When laser propagates in gas medium, the gas absorbs the laser energy and causes the refractive index to change, forming the gas thermal effect and reducing the beam quality. The laser has a high power density in the inner channel of the system, so the attenuation of Gaussian beam transmitted in the inner channel is far greater than that transmitted in the outer atmosphere. In addition, the thermal blooming effect of high-energy laser is not only closely related to the beam shape, but also very complex due to the interaction between the air flow and the absorption of laser energy. We hope to establish a more comprehensive thermal coupling effect model of laser transmission, which will provide strong support for the design and performance evaluation of the high-energy laser system. In this paper, the theory of optical-fluid-thermal coupling effect is introduced, and the simulation model of the thermal coupling effect of combined transmission of the elliptical Gaussian laser beams is established.
To solve the thermal effect of laser transmission in gas medium, flow field calculation, optical transmission calculation and optical-thermal coupling calculation are required. The optical transmission satisfies Maxwell equations. When simulating the electromagnetic field propagation of optical structures with long distance in a large size space, ray tracing is performed by solving the ray position and wave vector. Fluid flow and heat transfer follow the three conservation laws of mass, momentum and energy. The energy attenuation of laser transmission can be calculated according to Beer’s law, while the refractive index change of gas medium due to temperature and density changes satisfies Gladstone-Dale relationship. The wavefront aberration of the laser propagating in the enclosed space of the system is obtained by solving the above theoretical model with the finite element method, and the beam quality is calculated by comparing the results with those of the ideal Gaussian beam. The established model of laser thermal coupling effect is in good agreement with the experimental law, and can predict the degradation of beam quality caused by the thermal effect of gas medium.
Through numerical simulation, the natural convection phenomenon induced by laser heating in a cuboid enclosed space and the change process of the influence of the flow field on the beam propagation are analyzed, and the influential factors of the thermal effect are studied. After the light comes out, the surface temperature of the optical elements will continue to rise, while the air temperature will not change within a few seconds (Figs. 4 and 5). The optical path difference (OPD) distribution is elliptical Gaussian type at first, with obvious defocusing aberration, and its peak-to-valley (PV) value and beam quality show a trend of first increasing, then decreasing, and then stabilizing (Figs. 7 and 9). With the increase of the absorption coefficient, the OPD PV value caused by the thermal effect of gas increases (Fig. 12), but the absorption of the element surface has a low contribution to the thermal effect of gas medium (Fig. 13). The gas thermal effect can be suppressed and the beam quality can be improved by certain gas replacement methods (Fig. 15). The flow field distribution of each region in the spectral beam combining system is different, and the path of each sub-beam is different, so the aberration is also different. The number and power of sub-beams, the shape and distribution, the combining method, the transmission distance and the system layout will all affect the thermal aberration difference of the medium in the process of combined beam transmission.
In this paper, the theory of optical-fluid-thermal coupling effect is introduced, and the thermal coupling effect model of laser beam combining transmission in an enclosed space is established. The absorption of laser energy by the gas medium will affect the laser transmission, resulting in optical axis deflection, beam quality degradation, and changes in the shape of far-field spot. In this study, the influences of medium absorption, optical element absorption and gas replacement on the thermal effect of internal transport gas are analyzed. In the spectral beam combining system, all sub-beams will interfere with each other, the wavefront distribution will be different, and every sub-beam will have a dispersion trend. The model can analyze the thermal effects of different systems according to the actual situation, and the influence of structural parts and electronic devices on the flow field and laser transmission can also be considered, providing an effective reference for the design and performance evaluation of high-energy laser systems.
