Practical Techniques for the Measurement of Ultrashort Laser Pulses

超短激光脉冲的测量仍然是一项艰巨的任务。自相关器，几十年来的首选设备，是复杂的，包括许多组件。脉冲必须一分为二；然后，两个脉冲必须在空间和时间上重叠在一个精心排列的二次谐波产生（SHG）晶体中；在保持对准的同时，必须对脉冲相对延迟进行扫描。对准涉及四个敏感自由度，有很大的潜在误差。更糟糕的是，带宽方面的考虑需要非常薄的SHG晶体，这严重限制了器件的灵敏度。频率分辨光门控（FROG）产生完全随时间变化的脉冲强度和相位，需要在自相关装置上增加一个光谱仪，使本已复杂的装置进一步复杂化。FROG是经过验证的最简单的强度和相位测量设备。因此，我们开发了一种非常简单的，全强度和相位脉冲测量装置，1)不使用光谱仪，2)不使用分束器或延迟线，3)避免使用薄晶体，而是使用更方便的厚SHG晶体，4)相当灵敏，5)不涉及敏感的对准参数，6)可以测量单个脉冲，7)只包含几个简单的光学元件。它产生与SHG FROG相同的迹线，因此产生完全随时间变化的脉冲强度和相位。该装置包含两个关键的创新：1)菲涅耳双棱镜（一个顶点角为170的棱镜），取代了分束器和延迟线；2)一个厚非线性晶体，其输出光是角分辨的，产生光谱分辨率（以及提高灵敏度和通常的时间门控），取代了薄晶体和光谱仪。

The measurement of ultrashort laser pulses remains an arduous task. Theautocorrelator, the device of choice for decades, is complex, comprising many components. The pulse must be split into two; the two pulses must then be spatially and temporally overlapped in a carefully alignedsecond-harmonic-generation (SHG) crystal; and the pulse relative delay must be scanned while maintaining alignment. Alignment involves four sensitive degrees of freedom and has great potential for error. Worse, bandwidth considerations require a very thin SHG crystal, which severely limits device sensitivity.Frequency-Resolved Optical Gating (FROG), which yields the full time-dependent pulse intensity and phase, requires adding a spectrometer to an autocorrelator apparatus, further complicating an already complicated apparatus. And FROG is the simplest proven intensity-and-phase measurement device. As a result, we have developed a remarkably simple,full-intensity-and-phase pulse-measurement device that 1) uses no spectrometer, 2) uses no beam-splitter or delay line, 3) avoids the thin crystal and instead uses a more convenient, thick SHG crystal, 4) is considerably more sensitive, 5) involves no sensitive alignment parameters,6) can measure a single pulse, and 7) comprises only a few simple optical components. It yields traces identical to those of SHG FROG, and hence yields the full time-dependent pulse intensity and phase. This device incorporates two key innovations: 1) a Fresnel biprism (a prism with a 170 apex angle), which replaces the beam splitter and delay line, and 2) a thicknonlinear crystal, whose output light is angularly resolved, yielding spectral resolution (as well as improved sensitivity and the usual time-gating), and which replaces the thin crystal and spectrometer.