An optical information retrieval apparatus which utilizes the correlation detection function of a hologram to check coincidence matching between an interrogation signal and information recorded in a hologram memory is provided with an acousto-optic deflector serving as an input means for the interrogation signal. Both the interrogation signal and the hologram memory information are represented in the form of an M-out-of-N code, and identification of coincidence matching therebetween is effected on code-by-code basis. The interrogation signal to be digitally coded is applied through the acousto-optic deflector to an optical system for identification of coincidence matching. The acousto-optic deflector is driven by M different electric inputs having M different frequencies selected from N different frequencies so that a laser light beam incident onto the deflector is spatially modulated to simultaneously produce M deflected beams in M different directions determined by the M different driving frequencies, each of the deflected beams representing a '1' bit of an M-out-of-N code. The M deflected beams have differences in frequency corresponding to the differences between the individual driving frequencies, respectively, because upon deflection each of the deflected beams is deviated in frequency from the frequency of the incident laser beam by the amount of the corresponding driving frequency among the M driving frequencies. The M deflected beams representing an interrogation code are focussed onto the hologram to check coincidence matching between the interrogation code and the digitally coded hologram memory information. The result of coincidence matching occurs in diffracted light outputs from the hologram and is detected by photo diodes. The deflected light beams which were diffracted by the hologram are incident onto the photo diodes, and, consequently, due to an optical heterodyne phenomena, one or more beat outputs having one or more beat frequencies corresponding to the differences in frequency of the incident light beams can be obtained at the outputs of the photo diodes. If there exists coincidence matching between the interrogating and the interrogated codes, M different light beams having M different frequencies, respectively, impinge onto the photo diodes, while light beams having different frequencies in number equal to or less than (M-1) irradiate onto the photo diodes when no coincidence exists between the interrogating and the interrogated codes. Accordingly, for coincidence or non-coincidence the frequency components of the beat outputs differ, which means that the presence or absence of coincidence can be detected by analyzing the frequency of the beat oUtputs. When a 2-out-of-N code is used as an example of an M-out-of-N code, a beat output appears only upon existence of coincidence, rendering the simplest identification of coincidence matching.
Optical information retrieval apparatus