【作者】 戴小民；

【导师】 丁良恩；

【作者基本信息】 华东师范大学 ， 光学， 2010， 硕士

【摘要】 超短激光脉冲技术已经发展成为研究超快现象的强大工具,为了提高诊断的灵敏度和可靠性,人们总是希望获得关于激光脉冲尽可能多的特征参量,其中脉冲宽度的测量就是一个具有挑战性的关键性参量。对于紫外超短激光技术的发展,不仅取决于紫外激光技术本身的积累性发展,同时还得益于紫外脉冲测量技术的改进和新方法的问世。目前,人们常用的测量与诊断紫外脉冲的方法有自相关法、频率分辨光学开关法(FROG)和光谱位相相干电场重构法(SPIDER)等。自相关方法在概念上非常简单,也容易理解,但是由于大多数晶体在紫外波段不透明,非线性晶体的位相匹配条件限制了可测量光脉冲的范围。且被测激光脉冲都有一定带宽,而非线性晶体的有限相位匹配带宽又无法以相同的转换效率将全部频率分量转换,导致脉冲光谱畸变,因此自相关无法直接应用于紫外波段的脉冲测量。FROG可以给出脉冲宽度、光谱带宽、位相等比较详细信息,但是涉及到复杂的测量程序和计算程序,技术操作繁琐,运算速度慢,不适合实时测量,而且给出的只是近似的脉冲信息。SPIDER法作为一种自参考型的光谱干涉技术,最大优点是脉冲重建算法简单,运算速度极快,真正实现了脉冲的实时测量。同时,光谱相位重建的准确性对非线性晶体的相位匹配带宽和探测器的光谱响应都不敏感,只取决于光谱干涉条纹的间隔。而且它的灵敏度还很高,可以用于测量比较弱的脉冲,但是并不像FROG那样可以直观的给出脉冲宽度,为了计算脉冲宽度,还必须同时测量脉冲的光谱。总之,目前还没有一个公认简单而快速测量紫外极端超短激光脉冲宽度的方法。在这篇论文中,我们以实验室的现有条件为基础,讨论了一些简单实用的测量飞秒紫外激光脉冲宽度的方法,如光丝中双光子荧光显微镜法、互相关法等,拟通过这些方法得出强度对比度信息和脉宽信息,以期在检测激光系统的“健康状况”,对系统优化提供判据,给强场物理实验提供参考等方面有重要的实用价值。我们利用这些方法对实验室紫外光源进行了测试,实验结果表明：我们的测量紫外脉宽的双光子荧光系统时间分辨率达8.3 fs。在单丝的情况下测量脉宽时,该系统测量脉宽不受输入能量的影响,信号强度与背景光丝强度的对比度达3：1,且不需要相位匹配,避免了材料色散所带来的脉冲扭曲现象,能够快速采集数据,能够可靠和重复对紫外超短脉冲进行动态实时诊断,为紫外激光脉冲宽度的诊断与测量提供了一种简单而有效的方法。我们的互相关方法测量紫外飞秒脉冲宽度也得到了比较理想的结果,但是由于两束激光脉冲会存在差异,定标也存在一定的偶然误差,我们发现测量的脉宽结果差异很大,因此用该方法单独测量脉宽还不太可靠。但是我们通过对该测量系统进行了定标,分析了探针光和主激光相对延时与混频光信号峰移动量之间的关系,以及两光束夹角对混频光信号峰移动量的影响,因此在利用该方法测量脉宽的实验中,要尽可能的根据具体测量要求选择合适的夹角及晶体,以期减小测量误差。另外,我们还可以根据上述系统对两束光的延时实现飞秒量级的精确测量和利用光光转换实现对两束光延时的准确控制。更多还原

【Abstract】 Ultrashort laser pulse technology was powerful tools for the study of ultrafast phenomena. Nevertheless the measurement of pulse duration which is important to the development and the application of laser technology is a challenging problem.the development of ultrashort UV laser technology profits from the progress of laser technology itself, as well as the advance of the measurement methods for the uv pulse duration. So far, the autocorrelation, frequency-resolved optical gating (FROG) and spectral phase interferometry for direct electric-field reconstruction (SPIDER) are the general methods for the measurement of optical pulse duration. In the autocorrelation method, pulse was split into two beams, and intersected non-collinearly in the nonlinear super-sensitive crystal. pulse duration can be estimated from the relevant signal. The autocorrelation method is simple, however, it is inapplicable to the UV region due to the phase mismatching. In addition, nonlinear crystal phase matching bandwidth is limited and can not have the same conversion efficiency for all the frequency components, resulting in the distortion spectrum. Thus, although the autocorrelation method is very simple and easy to understand, it can not be directly applied to ultraviolet pulse measurement. FROG is a derivative of autocorrelation, it is a completely background-free auto-correlator, which can measure accurately the UV pulse duration, spectral Bandwidth and phase in theory. but it involves complex measurement procedures and calculation procedures, cumbersome technical operations, and the pulse is also given only approximate information. Therefore, FROG is not conducive to measure in real-time. SPIDER is a self-reference type of spectral interference technique, it’s advantage is that the reconstruction algorithm is convenient, the accuracy of spectral reconstruction which is not sensitive to the nonlinear crystal phase-matching bandwidth and spectral response of the detector depends only on the interval of spectral interference fringes. However, it can not be given pulse duration. In short, there is no simple and fast method for the measurement of extreme ultraviolet ultrashort laser pulse duration.In this paper, the measurement of UV ultrashort pulse based on the two-photon ionization and cross-correlation in air is demonstrated. It is shown that our experiments can obtain data reliably in real-time. The intensity of signal is threefold higher than that of the background. The resolution of 8.3fs is demonstrated. The relatively good results are also acquired with the cross-correlation measurement system. But the accidental errors in calibration cannot be able to avoid due to the deviation between two pulses, which caused the instability of the experimental results. The measurement of UV pulses duration only based on the cross-correlation is not reliable. However, we analyzed the signal peak position versus the delay between probe pulses and pumps, as well as the dependences of signal peak position on the intersection angle between the two light beams. The experimental results show that the appropriate intersection angle and crystals are necessary for the accuracy of data. In addition, we can measure and control the precision delay of femtosecond magnitude by this system, and achieve light to light conversion in delay measurements of two beams light.更多还原