Investigation On Temporal Contrast And Intensity Improvement In Femtosecond Ultra-High Intensity Laser By Nonlinear Optical Effects

Since the invention of laser,higher light intensity has always been one of the most important research goals in the field of optics.Nowadays,through the development of ultra-short and ultra-high intensity laser technology,extreme light fields with light intensity greater than 10²³W/cm² have been generated in laboratories.Such a light field can be used to drive strong field physics research,such as electron proton acceleration,ultra-fast X-ray generation,etc.,and deepen the understanding of material nonlinearity.Thusly,ultra-short and ultra-high intensity laser become important experimental facilities developed by various countries.However,with the increase of light intensity of the femtosecond pulse,the intensity of noise such as the pre-pulse before the main pulse and the amplified spontaneous emission substrate will also increase,which will seriously affect the interaction between the main pulse and the material.So,the temporal contrast has become one of the key parameters of femtosecond ultra-high intensity laser system.In view of the close relation between nonlinear optical effects and light intensity,utilizing nonlinear optical effects to improve temporal contrast is an important research topic in femtosecond ultra-high intensity lasers.This paper focused on the application of optical parametric oscillation,self-diffraction effect and back-stimulated Raman scattering effect in the temporal contrast and light intensity improvement of femtosecond ultra-high intensity lasers.Theoretical analysis and experimental research have been carried out,and the following results have been obtained:1.Starting from the second-order nonlinear optical effect,the temporal contrast characteristics of the femtosecond laser synchronously pumped optical parametric oscillation signal light are analyzed,and the optical parametric oscillation signal light is used as a seed to inject non-parametric chirped pulse amplification.Using the 515nm femtosecond pulses as the pump light,optical parametric oscillation output with center wavelength near 800nm was obtained by LBO crystal,which was injected as a seed into the chirped pulse amplification system based on Ti:Sapphire regeneration amplifier,with output energy of 1.8m J.The highest measured temporal contrast within 30ps is10⁷,which was improved by nearly 2 orders of magnitude compared with the ordinary Ti:Sapphire oscillator as a seed for amplification.Finally,a multi-wavelength synchronous high-contrast femtosecond laser system scheme based on optical parametric oscillator is proposed.2.Starting from the third-order nonlinear optical effect,the characteristics of temporal contrast improvement and pulse width compression of the degenerate four-wave mixing process are analyzed,and the spectral broadening and angular dispersion of the signal light of the self-diffraction effect are simulated.Experimental research on temporal contrast improvement based on self-diffraction effect was conducted.Under the driving of 35fs incident pulse,a self-diffracted signal light of 20.4fs was obtained.The energy reached 35μJ and the spatial quality was excellent.The time contrast was higher than 10¹⁰,which was improved by more than 4 orders of magnitude relative to the incident pulse.The result shows that the first order self-diffraction signal light has satisfied the requirements of high temporal contrast seed for femtosecond ultra-high intensity laser systems.3.The time contrast characteristic in the double-chirped pulse amplification system using the self-diffraction effect is studied.The angular dispersion of the Martinez-type stretcher is analyzed,and in the experiment,it is used to stretch the first-order self-diffraction signal light while compensating its angular dispersion,which improves the practicability of the self-diffraction effect in the chirped pulse amplification system.The self-diffracted signal light is used as a seed and injected into the post-chirped pulse amplifier to stably output ultra-short and ultra-strong laser pulses with energy of 900m J and pulse width of 29.7fs.The measured temporal contrast is as high as 10¹⁰,which can be used for laser plasma interaction experiments.4.Based on the third-order nonlinear effect produced by the electron oscillation in the plasma,a theoretical analysis of the stimulated Raman scattering process is carried out.The formation of Langmuir waves in plasma is introduced,combined with the stimulated Raman scattering effect in ordinary media,and the feasibility of plasma-based back Raman amplification technology is analyzed.The experimental platform was demonstrated and preliminary experimental research was conducted on it.