Abstract

One kind of optical tracking novelty filter is a phase-conjugate Michelson interferometer with each (two) arm having different time responses. The minimum detectable velocity of this device depends on the difference in response time between the two arms. The conventional scheme uses one photo refractive crystal to serve as two phase-conjugate mirrors, so that the identity of the phase shifts of the two phase conjugates is ensured. The limited difference in response time comes from the disparity of the intensity of the interacting beams and fringe spacing; it is difficult to achieve low minimum-detectable velocity. In this paper, we propose that different photorefractive crystals are used in each arm. By doing this, a substantial difference between response times can be obtained and the minimum detectable velocity can be greatly reduced so that the system is able to process a low-speed moving or time-varying object, without compromising the response time of the whole system to changes in input image. By adjusting the intensity and the phase shift of the readout beam of the crystal which yields a stronger phase-conjugate output, the phase shifts and amplitudes of the two phase conjugates are equalized. The response of such a system to time-varying objects is studied and the analytic expression of the relationship between object and output as well as factors determining the maximum and minimum detectable velocity are presented. Experiment results show that the minimum detectable velocity of this method can be three times lower than that formerly reported.

© 1993 Optical Society of America