【作者】 张光勇；

【导师】 刘劲松；

【作者基本信息】 华中科技大学 ， 物理电子学， 2007， 博士

【摘要】 光折变空间光孤子是指当光折变材料的自聚焦效应与光束的衍射发散作用相平衡时,在介质内无衍射向前传播的光束。由于光折变空间孤子在集成光学、光信息处理以及光通信等领域广阔的应用前景,它越来越成为人们关注的焦点。光折变空间孤子的形成机制可分为两类:自相位调制自聚焦和交叉相位调制自聚焦。本论文工作主要研究了基于自相位调制自聚焦的空间光孤子的演化偏转特性、温度特性、串连晶体回路中独立屏蔽空间明暗孤子对的温度特性以及基于交叉相位调制自聚焦的耗散全息光孤子的温度特性,初步研究了双光子光折变晶体中的空间光孤子等问题。基于光折变效应物理过程,同时考虑了漂移效应和光伏效应,推导出了光折变空间光孤子的理论模型,讨论了孤子的物理特性。利用数值模拟的方法,研究了光折变空间孤子的演化偏转特性。结果表明,光折变孤子能在晶体中以稳定的孤子态沿直线传播;当考虑到晶体扩散项影响后,孤子中心在传播过程中会偏离初始位置而发生偏转,偏转量的大小和晶体自身光伏场、外加电场以及入射光强比有关。当空间电荷场高阶项不能忽略时,数值研究了高阶空间电荷场对光伏明孤子偏转特性的影响。结果表明,对于光伏明孤子来说,存在一个特征光伏场,当光伏场强度小于该特征场时,孤子总是在逆光轴方向偏转,并且由低阶项引起的孤子中心偏转的距离总是大于低阶和高阶项共同作用时引起的偏转距离。如果光伏场强度大于特征场,孤子中心既可以在光轴方向偏转,也可以在逆光轴方向偏转。孤子中心的偏转方向和距离取决于光伏场强和入射孤子波的强度。在合适的光伏场和入射孤子强度下,孤子中心不表现出偏转特。微扰法的结果进一步证实了数值计算的正确性。研究了温度对光折变明空间孤子演化和自偏转特性的影响。结果表明,明孤子传播特性强烈依赖于晶体温度;明孤子中心偏转距离随温度增加而增加,在一个特征温度处达到最大值,而后随温度增加逐渐变小,在低温和高温区域,这一偏转接近于零。用数值方法研究了光折变晶体回路中暗孤子晶体温度对明孤子自偏转的影响。结果表明,考虑温度对扩散效应和暗辐射强度的影响,光折变明空间孤子的演化强烈依赖于晶体的温度,当晶体温度变化不大时,入射孤子波能在晶体中稳定传播,而当晶体温度变化足够大时,入射孤子波呈现周期性的压缩和膨胀,且光束中心偏转距离随温度变化而变化。通过调节独立空间孤子对中暗孤子的温度可以明显改变明孤子的演化和偏转特性。理论研究了双光束耦合光折变耗散系统中全息孤子的温度特性。结果表明,耗散全息光孤子对小的温度漂移是稳定的,当晶体的温度变化足够大时,孤子的峰值强度和宽度都随传播距离变化,入射孤子不能演化成稳定的孤子态传播。采用顶侧面观测法,实验研究了光伏明孤子的动态演化过程。这种方法能直观显示出入射光束在晶体中传播时,在水平及垂直方向上不同位置处的轮廓图。结合光闸瞬间成像法,采用顶侧面观测法成功观测了二维耗散光伏明孤子。最后,理论研究了光折变晶体中的双光子屏蔽光伏空间孤子。在一维稳态条件下,我们得到了双光子空间孤子的动态演化方程,求得了明、暗空间孤子解。考虑晶体材料的扩散效应,采用微扰法研究了双光子光折变明孤子的自偏转特性。结果表明,孤子光束的中心在传播过程中沿一条抛物线轨迹移动,而光束的中央空间频率分量随传播距离线性变化。更多还原

【Abstract】 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.更多还原