Phase-insensitive hologram readout technique

Abstract

A hologram-containing storage medium is rapidly scanned or swept by a sinusoidal spatial fringe pattern, the spatial frequency of which varies with time. This may be achieved by the use of a suitable light beam deflection system and frequency shift arrangement disposed up-stream of a focussing lens. The deflection system and frequency shift arrangement may incorporate a pair of light beams, such as those obtained from a laser, a source of coherent monochromatic light. A single deflector-frequency shifter, such as an acousto-optic element and an electro-mechanical deflector may be disposed in the path of one of the beams, with a prescribed carrier modulation frequency applied to shift the frequency of that one beam relative to that of the other beams. The modulated beam is deflected and is then directed along with the unmodulated beam through the focussing lens so that the beams combine with each other to form a read-out interference pattern on the hologram. Alternatively, a respective frequency shifter, such as an acousto-optic modulator may be disposed in the path of each of the pair of beams and a separate modulation frequency signal applied to each modulator. The frequency difference between the two signal sources represents the temporal frequency carrier component of the interference pattern. The beams are also deflected relative to one another and the light which is then diffracted by the hologram impinges upon a single photo-detector, whereby a signal representative of the original data, together with the imparted modulation components, is derived. The temporal and spatial modulation components introduced in the signal beam by the frequency-shift, deflection system are then removed, to obtain only the original data. In accordance with the invention, a constant scan frequency may be used or the frequency shift of a light beam may be caused to vary with time (e.g. .omega.(t)=kt) so as to effectively create a travelling optical chirp. In each case, the interference pattern created on the hologram for reading out the data will contain a translating sinusoidal fringe system with a time varying spatial frequency, and the output of the photo-detector represents the integral of the product of hologram transmissivity and the illumination intensity pattern, which may then be demodulated to obtain the original data.