【作者】 张菁；

【导师】 陈建国；

【作者基本信息】 四川大学 ， 光学， 2003， 硕士

【摘要】 钛宝石激光自锁模的发现使得超短脉冲技术发生了革命性的变化，利用自相位调制和负群速度色散结合的类孤子产生方式，人们已经在钛宝石锁模激光器上得到了脉宽只有几个飞秒的光脉冲。随着周期量级激光脉冲的产生，脉冲载波及包络之间的相对相位漂移逐渐成为不容忽视的问题。在本文中，我们从几周期脉冲的载波相位出发，对超短光脉冲的一些基本问题进行了探讨。从脉冲电场的表达式出发，计算得到几周期脉冲平均频率、脉冲能量等参量的解析表达式，在此基础上，分别从频域和时域分析了载波相位的作用大小和范围。数值分析的结果表明，当脉宽小于光波周期时，相位的作用开始显现出来，当脉宽比光波周期长时，载波相位的作用可以忽略，并分析了相位起作用的原因，用解析的方法说明正是相延迟和群延迟之间的差别造成了光载波在脉冲包络下的滑动。考虑到啁啾对载波相位作用程度的影响，导出了啁啾脉冲的频率重心、脉冲携带的能量、脉冲时间重心以及有效宽度等参量的解析表达式，分析表明，啁啾使得载波相位比无啁啾的情况提前起作用了，即啁啾在很大程度上增大了载波相位发挥作用所对应的脉宽范围。包络对载波相位的作用范围也是有一定影响的，我们对高斯、洛仑兹、双曲正割和Sinc四种不同包络的脉冲，研究了单周期脉冲能量随载波相位的变化规律，发现以上四种脉冲的归一化能量均可在形式上用同一表达式表示，并发现对于这四种不同包络的脉冲，高斯脉冲相位的作用最早体现出来，然后是双曲正割和洛仑兹脉冲，Sinc脉冲最迟。激光对材料的破坏在很大程度上制约了激光系统的进一步发展，对材料破坏的研究成为高功率固体激光工程中讨论的热点。我们从光与物质相互作用的角度出发，分析了在飞秒脉冲对介质的破坏过程中，多光子电离、雪崩电离与电子密度之间的关系。发现脉冲宽度越窄，多光子电离过程提供的自由电子比例就越大，而对于脉宽较宽(如:，之10Ps)的脉冲，雪崩电离过程将提供大部分的电子。在这个基础上，提出了一个研究脉冲破坏闭值的模型，对三种不同包络脉冲的分析表明，在相同的脉宽、波长和入射光强下，高斯脉冲的破坏闭值较高，双曲正割和洛仑兹脉冲的破坏闽值相对较低，即高斯脉冲最不容易对介质造成破坏。同时我们发现随着脉冲宽度变窄，包络对破坏闭值的影响将越来越小，这表明对于飞秒脉冲，破坏阑值主要是由介质的内在特性决定的，而与其他外部因素关系甚小，这一结论与近年来飞秒激光破坏实验的结果是一致的。更多还原

【Abstract】 There has been a revolutionary advance in ultrashort pulse technology since the discovery of self-mode locking of Ti: sapphire lasers. Few-femtosecond pulses have already been generated from Ti: sapphire lasers, by means of combining self-phase modulation with negative group velocity dispersion, which is similar to the way of soliton generation. With the development of few-cycle pulses (FCP), the phase shift of the optical carrier with respect to the pulse envelope becomes non-negligible. Some fundamental issues of ultrashort pulses will be studied in this dissertation, concentrating on the carrier phase of FCPs.Some analytical expressions of FCPs, such as frequency gravity, pulse energy, etc. have been calculated, beginning with the description of pulse electric field. Range of the influences of carrier phase is analyzed subsequently, both in the time and frequency domain. Calculations indicate that the carrier phase takes effect till the time width of the pulse envelope becomes smaller than the period of the carrier oscillation, and the interpretation of this phenomenon is also presented. It is the difference between phase delay and group delay that causes the sliding between the carrier and the envelope. Realizing the impact of chirp on the effect of carrier phase, some analytical expressions of chirped few-cycle pulses are also calculated, such as frequency gravity, pulse energy, center of gravity of time, effective pulse width, etc.Results show that chirp makes the pulse characteristics more sensible to carrier phase, and chirp has greatly increased the pulse width when the carrier phase takes effect.Shapes of pulse envelope are found to have some effect on the degree of phase influences. Dependence of single-cycle pulse (SCP) energy on the carrier phase has been studied for Gaussian, Lorentzian, hyperbolic secant and Sine shapes respectively, which shows that these energies can be expressed in the same form. Phase effect on pulse with Gaussian envelope is the first to show, then are hyperbolic secant and Lorentzian pulses, and Sine the last.Laser damage to dielectrics has greatly limited the performance of laser systems, and nowadays research on laser damage has become the focus of high power laser technology. The relations between impact ionization, avalanche ionization and free electron density have been studied. Calculations show that the shorter of the pulse duration, the larger percentage of electron density produced by impact ionization will be, while for longer pulses (e.g. Tp > 10ps), avalanche ionization process alone will provide most of the electrons needed to induce breakdown. On this basis, a model of pulse damage threshold Fth (J/cm2) was proposed. Calculations on pulses of three different envelope shapes indicate that, with the same pulse duration, wavelength and incident light intensity, Fth of pulse with Gaussian envelope is higher that that of the pulses with hyperbolic secant and Lorentzian envelope, i.e. Gaussian pulse would need a little higher intensity to induce breakdown than the other two. It is realized that with the decrease of Tp, the effect of envelope on Fth will decrease too, which confirms the previously reported experimental results that Fth is mainly determined by intrinsic properties of dielectrics, rather than some external factors.更多还原