Diffraction Properties of Volume and Layered Photorefractive Gratings with Application to Holographic Storage

摘要

In the past few years there has been growing interest in photorefractive crystals because their unique optical properties make them excellent recording media for holograms. To record a hologram two beams, a reference beam and an information-carrying signal beam, are incident on the crystal. These two beams create an interference pattern that induces, through the photorefractive effect, a spatially-variant index of refraction. When a readout beam properly illuminates this recorded hologram, part of the incident light is diffracted such that, ideally, the diffracted beam is a perfect reconstruction of the original signal beam. The illumination condition that yields the optimum diffraction efficiency is called the Bragg condition. As soon as one moves away from the Bragg condition, for example, by changing the angle or the wavelength of the readout beam, by modifying the index of refraction of the crystal, or by inducing strains inside the crystal, the diffraction efficiency drops to almost zero. All these changes can formally be described by a single parameter Xi, known as the Bragg detuning parameter, such that, in most practical situations, the diffraction efficiency is proportional to (sin Xi/Xi)2 where Xi = 0 if the Bragg condition is satisfied. This selectivity behavior is the most important property of thick holograms and is the main reason why these photorefractive gratings are used in multiplexed holographic data storage and wavelength filtering. In the first part of this thesis, we review the fundamental principles and the basic physics of holography and of the photorefractive effect. In the second part of the thesis, we develop a general formalism to study Bragg detuning effects that takes into account electric field effects, temperature effects, polarization effects as well as changes in readout angle and wavelength.