Research On Thermo-Optic Effect Of Multilayer Dielectric Grating And Combined Beam Quality

Fiber lasers have been extensively used in the operation,communication,and scientific research fields for high conversion efficiency,compact structure,convenient thermal management,and good beam quality.However,due to the limitation of nonlinear effects,fiber end-face damage,mode instability,and other factors,the output power improvement of a single fiber laser is limited.In order to break through the output limit of a single fiber laser and increase the power,beam combination technology emerges.In the combination scheme of optical devices,with the increase of the number and power of the beams,the temperature and thermal distortion of the devices gradually increase.It will affect the combining efficiency and beam quality,and even damage the device itself.Therefore,it is important to study the mechanism of thermal distortion of the beam combining device in the beam combining system.In addition,it is also essential to propose a method that can reduce the thermal distortion of the optical devices used for beam combining and further increase the combined beam power and beam quality of the combining system.The main research contents of the dissertation are as follows:By combining model simulation and experimental verification,the method of decreasing thermal deformation is proposed and the grating deformation measurement system is constructed.Firstly,the temperature model of laser irradiation is established,the internal heating mechanism of the grating under high power laser incident is simulated,and the experiment of ten thousand watt laser irradiation is designed to verify the simulation results and clarify the accuracy of the heating model.Then,combining the relationship between the thermal elasticity equation and stress-strain,the thermal deformation model is constructed,and then the evolution of internal stress and thermal deformation is studied.Then,the internal connection between laser irradiation time,laser power,substrate size,and material and grating thermal deformation is discussed.Then the grating surface test platform is built based on the Twyman-Green interferometer and Zernike polynomial fitting technology,the grating surface is reconstructed in real-time,and the thermal deformation simulation results are verified.Finally,based on the simulation and experimental results,it is found that the temperature performance and thermal deformation degree can be improved by optimizing the grating size and considering the use of materials kind.Aiming at the problem that the thermal deformation of the grating in the double-grating spectral beam combining system causes the near-field phase modulation of the combined beam to degrade the quality of the combined beam,a theoretical modeling and simulation research of the grating thermal deformation and the quality of the combined beam in the double-grating spectral beam combining system is carried out.Firstly,based on the thermoelastic equation and Huygens-Fresnel principle,a theoretical analysis model of double grating spectral beam combining is established,and the effect of the grating thermal deformation on the quality of the combined beam and the light field distribution is analyzed.Secondly,the effects of array spacing,laser power density,and other factors on the grating deformation and the quality of the combined beam are expounded.Then,the adjustment effects of different clamping methods on the grating deformation surface are discussed and compared.Finally,on this basis,a method based on optimizing the array arrangement and selecting a specific clamping method to adjust the grating surface deformation distribution is proposed,thereby alleviating the decline of the beam quality caused by the grating thermal deformation,and improving the quality of the combined beam.Based on the power arrangement of the near-field beamlet array with Gaussian gradient distribution,the far-field coherent combined beam output with sidelobe energy suppression and beam quality improvement is realized in the fiber laser spatial coherent combining system.First,the connection between the fill factor and coherently combined beams is explained.Theoretically,through the iterative calculation method,the array power distribution gradient ratio is obtained when the beam propagation factor of the far-field composite beam is optimized.Then,the effects of different degrees of piston phase error interference on the quality of the composite beam were compared and analyzed in the two cases of equal power arrangement and power gradient arrangement.The optical field distribution and composite beam quality of far-field coherent beams produced by different spatial arrangements of fiber laser arrays are discussed and compared,and it is found that the hexagonal arrangement is more suitable for producing coherent composite beams with good beam quality.Based on the adjustment of the distortion surface and the optimization of the internal thermal-induced refractive index distribution,the optimization of the combined beam quality is realized in the coherent combining system of spatial diffractive optical elements.Firstly,according to the relationship between the absorptivity of diffractive optical elements and the temperature field in the heat conduction equation,a method for measuring the absorptivity of diffractive optical elements is proposed,and an experimental platform for measuring the absorptivity of diffractive optical elements is built.The absorptivity of diffractive optical elements is successfully measured.Secondly,according to the simulation results,the connection between factors including the element size,material,and clamping method and the thermal distortion of diffractive optical elements,and the internal thermal-induced refractive index distribution is studied.The effects of thermal distortion of diffractive optical elements and thermal-induced refractive index changes on the quality of the combined beam in the coherent combining system based on diffractive optical elements were further explored.The results show that the thermal distortion can be effectively alleviated and the beam quality can be improved by optimizing the clamping method.