Photorefractive Higher-Order Diffraction Properity And Phase-Code Multiplexed Holographic Storage

Photorefractive effect was discovered in 1966 by A. Ashkin et al in Bell Laboratory when frequency double experiments was done with the inorganic crystals LiNbO₃ and LiTaO₃. The refractive index of the crystal was changed by the incident light. After analyzing it more deeply, the Photorefractive effect is recognized to be significant for holographic data storage and information process.In this thesis, we study the higher-order diffraction properties of photorefractive materials. The relationship between the changes of refractive index and diffraction light intensity is given. The maximum variation of the refractive index versus the relative intensity of diffraction light is discussed using Raman-Nath theory in two-wave coupling experiment at small incident angle With and without the phase transfer, the distributions of diffraction light intensity and angle position are presented when the probe light illuminates the photorefractive materials with different input angles and wavelengths. In two-wave coupling, the intensity of higher-order diffraction is derived with ignoring the absorption. The fluctuation of higher-order diffraction intensity is discussed theoretically. In thin photorefractive materials, it is confirmed experimentally that the diffraction of higher-order diffraction is belong to Raman-Nath diffraction in two-wave coupling at small incident angle.In PVK:5CB:C₆₀, the holographic storage results were given with small input angle when the film was placed behind the focal plane, in front of the focal plane and in the focal plane. It is found that higher-order diffraction images were amplified, reduced and rotated images compared with signal image. The permanent grating was recorded in MR doped 5CB at small incident angle. The higher-order diffraction images were reconstructed when different wavelength probe light input with direction perpendicular to the surface of the sample. A theory of higher-order diffraction images storage and reconstruction is developed. The theory is in good agreement with the experimental results. It is proved that higher-order diffraction images can be used in optical image processing. The superposition of higher-order diffraction images, the energy transfer between the higher-order diffraction images, the energy transfer between higher-order diffraction lights and incident beams were observed. A photorefractive higher-order diffraction optical beam splitter has been designed based on the higher-order diffraction of photorefractive materials. The one-dimension splitter was produced by two-wave coupling in photorefractive crystal at a small incident angle with He-Ne laser. The two-dimension splitter was produced by four-wave coupling in photorefractive polymer at a small incident angle with semiconductor laser. Three different wavelength signal lights (632.8 nm, 532.0 nm and 488.0 nm) were split into multi-output beams by the splitter. The experimental results indicate that the splitter can split signal beam into several diffraction beams with equal size and distance very well. We discuss the position and intensity distribution of higher-order diffraction beams when the film was placed in front of the focal plane, behind the focal plane and in the focal plane. The effect of photorefractive material thickness and the incident angle on the higher-order diffraction beam position and intensity distribution are discussed. The theoretical and experimental results show that the photorefractive higher-order diffraction optical beam splitter can provide a practical way to split the signal beam.A new phase-code multiplexed holographic storage method has been realized by using a rotated cylindrical-collimating lens system. The cross-talk is discussed theoretically and experimentally. The minimum value of angular selectivity is given experimentally. A method is proposed and the experimental results testified the validity to decrease the angular selectivity and increase the correlation accuracy. In Zn:Fe:LiNbO₃ (0.03 wt. % Fe, 3 mol. % Zn), 36 holograms have been successfully stored with phase-coded and angular multiplexing. The correlation recognition was finished for 36 holograms. The correlation accuracy was 100%. The advantage of rotationally phase-code multiplexed storage is used for correlation recognition, but the other phase-code multiplexed storage can't realize correlation recognition. Finally, the using of higher-order diffraction in correlation recognition was discussed.