<title>Feedback In Analog And Digital Optical Image Processing: A Review</title>

Abstract

introduce versatility and additional degrees of flexibility in attain-ing a processing function ii) utilize gain to extend processing capabilities and iii) in-troduce new nonlinearities as well as enhance those inherent.The evolution and classification of various optical processors with feedback were describ-ed in an earlier review paper.l Some of the new developments in this field in the last twoyears with special emphasis on the three operational characteristics mentioned above arediscussed here.In the first section, four separate systems are discussed, each demonstrating a particu-lar aspect of versatile processing. The first is a coherent system which uses feedback tocreate flexibility in filter selection, and to achieve a more accurate representation offilter functions.2. In the second system the phase of the feedback is used to obtain dif-ferent transfer functions with the same spatial filter.3 Processor three employs opticalorientation sub -systems in the feedback loop to achieve versatility in pattern recogni-tion.4 The fourth processor is an incoherent system which gains versatility from a combin-ation of temporal feedback characteristics with optical transfer function controls.5Several optical systems with feedback for performing nonlinear intensity transformationson images have been reported in the past. Although these systems differed considerably intheir structure, they all contained Fourier transforming optics. Some of them involved theuse of nonlinear optical materials. In the third section we will discuss three new feed-back systems for performing nonlinear operations useful in digital optical processing (eg.analog -to- digital conversion, thresholding, bistability, etc.). These three systems arenot based on modifying the Fourier transforms of the input images.The first system uses a plane parallel Fabry -Perot interferometer with the input imageinformation inside the resonator. Depending on whether the input image information is pro-vided as amplitude modulation or phase modulation, two different types of nonlinear opera-tions are obtained.8 The second system consists of two spatial light modulators with theirinputs and outputs cross -coupled optically. This system exhibits bistable characteristicsof a two -dimensional optical flip- flop.9 The third type of system is obtained by employingoptoelectronic feedback to an electrooptic light modulator. The nonlinear characteristicsof threshold and hysterisis are obtained, making the system useful as an optical latch forbinary images.10References