UCL Discovery
UCL home » Library Services » Electronic resources » UCL Discovery

Spatio-angularly multiplexed (SAM) holographic storage in photorefractive crystals

Tao, Shiquan; (1993) Spatio-angularly multiplexed (SAM) holographic storage in photorefractive crystals. Doctoral thesis (Ph.D), UCL (University College London). Green open access

[img] Text
Spatio-angularly_multiplexed_(.pdf

Download (14MB)

Abstract

In this thesis a novel multiplexing scheme for dense holographic storage in photorefractive crystals, Spatio-Angular Multiplexing (or SAM), is described in detail. In SAM Fourier transform holograms are formed in spatially overlapping regions of a crystal and are distinguished from one another by using variously angled reference beams. SAM takes advantage of both the high storage density possible using angularly multiplexed volume holograms and also the low crosstalk possible using spatially multiplexed Fourier transform holograms. Compared to pure spatial multiplexing, SAM increases the storage capacity by fully utilising the volume of the storage medium. On the other hand, SAM reduces the number of holograms overlapping any one hologram in a given volume, and so increases the diffraction efficiency achievable as compared to pure angular multiplexing. SAM offers the possibility of incorporating the recorded crystal into a content addressable memory (CAM) system for parallel access of all stored patterns. In order to obtain the maximum diffraction efficiency and signal to noise ratio, the hologram must be replayed by a readout beam incident at the correct angle of readout beam. The optimum angle may be shifted away from the angle used in recording by a ''Bragg-shift", caused (under certain conditions) by phase coupling between the two writing beams during recording. Although this Bragg shift is small, a large diffraction efficiency enhancement is obtained when the grating is read out at the optimum angle. We have calculated the Bragg shift, using a numerical calculation based on an earlier theory, and have obtained good agreement with experiment. Using the novel SAM scheme, we have succeeded in storing 756 high resolution binary patterns in an Fe:LiNbO3 crystal of volume 1cm3, with an average diffraction efficiency of 0.5%. This large database is designed for practical use in a novel associative memory system, called a high order feedback neural network (HOFNET). We also present the readout scheme required for parallel access of the information stored within the crystal. This is essential for operation of the HOFNET.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Spatio-angularly multiplexed (SAM) holographic storage in photorefractive crystals
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: Pure sciences; Photorefractive crystals
URI: https://discovery.ucl.ac.uk/id/eprint/10104455
Downloads since deposit
17Downloads
Download activity - last month
Download activity - last 12 months
Downloads by country - last 12 months

Archive Staff Only

View Item View Item