Theoretical Research On Transmission Of Gauss Beam In Photorefractive Crystals

In photorefractive crystals, photorefractive effects can produce space-charge field which can result in the change of refractive index of Photorefractive material, then a distribution of refractive index similar to lens will be formed to scatter the incidence of light. Now photorefractive effects have been widely applied to real-time optics information processing. As we know, Gauss beam is still Gauss beam after transmition through general lens, its compression ratio is not high (2~3). So for the sake of higher compression ratio, we do research on transmission of Gauss beam in nonphotovoltaic photorefractive and photovoltaic photorefractive crystals.Based on some relevant theories of photorefractive effects, this thesis starts from band transport model firstly developed by Kukhtarev, combining with the standard set of rate and continuity equations and Gauss's law, obtains space-charge field produced by photorefractive effects and dynamic evolution equation of envelope of Gauss beam. We do numerical simulation on dynamic evolution of Gauss beam in nonphotovoltaic photorefractive and photovoltaic photorefractive crystals based on corresponding physical models, and investigate the influence of biased electric-field, crystal loss and power-density of incidence on waist and power-density of Gauss beam.The result shows: First, Gauss beam can be compressed periodically under the proper changes of biased electric-field, crystal loss and power-density of incidence, and the maximum power-density decreases periodically along the length of crystal too. With the increasing electric-field or crystal loss, the length of compression period gets shorter; and with the increasing power-density of incidence, the period gets longer. Second, in photovoltaic photorefractive crystals when photovoltaic parameterα> 0, we can change not only electric-field intensity, but also electric-field polarity to compress Gauss beam periodically; in photovoltaic photorefractive crystal whenα< 0 or nonphotovoltaic photorefractive crystal, we only can change electric-field intensity to compress Gauss beam periodically. Third, we calculate the maximum compression ratio in SBN, LiNbO3, BaTiO3 crystal, and we get: when biased electric-field, crystal loss and power-density of incidence change, the compression effect in SBN is the best, the maximum compression ratio can reach 4.5.