Weak femtosecond pulse characterization based on Self-Referenced Spectral Interferometry

飞秒激光脉冲的测量有很多种方法，目前比较主流的方法仍是FROG和SPIDER，然而不得不提的是该两种方法在算法上仍不为理想。前者属于光谱图像处理，通常用盲目迭代算法，需要的图像采集时间和算法迭代时间都比较长，难以对脉冲进行实时测量和监测，并且在迭代过程中的比较残存误差。后者虽然具有比较直接的光谱相位算法，然而其完全的重建也需要一个积分过程，需要一定的时间，并对参考光脉宽和光谱形状作了一定的假设。除了算法外，两者的装置结构都相对比较复杂。 SRSI于2010年提出，该方法具有线性、解析、灵敏、准确的特性，更为重要的是具有直接的光谱重建和相位求解迭代算法，可以实现对飞秒激光脉冲进行实时测量和监测。 在弱飞秒激光脉冲测量领域，针对不同应用，也存在很多比较好的方法并将所能测量的单脉冲能量拓展到了飞焦甚至阿焦量级，而对于直接从振荡器输出的单脉冲能量为几纳焦到几十纳焦的飞秒激光脉冲，在考虑装置的简单易调，算法的直接快速上，TG-SRSI装置仍具有一定的优势和探索潜力。 本文以测量单脉冲能量为几纳焦到几十纳焦的弱飞秒脉冲为目标，利用基于瞬态光栅效应的自参考光谱干涉方法搭建了一台实验装置。 首先，我们简要的概述了自参考光谱干涉方法目前所用的三种装置结构XPW-SRSI、SD-SRSI、TG-SRSI，对三种装置结构进行对比，展现出基于瞬态光栅效应自参考光谱干涉方法在结构上不光具有较其它两种方法更具优势（相位自动匹配，参考光无角色散，参考光与信号光方向一致，无需光学偏振元件，不受波长范围限制等），同时具有相对较高的灵敏度（约为10nJ），这为测量单脉冲能量为几纳焦到几十纳焦的弱飞秒脉冲提供了可能。 其次，我们设计了两种TG-SRSI的原理性装置，并依此提出了装置的优化方案，利用反射式显微物镜，搭建了一台TG-SRSI实验装置。 再次，我们探索了SRSI方法的算法，分析怎么实现光谱的重建和相位的求解。 最后，我们利用所搭建的实验装置，成功的实现了对中心波长为800nm、脉冲宽度约为43fs、单脉冲能量为65nJ的钛宝石系统输出光的测量，并用SHG-FROG方法对同样的脉冲进行测量，验证了实验装置的可靠性Many techniques have been developed for femtosecond pulses characterization, the most widely used techniques are Frequency Resolved Optical Grating (FROG) and Spectral Phase Interferometry for Direct Electric-field Reconstruction (SPIDER), while, it should be noted that their algorithms are not ideal. FROG belongs to the class of spectrographic measurements and relies on a blind iterative retrieval algorithm. It costs long time to gain the spectrographic and retrieve the phase, so FROG is hard to be used to measure or monitor the output pulses in real time, what’s worse, there may exist error during the iteration. SPIDER uses an analytic function of the spectral phase is directly measured but complete phase retrieval is obtained through an integration or concatenation step. Furthermore, it makes some assumptions on the pulse duration and spectral shape. Besides algorithm, these two techniques’ devices are usually complex. Self-referenced spectral interferometry (SRSI) appears in 2010. TG-SRSI is linear, analytic, sensitive and accurate, what’s more its direct algorithm for pulse and phase reconstruction is direct. This makes it possible to measure or monitor the output pulses in time. In the area of weak femtosecond pulse characterization, there are also many good methods, and the pulses with energy as low as femtojoule and even attojoule have been characterized successfully. Take the device’s briefness and algorithm’s efficiency in consideration, TG-SRSI is still a good method for the characterization of pulses from few nanojoules to tens nanojoules level. The thesis is proposed for the characterization of pulses from few nanojoules to tens nanojoules. We build a device based on transient-grating self-referenced spectral interferometry. At first, we give a simple introduction of the three geometries of SRSI: XPW-SRSI, SD-SRSI and TG-SRSI. It can be concluded that TG-SRSI has more advantages than XPW-SRSI and SD-SRSI, the BOXCARS beam geometry of TG-SRSI leads the phase self-matched, the transient-grating single is without angular dispersion and has the same direction with the test beam, no polarizers is used and wavelength range is not limited, what’s more, TG-SRSI has a sensitivity as high as 10 nJ, and it is possible to be used characterization of pulses from few nanojoules to tens nanojoules. According to the analysis proposed before, we design two proof-of-principle devices of TG-SRSI, then we optimize the device and build a TG-SRSI device. Self-referenced spectral interferometry algorithm is discussed, we analyze how to reconstruct the pulse and phase. At last, the TG-SRSI device built is used to characterize a 65 nJ pulse from a Ti:sapphire laser system (Coherent Legend Elite) with center wavelength 800nm, its pulse duration is about 43 fs, and a second harmonic generation (SHG) based FROG (SHG-FROG) is used to measure the same pulse for comparison, both of the characterization results agree well with each other