Study On Several Problems Of Spectral Beam Combining With External Cavity For Fiber Laser

Laser intelligent manufacturing is a research hotspot in the field of information technology,especially in the background of "carbon neutral" strategy,intelligent manufacturing is the key to achieve the goal of dual carbon.Fiber laser is known as the most advanced third-generation industrial processing equipment because of its small size,light weight,simple and compact structure.However,due to the limitation of optical thermal damage,nonlinearity and optical thermal lens effect,the output power of narrow linewidth single-mode fiber laser will not exceed 2 k W.In view of this,constructing laser array and combining multi-channel laser becomes an effective way to obtain high power fiber laser at low cost.Spectral beam combining(SBC)technology has high output power and good beam quality,especially the SBC system with external cavity is more robust,which plays a leading role in laser combined technology.However,the following problems still exist in the SBC system with the external cavity:(1)the output performance of the SBC system for a single laser;(2)The optimization parameter setting of each component in the SBC system;(3)The dynamic effect of grating thermal deformation on the beam quality of the SBC system;In view of the above problems,the main research work and innovation points of this thesis are as follows:?.Based on the output performance requirements of a single fiber laser in the SBC system,the limitations of output performance of a single fiber laser due to different output wavelengths and fiber bending are analyzed theoretically and experimentally.For the laser output of different wavelengths in the combining system,the tuning effect of the output coupled mirror cavity and Littrow cavity scheme is compared experimentally.The results show that the output coupling scheme can obtain higher power and better beam quality,but the tunable range is slightly smaller than that of Littrow scheme.According to the curvature loss theory and the laser rate equation,the power carried by each mode is analyzed and the experimental system is built.The results show that,under the same bending diameter,the power carried by the lower order modes are larger than that of the higher order modes.The correspondence between the bending diameter and the laser modes are illustrated,that is,the laser beam close to the diffraction limit can be obtained by bending the fiber.?.Aiming at the problem of combining efficiency in the SBC system,the analysis models of coupling efficiency and combining efficiency are established.Based on the model,the effects of the focal length on coupling efficiency and combining efficiency are analyzed.The results show that in order to consider both high power and high efficiency,a high reflectivity front mirror and a low reflectivity output coupling mirror should be selected when designing the SBC system with external cavity.The longer the focal length of the transformation lens,the more elements can be used in combining system,and the number of elements has minimal influence on the combining efficiency.?.Aiming at the dynamic effect of thermal deformation of grating on the quality of the combined beam,the temperature distribution and thermal deformation of thermal deformation grating are analyzed by finite element method.The beam transmission model of the SBC system after thermal deformation was established,and the beam quality of the combined beam after grating deformation was calculated.On this basis,the experimental system of grating thermal deformation affecting the combined beam quality is designed and established.The results show that the thermal deformation of the substrate is larger than that of the relief structure at the same irradiation time,which is related to the material constants of the two structures.With the increase of irradiation time,the beam profile appears distortion,as well as the beam quality gradually deteriorates,which changes rapidly at the beginning of heating.