Photorefraction is a nonlinear optical effect where large nonlinearities can be observed at relatively low optical power levels. This phenomenon has evolved from being just a laboratory curiosity to being a useful device technology for optical information processing applications. In particular, the abilities to form holograms in real time and also store them for extended periods of time have aided the implementation of several pattern recognition machines which are adaptive and possess a high degree of parallelism. This paper reviews the photorefractive effect as a device technology for the problem of pattern recognition and describes several photorefractive realizations of pattern classifiers.
"Photorefractive devices for optical pattern recognition", Proc. SPIE 10262, Optical Pattern Recognition: A Critical Review, 102620B (1 April 1992); doi: 10.1117/12.59855; https://doi.org/10.1117/12.59855