Optical processors employing feedback can perform operations which are difficult or impossible to achieve with conventional optical techniques. As in electronic systems, optical feedback can i) introduce versatility and additional degrees of flexibility in attaining a processing function ii) utilize gain to extend processing capabilities and iii) introduce new nonlinearities as well as enhance those inherent.
S. H. Lee,
"Feedback In Analog And Digital Optical Image Processing: A Review", Proc. SPIE 0202, Active Optical Devices, (30 January 1980); doi: 10.1117/12.958099; https://doi.org/10.1117/12.958099