【作者】 张美志；

【导师】 卢克清；

【作者基本信息】 中国科学院研究生院（西安光学精密机械研究所） ， 光学， 2009， 博士

【摘要】 光折变空间光孤子是光折变效应与衍射效应相平衡的结果。光折变光孤子具有光强阈值低、存储时间长,二个横向维稳定和依赖于波长等特性,在光学信息处理和集成光学中具有广阔的应用前景。围绕光折变空间光孤子及其诱导的波导特性,本论文主要完成了以下工作:1.研究了屏蔽亮、暗光伏孤子诱导波导的传输特性。通过打靶法对方程进行求解,发现亮屏蔽光伏孤子诱导波导的导模数随着孤子强度比值的增加而单调增加,这一比值为孤子峰值强度与暗辐射之比。另外,暗屏蔽光伏孤子诱导波导的导模在任意强度比值下均为单模,限制在暗屏蔽孤子中心附近的能量随着强度比值的增加而增加。当光伏效应可以忽略时,这一波导变为屏蔽孤子诱导的波导。当没有外加电场时,这一波导变为光伏孤子诱导的波导。2.研究了一维屏蔽灰孤子诱导波导的传输特性。通过数值解孤子方程发现,灰屏蔽孤子诱导的波导在任意光强比率下均为单模。分析表明限制在灰孤子中心附近的能量和波导导模的传输常数随着强度比值的增加而单调增加。另外,当孤子灰度值增加时,限制在灰孤子中心附近的能量和波导导模的传输常数单调递减。3.基于光束的非相干特性,对一维部分相干暗光伏孤子及其诱导的波导进行了实验研究。实验结果表明利用部分相干光束,暗光伏孤子可以在非瞬时LiNbO₃:Fe晶体内形成。它诱导的波导可以对强相干光束进行导光。这一结果证明了用低功率非相干光源控制导引高功率激光光束的可能性。4.对屏蔽光伏孤子、屏蔽孤子和光伏孤子的时间特性进行了理论和实验研究。基于和时间有关的带输运模型,建立了光折变孤子的演化方程。通过数值模拟的方法发现,当电流密度J₀≤0时,稳态屏蔽孤子的形成时间随着|J₀|的增加而增加,在J₀＞0时,稳态孤子的形成时间随着孤子振幅的增加而减少。另外发现稳态屏蔽孤子的形成时间随着振幅的增加而减少,而开路光伏孤子的时间行为与孤子峰值强度与暗辐照的比值有关。当光伏孤子强度比值较小时,孤子的半高宽度随着时间单调递减达到一个稳定常数;当孤子比率较大时,孤子的半高宽度减少到一个最小值后增加到一个常数。进一步实验研究了暗光伏孤子的演化特性。实验结果证明在低功率下可以形成稳态孤子,在高功率下可以形成准稳态孤子。实验结果与理论分析很好地吻合。5.对折射率变化为负的LiNbO₃晶体中形成的亮光伏孤子进行了理论和实验研究。由于光折变晶体的光伏常数依赖于波长,因而选则不同波长的背景光和信号光进行理论分析和实验研究。结果表明,在背景光的照射下,当光伏常数κ＞1时,可以形成亮光伏孤子,其中κ为背景光的光伏常数与信号光光伏常数之比。实验中选用不同波长和相干度的背景光进行实验,得到孤子的存在曲线,与理论预测相吻合。更多还原

【Abstract】 Photorefractive （PR） spatial solitons is the result of diffraction balanced the PR effect. For the merits of low optical power, long storage time, two dimensions stable and depend on wavelength, PR solitons are prospective in optical information processing and integrated optics. Focusing on the photorefractive spatial solitons and its waveguides, the author mainly completed the following works:1. The propagation properties of waveguides induced by dark and bright screening photovoltaic （PV） solitons are studied theoretically. Using the shooting method, we show that the number of guided modes in a waveguide induced by a bright screening PV soliton increases monotonically with the increasing intensity ratio of the soliton, which is the ratio between the peak intensity of the soliton and the dark irradiance. On the other hand, waveguides induced by dark screening PV solitons are always single mode for all intensity ratios and the confined energy near the center of a dark screening PV soliton enhances monotonically with increasing the intensity ratio. When the bulk photovoltaic effect is neglectable, these waveguides are those induced by screening solitons. When the external field is absent, these waveguides predict those induced by photovoltaic solitons.2. The propagation properties of waveguides induced by one dimensional grey screening solitons are studied. Solving the soliton equation numerically, we find that waveguides induced by grey screening solitons are always single mode for all intensity rations. On the other hand, our analysis indicates that the energy confined near the centre of the grey soliton and the propagation constant of the guided mode of the waveguide increase monotonically with intensity ratio increasing. When the soliton grayness increases, the energy confined near the centre of the grey soliton and the propagation constant of the guided mode of the waveguide decrease monotonically.3. Based on the beam incoherent character, one dimensional partially incoherent dark PV solitons and its waveguides are studied experimentally. The experimental results show that dark PV solitons can form with partial incoherent beam in noninstantaneous PR crystal LiNbO₃:Fe. The waveguides induced by this type of solitons can guide strong coherent beam. These results prove that the possibility of controlling and switching high-power laser beam with low-power incoherent sources.4. The temporal behavior of screening PV solitons, screening solitons and PV solitons is studied theoretically and experimentally. Based on time dependent band transport model, the evolutional equation of PR solitons is established. Using numerical method, we find that the formation time of screening PV solitons increases with increasing |J₀| for J₀≤0, while decreases with increasing amplitude for J₀>0. Moreover, we find that the formation time of screening solitons decreases with increasing amplitude, while the PV soliton’s is relative to the intensity ratio. When the intensity ratio of PV soliton is smaller, the full width of half maximum （FWHM） of PV solitons decreases monotonically to a constant value; when the intensity ratio of the soliton is bigger, the FWHM of PV solitons first decreases to a minimum before it increases to a constant value. We further study the evolution of dark PV solitons in experiments. The results indicated that steady solitons can be observed at low optical power, while quasi-steady-state solitons can only be generated at higher optical power. Good agreement is found between theory and experiment.5. Bright PV solitons in LiNbO₃ crystal with negative refractive index change are studied theoretically and experimentally. For PV constant of PR crystal depending on wavelength, we choose different signal and background beam in theory analysis and experiment. We find that bright PV solitons can be obtained with background illumination forκ> 1, whereκis PV constant of background beam to that of signal beam. We also choose different coherent degree background beam in experiments and get soliton existence curve which has good agreement with theoretical prediction.更多还原