【作者】 易传祥；

【导师】 宋瑛林；

【作者基本信息】 苏州大学 ， 光学， 2011， 硕士

【摘要】 当激光与物质相互作用时,热效应往往不可避免,热致非线性效应也是最早研究的光学非线性效应之一,以往的研究多集中于稳态,近年来随着脉冲激光器的广泛应用,瞬态热致非线性效应的研究就显得尤其重要,也有相当一部分文献对此进行了报道。但关于瞬态热致非线性的动力学过程的研究并没有见到相关报道。本文采用4f泵浦探测技术着重研究了瞬态热致非线性的动力学过程。从热致非线性原理出发,在瞬态条件下,采用有限差分的方法对光声方程和热传导方程进行数值求解,计算了瞬态热致非线性折射随时间变化的动力学过程。同时研究了脉冲宽度、束腰半径、激光能量等物理参数对瞬态热致非线性动力学过程的影响,利用4f泵浦探测技术,通过不同的延迟时间,可以有效的区分介质的非线性机制。在实验上,利用4f泵浦探测技术来观察C70/甲苯溶液的瞬态热致非线性的动力学过程,用调Q倍频纳秒Nd:YAG激光器为光源,输出波长532nm,脉冲宽度为4ns,在延迟时间分别为5ns,8ns和10ns的条件下,用CCD拍摄不同延迟时间下的C70/甲苯溶液的线性和非线性图像,将线性光斑作为入射光斑拟合实验曲线,发现随着延迟时间的增加,吸收引起的热量积累增加,温度变化的影响增大,使得瞬态热折射明显增强。这与我们在理论上的计算符合一致。更多还原

【Abstract】 When the laser propagates through the material, the thermal effect is always existent, and the thermal nonlinear effect was the earliest one of the optical effect researches. Previous research focused on the steady. In recent years, with the wide application of pulsed lasers, the research of the transient thermal appears especially important; it is also reported by some literatures. But there is no report about the dynamic process of the transient thermal nonlinear refraction.We use the 4f pump-probe to study the process of the transient thermal nonlinear refraction. We start from transient thermal nonlinear principle, and use the Crank-Nicholson finite difference method to solve the acoustic-wave and thermal propagation equation, simulating the change by the time. At the same time, the influence of the thermal effect is simulated by altering such parameters as the laser width, beam radius, pulse energy etc.In the experiment, we introduce a method of measure the optics nonlinear index: 4f coherent imaging system with phase object. This method is very simple and accurate, and compare to the traditional method-z scan. The process of transient thermal nonlinear effect was studied using the time-resolved pump-probe system based on 4f coherent imaging system with phase object with nanosecond laser pulse at the wavelength of 532nm. The laser source we used is a Q-switched and mode locked Nd: YAG laser (Continuum Surelite). The delay time is 5ns 8ns and 10ns. The sample is C70/toluene, we get different linear and nonlinear image at the diverse delay time by using the CCD. We choose the linear flare as incident flare to fit experimental curves, and find when the delay time increases, the heat of absorption is growing, the influence of temperature variations increase, and make the transient thermal refraction obvious enhancement. The result is good fit to our theoretical numerical simulation.更多还原