Study On Photorefractive Properties And Optical Correlation Recognition Applications Of Codoped Hf:Fe:LiNbO_ 3 Crystals

Optical volume holographic data storage is the promising storage technique because of its high data storage density, high transfer speed, fast parallel access, and so on. Ideal volume holographic storage material should have high refractive index modulation, low scattering noise, high recording sensitivity and large dynamic range. Lithium niobate (LiNbO3) crystal has become a preferred holographic storage material because of its well-known photorefractive performance. However, LiNbO3 crystal exhibits the relatively low response speed and strong light-induced scattering, which limit the application of LiNbO3 crystal in volume holographic storage. So, improving and optimizing the photorefractive performance of LiNbO3 crystal have become one of important research subjects in volume holographic storage. In this thesis, a series of co-doped Hf:Fe:LiNbO3 crystals were grown and investigated. Based on the defect structure, the photorefractive properties of Hf:Fe:LiNbO3 crystals at 488nm wavelength were investigated in detail. The optical filtering correlation recognition system was designed and built up using Hf:Fe:LiNbO3 crystals as the recording materials, and the optical correlation recognition results of edge-enhanced images were investigated.The defect structures and occupied sites of Hf:Fe:LiNbO3 crystals were investigated detailedly by using infrared OH- absorption spectra, UV-Visible absorption spectra, ICP-AES analysis and Raman spectra. From the analytical results, its can be seen that Hf4+ ions take priority of replacing the antisite NbLi4+ defects when Hf4+ doping concentration is below its threshold concentration. While, when Hf4+ doping concentration exceeds its threshold concentration, Hf4+ ions begin to occupy the normal Nb sites. In our samples, Fe2+/3+ ions always occupy the normal Li sites.The optical damage resistance ability of Hf:Fe:LiNbO3 crystals was studied at 488nm wavelength by using the transmitted light spot distortion method as well as the photo-induced birefringence change method. The experimental results show that for congruent Hf:Fe:LiNbO3 crystals, the optical damage resistance ability of the crystal is the strongest when Hf4+ doping concentration is its threshold concentration (4.0mol.%). For Hf:Fe:LiNbO3 crystals with Hf4+ doping concentrations of 1.0mol.% and 4.0mol.%, the influence of the [Li]/[Nb] ratio on the optical damage resistance ability of crystals is different. When Hf4+ doping concentration is 1.0mol.%, the optical damage resistance ability of the crystals enhances remarkably with the [Li]/[Nb] ratio increasing. While, when Hf4+ doping concentration is 4.0mol.%, the optical damage resistance ability of the crystals decreases with the increase of the [Li]/[Nb] ratio.The photorefractive properties of Hf:Fe:LiNbO3 crystals were investigated experimentally at 488nm wavelength by using two-wave coupling experiment. The experimental results show that the photorefractive properties of the crystals descend with the increase of Hf4+ doping concentration. While, when Hf4+ doping concentration exceeds its threshold concentration, the photorefractive properties of the crystals return to increase. For Hf:Fe:LiNbO3 crystals with various [Li]/[Nb] ratios, the photorefractive properties of crystals is related to Hf4+ doping concentration. When Hf4+ doping concentration is 1.0mol.%, the photorefractive properties of the crystals descend with the [Li]/[Nb] ratio increasing. While, when Hf4+ doping concentration is 4.0mol.%, the photorefractive properties of the crystals enhance with the increase of the [Li]/[Nb] ratio. In addition, by optimizing the recording light intensity, preferable photorefractive properties can be obtained.Edge feature extraction of target images was achieved by using optical high-pass filtering method, and the influence of image edge enhancement on optical correlation recognition results was investigated theoretically and experimentally. The optical filtering correlation recognition system was designed and built up using Hf:Fe:LiNbO3 crystals as the recording materials. Theoretical analysis and experimental results show that the recognition rate of edge-enhanced images has a remarkable improvement compared with those of the original images.