Prof. Qian Liejia, Department of Physics and Astronomy, Shanghai Jiaotong University, made a major breakthrough in the measurement of signal-to-noise ratio and space-time noise in the ultra-short and super-intense laser pulses. The new measurement ability of nonlinear cross-correlation in space-time domain was established and a new type of physical essence of strong laser noise with spatio-temporal coupling characteristics was found. It provided an important solution to further improve the signal-noise ratio of ultrashort superlaser laser pulses . Relevant results published in Nature Communications [6, 6192, (2015)].
The pulse signal-to-noise ratio (ie, the ratio of the main peak intensity to the noise) of an ultrashort superlaser is a major technical bottleneck that restricts the laser light intensity towards an extremely strong field. SNR measurement is a precondition for achieving ultra-high SNR . Before the international research is limited to the pulse near field time domain signal to noise ratio measurement, but the vast majority of strong field physics experiments at the laser focus. The team found that the near-field measurement of pulse signal-to-noise ratio can not be true, fully reflect the pulse signal-to-noise ratio at the far field target surface. The research method is to expand the cross-correlation from the time domain to the space-time domain, and to measure the pulse far-field signal-to-noise ratio in high dynamic range from two dimensions of time and space, which promotes the research and understanding of super laser noise. Using a new cross-correlation in time and space, the team has experimentally found that surface scattering of a grating isotropic element causes noise with spatially and temporally coupled features that, along with the superlattice laser beam focus, disperse at different positions in the focal plane . Therefore, the measurement result in the range of strong laser focal spot is the pulse signal-to-noise ratio with physical significance. Utilizing the spatial-temporal cross-correlation of noise sources, the team found that optical elements located in the far field in the broadening / compressors can severely impact the pulse signal-to-noise ratio on the far-field axis. This finding provides a design guideline for high power laser systems: high signal-to-noise ratios require the use of far-field optics in broadening / compressors.
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