Alan R. Johnston

Optical links for cryogenic focal-plane array readout

An optical link can provide an interface channel for a focal plane array that is immune to electro-magnetic interference (EMI) and can lower the heat load on the dewar. Our approach involves the use of fiber optics and an on-focal-plane optical modulator to provide an interface to the focal plane array (FPA). The FPA drives the modulator with an electrical signal. We evaluated specially fabricated AlGaAs/GaAs multiple-quantum-well (MQW) optical modulators, operating near 840 nm, for analog modulation, and we have used the results to calculate the performance of an optical interface link using experimentally determined device parameters. Link noise and dynamic range for an analog link were estimated from a separate experiment using pigtailed fiber components. The performance of the MQW modulator system is compared to alternative strategies. Significant improvement in performance in comparison to conventional electronic interfaces appears to be possible.

Overview of fiber optics in the natural space environment

The potential applications of fiber-optic (FO) systems in spacecraft which will be exposed to the space radiation environment are discussed in view of tests conducted aboard the Long-Duration Exposure Facility and the Comet Rendezvous and Asteroid Flyby spacecraft. Attention is given to anticipated trends in the use of FO in spacecraft communications systems. The natural space radiation environment is noted to be far more benign than the military space environment, which encompasses displacement-damage effects due to significant neutron influences

Optical links for cryogenic focal plane array readout

An optical link can provide an interface channel for the focal-plane array that is immune to electromagnetic interference (EMI) and can lower the heat load on the dewar. Our approach involves the use of fiber-optics and an on-focal-plane optical modulator to provide an interface to the focal-plane array (FPA). The FPA drives the modulator with an electrical signal. We evaluated specially fabricated AlGaAs/GaAs multiple quantum well (MQW) optical modulators, operating near 840 nm, for analog modulation, and we have used the results to calculate the performance of an optical interface link using experimentally determined device parameters. Link noise and dynamic range for an analog link were estimated from a separate experiment using pigtailed fiber components. The performance of the MQW modulator system is compared to alternative strategies. Significant improvement in performance in comparison to conventional electronic interfaces appears to be possible.

Single-event effects on associated electronics for fiber optic systems

As spacecraft begin to unlock the potential of fiber optic systems for spaceflight applications, system level bit error rates (BER) become of great concern to the system designer. Fiber optic data busses running up to 1 Gbps data rates require an uncorrected BER to meet system specifications. With emerging high speed and low power devices such as 3 V CMOS and GaAs ICs being utilized in the system design to interface the electronic subsystems to the fiber optics, the BER contribution of these emerging technology devices in the space Single Event Effect (SEE) arena may outweigh that of the fiber optic components. We present data on several emerging technology devices and the implication of their usage in fiber optic systems in the space SEE environment.

Fiber Optic Experiment For The Shuttle Long-Duration Exposure Facility

This paper describes two complementary fiber-optic experiment packages that are under development for orbital exposure on the Shuttle Long Duration Exposure Facility. The intent of the experiment is to take the first step toward providing the experimental confidence and design data necessary for application of optical fiber technology on NASA spacecraft and military. satellites. Four active fiber links will be monitored at predetermined intervals on each experiment, and other components and fibers will be exposed passively. Results from the experiment will be presented in planned future publications.

Space-Environment Effects on Optical Cables

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.

Bit error rate measurement for evaluation of a fiber optic link

Digital fiber optic data link terminal modules are being developed in a number of laboratories, and often the modules take the form of black boxes without convenient access to internal signals. This paper discusses types of measurements which can be made using only the digital terminals of the link. Bit-error-rate (BER) vs signal power data can, of course, provide a means for proving overall link performance, but it can also be used to determine certain internal parameters of the receiver. The theoretical dependence of the BER vs light signal power curve shape on receiver input noise equivalent power and comparator slicing level is discussed. BER vs light signal power measurements obtained with an experimental link are presented and interpreted with the aid of the theoretical curves. The effect of other problems, such as hum or oscillation on the BER data, are mentioned. Sensitivity of the system BER to bit rate is also discussed, with application to determining system operating limits. Since BER measurements are simple to make and can easily be automated, they may prove to be a useful means for either laboratory evaluation or production-line acceptance testing of sealed terminal modules.

Power-over-fiber sensor network

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.

Theory of radiation-induced absorption in optical fibers

A new and simple mathematical model for describing radiation-induced absorption in optical fibers is presented. It treats the radiation-induced defect-generation and the decay process as a series of superposable infinitesimal growth and decay events. Unlike the existing power law growth equation, the new equation is non-empirical, dose-rate dependent, and describes the growth and decay of the induced defect at the same time. It is capable of predicting long-exposure, low-dose-rate induced fiber loss normally taking place in a space mission, using short-exposure, high-dose-rate results produced in a ground-based laboratory. Numerically, the derived equation is also capable of simulating those effects caused by irregular radiation events such as solar-flare radiation burst. In the case of constant dose rate, the general equation reduces to a simple analytical form which agrees reasonably well with the experiment.

Limitations Of Opto-Electronic Neural Networks

Neural networks are composed of three components: neurons, weights, and connections. Limitation on implementing these components in electronics and optics are discussed. We show that using electronically fabricated neurons and a combination of optics and electronics for the weights and connections efficiently utilizes the advantage of each technology. The choice of the technology used to implement the weights and connections depends on the type of neural network being built.