The role of adaptive photorefractive power limiting on acousto-optic RF signal excision.

摘要

Adaptive RF interference reduction for broadband communication systems continues to be problematic. The acousto-optic RF signal excision system offers real-time, narrowband RF interference, frequency-domain filtering for broadband communication systems, but has been limited in its capability for adaptive processing and rejection of broadband RF interference. As a means to reduce these limitations, this dissertation examines the application of a novel photorefractive optical power limiting device to achieve adaptive notch filtering, and multi-channel acousto-optic deflection to achieve angle-of-arrival signal discrimination at the notch filter.; This dissertation describes basic principles of acousto-optic RF signal excision, including linear phased array antenna, angle-of-arrival processing using a multi-channel acousto-optic deflector. The principles are verified experimentally by demonstration of frequency-domain filtering according to frequency and angle of arrival. This dissertation also describes basic principles of photorefractive optical power limiting, which is a new approach for achieving adaptive, frequency-domain notch filtering. Photorefractive field shielding of electro-optic birefringence is examined by numerical solution of the band-transport model, and experimental measurement of notch-filter amplitude and spatial profiles for high-intensity, Gaussian-beam optical illumination and high-intensity, externally applied electric fields. Results indicate that for illumination-beam radius larger than a few microns, power limiting notch widths follow the illumination intensity and terminate at the dark-to-light illumination interface where excess charge accumulates. Also, when sufficient light intensity is applied, a space-charge field develops that will completely screen the externally applied E-O modulator half-wave voltage, at which point only a very small diffusion field remains, and the notch-depth limit is the extinction ratio of the polarizer-crystal-analyzer combination.; The theoretical models of the optical power limiting device and acousto-optic RF signal excision system agreed with the experimental results to within 18% for the frequency-domain, notch-filter width. The results show that space and time excision of interference may be achieved, but the adaptive notch-filter width developed by high-intensity RF interference can overwhelm the angle-of-arrival resolution when small antenna arrays are used.