This paper presents key operational elements which impact upon performance characteristics for terrestrial free-space laser communications systems within the 1550 nm window. These elements include; optical transmit and receive functionality of the telescope terminals, electro-optic components as well as atmospheric conditions affecting link performance.

A low earth orbit (LEO) satellite laser communication (lasercom) terminal built under funding by the Ballistic Missile Defense Organization will be flown as part of the Space Technology Research Vehicle 2 (STRV-2) experiment module. The STRV-2 experiment module will be housed on the Air Force Tri-Service Experiments 5 spacecraft, which is scheduled to launch on April 15, 2000. The lasercom flight hardware weighs 31.5 pounds, with a volume less than a cubic foot, and is capable of communication at data rates up to 1 Gbps. The LEO satellite-to-ground lasercom experiment is designed for slant ranges up to 2000 km and elevation angles above 15 degrees over the horizon. This experiment will demonstrate a successful satellite lasercom link that will validate the capability and readiness of lasercom for inter- satellite crosslinks, and data downlinks from LEO earth sensing satellites. Two transportable ground terminals have been built and are presently being characterized. This paper describes the results from open loop pointing tests of an STRV-2 ground terminal to currently orbiting LEO satellites illuminated by the sun, such as the Hubble Space Telescope.

This paper presents a bread-board model of an optical BPSK homodyne system. A new phase synchronization method--the so- called Virtual Pilot-tone method--enables a dramatic reduction of the system completely. Although fully compatible to the transmit signal of a high data rate Costas Loop system, the receiver design is enormously simplified. Neither an optical 90 degree(s) hybrid nor digital high speed electronics are needed. An additional low data rate auxiliary channel is provided by using a Modulated Residual Carrier and envelope detection.The data rate of this auxiliary channel has to be well below that of the main channel, but is sufficient for transmitting e.g. telemetry data. Both methods and their realization are explained. Simulation and practical results are displayed and compared to a standard Costas Loop design.

fSONA has developed an optical wireless line of products that will enable high bandwidth wireless connectivity, much like fiber optic technology has done for wired connectivity. All of the fSONA products use wavelengths around 1.5 micrometers both for eye safety and for maximum commonality with fiber optic technology. Initial products provide either OC-3 (155 Mbps) or Fast Ethernet (125 Mbps). The 2 km unit is fixed mounted and provides a relatively large beamwidth to compensate for building motion. The 4 km unit utilizes a narrow transmit beamwidth with active pointing for motion compensation. Trials of the units with key customers begin in second quarter 2000, with volume production of the 2 km- unit beginning in third quarter and the 4 km unit in the fourth quarter. Product designs for 622 Mbps and 1.25 Gbps should be completed prior to the end of the 2000.

The results from a low cost 622 Mb/s, free-space laser communication link operating at 850 nm for short distance commercial applications is presented. The test results demonstrate the use of a free-space laser communications transceiver for building to building applications such as LAN, WAN and ATM operations, etc. This illustrates the potential for wide-use commercial computer network applications. The transceiver is constructed of commercial off-the-shelf materials for the development of a low-cost laser communications data link. The test system configuration utilizes standard Personal Computers with network cards and signal conversion cards for the copper to optical medical conversion. These tests precede the development of an increased data rate device operating at 2.5 Gb/s.

LDSC is developing the POCIT^TM (Portable Optical Communication Integrated Transceiver) family of products which includes VideoBeam^TM and the latest addition, EtherBeam^TM. Each is a full duplex portable laser communicator: VideoBeam^TM providing near-broadcast- quality analog video and stereo audio, and EtherBeam^TM providing standard Ethernet connectivity. Each POCIT^TM transceiver consists of a 3.5-pound unit with a binocular- type form factor, which can be manually pointed, tripod- mounted or gyro-stabilized. Both units have an operational range of over two miles (clear air) with excellent jam- resistance and low probability of interception characteristics. The transmission wavelength of 1550 nm enables Class 1 eyesafe operation (ANSI, IEC). The POCIT^TM units are ideally suited for numerous military scenarios, surveillance/espionage, industrial precious mineral exploration, and campus video teleconferencing applications. VideoBeam will be available second quarter 2000, followed by EtherBeam in third quarter 2000.

Future applications for the Optical Ground Station (OGS) on Tenerife, Spain, are currently being investigated, e.g. the use as a ground terminal in deep space missions. The impact of atmospheric effects on the sensitivity and reliability of an optical link depends heavily on the wavelength and the kind of transmission, mainly if direct or coherent detection is employed. To simulate and evaluate the link quality in the case of coherent reception (heterodyne or homodyne), measurements of the wavefront and intensity distributions have been carried out at the OGS telescope with a Shack- Hartmann sensor, using bright stars as reference sources. A representative set of normalized measurement samples has been generated and evaluated in respect of basic parameters like scintillations, tip-tilt, and wavefront error. The complex speckle patterns at the focal plane have been compared and a superposition with a local oscillator has been simulated. The outcome of these simulations (i.e. heterodyning efficiency) depending on several parameters is presented and compared with environmental conditions.

The capacity is determined for an optical channel employing Pulse Position Modulation (PPM) and an Avalanche PhotoDiode (APD) detector. This channel is different from the usual optical channel in that the detector output is characterized by a Webb-plus-Gaussian distribution, not a Poison distribution. The capacity is expressed as a function of the PPM order, slot width, laser dead time, average number of incident signal and background photons received, and APD parameters. Based on a system using a laser and detector proposed for X2000 second delivery, numerical results provide upper bounds on the data rate and level of background noise that the channel can support while operating at a given BER. For the particular case studied, the capacity-maximizing PPM order is near 2048 for nighttime reception and 16 for daytime reception. Reed-Solomon codes can handle background levels 2.3 to 7.6 dB below the ultimate level that can be handled by codes operating at the Shannon limit.

Because of the use of radio frequency, free space lasers and wired terrestrial communication systems are often separate, and at times conflicting systems, a proposed unifying system of all three modes of receiving and transmitting will be addressed under the collective title of Tripartite Systems. Because of the vast network of wired terrestrial communication systems in the Western world communications systems of the future will still use wired systems as the backbone of signal transmission for most data. Such a network does have limits, i.e. the need to replace existing copper wire with fiber optics, but with the advent of free space lasers, lasecom systems, that have bit-error rates at acceptable levels for commercial and military applications, along with radio frequency systems, will give wired systems a `complimentary' competition that will improve the overall efficiency of modern communication systems.

JPL is constructing an Optical Communications Telescope Laboratory (OCTL) at its Table Mountain Facility complex in the San Bernadino Mountains of Southern California. The OCTL will house a 1-m class telescope and serve as an R&D ground station supporting future optical communications demonstrations with Earth-orbiting satellites and deep space probes. It will also support research in adaptive optics, optical receiver technologies, and help in developing spacecraft acquisition and tracking strategies from future optical ground stations. The OCTL building was completed in November 1999, and Brashear-LP of Pittsburgh, PA has been selected to build the telescope. First light is expected in July 2001.

Quantum cryptography is an emerging technology in which two parties may simultaneously generate shared, secret cryptographic key material using the transmission of quantum states of light. The security of these transmissions is based on the inviolability of the laws of quantum mechanics and information-theoretically secure post-processing methods. An adversary can neither successfully tap the quantum transmissions, nor evade detection, owing to Heisenberg's uncertainty principle. In this paper we describe the theory of quantum cryptography, and the most recent results from our experimental free-space system with which we have demonstrated for the first time the feasibility of quantum key generation over a point-to-point outdoor atmospheric path in daylight. We achieved a transmission distance of 0.5 km, which was limited only by the length of the test range. Our results provide strong evidence that cryptographic key material could be generated on demand between a ground station and a satellite (or between two satellites), allowing a satellite to be securely re-keyed on orbit. We present a feasibility analysis of surface-to-satellite quantum key generation.

The opto-electronic receiver (detector and pre-amplifier) necessary to meet the demands of high capacity deep space missions is designed for a Mars-Earth optical communication link. The receiver requirements are driven by link performance (data rate, bit-error rate, margin), delivered power, pulse width, background signal, telescope quality, and atmospheric effects. Meeting these requirements becomes more challenging as the mission range and the demand for link capacity increases. In this article, the detector's characteristics (e.g. quantum efficiency, noise, gain, and diameter) are designed to address these various requirements. The receiver sensitivity's dependence on the background noise power and on the APD detector's characteristics is analyzed. The improvement in opto- electronic receiver sensitivity is quantified for improvements in APD quantum efficiency, ionization factor, and bulk dark current. It is also found that as the background signal increases the improvement on the receiver sensitivity from an improved detector is diminished due to the quantum noise limit. An opto-electronic receiver is designed based on a Silicon APD to meet the mission requirement of a PPM (M equals 256) 30 kbps data rate (BER of 10^-5) link. Improvements to the APD detector are also studied to describe a design that would achieve over 50 kbps data rates for a Mars-Earth optical communication link.

Because free lasers offer faster transmittal of data than radio frequency, but are less reliable because of cloud cover or fog, a two stage system can be developed using HALE, high altitude long endurance, platforms as a transitional link between space and terrestrial communication nodes. Because HALE platforms will fly above cloud layers, 35,000 feet, such platforms could use either radio frequency or free space lasers for `air' to `ground' transmissions and free lasers for `space' to `air' phase of transmissions. The use of radio frequency or free laser would depend on the type of environment during operations. Vehicles that are remotely operated, solar powered platforms that function at 100,000 feet indefinitely.

We present a preliminary design for a tracking and pointing subsystem for the optical communication link between the International Space Station and a ground receiver at Table Mountain Facility. The link is intended to demonstrate high rate downlink capability of rates up to 2.5 Gbps. The design objective of tracking and pointing subsystem is to limit the pointing loss to within 4 dB to ensure the maximum downlink capability with 3 dB link margin. We will show the underlying tracking and pointing subsystem design and present analysis that shows the allocated error budget can be met.

Optical Intersatellite links have been investigated for many years, but to date have enjoyed few spaceborne applications. The literature is rich in articles describing system issues such as jitter and pointing effects, but this author believes that simplifications generally made lead to significant errors. Simplifications made, for example, due to the complexity of joint distribution functions are easily overcome with widely available computer tools. Satellite- based data transport systems must offer similar Quality of Service (QoS) parameters as fiber-based transport. The movement to packet-based protocols adds additional constraints not often considered in past papers. BER may no longer be the dominant concern; packet loss, misdelivery, or severely corrupted packets can easily dominate the error budgets. The aggregation of static and dynamic pointing errors on both ends of such a link dramatically reduces the QoS. The approach described in this paper provides the terminal designer the methodology to analytically balance the impacts of these error sources against implementation solutions.

Optical communications is a very complex task between satellites because of the small laser beam divergence and the large distance. Also, the vibrations of the satellites due to satellite internal systems and due to external sources may cause the laser beam to miss the intended satellite. In order to start the communication one of the satellites should start acquisition to compensate for possible large deviation angle between transmitter and receiver telescope lines of sight (LOS). During the communication if in the tracking process the satellite loses the connection the acquisition system should establish LOS quickly. In this paper are presented various acquisition search pattern methods such as raster and spiral, and different search methods such as scan/scan or scan/stare are analyzed. The influence of vibrations on these methods and compensation systems is discussed. Analysis of several components of the system is obtained through simulation.

Highly efficient laser sources are required for deep space optical telecommunication. This paper investigates the efficiency components for pulsed diode pumped solid state laser transmitters and determines the overall wall-plug efficiency applicable to a space-borne system. Thermal control of the pump diodes is critical to achieving optimum photon efficiency. Hence, a thermal model involving either a thermo-electric cooler or loop heat pipes is applied to the efficiency calculations. The electro-optical conversion efficiency for an optimized bulk design is expected to be up to 28% with the overall wall-plug efficiency being in the 12 - 16% range depending on the radiator temperature. A fiber based master-oscillator power-amplifier design is also investigated with passively cooled pumped diodes and promises high efficiency operation. Preliminary results from an evaluation model are also discussed.

The noise characteristics of high-power operated erbium- doped fiber amplifiers are evaluated experimentally and theoretically. Optical high-power post-amplifiers are indispensable for the transmitter in high-bit rate optical inter-orbit communication (OIC) links applications in which repeaters is not applicable. Thus, the effects of optical amplification noise on communication performance must be analyzed to establish reliable optical links. The dominant noise component and the influence on receiver sensitivity are discussed for various kinds of OIC links in our study. In addition, intensity modulation direct detection (IM-DD) scheme utilizing optical amplifiers are evaluated for OIC link applications. Experimental demonstrations of coherent detection are also performed with optical fiber amplifiers. The performance of IM-DD and coherent optical communication system are discussed in case of operation under the heavy amplified spontaneous emission (ASE) noise environment induced by the optical amplifiers. The requirements for optical band-pass filters to reduce ASE noise are clarified for IM-DD scheme and the superior feature of coherent systems are shown in aspects of tolerance for bandwidth of optical filter. High-sensitive receiving operation under 50 photons/bit at bit error rate of 10^-9 is achieved in the system with a 500 mW optical booster-amplifier assuming GEO-GEO links (approximately 40,000 km).

The problems of the investigation of a deformations of the lunar surface and another planets (without an atmosphere) lunarquakes and vibrations of Moon and another planets a consequence of the influence endogenous and exogenous factors (for making discoveries of oscillations; lunar and planetar periodicity in oscillations, detection of gravitational waves, galactical and global periodicity in oscillations, etc.)--are urgent problems. The interferometers are proposed with this aims.

High pointing accuracy in a ground-to-satellite laser beam transmission is required because of a laser beam divergence limitation. This paper presents a laser beam transmission direction adjustment method from a ground station to a satellite. Laser transmitting direction is adjusted using angular relation between the image of back-scattered laser beam and a target object. Intensity distribution of an image of back-scattered laser beam is theoretically calculated. The method was applied to transmit a laser-beam from a ground station to Engineering Test Satellite-VI. The accuracy of pointing the laser-beam was about 10 (mu) radians in this experiment.

We simultaneously transmit 16 separate 2.5 Gb/s wavelength data channels, with a 200 Ghz channel spacing, error-free, over a horizontal free space distance of 4.4 km. We believe this result represents the largest bandwidth transmitted at one time over such a distance, without the use of optical transmission fiber.

NASDA (National Space Development Agency of Japan) have developed (OICETS (Optical Inter-Orbit Communications Engineering Test Satellite) since 1993 in order to get the acquisition, tracking and pointing technology for optical inter-satellite communications through the in-orbit experiment with ARTEMIS (Advanced Relay and TEchnology Mission Satellite) of European Space Agency. The engineering model of the optical terminal called LUCE (Laser Utilizing Communications Equipment) was tested and its optical performance was evaluated during the thermal vacuum test. The on-axis intensity transmitted from the terminal was about 560 MW/Sr with 90 mW of the averaged semiconductor laser output. The full width of the far field pattern of the emitted beam at the point of 1/e² was about 9.4 (mu) radian.