Optimization and improvement of the performance of ultrashort and ultraintense laser system and its applications

随着激光技术及相关器件单元（如啁啾镜，半导体可饱和吸收体）的提出及发展，获得短至2个光学周期的激光脉冲已经成为现实。同时，超短超强啁啾脉冲放大技术和光参量啁啾脉冲放大技术也得到了迅猛发展，目前已经可以获得1022Wcm－2的聚焦功率，这为人类提供了前所未有的全新的实验手段及物理条件。激光系统正向着产生更短的脉冲宽度、更高的峰值功率、更强的可聚焦峰值功率密度、更高的重复率、更好的稳定性等方面发展。 本论文围绕上海光机所强场激光物理国家重点实验室已建成的23TW的啁啾脉冲放大钛宝石激光系统开展了一系列研究工作，包括提出一些有力措施及建议，从而可以改进目前的激光系统性能，使得激光系统可以输出重复率高达（100MHz）、高对比度的超短脉冲。此外，开展了三项应用飞秒激光与物质相互作用的工作，诸如提高等离子体通道的寿命，多光子吸收引起的频率上转换现象以及铁电晶体的飞秒激光全息存储特性。本论文工作的主要结果与创新点包括： 1. 提出一种由钛宝石再生放大器获得高重复率的啁啾脉冲序列的新方法：通过改变钛宝石再生放大器中泡克耳斯盒电光开关的传统工作模式，从而获得了高重复率（100MHz）的啁啾脉冲序列，经过放大和压缩后，可以获得高重复率的飞秒太瓦级激光脉冲序列。实验证明，用此高重复率的激光脉冲在空气中聚焦产生的等离子体通道，与低重复率的激光脉冲序列相比较，等离子体通道中的电子可以在较高的密度下维持较长的时间。 此100MHz的飞秒太瓦级激光脉冲列是当前国际在飞秒太瓦级激光输出上最高的重复率。本论文提出并演示的这种获得高重复率脉冲列的方法可以推广到含有泡克耳斯盒电光开关的再生放大器的所有单次运行（如钕玻璃激光装置）或低重复率运行的高功率激光系统，从而进一步提高实际应用性能。 2. 对再生放大器内的石英双折射晶体片的腔内光谱整形进行了理论和实验研究。通过调节一定厚度的双折射晶体片在空间的光轴取向，对再生放大器的增益中心进行合适的衰减可以有效地补偿增益窄化的影响。根据石英晶体双折射率的色散特性，对石英波片的偏光干涉谱进行了理论分析和数值模拟，首次基于偏光干涉法提出了一种测量石英波片延迟量和厚度的方法。 3.对啁啾脉冲放大过程中的放大自发辐射引起的激光脉冲对比度下降问题进行了研究。基于在立方晶体中产生正交偏振波的机制可以提高脉冲的对比度，我们建立了双晶体脉冲净化器并应用该脉冲净化器来提高激光脉冲的对比度，并取得了一些有意义的结果。在此基础上，提出了针对本实验室现有的超短超强钛宝石激光系统对比度提高的结构建议。 4.与同事合作开展了飞秒激光与物质的相互作用方面的工作。开展了稀土Ce3＋离子掺杂的Lu2Si2O7和Gd2SiO5晶体的飞秒激光辐照的多光子吸收上转换发光机制和新型上转换材料的初步研究和探索工作。此外，还开展了铁电晶体的飞秒激光全息存储特性的研究。旨在用飞秒激光脉冲制作周期微结构、及全息光栅。由于飞秒激光本身所具有的特点，形成的光栅结构可能具有较长的寿命，而不需要额外的热固定步骤。目前实验还在进一步的开展当中。With the advent and development of the laser technology and corresponding laser elements such as chirped mirror and semiconductor saturable absorption mirror (SESAM), it comes true that laser pulse as short as two optical cycles can be obtained. At the same time, with the rapid and big progresses of chirped pulse amplification (CPA) and optical parametric chirped pulse amplification (OPCPA) techniques, the focused power of 1022Wcm-2 can be obtained, which provide unheard of and newly experimental methods and physical conditions for the researchers. Now, the laser system is directed towards supporting shorter pulse width, higher peak power, stronger focused peak power density, higher repetition rate and better stability. The research works of this thesis are carried out on the 23TW CPA Ti: sapphire laser system at State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. We give some efficient means and advice, which can improve the performance of the exsiting laser system. Now, the repetition rate of the output laser pulse can reach 100MHz, the pulse width can be shortened and the contrast ratio will be expected to be improved. In addition, we carry out the works on the interaction between ultrashort and ultraintense laser pulses and media, such as lengthening the lifetime of the plasma channel, the effect of the femtosecond laser induced multi-photon absorption upconversion of crystal and the femtosecond laser holographic storage characteristic of the ferroelectric crystal. Main results and innovation points in the thesis can be summarized as follows: 1. We develop a method to generate chirped pulses at high repetition rate with a Ti: sapphire regenerative amplifier: the conventional working mode of the Pockels cell in the Ti:sapphire regenerative amplifier is changed and then the chirped pulse train at high repetition rate (100MHz) is obtained. After amplification and compression, the femtosecond pulse train with the peak power of terawatt (TW) per pulse is obtained, which is focused used for generating plasma channel. Comparing with the 10Hz femtosecond pulses, the experiments show that electron in this plasma channel can last longer time at higher density. This 100MHz femtosecond terawatt laser pulse is the highest repetition rate among the femtosecond terawatt laser systems all over the word. The method of generation of high repetition rate pulse train presented and demonstrated in this thesis can be used on single shot (neodymium glass laser facility) or low repetition rate high power laser systems, which both contain Pockels cell electro-optic switch element, and then the practical application performance of laser systems can be promoted further. 2. The theoretical analysis and experimental results about intra-cavity spectrum shaping of birefringent quartz crystal in regenerative amplifier are given. The filter of quartz crystal with certain thickness is inserted into a Ti:sapphire regenerative amplifier for the intra-cavity spectrum shaping by adjusting the optical axis orientation of the crystal, which can make an appropriate attenuation at the centre of gain, so that it can compensate the effect of the gain narrowing effectively. According to the birefringence dispersion character of quartz crystal, the theoretical analysis and numerical simulation about the polarization interference spectrum are given，and then for the first time a systematic way of measuring the phase retardation and the thickness of quartz wave plate, which is based on the principle of polarization interference, is put forward. 3. The degeneration of contrast ratio of a laser pulse due to the amplified spontaneous emission during the chirped pulse amplification process is studied. The mechanism of the cross-polarized wave (XPW) generation in cubic crystal can applied to improvement of the contrast ratio of laser pulse. We construct a double crystal pulse cleaner and apply it to the improvement of the contrast ratio and have achieved some significant results. Based on these results, some suggestion on improvement the contrast ratio of the laser pulse of the ultrashort and ultraintense laser system at our laboratory is proposed. 4. Cooperated with colleagues, we carry out the works on the interaction between ultrashort and ultraintense laser pulses and media, including the preliminary heuristic work of femtosecond laser induced nonlinear multiphoto absorption upconversion luminescence in rare-earth ions Ce3+-doped Lu2Si2O7 and Gd2SiO5 crystals and the work of late-model upconversion material. In addition, the work of femtosecond laser holographic storage characteristic of the ferroelectric crystal is studied, which is aimed at fabricating periodic microstructure and holographic grating, which may have long lifetime and need not extra heat fixation owing to the feature of the femtosecond laser pulses. Now, the relevant experiments are in process