Study On Propagation Properties Of Femtosecond Laser Diffracted By "Surface-volume-surface" Gratings

With the advantages of high-peak power,ultra-short pulse duration time and no-thermal effect,the femtosecond laser has been widely used in numerous fields,such as high-field physics,nonlinear optics,laser nanomachining,laser medical science and so on.Therefore,the research on the propagation and control of femtosecond laser is one of the most popular problems in the field of ultrafast laser technology.In the femtosecond laser system,with the increase of the output peak power,the higher contrast and the better beam quality of amplification pulse laser are required as well.With the advantages of near field spatial filtering for a large bandwidth light source,the “Surface-Volume-Surface” gratings(SVSGs)is possible to improve beam quality of the femtosecond lasers.However,the femtosecond laser contains wide spectral bandwidth,and spatio-temporal distortions can be introduced when femtosecond laser diffracted through SVSGs due to the group delay dispersion(GDD)and angular dispersion.In this thesis,the time-domain and spatial-domain properties of femtosecond laser diffracted through SVSGs are studied.Firstly,the time-domain intensity distribution equation of sech2 femtosecond laser diffracted through SVSGs is obtained with combining the diffraction equation of transmitting surface gratings(TSGs)and volume Bragg gratings(VBGs).Then,the additional phase and GDD of femtosecond laser diffracted through the TSG,VBG and SVSGs are derived with the ray-tracing method.The time-domain intensity distribution of Fourier transform limit and chiped femtosecond laser with different pulse durations diffracted through the TSG and SVSGs are analyzed,respectively.Results show that the pulse broadening decreases with the grating period increases.While the pulse broadening increases with the grating spacing and grating thickness increases.Finally,in the case of grating period matching and mismatching between TSG and VBG,the spatial properties of femtosecond laser diffracted through SVSGs are discussed with the coupled wave theory,respectively.Results show that the femtosecond laser diffracted through SVSGs will not generate spatial dispersion,and the propagation of the diffraction pulse will be coaxial with that of the incident pulse in the case of grating period matching between TSG and VBG.However,In the case of grating period mismatching between TSG and VBG,the femtosecond laser diffracted through SVSGs will generate spatial dispersion,and the diffracted angle and angular dispersion analytic expression are given.The results in this thesis may provide a theoretical solution in the near field filtering for femtosecond laser.