Propagation Characteristics of Rotating Beam Under Multi-field Coupling in Inner Channel
Objective The absorption of laser energy by gases and optical components in inner channels of high-power laser systems leads to complex interactions among laser,fluid,and solids.This interaction causes uneven optical path differences as laser beams propagate,significantly degrading beam quality.As laser power continuously increases,this degradation,driven by thermal effects in the inner channel,becomes more severe.Understanding the mechanism of thermal effect and developing strategies to reduce beam quality degradation are essential.However,most existing studies on mitigating the thermal effect of the inner channel focus primarily on the laser-fluid interaction,often overlooking the optical-structure interaction.In this study,a physical model of the multi-field coupling interaction of the laser-fluid-solid in the inner channel is established to reveal the propagation characteristics of a rotating beam in the inner channel and mitigation methods are proposed for the degradation of beam quality caused by the thermal effect.Methods In this study,a physical model is established to investigate the propagation characteristics of a rotating beam in an inner channel,considering the laser-fluid-solid multifield coupling effect.To achieve this,a split-step Fourier algorithm is used to simulate the optical field in the inner channel at each time step.The resulting heat source distributions of the optical elements and fluid are then integrated into the calculations of the flow field and solid mechanics using the finite-element-method(FEM)method.By employing ray tracing and power spectrum inversion method,the aberrations due to the nonuniform distribution of temperature and velocity within the fluid,as well as the deformation distribution of the solid,can be accurately calculated.These results are subsequently incorporated into the analysis of the laser propagation,enabling the calculation and analysis of the propagation characteristics of the rotating beam within the inner channel.In the subsequent time step,the heat source distribution is updated based on the newly calculated optical field,and the optical field as well as the temperature,velocity,and deformation in the next moment can be obtained by repeating the iteration process.Based on this,we propose a novel approach to suppress the thermal effect of a laser via a rotating beam,which is generated by the coherent superposition of two Laguerre-Gaussian(LG)subbeams with different wavelengths and opposite topological charges.The effects of the rotation rate,topological charge number,and gas absorption coefficient on the propagation characteristics of the rotating beam in the inner channel are quantitatively analyzed.Results and Discussions During the evolution of the laser beam within the inner channel,heat in the fluid primarily accumulates along the beam path,resulting in the highest temperature near the surfaces of the mirrors(Fig.4).Owing to the influence of natural convection,a gas with a higher temperature tends to flow in the opposite direction to gravity,leading to centroid drifting of the phase screen.Additionally,the centroids of the thermal deformation distributions of the optical elements exhibit different degrees of deviation because the optical elements exhibit different angles between the normal line and gravity direction(Figs.4 and 5).It is worth noting that the optical path difference induced by the gas thermal effects is several micrometers,whereas the thermal deformations of the window mirror and reflection mirror are several sub-nanometers and tens of nanometers,respectively(Figs.4 and 5),which are significantly smaller than those caused by the gas thermal effects(Fig.6).The rotating beam effectively mitigates the laser thermal effect in the inner channel.The optical path differences caused by the thermal effect of the gas and optical elements heated by the rotated beam is more uniform than those heated by the unrotated beam(Figs.7 and 8).Moreover,the peak-valley(PV)and root mean square(RMS)values of the optical path differences and deformations induced by the gas thermal effects are minimized when compared with those of the LG beam(Table 1).Furthermore,increasing the rotating beam angular rate and topological charge number and decreasing the gas absorption coefficient can lead to reductions in the Zernike coefficients of the additional phase.This phase is induced by the combined effects of gas thermal effects and mirror thermal deformations.Furthermore,it leads to improvements in the beam quality(β)of the output beam(Figs.10,11,and 12).Conclusions Based on the physical model of the multi-field coupling interaction of the laser-fluid-solid in the inner channel,in this study,the propagation characteristics of the rotating beam are investigated in the inner channel.Under the specific boundary conditions considered in this study,the primary factor affecting the output beam quality is the nonuniform optical path difference induced by the thermal effects in the gas,which is significantly more pronounced than the thermal deformations on the mirror surface.The use of a rotating beam has been proven to be an effective method for suppressing these thermal effects during laser propagation in the inner channel,particularly for mitigating beam astigmatism and comet-like distortions.Additionally,increasing the rotation rate and topological charge can significantly improve the ability to suppress thermal effects.Reducing the gas absorption coefficient can further improve the quality of the output beam.
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