Processeurs Parallèles Optoélectroniques Stochastiques pour le traitement d'images en temps réel

Abstract

The purpose of this work is to highlight the interest in using dedicated optoelectronic stochastic parallel processors (OSPP) within real-time low-level image processing systems. An OSPP (a sort of Smart Pixel Array sometimes called stochastic artificial retina) is composed of a (regular) matrix of simple processing elements (PE) whose binary state depends on a short-range neighborhood. The update of each processing element is made stochastic thanks to a laser speckle based random number generator. A first VLSI prototype was built on standard silicon technology (CMOS 0,8 μm), providing mesh-like electronic linking between PEs. The optical interconnection issue was explored afterwards and showed it to be a powerful tool for providing both easy extension and rearrangement of the interconnection pattern. Experimental results using a compact prototype demonstrator (21x35x14 cm3) based on a silicon chip, two spatial light modulators and a convolution setup relying on interchangeable Dammann gratings (in charge of the interconnection pattern) enabled us to simulate stochastic relaxation in a lattice of spins (2-D Ising model), and then successfully demonstrate cleaning noise on binary images and motion detection in a sequence of gray level images using stochastic optimization techniques (simulated annealing). Further results on this topic are reported in a separated paper [Cassin01], where a discussion about the advantages of using a hybrid CMOS/SEED Smart Pixel Array to monolithically integrate photodetectors and modulators on the same chip, providing compact, high bandwidth intrachip optoelectronic interconnects can be found. Modelling of this device clearly showed system performance improvement (the prototype should be able to process more than ten thousands images per second while 2 to 5 seconds where needed to process each on the previous prototype). Equally important is the fact that the size of the whole system would be compatible with the standards of electronic packaging technologies.