Optical data storage by using orthogonal wavelength-multiplexed volume holograms

Abstract

We propose a volume holographic data storage scheme that employs counterpropagating reference and image beams and wavelength multiplexing for page differentation.This method is compared with that based on angular multiplexing. A reduction in holographic cross talk is predicted. Further cross-talk reduction that is due to sidelobe suppression is observed in experiments by using photorefractive crystals and the proposed orthogonal data storage.Optical data storage in volume holograms has been an exciting prospect since the early days of holography.' This is due mostly to the theoretical storage capacity of -V/Al bits in a volume V. This, as an example, translates to 8 X 1012 bits in a 1-cm3 volume at A = 0.5 ,.m.Early attempts at exploiting this potential have been disappointing.t"10 Even though hundreds of holograms have been successfully stored in a single volume in these experiments, the data content of the individual holograms was minimal, and, therefore, so was the overall volumetric information density. This was due partly to material limitations"1", 2 but, more fundamentally, to the intolerably high cross talk resulting from the angular multiplexing method used to record the large number of holograms.'-' In this traditional method, many twodimensional images are recorded sequentially, each with its unique angularly multiplexed reference wave. Recently, however, phase-encoded or spatially orthogonal reference beams have been used to multiplex many holograms."31 4 These approaches, nonetheless, are variations of the basic angular multiplexing technique and differ fundamentally from the orthogonal wavelength-multiplexed storage technique proposed here.To appreciate the nature of the problem we recall that the basic property, and virtue, of holography is the essentially uniform distribution of the recorded information throughout the physical volume of the recording medium. We assert here the self-evident, but heretofore ignored, fact that it is equally important to distribute the information throughout the whereSuch a grating can be identified by the point K in KXIKZK space. If the wave E, is spatially modulated, say by image information, the corresponding K point becomes a surface whose extent (or solid angle) is determined by the smallest feature size of the information. Such a surface can be labeled by the reference wave used to record it and is shown as K(rl)ing in Fig. 1 along with the responsible kr$t. The same figure also shows the surface K Lting that is due to a second image recorded with a second, angle-multiplexed reference wave kr.2et Reconstruction of, say, image 2 by illuminating the stored holograms with a wave along kr(2} will inevitably lead to undesirable parasitic scattering

Modulating Optical Signal to Improve the Images by Using a Self Crossed Diffraction Grating

“…If the object has a phase information, reference beam must be introduce to interferes with the signal beam. The high-frequency fringe system was formed on a photographic plate [1][2]. If the object has only real amplitude variations, the amplitude diffuser [3][4][5][6][7] or amplitude diffraction grating [8][9][10][11][12][13][14][15] was used to modulate the signal to be stored.…”

Implementation of single-beam multiplexing encoding with a dually modulated spatial light modulator

“…On the other hand, due to the high-density storage capacity of a volume hologram and its parallel processing property, holographic data storage has become the most promising candidate to meet the challenge. To increase the data storage capacity, many techniques based on the Bragg diffraction condition have been proposed, including angular multiplexing [11], phase multiplexing [12,13], wavelength multiplexing [14], and shift multiplexing [15,16].…”

Optical Data Storage in Photosensitive Glasses and Spin State Transition Compounds

“…The physical mechanism is photochromism, which is defined as a reversible transformation of a single chemical species between two states that have different absorption spectra and refractive indices. This allows for holographic multiplexing recording and reading, such as wavelength (Rakuljic et al, 1992), angular (Mok, 1993), shift (Psaltis et al, 1995) and phase encoding. Another promising 3D optical data storage system is the bit-by-bit memory at the nanoscale (Li et al, 2007).…”