Optical processing using optical memories

The spatial- and wavelength-selective properties of volume holograms make it possible to overlap multiple holograms within the same volume, yet access them independently. By recording the interference fringes between coherent reference and object beams, information displayed on a spatial light modulator (SLM) can be densely packed into a photosensitive material. Reilluminating with the original addressing reference beam reconstructs a weak copy of the original data-bearing object beam, which can then be detected with a pixel-matched camera. Alternatively, these holograms can be read out with a new data-bearing object beam, performing optical correlation between the input and stored data pages. The volume nature of the holograms produces many optical correlations in parallel; however, one dimension of the 2-D correlation function is lost. This multi-channel optical correlation can be used to implement a content-addressable memory, in which an entire database is compared against an input pattern simultaneously. If the patterns displayed during storage represent the records of a digital database, and the input pattern represents a search query, then the content-addressable holographic memory becomes a parallel database machine. Exact searches through the digital database can be implemented holographically by positionally encoding the data into blocks of SLM pixels