Simulation Model Of High Power Laser Propagation In The Inner Channel

Because of its small diameter and dense intensity, when high power laserpropagates through the optic inner channel, it would lead thermal distortion in beamsplitters, high power mirrors and thermal blooming in the air. There would be a declinein the power, beam quality and the far-field transmitting ability. An accurate assessmentof the impact of the inner channel is a tedious and difficult calculation requiring waveoptics, Fourier analysis, finite-element method, might consume a considerable amountof time and resources, with the result only fit for a particular situation. The calculationprocess proposed here would estimate the influence of the mirrors’ thermal distortionand thermal blooming to the laser beam quickly and conveniently, evaluate the innerchannel transmission performance efficiently.The research is carried out on the base of OPTRAIN model. With the methodsdescribed in the model, more detailed calculations of the beam splitter’s influence to thebeam quality are worked out. Computing software is written on the base of OPTRAINmodel. Some shortages of OPTRAIN as well as improvements are pointed out.In the discussion of the mirror thermal distortion, the physical model of annularbeam illuminating on the mirror is built. Some temperature distribution principles arefound out by solving heat-conduction equation with energy absorption as body heatsource under different situations. The temperature profile is fitted by quadraticpolynomial according to these principles; the correct coefficient calculating absolutetemperature rise is defined. It is demonstrated that the temperature profiles obtained bythe fitting curve and computed from heat-conduction equation are quite similar.In the discussion of the gas thermal effects in the pipe, radial temperature profile isestablished through the steady-state heat-conduction equation. The way to computebeam quality variation brought by the gas temperature rise is found out. The obscurationratio’s and the axial flow’s effects to beam quality improvement are calculated.With the conclusion listed above, when the inner channel transmission part isadded to the visual evaluation software of the high power laser system, after settingproper parameters of the inner channel, with the Fresnel diffraction integral, we cancalculate peak intensity on the target considering the beam power and quality declinebrought by the inner channel. At last the impacts of various factors within the channel tothe beam quality are calculated respectively.