Optical signal processing by filtering fresnel images of acoustic light modulators

Spatial filtering of the instantaneous interference pattern displayed by an ultrasonic light modulator (ULM) in a plane of its Fresnel diffraction field was recently devised as a means of achieving real-time optical signal processing. This approach has been used to obtain the correlation of an electric signal, propagated in a ULM, by filtering a Fresnel image of the acoustic wave through an amplitude replica of the expected waveform, recorded on an optical mask. Careful investigation of the ULM interference patterns is needed, however, since Fresnel images vary with the distance from the ULM, and, accordingly, the correlator response critically depends on the ULM-signal replica separation. In this paper the interference patterns formed by a ULM fed with a linear FM signal are analyzed, and the performance of the device is investigated by computing the correlation response as a function of the ULM-replica separation.