## 1 Introduction  
Since the 1990s, ultrashort pulse laser technology has greatly progressed. Nowadays, the peak power of laser pulse reaches up to PW [1], the intensity goes beyond \(10^{22}\) W/cm\({}^{2}\)[2, 3], while the pulse width can be controlled at few-cycle even sub-cycle of light [4]. Simultaneously, ultrashort pulse lasers are widely applied in many subjects, such as physics, chemistry, biology, medicine, materials, and so on. Correspondingly, characterizing temporally laser pulse becomes more and more important no matter for the developments of ultrafast laser technology or its applications. To measure a transient physical quantity, we need another controllable faster quantity as a \"time ruler\" to compare with it. For example, in a streak camera [5], the time ruler is the electric-field gradient in the picosecond scale. However, ultrashort laser pulses in the femtosecond scale are so fast that it is difficult to arrange their ruler except themselves. So practically, characterizing ultrashort pulses usually works by \"light measures light\", or by the comparison between a test pulse and a reference pulse. The reference can be the frequency-conversion [6], or spectral modulation [7, 8] from the test pulse, as well as the test pulse itself [9]. Spectral interference [7, 8] or optical nonlinearity [9] is induced for the comparison between the test and the reference. The measurement can be carried out in the time-domain [6, 10], frequency-domain [7, 8], or both [11, 12]. In the time-domain, the most important parameter is the delay time between the test and the reference, which is controlled by their fly-time difference in the light paths. In the frequency-domain, spectrograph can be directly applied to spectral measurements for the output light, which benefits from the wide bandwidth of the ultrashort pulses. In the ultrashort pulse measurement, optical nonlinearity, as an all-rounder, e.g., frequency-conversion, correlation, spectral convolution, or time-filtering [6, 7, 8, 9, 10, 11, 12, 13], plays a very important role.  
In this article, we review the developments of the technologies to measure ultrashort light pulses in recent years, which divides into three types: correlation [6; 10], spectrogram [11; 12], and spectral interferometry [7; 8], which are based on time-domain, time-frequency combining, and frequency-domain measurements, respectively. Correlation is a simple, robust, and high signal-to-noise ratio (SNR) method to obtain the pulse intensity information but without the phase information. It is not suitable for characterizing pulses with complicated structures, e.g., asymmetric air pulses [14], white-light supercontinuum [15]. Spectrogram and spectral interferometry can measure the complete information of the pulse with both intensity and phase. Spectrogram usually can be seen as a frequency-resolved correlation measurement, light field information is retrieved from the 2-dimension (2D) time-frequency spectrogram by an iterative algorithm. Spectral Interferometry is based on direct spectral phase measurement by self-reference interferometry. All the three techniques have their single-shot versions which are suitable for pulses those with low repetitive rate. The article is arranged as follows: Section 2 shows the basics of the ultrashort laser pulse; Sections 3-5 introduce the techniques based on correlation, spectrogram, and spectral interferometry, respectively; Section 6 is the Conclusion and Prospects.