## 5 Discussion and Prospect  
We have discussed several methods for characterizing femtosecond pulses. As a relatively novel method, SRSI has many advantages over other methods used to characterize the temporal profile of femtosecond laser pulses. The frequency-conserved third-order nonlinear processes XPW, SD, and TG are used to generate the reference pulse for SRSI, of which TG is the most advantageous. The combination of TG-FROG and TG-SRSI comprises the best properties for pulse measurement. Great progress has been made in improving SRSI, including extending the spectral range, improving its sensitivity to input pulse energy, simplifying the setup, and increasing the dynamic range. We hope to develop microdevices based on TG-SRSI similar to sensors that can be located inside a femtosecond laser system to monitor or control via feedback the temporal profile of output pulses.  
SI can characterize pulses with an average energy of 42 zJ per pulse, i.e., less than one photon per pulse [50]. Furthermore, SRSI with a 25 dB SNR spectrometer can characterize a temporal profile with a 50 dB dynamic range during pulse measurement [39]. These properties allow SRSI to be used for the single-shot characterization of the temporal contrast of ultraintense femtosecond laser pulses. In 2013, researchers achieved a temporal contrast of up to 60 dB in a 25 ps window using SRSI [64]. However, this dynamic range is far lower than that required for the single-shot characterization of an ultraintense femtosecond laser pulse. We expect to extend the use of SRSI to the single-shot characterization of a femtosecond laser pulse with a considerably higher dynamic range. This should be possible, because the temporal resolution of SRSI is significantly higher than that of the third-order correlator,  
Figure 19: Characterization of femtosecond pulses at 84-MHz repetition rates. (**a**) Reconstructed (R) and directly measured (D) spectra of the input unknown (test) pulse and the TG reference pulse. (**b**) Reconstructed temporal profile (black solid line) and phase of the input unknown pulse (purple line), and the FTL temporal profile of the input unknown pulse (red dashed-dotted line). (**c**) Reconstructed temporal intensity profile on a logarithmic scale. (**d**) Fluctuation of pulse duration at different input pulse energies [63].  
the temporal contrast resolution of which is usually hundreds of femtoseconds, or picoseconds. For a PW femtosecond laser system, with a pulse duration typically of tens of femtoseconds, precisely showing the satellite prepulses of the ultraintense laser by SRSI may assist in building or using such systems.  
This work was supported by the National Natural Science Foundation of China (NSFC) (grants 11274327, 61521093, and 61527821), the Instrument Developing Project of the Chinese Academy of Sciences (grant YZ201538), and the Strategic Priority Research Program of the Chinese Academy of Sciences (grant XDB160106).  
[PERSON] conceived and designed the experiments; [PERSON], [PERSON], and [PERSON] performed the experiments and analyzed the data; and [PERSON], [PERSON], and [PERSON] supervised the project. All authors contributed to the preparation of the paper.  
The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; collection, analyses, or interpretation of the data; writing of the manuscript; or the decision to publish the results.