The Research On Thermal Fixing Technique Of Holographic Storage In Photorefractive Crystals

Photorefractive crystals such as Fe doped LiNbO3, owing to their outstanding optical properties, are widely used in holographic storage. Nevertheless, the recorded holograms will be erased by further illumination, leading to the loss of stored information during readout, which is a serious impediment to the practical realization of high-capacity memories. Thermal fixing technique, using the ionic compensational mechanism, is a promising solution to the volatility problem in holographic storage systems that use photorefractive crystals. In the thesis, a study is carried out, both theoretically and experimentally, on off-line thermal fixing method.The theoretical model and dynamic characteristics of thermal fixing are investigated. The erasure effect on electronic gratings in three cases of illumination is analyzed as well. We discuss the relation between different heating conditions (temperatures, timing) and compensation effect of ions. During the revealing process, the time evolution of the diffraction efficiency is obtained, and the developing of retrieved holograms is also observed through a CCD. The experimental results testify the theoretical prediction. The appropriate conditions and convenient procedure for off-line thermal fixing experiment is present.We stored and thermally fixed holograms in a Fe:LiNbO3 crystal using 90°geometry setup and angular-multiplexing system with a 532nm wavelength laser. All the stored images, retrieved and readout by using the original reference light, have good quality. The fixing efficiency of one hologram can be 50%, after 4 hours'illumination or 10 days'preservation in darkness the fixed images go without decay, which indicate that thermal fixing technique can prolong the lifetime of holographic storage in photorefractive crystals.The asymmetric dividing scheme, on the basis of batch heating fixing method, is proposed to optimize the fixing of multiple holograms. In an experiment, 20 pages of holograms which are divided into 4 groups using the new scheme are fixed in one location. The result indicate that batch fixing can reduce the erasure of further recording lights, and hereby the density of multiple storage will be increased. Furthermore, the asymmetric dividing scheme can reduce the scatter noises caused by long exposure in front groups. Therefore, the image qualities of the fixed holograms are improved.