Recording and reading synthetic holograms

Abstract

A byte of binary data to be stored is recorded as a synthetic Fourier transform hologram of the byte of data. A time varying control signal representing the synthetic hologram is used to intensity modulate a coherent light beam as the beam scans transversely across a photosensitive recording film. The amplitudes of different spatial frequencies in the data band are differently altered to compensate for signal to noise roll-off over the data band. The thus compensated signals are also modified to compensate for nonlinear gain characteristics. The thus modified signals are processed to provide the modulating control signal. During the readout process an inverse Fourier transformation is performed optically on the light diffracted by the synthetic hologram in order to produce the original byte of data in the form of an optical intensity pattern. This pattern is transversely distributed on an array of photosensitive detectors which converts the optical intensity pattern to an electrical data signal. During this readout process a tapered neutral density filter, i.e. a wedge filter, is employed in front of the photosensitive detectors to provide attenuation or gain across the spatial frequency bandwidth to compensate for system modulation transfer function.