Compact lens-less holographic memory

A common problem of many holographic memory systems is that they must occupy relatively large areas in order to accommodate the various imaging optics necessary for system operation. A typical memory system requires an imaging lens system between the input and output devices, such as a spatial light modulator (SLM) and charge-coupled device (CCD). The memory system we describe here integrates the various system components very closely to the crystal by using a smart pixel array and making use of phase-conjugate read-out to eliminate the need for imaging optics. This significantly reduces the alignment requirements and the number of required components. Our angularly multiplexed integrated read/write memory system is shown. An optoelectronic integrated circuit, which we refer to as the dynamic hologram refresher (DHR), functions as both a spatial light modulator and a detector array. Liquid crystal beam steerers are attached to the two faces of the crystal perpendicular to the signal beam path. These beam steerers address the angularly multiplexed memory. Holograms are recorded by the interference of the signal beam (from the DHR) with the forward reference wave entering the top face of the crystal; when writing holograms, the DHR acts as a spatial light modulator. The counter-propagating reference wave (illuminating the crystal from the bottom) is employed to reconstruct holograms. The reference beam is steered into either the forward or conjugate paths by means of a twisted-nematic liquid crystal (TNLC) device which switches the polarization in the reference arm.