The laser as nonlinear element for an optical associative memory

We discuss an optical associative memory architecture, which combines a nonlinear part - the laser - and a linear part formed by lenses, holograms and a pinhole mask. The laser works as the discriminator element in the system, and operates in a regime of spatial multistability, i.e. of coexistence of different stable stationary states. Numerical calculations in the simplest situations indicate that the laser is able to decide which of its stationary states is most similar to a field pattern that is injected into the laser itself. The memory is constituted by a certain number of images which are stored in one of the holograms and are in one-to-one correspondence with the stationary states of the laser. The task of the linear part is (i) to convert an arbitrary image, which is offered to the system, into an appropriate field pattern which is injected into the laser and (ii) to convert the stationary beam, which emerges from the laser, into the corresponding image in memory. We show that the system as a whole works as an associative memory. We illustrate some experimental results, obtained using a HeNe laser, which represent an initial step in the direction of a realization of this system.