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

Transflective multiplexing of holographic polymer dispersed liquid crystal using Si additives

“…This indicates that two different gratings which are simultaneously formed with three beams disturb the formation of the other [23]. However, it is mentioned that the two gratings give similar diffraction efficiencies of about 35 (G12) and 22% (G13) though no exposure time scheduling has been used [20,22]. Diffraction efficiency shows a maximum at 0.8 (RS, MS) or 1.2 wt% (VTMS) due to the enhanced phase separation [24].…”

“…There are a number of multiplexing schemes to store many holograms in the same volume such as angular [12][13][14][15], peristrophic [16,17], shift multiplexing [18,19] or by more than one of these techniques in combination [20][21][22], which are used to store multiple holograms at the same location. In these methods, angular multiplexing is probably the most common in which the reference beam is incident upon the sample at difference angle to record each hologram.…”

Information capacity of 3-D holographic data storage

“…Wavelength multiplexed holograms are an alternative to angularly multiplexed holograms [13]. The basic geometry for a WDM storage system is sketched in Fig.…”

Electric-field multiplexing/demultiplexing of volume holograms in photorefractive media

“…Holographic data-storage systems typically use angular," 2 wavelength, 3 or phase-coded 4 ' 5 multiplexing. Of these, the first two techniques exploit the dependence of the Bragg condition on the angle and wavelength of the writing beams:…”