Real-time holographic display devices for image projection and spatial light modulation

Clapham, Sarah Louise (1991) Real-time holographic display devices for image projection and spatial light modulation. University of Southampton, Doctoral Thesis.

Abstract

Liquid crystal and photorefractive materials and devices are investigated for application to holographic storage, real-time holographic image projection and display, and spatial light modulation.

The material requirements and methods of twisted nematic liquid crystal (LC) cell fabrication are presented, and associated problems and characteristics are discussed. The technique of photolithography is used to produce high quality, well-defined electrode patterns on cell substrates for the construction of LC devices to demonstrate controlled modulation of laser light. These technologies are combined to produce miniature, pixel-lated LC screens for use as electrically addressable spatial light modulator (LC SLM) devices. Results of the use of three LC SLMs with varying pixel dimensions in image display, and as updateable holographic recording devices in the real-time reconstruction and display of holograms are presented.

The characteristics of a technique for storing holograms in crystals of bismuth silicon oxide (BSO) are described that combines reversible pho-tochromic effects with the more usual real-time photorefractive properties of the material. Photochromatic effects have been observed before in these sillenite crystals but have generally been considered as a problem rather than as a possible mechanism for holographic storage. As the normal photorefractive behaviour is unaffected, simultaneous spatial multiplexing of both photorefractive (real-time) and photochromic (permanent) holograms is possible in the same crystal volume.

Several applications are demonstrated ranging from image synthesis to holographic interferometry. These operations arc based on the inherently separate nature of these two holographic gratings which can lead to relative phase shifts between the two simultaneously scattered fields by shifting the real-time grating with respect to the permanent one, using optical techniques.

Permanent photochromic holograms recorded in BSO also show unexpected dynamic behaviour in which a fast increase in diffraction efficiency is observed upon illumination by a beam at the highly absorbed blue wavelengths. A novel scheme for spatial light modulation based on the spatial enhancement of the diffraction efficiency of photochromic holograms stored in BSO is demonstrated. The enhancement effect is dependent on a number of parameters, for example, crystallographic orientation, intensity and wavelength of the illuminating beam, angle of polarisation of the holographic readout beam, and externally applied high electric fields. These observations are consistent with the formation of a secondary, or parasitic, photorefractive grating upon the illumination of the permanent hologram by the additional blue beam. A theoretical model combining the effects

of interference between contributions of diffraction from these two separate gratings (permanent and induced photorefractive) with different phase and polarisation properties is shown to be in good agreement with the experimental data.

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