Semiconductor lasers have been packaged and qualified for various applications in space. In this paper we discuss two subsystems, both of which will be launched shortly. The first subsystem employs a partially coherent semiconductor laser array coupled to an optical fiber, and monitored by a photo-diode. This package is used in a highly reliable, moderate data rate, moderate length communications link. The fiber coupled output power is 28 mW and the package has been fully space qualified. That is, it is free of organics, hermetically sealed, and has been subjected to die shear, acceleration, thermal cycling, and other tests. The laser longevity exceeds 10 years of continuous operation. The second subsystem consists of 27 fiber coupled space qualified packaged lasers of the same type as above, but designed to operate at 200 mW power output from the fiber. Twenty-seven fibers are bundled to produce a source with a total power emission in excess of 5 watts cw. The subsystem is part of a beacon, which will be used very shortly in a polar satellite experiment. The lasers employed in the two subsystems described above are only partially coherent and therefore are insufficiently bright to operate in a long distance, high data rate link, except with a prohibitively large lens.

Free space optical communication systems require high quality laser diode collimators capable of long term operation in the space environment. A four-element collimator with all-spherical surfaces was designed and built using conventional fabrication technologies and specialized assembly techniques. Final assemblies demonstrated 1/50-wave rms wavefront errors and no destabilizing feedback to the laser diode source. Advanced collimator designs employing aspheres reduce the collimator design complexity and cost while maintaining optical performance. Examples of these advanced designs, based on lens molding technologies, demonstrate suitability to space applications.

NASA Goddard is developing a free-space laser communication transmitter. Each of the 10 GaAlAs diode lasers in the transmitter must be digitally modulated by individual current drivers from a common data source. In order to fit the miniature laser header modules, a hybrid integrated laser current driver was required. The hybrid, smaller than a postage stamp, can drive 100 mA modulation current at a data rate of 200 Mbit/s NRZ.

Thermoelectric coolers (TECs) are solid-state devices with no moving parts, and therefore are inherently very reliable. The inherent reliability of the cooler, however, is determined by the electrical arrangement of the p- and n- type thermoelements (elements). Typically, TECs are manufactured with the elements in series electrically, resulting in the highest voltage and lowest current requirements for a given number of elements, but also the lowest possible inherent reliability. For maximum TEC reliability, all of the elements should be connected electrically in parallel, but this results in the need for an expensive high current power supply. By using a redundant element design, the inherent reliability of the TEC can be dramatically improved with very minimal impact on TEC cost and performance, and more acceptable increases in current requirements. A brief description of the exponential reliability model is presented followed by an analysis of various electrical arrangements of the TEC elements in order to optimize the TEC design as it relates to 1) reliability, 2) electrical requirements, and 3) impact on system design and cost. Comparisons are made between the configurations and the results are tabulated and graphed.

A space-qualified module incorporating a high power semiconductor diode laser, collimating lens, thermal control, and driver circuit is described. Three such modules are used in an optical transmitter designed for use in NASA space laser communications experiments. Details of the various components are discussed, as well as laser diode lifetest results.

A laser diode collimator objective was developed in support of the Direct Detection Transceiver program. Close attention to optomechanical design issues including athermalization, alignment, selection of materials, mounting of elements, and hermetic sealing of the assembly was necessary to insure that the desired optical performance was maintained in space deployment.

The design of a compensated grating power combiner for use with semiconductor laser diodes in a satellite communications crosslink application is presented. This novel combiner concept contains the angular compensation associated with a conventional grating rhomb as well as a feature that mitigates pupil shear. A proof-of-principle version of a conventional combiner was fabricated and tested at TRW, and the limitations of this approach will be discussed and contrasted with the more novel compensated grating rhomb technique.

A laser diode header has been fabricated for a grating laser beam combiner (GLBC). The laser diode header provides the thermal control, the drive electronics, and the optical system necessary for proper operation of the beam combiner. The diode header is required to provide diffraction limited optical performance while providing correction for worst case defocus aberration, 0.6 mrad excess divergence, and worst case decenter aberration, 1.0 mrad pointing error. The design of the header considered the mechanical design and the optical design together resulting in a small, self-contained header with 0.7 mrad range for focus correction and +1- 2.5 mrad of beam steering. The complete diode header is currently undergoing optical and mechanical performance testing.

Many applications of laser diode simultaneously require 1) a single longitudinal mode with 2) satellite mode discrimination of better than 20 dB, 3) line-center stability approaching a MHz or better. Monolithic chips satisfying all these requirements are very difficult to fabricate. We have designed and demonstrated a hybridized 1 cm grating cavity that is capable of satisfying all these requirements using simple aluminum mounts and simple thermally controlled environment.

Control of the longitudinal mode of a laser diode is necessary in any wavelength dependent beam combining system. External and internal etalons can provide periodic optical feedback to a laser diode which modifies the laser diode gain curve, resulting in improved control of the lasing mode. We have obtained single mode lasing without mode hops for VSIS and CSP laser diodes with an external etalon attached to the laser's front facet for up to an 8°C range CW and a 4°C range pulsed, with .07 nm/°C tuning. Our tests of thin tapered-thickness (TT I) laser diodes show CW and pulsed single mode lasing over 10°C and 2°C ranges, respectively, with .08 nm/°C tuning. An analysis of the TTT structure reveals the equivalent of an internal etalon. The time resolved pulsed behavior for both types of lasers show single mode lasing within the proper temperature ranges with minor modes present only early in the optical pulse, if at all. The external etalon produces noticeable interference fringes in the farfield pattern, while those of the TTT lasers are smooth. Ongoing CW lifctest results indicate stability to within one longitudinal mode after a few hundred hours of operation, along with at least several thousand hours lifetime.

Buried heterostructure window lasers with micron wide waveguides suffer catastrophic damage at approximately 10 mW. When modified to incorporate nonabsorbing facets, the damage level can be increased by between one and two orders of magnitude. A lifetest of such "window" buried heterostructure lasers has been performed at 10, 20, 30 and 50 mW peak power and at 50°C and 70°C. The results project lifetimes of 180,000 hours at 20°C independent of power level, and show an activation energy of 0.48 eV. Undesirable fringes in the far field are caused by interference between the beam and waveguide sidewall scattered light. This problem is eliminated by adiabatically widening the waveguide near the output facet.

TRW has developed a prototype driver circuit for GaAs laser diodes as part of the NASA/Goddard Space Flight Center's Direct Detection Laser Transceiver (DDLT) program. The circuit is designed to drive the laser diode over a range of user-selectable data rates from 1.7 to 220 Mbps, Manchester-encoded, while ensuring compatibility with 8-bit and quaternary pulse position modulation (QPPM) formats for simulating deep space communications. The resulting hybrid circuit has demonstrated 10 to 90 percent rise and fall times of less than 300 ps at peak currents exceeding 100 mA.

The advent of space-based coherent diode-laser communication systems requires development of transmitter assemblies capable of operating under harsh conditions after exposure to the launch environment. This paper presents the optomechanical design and expected performance of a transmitter for use in a heterodyne system.

Several optomechanical design trades have been conducted for the NASA Goddard Space Flight Center (GSFC) Direct Detection Laser Transceiver (DDLT) program. DDLT is an experiment designed to demonstrate high data rate direct detection optical communication technology for space applications. Weight, resonance frequency, and the accommodation of a complex thermal design are the primary transmitter design drivers. The receiver hermetic seal is the major optomechanical design issue. In addition to the transmitter/receiver trade studies, a flight qualified third mirror mode stabilized laser diode was developed.

The application of variable conductance heat pipe technology for achieving precise temperature control to + 0.1°C for a space based laser diode transmitter is described. Heat pipe theory of operation and test data are presented along with a discussion of its applicability for NASA's Direct Detection Laser Transceiver (DDLT) program. Our design for the DDLT transmitter features a reduction in space radiator size and up to 42% reduction in prime power requirements.

A low noise avalanche photodiode (APD) receiver has been developed for the NASA Direct Detection Laser Transceiver Program. The hermetically sealed receiver includes the focusing lens, solar reject filter, APD and low noise amplifier. A low-noise silicon APD was integrated with a hybrid low noise amplifier. The radiation-hardened APD/LNA provides superior wideband receiver performance. The receiver sensitivity is -52 dBm (123 photons per bit), at 220 MBPS manchester with a 10-6 BER. This is the most sensitive manchester receiver reported in the technical literature, at 220 MBPS.

The Pyrolaser is a self-contained hand-held device which permits accurate measurement of temperature from a remote position. It incorporates the ability to measure the diffuse reflectivity of the target, and hence to estimate the emissivity for the wavelength at which the radiant energy is detected. The system design of the instrument is presented, with specific details of the opto-mechanical design of the four optical channels which share common optical components. The methods of alignment and testing for series production are also discussed.

The future demand for large multimission spacecraft and space platforms such as space station has warranted research into modular disconnects which incorporate thermal, electrical and data transfer into a single device. The modular devices which could benefit from these mechanisms would be electrical power conditioning units, experimental and communication payloads, orbital maneuvering vehicles and other free fliers. These devices must avoid the use of connector pins since this type of connector cannot provide the necessary reliability for hundreds of connect cycles. Reliability is a key issue in a space based interconnect. At present separate connections must be made for heat sink, electrical power and data linking. Attachment of equipment to large cold plates is difficult and variations in clamping forces will affect the capacity for thermal control. The requirements from NASA were to develop a generic space based interconnect that combines heat, electrical, and data linking in one structure. The contact heat transfer requirement was 500 watts, and the electrical power transfer requirement was also 500 watts at 20 kHz single phase. The data links were to have six channels communicating at a maximum of 100 Mbps, Manchester-encoded, with a bit error rate of less than 10-10. The operational requirement was to achieve 1000 reliable interconnects.

It is proposed that a wide field of view optical system, fitted with multiple fovea of optical receivers and transmitters, could serve multiple ground and near earth clients from geosynch platforms. Payload advantages over a system with separate servers is discussed.A number of optical forms are suitable for this application. Approaches to interfacing the transceivers with the telescope and to moving them to maintain track of each client are suggested.

Hemispherical data coverage for remote sensing satellites with a single ground station is opened up using a lightwave communication link. The paper presents the design of the semiconductor diode laser system for the low-earth-orbiting remote sensing satellite data relay through a geostationary satellite with data rates of 500 Mega bits per second. The laser type selected is the semiconductor GaAlAs diode laser operating at 810-840 nm. The detector type selected is an avalanche photodiode detector with direct detection. The optics design is described for the laser transmitter on the low-earth-orbiting satellite and on the geostationary satellite, along with the corresponding receivers. Consideration is given to the pointing, acquisition and tracking subsystem, as the tracking precision should be of the order of 0.3 urad. From bit-error-rate considerations, a CW laser power of 50 mW with direct detection using avalanche photodiodes is used for transmission, with a low-earth orbiting satellite transmitter optics diameter of 200 mm and a geostationary receiver optics diameter of 350 mm. A laser beacon operation is an integral part of the acquisition procedure to establish intra-orbit links.

A spatial diversity transmitter consisting of multiple diode lasers with individual collimating lenses has been demonstrated to be effective in reducing fading caused by scintillation. The normalized standard deviation of the intensity at the receiver is reduced by the sware root of the number of transmitting elements, when they are separated by at least the correlation distance for intensity fluctuations.

A study has been made to combine the functions of acquisition, tracking, and point-ahead in space optical com-munications into a single system utilizing an area array detector. This paper presents an analysis of the feasibility of the concept. The key parameters are: (1) optical power less than 1 pW at 0.86 μm, (2) acquisition in less than 30 seconds in an acquisition field of view (FONT) of 1 μrad, (3) tracking with 0.5 μrad rms noise at 1000 Hz update rate, and (4) point ahead transfer function precision of 0.25 μrad over a region of 150 μrad. Currently available array detectors have been examined. The most demanding specifications are low out-put noise, a high detection efficiency, a large number of pixels, and frame rates over 1 kHz. A proof-of-concept (POC) demonstration system is currently being built utilizing the Kodak HS-40 detector (a 128x128 photodiode array with a 64 channel CCD readout architecture which can be op-erated at frame rates as high as 40,000/sec). The POC system implements a windowing scheme and special pur-pose digital signal processing electronics for matched filter acquisition and tracking algorithms.

Due to the increasing number of applications for optical communications, methods such as computer simulation are needed for the performance analysis of these systems. The objective of this paper is to propose a system level model for simulating the Earth's atmosphere as an unguided optical communications channel. The major degradations in received optical intensity introduced by the atmosphere are scintillation, beam spreading, beam wander, and atmospheric transmissivity. The model presented here considers scintillation and beam wander to impose random fading in the received signal while beam spreading is a constant loss in intensity. Atmospheric transmissivity is treated as a filter-like channel transfer function. Relationships for the parameters of the model are given in terms of parameters which characterize the optical link. Also included is a description of an implementation of the model.

Laser communication offers many advantages in space- and weight-critical applications. Swept volume and weight of the receiver are reduced through use of laser communications, and communications security is enhanced. A critical parameter in the performance of a laser communications system is receiver gain, given by:1 GR = π2 dR_ λ2 (1) where dR is the receiver aperture and X is the receiver operating wavelength. For maximum receiver gain, the receiver aperture should be as large as possible and the wavelength as short as possible. Wavelength is frequently decided by considerations of propagation, interference, and the availability of lasers of the appropriate characteristics. Increased gain therefore requires more receiver aperture. To maintain the advantages of light weight and minimum swept volume, the laser communications receiver must have the lightest possible receiver for a given aperture.

An extremely sensitive, accurate, and versatile extensometer has been demonstrated for measuring minute thermally induced dimensional changes in large structures fabricated from materials with a near-zero, but difficult to predict, coefficient of thermal expansion (CTE) -such as laminated graphite-epoxy composites. The method employs a commercially available Doppler laser interferometer designed for extremely accurate translation measurements in large machine tools. Operation of the laser device is described together with the special precautions required, and results of thermally cycling a near-zero CTE graphite-epoxy I-beam.

The design of a laser diode objective composed of a Schwarzchild reflecting objective and a two element refracting cylindrical telescope is described. Good wavefront quality is obtained over a large spectral bandwidth (with refocusing). Some amount of oblateness of the output is a consequence of the tilted reflecting elements.

The Optical Test Unit (OTU) for the NASA Direct Detection Laser Tranceiver (DDLT) Experiment was designed to perform a wide variety of electrical and optical tests with minimum cost and risk. Choosing instruments that could do their own processing and carefully examining each step of the integration and test sequence helped reduce the cost and complexity of the system, allowing us to meet a very ambitious schedule. For some tests, less elegant schemes can be used than the traditional system test methods, while still maintaining the integrity of the test system. This paper explores the problems encountered and options that were available in designing the system test equipment for this optical communication system.

A diffractive optical element has been fabricated using VLSI techniques for use in multi-channel infra-red optical systems. This diffractive micro-optic demonstrates the flexibility and resolution available with current lithographic techniques. The optical element has a diffraction efficiency of 47% and a noise floor of -30dB. As lithographic techniques continue to improve, diffractive optics will become available for systems operating throughout the visible spectrum.

We have demonstrated that ultra-lightweighted telescope primaries of nominal surface figure can be used to generate diffraction limited beams for laser communications. Compensation for optical surface misfigure was provided by a phase conjugating mirror to construct a phase conjugated transmitter subsystem. For our demonstration we matched a cw 514 nm beam with a self pumped photorefractive phase conjugating mirror. (Laser diode beams at 820 nm are also well matched to this phase conjugating mirror [1].) Using a primary with an RMS surface misfigure of μ/3, we produced a six inch beam with an RMS OPD of λ/17.

The minimization of laser diode relative intensity noise (RIN) is an important consideration for obtaining the optimum system performance in many optical communications systems. RIN is a catch-all term that defines the laser's noise floor which results in increased system noise figure and limited dynamic range. This paper is a tutorial on RIN, explaining what it is, how it can be measured, and what can be done to alleviate it. The laser structure parameters and dynamic processes which affect RIN and the methodology to reduce the RIN of an existing laser will be addressed. The general characteristics of a low-noise semiconductor laser diode, best exemplified by a multiple quantum well laser are also discussed in detail.

Recent investigations have demonstrated the applicability of dichromated gelatin holograms to a number of optical communication, sensor and solar energy applications. This has prompted interest in using these elements in space based optical systems. In order to establish the suitability of using DCG holograms in space based optical systems an investigation was undertaken to determine the effects of simulated space environments on the performance of these elements. Testing was conducted to determine the effects of UV radiation, charged particle radiation and vacuum effects. Further testing established methods of protecting DCG holograms from humidity prior to launch.