Acousto-Optic Implementation Of Real- And Near-Real-Time Signal Processing

Abstract

Acousto-optic devices have been developed for implementing advanced signal processing functions required for real-time and near-real-time analysis of both high-frequency radar signals and low-frequency sonar signals. These devices use the Bragg interaction between laser beams and surface acoustic waves (SAWs) to perform waveform convolution and correlation, achieving time-bandwidth products of 3000 to 10,000 with dynamic bandwidths of several hundred megahertz. Using these devices, wideband (about 100 MHz) signals have been extracted from extremely noisy environments (-30 dB signal-to-noise ratio) and a real-time Fourier transformation with a linear dynamic range exceeding 60 dB has been performed. A programmable correlator and a programmable filter have been demonstrated that use the newly discovered acousto-photorefractive memory effect. This effect is based on a nonlinear interaction between intense, short-duration laser pulses permanent index-of-refraction pattern corresponding to the SAW signal. An implementation of the triple-product convolver architecture (proposed by Whitehouse et al.) for perform-ing either a long one-dimensional or a two-dimensional discrete Fourier transform can be obtained by combining an acousto-optic convolver with many CCD chirp-Z transform modules. This triple-product convolver would have extremely rapid data handling capability and large dynamic range, and would be useful for applications such as w-k beamforming for sonar signal process-ing. Further, a programmable 1-, 2-, 3-dimensional beamformer with a one second update capability may be feasible if the acousto-photorefractive memory effect is used to store SAW signals corresponding to the position of nonstationary sensors.