I wasn't sure how to start this talk. I could start by saying I gave a paper on Space Laser Communications in Sept. 1962 at a space conference over a quarter century ago, or I could start by noting that this conference on lasers and laser systems is far larger than the whole conference at which my paper in 1962 was the only one on laser communications. I was sandwiched in my session between a microwave link and a UHF system. Or I could start by saying here it is 25 years later, and there still is no operational laser comm system up there working away. Or I could point out there are more people working on laser communications today than understood what a laser was in 1962.
The Laser Communications Airborne Testbed (LCAT) offers an excellent opportunity for testing of an air-to-satellite laser communications link with the NASA Advanced Communications Technology Satellite (ACTS). The direct detection laser portion of the ACTS is suitable for examining the feasibility of an airborne terminal. Development of an airborne laser communications terminal is not currently part of the ACTS program; however, an air-to-satellite link is of interest. The Air Force performs airborne laser communications experiments to examine the potential usefulness of this technology to future aircraft. Lasers could be used, for example, by future airborne command posts and reconnaissance aircraft to communicate via satellite over long distances and transmit large quantities of data in the fastest way possible from one aircraft to another or to ground sites. Lasers are potentially secure, jam resistant and hard to detect and in this regard increase the survivability of the users. Under a contract awarded by Aeronautical Systems Division's Avionics Laboratory, a C-135E testbed aircraft belonging to ASD's 4950th Test Wing will be modified to create a Laser Communications Airborne Testbed. The contract is for development and fabrication of laser testbed equipment and support of the aircraft modification effort by the Test Wing. The plane to be modified is already in use as a testbed for other satellite communications projects and the LCAT effort will expand those capabilities. This analysis examines the characteristics of an LCAT to ACTS direct detection communications link. The link analysis provides a measure of the feasibility of developing an airborne laser terminal which will interface directly to the LCAT. Through the existence of the LCAT, the potential for development of an air-to-satellite laser communications terminal for the experimentation with the ACTS system is greatly enhanced.
A series of three experiments proposed for advanced optical deep-space communications is described. These proposed experiments would be carried aboard the Space Station to test and evaluate the capability of optical instruments to conduct data communication and spacecraft navigation for deep-space missions. Techniques for effective data communication, precision spacecraft ranging, and accurate angular measurements will be developed and evaluated in a spaceborne environment. The technology needed to assemble large space structures will be applied to construct a large aperture, segmented mirror, non-diffraction limited photon bucket capable of receiving downlink data from as far as 1000 astronomical units. In addition to testing the basic functions on communication and spacecraft navigation, these experiments will also evaluate the performance of supporting subsystems needed to carry out spacecraft acquisition, tracking, laser modulation, and data detection. Successful completion of these experiments is needed to guarantee the eventual deployment of an orbiting optical receiving station. The latter will be capable of supporting deep-space communications activities for the next century.
Successful design of an atmospheric laser communications terminal requires an understanding of atmospheric turbulence induced scintillation of the optical signal. Laser beam scintillation is small scale interference within the beam cross section due to turbulence induced fluctuations of the refractive index of the atmosphere, causing variations in the spatial power density at the receiver. The variations in the spatial power density at the receiver manifest themselves as fades and surges of the detected optical signal. By understanding the statistics and power spectrum of the fades and surges, communication terminals can be designed to achieve needed levels of performance by employing optimized choices of increased link margin and error coding. As part of the HAVE LACE (Laser Airborne Communications Experiment) program, optical scintillation data was collected and analyzed. The results from this analysis will aid in the future determination of the degree of scintillation, and the effect on communications performance. The HAVE LACE terminals use direct detection of pulsed laser energy; therefore, the random variations in the received signal strength can be used to evaluate the atmospheric turbulence induced amplitude scintillations. The collected data has been reduced and compared with a model for the air-to-air communications channel. The objective of the analysis was to verify the performance of the model against the collected data. The results are presented in a form consistent with a straightforward understanding of the effects on communications performance.
JPL is actively pursuing the use of optical communications with deep-space probes using the extremely energy-efficient pulse-position modulation (PPM) format. We are studying and developing the critical technologies for this, including the use of different modulation schemes for efficiently producing laser pulses over a broad range of repetition rates. Diode-pumped Nd:YAG is envisioned to be the source, and two principal types of modulation are available: Q-switching and cavity dumping. These two techniques are discussed, and numerical calculations of cavity dumping are described.
Certain space communications applications have very high link availability requirements. These real-time missions would benefit from laser crosslinks in space that do not experience solar outages when the receiver must view the transmitter against the background of the solar disc. We have examined the performance penalty for a direct detection avalanche photodiode (APD) receiver with the sun in the FOV. The increased signal strength required by the solar background varies with datarate and other system parameters, but typically ranges from 10 to 15 dB. A narrower optical passband centered on a solar Fraunhofer absorption line has been found to offer an order of magnitude decrease in the required signal strength at the receiver. Several options are available to implement this performance improvement, and we summarize the potential and advantages of candidate techniques.
ESA is developing an optical communication system for data links between two geostationary satellites or between a geostationary satellite and a low earth orbiting satellite within the Payload and Satellite Development and Experimentation (PSDE) program. With these payloads an experimental and preoperational phase is foreseen in order to demonstrate the advantages of optical intersatellite data links over conventional microwave data links. The in-orbit experimental arrangement as well as the test program will be described. The most important difference between optical and microwave intersatellite links is the statistical distribution of bit error probability. For microwave links mainly the thermal noise of the LNA determines the statistic of bit error distribution whilst antenna pointing error influence is negligible whereas for optical intersatellite links the antenna mispointing as well as the noise of the optical receiver front end have great influence on bit error probability distribution. The investigation and evaluation of the influence of satellite attitude motion on antenna pointing error and bit error rate is a major part of the test program which further includes the measurement of relevant parameters like interference between different wavelength channels, change of optical surfaces,... after some time in orbit. The GEO-2 terminal will be installed on board of SAT-2 which is developed within the PSDE programme and the first European LEO spacecraft which will carry an optical data link terminal is the French SPOT 4 earth observation spacecraft. The whole mission scenario including the microwave feeder link between ground and geostationary spacecraft SAT-2 will be considered from the communication systems point of view.
Many of the systems issues associated with the development, integration and on-orbit operation of an optical intersatellite link may not be widely known due to use of a new technology. An evaluation of these systems issues has shown that they may be classified into six general categories. These include: (1) system engineering issues, (2) technology and development issues, (3) system and subsystem space qualification and reliability verifica-tion issues, (4) system integration and test issues, (5) satellite integration and test issues, and (6) system use architectural issues. These issues are discussed and the scope of the difficulties identified. It is concluded that the systems issues must be properly addressed so that optical system development may proceed to the advanced stages of development needed for space applications.
The performance of an optical ISL, and in particular its tracking sub-system, is examined in terms of its ability to carry commercial digital traffic. To obtain possible criteria to judge the performance of the link, an allocation of 25% of the degradation permitted for the satellite portion of the link by CCITT Rec. G.821 and CCIR Rec. 614 is assumed here. It is found: (1) that the link meets all of the derived criteria if the average BER < 2.5E-8 and (2) that an optical ISL should have little difficulty in meeting these requirements.
A major advantage of communication satellites over cable systems is their ability to cover and serve vast areas of the earth, thereby avoiding the extensive terrestrial networks that are required to feed the cable head-ends. An Intersatellite Link (ISL) would further increase both coverage and flexibility in locating the satellites. This paper examines the pros and cons of the ISLs in an operational environment. System advantages, system disadvantages and certain restrictions placed on satellites using ISLs are discussed.
An overview of the critical component technology and subsystems to be implemented in future optical intersatellites links using laser diodes and heterodyne detection is given. It explains typical problems related to heterodyne detection schemes and the interfaces principle between the optical link payloads and conventionnal traffic payloads.
In free-space optical data transmission systems illumination of the receive antenna by background radiation will decrease the signal-to-noise ratio. We derive expressions for that degradation both for direct and for heterodyne/homodyne receivers. Examples are given for the case that the Sun, the Moon, the Earth, and Venus illuminate earth-orbiting receivers operating at wavelengths of 0.85μm, 1.3μm, and 10.6 μm. Direct detection receivers will typically suffer a degradation between 5 and 15dB at λ=0.85p.m and λ=1.3pm when hit by the Sun. Heterodyne/homodyne receivers at 10.6 μm degrade stronger with Sun radiation (typically 4dB) than at the smaller wavelengths (z0.3dB). Moon, Earth, and Venus cause negligible reduction of signal-to-noise ratio.
A model is described for analyzing the burst error probability and the tracking error statistics of two mutally tracking optical transceivers. The calculations are based on the tracking error statistics of a single "non-coupled" transceiver. It is shown that, for mutually tracking transceivers, the tracking error statistics are different to the non-coupled case, leading to an increase of the burst error probability. A testbed configuration is proposed for the experimental evaluation of both the tracking and data channel performance. Preliminary experimental results on the tracking error statistics of a coupled two-transceiver system show good agreement with the calculations.
The status of high-power (50-1000 mW) diode lasers for space communications is reviewed, with emphasis on monolithic devices. The current performance and future potential of single-element devices, few-element (2-3) phase-locked arrays and many-element (>10) phase-locked arrays are presented and discussed. A brief discussion of hybrid devices for achieving watt-range powers is presented as well.
The results from an extensive life test of wide stripe laser diodes operating in the 808 nm wavelength region and at a heatsink temperature of -20°C are reported. Double heterostructure laser diode wafer material, grown by metalorganic chemical vapor deposition (MOCVD) was processed with 60 micron wide oxide stripes and 150 micron long high reflectivity/passivated cavities. Devices were mounted p-side down on BeO heatsinks with indium solder. Stringent screening and burn-in criteria were applied to the device population prior to selection for long term test. The lasers were operated constant current at 1.4 times threshold (approximately 50 mW initial output power). Based on gradual degradation of output power, during the 10,000 hour test, a median lifetime of greater than 30 years is projected.
High-power single mode channeled-substrate-planar AlGaAs diode lasers are being developed for reliable, high-power operation for use as sources in spaceborne optical communications systems. Most work on AlGaAs semiconductor lasers has been focused on devices with an emission wavelengths less than 8400A where both high power and reliable operation have been previously demonstrated. In spaceborne communication systems, the output wavelength is optimized at 8700A to avoid absorption of the emitted light by the atmosphere when communicating with ground-based terminals. The CSP laser structure has been optimized for operation at an emission wavelength of 8700A. Such devices have exhibited output powers in excess of 80 mW cw at an operating temperature of 80 C. Single spatial mode and substantially single longitudinal mode operation has been obtained at output powers greater than 50 mW cw and 100 mW (50% duty-cycle). The phase-front of the high-power devices has been examined and has shown rms aberrations to be≈ λ/50. Lifetesting of these devices at 50mW (50% duty-cycle) has shown reliable operation in excess of 5,000 hrs.
Free space laser intersatellite links (ISL) require terminal pointing, acquisition, and tracking (PAT) subsystems that are capa-ble of high-speed, high-accuracy pointing control for acquisition and tracking to support communications operations. Typical links will require 16- to 30-cm telescope apertures, which translate to communication beam divergences of approximately 13 to 7 μrad, respectively, assuming a GaAlAs laser system operating at 0.85 um. Based on analyses of the effects of beam jitter on bit error rate (BER), communication beams with pointing accuracies of approximately 0.6 and 0.3 μrad (1o), respectively, are required to provide for reliable ISL communication with burst error rates of -10 -6. Ball Aerospace Systems Division (BASD) has designed, developed, and tested a PAT subsystem with these capabilities that uses current technology. This paper describes the tracking control system, the track detectors, and the key subsystem component: a two-axis, reactionless fine-steering mirror (FSM). Also shown are experimental results of the pointing error distribution of the tracking subsystem, both with and without an injected typical satellite disturbance spectrum. Test results agree very well with computer models.
The launch of a laser communication transmitter package into geosynchronous earth orbit onboard the Advanced Communications Technology (ACT) satellite will present an excellent opportunity for the experimental reception of laser communication signals transmitted from a space orbit. The ACTS laser package includes both a heterodyne transmitter (Lincoln Labs design) and a direct detection transmitter (Goddard Space Flight Center design) with both sharing some common optical components. Lewis Research Center's Space Electronics Division is planning to perform a space communication experiment utilizing the GSFC direct detection laser transceiver. The laser receiver will be installed within an aircraft provided with a glass port for the reception of the signal. This paper describes the experiment and the approach to performing such an experiment. Described are the constraints placed upon the LeRC experiment by the performance parameters of the laser transmitter and by the ACTS spacecraft operations. The conceptual design of the receiving terminal is given; also included is the anticipated performance capability of the detector.
Hughes Aircraft Electro-Optical and Data Systems Group designed and built two automatic tracking lasercom terminals during the 1983-84 IRAD program. These terminals were intended to serve as proof-of principle prototype hardware to demonstrate the capability of current technology to support aircraft and ship laser communications applications. The low probability of intercept (LPI) and jam-resistant (JR) properties of laser communication systems offer potential advantages over conventional RF communication technologies for some important missions such as aircraft refueling, SAC airborne command post computer data dump and ship-to-ship communications during EMCON conditions. The terminals were first described at MILCOM '84 . Since that time they have been upgraded to include separate apertures for the transmit, receive, and tracking functions, as well as the ability to handle tRZ data at a 19.2 Kbps data rate. These terminals demonstrate that a CCD video camera, gyro-stabilized gimbal and servo electronics can perform precision tracking in support of aircraft laser communication. We believe Hughes testing has shown that video tracking is a legitimate alternative to a previously described quadrant detector approach [2,3].
The laboratory test configuration of an engineering model of a CO2 laser homodyne communication system is described. The system is based on a full-scale transceiver and a simulated opposite terminal (SOT). The transceiver technology development has been initiated and sup-ported by the European Space Agency and is now going to be completed. The SOT has been designed as a transmitter including front-end optics (telescope) and pointing elements. Fully representative functional and operational tests of the transceiver and the whole system are possible.
In the early 1990s NASA, with collaboration from the French national center for space studies (CNES) will launch the ocean topography experiment (TOPEX) satellite. The principal function of TOPEX is to use two onboard altimeters to precisely measure the altitude of the satellite's circular orbit from the ocean surface. The altimeters will be capable of measuring the satellite's altitude with a relative precision of 2-3 cm, and an absolute accuracy of 14 cm. This capability dictates the necessity of calibrating the altimeters to the same level of accuracy and precision. Several means are being considered for accomplishing this calibration. The Navy's TRANET system is the prime candidate under consideration, though questions have been raised as to whether it will have sufficient availability in the early 1990s time frame. Another radio-frequency system is the Global Positioning System (GPS). Though having the requisite accuracy, the availability of the GPS is also in doubt because of the Space Shuttle launch schedule. Finally, there exists a sparse, but world-wide laser tracking network. Satellite laser ranging is a LIDAR technique that works by retroreflecting a laser pulse off of a set of cube corner retroreflectors which are precisely located relative to the satellite center of mass. Principally, there are the Nd:YAG mobile laser (MOBLAS) stations operated by the Goddard Space Flight Center (GSFC). These laser systems have the necessary ranging accuracy and will be available at well surveyed sites throughout the 1990s. Based on previous work done at GSFC, we have developed a computer program to model the result of using different ground-based LIDAR systems in conjunction with various retroreflector array designs on the TOPEX satellite. This PC-based software model is designed to accommodate a wide range of satellite viewing angles, altitudes, retroreflector arrays and cube corner parameters. The program is to be used as a design tool to help understand how the LIDAR return signal strength varies with satellite elevation angle, number and geometry of cube corners, laser strength, transmitter size, etc., and how these parameters are traded off against each other.
An array of discrete semiconductor lasers utilizing graded index rod lenses as collimators, were fabricated to form a noncoherent laser beam combiner. 600 mW cw output power and 5.25% total efficiency was achieved in a 5 milliradian beam from a ten diode laser array. A technique of dichroically combining five arrays is also described.
Free-space laser communications requires extremely accurate tracking to keep the transmitter and receiver well aligned in order to prevent data errors. This paper describes a demonstration of diffraction limited tracking for free-space laser communications. Diffraction limited tracking requires the laser beam be held to its undisturbed location to within a fraction of the angular resolution of the system. The demonstration consists of a transmitter, receiver, and far-field simulator. The far-field simulator is used to simulate the laser beam traversing a large distance in space and to introduce a disturbance representative of the spacecraft base motion effects. TRW has developed a biaxial mirror articulator for tracking out this disturbance. This fine and medium track mechanism (F/MTM) consists of a flexurally-suspended gimbal assembly, four voice coils, a caging mechanism, on-board signal conditioning circuit, and outboard electronics. A fine track detector (FTD) consisting of a quadrant photodetector and fine track processing electronics (FTPE) provides feedback to the F/MTM. This paper discusses the major components of the demonstration in detail. In addition, data on the performance of the tracking system is presented and discussed.
This paper presents an overview of solid state laser sources which have the greatest potential for space laser communications. The general capabilities necessary for such sources and the leading source technologies are reviewed.
This paper reports on observed preliminary transmission performance of frequency-division multiplexed NTSC video and DS1 channels modulated on to an optical carrier and transmitted over a two-kilometer atmospheric path in New Jersey, USA, with emphasis on measurements made during the fall of 1987. At a distance of two-km, the received laser beam footprint was set at 8 feet in diameter to accommodate temperature related beam wander. The received signal was processed to retrieve the video and data signals. Transmission path fading was monitored using a tone signal over an adjacent parallel path. DS1 bit-error ratio and video signal parameters were monitored according to established standards. Link deployment issues, optical power budgets and preliminary performance results are reported. The Conditions under which system failures were observed are described.
A novel technique of digital laser space communication, in which a device of CO2 optical bistability is utilized as the optical switch, is presented. In this paper, the theory of optical bistability (03) in 10.6um with the Fabry-Perot (F-P) interferometer device and intracavity electrooptic modulation (IEM) device are introduced. Corresponding experimental results, which show the consistency with the theoritical analyses, are observed. Choosing the scanning F-P interferometer device and IEM device as our experimental components for CO2 laser digital communication, the elementary experiments of this technique in 10.6um laser communication link are achieved.
Architecture of a real time acousto-optic synthetic aperture radar processor is reviewed and recent efforts to develop a compact processor are presented. It employs an acousto-optic device operated in the space integrating mode to compress the signal in range. Two CCD cameras are used in the time-delay and integrate mode to focus data in azimuth and to remove unwanted bias from the image.