The Mechanism Of Regenerative Amplification With Electrical And Optical Hybrid Pumping For Picosecond CO₂ Laser System

The Picosecond Terawatt CO₂ laser is rapidly developed due to the requirements of laser plasma acceleration.However,the dispersion and narrow linewidth of the CO₂ molecular gain spectrum have become the biggest obstacles to the development of this laser.To solve the spectrum modulation and pulse splitting effect caused by the discrete gain spectrum in regenerative amplifier,an electrically and optically hybrid pumped picosecond CO₂ laser amplifier is innovatively proposed in this thesis.This amplifier can increase the gain of the sequence band of CO₂,make the sequence band spectrum lines overlap with those of the regular band to smooth the gain spectrum,and finally solve the problems.The structural design and theoretical model of hybrid pumped picosecond CO₂ regenerative laser amplifier are proposed and established.The energy concentration and energy concentration of m J-level output pulse can be effectively improved by using the hybrid pumped regenerative laser amplifier scheme.This kind of amplifier can help the picosecond terawatt CO₂ laser system to overcome critical bottlenecks.The main conclusions of this thesis are summarized as follows:(1)The gain characteristics of the sequence band and regular band of the picosecond CO₂ laser are analyzed.The vibrational-rotational energy level structure of CO₂ molecules and its energy transfer are calculated and modeled.By combining the intensity of absorption spectrum,the stimulated radiation cross-section,the line shape function and the collisional linewidth at different frequencies,the characteristics of the CO₂ gain spectrum under gas discharge are obtained.It is shown that the spectral lines of the regular band and the sequence band of CO₂ molecules overlap each other in frequency domain.Under gas discharge,the regular band gain accounts for more than 80%of the total gain,the regular band gain accounts for less than 15%,and the gas discharge scheme limits the characteristic temperature ₃T to below 1800K.These characteristics reveal the limitation of discharge pumping on the broadening of the gain spectrum lines,and provide a theoretical basis for proposing the hybrid pump scheme.(2)The five methods of broadening the spectral lines are compared.In the pumping process,the pressure of the gain medium will broaden each gain spectrum line of CO₂molecules as the pressure increases.The Stark effect and Zeeman effect will split the energy level to smooth the distribution of the spectrum.The isotope doping and branch selection will change the spectrum distribution in frequency domain.Increasing the sequence band gain will make its spectral lines overlap with the regular band spectral lines,and smooth the overall gain spectrum.The comparative analysis shows that it is the easiest to broaden the gain spectrum by increasing the sequence band gain,and the effect is the most obvious.Based on the conclusions,this thesis proposes a hybrid pumping method by combining discharge pumping and optical pumping to increase the sequence band gain directionally,and smooth the overall gain spectrum.(3)The overall structure of a hybrid pumped picosecond CO₂ regenerative laser amplifier is designed.Aiming at the current situation that the pumping energy cannot be absorbed effectively in the existing optical pumping structure,a picosecond CO₂ laser regenerative laser amplifier system based on a hybrid pumping structure and its cavity structure are designed.The designed system is able to meet the magnification of 10² to 10⁵.After calculation,the designed discharge area is 1×1.5×80cm³,the distance between the discharge plates is 1cm,and the initial E/N value is 2×10^-16V/cm².The set-up time of the gas discharge is 724ns,the time required for the optical pump from the pump pulse incident to be fully absorbed is 213ns,and the total delay time for other optical components is 114ns.When the injection time of the pump pulse is set at 625ns,a fully broadened gain spectrum can be obtained in the regenerative laser amplifier.(4)A theoretical model of the hybrid pump to increase the gain spectral density is established,and the pump dynamics process is analyzed.Based on the hybrid pumping theory,a theoretical model of the CO₂ laser rate equation under the influence of the light absorption process is proposed and established.At the same time,the dynamic of gas discharge based on this theoretical model is comprehensively analyzed.When the pump pulse is injected at the optimal injection time of 625ns,the characteristic temperature ₃T can break through the bottleneck and reach 2578K.this result shows that the hybrid pumped CO₂ regenerative laser amplifier do effectively broaden the gain spectrum,and is a potential technical solution to improve the output power and the energy concentration of the amplified pulse.(5)The effect of gain broadening effect and the regenerative amplification of picosecond pulse under hybrid pumping are explored.The results show that comparing with the regenerative amplifier of the ps-TW CO₂ laser system,the one pumped by the hybrid pump structure can increase the proportion of the sequence band gain by more than 20%,compared with the traditional discharge pumping structure.The energy proportion of the main pulse after the amplification can be increased by more than 30%.When the pressure in the cavity is 5atm,the enegry concentration of the main pulse can be increased to more than 30%,and the degree of improvement has reached 385%.The above series of results show that the hybrid pump structure do can effectively improve the power and energy concentration of the amplified picosecond pulse at high magnification.This picosecond CO₂ regenerative laser amplifier lays the foundation for the multi-stage amplification scheme of the CO₂ laser system to output terawatt-level picosecond pulses.