Photorefractive processes at near infrared wavelengths
This thesis presents the results of a three year study of the properties and applications of photorefractive materials. Attention is focused in particular on barium titanate, and experimental work examines the photorefractive processes and optical characteristics at near infrared wavelengths around 800nm - an important wavelength range compatible with solid state laser diodes. A comparison of properties is made between crystals of nominally undoped barium titanate and samples with impurity enhanced absorption in the red and near infrared which appear blue in colour. Throughout the thesis, photorefractive theory is developed to explain the experimental observations, and beam coupling experiments demonstrate energy exchange between laser beams within the photorefractive crystal in order to establish model parameters. Experiments investigating optical phase conjugation using the blue crystals demonstrate effective self-alignment and distortion correction, with high fidelity and good temporal and frequency stability. Wide wavelength response, laser diode compatibility, low power operation and highly efficient reflectivities of up to 76% at 1µm and 38% at 1.064µm make the new blue crystals promising for near infrared applications. Throughout, comparisons are drawn with the undoped crystal. Brightness enhancement schemes are also considered and a phase conjugate fibre laser demonstrated. Coupling of mutually incoherent laser beams is also reported at near infrared wavelengths with the demonstration of a self-aligning holographic interconnect behaving as a phase conjugate mirror with an effective reflectivity of 600%. Using laser diode sources, the wavelength tolerance of the coupling scheme is investigated and the effect of feedback during coupling examined.