Invited Paper Optical Resonators For Associative Memory

Invited PaperOptical resonators for associative memoryDana Z. AndersonDepartment of Physics, University of Colorado, andJoint Institute for Laboratory AstrophysicsUniversity of Colorado and National Bureau of StandardsBoulder, Colorado 80309 U.S.A.AbstractOne can construct a memory having associative characteristics using optical resonatorswith an internal gain medium. The device operates on the principle that an optical reso-nator employing a holographic grating can have user prescribed eigenmodes. Informationthat is to be recalled is contained in the hologram. Each information entity (e.g. animage of a cat) defines an eigenmode of the resonator. The stored information is accessedby injecting partial information (e.g. an image of the cat's ear) into the resonator. Theappropriate eigenmode is selected through a competitive process in a gain medium placedinside the resonator. With a net gain greater than one, the gain amplifies the field be-longing to the eigenmode that most resembles the injected field; the other eigenmodes aresuppressed via the competition for the gain. One can expect this device to display sever-al intriguing features such as recall transitions and creativity. I will discuss some ofthe general properties of this class of devices and present the results from a series ofexperiments with a simple holographic resonator employing photorefractive gain.IntroductionAs digital serial processing general purpose computers find their limits in speed andcomputational power, computer architects are turning to special purpose interconnectedmulti- processor machines. New device technology will play an essential role in the de-velopment of these new architectures. It seems inevitable that light will play a profoundrole in future generations of sophisticated computers.Optical processors can access the inherent parallel interconnect characteristics of op-tical components without being limited by the essentially two -dimensional topology of thesilicon wafer. There has been a recent upsurge of interest in the kinds of processing inwhich optics should excel. The application I will discuss here is that of associativememory.An associative memory is one in which a body of information is retrieved from storageby a small part of that body. The recall process associates the small amount of informa-tion with the larger whole. One might, for example, have stored in memory images of acat, a dog, and a mouse. One retrieves the image of the cat by entering an image of onlyits ears. I've used images in the example but the stored contents could be data or infor-mation of any type.The attraction of associative memory for computing applications lies in its resiliencyagainst device element failure and its power in certain kinds of problems such as patternrecognition and signal processing. However, the interest in associative memory does notstem only from the computing community. Models for associative memory exhibit many humanmemory -like characteristics. For example, under the right circumstances they can forgetand make mistakes (associate things that were not intended to be associated). Furthermoreas the elements composing the memory structure become decayed or destroyed, the memory be-comes more or less senile, rather than completely inoperative as is usually the case withthe silicon device. Thus interest in this field has been generated by the neurophysiologycommunity as well, where there is a desire to better understand human memory.Associative memory is by no means a new field. Pioneering contributions have been madeby many, in particular by T. Kohonen of the University of Helsinki and by J. Anderson ofBrown University. 1,2 The current surge of interest on the part of the physics and opticscommunities, however, seems to have been stimulated by some recent work by J. Hopfield atCaltech.3 An electrooptic implementation of associative memory based upon Hopfield's workhas been realized by Psaltis and Farhat.4 The concept discussed below represents a con-siderable departure from previous models; it nonetheless should exhibit the behavior ofthe generic associative memory.