Liquid crystal devices are among the most important elements in today's photonic technology. However, little is known about the effects of ionizing radiation on the electro-optic properties of nematic liquid crystal devices. Therefore, we have performed gamma irradiation experiments on commercially available liquid crystal variable retarders (LCVR). We monitored the voltage tunable birefringence of the LCVRs at 633 nm, using a real-time polarization analyzer, as a function of total dose. The first experiment was conducted at room temperature, using a ⁶⁰Co source with a dose rate of 3.5 Gy/h, up to a total dose of about 0.5 kGy. A second experiment was conducted at a higher dose rate (85 Gy/h), using a spent fuel irradiator. For voltages exceeding the Frederiksz transition threshold, there was no considerable radiation effect on the birefringence. The sub- threshold birefringence, however, was influenced by radiation and showed post-irradiation recovery. A final experiment at 4.5 kGy/h allowed doses of up to 500 kGy to be reached, for which the LCVR still showed satisfactory operation. Further investigations aim at performing parameter estimations for identifying which liquid crystal material parameters are most affected by radiation.

Radiation induced changes to the polarization states of light propagating in various photonic devices have been observed. This paper reports on the nature of the polarization changes in one particular photonic device -- the acousto optic Bragg cell. A short description of the detection methodology used to quantify transient changes to the polarization states and to the degree of polarization in tellurium dioxide and gallium phosphide Bragg cells are presented. A discussion of the Stokes parameter analysis used to interpret the data is also presented.

Polymer photonic modulators and switches offer several attractive features for space systems, including rf bandwidth of 50 GHz or higher, and the potential for high sensitivity and low cost. We have shown that polymer modulators experience little if any degradation (less than 2%) in the half-wave voltage due to total dose exposure of up to 5 MRad from a Co-60 source. An outgassing test at 125 degrees Celsius in vacuum for 24 hr resulted in a mass loss of less than 0.2% of the polymer material.

The effect of (gamma) and proton radiation on the optical attenuations at 980, 1300, and 1550 nm has been measured in two Lucent Technologies erbium-doped fibers (EDFs) designed for use in amplifiers pumped at 980 nm. Growth of the induced loss at the 3 wavelengths followed a power law of dose with an exponent of approximately 0.8 independent of fiber, radiation type or dose rate. The incremental attenuation was found to have only a weak dependence on dose rate. The recovery could be described by the sum of 3 exponential decay terms, but simple color center models overestimated the effect of dose rate.

A model for the numerical calculation of the effect of surface heating on the diffraction efficiency of an acousto- optic Bragg cell is presented. The temperature distribution is obtained by fitting the boundary conditions at the surface of the crystal with a solution consisting of a two dimensional Fourier series multiplied by a hyperbolic sin function in the third dimension. Calculated temperature contours for an example are shown. We also present equations for obtaining the complex amplitude of the diffracted wave and the diffraction efficiency resulting from the interaction of an optical light beam of limited cross section with the aberrated acoustic wave. A prediction of the decrease in the diffraction efficiency of a heated PbMoO₄ Bragg cell is given.

Nematic liquid crystals are used in a variety of applications including polarization interference filters, Fabry-Perot etalons, spatial light modulators, and polarimeters. Due to substantial interest in utilizing these devices in space, we have performed numerous tests to indicate the potential suitability of nematic liquid crystal components in a space environment. We report on liquid crystal survivability under extreme vacuum and temperature conditions, as well as their long term functionality after exposure to gamma, ultraviolet, and free electron radiation. The only damage occurred with UV exposure. No damage was observed due to a gamma radiation dose of 2.3 krad or from electron beam dose of 2 Mrad. We report on continued tests designed to determine damage thresholds of liquid crystals to radiation.

The NASA Radiation Health Program has supported basic research over the last decade in radiation physics to develop ionizing radiation transport codes and corresponding data bases for the protection of astronauts from galactic and solar cosmic rays on future deep space missions. The codes describe the interactions of the incident radiations with shield materials where their content is modified by the atomic and nuclear reactions through which high energy heavy ions are fragmented into less massive reaction products and reaction products are produced as radiations as direct knockout of shield constituents or produced as de-excitation products in the reactions. This defines the radiation fields to which specific devices are subjected onboard a spacecraft. Similar reactions occur in the device itself which is the initiating event for the device response. An overview of the computational procedures and data base with some applications to photonic and data processing devices is given.

We have used ab initio Hartree-Fock theory to characterize the microscopic structure and the nonlinear optical (NLO) properties of the over-coordinated oxygen hole center observed in radiation-exposed SiO₂ films. Our calculations indicate that a proton (H⁺) forms a stable bond with a divalent oxygen atom in the Si-O-Si network at an equilibrium r(O-H) approximately equals 1.005 angstrom and also leads to an enhancement in the microscopic NLO response of the local structure by a factor of 4 or more. In the absence of the over-coordinating H⁺, the dipole, moment and the second-order NLO response of the Si- O-Si cluster is extremely small. Protonation of a bridging O atom distorts the electron charge cloud in the direction of the O-H bonding and also reduces the gap between the filled and the vacant energy levels. This leads to a substantial increase in the magnitude of the dipole moment vector and the component of the second-order NLO susceptibility along the O-H bond.

The radiation-induced loss of single mode fibers with undoped and Ge-doped core material was measured at 1300 nm and 1550 nm in the time range from 0.1 seconds to approximately less than 500,000 seconds at room temperature (plus 25 degrees Celsius). With the Ge-doped fiber, measurements were also made at minus 50 degrees Celsius and plus 80 degrees Celsius. The ratio of the losses at 1300 nm and 1550 nm increased at plus 25 degrees Celsius with both fiber types within about 10 seconds to a maximum value of approximately equal to 2 (Ge-doped) or approximately equals 2.7 (undoped). Then the ratio decreased continuously and became less than 1 (equals higher loss at 1550 nm) after about 35,000 - 70,000 seconds (Ge-doped) or approximately greater than 350,000 seconds (undoped). After approximately equal to 500,000 seconds a value of approximately equal to 0.75 was reached with the Ge-doped fiber, with an observable tendency to fall further. At minus 50 degrees Celsius the ratio increased up to about 2.2 and remained constant (as if frozen) during the whole irradiation time of 500,000 seconds. At plus 80 degrees Celsius, however, the radiation- induced loss at 1550 nm was higher at the beginning and became lower than the one at 1300 nm only after an irradiation time of approximately greater than 10⁴ seconds. Additionally the annealing time of loss was measured after the end of irradiation for varying irradiation times between about 3 seconds and 400,000 seconds. The results can qualitatively explain the radiation-induced loss curves at 1300 nm and 1550 nm as well as their ratio.

Radiation testing of 980 nm single mode optical fiber, at 120 krads total dose was performed at three temperatures (minus 30 degrees Celsius, plus 96 degrees Celsius, and room temperature). Testing was also performed on samples from different sections of the optical fiber drawn from a single preform that was designed for a lower cutoff wavelength this fiber is referred to as (Flight), and an off-the-shelf (OTS) 980 nm optical fiber from the same manufacture. The results of this data are discussed in this paper and a projection of the damage to the optical fibers over a five year period was done using the n^th order kinetic model developed by J. Friebele et al.

This paper summarizes the results of recent gamma irradiation tests on InGaAsP surface-emitting LEDs. The spectral output, total output power and frequency response of the devices were measured during irradiation.

Radiation-induced losses of SpecTran and Corning optical fibers were measured in a proton beam at the UC Davis Cyclotron facility. The tests were conducted at different temperatures in the range 150 degrees Celsius to minus 150 degrees Celsius. The paper discusses the measurement setup, the environmental control equipment and the results. A comparison with prior data and theoretical models is presented.

In 1993 Boeing completed and delivered a package of experiments designed to monitor long term space environment performance of selected fiber optics and photonics components. The five segment package, known as the Photonics Space Experiment (PSE), was built from existing and emerging technology components selected for space. A preliminary analysis of the PSE on-orbit data is presented here.

The dual-rate 1773 (DR1773) fiber optic data bus (FODB) experiment is one of twenty four space fight experiments on the Naval Research Laboratory's (NRL) microelectronics and photonics test bed (MPTB). MPTB is an NRL satellite payload that will be composed of modern technology microelectronics and photonic experiments. The Goddard Space Flight Center (GSFC) experiment on MPTB will evaluate the in-flight performance characteristics of the Society of Automotive Engineers (SAE) Aerospace Standard (AS) 1773 FODB in the space radiation environment. AS1773 is a passive star coupled FODB that operates at data rates of Mbps and 20 Mbps. This protocol is commonly referred to as 'dual-rate 1773 (DR1773).' Two different DR1773 fiber optic transceiver designs are tested on the experiment. These devices are the Boeing DR1773 and the SCI DR1773 transceiver. The transceiver devices on the experiment are implemented in a star coupled system and are tested at the higher 20 Mbps data rate. It is estimated that DR1773 at 20 Mbps will meet approximately 80% of NASA's current data rate requirements. DR1773's predecessor, Mil-Std-1773, is currently being flown on several NASA spacecraft. Mil-Std-1773 operates at a single rate of 1 Mbps. Because DR1773 is based on an existing bus protocol, incorporating DR 1773 into the NASA spacecraft program would be both time and cost effective. Success of the DR1773 experiment will provide valuable data on the DR1773 FODB space radiation performance as well as proof of concept for use of the DR1773 FODB for future NASA spacecraft systems.

NASA Goddard Space Flight Center (GSFC) along with the Naval Research Laboratory (NRL) has been at the forefront of the space community in terms of the use of fiber optic data busses and links in the space radiation environment. Previously, we have described the ground radiation test program of the small explorer data system (SEDS) 1773 1 Mbps fiber optic data bus (FODB), as well as its associated in- flight space radiation-induced performance. Further work has also been presented covering higher speed photonic components utilizing III-V materials. Because of the success of the SEDS 1773 FODB coupled with the radiation testing of III-V devices, a second generation FODB capable of both 1 Mbps and 20 Mbps operation is being developed for spaceflight utilization. We present herein preliminary ground test radiation results of hybrid transceiver devices manufactured by Boeing Space Systems that perform the electro-optic and opto-electric translations in support of this medium rate FODB, the AS1773 bus. These devices, designed to be radiation hard (or rad hard), will be flying on NRL's Microelectronics and Photonics Testbed (MPTB) payload as a NASA experiment. This experiment is described in detail elsewhere in this proceedings.

A direct-detection laser altimeter is one of five instruments supporting the one-year scientific investigation of the near-Earth asteroid, 433 Eros, the subject of the near-earth asteroid rendezvous (NEAR) mission. While orbiting Eros at an altitude of 50 km, the NEAR laser rangefinder will continuously sample Eros' surface. Evaluation of altimeter performance requires an understanding of pertinent asteroid characteristics, mission geometry, and rangefinder implementation of the Neyman- Pearson detection criterion. Analysis indicates performance margin of 9.8 dB at 50 km in the presence of speckle. The altimeter is a bistatic configuration that uses a 15 mJ/pulse Cr:Nd:YAG solid-state laser and 3.5-inch aperture Dall-Kirkham receiver telescope with low-noise, high-speed detection electronics. This paper presents pertinent mission requirements and highlights the altimeter design. Our analysis is described and results from altimeter testing are provided demonstrating 9 - 12 dB performance margin, in agreement with prediction.

The near earth asteroid rendezvous (NEAR) laser rangefinder (NLR) is a bistatic system using a diode-pumped Nd:YAG laser and a Dall-Kirkhamm telescope for a receiver. The NLR is one of a suite of five scientific data gathering instruments on the NEAR spacecraft. The NEAR mission is the first of NASA's Discovery Series of spacecraft. The NLR transmitter emits a 15.6 mJ, 15 ns pulse at 1064 nm. The receiver is capable of reliably detecting return signals from the asteroid as low as 1 fJ per pulse, which corresponds to an average power of 50 nW (20 ns pulse). The development and alignment approach of the bistatic system are discussed. The performance test results of the receiver, transmitter, and integrated rangefinder system are presented. Particular attention is given to the system alignment tests and an open air range verification test.

The near earth asteroid rendezvous (NEAR) mission includes a laser ranging instrument which will be used to aid in the measurement of the physical properties of the asteroid Eros. The NEAR laser rangefinder (NLR) is comprised of a laser transmitter, receiver optics and electronics, and a digital control subsystem. The digital component is implemented in a single-board radiation-hardened architecture containing a microprocessor, a custom time-of-flight device, a 16-channel data acquisition system, and a MIL-STD-1553 command and telemetry interface to the spacecraft. This paper covers the digital subsystem design, and discusses approaches taken to maximize the integration of the digital elements, to compress the desired functions onto a single printed circuit board.

The near earth asteroid rendezvous (NEAR) laser rangefinder (NLR), an instrument on the NEAR spacecraft, was designed to measure range from the NEAR spacecraft to the surface of the asteroid 433 EROS. The instrument consists of a laser transmitter, a calibration fiber, an optical receiver, analog electronics, power converting and conditioning electronics, and a digital processing unit. The digital processing unit controls configuration and operation of the transmitter and analog electronics. Software running in the processor handles communication between the spacecraft data bus and the NLR. The software includes functions for command handling, telemetry data formatting and data transfer to the command and data handling computer, transmitter control, measurement of the receiver noise floor, and correction of some timing delays. A brief overview of the software is given along with descriptions of auto-calibration sequences and test results.

The NEAR laser range finder (NLR) design is a compact, light weight design with a high power laser transmitter and a high performance mirror receiver system. One of the main objectives of the NLR is to provide the in-situ distance measurement from the spacecraft to a near earth asteroid. An on board computer will compile this information to provide necessary navigation requirements for the NEAR satellite. Due to the weight budget constraint, the maximum weight limitation of the NLR has been a critical issue from the beginning of the program. To achieve this goal and meet the system design objectives, innovative designs have been implemented in the development of light weight optical, mechanism, and electronic packaging hardware. This paper provides details of the NLR electronic packaging design, thermal and structural designs.

The qualification of the NEAR laser transmitter is discussed with emphasis placed on the three major problem areas encountered: (1) use and derating of discrete power supply components; (2) application of a non-hermetic, high voltage hybrid to the space environment; and (3) vibration testing of the laser optics train. Summary comments are made with respect to the predictability of these quality/reliability problems.

The NEAR spacecraft was launched on February 17, 1996. Qualification tests conducted on the NEAR laser rangefinder allowed evaluation of the instrument's performance and provided calibration data prior to launch. From these data, we were able to determine the system electronic delays, the receiver rangewalk, and the receiver noise floor. The first operational test occurred on April 25, 1996. This post- launch test of the rangefinder verified survival of the instrument and provided data on the calibration parameters listed above. This paper describes these parameters and their significance to rangefinder operations. An interference test was conducted on May 22, 1996. This test allowed engineers to evaluate the effect of laser operations on data from other instruments. The post-launch test and interference are described and the results from these test are presented.

Vertical cavity surface emitting lasers (VCSELs) offer substantial advantages in performance and simplicity of packaging over the edge emitting lasers currently being applied to state-of-the-art photonic interconnects. We have demonstrated operation of VCSELs at cryogenic temperatures and at temperatures as high as 200 degrees Celsius, with a single device operating from minus 55 degrees Celsius to plus 125 degrees Celsius. The devices operate to 14 GHZ and can be operated in excess of 1 GHZ with bias-free operation. Initial radiation tests indicate an order of magnitude improvement in hardness with respect to neutron damage over an LED which is currently used in spaceborne photonic interconnect modules. We also describe the packaging of VCSELs in compact multichip modules. By using passive alignment techniques, optoelectronic devices can be packaged in established multichip module fabrication schemes without adding costly high precision assembly techniques.

We report the performance of a low-cost fiber-optic link used for distributing high-quality reference frequencies to several satellite ground stations. We demonstrate that the phase-noise performance of the fiber-optic link is adequate for supplying an S-band telemetry receiver with a 5 MHz reference signal.

The basic configuration of free space satellite optical communication includes a transmitter and a receiver. The transmitter satellite must point the optical information beam to the receiver satellite in order to establish communication. An important aspect in satellite optical communication is to obtain minimum bit error rate using minimum power. This aim can be achieved with very small transmitter beam divergence angles. The disadvantages of too narrow a divergence angle are that the transmitter beam may sometimes miss the receiver satellite due to pointing vibrations, and that the transmitter optics aperture required is large and expensive. The optimum value of the received power as a function of the pointing vibration displacement determines the optimum bean divergence angle. The performance of the tracking system determines the amplitude limits of the vibrations of the transmitter beam in the spatial domain. A mathematical model including the performance of the communication system as a function of the performance of the tracking system is derived. From this model we derive the optimum transmitter telescope gain. An example for a practical communication system between a low earth orbit satellite (LEO) and a geostationary earth orbit satellite (GEO) is presented. Using this model makes it possible to choose appropriate optics for the transmitter with reduced size and weight.

There exist commercially available fiber optic connectors which are capable of extremely low insertion losses per mated pair due to the utilization of a unique process of optical centering and crimping during termination. We have identified both single-fiber and multi-fiber connectors which utilize this process and have acquired a fabrication line which is capable of performing the active alignment termination process. This paper discusses tests and results required for space usage.

Feasibility of photonics in beam forming and steering of large phased-array antennas onboard communications satellite/avionics systems is addressed in this paper. Specifically, a proof-of-concept demonstration of phased- array antenna feed network using fiber optic true time-delay (TTD) elements is reported for SATCOM phased-array antennas operating at C-band. Results of the photonic hardware design and performance analysis, including the measured radiation patterns of the antenna array fed by the photonic BFN, are presented. An excellent agreement between the analysis and measured data has been observed. In addition to being light- weight and compact, several unique characteristics such as rf carrier frequency agility and continuous steerability of the radiated beam achieved by the fiber optic TTD architecture are clear evidences of its superiority over other competing photonic architectures.

Optical interconnects will play an increasingly significant role in future space applications, because of their high transmission bandwidth potential and radiation hardness. This paper presents the development results of a hybrid optoelectronic system comprising four electronic processing boards that communicate through an optical 3 by 3 non- blocking crossbar switch. Each processing board contains Tx and Rx modules pigtailed to multimode fibers. An optical fan-out of 1-to-3 is implemented by means of a holographic grating. The fan-in is implemented as a lens array. The system has been tested using dedicated automatic target recognition (ATR) software, and space testing is planned.

An economical, fiber optic sensor which monitors atomic oxygen erosion of polymer matrix composites has been developed into a functional prototype. The self-contained prototype featuring automated data collection has been tested in two different atomic oxygen beam facilities. Results show very good correlation of the sensor's response under different operating conditions.

The microelectronics and photonics test bed (MPTB) operated by NRL is intended to provide an experimental platform for the evaluation of the effects of radiation damage on advanced microelectronic circuits and photonic devices. While several systems exist to monitor the accumulation of dose from ionizing radiation in space, little attention has been given to the accumulation of bulk damage in semiconductors. This is of increasing concern when using sensitive detectors for example for astronomical imaging or star-sensing. The bulk damage monitor experiment is designed as a prototype module to monitor the accumulation of bulk damage and uses a sensitive linear charge coupled device in a system designed to accurately monitor the effect of non- ionizing radiation. The experiment, which is light weight, with very low average power consumption and telemetry volume, may form one element in future dosimetry systems on- board spacecraft.

Boeing has first developed a space-qualifiable, radiation- hardened, dual-rate MIL-STD-1773A fiber-optic transceiver for military command/response time division multiplexed data bus applications. This will provide an enhancement path from a fixed low data rate MIL-STD-1553 bus to a high data rate MIL-STD-1773 bus with minor modification of terminals supporting the existing function. The transceiver operates at dual-rates of both 1 and 20 Mbps over temperature (minus 25 to plus 85 degrees Celsius), and it shows a large dynamic range of greater than 20 dB in support of 32-node star network; a sensitivity of up to minus 38.7 dBm and a saturation of minus 16.1 dBm (limited mainly by test source power) at BER less than or equal to 1 multiplied by 10^-10. These transceivers are currently being utilized for space and military applications at DARPA, NASA, and JPL. In this paper we describe the dual-rate transceiver design requirements, ASIC and hybrid design, and performance test results.