Acousto-optic adaptive signal canceller

Abstract

The conventional approach for implementing adaptive signal cancellers to reject radar noise is to build a separate electronic loop for each required degree of freedom. The process consists of cross-correlating the radar signal with noise samples received by wide-beamwidth auxiliary antennae. These correlation values are then used to adjust the noise waveforms in amplitude and phase so that a precise subtraction of the offending noise can be accomplished. The noise components can arrive reflected from various objects causing “multi-path” delay. Each resolvable time delay requires a new degree of freedom in the processing, and in many cases an even denser time sampling is required. In addition, if there are independent noise sources in the radar space, a new set of adaptive loops with multi-path capability must be provided for each of these sources. The result is often an expensively large number of electronic loops. An optical implementation of such a processor is described. The required operations of multiplication and time delay are provided by AO delay lines. The required time integration is provided by an image converter having a suitable decay time. In the optical realization, each resolvable optical element along the AO lines represents an additional delay time degree of freedom. Thus we find a “continuum of weights” developed spatially along the delay line. The optical dimension transverse to the direction of acoustic propagation can be used to provide another array dimension. Each additional noise source can be properly cancelled by providing another AO column in the optical aperture. Thus with an array of delay lines in the aperture, a single optical adaptive system can be equivalent to many hundreds of electronic control loops. In this paper, the details of the configuration will be described. An analysis of the system will be presented which will investigate effects of the system parameters and the loop gain on cancellation performance. An experimental effort to implement a feasibility model of this system will be described, and the results presented.The conventional approach for implementing adaptive signal cancellers to reject radar noise is to build a separate electronic loop for each required degree of freedom. The process consists of cross-correlating the radar signal with noise samples received by wide-beamwidth auxiliary antennae. These correlation values are then used to adjust the noise waveforms in amplitude and phase so that a precise subtraction of the offending noise can be accomplished. The noise components can arrive reflected from various objects causing “multi-path” delay. Each resolvable time delay requires a new degree of freedom in the processing, and in many cases an even denser time sampling is required. In addition, if there are independent noise sources in the radar space, a new set of adaptive loops with multi-path capability must be provided for each of these sources. The result is often an expensively large number of electronic loops. An optical implementation of such a processor is described. The required operations of mult...