Fast update of dynamic photorefractive optical memory

Nondestructive readout of a photorefractive hologram by phase-conjugate copying in a one-crystal configuration

We propose a novel phase-conjugate copying method for nondestructive readout of a volatile photorefractive hologram. In the one-crystal configuration, two holographic memories and a mutually pumped phase conjugator (MPPC) are formed within a single photorefractive crystal, instead of using multiple crystals. Two memories share the same hologram and complement each other in refreshing the hologram. A MPPC suppresses fanning noise and automatically aligns the wavefront of the reference and readout beams. We found the optimum configuration to achieve nondestructive readout from calculations and geometric consideration. In the experiments with a BaTiO(3) crystal, a continuous readout of 20 times longer than the recording time was achieved.

Selective erasure of speckle-multiplexed holograms by use of a double Mach-Zehnder interferometric arrangement

A novel method to selectively erase and update speckle-multiplexed holograms in photorefractive crystals by use of a double Mach-Zehnder (DMZ) interferometric arrangement is presented. The DMZ arrangement automatically produces a pair of pi-phase-shifted interference patterns used for holographic recording, erasure, and update operations with a fairly simple optical configuration that consists of a commonly used dielectric multilayer beam splitter and two mirrors. The recording and the erasure conditions required for erasing a photorefractive hologram quickly and completely are discussed by calculating the temporal property of the hologram buildup and decay using the time-dependent coupled-wave equations. An experiment is also performed, in which arbitrary holograms in speckle-multiplexed holograms are selectively erased and updated with the simple DMZ optical configuration.

Dynamic digital photorefractive memory for optoelectronic neural network learning modules

Neural network modules based on page-oriented dynamic digital photorefractive memory are described. The modules can implement two different interconnection organizations, fan-out and fan-in, depending on their target network applications. Neural network learning is realized by the real-time memory update of dynamic digital photorefractive memory. Physical separation of subvolumes in the page-oriented photorefractive memory architecture contributes to the low cross talk and high diffraction efficiency of the stored interconnection weights. Digitally encoded interconnection weights ensure high accuracy, providing superior neural network system scalability. Module scalability and feedforward throughput have been investigated based on photorefractive memory geometry and the photodetector power requirements. The following four approaches to extend module scalability are discussed: partial optical summation, semiparallel feedforward operation, time partitioning, and interconnection matrix partitioning. Learning capabilities of the system are investigated in terms of required interconnection primitives for implementing learning processes and three memory-update schemes. The experimental results of Perceptron learning network implementation with 900 input neurons with digital 6-bit accuracy are reported.

Sparse-wavelength angle-multiplexed volume holographic memory system: analysis and advances

We present and analyze our hybrid wavelength-and-angle-multiplexed volume holographic memory system. The hybridization of wavelength and angle multiplexing relaxes demands on spectral-tuning sources, angle-tuning devices, and optical system numerical apertures while maintaining a large K-space addressability. We consider realistic properties of our volume holographic memory system, addressing practical issues such as storage density and material-dependent photon-limited information throughput. Finally, we present experimental results of the storage of 2000 sparse-wavelength angle-multiplexed volume holograms in a 1.86-cm3 volume of lithium niobate.

Spatiotemporal dynamics of self-pumped phase conjugation in a photorefractive crystal: multipattern storage using incremental recording

We investigate the spatiotemporal dynamics of self-pumped phase conjugation for superimposed incrementally recorded images. Experimental evidence of time delay in the appearance of phase-conjugation patterns was observed during both the recording and the readout phases.

Optical parallel-access shared memory system: analysis and experimental demonstration

An optical implementation of a parallel-access shared memory uses a single photorefractive crystal and can realize the set of memory modules in a digital shared memory computer. This implementation uses two arrays of sources that are individually coherent but mutually incoherent from region to region across each array, and it permits incoherent/coherent double angular multiplexing of data in the crystal. A complete instruction set for its memory access consists of four operations, READ, WRITE, SELECTIVE ERASE, and REFRESH, which can be applied to any memory module independent of (and parallel with) instructions to the other memory modules. In addition, a memory module can execute a sequence of READ operations simultaneously with the execution of a WRITE operation to accommodate differences efficiently in optical recording and readout times common to optical volume storage media. An experimental shared memory system demonstrates two memory modules, each consisting of up to two 5-bit data blocks, implemented in a single Fe:LiNbO(3) crystal. The projected performance of the optical parallel-access shared memory system is analyzed and compared with conventional page-addressed volume holographic memories.