Study On The Thermal Blooming Of Beam Propagation In The Inner Channel

As a laser beam propagates through a pipe in which the air would absorbe some energy of the laser beam, the absorbed laser energy heats the air and form a distributed thermal blooming effects along the heated air path. As it can limit the maximum power that can be efficiently transmitted through the air, thermal blooming is one of the most serious problems encountered in the propagation of laser. Many theoretical and experimental works have been concerned with it.In this paper, based on the available research of the blooming thermal effect, thermal effects of the inside and outside channel are analyzed by the simplified model. For the outside laser path, the expressions of the angular anisoplanatic error and the focus anisoplanatic error are derived for the beacon adaptive correction system. For the laser beam path indoor, using the simplified straight round pipe model with the axial pipe flow, scaling laws for the thermal effects of the beam path indoor is derived theoretically, the description of the energy scaling parameters N_E = n_TαPLR²/ρC_pεπR₀²and geometric scaling parameters N_G= vR²/εL are expressed. The effects of the ratio of the pipe radius to the beam radius on the thermal blooming are also analyzed based on the flattened hollow gaussian beam.Using the user-defined function provided by the fluid dynamics software FLUENT, a more completed simulation platform for the laser interacting with the fluid is established. In the simulation platform, the laser propagation equation is solved by the self-compiled program. For the discrete equations of the flow field, the fluid dynamics software FLUENT is used. The coupling effects between the flow field and the laser are solved by the user-defined function.In view of the uncertainty of the gas absorption coefficient in the experiment, the theoretical analysis is compared with the numerical simulation results for the simple mode of the straight pipe, and it is verified that the results derived from the simulation platform are reliable. Based on the numerical simulation platform, scaling law of gas thermal effect of the inside channels is verified by simplified theoretical analysis.The result shows that the established scaling law not only can be applied for straight circular pipe structure, but also for the more complex"L"shaped pipe structure.By studying gas thermal effect in the complex channels for laser propagation, it is shown that the gas velocity distribution in the pipe cross-section is very nonuniform, and furthermore the nonuniform velocity distribution changes the gas concentration distribution in channels and finally brings about optical phase distribution into the"double fishes eyes type". In order to weak the affect of the beam distribution to the beam quality, the problem on the gas thermal effect of the rotation beam in the inside channels is simulated and calculated based on the rotation beam model mentioned in the previous literatures,. Simulation results show that the beam rotation does not change the time duration which the gas thermal effect reaches steady state, and symmetrical defocus and spherical aberration along the optical axis remains constant. But the second-order astigmatism caused by the uneven beam intensity distribution is disappeared. Therefore, the way of the beam rotation can weaken the asymmetrical distribution of optical path difference caused by the thermal blooming of asymmetrical beam intensity distribution.By introduction of adaptive conjugate correction model to the established simulation platform of thermal effects, the change of the peak far-field Strehl-ratio with output is studied. The threshold property of conjugate adaptive phase correction for the thermal effects is obtained. Without considering the thermal deformation of the mirror, alone heating effect of the mirror, alone heating effect of the gas and heating effect of simultaneously considering mirror and gas are separately simulated. Simulation results show that the optical path difference caused by the heating of gas is meanly composed by the piston items and focus items, the optical path difference caused by the heating of mirror is meanly composed by the high frequency component. Taking into account of the gas heating effect and the mirror heating effect synchronously, the optical path difference caused by the heating effect of gas and mirror can be linearly superposed.Acording to the thermal effect treatment manner of pipe filled with low absorption coefficient gas, the numerical and experimental studies on air curtain and thermal effects of different gas are carried out. The results indicate that the method of vacuum pipe filled with low absorption coefficient gas with the air curtain as the seals can reduce the optics phase difference caused by the heterogeneous beam intensity distribution. However the optics phase difference induced by the air curtain is small. With the axial flow exists, with the air, nitrogen and nitrogen-oxygen mixture as the injected gas respectively, the corresponding thermal effects experiments in the pipe are carried out. The results indicate that the heating effect of air is serious as the laser transmitts through the pipe, while the thermal effects are very weak as the laser transmitts though the pipe filled with nitrogen or nitrogen and oxygen mixed gas. The optical path difference caused by thermal effect of the gas is much smaller than that caused by the permeating thermal deformation of the splitter. Therefore, the transmission path of the laser system filled with the low absorption coefficient of nitrogen or nitrogen and oxygen mixed gas flow will be a more effective thermal treatment manner.