【作者】 刘岩；

【导师】 刘伟伟；

【作者基本信息】 南开大学 ， 光学工程， 2011， 博士

【摘要】 飞秒激光在空气中的成丝现象是当前科研领域的热门话题,其复杂的物理机制以及巨大的应用潜力引起了人们的广泛关注。一方面,飞秒激光成丝过程中存在复杂的非线性过程,例如自聚焦、光致电离、自相位调制、自陡峭等,这导致飞秒激光在光丝内传输时脉冲形状发生复杂的时空演变;另一方面,脉冲性质的演化过程携带了成丝现象中非线性光学过程的重要信息。如果测量出飞秒激光脉冲在成丝中的演化过程,可以更加深入地了解光丝内部所发生的各种复杂的物理过程。因此,成丝过程中激光脉冲的诊断技术研究具有重要的理论和实用价值。本文针对空气飞秒激光成丝过程中激光脉冲的诊断问题进行了理论以及实验研究,在对现有测量技术进行系统介绍和理论分析的基础上,提出了三种新颖的测量技术并对它们的测量原理进行了详细的阐述,给出了相应的理论推导过程,并通过实验测量验证了三种测量技术的可行性。本文提出的第一种测量技术是基于双光子荧光测量原理的空气飞秒激光成丝过程中激光脉冲时空特性测量法,它通过对飞秒激光脉冲在色散介质中激发产生双光子荧光信号空间分布的测量来反演激光光束的直径、激光脉冲的宽度和啁啾率。本文首先从理论上推导了样品池中荧光信号强度的空间分布模型,然后实验测量空气成丝过程中飞秒激光脉冲的时空特性,获得了激光光束直径、脉冲宽度和啁啾率等重要参数的演变过程。本文提出的第二种测量技术是基于双色场频率分辨光快门技术的空气飞秒激光成丝过程中激光脉冲时域特性测量法,它通过测量基频飞秒脉冲和倍频飞秒脉冲所合成的双色场在空气中聚焦后产生的三次谐波的频谱分布随两脉冲之间时间延迟变化的演变规律,并结合相应的反演算法来实现激光脉冲强度和相位的测量。本文首先对空气中双色场飞秒激光泵浦产生三次谐波的物理机制进行了理论研究,总结了时间延迟、激光脉冲的啁啾率和自相位调制效应给三次谐波频谱分布带来的影响。然后根据所建立的三次谐波频谱分布模型设计了相应的反演算法,并利用该算法对聚焦区域中激光脉冲的强度和相位分布进行了反演计算,给出了反演结果与理论数据的误差分析。本文提出的第三种测量技术是基于氮气荧光测量原理的空气飞秒激光成丝过程中激光脉冲宽度测量法,它通过测量两束飞秒激光在交叉区域所产生的氮气荧光随两脉冲之间时间延迟变化的分布来实现激光脉冲宽度的反演。本文首先从理论上推导了氮气荧光信号的强度随泵浦脉冲和探测脉冲之间的时间延迟变化的分布模型,然后对两束飞秒激光交叉位置处泵浦脉冲的脉宽进行了实验测量。最后,本文对所做工作进行了总结,讨论了下一步工作需要改善的地方:一、基于双光子荧光测量原理的空气飞秒激光成丝过程中激光脉冲时空特性的测量技术,在光束截面可分辨的前提下实现对飞秒脉冲的三维检测。二、基于双色场频率分辨光快门技术的空气飞秒激光成丝过程中激光脉冲时域特性的演化过程的实验测量。三、利用基于氮气荧光测量原理的空气飞秒激光成丝过程中激光脉冲宽度测量方法,对光丝内部激光脉冲宽度的演化过程进行远程实验测量。更多还原

【Abstract】 Femtosecond pulse filamentation in air has attracted considerable interest because of its complex physical mechanisms and wide range of applications. The femtosecond pulse undergoes strong transformation during filamentation, which also carries key information of the nonlinear optical processes. Therefore, the pulse characterization during femtosecond laser filamentation in air is crucial for both theoretical and experimental investigations, providing a deeper and more thorough understanding of the complex underlying physical processes inside the filament. This dissertation is devoted to study the femtosecond pulse characterization technique during the filamentation in air. Three measurement techniques have been discussed.The first technique studied is based on the two-photon fluorescence measurement. It retrieves the pulse duration, chirp rate and beam radius by measuring the intensity distributions of the two-photon fluorescence induced by the femtosecond pulse. The experimental results show that the pulse duration, chirp rate and beam radius could be simultaneously quantitatively retrieved and this simple technique would be useful in practice to trace the underlying dynamics of filamentation in air.The second technique studied is named two-color-field frequency-resolved optical gating method (TC-FROG). It retrieves the pulse intensity and phase by measuring the spectral distributions of the generated third harmonic pumped by two-color laser field. First, the physical mechanisms of the third harmonic generation in air by a two-color laser field are studied. It is found that the third harmonic spectrum varies with the temporal delay, chirp rate and the self-phase modulation of the fundamental and the second harmonic wave. Then, an amplitude-phase reconstruction algorithm is designed with the adaptation of Genetic Algorithm. Finally, the theoretical simulation is performed for the electric field reconstruction of Gassian pulse, and the uncertainty between the fitting data and the measurement data is discussed.The third technique studied is based on the nitrogen fluorescence measurement. It retrieves the pulse duration by measuring the intensity of the nitrogen fluorescence in the overlapped zone of the pump pulse and probe pulse, which varies as a function of the temporal delay between the two pulses. First, the distribution model of the nitrogen fluorescence intensity according to the temporal delay is derivated. Then, the pulse duration measurement of the femtosecond laser pulse in the overlapped zone is carried out.In the end, the future work of the thesis is discussed: first, by improving the spatial resolution of the laser beam’s cross-section, a 3D measurement of the femtosecond pulse could be realized; second, TC-FROG technique needs further practical confirmation; third, the remote-measurement of the laser pulse duration during the femtosecond laser filamentation in air could be performed by using the technique based on the nitrogen fluorescence measurement.更多还原