Theoretical Study On Photorefractive Spatial Solitons

Photorefractive (PR) spatial solitons form when the self-focusing of a light beam inside the PR material exactly balances the diffraction of the beam. For their wide potential application in many fields such as integrating optics, optical signal processing and optical communications, they have attracted more and more attention of scientists home and abroad. On the basis of our previous theory of screening-photovoltaic (SP) solitons, this dissertation investigates theoretically the properties of the SP solitons formed in a single PR crystal and some new phenomena of the separate spatial soliton pair formed in a serial PR crystal circuit.The forming mechanism of the SP soliton is first derived based on the Kukhtarev-Vinestskii model and the numerical integral solutions of the bright and dark solitons are given in a PR crystal. Then, the evolution properties of the SP soliton propagating in PR crystal are numerically studied with the result that the SP soliton can propagate stably in the crystal and are able to stand against small perturbations. When the perturbation is large, the incident solitary beam will not retain its shape but instead alternate between expansion and compression or disperse quickly in the crystal. When the diffusion effect of the crystal is taken into account, the solitary beam will move on a parabolic trajectory, which is called the self-deflection process, and the spatial shift of the beam center depends on the parameters of the crystal, the biased electric field and the incident beam. The higher-order space charge filed effects on the self-deflection of bright SP solitons are also investigated. The case for SP bright solitons is different from that for screening bright solitons, and the solitons can sometimes bend in the direction of the crystal's c axis and sometimes in the opposite direction. The absolute value of spatial shift due to the first-order diffusion effect alone is not always smaller than that due to both first-order and higher-order diffusion acting together, which holds for the screening or photovoltaic bright solitons, while sometimes for the SP bright solitons, the former can be larger than the latter.The temperature effects on the evolution of the SP solitons are also discussed. The result shows that the stabilities of the SP solitons are strongly influenced by the crystal's temperature. Changing temperature of the PR crystal can make the previously stable soliton become unstable and vice versa.The separate spatial soliton pair, formed in a serial PR crystal circuit consisting oftwo PR crystals connected by electric leads with or without a biased source, is first predicted and studied. Three sorts of separate soliton pair can be formed in the circuit, namely bright-bright, bright-dark and dark-dark. The two solitons in one pair can interact with each other. However, in the limit of the spatial extent of the optical wave being much less than the width of the crystal, the dark soliton can affect the other one whereas the bright soliton cannot.Based on the asymmetric property of the soliton pair, the effect of the dark soliton on the properties of dynamic evolution, first-order self-deflection and higher-order deflection of the bright soliton in a bright-dark soliton pair are investigated in detail. The results show that changing the input amplitude of the dark soliton can adjust the spatial shift of the center of the bright soliton effectively and accurately.At the end of the thesis, the stability and the coupling effects between the two solitons composing the PR bright holographic soliton pair when one or both initial input solitary beams are perturbed are numerically studied. The potential applications of the spatial soliton are discussed.