A brief overview of ongoing radiation effects research in photonics is presented focusing on integrated optic and acousto-optic components. A short summary of radiation-induced effects in electro-optic modulators, detector arrays, and other photonic technologies is presented along with extensive references. The coordinated radiation effects studies among researchers within the Tri-Service Organizations and international experimental teams are beginning to demonstrate consistent measurements of radiation-induced effects in photonic components and confirming earlier reported data. This paper will present an overview of these coordinated investigations and focus on key research being conducted with the AFMC Phillips Laboratory, Naval Research Laboratory, Defence Nuclear Agency, NATO Nuclear Effects Task Group, and the Tri-Service Photonics Coordinating Committee.

Experimental data of the first observed radiation-induced transient changes to the bandwidth responses of an acouso-optic (AO) Bragg device is reported. A brief description is presented of a procedure for measuring radiation-induced transient changes to the first-order diffracted beam and the zero-order transmitted beam to determine the diffraction efficiencies of PbMoO₄, TeO₂, and GaP AO deflectors and modulators. A brief analysis of the effects of radiation-induced color centers and heating effects in a PbMoO₄ AO Bragg device is presented. It was determined that a significant portion of the changes to the PbMoO₄ swept frequency reponses were due to radiation-induced heating, and that radiation-induced color center effects played a secondary role. Changes to the PbMoO₄ diffraction efficiency were induced by CO₂ laser heating of the PbMoO₄ crystal. The laser-induced changes correlated highly with electron-induced changes indicating alteration of the acoustic diffraction grating which resulted in changes to the bandwidth and bandshape responses. The presence of thermally induced spatial index gradients across the acousto-optic interaction region are believed responsible for these changes.

Radiation testing of photonic components is not new, however component level testing to date has not completely addressed quantities which are important to system behavior. One characteristic that is of particular importance for optical processing systems is the frequency response. In this paper, we present the results of the analysis of data from an experiment designed to provide a preliminary understanding of the effects of radiation on the frequency response of acousto-optic devices. The goal is to present possible physical mechanisms responsible for the radiation effects and to discuss the effects on signal processing functionality. The experiment discussed in this paper was designed by Sandia National Laboratories (SNL) and performed by SNL and Phillips Laboratory (PL) personnel at White Sands Missile Range (WSMR). In the experiment, a TeO₂ slow shear-wave aocusto-optic cell was exposed to radiation from the WSMR linear accelerator. The TeO₂ cell was placed in an experimental configuration which allowed swept frequency diffracted power measurements to be taken during radiation exposure and recovery. A series of exposures was performed. Each exposure consisted of between 1 to 800, 1 microsecond(s) ec radiation pulses (yielding exposures of 2.25 kRad(Si) to 913 kRad(Si), followed by recovery time. At low total and cumulative doses, the bandshape of the frequency response (i.e. diffracted power vs. frequency) remained almost identical during and after radiation. At the higher exposures, however, the amplitude and width of the frequency response changed as the radiation continued, but returned to the original shape slowly after the radiation stopped and recovery proceeded. It is interesting to note that the location of the Bragg degeneracy does not change significantly with radiation. In this paper, we discuss these effects, and we discuss the effect on the signal processing functionality.

A carbon dioxide laser was used to heat an acousto-optic deflector operating at the Bragg angle in order to simulate heating effects induced by linearly accelerated electron irradiation. Temperature gradients induced by the laser heating independently confirmed an earlier hypothesis advanced to explain radiation-induced spatial intensity shifts in acousto-optical deflectors and modulators. Acousto-optic (AO) devices are under consideration for a variety of space and terrestrial applications where radiation environments are present, and an understanding of the limitations of using AO devices in these environments is required.

A review is given of the effects of radiation on various photonic modulator materials and devices including polymer-based structures, insulator-based devices, semiconductor-based devices, and spatial light modulators. We conclude by providing recommendations for further work in the area of radiation effects in photonic modulators.

Pulsed electron-induced responses observed in a proton exchanged LiNbO₃ optical directional coupler waveguide are presented. The proton exchanged coupler was observed to be less sensitive to electron radiation than a previously studied titanium indiffused LiNbO₃ directional coupler. A brief comparison of the attenuation and crosstalk behavior experienced by both couplers exposed to comparable electron doses is presented. The responses of the couplers are believed to depend on their respective photorefractivity and on the extent of radiation-induced heating.

This paper describes the effects of combined neutron and gamma radiation as produced by a pulsed TRIGA reactor on a 100/140 micrometers 1 X 2 fused biconical tapered coupler and a LiNbO₃ polarization maintaining directional coupler. Measurements for both devices are described for two fluence levels, with the transverse electric and transverse magnetic output signals separately measured for the directional coupler.

Recent advances in Interferometric Fiber Optic Gyroscope (IFOG) technology have enabled these devices to equal, and in some respects exceed, the performance of the floated, spinning wheel rate integrating gyroscope. However, their ability to perform in a space radiation environment has been a significant concern. Test results are presented addressing the effects of space radiation on the performance of a high precision pointing grade IFOG. Proton-induced degradation of the optical components of an IFOG is evaluated based on testing performed at the Harvard Cyclotron Laboratory (HCL). Rationale is provided for using the HCL proton accelerator as a reasonable simulation of the space environment. An analysis is presented which prioritizes the component-level dose tests based on expected radiation sensitivities. The evaluation addresses both total dose (to about 12 krad) and dose rate effects. Testing was performed at the component level as well as the system level with an expanded version of a closed-loop operational IFOG. Primary concerns include permanent attenuation and spectral transmission (wavelength) sensitivity to total dose, and angle random walk and bias stability degradation as a function of dose rate. Component level results are presented for a superfluorescent light source, integrated optics chip (IOC), coupler, and polarization maintaining fiber coils. Closed-loop transient noise results are evaluated based on dose rate testing of the IOC, coupler, and fiber coil.

Transitioning optical processors from the laboratory to rugged environments requires special care during the design of the optical and mechanical components as well as the total package configuration. Processors exist today that were developed to operate in various rugged environments while maintaining performance specifications. Designing optical processors for space environments requires additional consideration of features such as radiation, launch vibration, thermal cycling, heat dissipation, self calibration, and autonomous operation. This paper presents the design of processors which operate in rugged environments, and the rules that must be adapted/extended space operation. Specific experience and general conditions are presented on existing processors and those under development. The status of radiation hardened components is also presented.

In response to the need for radiation hardened optoelectronic integrated circuits (OEICs) and systems, Physical Optics Corporation (POC) in cooperation with the University of New Mexico proposed to design, develop, and fabricate special OEIC units based on an open modular Light Distributing Interconnective Active (LIDIA) architecture. This design will alleviate most problems associated with electrical interconnects, and will be optimized from the perspective of system survivability, radiation hardening, high I/O bandwidth, built-in testing (BIT), packaging density, electrical power budget, thermal (heat) management, and electrical isolation. The proposed concept will include GaAs waveguides, vertical cavity surface emitting lasers (VCSEL--developed by the University of New Mexico), electro-optical modulators and all optical switches. The proposed optical interconnect concept will be applied for chip-to-chip data communications as well as for board-to-board data transmission in a 3D packaging configuration.

Acceptance of optical interconnects into spaceborne and ground-based military systems will be limited by the risk and maturity of the technology. Multimode fibers are currently the interconnect medium of choice for military application due to the availability of qualified parts. Multimode optical interconnects also offer lower assembly cost than singlemode interconnects, making them attractive for use in commercial computers. In both applications, modal noise may limit the attainable bit error rate in a digital system. We report on an investigation of the validity of simple expressions for determining modal noise in multimode systems, and consider in particular waveguides supporting fewer modes than typical multimode fibers and waveguides with incomplete modal excitation. We conclude that the simple expressions are valid for losses greater than approximately 0.5 dB per interface, but that the actual signal-to-noise ratio is significantly poorer than that predicted by theory for large displacements of waveguides in which a small fraction of the total modal volume is excited. We describe two simulation techniques for determining the statistics of the transmission associated with a waveguide bend, and find that the simple formula widely used for straight waveguide intersections do not describe curved waveguide interconnects accurately. We also describe a practical demonstration of two optical interconnect systems in which multimode interconnect media incorporating several imperfect interfaces and excited with coherent sources can achieve useful bit error rates in digital systems. In the first interconnect, a fiber optica data bus for satellite use employs six multimode connectors and multimode fiber to transmit data at 1.2 Gbps over distances from 1 meter to 100 meters. In the second interconnect, passive multimode polymide waveguides compatible with intracabinet optical interconnects are used to implement a multichip module (MCM)-to-MCM interconnect in which optoelectronic die are incorporated in a 'chip first' multichip module technology.

Previous experiments have shown that fiber-optic technology is capable of UHF (5 MHz) signal transmission having a single-sideband phasenoise of - 120 dBc/Hz in a one Hertz bandwidth. These experiments also demonstrated that radar chirp signals distributed by these fiber-optic links meet system requirements with respect to flatness and stability. The major drawback of a fiber-optic system appears to be the high insertion loss. This paper reports on progress made in the developments of fiber-optic links as well as the adaptation of fiber optics for the transmission of precise timing pulses (1 pps) and 5-MHz UHF signals generated by an atomic hydrogen maser over distances of several thousand feet. A fiber-optic system which transmits these precision time and frequency signals from an atomic frequency standards laboratory to a satellite tracking station has been implemented. This system will play an active part in the tracking of spacecraft. Details on its performance are presented and comparisons between systems using laser diode and LED sources are briefly discussed.

We report the results of our experimental study using sensitized Polymethyl methacrylate as a recording material for storing multiplexed holograms. Such holograms form the basic building block of an optical neural network processor for image recognition.

Boeing has successfully concluded a 2 1/2 year, two phase developmental contract for the STAR-Fiber Optic Data Bus (FODB) that is intended for future space-based applications. The first phase included system analysis, trade studies, behavior modeling, and architecture and protocal selection. During this phase we selected AS4074 Linear Token Passing Bus (LTPB) protocol operating at 200 Mbps, along with the passive, star-coupled fiber media. The second phase involved design, build, integration, and performance and environmental test of brassboard hardware. The resulting brassboard hardware successfully passed performance testing, providing 200 Mbps operation with a 32 X 32 star-coupled medium. This hardware is suitable for a spaceflight experiment to validate ground testing and analysis and to demonstrate performace in the intended environment. The fiber bus interface unit (FBIU) is a multichip module containing transceiver, protocol, and data formatting chips, buffer memory, and a station management controller. The FBIU has been designed for low power, high reliability, and radiation tolerance. Nine FBIUs were built and integrated with the fiber optic physical layer consisting of the fiber cable plant (FCP) and star coupler assembly (SCA). Performance and environmental testing, including radiation exposure, was performed on selected FBIUs and the physical layer. The integrated system was demonstrated with a full motion color video image transfer across the bus while simultaneously performing utility functions with a fiber bus control module (FBCM) over a telemetry and control (T&C) bus, in this case AS1773.

The potential applications of space systems employing laser diodes in guidance, control, and scientific instruments have begun to emerge. For example, the Near Earth Asteroid Rondezvous (NEAR) project sponsored by NASA will contain a laser rangefinder. NEAR is a 4-year mission. For the first 3 years (about 26,000 h) it will cruise to the asteroid Eros, and its laser diodes will be dormant. A fourth year will be spent in orbit around Eros and will require the diodes to operate reliably for about 10,000 h. The array of laser diodes will be used to pump a Nd:YAG element in the Near laser transmitter.

We present the characteristics of a prototype all-fiber sensor network that is useful in structure-health management and distributed sensor data acquisition. In this network, each remote sensor node is powered over a fiber by a laser in the base station. The sensor data are sent back to a base station through a different fiber. Issues concerning power consumption per node, data rate, fault-tolerance, packaging, cost, and network expandability will be discussed.

Two optical beam forming network (BFN) architectures that are deemed viable for on-board satellite phased-array antenna applications are assessed for functional capacity and technology feasibility: specifically, the implementation issues, reliability, and long-term performance are discussed for an M-beam, N-element phased-array antenna operating at Ka-band. Also included are the results of recently demonstrated proof-of-concept BFNs employing fiber optic true-time delay elements and coherent optical processor (COP) based approaches. The details of the trade-off study results and relevant POC hardware developed will be presented in the conference to demonstrate the advantage of light weight and large bandwidth capability of photonic beamforming which are at premium in large antenna arrays. Coherent optical processor using Fourier transform has many advantages.

The major part of the studies for introducing photonics in space are performed in the USA, but a few years ago, Saab Ericsson Space AB started a study project under a European Space Agency (ESA) contract with Ericsson Microwave Systems AB as a subcontractor. The main objectives of the project were to investigate the requirements on photonics systems and hardware for space applications and to develop space qualifiable fiber optic hybrids for a spacecraft data network. The system analysis, as well as the specifications for the fiber optic hybrids, was performed to meet the performance and environmental requirements of the Columbus Data Management System (DMS) (the fiber optic version of 10 Mbps ISO 8802-4 Token Bus). The requirements, design, and performance of these fiber optic transmitter and receiver hybrids are presented. The most important characteristics are: optical output peak power of -8.5 dBm, receiver sensitivity of -44 dBm, acquisition time of 2 microsecond(s) and a dynamic range of 27 dB.

A compact programmable tuning external cavity laser has been developed using a microprocessor as the controller in which a stepper motor is employed to execute the motions of the grating for wavelength continuous tuning and a dual stage Peltier cooling system as precise temperature controller. A tuning range of 77 nm and 1GHz/microstep tuning resolution are achieved. This device can be widely applied to spectroscopy, spectral measurements, metrology, remote sensing, and so on.

In-situ measurements of proton-induced system level errors are reported for newly designed and fabricated hardware implementing an operational 200 Mbps linear token passing fiber optic data bus. Parametric analysis of key single-particle-event and total-ionizing-dose variables affecting components is used to quantify system response and predict on-orbit performance.

The Microelectronics and Photonics Test Bed (MPTB) is a space experiment which will evaluate the performance of components and sybsystems of important new technologies is advance of their deployment of future spacecraft. Devices aboard MPTB will monitor the environment, and the radiation effects data obtained on components will be compared to ground tests and predictions. We present a description of the proposed high performance fiber optic data bus (FODB) experiment for MPTB which will feature the newly available 200 Mbps Boeing STAR-FODB hardware which is designed for space applications. This bus uses a passive star architecture and implements a Linear Token Passing Bus (LTPB) standard. The existence of extensive ground radiation test results for the STAR-FODB will enable high confidence predicition of its on-orbit performance to be made prior to launch.

During the past year, Martin Marietta Astronautics conducted an intensive trade study to compare optical fibers for radiation performance, to determine the availability of buffer materials for space, and to ascertain current capabilities of cabling manufacturers. In addition, we investigated single-fiber and multi-fiber connectors and backshells for multimode and single-mode optical fibers for flight hardware. The trade study included numerous literature searches and discussions with vendors and experts in the field to determine product availability of components and the manufacturing of flight harnesses.