This paper presents the qualification history of the Silex flight hardware. The two flight model terminals, the GEO to be flown on ARTEMIS, the LEO to be flown on SPOT 4, have been or are about to be delivered for integration with their host spacecraft. Both terminals have undergone a stringent environmental test program, including optical performance measurements in vacuum, requiring complex test equipment. The flight hardware qualification was supported by development models that demonstrated the structural and thermal integrity of the design, the opto-mechanical stability of all optical paths within the terminals and the communication performances of the optical and electronic equipment. Due to the agreed SPOT 4 and ARTEMIS launch dates SILEX will not be in orbit before 1998 and 2000 respectively, such that the in orbit demonstration of the overall system can, most likely, not start before mid 2000, requiring the storage of the GEO terminal after the completion of the spacecraft testing. In the mean time, the construction of the optical ground station in Tenerife is well advanced. The building with the 1m telescope was handed over to ESA in Summer 1996, the optical bench will be ready in early 1997 and the whole station will be operational by mid 1997. It will first be used for in orbit debris observation. The preparation of the experiment between ARTEMIS and the Japanese spacecraft OICETS is proceeding well. A detailed link simulation has demonstrated the feasibility of the undertakings, the in orbit test plan is agreed, the ground interfaces are defined. This experiment will allow to control a LEO spacecraft via a data relay satellite through a S-band link with the simultaneous bi- directional optical link allowing data transmission at a rate of 50 Megabit per second in the return direction and 2 Mpbs in the forward direction.

The ESA program dedicated to Optical Communications development has allowed to qualify two terminals in 1994 and 1995. In 1995/96, the respective proto flight models have been integrated and tested, leading to in depth knowledge of the behavior of optical terminals in optomechanical, pointing and telecommunications fields for various environmental conditions. The paper describes the program and the associated major tests results at terminal level or in coupled configuration on SPOTIV spacecraft. Considerations for future application are given in order to identify SILEX feedback on MMS optical terminal product line for commercial market.

The ground-to-orbit Lasercom Demonstration conducted between the ETS-VI spacecraft and the ground station at JPL's Table Mountain Facility, Wrightwood CA was the first ground-to- space two-way optical communications experiment. The demonstration was conducted over a period of seven months and required simultaneous and cooperative operations by team members in Tokyo and California. A key objective was to measure the atmospheric attenuation and seeing during the demonstration to validate the performance of the optical link. The telemetry downlinked from the laser communications equipment provided information on the in-orbit performance of the onboard laser transmitter. Downlinked PN data enabled measurement of bit error rates. BERs as low as 10^-4 were measured on the uplink and 10^-5 on the downlink. Measured signal powers agreed with theoretical predictions.

OICETS is under development by NASDA to verify the optical data link technology. The experiment in orbit will be done establishing optical link between the OICETS and ARTEMIS in cooperation with ESA. This presentation explains current OICETS Laser Terminal development status. The LUCE EM is under assembling phase and the test will be finished in 1997. LUCE uses wto 200m LD's for transmitting. Selection procedure of the flight parts and some of the performance requirements are also presentation.

Laser communication hardware being built under funding from the Ballistic Missile Defense Organization will be flown on the Space Technology Research Vehicle 2 to be launched as part of Air Force mission TSX-5 in 1998. The flight hardware, which weighs 31.5 pounds and is designed for satellite-to-ground laser communications at data rates up to 1.24 Gb/s and ranges up to 2000 km, will be delivered to JPL for integration in March, 1997. It is hoped that a successful satellite demonstration will validate the capability and readiness of lasercom for inter-satellite crosslinks and low Earth Orbit satellite downlinks to the ground. This paper describes the hardware with photographs.

Laser communications between high flying aircraft such as high altitude Unmanned Aerial Vehicles and between the aircraft and the ground 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 US Army Space and Strategic Defense Command (SSDC) 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. In 1996, BMDO and SSDC have supported an effort to demonstrate operation of the lasercom system on the Boeing Airborne Surveillance Testbed aircraft by establishing a long range link to another lasercom terminal on the ground. Link acquisition utilizes interfaces to global positioning systems, an inertial navigation unit for initial pointing and atomic line filter technology for background light rejection. We present the development of the acquisition scenario algorithms and results of the experiment performed on December 4, 1996, with the aircraft circling ThermoTrex in San Diego. This first test had the aircraft at a range of 20 to 30 km, at an altitude of 38,000 feet.

Laser communications hardware being built under funding from the Ballistic Missile Defense Organization (BMDO) will be flown on the Space Technology Research Vehicle 2 to be launched as part of Air Force mission TSX-5 in 1998. The flight hardware, which weighs 31.5 pounds and is designed for satellite-to-ground laser communications at data rates up to 1.24 Gb/s and ranges up to 2000 km, will be delivered to JPL for integration in March 1997. It is hoped that a successful satellite demonstration will validate the capability and readiness of lasercom for inter-satellite crosslinks and low earth orbit satellite downlinks to the ground. This paper describes the operational scenario for satellite-to-ground lasercom experiments. Experiments at other facilities, including those in Europe and Japan, are encouraged.

Analyses of uplink and downlink data from recent free-space optical communications experiments carried out between Table Mountain Facility and the Japanese ETS-VI satellite are presented. Fluctuations in signal power collected by the satellite's laser communication experiment due to atmospheric scintillation and its amelioration using multiple uplink beams are analyzed and compared to experimental data. Downlink data was analyzed to determine the cause of a larger than expected variation in signal strength. In spite of the difficulty in deconvolving atmospheric effects from pointing errors and spacecraft vibration, experimental data clearly indicate significant improvement in signal reception on the uplink with multiple beams, and the need for stable pointing to establish high data rate optical communications.

In this paper we describe recent tests of a unidirectional laser communication link operating in strong atmospheric turbulence conditions, near ground level, over horizontal path distances up to 2.4 km. The source is a 1.3 micrometers Nd:YAG, diode-pumped laser which is fiber-coupled to an electro-optic modulator. The transmitter includes an active tip-tilt system which maintains rough link alignment and compensates for slow pointing drift. The transmitted light is collected by a 7 cm diameter receiver aperture and focused onto a InGaAs p-i-n photodetector. The receiver includes a CCD camera for coarse alignment of the link. The optical carrier is modulated by a 160 Mbps digital signal using quadrature amplitude modulation (QAM) at a subcarrier frequency of 140 MHz or by a 678 Mbps digital on-off keyed (OOK) signal. The link receiver includes an electronic automatic gain control circuit to compensate for atmospheric induced signal fading. The electronic test equipment at the transmitter and the receiver is connected through an RF ethernet link to enable automated measurements of symbol error rate as a function of optical power, modulation format, and energy-per-bit to noise-density ratio. We report on the measurement of bit error rates and demonstrate error- free operation of the communication link using 160 Mbps QAM over 0.9 km and 678 Mbps OOK modulation over 2.4 km under certain atmospheric conditions.

The Optical Communications Demonstrator (OCD) is a laboratory-based lasercom demonstration terminal designed to validate several key technologies, including beacon acquisition, high bandwidth tracking, precision beam pointing, and point-ahead compensation functions. It has been under active development over the past few years. The instrument uses a CCD array detector for both spatial acquisition and high-bandwidth tracking, and a fiber coupled laser transmitter. The array detector tracking concept provides wide field-of-view acquisition and permits effective platform jitter compensation and point-ahead control using only one steering mirror. The use of a fiber- coupled transmitter modularizes the transmitter design and de-couples its thermal management problems from the main system optics. The reduction in design complexity can lead to a reduced system cost and an improved system reliability. This paper describes recent progress on the development of the OCD terminal.

Uplink scintillation is a serious issue for laser communication between a satellite and the ground. Fluctuations on the uplinked beacon and communications laser can be minimized by transmitting multiple independent lasers from separate apertures which then sum incoherently at the satellite. The objective of the experiment described here was to determine the number and spacing required for separate transmitters to reduce fluctuations in the received power due to atmospheric scintillation to acceptable levels. Received power vs. time data were collected for horizontal laser links established between a laser transmitting platform and a receive telescope assembly separated by distances of 1.2 and 10.4 km to mimic the expected atmospheric effects of an uplink slant path to a satellite. Reduction in signal fluctuations was observed as the number of laser transmitters was increased from 1 to 16. A ground terminal design with 16 lasers on an 18 inch diameter circle with a 7 dB fade margin should be adequate to compensate form sot scintillation fades, while the remaining deep fades may be corrected by using forward error correction techniques.

The objective of the Atmospheric VIsibility Monitoring (AVM) program at the JPL is 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 US, that observe the stars throughout the year. Data from the three sites are collected and processed on a regular basis to obtain cumulative distribution functions of atmospheric attenuation at different spectral regions. These data will be used to determine site diversity information. In addition, work is underway to build a database of atmospheric transmission data collected by the AVM program and to upgrade the AVM systems to obtain improved data at the important 1.06 micrometers wavelength.

Large commercial satellite programs requiring ISLs are growing in number and maturing. An important segment of the commercial satellite market, and its ISL needs, is discussed in the paper. ISL value will increase as long-haul terrestrial backbones become increasingly congested. Providing interregional and intercontinental connectivity via ISL presents far lower cost and fewer problems than relying on terrestrial fiber-optic networks. To demonstrate this, a new metric is proposed which allows 'apples-to- apples' cost/performance comparisons between laser communications in GEO, LEO, and terrestrial fiber-optics. ISL requirements in to the next decade are predicted >= 50-100 Gb/s full duplex. Many attitudinal changes must be embraced among those who choose to focus on this new commercial business. Foremost among these is a preponderance to delivering fully acceptable hardware fast and at low cost, as opposed to merely designing such. Considerable attention must be given business considerations foreign to professionals who have spent time in the government or government contracting sectors. Successful ISL customers will come to recognize that ISLs are not commodity products. Failure to embrace these attitudes will nonetheless constitute decision to which the commercial market, and particularly the financial market, will appropriately respond.

Increasing demands for handling large amounts of information necessitates a new and expanding array of services within the private and government sectors. Terrestrial communications needs will be met by high bandwidth fiber optic communications. The bridging of mobile and geographically diverse or separated land networks can most economically be met by satellites. Increasing the capability of communication satellites while at the same time meeting the demands for smaller and less expensive launches presents new challenges. Laser communications offers new options for future satellite communication system suppliers. This paper presents an overview of the developing National Information Infrastructure and the Global Information Infrastructure, the present and projected trends in communications, and examines the impact of laser communications as an enabling technology for future satellite systems.

We propose a satellite laser communication system between a ground station and a geostationary satellite, named high- speed optical feeder link system. It is based on the application of (a) high-speed optical devices, which have been developed for ground-based high-speed fiber-optic communications, and (b) the adaptive optics which compensates wavefront distortions due to atmospheric turbulences using a real time feedback control. A link budget study shows that a system with 10-Gbps bit-rate are available assuming the state-of-the-art device performance of the Er-doped fiber amplifier. We further discuss preliminary measurement results of the atmospheric turbulence at the telescope site in Tokyo, and present current study on the design of the key components for the feeder-link laser transceiver.

Full-up pre-launch characterization of a lasercom terminal's communications and acquisition/tracking subsystems can provide quantitative characterization of the terminal and better realize the benefits of any demonstration. The lasercom test and evaluation station (LTES) being developed at NASA/JPL is a high quality optical system that will measure the key characteristics of lasercom terminals that operate over the visible and near-IR spectral region. The LTES's large receiving aperture will accommodate terminals up to 20 cm. in diameter. The unit has six optical channels and it measures far-field beam pattern, divergence, data rates up to 1.4 Gbps and bit-error rates as low as 10^-9. It also measures the output power of the laser-terminal's beacon and communications channels. The 1 kHz frame rate camera in LTES's acquisition channel measures the point-ahead angle of the laser communications terminal to a resolution of 1 (mu) rad. When combined with the data channel detection, the acquisition channel measures acquisition and reacquisition times with a 1 ms resolution.

This paper presents an overview of a cooperative research and development program between Ball Aerospace and Technologies Corp. and the Air Force Phillips Laboratory for laser communications. This effort employs hardware and equipment originally developed to support the crosslinking between geostationary Defense Support Program surveillance satellites. This joint activity modifies the existing hardware for ground based demonstrations and simulations focuses at risk reduction for future applications and technology insertion into operational architectures meeting future commercial, civil, and DOD communications requirements. The ultimate goal of the program is to produce hardware for a near term flight demonstration. A brief overview of the capabilities of the existing hardware will be presented followed by a status of the development efforts and future plans.

We would like to report our research and development on InGaAs-QD as a part of our research on optical inter-orbit communication technology. Optical transmitter subsystems using Erbium doped fiber amplifier (EDFA, wavelength 1550nm) and Neodymium doped fiber amplifier (NDFA, wavelength 1060nm) have been candidate subsystems of future optical-IOC system. However, present fine pointing sensors, such as Si- QD and Si-CCD, have little sensitivity at 1060nm, and no sensitivity at wavelengths longer than 1100nm. There is a need for a fine pointing sensor that has a high sensitivity at 1060nm and 1550 nm. Thus, we tried to manufacture InGaAs- pin-QD, adding optical surface separation processes into conventional production processes of commercially available InGaAs-pin-PD. InGaAs-QD with high sensitivity and low dark current was produced. Our analysis shows that these devices are suitable as a fine pointing sensor of optical-IOC systems using EDFA or NDFA as a transmitter subsystem.

Disturbances to the line-of-sight induced by satellite microvibrations pose a major design challenge for optical terminals. The conventional approach is to hard-mount the terminal to the satellite so that the fine pointing control system has to cope with disturbance spectra extending above a hundred Hz. The need for a wideband tracking sensor and fine pointing mechanism and fast control electronics results in a substantial mass, power and cost overhead. An alternative approach which offers considerable advantages has been pursued by MMS. This is to attenuate the high frequency components of the microvibrations by introducing an anti-vibration interface between the terminal and the satellite. A set of elastomeric elements with associated launch locking devices performs this filtering function. MMS has employed this technology in a number of small optical terminal designs including the SOUT and the SOTT intended for LEO-GEO and GEO-GEO intersatellite links respectively. This paper provides an overview of the softmount approach including the benefits, hardware implementations and design constraints on the terminal. The SOUT and SOTT terminals are used to illustrate implementations for both fixed and gimballed terminals and pertinent measurements on the SOUT breadboard model are reported. Finally, a program to verify softmount performance experimentally for a terminal configuration dedicated to future earth observation missions is described. This successful program has led MMS to baseline this type of interface for future missions when the terminals are mounted on highly perturbed satellites.

We proposed the use of model reference control for the improvement in the performance characteristics of fine tracking mirrors. We present both analog and digital implementations of model reference control on a specially designed high bandwidth steering mirror and a commercially available lower bandwidth mirror. Improvement in system response and immunity to environmental jitter are demonstrated.

The extended target that the size is larger that the diameter of the light beam can be tracked in the laser tracking radar designed on the basic of the method and the algorithm of the staring edge tracking. A moving aluminium plate is tracked by a coherent CO₂ laser tracking radar facility with transmitter power 5W and the divergent angle of the transmitter light beam less than 0.18 mrad at a n approximate range of 1 km. The error signals of the azimuth and the elevation are generated from the quad detector. This successful experiment results indicates that the question that the traditional tracking method of quad detector is vain to the extended target has been overcome and testifies that our theory of the staring edge tracking is correct. This tracking method has many advantages. For example, is we choose the tracking position at the top of the target, then the ground target is not easy to lose. So it can avoid tracking the ground. On the other hand, the range of the laser radar using this method is longer than the range of the radar using the point tracking, because the target using narrow light beam illumination is taken as an extended Lambertian target.

In this part-II of the advanced communications benefits study, tow critical metrics for comparing the benefits of utilizing X-band, Ka-band and Optical frequencies for supporting generic classes of Martian exploration missions have been evaluated. The first of these is the overall equivalent communications system mass on the spacecraft. The second comparison metric is the overall cost impact. This 'overall' cost assessment has considered the costs for both the spacecraft end of the link and the ground end. In both cases the metrics indicate that higher frequency communication bands have favorable mass and cost, particularly at higher data volumes transmitted daily to the earth. The same metrics are also applied to telecommunication for a hypothetical Neptune mission, extrapolating from the designs for the Mars case.

The present literature on the link engineering of optical repeater systems is dominated by the methods and techniques required for link design of optical fiber systems. Such systems are characterized by short transmission spans and optical amplifiers with low output power. The worked examples in the literature usually have equal transmission losses and gains on each section. The link engineering of free space systems with irregular ranges, differing transmission losses and gains, and high power optical amplifiers presents a unique problem in determining the way to allocate system performance in a way which allows for the application of excess engineering link margin to be applied to the system in the pre-hardware development phase. A method has been developed which allows for such allocation and is presented in this paper.

Since Nd-doped fiber amplifiers could be used as power amplifiers for high data rate optical inter-orbit communications, we have fabricated and studied a square shaped double clad Nd-doped fiber for the 1.06 micrometers amplifier characteristics. The output signal power is not saturated up to 1 watt level using the single frequency broad linewidth signal source. It is, however, saturated at a few hundred milliwatts by a stimulated Brillouin scattering effect using narrow linewidth non-modulated linewidth signal source. The maximum output power of 1.17 Watts is obtained with extraction efficiency of 37 percent from absorbed pump power. The net gain is estimated more than 25dB.

The transferred electron-intensified photo-diode (TE-IPD) is a photomultiplier-like device that provides superior detection capability in the near IR. It was developed at Intevac ATD, and the fundamentals of its implementation have been discussed. Under NASA Goddard sponsorship, we have been conducting laboratory tests and evaluations on all aspects of this device, as well as characterizing its potential optical communication and laser radar applications. The TE- IPD can be optimized to have as much as 20 percent quantum efficiency at 1060 nm, or to provide a nearly flat response of 10 percent quantum efficiency out to 1600 nm. The internal gain can be of order 1000x or as much as 20,000x, depending on the choice of anode. We have evaluated all aspects of these variants, including spectral quantum efficiency, dark current as a function of temperature, active cathode area, gain, nose factor and spatial uniformity. We will present detailed laboratory test results and discuss device characterization s for specific system applications in terms of the sensitivity and required signal power at the detector cathode to achieve a given quality of service.

It is well known that atmospheric turbulence causes degradations in the performance of atmospheric links. The focus of this study is the effect of scintillation on the performance of optical On-Off keying (OOK) links in horizontal atmospheric paths. A new approach to improving the performance of OOK links in the atmosphere is presented in this paper: optimum adaptive thresholding for anticipated, future scintillation values. We look first at the non-time varying irradiance case. The theory of minimum probability of error receivers for this case is well established. When the link operates in a turbulent atmosphere, however, this theory will not necessarily hold. A new adaptive predictive control (APC) approach is described, and to test its effectiveness, a specific link is assumed and the expected BER performance with and without APC was determined by computer simulation. We see that all measures of the BER are better for the APC receiver than the fixed threshold receiver. APC is shown to illustrate a simple way to improve the effectiveness of atmospheric links. Other areas of laser communications that APC might potentially benefit are all processes that satisfy the two criteria of slow time variation and sample-to-sample correlation.

The first ground-to-satellite laser communication experiments were performed during December 1994 and July 1996 to demonstrate basic technologies for space laser communication systems. It used an optical communication package onboard the Engineering Test Satellite VI and its companion ground optical terminals. A bi-directional optical communication link over 40000 km was demonstrated with precise transmission control of extremely narrow laser beams at both onboard and ground terminals. In the paper, the experimental operations performed in the demonstration are introduced in context with acquisition method for various technical data useful for evaluating the terminal characteristics.

A far-field pattern of an onboard laser transmitter was measured using a trans-atmospheric optical link over 35,000 km range between a satellite and an optical ground station. The acquisition of the far-field pattern is made in use of a new proposed statistical analysis of downlink irradiance data obtained at the ground station. Tracking/pointing statistical characteristics of the transmitter is taken into account in order to estimate downlink irradiance when there would be no atmospheric scintillation effects. The peak directive gain of the downlink laser beam was 104.3 dB. The beam width was 28.5 X 17.5 (mu) rad. These results were consistent with the results at a laboratory test performed before launch of the satellite.

Laser beam scintillation of a ground station to a satellite path are presented. After the up-link laser beam propagated to the satellite, the laser beam intensity distribution was varied by atmospheric turbulence. The variance of the up- link logarithmic-amplitude intensity fluctuation was about 0.5 and the maximum duration for the satellite receiver to be able to receive an optical power more than the communication detector sensitivity was about 50 mili-seconds in weak atmospheric turbulence condition. The maximum down- link time duration was about 0.3 seconds. The main cause of the down-link fluctuation can be considered as a fine pointing mirror angle variation in case of an up-link received power fading.

On September 15, 1996, researchers from Utah State University/Space Dynamics Lab in conjunction with Phillips Lab/Starfire Optical Range and Kjome Research successfully flew and tested a retromodulator laser communication package on a high altitude balloon. This paper addresses the layout and hardware used for the communication link, as well as presenting some preliminary data collected during the 6 hour flight of the balloon. The package was a proof of concept demonstration system for a low-power laser communications systems for small, low Earth orbiting satellites. The ferroelectric liquid crystal based retromodulator design of Utah State provided test patterns for modulation rates up to 20 kilo bits per second. Data was successfully downlinked using a 1200 bps RS232 format and a simplistic receiver. The Starfire Optical Range 1.5-meter telescope located on Kirtland AFB, tracked the balloon, which reached a float altitude of 31 km and collected the modulated light reflected from the payload.