The Research On The Effect Of Knife-edge Diffraction For Ultrashort Laser Pulse During Nonlinear Propagation

Spatial diffraction is the main reason for generating small-scale self-focusing(SSSF)during nonlinear propagation.SSSF would lead to off-axis energy gathering in some spatial positions.The intensity of the positionsincreasedue to the aggregation of energy,which would damage the transmission medium and laser components.During the SSSF,the distribution of temporal for laser would change due to the transfer of energy.Because of the interaction between dispersion and nonlinear effect in nonlinear medium,the laser would produce temporal instability.Temporal instability lead to distribution of temporal for laser became uneven.When the temporal instability increase continually,the laser pulse would split.In this context,we use knife-edge as spatial diffraction and we investigate the effect of knife-edge diffraction for temporal-spatial instability.And then,we obtain the evolution rule of temporal-spatial for ultrashort pulse laser during nonlinear propagation through experiment.Firstly,we obtained the expression of gain spectrum for temporal-spatial instability according tothe nonlinear Schr?dinger equation.Wefindthe range of gain spectrum increase with the increment of the power.At the same time,the fastest growth frequency of gain spectrumand the biggest gain would also increase with the increment of power.In addition,we study the linear propagation theory of spatial diffraction.We obtain the expression of spatial intensity and the expression of spatial spectrum for knife-edge diffraction.Secondly,we theoretical and experimental analyzed the evolution rule of SSSF for ultrashort pulse laser with different of diffraction parameters.We found that with the increase of truncation coefficient,the effect of diffraction modulation would enhance.Whilethe effect of diffraction modulation would decrease with the truncation coefficient continually increase.In addition,the parts of laser pulse which blocked by knife-edge decline with the decrease of truncation coefficient.So the effective power of laser pulse would increase and the spatial instability strengthen,so the effect of SSSF strengthen.As the truncation coefficient continually decrease,the effect of SSSF gradually augmented and the laser pulse split.Besides,the diffraction distance also affect the spatial distribution of SSSF.With the increase of diffraction distance,the diffraction stripe broaden and the new growth point is far away from the main peakof knife-edge diffraction.Finally,we studied the temporal evolution rule of ultrashort pulse laser during the knife-edge diffraction.We acquired the broaden factor of spectrum according to the nonlinear transmission equation.It can be seen from the expression that the width of spectrum increase with the increase of the input power.The spectrum broadening would lead to the pulse compressing.And then,we verify these theoretical analysis through experiment.We also analyzed the evolution rule of pulse width with different truncation coefficient of knife-edge diffraction.We found that the part of the laser which blocked by knife-edge increased with the increase of truncation coefficient,and then the power would decline.So the spectrum would n arrow and the pulse would broaden.