Study On Beam Self - Trapping And Self - Deflection In Lithium Niobate Crystal

In recent decades, a series of nonlinear phenomena were studied in varieties of branches such as in biology, electrology, material science, optics etc. The optical soliton and the beam self-deflection are the most classical examples in optics. The self-trapping and self-deflection of a light beam in homogeneous and continuous bulk media have been widely studied, for example in the lithium niobate and strontium barium niobate crystal. The characteristics of the local energy and the deflection have certain application values in optical communication, optical interconnection, optical switch etc. In addition, due to the photonic bandgap, the photonic lattice with the periodic discrete characteristics can also influence the behavior of a light beam. The behavior of the light beam in such systems is quite different from what in the common continuous uniform media. It can occur some phenomena such as discrete diffraction, abnormal diffraction, abnormal refraction and asymmetric transmission. Therefore, the photonics lattice can be used for controlling the behavior of the light beam. This thesis mainly studied the self-trapping and the self-deflection of a light beam in the lithium niobate crystal, as well as photo-induced one-dimensional photonic lattice. The results are as follows.Firstly, we used the band transport model, considering the co-action of the photorefractive effect, the pyroelectric effect and the thermal-optics effect, to study the distribution of the space charge field and the refractive index in the bulk lithium niobate crystal and one-dimensional photonic lattice.Secondly, according to the electric field and the refractive index distribution, we numerically studied the self-trapping and self-deflection of the light beam. The Nonlinear Schrodinger Equation, FFT-BPM method were used in our simulations.Thirdly, we experimentally studied the light beam self-focusing in bulk pure lithium niobate crystal. The crystal shows the self-defocusing nonlinearity effect when it was illuminated. If there was no temperature gradient between the two facets of the crystal when increasing the temperature, we observed the light beam self-trapping and ultimately forming a circular bright spatial soliton, It was because the photovoltaic self-defocusing effect was suppressed by the pyroelectric effect. The forming time and the state of the soliton were dependent on the intensity and the polarization direction of the light and the temperature change.We also experimentally studied the self-deflection and self-trapping of the light beam in photo-induced one-dimensional photonic lattice. Firstly, the one-dimensional photonic lattice was fabricated by the photo-induced method. If there was no temperature gradient between the two facets of the crystal, when increased the temperature, we also observed the light beam self-trapping and ultimately forming a circular bright spatial soliton. Otherwise, if it had a temperature gradient between the two facets of the crystal, the light beam deflected and self-trapped, and then it formed a discrete bright circular soliton under the co-action of the photovoltaic effect, the pyroelectric effect and the thermo-optic effect. When the temperature was decreased, the pyroelectric effect strengthened the self-defocusing effect, resulting in the scattering effect of the light beam more apparently. Under that condition, the light beam can’t be self-trapped. The research results showed that the simulations were in good agreement with the experiments. Our study provided a simple way to manipulate the beam propagation. In the future, it is expected to be applied in the beam deflectors and limiters etc.