Theoretical Study On Spatial Optical Solitons In Centrosymmetric Photorefractive Crystals

Since their existence was first predicted and observed, photorefractive optical spatial solitons have been the subject of very intense experimental and theoretical studies for their feature of formation at low laser power and potential important applications. Thus far, several different kinds of photorefractive spatial solitons have been investigated in non-centrosymmetric photorefractive crystals. In this dissertation, the spatial optical solitons in centrosymmetric photorefractive crystals are investigate systematically.The effects of diffusion on self-deflection of steady-state bright spatial optical soliton are studied systematically by both numerical and perturbation methods. The center of the soliton moves on a parabolic trajectory, and the deflection effects vary cubically with applied external field. Moreover, a theoretical investigation of temperature effects on bright solitons in centrosymmetric photorefractive crystals is provided. Three physical factors related to the temperature, i.e., the dielectric constant, diffusion process and dark irradiance, are considered. It is shown that the dielectric constant temperature dependence dominates the process.A one-dimensional time-dependent wave propagation equation wihich in centrosymmetric photorefractive crystals is derived. The equation exhibits the bright and dark soliton solutions whose evolutions against time are investigated. It is shown that the screening bright soliton exists in both quasi-steady-state and steady-state regimes for high intensity saturation values. When the intensity ratio is smaller than unity, only steady-state regime is shown. Moreover, the wave equation exhibits only steady-state screening dark spatial optical solitons.The coupling between two mutually incoherent Gaussian beams that propagate collinearly in centrosymmetric photorefractive crystals is investigated. The centrosymmetric photorefractive crystals with a positive effective quadratic electro-optic coefficient can support bright-bright Gaussian spatial soliton pairs. The physical properties of these soliton pairs are also discussed in detail.A comprehensive theoretical study on holographic solitons is provided in centrosymmetric photorefractive crystals. It is predicted that both dark and bright spatial soliton states can exist in centrosymmetric photorefractive Hamiltonian systems and that the signal beam alone can evolve into steady-state bright and dark holographic solitons in a centrosymmetric photorefractive dissipative system.Based on Castro-Camus model, the evolution equation of one-dimensional steady-state spatial solitons is presented in two-photon centrosymmetric photorefractive crystals, which predicts not only bright but also dark solitons. By using a perturbation method, the effect of diffusion on the self-deflection of bright solitons is also investigated. Furthermore, it is shown that incoherently coupled dark, bright and bright-dark soliton families are possible in two-photon centrosymmetric photorefractive crystals and that Manakov soliton pairs can be established provided that the total intensity of two mutually incoherent optical beams is much lower than the dark irradiance.By global and locally treating the space-charge field, one-dimensional modulation instability of broad optical beams in biased centrosymmetric photorefractive crystals is studied under steady-state conditions. The expressions of global and locally modulation instability gains in single-photon centrosymmetric photorefractive crystals and locally modulation instability gain in two-photon centrosymmetric photorefractive crystals are provided.