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The authors have described the design and demonstrated the performance of two types of lensless array generator. The Self-focusing Fresnel-Dammann grating (SFD), and the Fresnel hologram. The SFD is based on a conventional Dammann grating, but obviates the need for extraneous Fourier transform lenses, by combining the grating with a Fresnel zone plate, in an exclusive-OR fashion. No alignment is required and no bulky or dispersive refractive elements are used. The Fresnel hologram can be designed for high fanout, but the design must take into account the phases of the light spots in the image array. The authors have demonstrated this for an 8times8 fanout element, where a set of random phases in the output gave rise to a satisfactory performance, whilst another set of phases gave an undesired, although interesting, result. The authors have also demonstrated a 30deg slanted axis, random phase design, generating a two dimensional array of 8times8 beams, used in reflection, and operating in a multiple reflection, planar-optic configuration

Most neural networks proposed for pattern recognition sample the incoming image at one instant and then analyse it. This means that the data to be analysed is limited to that containing the noise present at one instant. Time independent noise is therefore, captured but only one sample of time dependent noise is included in the analysis. If however, the incoming image is sampled at several instants, or continuously, then in the subsequent analysis the time dependent noise can be averaged out. This, of course, assumes that sufficient samples can be taken before the object being imaged, has moved an appreciable distance in the field of view. High speed sampling requires parallel image input and is most conveniently carried out by optoelectronic neural network image analysis systems. Optical technology is particularly good at performing certain operations, such as Fourier Transforms, correlations and convolutions while others such as subtraction are difficult. So for an optical net it is best to choose an architecture based on convenient operations such as the high order neural networks.

An optically addressable spatial light modulator comprises an electrically conducting dielectric mirror 3 with a phototransistor array 1 at one face and a semiconductor light modulator 2 at the other face. The light modulator may be a quantum well structure or a bulk semiconductor Franz-Keldysh modulator. The spatial light modulator is preferably formed of layers of gallium arsenide and gallium aluminium arsenide. An optically addressable spatial light modulator comprises an electrically conducting dielectric mirror 3 with a phototransistor array 1 at one face and a semiconductor light modulator 2 at the other face. The light modulator may be a quantum well structure or a bulk semiconductor Franz-Keldysh modulator. The spatial light modulator is preferably formed of layers of gallium arsenide and gallium aluminium arsenide.

Ieee Journal of Quantum Electronics