adaptive optic servo loop cutoff frequency f_c, and the ratio D/r₀. From these statistics, link bit-error rate (BER) performance is calculated. The results reveal that conventional performance measures such as Strehl ratio or mean signal-to- noise ratio loss can significantly underestimate receiver BER losses. Only when the ratio f_g/f_c is 0.1 or less will communications losses be small (about 0.5 dB) over a wide range of D/r₀.

The flight hardware for the European Semiconductor laser Intersatellite Link EXperiment, SILEX, is presently under integration. Qualification at equipment level has been performed during 1994 and performance testing at subsystem level is being undertaken. This paper describes the design requirements, the actual design, and the tests of the Pointing Acquisition and Tracking subsystem. It can be summarized in the following headlines: Mission overview, system tracking and pointing strategy, terminal design, PAT subsystem requirements, PAT equipment design, PAT subsystem design, and on ground verification approach.

This paper describes the Acquisition Sensor Detection Unit (ASDU) and the Tracking Sensor Detection Unit (TSDU) developed by Sira Ltd to be flown in the European Space Agency's SILEX (Semi-conductor Inter satellite Link Experiment) terminals. These units were designed, manufactured and qualified by Sira under contract to Matra Marconi Space. Stringent vibration, thermal stability and video waveform noise and timing specifications were critical elements in the design. The ASDU consists of an optical head containing a 385 X 288 CCD and video buffer, and separate electronics box containing clock decoding electronics, CCD clock drivers and video amplifiers. The ASDU operates at a frame rate of 33 or 135 Hz and its noise performance allows the brightest pixel to be detected with an accuracy of +/- 0.5 pixel for input powers in the range 9 - 55 pW (33 Hz) or 16 - 240 pW (135 Hz) during acquisition. Using the ASDU signal, the SILEX pointing, acquisition and tracking system controls the pointing mirrors to centralize the beam, at which point it falls within the smaller TSDU field of view. The TSDU comprises a single 480 g unit containing a 43 mm focal length lens, a 14 X 14 CCD, and clock receiver and video amplifier electronics. During tracking the central 4 CCD pixels are used as a quadrant detector, and the TSDU has demonstrated a noise equivalent angle of 2.7 (mu) rad (1 (sigma) half cone, 180 pW optical input power) at a frame rate of 8 kHz.

A new combined modulation and coding scheme for optical PSK-homodyne transmission is proposed. The new system is designed with scope to the realization of a very compact and high sensitive coherent PSK homodyne system with considerable small hardware complexity. This is achieved by using a combined coding scheme for synchronization information (syncbit method), spectral shaping and forward error correction. Applications of this technique could be in the future small and lightweight optical terminals for space communications, but also possibly in the future coherent optical FDMA-fiber networks. Without using the syncbit- coding option, this method is also very well suited for optical direct detection schemes, in which an optimal combination of line- and channel coding is needed.

Wavelength Division Multiplexing (WDM) is currently being investigated for ground-based fiber optic networks as a means to achieve high aggregate data rates and/or to allow the use of lower power lasers at a given rate by adding additional channels. A drawback to using WDM techniques for space applications has been that most of the WDM devices used to combine wavelengths are lossy, particularly when implemented with integrated optics or fiber gratings. The additional loss is not important for ground-based applications because it can be overcome by adding gain (with an erbium-doped fiber amplifier, for example). For space applications, where size, weight, and power consumption are critical, the excess loss is a serious drawback. We describe here a very low-loss technique for wavelength combining using standard fiber components. The technique is scalable to moderate numbers of wavelengths while maintaining low loss. Low loss means that these multiplexers can be used for applications such as power combining that are not feasible with traditional techniques. Experimental results confirming low loss power combining will be presented.

The CNES (Centre National d'Etudes Spatiales, French space agency) established the laser system characteristics for Mars-Earth spatial communications using the Pulse Position Modulation technique. Today, diode-pumped solid-state lasers are under intense research and development. Indeed, in comparison with flash-lamp pumped laser, they offer significant advantages in terms of efficiency, compactness, lifetime and high beam quality. We have demonstrated that gain-switch operation is preferable to Q-switching technique to control and to obtain a good pulse width and amplitude reproducibility. The pulse width requirement and the laser-diode pumping scheme lead to a preferred configuration based on a microchip laser oscillator coupled to an amplifier. Best results were performed with a Nd:YVO₄ microchip longitudinally-pumped by fiber-coupled laser-diode bar. Diffraction limited beam, 15 ns pulse width and 100 kHz repetition rate were achieved. The output beam was actually also single frequency and linearly polarized. A transversally-diode-pumped 7-pass amplifier is also demonstrated.

We would like to report present status of our research on future optical inter-orbit communications. First, we will discuss the key subsystem, that is high power laser transmitter subsystem, making comparison among EDFA, Nd:YAG laser, NDFA and MOPA-LD. Second, we will describe the results of our study that optical downlink would be required in future, mentioning its bottle neck technologies and transmittance laser subsystem. Finally, we will raise a question on a space interoperability, which will be the next item of consideration among concerning entities organization.

We have built a diode-pumped Nd:YAG optical amplifier with 52 dB small-signal gain and 1.125 Watts cw extractable power as gain approaches 0 dB. The amplifier is compact (15 cm X 10 cm X 5 cm). Two 2-Watt laser diodes provide the pump power for the amplifier. Gain is at the wavelength of 1.064 micrometers . We have modulated the output of an Nd:YAG diode-pumped cw oscillator with a waveguide modulator and amplified the modulated output. With an M equals 4 PPM modulation, the transmitted pulse power was 1.8 Watts increasing to 6.5 Watts at M equals 64. A greater increase would be possible if the modulator had a better contrast ratio. For pulse rates greater than 200 kHz inter-symbol interference was not a problem.

A method of efficient, high peak power transmitter operation for communications and radar applications is described. The approach provides a means of energy storage for laser diodes using an external resonant cavity with a CW pump source. This technique enables laser diodes to offer higher efficiency alternatives to cavity-dumped ionic solid state lasers (e.g. Nd:YAG) for medium data rate (1 - 50 Mbps) communications using pulse position modulation, or for moderate power laser radar applications. This technique was awarded U.S. patent no. 5,283,801.

The Beacon is a powerful non-coherent CW infra-red laser source which is developed under the Semi-conductor Inter-satellite Link Experiment (SILEX). It 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 during unit life using an Optical Monitoring System and a Beacon output Tele-Command. The Engineering Qualification Model is now complete and overall performance with respect to the SILEX requirements is presented.

We have designed, realized and tested a coherent optical 565 Mbit/s PSK heterodyne receiver demonstrator for space communications which employs a diode-pumped Nd:YAG ring laser at (lambda) equals 1064 micrometers as local oscillator (LO). The entire system was built as a robust and compact unit, with a fiber connector input for the received signal. The required optical input power is 22 photons/bit for a bit error probability of 10^-6, corresponding to a sensitivity degradation of 3 dB compared to an ideal, shot noise limited PSK heterodyne system. Reliable, fully automatic frequency acquisition is achieved employing a microcomputer controlled two-speed algorithm. For initial frequency differences of up to 30 GHz the average acquisition time is 140 s. A frequency tracking subunit controls the LO laser frequency to keep the intermediate frequency constant, even in the presence of Doppler shift. Doppler shift rates of up to 11.5 MHz/s can be compensated.

We report here on measurements made on a lasercom crosslink hardware testbed built on internal MITRE funds. Laser diodes rated at 150 mwatts were characterized to be flat to 100 MHz, rolling off 5 dB at 500 MHz. A microstrip laser driver with equalizer was implemented to provide flat, highly linear frequency response for analog modulation out to 550 MHz. Microstrip and hybrid versions of APD receivers were also fabricated with flat, linear response to 700 MHz (typically -3 dB at 850 - 1000 MHz), suitable for datarates up to 1.25 Gbps. The optical crosslink testbed with equalized driver, laser, and APD receiver exhibits +/- 0.25 dB flatness, +/- 2.5 degrees phase linearity deviation, and +/- 0.25 (eta) sec group delay variation over the full bandwidth for 650 Mbps. This testbed was evaluated with two modulation approaches: analog laser intensity modulation using an OQPSK subcarrier scheme, and baseband digital NRZ OOK laser intensity modulation, at datarates from 266 to 1244 Mbps. The QPSK subcarrier hardware characterization includes the high speed modem/demodulator and a pair of frequency converters. The digital NRZ hardware characterization includes the clock recovery and amplitude/timing decision circuit. Both the subcarrier QPSK and the digital OOK tests used the same laser and equalized laser driver, the same modulated laser power with 95% intensity modulatino depth, and the same APD receiver for these tests. As such, the comparison between these modulation schemes is an even-handed comparison of the end-to-end performance. Results and conclusions are presented.

We developed a multichannel receiver concept for densely packed optical channels which are to be demodulated simultaneously. In each channel, the input is a binary phase modulated optical signal of approximately the same optical power. For experimental verification of the concept we designed, realized and tested a dualchannel system working at 2 X 8.45 Mbit/s, employing DPSK modulation of Nd:YAG ring lasers at 1.064 micrometers . In the transmitter, the channel distance is set by means of a synthesizer-stabilized optical PLL. In the receiver, a balanced frontend based on a polarization-preserving fiber directional coupler, InGaAs photodiodes, and a 5 mW local laser oscillator transposes the optical input channels to a first intermediate frequency. To achieve good channel separation and reliable simultaneous demodulation, conversion to a second and third intermediate frequency is implemented. If only one channel carries information, a sensitivity of 3.2 dB above the shot noise limit is achieved. The crosstalk penalty depends on the optical channel distance, on the channel selection filter, and the post-detection filter. The power penalty due to crosstalk is investigated experimentally and is compared with theoretically predicted results. For a channel distance of twice the data rate a crosstalk penalty of less than 2 dB is achieved.

The theory is proposed which deals with physical grounds of a new type detection process. This means to use the following experimental conditions. A crystal platelet, e.g., CdS, is excited by optical or electron beam pumping at near or above visible laser generation threshold. If a surface of the platelet will be illuminated by two IR rays with different frequencies and angles of incidence, then the running wave of beatings between these rays will arise. It is assumed that the excitation is capable to create the sufficient concentration of spherical excitons having absorption lines with frequencies that correspond to incident IR radiation ones. Thus, the running wave of absorption and, consequently, the net laser gain change, moving with velocity v determined by difference of frequencies divided by wave vectors difference module for two IR (signal and reference) rays, appears. Here we have taken into consideration the possibility for spherical excitons at impurity atoms of the crystal to participate in IR-induced absorption and amplification of laser (visible) radiation due to indirect process. The amplitude and (or) frequency of the laser generation can be changed if initially the group velocity v_g of the laser radiation equals to v. In the case, when the external probe laser beam is used, instead of intrinsic laser generation one, for read-out of the incident IR-radiation information, the output visible radiation will be only changed in the amplitude. As it follows from above, modulated in frequency coherent incident IR radiation will lead to amplitude modulated signal in visible range. The results show that this way open the possibilities to design one-mode fast operating excitonic FM and AM receiver with highest sensitivity that meet all the demands for free space laser communication and LIDARs in spectral interval near 3 mkm wavelength.

For NASA deep space missions, an optical subnet can provide significantly increased telecommunications performance (greater than 6 dB above the DSN 70 meter at Ka-Band) and provide full line-of-sight coverage of the ecliptic, 24 hours a day, with high weather availability. This paper describes the results of recent work in defining such a network including the ground terminal and network design, its projected performance as well as a rough order definition of its development, implementation and operations costs.

Fine steering mirrors of satellite-based laser communication systems operate at high frequencies. Performance of such mirrors are adversely affected by bending modes of their moving parts. Application of feedforward compensation of bending modes, resulting in a model reference control scheme, is considered for 5 KHz and 500 Hz optical mirrors.

This paper describes the design and the implementation of a high speed optical homodyne transmission system for the high data rate channel YKS of SOLACOS. The SOLACOS YKS (YAG Kommunikations System) is based on the Syncbit transmission scheme and uses diode pumped Nd:YAG lasers and an external phase modulator. The results of the Syncbit algorithm simulations, of a 2 MBit/s breadboard implementation and the state of the high speed system implementation (650 MBit/s net data rate) are presented. Special emphasis is put on the design of the digital electronics for the Syncbit transmission scheme at a total channel data rate of 845 MBit/s.

This paper describes the design and the current implementation state of the SOLACOS breadboard. SOLACOS is the German national program to develop a high data rate inter- satellite communication system. The breadboard comprises two communication terminals and experimental ground infrastructure. Each terminal basically consists of the `Optical Head', which is close to a flight system implementation and the remote `Electronics Rack' with the lasers and electronics. The optical assemblies are connected via fibers. The experimental ground infrastructure will implement a test setup for the verification of the pointing acquisition and tracking subsystem performance.

Laser communication systems can be based on a wide range of wavelengths in the optical and infrared regimes. This article provides a rationale for the selection of wavelengths particularly suited for free space laser communication systems. The choice of wavelengths is based on an analysis of propagation issues, especially through the atmosphere, optical background noise, and the necessary technologies that include lasers, detectors, and spectral filters. The maturity of technology is assessed and given due consideration to identify suitable wavelengths for today's laser communication systems. Figures of merit are also developed where useful to provide a comparative estimate of expected system performance as a function of wavelength.

This paper shows the most recent result of the laser communication experiment using ETS-VI satellite. The satellite failed into geostationary orbit and it is currently in a high elliptic three- day recurrent orbit, but from December 1994, we have performed laser transmission experiment for both uplink and downlink.