【作者】 常彦琴；

【导师】 黄金哲；

【作者基本信息】 哈尔滨理工大学 ， 光学工程， 2009， 硕士

【摘要】 超短脉冲倍频是获得短波长超短脉冲激光行之有效的方法。由于超短脉冲具有较宽的光谱范围及较高功率密度,倍频转换效率和谐波脉冲波形将受到许多因素的影响。为了提高转换效率和改善脉冲光束质量,有必要对相关的各种物理因素进行研究。本文研究了飞秒脉冲在BBO晶体中的I型倍频特性。首先,根据麦克斯韦方程组建立了超短脉冲I型倍频过程的物理模型,其中考虑了空间离散、衍射、群速失配、群速度色散、三阶非线性效应等多种物理因素。介绍了BBO晶体的物理光学性质以及一般的数值求解算法,并通过对耦合波方程组进行归一化处理并且编写了相应的数值模拟程序。其次,模拟了飞秒脉冲的I型匹配倍频过程;详细分析了高转换效率下各个因素如晶体长度、脉宽、脉冲能量、束腰半径在单独或组合情况下对倍频脉冲波形、空间近场分布(在晶体出射面上)及转换效率的影响。最后,用初始角度失谐和基频脉冲啁啾两种方法来补偿倍频过程中产生的相位失配,提高了倍频转换效率。在此基础上,设计了超短脉冲三倍频群速度补偿方案,方案采用了晶体双折射群速度补偿、以及极化旋转技术。分析发现,各种因素对倍频过程有不同的影响,且倍频脉冲时间波形和空间近场分布十分相似。啁啾补偿方法相对来说能较好地补偿倍频过程中的相位失配,从而为实际应用提供了一定参考。更多还原

【Abstract】 Frequency doubling of ultra-short pulses is an effective method to obtain short-wavelength ultra-short pulses. Conversion efficiency and pulse wave-forms will be affected by many factors due to the wide spectrum range and high intensity density of ultra-short pulses. To increase conversion efficiency and improve beam quality, it is necessary to study various physical factors involved.Properties of frequency doubling of femtosecond pulses in type-I phase-matched BBO crystal are investigated in this thesis. First, a physical model on the frequency doubling of ultra-short pulses in type-I phase-matching case has been built up on the basis of Maxwell’s equations, where various physical factors such as spatial walk-off, diffraction, group velocity mismatch(GVM), dispersion (GVD) and third order nonlinear effect are considered. Physical and optical properties of BBO and general numerical solving methods are introduced, these coupling equations are normalized and a final numerical simulation program is programmed. Second, the frequency doubling process of femtosecond pulses in type-I phase-matching case is simulated, where the influence on conversion efficiency, pulse waveforms and spatial near-field distribution (in the exit plane of crystal) of harmonic pulses with different physic factors by pulse width, energy and beam radius is studied in detail in high conversion efficiency case. At last, two methods of initial angular mismatching and pulse chirping are used to compensate phase matches in frequency doubling processes, which really increases conversion efficiency. Based on phase mismatch compensation, a scheme of group velocity compensation in frequency tripling is designed by compensation principle of birefringence and polarization rotation. It is found that different factors have different decent influences on frequency doubling processes; where the pulse spatial distributions are quite similar to temporal waveforms. The chirping method is better in compensation for phase mismatch comparatively, thus can provide a practical reference in application.更多还原