The Air Force Space Laser Communications (LASERCOM) Program started with system concept and component design in the early 1970's at the Air Force Avionics Laboratory. The communications system that evolved demonstrated, in 1973, data rates up to one gigabit per second with a bit error rate of 10-6 for 40,000 kilometer simulated links. System capabilities were demonstrated during the period 1975 to present using an engineering feasibility model of a gigabit-per-second space-qualifiable transmitter and a brassboard receiver. The next phase of the program will start in September of this year when the LASERCOM system begins operation outside of the laboratory environment at the White Sands Missile Range in New Mexico. This six-phase demonstration will include ground-to-ground links up to approximately 20 kilometers and aircraft-to-ground links up to approximately 50 kilometers. During these demonstrations, dynamic far-field acquisition, tracking, and two-way communications will be demonstrated. The performance characteristics of the LASERCOM system make its potential application to certain satellite-satellite and satellite-aircraft links unique, while other potential LASERCOM links require a detailed cost analysis of the current investment in radio frequency terminals and systems versus the cost of developing and deploying LASERCOM terminals and systems. There are also some communications links that can be most effectively satisfied by a hybrid LASERCOM and radio frequency system.
Design approaches for acquisition and tracking for a narrow-beam cooperative space laser communications transmitter and receiver are presented. Rationale for selection of key components and system test data are given which demonstrate acquisition in less than 6.0 seconds and tracking with less than 0.6 µrad error.
In tracking optical beams from a source, a pointing error signal is derived from photodetecting the field in the receiver focal plane. This error signal is then used in some manner to control a gimballed system that continually points the receiver optics toward the source. When the source field undergoes turbulent transmission, the optical beam is attenuated and scattered, and the field is randomly defocused at the receiver. In this case the pointing error of the tracking system will evolve as a random vector process in time, statistically related to the random scattering, the photo-detector process, and the dynamics of the gimballing system. Such vector processes have probability densities that satisfy well known differential equations. In this paper this equation is derived in terms of accepted scattering models and tracking systems, assuming quadrant type error detectors are used in the focal plane. Approximate solutions are obtained and analyzed for typical operating conditions, and the manner in which the degree of scattering degrades the entire pointing operation is shown.
This paper describes two versions of a mode-locked, internally frequency doubled Nd:YAG laser for use in a high data rate spaceborne optical communications system. The two versions are similar except for the method of optically pumping the rod. One is CW pumped with an alkali-metal vapor arc lamp operating at 250W input, while the other is pumped using sunlight collected by a 24-inch cassegrainian telescope. Mode-locking at a pulse repetition rate of 500 MHz and frequency doubling are accomplished using a single crystal of barium sodium niobate. Laser output powers in the TEM00 mode are 200mW and 400mW, respectively, for the lamp-pumped and sun-pumped versions with pulse widths of 300-400 ps (10% points). The mechanical and optical design of these lasers are discussed, and their pricipal performance characteristics are described.
Although high speed ( > 100 MHz) photomultipliers (PMT's) have been investigated and produced in the past few decades, only in recent years have practical high performance devices. become available. Significant advances in this technology have occurred largely due to the demand for optical receivers for high data rate laser communication systems. In this paper the spectrum of available high speed PMT's is discussed. This spectrum of devices is first broken down into four design classes: All Electrostatic, Static Crossed Field, Dynamic Crossed Field, and Hybrid (EBS). Then the design, construction and performance parameters as well as limitations of each class of device are presented, along with the prognosis for its future in high speed light detection.
Performance data taken on several candidate high data rate laser communications photo-detectors is presented. Measurements of bit error rate versus signal level were made in both a 1064 nm system at 400 Mbps and a 532 nm system at 500 Mbps. RCA silicon avalanche photodiodes are superior at 1064 nm, but the Rockwell hybrid 3-5 avalanche photodiode preamplifiers offer potentially superior performance. Varian dynamic crossed field photo-multipliers are superior at 532 nm, however the RCA silicon avalanche photodiode is a close contender.
A potassium vapor arc lamp has been developed for use as the optical pump source in a Nd:YAG space communications laser. The 250 W dc, sapphire envelope lamp is significantly more efficient than conventional optical pumps such as krypton arc lamps and tungsten in-candescent lamps at laser power levels required for the space communications application, is longer lived, and is ruggedized to survive a spacecraft launch environment. The lamp has a compact single envelope configuration and is capable of operation in ambient air. In this paper features of the lamp design are described and lamp performance characteristics are discussed.
A study has been conducted to develop and evaluate the optical coatings required for a space laser communications system. The requirements for the Nd:YAG laser, solar collector optics, and other optical components are discussed. The approach to the design, fabrication, and evaluation of the coatings is outlined, including the environmental tests planned to determine the stability of the coatings in space. Selected experimental results are included.
Repetitive pulse train of high peak power and very narrow pulse width from a mode-locked laser has been shown to be a viable candidate as a carrier wave for optical digital communication systems. In addition, the bit rate capacity of such a system can be increased many times by means of optical time-division multiplexing and demultiplexing. In this tutorial paper, a brief review of the optical multiplexing/demultiplexing scheme which utilizes ultrasonic standing wave Bragg cells (SUBC) is given. First, basic interaction mechanisms, working principle and modes of operation, major performance figures and design considerations, and terminal configurations for this acoustooptic scheme are described. Measured performance figures that have been obtained with some of the very high data rate multiplexers/demultiplexers using PbMo04 crystals are then discussed. This acoustooptic multiplexing/demultiplexing scheme should be highly useful for the implementation of multigigabits/sec optical space communication systems.
In order to overcome the bandwidth limitations of lumped element electro-optic modulators, a velocity matched traveling wave modulator has been designed, built and is being tested. The modulator, which utilized two lithium tantalate crystals in a 12.5Q transmission line structure, had a switching voltage of 12 volts. Preliminary data taken using a CW krypton laser and a 1 Gbps data source indicates that the driver rather than the modulator is limiting the response of the system. A 2 Gbps data source and a 2 Gpps optical input is being prepared for further testing. When operating at 2 Gbps, the modulator should require 25 watts of operating power in order to achieve full switching.
Optical communication in the atmosphere, space, the marine boundary layer, and underwater are being investigated for a variety of applications. Three classes of optical communications systems will be addressed: OCULT (Optical Communications Using Laser Transceivers), ELOS (Extended Line-of-Sight) optical communications, and satellite to subsurface optical communications. OCULT is a 10.5Âµ high rate reciprocal tracking heterodyne laser communications system designed for nearly all-weather duplex video bandwidth communications to horizon limited ranges. Of special interest are effects of coherent propagation through fogs and turbulence. The ELOS system is a 1.06μ optical aeorsol scatter communications system for ranges of 30 to 300 miles. Scattering measurement at 40 to 80 miles through the marine boundary layer will be presented. The satellite to subsurface communication efforts deal with blue/green transmission from a satellite, through the atmosphere (including clouds) to a submerged receiver, exploiting the blue/green "window" in ocean water. The multiple forward scattered and diffusion transport of semi-plane waves through clouds and ocean waters will be discussed.
Water absorption characteristics limit the useable electromagnetic carrier frequencies for underwater communication to either less than a few hundred hertz (ELF) or the blue-green optical wavelengths. This paper considers the case of a satellite-borne blue-green laser transmitter communicating at low data rates to a deeply submerged receiver. One dif-ficulty in designing such a system is the uncertainty in propagation losses. This paper discusses the calculation of propagation loss considering both the case of a cloud free LOS (line-of-sight) and the case where the LOS passes through a thick cloud.
Motivation for the use of spatial diversity arrays in both direct detection and optical' heterodyne communications receivers in the presence of atmospheric turbulence is discussed. Procedures for specifying optimum receiver structures are described and the statistical characterization of the received field necessary for utilizing these procedures is presented.
Currently the only cost effective approach to remote utility meter reading appears to lie in a diode laser system called Infra-Red Meter Access (IRMA). The IRMA system combines electro-optical and large scale integrated (LSI) circuit technology to provide a low cost solution to the remote meter reading problem. The IRMA system consists of two diode laser transceiver units which communicate utility consumption data from the meter to a mobile location. The meter monitor unit transponder, which is permanently located in the proximity of the meter, utilizes a light emitting diode (LED) scanner to accumulate the meter reading. When the monitor unit is interrogated, it transmits the meter information to the mobile interrogator transceiver. The mobile transceiver, which is either a handheld unit or located in a moving van, interrogates each monitor unit and receives and processes the meter data. Electrical utility meter information can be bracketed in the monitor unit memory to provide for direct meter reading, peak demand smoothing incentive billing functions, load management signals, and meter tampering detection. A combination of these functions allows the conservation of millions of barrels of oil and an average electric utility bill reduction on the order of 35 to 45 percent.
On many occasions it is required to modify a USAF aircraft to render it capable of supporting a Research, Development, Test and Evaluation (RDT&E) effort. This modification usually involves the addition of wires in the aircraft to gather and transmit data to an external instrumentation pod. This problem has limited the types of aircraft that could be modified to support tests and has been the cause for lengthy and expensive delays in test support. The Optical Communicator System (OCS) will minimize the previously mentioned problems and supply capabilities not now realizable. With the OCS, data is transmitted from the data source to the instrumentation pod via a modulated, non-coherent light beam. The OCS is capable of handling 140 independent, simultaneous data channels under realistic airborne environmental conditions. Features of the system include: visible light beam utilized for alignment, a memory to assure logical data output at all times; a "test word" to assure optical path integrity while in-flight; sufficient system beamwidth to allow for relative motion between the instrumentation pod on the aircraft wing and the data initia-tion point. The OCS is completing airborne acceptance testing at this time.
Fiber optic point-to-point links show many advantages over conventional wire systems. These include wide bandwidth, long inter-repeater spacing, secure information transfer, elimination of ground loops and electromagnetic pulse problems. A brief discussion is given of fiber optic components, temporal dispersion, link length, and nuclear hardness. Bit error rate results are given for a 400 Mbps NRZ 200 meter fiber optic link. Some practical aspects are given for fiber system installations.
From a user viewpoint, the main optical properties of fiber waveguides are reviewed. Attention is given to the more subtle secondary effects that can influence the interpretation of attenuation, dispersion and numerical aperture data given in typical fiber or optical cable specification sheets. Based upon such optical specifications, systems designers can usually do only a first order estimate of how the fiber will perform in their particular application. Numerous references are given, emphasizing recent rather than original contributions. Secondly, the fiber is modelled as an interactive "black box" specified by a minimum number of phenomenological parameters. With transmitter and receiver characteristics held fixed, these parameters are independently varied to calculate their relative importance in systems performance. Multimode and monomode guides are considered, both at conventional and longer wavelengths. It is found that toward increasing transmission distances at lower information rates, the significant improvements in decreasing order of effectiveness are: longer wavelength operation, lower attenuation, higher launched power. At higher rates, mode mixing, an increase in multimode bandwidth and narrowing of source linewidth assume importance.
The measurement sequence for the characterization of graded-refractive-index optical waveguide fibers in a manufacturing environment is presented. The sequence has been designed for minimum human interaction and high throughput. The optical fibers, characterized by attenuation rate, bandwidth, numerical aperture, and optical core diameter, are measured on a quality assurance system driven by a mini-computer. This semi-automated system, consisting of optical and electronic hardware and software for complete characterization, is described. Finally, the measurement methods and current test conditions are presented.
The use of active components for data distribution in multi-terminal fiber networks is reviewed in this paper. Our approach is to place electro-optic switches at nodes of the network. These signal-routing devices are interconnected by individual strands of multimode fiber. We shall present recent results on multimode switching in the areas of optical data busing, multiplexing, and circuit switching. A new "mirror terminal" has been developed as a low-loss means of coupling information on and off a serial bus, while time-division multi-plexing has been implemented with multipole LiTaO3 switches. The "optical crossbar" or matrix is a first step toward telecommunication switching. The matrix provides communica-tion between two groups of N terminals. This can also be done with a passive star coupler, but the matrix does not suffer a 1/N power-division loss, it sets up N independent "con-versations" simultaneously, and the circuits can be reconfigured rapidly. Multimode crossbars 3 x 2 in size are now operating in our laboratory. Fiber guides are butt-coupled to the crossbar crystal at an angle in order to collimate the 0.15-NA fiber light in the crossbar.
The stringent alignment requirements for connecting graded index multimode optical waveguides have resulted in detachable connectors which are mostly laboratory curiosities and are not suitable for app-lication in the field. Achieving accurate alignment does not necessarily guarantee less than 1.0 dB coup ling loss. In this paper the interconnection problem is analyzed from a model which is concerned with the fiber transmission properties thus permitting the analysis to deal with losses in the system. Losses due to misalignments are shown to be dependent on power distribution in the waveguides. The common inter connection concepts of end to end joining and a simple collimating lens are analyzed. Expected losses for these systems make a 1.0 dB detachable connector difficult to achieve. A new concept which forms an index of refraction fluid into a compound lens is shown to surmount the problems encountered in typical connector systems.
Much of the data transfer in computer and process control systems takes place by half duplex data transmission. Conventional approaches to implementation of half duplex fiber optic links use a separate fiber for each transmission direction or a single fiber for both directions and a Y branch. If an LED is used as a semiconductor junction transceiver (SJT) for both emission and detection of optical signals, half duplex data transmission links can be implemented with a single fiber and a single optoelectronic device at each end of the fiber. Investigation of the suitability of reverse biased LED's for use as detectors shows that responsivities higher than 1/5 that of a PIN diode can be achieved. Although the reverse biased capacitance of the LED is larger than the PIN diode, rise times as short as 3 nanoseconds have been measured for LED's used in the detector mode. Comparison of Burrus, edge emitting and large area surface emitting diodes shows that the Burrus and large area surface emitting diodes offer the best detector performance. Experimental results for a half duplex transceiver using an SJT will be presented.
LED driver and PIN diode receiver ICs have been developed for application in fiber optic communication systems. The LED driver provides a drive current adjustable from 25 to 150 mA in 25 mA incre-ments. The current is stabilized against changes in supply voltage and changes in the LED voltage drop due to temperature variations by an internal regulator. When coupled with a suitable PIN diode, the receiver amplifies signals as small as 250 nanoamps to TTL logic levels. The receiver IC includes automatic gain control circuitry that enables it to operate over a 36 db range of optical input signal. Both ICs operate with a single 5-volt supply. These ICs can be used with state-of-the-art fiber optic components to construct fiber optic communications links operating at data rates greater than 10 Mb/s with Manchester encoded data or 20 Mb/s with NRZ data.
The microwave, low-noise GaAs-metal-semiconductor, field-effect transistor (MESFET) has a cutoff frequency higher than 80 GHz and possesses a good optical responsiveness over a very wide spectral range, from visible to near-infrared wavelengths. The drain-current variation of the MESFET is proportional to the input optic intensity. If the light is modulated, MESFET detects the high-speed subcarrier. Besides the low-noise and amplification-gain advantages, the MESFET photodetector operates at a low bias voltage, normally, below 6 V. The common-gate configuration at zero drain voltage also can function in a photocapacitive mode in which the capacitance changes with the change in polarized light intensity. The S-parameter of the device has been characterized up to Ku-band. The high-frequency optical receiver is ready to be designed and optimized using this charaterization. For high-speed fiber-optic communications, the monolithic receiver, the ILD driver, and the low-cost repeater all can be fabricated by using the MESFET's.
The application of fiber optics (FO) to modern radar system design is examined. This paper presents the more conventional uses of fiber optics to transmit wideband signals as well as the less common use of their stable transmission delay characteristics to form active circuit elements within a radar. The stable delay characteristics of fiber optic devices are exploited in the design of such radar circuits as a high frequency oscillator, phased array antenna, precision pulse generator, AID converter, MTI system and chirp generator. The capability of fiber optics to transmit wideband information over long distances is being exploited to develop a radar remoting system for control, power, status, voice and video signals. Three hundred signals are time division multiplexed over seven optical fibers for a distance of 1200 feet. Future development of new materials for fiber optics will permit operation at IR frequencies and lower line loss for new applications.