## 6 Conclusions and Prospect  
In the paper, we classify the self-reference measurements for ultrashort pulse into three categories: Correlation, Spectrogram, and Spectral Interferometry. Correlation is a time-domain technique. It obtains rough time information of the pulse by delaying the interactional pulses in a nonlinear crystal. For its simple geometry and measurement, correlation has very high measuring SNR, which makes it irreplaceable in the measurement of contrast for high power lasers. The Spectrogram technique is a 2D \(\omega\)-\(\xi\)-combined measurement, \(\xi\) can be the time delay (FROG) or other scanning parameters (e.g., dispersion in d-scan, phase of spectral modulation function in MIIPS). Benefiting by their time-frequency over-determine, FROG has its superiority in drawing complex pulses, meanwhile, the pulse retrieval needs a time-consuming iteration algorithm. Spectral Interferometry (including SPIDER and SRSI) is based on spectral measurement by coding the spectral phase into the interferential fringes. Spectral Interferometry has a direct, robust retrieval algorithm supporting real-time display at a very high frame rate. Interferometry, especially shearing interferometry, has a high requirement for the resolution of the spectrograph when applied to complex pulses. In the future, ultrashort pulse lasers will have higher peak power, more complex pulses, and/or more diverse wavelengths, which bring higher and new requirements for their measurements. In high power laser region, 10 PW level lasers with a focused intensity of \(\sim\)10\({}^{22}\) W/cm\({}^{2}\) have been realized in many countries or regions, and to build 100-PW-level lasers is on the agenda. This demands the Correlator can work with ultrahigh SNR (\(>\)10\({}^{12}\)), single-shot, space-resolved technique (a recent study [80] shows that spatiotemporal noise also impacts the far-field contrast). Nowadays, with the developments of the ultrafast laser technology and the optical manipulations, the forms of ultrashort light fields become more and more abundant. In some scenes, such as few-/single-cycle pulses generation, white-light supercontinuum induced by Kerr effects, spectral coherent synthesis, and spectral modulation for quantum coherent control, the pulse structures have comparative complex in the temporal domain, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**SPIDER Geometry** & **Traditional** & **TSPS/CTP [65, 68]** & **CP [71]** & **2 DSI [13]** \\ \hline Operating Mode & Single-shot & Single-shot & Single-shot & Multi-shot \\ Acquire data & 1D & 1D \(\times\) 2\({}^{1}\) & 1D/1D \(\times\) 2 & 2D \\ Spectral resolution 2  & 5\(\sim\)10 \(\Delta\omega_{\text{res}}\) & \(\sim\)2 \(\Delta\omega_{\text{res}}\) & \(\sim\)2 \(\Delta\omega_{\text{res}}\) \\ Shear 3  & 0.1–0.2 \(\Delta\omega_{\text{BW}}\) & \(\sim\)0.015 \(\Delta\omega_{\text{BW}}\) & \(\sim\)0.03 \(\Delta\omega_{\text{BW}}\) & \(\sim\)0.05 \(\Delta\omega_{\text{BW}}\) \\ Phase error (3 dB bandwidth) & \(\sim\)44 mrad [74] & 74.2 mrad (TSPS) & 2.6 mrad & Unspecified \\  & & 19.3 mrad (CTP) & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of some SPIDER geometries. which requires the measurements to have a high temporal resolution. While in some other scenes, the identity of the pulse characteristics in space is broken, where spatiotemporal measurement capability becomes important. For example, in optical parametric amplification (OPA) or optical rectification for THz generation, the pulse-front title is induced to realize velocity-matching to improve the energy conversion efficiency or the bandwidth; for vortex beams or other optical fields by spatial manipulation, the pulse characteristics depend on spatial distribution; for few-cycle pulses, the spatiotemporal coupling effect becomes sensitive for its very broad bandwidth. In the past, measurement techniques are mostly designed for ultrafast lasers in the near-infrared (IR). Nowadays, more requirements in other wavebands are put forward.