Research On Energy Transmittance Of Femtosecond Laser Filamentation In Air

In recent years,the nonlinear effects caused by femtosecond laser pulse propagation in air,such as filamentation,have attracted much attention.When femtosecond laser pulse propagates in transparent medium,the intensity of laser beam will be stronger and stronger due to Kerr self-focusing effect.When the intensity of laser beam reaches the ionization threshold,the laser pulse will ionize the air and produce a large amount of plasma,and the existence of plasma will produce defocusing effect on the laser beam and make the laser beam diverge,When Kerr self-focusing effect and plasma defocusing effect reach dynamic equilibrium,a long plasma channel can be obtained,which is called “filament”.In this paper,we mainly study the energy transmittance at the central wavelength of femtosecond laser pulse,which is helpful to understand the excitation,ionization and other dynamic processes of atoms and molecules in the process of laser filamentation.In the experiment,we changed the repetition rate of laser,laser energy,gas pressure,and focal length of external lens,and discussed the energy loss of laser beam by collecting the transmitted energy of laser beam.The nonlinear effects(such as terahertz radiation,air fluorescence and high-order harmonics)in the process of femtosecond laser filamentation,the plasma formed by laser beam ionizing air,and the sound wave excited by the filament will cause energy loss in the process of femtosecond laser pulse propagation.The spectrum broadening caused by self-phase modulation will transfer the energy of the central wavelength to other wavelengths,The peak intensity of the central wavelength of the laser pulse decreases.At low pressure,we find that the energy transmittance of the central wavelength of the laser pulse decreases with the increase of the pressure.The higher the energy of the laser pulse,the lower the transmittance of the central wavelength.However,when the pressure is close to normal pressure,the transmittance of the laser energy will increase.On the contrary,the energy transmittance of the central wavelength of the high-energy laser pulse is higher than that of the low-energy laser pulse.We think that the competition between the energy loss of the central wavelength of the laser pulse in the filament and the recovery of the 800 nm spectral intensity caused by the broadening of the laser pulse spectrum leads to the recovery of the energy transmittance of the central wavelength of the laser pulse,and the higher the repetition frequency of the laser pulse,the more obvious the recovery of the energy transmittance of the laser pulse.The energy transmittance of the central wavelength of laser pulse does not simply decrease with the increase of energy and pressure.