毛细管放电泵浦谐振腔自反馈多波长软X射线激光及其动力学研究

The soft X-ray or extreme ultraviolet (EUV) lasers, whose wavelengths are shorter than 100nm, is mainly obtained with single-pass amplification without feedback by using plasma as gain medium. The beam qualities of these lasers are poor. The realization of resonator self-feedback soft X-ray lasers is beneficial to improve the laser beam quality, increase the laser spatial coherence and expand their applications. On the basis of our first demonstration of multi-wavelength soft X-ray laser of 46.9nm, 69.8nm and 72.6nm pumped by capillary discharge, we will realize multi-wavelength soft X-ray laser with resonator self-feedback in this project. In theory, in combination with the simulation of plasma Z-pinch process and laser kinetics, the gain duration and intensity distribution of multi-wavelength laser will be calculated. In experiment, the SiC mirrors as rear mirror and output mirror will be used to realize the resonator self-feedback of multi-wavelength laser. The influences of experimental parameters on the laser will be researched by changing cavity type, transmissivity of output mirror, amplitude of main pulse current, initial pressure, amplitude of pre-pulse current, delay between pre-pulse and main pulse and so on. The laser beam quality will be improved by self-feedback, and a quasi-Gaussian multi-wavelength laser of 46.9nm, 69.8nm and 72.6nm will be realized finally.

目前以等离子体为增益介质，波长小于100nm的软X射线或极紫外激光，主要采用的是无反馈单程放大方式，光束质量差。实现光学谐振腔自反馈软X射线激光，对改善激光光束质量、提高激光空间相干性、拓展激光的应用具有重要的意义。本项目以本课题组首次实现的毛细管放电类氖氩46.9nm、69.8nm和72.6nm多波长激光为基础，采用光学谐振腔自反馈，拟实现多波长激光输出。理论上，结合等离子体Z箍缩过程和激光动力学模拟结果，计算多波长激光的增益持续时间和光强空间分布。实验上，采用SiC为后反镜和输出镜的谐振腔，实现多波长激光的自反馈。通过改变谐振腔腔型、输出镜透过率、主脉冲电流幅值、初始气压、预脉冲电流幅值和预主脉冲延时等参数，研究各参数对谐振腔自反馈多波长激光的影响。通过自反馈方式改善光束质量，最终获得近高斯分布的46.9nm、69.8nm和72.6nm多波长激光输出。

由于受到腔镜反射率低、等离子体轰击易损坏腔镜、增益持续时间短等因素影响，很难将谐振腔应用于以等离子体为增益介质的软X射线或极紫外激光中。目前该波段主要采用无反馈单程放大方式获得激光。国际上只有一篇采用谐振腔获得软X射线激光三程放大的实验报道，由于等离子体轰击，输出一个激光脉冲后多层膜腔镜即会被损坏。本项目采用无需镀膜的SiC反射镜为后反镜，采用带有中心孔和带尖角的SiC反射镜为输出镜，解决了在等离子体轰击下谐振腔寿命很短的问题，实现了谐振腔自反馈多波长激光多程放大。.理论上，采用一维磁流体力学模型，模拟了等离子体Z箍缩过程和激光产生时刻的等离子体状态分布。计算结果表明，增大初始等离子体直径可以增大增益体积，有利于提高激光能量。建立了毛细管放电类氖氩多波长激光器动力学理论模型，计算了类氖氩离子能级参数和69.8nm、72.6nm激光增益系数。计算结果表明，69.8nm激光增益系数可达0.47cm-1，增益系数较小是导致实验上没能实现较高强度72.6nm激光的主要原因。建立了单程、双程和多程放大类氖氩激光光斑的理论计算模型，研究了单程放大激光光斑形状、谐振腔腔型、输出镜开孔大小等参数对多程放大激光的影响规律。.实验上，在两台毛细管放电装置上，研究了主脉冲电流幅值、初始气压、毛细管内径、预脉冲电流幅值和预主脉冲延时等参数对单程放大46.9nm、69.8nm和72.6nm激光的影响规律。获得的46.9nm激光能量达1.5mJ，69.8nm激光增益系数达0.47cm-1，大于国际上其他小组报道的结果。采用平面SiC反射镜为后反镜获得了46.9nm和69.8nm激光双程放大，根据双程放大激光脉冲，研究了增益系数随时间的变化规律。以上实验为实现多程放大奠定了基础。分别采用带中心孔的SiC反射镜和带有尖角的SiC反射镜为输出镜，实现了平-平谐振器自反馈46.9nm和69.8nm激光的多程放大。研究了实验参数对多程放大激光的影响规律，获得了近高斯分布的46.9nm和69.8nm多波长激光输出。实验上还研究了46.9nm激光的空间相干性，并用该激光开展烧蚀铜靶的应用研究。本项目获得的高光束质量的谐振腔自反馈46.9nm和69.8nm激光，在高分辨率微纳加工、显微、光刻等领域有广泛的应用价值。