Chang, Chi Ching; (1997) Optical photorefractive phase-conjugate mirrors and resonant systems. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

This thesis describes and assesses experimentally novel configurations for various types of phase conjugation in the barium titanate crystals and novel designs for optical resonant systems with photorefractive phase-conjugate mirrors (PCMs) and holographic memory for use in optical associative memories (OAM). The thesis describes 3 novel self-pumped phase-conjugate mirror (SPPCM) configurations and 4 novel mutually pumped phase-conjugate mirror (MPPCM) configurations and 1 novel optically switchable phase- conjugate mirror with several beams conjugated over a wide range of angles. A novel compact 2-f coherent correlator design for use in optical neural networks (ONNs) is described. The thesis describes how to record improved efficiency volume holograms in a novel setup using 2 BaTiO3 phase-conjugating mirrors (PCMs). The thesis also describes our discoveries which is the extending the earlier research began by Dr. Tao of UCL on the improvement of holographic diffraction efficiency (DE) possible arising from Bragg shifts occurring during recording. The thesis presents experimental work on the dependence of DE on the incident beam intensities and the beam ratio. The thesis describes a novel optical resonator in which optical eigenmodes are established between SPPCMs. This is the first time to our knowledge any resonator has been demonstrated and reported between 2 PCMs. In addition 21 angularly multiplexed volume holograms were inserted into the resonator to give 21 clearly defined eigenmodes or memories. The optimum results were obtained for our "-c curve configuration" in which a SPPCM efficiency of 30[percent] was obtained and phase conjugation was maintained over a range of 4mm in lateral translations of the beam and over a range of 40 degrees in the incident angle of the beam. We have proposed a simple model with Four Interaction Regions (FIR) to fiirther account for our discoveries. The "Arch", "Fish-Head", "Fish" and "Manta Ray" configurations are our four new configurations that we have discovered for MPPCM of two incoherent beams which are all remarkably insensitive to angular and lateral positional changes in either of the two input beams or in the crystal itself. The four configurations are also notable in that they have high reflectivity efficiencies and fast response times. These three configurations are part of a family of MPPCMs all of which have similar internal beam paths within the crystal having three internal reflections. The phase conjugate behaviour of the Fish-Head is established relatively fast ([tilde]1sec) and has a high reflectivity efficiency (30[percent]) and no crosstalk is observable between the two beams.

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