Compact, integrated dynamic holographic memory with refreshed holograms

Abstract

An innovative architecture for compact, integrated volume holographic memories is described. It is based on phase-conjugate readout and on a modulator -detector -memory array implemented in a silicon integrated circuit. The lensless memory module sustains dynamic read-write holograms by periodic refreshing. The integrated circuit is described and experimentally characterized. Holograms were stored in a prototype storage module that uses a 30 ± -cut BaTiO 3 crystal and the 90 ± recording geometry. As many as three angularly multiplexed holograms were periodically refreshed and subjected to .40% decay from exposure to the reference beam over 50 to 100 cycles. Experimental data are presented. © 1997 Optical Society of AmericaThe use of counterpropagating reference beams for recording and readout in volume holographic memories permits compact storage modules 1 -3 with lensless signal-beam paths. The reference beam for readout is the conjugate of the reference beam for recording. We refer to this technique as conjugate readout. Figure 1 shows a compact module that uses angular multiplexing. The module is composed of a photorefractive crystal in which holograms are stored, a pair of liquidcrystal beam steerers (one of which is hidden behind the crystal) that is responsible for angularly multiplexing holograms in the crystal, and an optoelectronic integrated circuit (OEIC) that merges the functions of a ref lective spatial light modulator (SLM ) for recording holograms and a detector array for readout. One writes holograms by letting the signal beam modulated by the OEIC interfere with the plane wave coming from the back of the crystal. Holograms are read out by illumination of the crystal from the front with a counterpropagating plane wave, which is the conjugate of the reference wave used to write holograms. The resulting conjugate reconstruction self-focuses back onto the OEIC. The conjugate reference can be produced by a phase-conjugate mirror, 2 or it can simply consist of a counterpropagating beam in the case of plane waves.

Comparison of transmission and the 90-degree holographic recording geometry

“…The main advantages of the 90-degree geometry are insensitivity to holographic scattering and fanning 3,7,10 as well as the possibility of designing compact memory modules. 23 Strong fanning in sensitive singly-doped LiNbO 3 crystals has been a major obstacle in the implementation of holographic memories in transmission geometry. All the read-write memory systems demonstrated to date use the 90-degree geometry to avoid fanning, thereby sacrificing both M͞# and S. The holograms recorded by using both geometries are not persistent, i.e., they are erased during readout.…”

“…This allows the design of compact architectures for the holographic memory module. 23 It also makes the implementation of the phase-conjugate readout much easier compared to transmission geometry. 27,28 The choice of the recording geometry involves a trade-off between the larger capacity and sensitivity on the one hand and the architecture design on the other hand.…”

“…However, for the compact low-cost holographic memory module, the 90-degree geometry is a better choice at the expense of reduction in capacity and speed. 23 Fig . 13.…”

Volume Holographic Hyperspectral Imaging

“…Holography has been intensively investigated as a potential information storage technology [1][2][3][4][5][6][7][8] and for information-processing elements, 9 such as in neural networks, 10 -12 optical correlators, [13][14][15] optical interconnections, 16 -19 and diffracting elements. 20 -23 Optical information storage and optical information processing have two significantly different holographic philosophies.…”

Pixel size limit in holographic memories

“…1 This high density is a consequence of the wide recording bandwidth of the photorefractive medium and the phase-conjugate readout method. Figure 1 shows one possible implementation of the angle-multiplexed phase-conjugate holographic memory module 1 that consists of a spatial light modulator, a pixel-matched detector array, a photorefractive crystal, and multiplexing optical elements. Information is recorded by the interference between the spatial light modulator -encoded signal beam and the plane-wave reference.…”