Research On The Spatio-temporal Coupling Of High Peak Power Femtosecond Laser

Ultrashort and ultraintense laser has developed rapidly since the chirped pulse amplification technology was proposed,and it is widely used in many fields such as physics,chemistry,material science,biology and medical science.With the increasing requirements of these subjects,the ultrashort and ultraintense laser is developing towards high peak power,short pulse,high signal-to-noise ration and high stability.At present,many countries in the world have established their own high peak power femtosecond laser systems based on chirped pulse amplification technology which involves pulse stretching,beam expansion,pulse compression and other processes.These process may introduce some spatio-temporal distortions which will have great influence on far-field distribution,pulse-duration,peak intensity,signal-to-noise ratio,etc.This paper focuses on the common spatio-temporal coupling generated by beam expanding system,stretcher and compressor,which are analyzed theoretically,studying the influence of saptio-temporal coupling on the properties of pulses mainly by numerical simulation,and the improvement measures are put forward.The main contents include the following parts:1.Several kinds of spatio-temporal coupling in the high peak power femtosecond laser systems are introduced.The mechanism of spatio-temporal coupling and it's influence on the pulses are analyzed briefly.The results show that beam expansion,pulse stretch and pulse compress process will introduce a large spatio-temporal distortion and it will have a great influence on the properties of the pulses.2.For beam expansion system,the formula of time delay at different position in lenses is derived,and a distortion-free beam expansion unit has been designed based on the formula.Based on numerical simulation,the spatio-temporal coupling of pulses in the near-field and far-field is also calculated.The results show that by using this distortion-free beam expansion system,the spatio-temporal distortion can be effectively compensated and the far-field distribution can be improved.3.For several kinds of commonly-used stretchers in the high peak power femtosecond laser systems,the spatio-temporal coupling introduced by its' aberration is analyzed,the analysis indicates that the double-grating-offner-stretcher can solve the problem of serious aberration in the traditional stretcher.Three key factors of introducing spherical aberration in the double-grating-offner-stretcher is analyzed based on a three-dimensional model:off axis in the stretcher,the deviation of the incident pulse,the beam size,and the improvement measures are put forward for these factors.The influence of medium-high frequency and low frequency of surface defect on signal-to-noise ratio of pulses is analyzed in detail.4.The spatio-temporal distortion introduced by deflection of the reflective mirror in double-pass two parallel grating pair compressor is analyzed based on ray-tracing method,and the error tolerances according to strehl ratio and pulse-duration broadening ratio are given under three different conditions.The analysis indicates that ideal four grating compressor can output a fourier transform limit pulse.The dispersion process of four grating compressor and the focusing process of parabolic mirror are analyzed based on ray-tracing method and Fraunhofer far-field diffraction.The wavefront distortion for square aperture is described using Square-Radial polynomial.The effect of four different wavefront distortions in near-field on far-field spatio-temporal distribution is analyzed,and the error tolerances according to strehl ratio and pulse-duration broadening ratio are also given under different conditions.