Bias-free time-integrating optical correlator using a photorefractive crystal.

Time integration' has proved to be a powerful technique in optical signal processing and has been used in a wide variety of architectures. A major drawback of time-integrating processors is the buildup of bias in addition to the signal. This occurs because the photogenerated charge that is integrated on the detector is proportional to the intensity of the optical signal which makes it necessary to represent bipolar signals on a bias. The effective system dynamic range at the output is given by DR' = DR [SBR/(1 + SBR)] where DR is the dynamic range of the output detector and SBR is the signal-to-bias ratio on the detector.2 In most cases of interest, the SBR is much smaller than unity and thus the added bias significantly reduces the usable dynamic range of the system. The most frequently used method for separating the signal from the bias involves placing the signal on a spatial carrier and then electronically filtering the output of the integrator. This method of bias removal, however, does not solve the dynamic range problem since the processing is done after the detection of the signal. Also, an additional constraint is placed on the resolution of the detector, since the pixel separation must be less than one-half of the period of the carrier being recorded, which will result in a significant reduction in the available space-bandwidth product at the output. In this paper a new method for performing timeintegrating correlation is described using a photorefractive bismuth silicon oxide (BSO) crystal as the time-integrating element. The correlation is formed