Abstract

Previous photorefractive holographic filters are limited by the response time of the material. The time correlation filter recognizes a sequence of individual temporal features which are limited only by the speed of the phase modulators in the reference and input signals. This filter operates using photorefractive time-integrated degenerate four- wave mixing with a reference beam and signal beam incident on one side of the bismuth silicon oxide crystal and an incoherent counter-propagating beam to read out the time correlation hologram. When no modulation is applied to the signal, the phase variations in the reference beam erase the hologram. Experimentally measured signals include square wave phase and amplitude modulation for the detection of similar signals, constant longitudinal velocity signals, and periodic transverse signals. The time correlation filter performs a special form of correlation of temporal signals which is not linearly proportional to the modulation. For example, the velocity detection output is strongest at the reference encoded velocity but also gives smaller peak responses at odd integral multiples of the velocity. The device operation agrees with theory developed from time varying photorefractive theory for general signals.

© 1990 Optical Society of America