Acousto-optic two-dimensional coherent optical modulator

Abstract

Outputs of a linear phased array antenna can be employed for emitter field sorting, i.e., providing an indication of the frequency and angle of arrival of signals from a plurality of radiation sources, through the use of a two-dimensional optical processor that does not require a mechanism for correcting for acoustic spreading in a multi-channel Bragg cell. In effect, the multi-channel array of transducers of a Bragg cell forms a composite N-channel transducer the width of which is N times the spacing between channels. For a CW radiation source, the signal received by each array element undergoes an incremental phase shift associated with the tilt of the phase front across the array. When these signals are used to drive respectively adjacent transducers of a multi-channel Bragg cell, the acoustic effect within the bulk is equivalent to that of driving a large composite transducer with a constant frequency signal. This larger size of the effective composite transducer provides significantly reduced divergence of the composite acoustic wave, while the linear phase shift across the transducer results in a tilting of the composite acoustic wave. The two-dimensional acoustic wave modulation of a light beam passing through the Bragg cell is then imaged in the transform plane of a downstream Fourier transform lens, so as to produce a two-dimensional Fourier transform of the Bragg cell. In the transform plane there is provided a frequency display in the horizontal (parallel to the direction of travel of acoustic waves in the Bragg cell) direction and an angle of arrival (azimuth angle) display in the vertical direction. Multiple radiation sources at different locations may be processed independently through superposition.