The Effect Of Strong Focusing Parameters On The Four-Wave Mixing Intensity Of Femtosecond Filament

The emergence of ultra-fast and ultra-strong lasers gradually lifted the veil of the strong interaction between light and matter.Four-wave mixing,as an important thirdorder nonlinear optical effect,has attracted widespread attention because of its ability to extend the laser from the visible band to the infrared and ultraviolet bands and be tunable.The gas medium has a high damage threshold under light excitation,which makes people gradually shift from the solid medium to the study of four-wave mixing in the gas medium.This paper studies the effect of focusing parameters on the conversion efficiency of four-wave mixing in the filament.Using an intense 808 nm near-infrared pulse and a weak tunable infrared pulse(1.3-2.2 ?m)to generate filament in the air medium,and based on the four-wave mixing process,tunable ultra-short laser pulses in the visible light band are generated.The study found that the external focusing parameters strongly affect the four-wave mixing efficiency in the air filament,and changing the tilting angle and focal length of the focal lens can optimize the four-wave mixing intensity.The intensity of the four-wave mixing pulse first increases and then decreases with the increase of the tilting angle.The best tilting angle in this experimental study is about16 degrees.The intensity of the four-wave mixing pulse first increases and then decreases with the increase of the focal length.The maximum four-wave mixing efficiency is achieved when the focal length of the focal lens is 75 cm.The paper explains the effect of the above focusing parameters on the four-wave mixing efficiency by the combined effect of the filament plasma density and the volume of the filament.In addition,this paper also studies the effect of incident power,polarization matching,nitrogen pressure and gas medium on the four-wave mixing efficiency to further improve the four-wave mixing efficiency.This work has certain significance for optimizing the four-wave mixing in the femtosecond laser filament to generate visible and ultraviolet ultrashort laser pulses.