The microelectronics and photonics test bed (MPTB) is being developed to evaluate the performance of components planned for next generation operational spacecraft in the space radiation environment in advance of their scheduled deployment in operational missions. Important new technologies such as fiber optics, photonics, sensors, CCDs and room temperature focal plane arrays are space qualified and tested as well as new microelectronics devices such as advanced micro-processors, memories, logic gates, and analog to digital converters. It is expected that 50 to 60 microelectronic device types and a few photonics subsystems will be tested in space in the MPTB. Before launch, identical copies of the devices selected for space testing are tested on the ground at accelerators. Predictions of the behavior of the devices in the space radiation environment are made based on the ground test results and detailed models which are developed for each of the devices.

A brief overview of the potential space radiation and temperature induced responses in key photonic enabling technologies is presented. An example of the recent successful operation of optical fiber systems in space is used to discuss the effects of radiation and temperature on photonic components. Typical near-earth radiation environments are examined with a short discussion on how these environments can affect photonic technologies. Forecasts on the potential performance of integrated optic devices, spatial light modulators, charge-coupled devices, and other space applicable photonic components are presented.

The Microelectronics and Photonics Test Bed (MPTB) is a space experiment which evaluates the performance of components and subsystems of important new technologies in advance of their deployment on future spacecraft. Devices aboard MPTB monitor the environment, and the radiation affects data obtained on components will be compared to ground tests and predictions. We present a brief description of the candidate NRL (Naval Research Laboratory) photonics experiments for MPTB.

The Defense Critical Technologies Plan (DCTP) submitted to Congress has for several years listed photonics as one of the highest priority technologies within the DoD. Currently, several photonic architectures are being considered for space applications. As a demonstration of the functionality of photonics for DoD space applications, some photonic devices will be flown on the Microelectronics and Photonics Test Bed (MPTB) space experiment. The MPTB is planned to be an innovative space demonstration of new and emerging electronic and photonic technologies. This paper discusses possible photonics technology candidates for the MPTB mission. In particular, an optical spectrum analyzer experiment is presented as one such candidate.

The Small Explorer Data System (SEDS) is a spaceflight command and data handling system for the small explorer (SMEX) program at Goddard Space Flight Center (GSFC). A key component in this system is the SEDS MIL-STD-1773 Fiber Optic Multiplexed Data Bus. The 1773 bus provides a means of passing telemetry and commands between spacecraft subsystems. This bus is currently being considered for additional spaceflight programs inside and outside of the NASA realm. The SEDS 1773 bus uses integrated optoelectronics as part of its electrical subsystem (or user) to optical interface. Generic proton and heavy ion test results have been previously reported. Herein is presented proton test results for continuing this investigation under actual subsystem interface conditions (MIL-STD-1773) as well as for generic devices using the proton test facilities at University of California, Davis (UCD). This testing was undertaken as a joint effort between NASA/GSFC and the Naval Research Laboratories (NRL).

The Small Explorer Data System (SEDS) relies heavily on new technologies in the electrical designs. Among the key technologies utilized are fiber optics. The effects of the harsh space radiation environment on these spacecraft components can be quite severe. This paper takes a preliminary look at the single event upset (SEU) data seen during the early portion of SAMPEX flight (launched in July 1992) versus the ground test predictions. The new technologies are addressed along with the error handling abilities of the fiber optic system (MIL-STD-1773). The predicted SAMPEX radiation environment is discussed as well as the methodology of SEU rate prediction utilizing both cosmic ray and proton concerns. A comparison of the flight data to ground test predictions is discussed along with information concerning the significance of where and when the SEUs have occurred.

The small explorer (SMEX) project at Goddard Space Flight Center (GSFC) launched the first spaceflight implementation of the MIL-STD-1773 fiber optic data bus on the Solar Anomalous Magnetospheric Particle Explorer (SAMPEX) in July of 1992. The Small Explorer Data System (SEDS), of which the MIL-STD-1773 data bus is a part, has been successful. The MIL-STD-1773 bus is the implementation of the MIL-STD-1553 protocol using fiber optics. Advantages of the fiber optic bus over the electrical bus include lower power, lower weight, and immunity from EMI/RFI. It does not radiate electrical or magnetic fields. It is a nonconductor so it cannot conduct electrical noise into or from a subsystem. This is particularly advantageous on a spacecraft with very sensitive instruments which are often susceptible to electrical interference. Although the MIL-STD-1773 bus is a 1 Mbps bus like the MIL-STD-1553 bus, the fiber optics also provide a path to the much higher rate systems required in upcoming NASA missions.

We describe a technology experiment on the x ray timing explorer spacecraft to determine the feasibility of interferometric fiber optic gyroscopes for space flight navigation. The experiment consists of placing a medium grade fiber optic gyroscope in parallel with the spacecraft's inertial reference unit. The performance of the fiber optic gyroscope is monitored and compared to the primary mechanical gyroscope's performance throughout the two-year mission life.

The Space Station Freedom data management system uses a fiber distributed data interface (FDDI) backbone, which provides counter rotating rings with a data bandwidth of 100 Mbps. This data rate allows a potential for system growth not practical with a copper solution. Fiber's growth potential, coupled with obvious advantages in weight and EMI, perfectly matches the needs and requirements of the Space Station environment. The Space Station environment also provides some unique problems not normally encountered in a FDDI design. The hardware must perform in a vacuum and at nearly full military temperature range. Also, the assembly process of the Space Station causes a much larger than normal signal loss between stations. Finally, the system requires data integrity much higher than the normal FDDI specification demands. These problems are solved by deviating from the FDDI standard on several points. Custom fiber optic receiver and transmitter hybrids have been developed for the program. The result of these hybrids is a much higher sensitivity and larger dynamic range than an ordinary FDDI network would need. Another change from normal systems is the use of specially designed ring concentrator. This allows easier servicing of the units while in orbit, as well as facilitating system reconfiguration for future growth.

Charge coupled device (CCD) imaging arrays are becoming more frequently used in space vehicles and equipment, especially space-based astronomical telescopes. It is important to understand the effects of radiation on a CCD so that its performance degradation during mission lifetime can be predicted, and so that methods to prevent unacceptable performance degradation can be found. Much recent work by various groups has focused on the problems surrounding the loss of charge transfer efficiency and the increase in dark current and dark current spikes in CCDs. The use of a CCD as the fine error sensor in the Lyman Far Ultraviolet Spectroscopic Explorer (FUSE) is limited by its noise performance. In this work we attempt to understand some of the factors surrounding the noise degradation due to radiation in a space environment. Later, we demonstrate how low frequency noise can be used as a characterization tool for studying proton radiation damage in CCDs.

Two classes of quantum-well-based fiber-optic light sources were evaluated for degradation under 5.5-MeV proton irradiation as part of an evaluation of the survivability of fiber optic components for satellite applications. The first was an InGaAs/GaAs strained-layer quantum- well laser; the second was a broad-band light-emitting diode based on concurrent multiple- state transitions from two asymmetric quantum wells in the GaAs/AlGaAs system. In contrast to earlier reports comparing bulk active-region heterostructure light-emitting diodes with similar structured laser didoes, it was found here that these quantum-well light-emitting diodes are more tolerant of proton irradiation than the quantum-well-based lasers. Analysis revealed the quantum-well light-emitting diode allows operation far into gain saturation with the more lossy cavity structure as compared to quantum-well lasers. The lasers, in contrast, operate in a region where gain is more sensitive to current density, and therefore, the introduction of energetic proton-induced atomic displacement-related recombination sites have a greater effect than in similar structured light-emitting-diodes. In addition, it was found that the laser maintained constant slope efficiency, while current thresholds increased linearly with proton fluence. In contrast, both light-emitting-diode output-power and slope efficiency decreased with fluence. Experimental damage constants were determined for performance parameters and found to be similar to those previously reported for carrier removal rates on other GaAs- based electronic structures.

Results obtained from the Long-Duration Exposure Facility (LDEF) JPL fiber optics experiment, which remained in low-earth orbit for 5 3/4 years, are discussed in order to illustrate the effects of the adverse space environment on fiber optic cables. The results of tests performed on the ten fiber optic cable samples, flown on the LDEF, are then compared to data obtained from similar laboratory tests performed on currently available fiber optic cables. The effects of radiation exposure, temperature cycling, polymer aging, and micrometeoroid impacts on fiber optic cables applied in space are discussed. Overall, it seems that current commercially available fiber cables could be used for space missions, if kept in a controlled environment. Improvements in purity of silica glass, in buffer coatings, and in cabling materials are already visible in the new generation of fiber cables, bringing it one step closer to the ultimate `space qualified' fiber cable.

Current developments in high performance satellite data links rely on fiber optic systems to take advantage of light weight, electromagnetic isolation, low power, and high bandwidth. Indications are that fiber data links operate with little degradation or interference in the earth's trapped radiation belts. To quantify this, we report analyses of experimental investigations in which operating fiber bus components are subjected to proton bombardment at varying proton energy, proton flux, angle of incidence, data rate, and signal levels. Parameterization of bit error rate (BER) effects in terms of these variables offers insights into the physical mechanisms involved and suggests both circuit modification and device selection criteria to maximize link performance. We outline a method to predict BER in orbit and offer this as a basis for evaluating proposed hardening solutions. The method combines predicted trapped particle orbital environmental data, including spacecraft shielding effects, with the measured system response.

In 1991 Boeing was given an opportunity to fly an undefined fiber optics experiment aboard a spacecraft. The main restriction was that a qualified package had to be delivered in eight (8) months. Boeing took advantage of this opportunity and created the Photonics Space Experiment (PSE).

Fiber-optic ring architectures have an inherent reliability problem when considered for spaceborne applications: a single-node failure can interrupt communications to every other node. The Fiber Distributed Data Interface (FDDI) protocol solves this problem through use of a dual counter-rotating ring architecture. This provides communication between all nodes, even if a single fault has occurred. If more than a single fault occurs, the ring can become segmented and communication is not provided to all nodes. This paper presents a dual cross- strapped ring architecture that provides the degree of survivability required for many spaceborne applications.

We have utilized a semiconductor optical amplifier (SOA) as an optical gain/absorption switch and have demonstrated an array of such switches in the form of a monolithic 4 X 4 optical crossbar switch. This device is completely nonblocking and can be operated in either a point-to-point or broadcast mode, simultaneously linking any combination of four input/output ports on one side with any combination of ports on the other. It is housed in a standard hybrid flatpack, with external control leads and eight connectorized optical fibers. The device consists of ridge waveguides, etched-facet turning mirrors, and etched T-branches laid out into an all- optical-path network. It is fabricated from a single GaAs quantum well/AlGaAs GRINSCH epitaxial structure on an N+ GaAs substrate. Because the paths are optical, they transmit signals in either direction -- i.e., the device can operate in a duplex (bidirectional) mode. We have demonstrated switching functionality, simultaneous routing of two digital FM video signals, and routing of pseudo-random NRZ data.

We have developed semiconductor active (with gain) waveguide structures than can be used as building blocks at a monolithic device and multi-device level to configure a high transmission bandwidth space division switch. For space and aircraft systems, where weight and volume are a premium and reliability is essential, reconfigurable optical interconnects promise a quantum improvement in overall system performance and capability. Because of its inherent gain, our switch can split up an optical signal, to multiple destinations, many times and still achieve a net 0 dB loss. We have used this basic structure to design distributed gain matrix vector multiplier (DGMVM) 4 X 4 and 8 X 8 crossbar switches. These monolithic devices are completely nonblocking, bidirectional, and can be operated in either a point-to-point or broadcast mode. Multiple monolithic space division switches can be interconnected by inorganic or polymer waveguide arrays to form a large switching fabric on a single substrate. Our simulations for signal-to-noise (S/N) performance show that a -10 dBm 10 GHz optical signal can be split over thirty times and still maintain better than a 10^-10 bit error rate (BER) level. Optical interconnect devices are inherently immune to electronically generated noise -- e.g., EMI, RFI, EMP. Conversely, active waveguide interconnect devices are non-noise-intrusive in that they do not generate EMI or RFI. We discuss our concepts for applying semiconductor active waveguide devices to higher levels of network integration for space and avionics applications.

The design of the optical portion of a fiber optic data bus for space applications is described. A maximum NRZ data rate of 1.9 Gbps is predicted using low risk components which have either been demonstrated in space or for which sufficient qualification data are available. Substitution of lasers and fibers for higher bandwidth components are expected to extend the data rates to 4 Gbps but require additional qualification effort.

Satellite receiving terminals can benefit by locating the antennas on hill tops or away from interference and locating the receivers at a convenient, more accessible location. Wideband, high dynamic range, analog fiber optic links can replace conventional coax, wave-guide or microwave link techniques by using signal converters that offer superior performance, plus operational convenience and flexibility. This paper discusses a five mile ground link using single mode fiber and direct modulated DFB lasers. Complete C-band links have been built that achieve over 114 dB/Hz^(2/3) dynamic range and a low total group delay of less than 1 nsec p-p. Both the completed transmitter and receiver fit in 2 unit high 19 inch rack mount assemblies, use less than 12.5 watts total dc power, and the completed link MTBF was greater than 70,000 hours.

Aircraft and tactical communications systems have ever-increasing needs to distribute and switch voice, data and video services. These networks must be very flexible in protocols and format for the information and need to be modular to accommodate reconfiguration and changes in services. A broadband 50 MHz - 2.5 GHz analog, fiber optic network is an excellent solution for the system backbone. This paper describes a robust 50 MHz - 2.5 GHz analog backbone having almost 100 dB/Hz^2/3 suitable for both military airborne and tactical applications. The network operates in a similar manner to video cable distribution systems, but has full two-way capabilities for channelized voice, data and video.

Production lasers are available which offer a broadband, 2 - 18 GHz, microwave link capability. Lasers, detectors and fiber optic components are adaptable to aircraft and space environments in small, lightweight, self-contained modules. Tests and analysis of performance specifications show that a third order, spurious free, dynamic range of approximately 100 dB/Hz^2/3 can easily be achieved with production components which are reliable, rugged, and hermetically sealed. Future laser improvements are expected to achieve even greater dynamic range.

Fiber optic imaging bundles have been used for a number of years in medical and industrial remote sensing applications. Several NASA applications (such as Orbiter payload alignment for berthing/unberthing operations and Space Station Freedom construction) requiring multiple views have been identified. This paper reports on some of the proposed systems and on an optical switch which can connect any one of multiple imaging bundles to a single camera. The switch uses a rotating offset prism and collimated optics to relay the image to the camera. The offset prism merely displaces the image without dispersing, thereby acting as a periscope. By operating the prism in collimated light any one of several imaging bundles can be selected with negligible sensitivity to rotational precision. Located in the Orbiter payload bay, the input view would be selected by driving motors to rotate the prism. The same motors, used in a feedback mode, would be used to `stop-down' the input optics. Detailed mechanical and optical designs are presented as well as results from developmental studies.

We have adapted a fiber optic and television camera imaging system for operational use in the crew cabin on space shuttle orbiter (orbiter) flights. It allows on-orbit inspection into small or confined spaces. Its main use is in-flight maintenance procedures such as examining cabin under-floor air filters, but it has also been used to observe orbiter experiments. A recent change to less sensitive cameras has dictated that a brighter light be emitted from the end of the fiberscope. We have consequently upgraded the battery-powered self-illumination capability by several orders of magnitude, which was a challenge with respect to limited touch-temperature, small admittance (area and solid-angle product) of the illuminating portion of the fiber bundle, and efficiency of getting light from a tungsten-halogen source into the fiber bundle.

Optical correlators are just now finding their way out of a laboratory environment and are being considered for practical systems. Space application for optical correlators have been discussed and studied for several years; however, there has been no funded program to move beyond paper for space considerations. One of the first supporting efforts to evaluate fieldable optical correlators is being funded by DARPA under their Transfer of Optical Processing to Systems (TOPS) program. One of the TOPS projects is to build an optical correlator that will perform target recognition while being flown in a helicopter. Some of the packaging consideration for the helicopter environment will carry over to the space applications environment. This paper presents a brief description of optical correlators and their applications and potential value to space operations. Critical parts and components of the correlator are discussed in reference to the space environment and qualification for space systems.

To remain competitive with fiber telecommunications systems, advanced communications satellites will require increased bandwidth and capability to provide efficient economical services. On-board photonic subsystems could potentially reduce the payload mass and volume and increase processing bandwidth. Technical feasibility of a photonic satellite where optical subsystems replace the microwave and digital electronics on-board communications spacecraft is discussed in this paper.

Photonics technology offers a multitude of applications and major benefits in the development of future spaceborne communications systems with high performance and low mass/size requirements. These applications include not only signal distribution/control functions, but also optical signal processing, phased array antennas, sensors, instruments, and gyros.

As an integral part of its development of mm-wave phased array antennas, NASA Lewis Research Center has established an Optics-In-Antennas (OIA) program to develop optical technologies required for commercial communications satellite applications and future NASA missions. The thrust of this program is to utilize optical technologies to support the insertion of Ka-band MMICs into phased array antennas, and to identify novel optical techniques that enable improved antenna performance. The OIA program is organized into three basic components: optical control signal distribution, optical rf signal distribution, and optically processed beam forming networks. This paper discusses the developments in each of these areas and identifies future applications and requirements for these technologies.

Feasibility of space-fed optical beam forming networks for use on board satellite C-band multi-beam phased-array antennas is addressed in this paper. Two optical space-fed BFN architectures have been investigated: one using optical fibers as delay lines to simulate the required microwave phase shifts at the feed array elements and the other formed by reducing the microwave BFN to optical dimensions and then converting, in a 1:1 correspondence, the optical phase shifts to the microwave equivalent. BFN system design, trade-off and performance are evaluated for payload weight, size and power requirements.

Optically Processed Beam Forming Networks (OPBFNs) have been identified as an emerging technology for phased array antenna applications requiring rapidly reconfigurable multiple beams. OPBFNs have the potential for a considerable decrease in weight and volume over typical phased array antenna architectures. The potential compactness and flexibility of OPBFNs has made them an attractive candidate for an antenna role in the Mars Environmental Survey (MESUR) project. The NASA Lewis Research Center (LeRC) and the Jet Propulsion Laboratory (JPL) are jointly investigating the use of OPBFNs in this application. As its part in this joint effort, LeRC is developing an OPBFN testbed to evaluate OPBFN architectures and components, as well as developing system modeling programs to simulate OPBFN performance. JPL's role in the project is to develop a photonic transceiver that feeds the OPBFN. This paper discusses the modeling and development of the OPBFN testbed at LeRC.

In this paper, we present the theoretical investigation, design, and simulation of a new LiNbO₃ guided-wave optical correlator suitable for real-time SAR applications. It is based on a complex interferometric structure, involving four aperiodic phase-reversal traveling wave modulators. The electrode structure is designed in order to reproduce the product signal between the received and reference voltages, which is then time-integrated by a suitable photodetector. The filtered signal outgoing from the detector is proportional to the final correlation function, which can be electronically registered and multiplexed on a two- dimensional matrix by sum-and-shift procedure. Thus, the processor performs the correlation function between the reference signal and the received signal when they are applied to the laser diode and to the electrodes as driving voltage, respectively. Comparisons between two different LiNbO₃ waveguide fabrication techniques, i.e., proton exchange and titanium indiffusion, have been carried out in terms of circuit performances in order to reconstruct the SAR images.

This paper reports on the system design of an rf-modulated optical link for spaceborne radar altimeter applications and presents results of rf carrier phase noise and phase stability measurements. Our study involved the transmission of rf-modulated optical signals at 834 nm wavelengths using Bias-T and Mach-Zehnder modulators. The rf/microwave signals' phase coherence, modulation levels, and insertion loss are reported. Phase noise measurements revealed a noise floor of at least -118 dBc/Hz at frequencies greater than 100 Hz from a 5-MHz carrier with direct modulation. The phase noise was degraded by about 10 dBc/Hz for external modulation techniques. The rf insertion losses appear smallest for the Bias-T intensity modulators (approximately 30 dB at an 834 nm optical carrier with 5 MHz modulation). The results of modulation experiments with 320 MHz radar altimeter chirps are also presented with an emphasis on coherence, stability, and rise and fall time. The linear FM chirp signal ramps down at a rate of -3.125 MHz/microsecond(s) (+/- 0.5%) and is flat to within +/- 1 dB. Measurements show that this type of FM chirp modulated on optical carriers at 834 nm meets radar altimeter system requirements.

Data management aboard advanced spacecraft has become increasingly complex due in part to the many types of data present and an ever increasing volume of data to be managed. Data management has traditionally been a conservative practice, however, practical concerns such as system size, power, weight, and cost have created opportunities for the use of alternate technologies such as fiber optics. The use of fiber optics aboard the space station and the Small Explorer mission is evidence of this trend.