Progress has been made in the research of single shot characterization technology for complex combination laser pulses at Shanghai Institute of Optics and Fine Mechanics

Recently, the research team of the High Power Laser Physics Joint Laboratory at the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, has made significant progress in the study of single shot characterization technology for complex combination laser pulses. The research team utilized an improved broadband transient grating frequency resolved optical switch technology (TG-FROG) to achieve complete characterization of complex high-power laser pulses in a single shot, and revealed the dynamic evolution law of ultra short pulses during nonlinear frequency conversion. The related research results were published in Optics Express under the title "Single shot complete characterization of synthesized laser pulses and non-linear frequency conversion process".

The combination of laser fields (pulse combinations with different polarizations, center wavelengths, or pulse widths) has important applications in fields such as ultrafast spectroscopy and high-order harmonic generation, but its precise measurement faces multiple challenges. Traditional methods are limited by polarization sensitivity, insufficient measurement bandwidth, or the need for multiple measurements, making it difficult to meet the real-time diagnostic requirements of high-power, low repetition rate laser systems. In addition, the dynamic characteristics of the nonlinear frequency transformation process of complex pulses lack effective observation methods, which restricts the optimization and application expansion of laser systems.

In response to the above challenges, the research team based on improved TG-FROG measurement technology, designed a self referencing and reflective structure with a wideband imaging spectrometer, to achieve single measurement support for at least 460nm spectral range, with a time resolution of 5.81 fs and spectral resolution better than 0.13 nm. The synchronous observation of waveform and spectral evolution of fundamental frequency pulses and second harmonic pulses during nonlinear frequency conversion has been achieved, revealing complex modulation effects such as spectral broadening, redshift, and time-domain multi peak structure under high-energy injection. And successfully measured the dual color pulse with spectral time-domain separation generated by the cascaded second harmonic process, and analyzed its time delay (208.4 fs) and relative phase (0.29 rad), breaking through the phase ambiguity limitation. This method not only provides a good measurement method for optimizing the waveform and contrast of ultra wideband laser pulses, but also provides a powerful diagnostic tool for complex nonlinear optical physical processes.

Figure 1 (a) Single broadband TG FROG device; (b) The process of broadband nonlinear frequency transformation and the experimental optical path diagram of dual pulse measurement.

Figure 2 TG-FROG synchronous measurement results of fundamental frequency pulse and second harmonic pulse during SHG process under high injection energy

Source: opticsky