The STRV-2 lasercom terminal (LCT) was designed and developed by AstroTerra Corporation of San Diego, California, under funding from the Ballistic Missile Defense Organization (BMDO). Scheduled for launch in late 1998 it will be used to demonstrate, for the first time, high data rate bi-directional satellite-to-ground optical communications. Concurrently with the development of the STRV-2 lasercom NASA/JPL was assembling the lasercom test and evaluation station (LTES), a high quality test platform for pre-flight characterization of optical communications terminals. The respective development schedules allowed evaluation of the STRV-2 LCT using LTES, for a month, prior to integration of the LCT with the spacecraft palette. Final co-alignment of the transmitter lasers to within plus or minus 20 (mu) rads with respect to the receive axis was achieved. This in turn allowed the specified 76 (mu) rad transmit beam divergence to be realized. However, subjecting the LCT to expected on-orbit temperatures revealed that the co-alignment deteriorated causing beam spreading, a finding which prompted the recommendation to operate the lasers warmed up during ground encounters. The 'bent-pipe' operation bit-error rates (BER) at 155, 194 and 325 Mbps were less than or equal to 1E - 10 over an approximately 20 dB range of irradiance measured at the receive telescope aperture. At 500 Mbps BER's of 1E-6 were achieved over an approximately 6 dB irradiance range, suggesting greater vulnerability to atmosphere induced fades. A pointing offset between the acquisition receivers and transmitter lasers of 1 mrad was measured. The impact of this offset will be to limit acquisition camera framing rates to 87 and 251 Hz, thus limiting the tracking loop bandwidth. Tracking performance test of the lasercom terminal, though planned could not be carried out because the software was not ready at the time of testing with LTES. The test results obtained for STRV-2 lasercom terminal will be used for designing the ground receiver.

High data rate communications between satellites request to develop very specific electronic circuits. Very high speed, high current (more than 500 mA peak to peak) and low power consumption laser driver was realized and integrated into a specific low volume and low mass hybrid design. This paper reports also the realization of a complete receiver based on the integration of an Avalanche Photodiode (APD) into a very low noise preamplifier followed by an other hybrid module including a limiter amplifier, a clock recovery and decision circuit. The high voltage DC/DC convertor for the APD is also presented as a third hybrid circuit. Full bit rate is in the range of 622 Mbps. For the receiver, the sensitivity obtained is very closed to the theoretical possibilities. The opto- electronic modules under realization (or realized) are fiber pigtailed, with a single mode fiber on the transmitter side and a multimode fiber on the receiver side which allow their integration into the communication boxes. The present paper gives an overview of the modules development, including the main results, and situates these activities in a more complete realization of optical communication boxes.

Pre-launch integrated system characterization of a lasercom terminal's (LCT's) communications and acquisition/tracking subsystems can provide a quantitative evaluation of the terminal and afford a better rigorous assessment of the benefits of any demonstration. The lasercom test and evaluation station developed at NASA/JPL is a high quality optical system that possesses the unique capabilities required to provide laboratory measurements of the key characteristics of lasercom terminals operating over the visible and near- infrared spectral region. Over the past year LTES has been used to provide pre-flight testing of the STRV-2 lasercom terminal developed by AstroTerra Corporation of San Diego, CA, and is currently being used for testing of the Optical Communication Demonstrator (OCD) developed by NASA/JPL. Discussions of performance validation tests carried out on LTES and its diverse capabilities are reported in this paper.

The Optical Communications Demonstrator (OCD), under development at the Jet Propulsion Laboratory (JPL), is a laboratory-based lasercomm terminal designed to validate several key technologies, primarily precision beam pointing, high bandwidth tracking, and beacon acquisition. The novelty of the instrument is that it uses a single CCD array detector for both beacon acquisition and tracking, and a fiber-coupled laser transmitter. The resulting reduction in design complexity can lead to a reduced system cost and an improved system reliability. In this paper, we describe recent progress on the development of the OCD terminal, particularly the electronics packaging and optical characterization with the Lasercom Test and Evaluation Station (LTES).

Optical inter-orbit communications (Optical IOC) technology is necessary for the space activities of the next century, because of the wide bandwidth of the optical region, and the light weight and small size of Optical IOC equipment. Moreover, transmitter subsystems using optical fiber amplifiers are strong candidates for Optical IOC's high power transmitter. In this paper, R&D activities on optical fiber amplifiers in NASDA are described. Several high power optical fiber amplifiers which have been developed very recently are also discussed. At the end of this paper, we consider a trade off results, for example 'Which optical fiber amplifiers are best for Optical IOC?'

High power single-mode AlGaAs semiconductor lasers operating between 820 nm and 860 nm (SDL-5400 series diodes) have been successfully qualified for deployment in many free-space inter-satellite communication link programs. Traditionally these high power devices did not have sufficient bandwidth for direct high speed modulation because of large device and package parasitics. We have improved the device parasitics of the SDL-5430 laser diode, i.e. reduced the RC product, from 240 ps to about 40 ps. The initial measurements indicate that this device (SDL-5480) is suitable for high power optical inter-satellite link (OISL) applications at data rates greater than 1 Gbit/s. The preliminary life test indicates that the new device has better a reliability than the previous design.

An adaptive receive telescope array with 16 apertures has been designed and breadboarded. With respect to size and performance, such a telescope array is well suited for use as receive antenna in a coherent interorbit laser link. The laboratory demonstrator, designed to operate at a wavelength of (lambda) equals 1.064 micrometers, is completely independent of any subsequent receiver and of the data modulation format employed. The telescope array is self-phasing, i.e. the main lobe of the antenna pattern automatically follows the direction of the incident wave. It thus performs non- mechanical fine tracking. Our experimental setup comprises a subtelescope array and a digital control unit employing digital signal processors. Besides inertia-free tracking, the control unit also checks and, if necessary, restores parallel alignment of the subtelescope axes at regular intervals. Space-worthy concepts have been applied wherever possible, although experiments have been performed only in the laboratory. Automatic fine-tracking is achieved within a single subtelescope's field of view (30 (mu) rad) in the frequency range up to 730 Hz.

We have developed two kinds of small size and light weight mechanical devices which consist of 2-axis piezoelectronic driving actuators and elastic hinges for fine pointing mechanism (FPM) and point ahead mechanism (PAM) optical inter- orbit communication (optical IOC) equipment. These are 'Beam Trapping Mechanism (BTM)' and 'Beam Scanning Mechanism (BSM)' respectively. High sensitivity receive system using single- mode optical fiber should be needed to increase communication data rate, which are a receiver of heterodyne detection in optical fiber and a receiver using optical pre-amplifier (for example, Er doped fiber amplifier). Therefore, both improvement of precision of FPM and control of the end of optical fiber are needed. In order to realize these requirements, we have designed, developed and tested two kinds of mechanical devices. At first, results of BTM, which consists of an optical fiber, four piezoelectric actuators, elastic hinges and a housing which mounts them, are described. The end facet of optical fiber supported by elastic hinges is controlled by piezoelectronic actuators in order to trap spatial received beam into optical fiber with high efficiency and high speed. The functional test results of BTM show a moving range of 140 (mu) rad, a resolution of less than 0.2 (mu) rad and natural frequency of 390 Hz. These values show BTM has enough performance for low-loss received beam trapping into optical fiber core. Furthermore, small-sized and lightweight BTM was realized using piezoelectronic actuators. We designed a BSM which has function of piezoelectronic driving 2-axis FPM in order to improve FPM. BSM have mirror of which size is 15 * 12 mm². The mechanical concept of BSM is as same as BTM. In BSM, mirror is supported by elastic hinges, replace with optical fiber. Function test results show scanning angle range of more than 2.8 mrad at azimuth direction, 2.0 mrad at elevation direction, scanning resolution of 1.0 (mu) rad and natural frequency is 1.1 kHz. Furthermore, BSM is smaller and lighter compared with electro- magnetic devices.

The Free-space Optical Communication Analysis Software (FOCAS) was developed at the Jet Propulsion Laboratory (JPL) to provide mission planners, systems engineers and communications engineers with an easy to use tool to analyze direct-detection optical communication links. The FOCAS program, implemented in Microsoft Excel, gives it all the power and flexibility built into the spreadsheet. An easy-to-use interface, developed using Visual Basic for Applications (VBA), to the spreadsheet allows easy input of data and parameters. A host of pre- defined components allow an analyst to configure a link without having to know the details of the components. FOCAS replaces the over-a-decade-old FORTRAN program called OPTI widely used previously at JPL. This paper describes the features and capabilities of the Excel-spreadsheet-based FOCAS program.

Large commercial satellite programs needing high bandwidth inter-satellite links (ISLs) are growing rapidly in number. Precious few are visibly maturing. These commercial needs present greater customer diversity and opportunity for free- space laser communications application than the current plans of all the world's governments combined, multiplied manyfold. However, commercial customers generally do not have access to the independent, knowledgeable, but often heterogeneous laser communications expertise upon which government programs have historically relied. Moreover, commercial needs differ substantially from those of governments, particularly in the areas of price sensitivity and assured delivery on schedule and meeting all requirements. And the number of would-be laser ISL terminal suppliers also grows despite little verifiable expertise in actually delivering complete, working space-based laser ISL terminals, regardless of price or performance. Consequently, the opportunity for mistakes, disappointments, and outright failure is intensified. More 'red meatballs' are unfortunately on the horizon and neither customers nor suppliers recognize the warning signs. Is ignorance bliss? Virtually the entire space communications community appears oblivious to emerging terrestrial broadband communications projects which appear better backed with superior management far more attentive to time-to-market and other schedule and business considerations than any space venture. Space systems offer advantages through realizing global network operations not possible terrestrially, yet few promoters recogni the potential. Might these be omens worth capitalizing upon, or perhaps from which escape may be warranted? This paper provides a commercial market status update to that presented in preceding years' papers. Laser ISL applications are reviewed which enable commercial broadband satellite customer opportunities not yet recognized among most in the customer community, despite considerable public and private exposure. Industry participant soon will collectively and individually have tough decisions before them. Failure to address these issues will be decisions nonetheless. This could very well be the last hurrah for many, companies and individual laser ISL professionals alike.

Ideal detection techniques for laser communication are briefly reviewed. Laser communication systems using homodyne Manchester BPSK (MBPSK) and optically preamplified (OP) DPSK are compared on the basis of receiver sensitivity, temporal, spatial, and hardware requirements. It is found that a reduction of optical parts count at the receiver and a reduction in prime power also accompany the 3 dB benefit of homodyne. This occurs, however, at the expense of a more complex, although likely off-the-shelf, local oscillator laser and a doubling of the required signal bandwidth.

There are some effects of atmospheric turbulence on the laser radar, which include the transmitter beam spread, the target plane scintillation and the target return beam spread. When the turbulent layer is close to the target, there will not be essentially transmitter and the target return beam spread. However, there is always the turbulence-induced target scintillation which will produces a notable impact on laser radar image. This paper mainly studies the turbulence-induced target scintillation. A model of atmospheric propagation has been set up with Huggens-Fresnel principle. Meanwhile we have set up a computer simulating system with the principle of the sample statistics. The results shown that the clarity of the target was affected seriously by atmospheric turbulence. Especially when the scintillation is saturated or nearly saturated, the target can not be seen. To improve the image quality, we used the measurements of the more-frame overage and the range gating. For glint target and speckle target, this two methods were effective.

The Optical Communications Group at the Jet Propulsion Laboratory will deploy an optical communications transmitter demonstration and facility to the International Space Station in January 2002. The purpose of the demonstration is to establish a Gbps-class downlink from the International Space Station to ground and measure the flight terminal's performance. A large number of downlink passes are required to accurately measure the flight terminal's performance and the International Space Station provides such a long-term demonstration opportunity. The International Space Station simultaneously provides a payload-friendly environment. In this paper we discuss the objectives of the optical communications demonstration as well as the schedule for and issues related to the design, construction, and deployment of the flight terminal.

Constellations of satellites will perform a key role in the provision of multi-media services both on a continental and global scale. Such constellations are likely to involve satellites in GEO, LEO and MEO orbits -- either in a single orbital system such as GEO or LEO, or in combinations e.g. MEO and GEO. In some of these systems ISLs are fundamental to their operation while others derive significant additional benefit from the inclusion of ISLs. MMS is developing terminal designs to meet these various anticipated requirements under both MMS and ESA funded programmes. This paper outlines some of the European multimedia initiatives that MMS is involved in and discusses the technology choices for the ISLs. In particular, the terminal requirements for both long and short range GEO to GEO links are presented. Two critical technologies which MMS is developing for these applications are described -- namely, silicon carbide telescopes and acquisition and rear-tracking (ARTS) sensors. These are important building blocks for robust and cost-effective optical terminals for multimedia applications.

With the impetus towards high data rate communications in inter-satellite and space-to-ground links, the small size, low-mass, and low-power consumption of optical communications is seen as a viable alternative to radio frequency links. Recent NASA/JPL optical communications field demonstrations have shown some of the operational strategies needed for space-to-ground optical links. In preparation for the optical communications demonstrations planned for the turn of the century, NASA/JPL is building an Optical Communications Telescope Laboratory (OCTL) with a 1-m class telescope. The OCTL will be located at JPL's Table Mountain Facility complex in the San Bernadino mountains of Southern California and will be capable of supporting demonstrations with satellites from LEO to deep space ranges. In addition, it will support advanced optical communications research, astrometry and astronomy research.

A laser communications terminal built under funding from the Ballistic Missile Defense Organization (BMDO) will be flown as part of the Space Technology Research Vehicle 2 (STRV-2) experiment to be launched as part of Air Force mission TSX-5 in late 1998. The flight hardware, which weighs 31.5 pounds and is capable of communications data rates above 1 Gb/s, was delivered to JPL for integration in July 1997. The planned satellite-to-ground lasercom experiment is designed for slant ranges up to 2000 km and elevation angles above the horizon down to 15 degrees. The ground terminal, described in this paper, is being built now and will be used to measure various link parameters such as burst error rates due to scintillation in addition to demonstrating high speed communications.

ACLAIM is a multi-function instrument consisting of a laser communication terminal and an imaging camera that share a common telescope. A single APS-(Active Pixel Sensor) based focal-plane-array is used to perform both the acquisition and tracking (for laser communication) and science imaging functions. ACLAIM was designed based on all commercially available catalog items. The instrument opto-mechanical design and performance is described here.

A new program called the Advanced Deep-Space Systems Development Program (a.k.a. X2000 Program) has been initiated to develop new cutting-edge technologies for NASA's deep-space missions in an overall flight project environment. Several technologies have been selected for development under the first delivery of the X2000 Program. Optical communications is one of those technologies. This paper provides an overview of the X2000 Program and describes the optical communications terminal that will be developed therein. The spacecraft terminal will be a multi-function instrument, capable of not only two-way communications from distances out to Europa (Jupiter), but two-way ranging, science imaging and laser altimeter reception as well. The plan for the development of the terminal is also described.

The Atmospheric Visibility Monitoring (AVM) program at the Jet Propulsion Laboratory has been in place for the last few years to obtain atmospheric transmission statistics data to support free-space optical communications experiments and missions. Atmospheric transmission data is collected through a set of three autonomous systems, all located in the south-western U.S., that observe selected stars throughout the year. Data from these three sites are collected and processed on a regular basis to obtain cumulative distribution functions of atmospheric attenuation for different spectral windows of interest. In This paper, we describe recent work on creation of a database using Microsoft Access to analyze atmospheric transmission data collected by the AVM project. The database, which replaces time-consuming Matlab programs, offers a rapid and extremely easy way to extract and analyze AVM data.

The effects of scintillation on the performance of free-space laser communication (lasercom) systems can be significant. Atmospheric scintillation can introduce burst errors which can be the major contributor to the laser link's overall bit error rate (BER). As part of the limited-visibility lasercom experiment, independent measurements of scintillation were taken to determine how much of the overall lasercom system's performance degradation was due to scintillation as opposed to atmospheric attenuation. The scintillation measurements were also correlated to the suite of meteorological measurements (total attenuation coefficient, total backscatter, temperature, relative humidity, and wind speed and direction) recorded concurrently. Scintillation variation over the time of day was measured. Indications of a reduction of scintillation in conditions of poorer visibility (rain) were observed. All of these measurements will assist in providing more accurate predictions of lasercom performance where these routinely measured meteorological variables are available.

Atmospheric turbulence severely degrades the performance of uplinks. Employment of adaptive optics to enhance ground-space laser communication systems has recently been considered and possible benefits have been shown. Uplink scintillation reduction by using multiple transmitters is also being considered. What appears to be currently missing in these works is the realization that transmitter beam-size is a crucial design parameter and its optimum value changes continuously according to changing turbulence conditions along the propagation path. In this paper we consider a configuration where uplink transmitter beam-size is controlled in real-time in response to measured turbulence parameters to maximize mean intensity and minimize fluctuations on the satellite receiver. Controlling the beam-size, especially by a factor of two or so, in relatively long time scales (seconds) should not be a problem. Although it is obvious that this will greatly improve the performance of especially a variable data rate communication system, we find that the current state of knowledge is fragmentary and insufficient to quantify the improvement in any general and sufficiently accurate way. Semi-analytic simulations can be useful in exploring the improvement for different scenarios and site conditions

In this paper, analytic expressions for the temporal broadening of narrowband space-time Gaussian pulses propagating in weak optical turbulence are derived for both near and far fields. General results are presented for nominal parameter values characterizing laser communication through the atmosphere. Specific examples are calculated for both upper atmosphere UAV-UAV cross-links and uplink/downlink satellite communication paths. It is shown that for both the near and far fields, pulses on the order of 10 - 20 femtoseconds can broaden by more than 100% whereas pulses greater than 500 femtoseconds have negligible broadening.

Recently, there has been strong interest in the application of commercial-off-the-shelf (COTS) electronic and optoelectronic (O/E) components for free space laser communication application. Besides the space qualified packaging issues, the main problems of using COTS O/E transmitter are: (1) Telecom grade laser transmitters do not have sufficient power to meet the free space laser communication requirements; (2) COTS laser diode transmitter driver circuits have limited peak drive current, usually below 100 mA, which is too low for driving high power laser diodes; and (3) COTS high power laser diodes are usually not used for high data rate applications since the high speed performance of the laser/driver combination is usually inadequate. In this paper we will present our latest study results on the SDL 5430 and SDL 580 high power laser diodes driven by high current laser drivers at data rates from 600 Mb/s to over 1 Gb/s. Several models from the HY6000 family of high current and high speed laser diode drivers from Hytek Microsystems Inc., designed for free space laser communication applications, have been tested with the SDL high power laser diodes. Using direct drive technique with NRZ modulation, average output power over 100 mW at 622 Mb/s were obtained with these low cost Hytek drivers. For data rates over 1 Gb/s, the parasitic associated with the laser diode becomes an important limitation factor. We have measured the capacitance of the SDL 5430 and the new junction up SDL 580 laser diodes, an equivalent circuit model is developed to examine the effect of these parasitics on the speed of the laser diode. The results are consistent with our experimental observations.

An aircraft mounted laser communications system is being developed for an air to air communication experiment to be performed over a range of 50 to 500 kilometers at altitudes up to 40,000 feet. The purpose of the experiment is to demonstrate and characterize communication between two aircraft at various ranges, altitudes and atmospheric conditions. Communication data rates are up to 1 GBPS using four on-off keyed 810 nm diode lasers multiplexed into two circularly polarized transmitting apertures. The communication receiver uses a 134 mm diameter Matsutov telescope with APD's for the detectors. Two separate beacon lasers at 852 nm provide sources for tracking. Two T-39 aircraft each will be equipped with a laser communication system and experimental data collection equipment.