Simulation Study On The Thermal Effect And Wavefornt Pre-compensation In The Beam Path

With the increase of laser power, the thermal effect of mirror and gas in the beampath is serious, which causes the decrease of the optical performance of laser system.Based on the basic theory of the thermal deformation and gas thermal effect, by usingthe Comsol multiphysis simulation software and Matlab calculation software, contentsincluding the aberration caused by thermal deformation and gas thermal effect, thewavefront pre-compensation of whole thermal effect in the inner beam path areinvestigated in theory and simulation in this thesis.The thermal deformation aberration of mirrors in inner beam path is studied insimulation, and the scaling model for an inner optical system is designed. Firstly, theinfluence of deformation of reflection coating on the thermal deformation of mirror isanalysed. The thermal deformations on different absorbtion conditions are comparatedand the surface heat condition is choosed to calaulating of thermal deformationaberration. Secondly, the thermal deformation of reflector is calculated and analysed onvary laser irradiation condition. When the distribution of absorption intensity is even,the influence of absorptiton power, angle of incidence, irradiation time and relative sizeon thermal deformation is studied respectively, and the abrration ratio nmis defined todescribe this relation. When the distribution of absorption intensity is uneven, theinfluence of beam rotation on thermal deformation is analysed, and the result shows thatwith the increase of rotation velocity the high-order aberration is decreased. Finally,based on the angular spectrum diffraction theory of light, the heat conduction theory andthe thermal elastic theory, the thermal deformation aberration and the wavefrontpre-compensation are simulated for both models. The scaling model can be numericallytransformed to original model.The distribution and time characteristic of gas thermal effect in a L shaped innerbeam path is discussed, and the thremal effect containing thermal deformation and gasthermal effect is claculated in simulation.The gas thermal effect distribuytion in a Lshaped inner beam path is calculated by using Comsol muli-physics software. Onconditon that the irradiation time is long the peak value spot of temperature and densityis moved to exit of pipe and the characteristic of ’double pearl eye’ structure is disappear.The coupling of beam field, temperature field, stress/strain field and fluid field in aninner beam path is simulated. The thermal effect of L shaped path and the beam qualityof outgoing laser are analysed. With the increase of heat time the temperature rise ofmirror and gas in pipe is steady, and the thermal effect and beam quallity of laser arechanged no more.The wavefront pre-compensation of thermal deformation and gas thermal effect ininner bam path is studied in simulation and the thermal effect compensation instabilities is analysed. Firstly, the thermal deformation pre-compensation on vary laser irradiationcondition is calculated and studied respectively. The results show that on condition thatthe distribution of absorption intensity is uneven, the active influence of wavefrontpre-compensation on the optical performance is more obvious when the value ofirradiation power is higher. On condition that the distribution of absorption intensity iseven, based on the analysis conclusion of thermal deformation aberration of mirrors, thesimplification calculation model of thermal deformation pre-compensation in innerbeam path is established with Zernike polynomials. Secondly, the influence of distortionphase error of beacon, precisions of adaptive conjugation correction and general Fresnelnumber on thermal deformation pre-compensation in inner beam path is studiedrespectively. When the distortion phase of beacon is used as conjugation correctionphase the proportion of high-order aberration in outgoing laser phase is increasedbecause of thermo-optic effect. The residual phase is decreased with increase ofprecisions of adaptive conjugation correction N, and when N>20, the correction ofthermal deforamtion aberration is more obvious. With the increase of general Fresnelnumber the beam quality in far field of outgoing laser is increased. Then, the thermaldeformation pre-compensation instability in inner beam path is studied and analysed.When the irradiation time is little, the instability of compensation is obvious. With theincrease of irradiation time the increase of beam quality factor is slow down.Finally, on condition that the irradiation time is long the thermal effect compensation ofL shaped path is studied in simulation. The results show that with the long irradiationtime the residual phase is increased.