Study On Beam Transmission And Power Amplification Characteristics Of Fast-Axial-Flow CO₂ Laser Amplifier

With the development of integrated circuit lithography technology,in order to obtain a finer circuit process,the light source used for exposure has moved towards the extreme ultraviolet（13.5nm）.At present,the mainstream way to obtain high-power extreme ultraviolet light source is based on Laser Produced Plasma（LPP）technology,which uses high-power,high repetition frequency,and short-pulse CO₂ laser to bombard the tin target to generate plasma and radiate EUV light.Master Oscillator Power Amplifier（MOPA）technology is the mainstream technical route to obtain high-power CO₂-driven light sources.Based on fast axial flow CO₂ laser amplifiers,this paper uses experiments and simulations to analyze short-pulse lasers in the space and time domains.The research of traveling wave amplification in the 1990s aims to improve the energy extraction efficiency in the amplifier.On the one hand,the transmission and amplification of the super-Gaussian beam in the gain medium is studied,and its energy extraction process in the discharge tube is numerically simulated,and the influence of beam shaping on increasing the beam filling factor in the gain medium is studied.On the other hand,a dynamic model of short-pulse CO2 laser amplification is established,and the changes of pulse time-domain waveform are simulated during the amplification process,and the theoretical results are compared and analyzed in combination with experiments.The main work done is as follows:（1）The generalized diffraction integral model of super Gaussian beam propagating in a laser amplifier is established.Analyze the superposition integration problem between the intensity gain distribution in an amplifier using the variational method.When the distribution of the gain coefficient is determined,the solution of the seed light intensity distribution with the maximum energy extraction efficiency can be summed up as the solution of the conditional extreme value problem of the functional isoperimetric problem.Analyze the characteristics of light intensity and gain through numerical simulation.（2）A dynamic model for pulse time-domain amplification was established based on the time-domain characteristics of short pulse CO₂ laser pulses and the relaxation lifetime between various energy levels in a fast axial flow CO₂ amplifier.Based on this model,the pulse segmentation method was used to study the waveform evolution of CO₂ short pulses in the amplifier.The distribution of gain coefficient and extracted photon number in the pulse time domain under different single pulse energies was analyzed.It was pointed out that the reason for the change in pulse waveform was that in the pulse time domain,the base energy that first contacted the gain medium would consume the inversion particle number first,resulting in a continuous decrease in the proportion of main pulse energy.Analyze the base energy issue of CO₂ short pulse amplification.The Gaussian and Lorentz linetypes were used to simulate the changes in the energy proportion of the main pulse after amplification of pulses with different base energies.The model calculation results were validated using a CO₂ pulse laser amplification experimental platform.The results showed that after passing the pulse with a base energy proportion of 14.65%through the laser amplifier,the base portion increased by 4%.Removing the seed light pulse base energy can effectively improve the amplification efficiency of the main pulse portion.