Study On The Self-deflection And Temperature Dependence Of Photorefractive Spatial Solitons And Solitonic Observation

Photorefractive (PR) spatial optical solitons occur 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. The mechanism to form PR spatial soliton can be classified into two generic types, that is, the self-phase-modulation (SPM) self-focusing mechanism and the cross-phase modulation (XPM) self-focusing mechanism. This dissertation investigates theoretically the properties of solitons result from the SPM self-focusing, such as dynamical evolution, the self-deflectionA and the temperature property. We also investigate the temperature effect on the stability of spatial solitons formed in a PR two wave mixing (TWM), which result from the XPM self-focusing. At last, we initially investigate the spatial optical solitons in PR crystal due to two-photon (TP) PR effect.Based on the physical process of the PR effect, the theoretical model of PR spatial optical solitons is derived with the consideration of both the drift and photovoltaic effects. Then, the properties of such PR solitons are studied. By using a numerical method, the dynamical evolution and self-deflection properties of PR solitons are investigated. The results indicate that the PR solitons can stably propagate along a straight line in the crystal with unchanged profile. 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 PR solitons are also investigated. Our results indicate that there exits a characteristic value of photovoltaic fields for photovoltaic bright solitons. When the photovoltaic field is less than the characteristic value, these solitons always bend in the direction of opposite the crystal's c axis, and the absolute value of the spatial shift that is due to the first-order diffusion term alone is always larger than that which is due to both the first-order diffusion term and the higher-order terms acting together. If the strength of photovoltaic field is larger than the characteristic value, these solitons bend in both the same direction as and in the direction opposite the crystal's c axis is possible. Whether the direction is in the same or in the opposite direction will depend on the strength of photovoltaic field and on the input intensity. Specifically, self-deflection cannot occur for photovoltaic bright solitons if the strength of the photovoltaic field and the intensity of the input beam are appropriately selected. The self-deflection of bright solitary beam is further studied by perturbation technique, and the results are found to be good agreement with that obtained from numerical method.The temperature effects on the dynamical evolution and self-deflection of photorefractive bright soliton are investigated. The result shows that the stabilities of the bright solitons are strongly influenced by the crystal's temperature, the self deflection distance of the bright solitons centre increases and, reaches its maximum value at a character temperature and then decreases as temperature rises and, approaches zero at low and high temperatures.By using a numerical method, the dark soliton crystal temperature effect on the evolution and self-deflection of bright soliton are discussed in a separate soliton pair. The numerical results indicate that by taking into account the temperature dependences of diffusion effect and the dark irradiance, the dynamical evolution of the bright soliton depends strongly on the crystal temperature. The incident beam could be stable if the temperature changes little enough. However, the incident beam could be larger cycles of expansion or compression when the temperature changes large enough, and the spatial shift of the beam center changes with the temperature changes. The evolution and self-deflection of the bright soliton can be controlled by changing the temperature of the dark soliton crystal in a separate spatial soliton pair.It has been investigated that temperature effect on the dynamical evolutions of holographic bright solitons in photorefractive dissipative systems based on TWM. Numerical results show that the input solitary beam can evolve into a stable soliton and propagate in the crystal when the temperature drift is quite small, whereas it will not evolve into a stable soliton and its intensity becomes increase or decrease with increasing propagation if the temperature difference is big enough. By use of top- and side-view method, we investigated experimentally the dynamical evolution of bright photovoltaic soliton. This method has an ability to show the horizontal and veritical outline of the incident beam at any location of the crystal very intuitively. We also observed the two-dimensional dissipative bright photovoltaic solitons by the same method combined with optical shutter.At last, we investigate the screening-photovoltaic (SP) spatial solitons formed in PR photovoltaic crystal due to TP-PR effect. We present the one-dimensional dynamical evolution equation and find the soliton solutions under appropriate conditions in steady state. By use of perturbation techniques, we also investigate the self-deflection process of these bright solitons with taking diffusion effect into account. Our results indicate that the center of the TP-SP solitary beam moves on a parabolic trajectory, whereas its central spatial-frequency component shifts linearly with the propagation distance.