eprintid: 10104455
rev_number: 8
eprint_status: archive
userid: 695
dir: disk0/10/10/44/55
datestamp: 2020-07-08 15:39:32
lastmod: 2020-07-08 15:39:32
status_changed: 2020-07-08 15:39:32
type: thesis
metadata_visibility: show
creators_name: Tao, Shiquan
title: Spatio-angularly multiplexed (SAM) holographic storage in photorefractive crystals
ispublished: unpub
keywords: Pure sciences; Photorefractive crystals
note: Thesis digitised by ProQuest.
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.
date: 1993
oa_status: green
full_text_type: other
thesis_class: doctoral_open
thesis_award: Ph.D
language: eng
thesis_view: UCL_Thesis
primo: open
primo_central: open_green
verified: verified_manual
full_text_status: public
pages: 239
institution: UCL (University College London)
department: Electronic and Electrical Engineering
thesis_type: Doctoral
citation:        Tao, Shiquan;      (1993)    Spatio-angularly multiplexed (SAM) holographic storage in photorefractive crystals.                   Doctoral thesis  (Ph.D), UCL (University College London).     Green open access   
 
document_url: https://discovery-pp.ucl.ac.uk/id/eprint/10104455/1/Spatio-angularly_multiplexed_%28.pdf