A Compact Practical Thermal Fixing System For High-Density Holographic Storage

Holographic data storage is a subject of considerable current and commercial interest. Volume holographic storage (VHS) has attracted interest because of its potential in high-density and fast parallel readout. Photorefractive crystals (e.g., doped LiNbO3) have been considered as one kind of the most important materials for VHS owing to wider dynamic range and their commercial availability in large size. However, the volume holographic gratings in photorefractive crystals are liable to dark decay and erasure under readout illumination. Thermal fixing method is a feasible solution to this problem. A complete thermal fixing procedure includes two steps i.e., fixing and developing. In fixing stage, the ions in crystals move to form an ionic grating that compensates the electronic grating at elevated temperature. In developing stage after cooling crystals to room temperature, a homogeneous illumination erases the electronic grating and brings out the ionic grating. The ionic grating has a longer lifetime both in dark and under readout illumination, so nonvolatile storage comes true.This thesis is aimed at building up a compact high-density holographic storage system with thermal fixing function to make the nonvolatile storage technology practical. This system uses mainly post-recording fixing technique that involves recording at room temperature, fixing at elevated temperature, and developing at room temperature. The work of this thesis is divided into two parts. Firstly, based on holographic principles and the band transport atom theory for thermal fixing, given by Yariv, we investigated holographic characteristics and thermal-fixing characteristics of photorefractive crystals. Upon a lot of experiments, a practical assessment method is worked out for selection of crystals that are suitable for large-scale VHS and shows good thermal-fixing performance as well. Meanwhile, a method for rapid thermal fixing is presented, which has some advantages over conventional technique in stability and repeatability, while shortened the fixing time. Secondly, we have designed and made a compact practical holographic system that consists of angular-fractal multiplexing system and thermal fixingequipment, including a crystal repositioner with precision less than 0.001 . For a cubic crystal of 1cm 1cm 1cm, the horizontal and vertical selective angles are 0.008 and 0.41 respectively. So the system is capable of storing 10000 holograms. We also have made an off-line heater and an on-line heater, both of which are controlled by a temperature controller (model EUROTHERM). The temperature controller can control temperature variation less than 0.1 in the chamber of the heater.We stored 1000 binary pages in a 0.03% FerLiNbO3 crystal using this system and batch thermal fixing technique. All the stored images are retrieved, and the retrieved images have uniform diffraction efficiency and good quality. Experiments show that the holograms fixed in this crystal may last 5.4 years in dark and 43.6h under readout illumination. We believe that the successful building up of this system will make a great progress in high-density nonvolatile volume holographic storage.