**1. Introduction**

348 Modern Metrology Concerns

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An ultrashort optical pulse has an extremely short duration period, an extremely broad spectral bandwidth, and an extremely high peak power [1]; it has therefore been widely used in a variety of applications, such as ultrafast pumping and detection, time-resolved spectroscopy, optical telecommunications, ultra-fine microfabrication, nonlinear optics and femtosecond chemistry. Femtosecond optical pulses have also brought revolutions in contemporary metrology [2], including time and frequency standards [3], terahertz metrology [4], and ultrafast electric pulses characterization [5]. Time domain waveform and pulse width are key parameters of the ultrashort optical pulses, because they directly affect experimental results acquired by use of ultrashort optical pulses. Experimental data cannot be deemed credible unless the waveform and pulse width are determined. Accurate knowledge of the temporal shape of optical pulses is therefore crucial to scientific research.

In the past three decades, several measurement techniques for ultrashort optical pulse have been developed. Autocorrelation (AC) [6], frequency-resolved optical gating (FROG) [7] and spectral phase interferometry for direct electric-field reconstruction (SPIDER) [8] are most commonly used. Autocorrelation is simple and convenient, but can give only the autocorrelation width; the waveform and phase is difficult to obtain. FROG is a twodimensional measurement technique, pulse waveform and phase can be retrieved from the FROG trace, but an iterative procedure is needed. SPIDER can directly extract spectral phase and reconstruct pulse waveform; it is therefore suitable for accurate and fast measurement of ultrashort optical pulses, especially for the optical pulses in femtosecond range. In this chapter, we would introduce our technique for spectral phase retrieval to improve the accuracy of femtosecond optical pulses measurement, and demonstrate a versatile autocorrelator for ultrashort optical measurement, with which the time domain measurement range is greatly broaden.

<sup>\*</sup> Corresponding Author
