Multiplexed phase–conjugate holographic storage using an intermediate buffer hologram

Volume holographic storage combines fast, parallel readout (because each hologram stores a large data page) with high density (because many holograms are multiplexed within the same volume). Phase-conjugate readout has been proposed as a way to eliminate the precision optics that recent demonstrations have relied upon to image the pixels of the input spatial light modulator (SLM) onto those of the output detector array. However, hologram multiplexing with the phase-conjugate approach requires multiple pairs of phase-conjugate beams, which are extremely difficult to create and maintain. We have developed a two–step recording process which combines the advantages of phase–conjugate holography with the simplicity of using the same multiplexed reference beam for recording and readout [1]. The data–bearing object beam first passes completely through a long storage crystal, and is then temporarily stored in a second holographic storage material. This “buffer” hologram is immediately read with a phase–conjugate reference beam, reconstructing a phase–conjugate object beam which travels back into the storage crystal. This new object beam can now be recorded, and then later reconstructed, with a multiplexed reference beam at any of the spatial storage locations. We describe the advantages and limitations of this technique, the materials requirements for the buffer hologram, and describe a test platform designed to implement this technique.