Computer generated holograms with continuously varying grey level. Application to the synthesis of holographic memory

Facilities of computer generated Fourier holograms are envisioned with the synthesis of a computer holographic memory in view. The computing process described is applicable to a class of objects made up of binary points. A relatively small number of samples is used, and this without any phase encoding. A double threshold amplitude saturation is performed on the Fourier transform, thus optimising the efficiency and S/N ratio, both studied on a theoretical model and by numerical simulation. Non linearities are finally cut out by a very simple filtering process implemented in incoherent light, the result being synthesis of a continuously varying grey level function. A 3 x 3 mm Fourier hologram of a 64 x 64 binary point object restores a highly acceptable image. Nevertheless the photoreduction is slow and limits storage density. A direct point by point discretely recorded hologram is proposed. For this purpose, we describe two alternate systems of a real-time computer-controlled optical microprinter. Using the above methods, characteristics are outlined for computers holographic memories.