Research On Spectral Tuning And Diagnosis Methods Of Broadband Near Infrared Pulses

With the development of ultrafast science and technology,the generation and diagnosis of high-energy and frequency-tunable few-cycle laser pulse sources,which have a wide range of applications in ultra-high time-resolved detection of atomic and molecular ultrafast dynamics,the generation of isolated attosecond pulse,and industrial laser micromachining,have been a hot research topic.The techniques for the generation and diagnosis of few-cycle laser pulses are all based on nonlinear effects.Among them,hollow core fiber with wide spectrum and high beam quality for spectral broadening and second-harmonic frequency resolved optical gating with simple device are widely used.At present,frequency-tunable broadband pulses generated by hollow-core fibers are mainly concentrated in the ultraviolet and infrared bands.However,spectral tuning of high-energy near-infrared broadband pulses is of great interest for the generation of frequency-tunable few-cycle laser pulses in the ultraviolet to infrared range.At the same time,the traditional diagnosis scheme of the pulse has disadvantages such as complicated measurement and difficult machining of optical components.In view of the above research status,this thesis studies the spectral broadening process of high-energy femtosecond laser pulses in hollow-core fibers theoretically and experimentally,and proposes innovative schemes for spectral tuning and phase diagnosis of broadband near-infrared pulses.The main research contents are as follows:(1)A nonlinear spectral tuning technique of near-infrared pulses in pre-aligned molecules is proposed.The numerical simulation results based on the nonlinear Schr(?)dinger equation show that the mechanism of spectral tuning is the competition among nonlinear effects such as stimulated Raman scattering,Kerr effect and molecular alignment.By properly adjusting the relative delay between the pump and probe pulses,decoupling between different nonlinear effects can be achieved while spectral tuning can be achieved in a controllable manner.Experimentally,the near-infrared pulse has achieved frequency shift of more than 100 nm in a hollow core fiber filled with pre-aligned nitrogen,nitrous oxide and carbon dioxide.And the spectral shaping of the pulse has been achieved by controlling experimental parameters such as gas pressure and pulse energy.This scheme has good application prospects in generating frequency-tunable few-cycle laser pulses,spectral shaping of broadband pulses,and strong-field physics.(2)A novel diagnostic method of few-cycle laser pulses,namely phase-mismatching technique,is proposed.It is found that in the case of phase-mismatching in thick nonlinear crystals,not only the complete spectral width of the second harmonic signal can be obtained,but also the efficiency of the second harmonic signal can be greatly improved.This phenomenon has been applied to a home-made second-harmonic frequency-resolved optical gating,and a few-cycle laser pulse of 8.6 fs has been successfully measured using a 150 μm BBO.At the same time,the measurement results are consistent with the measurement results of a 5 μm BBO with sufficient phase matching bandwidth,which further proves the correctness of this scheme.This scheme is also applicable to other pulse diagnostic methods based on second-order nonlinear processes,such as spectral phase interferometry for direct electric field reconstruction and dispersion scan.