Research On High Power Spectral Beam Combination Technology Based On Multilayer Dichroic Mirrors

With the invention and technical realization of low-loss large-mode-area（LMA）fibers and high brightness semiconductor-based pump sources,the output power of fiber laser has shown a dramatic increase in the recent 20 years.Owing to its high conversion efficiency and good beam quality,the fiber laser has been widely applied in scientific research,advanced manufacturing,aviation,medicine and sanitation,military defense and other aspects.Especially with the advent of the 10 k W level fiber laser,the application prospect of laser manufacturing has been further enhanced.However,limited by the thermal effect,nonlinear effect and mode instability,the scaling of a single fiber laser exists theoretical limit,cannot meeting the demand of modern and technologically advanced industry.A promising approach to break through the physical limit and achieve higher output power is laser beam combination technology.Although there are some schemes available to achieve a combined output beam beyond 10 k W level,existing laser beam combination methods still face many problems and difficulties.For instance,the geometric beam combination and fiber beam combination could not improve the brightness of the combined beam and would lead to a deterioration of beam quality,which is inconvenient for practical application;The grating-based spectral beam combination（SBC）has a strict requirement to the narrow linewidth of the incident beams and thus it could not combine the multimode lasers with high power and broad spectrum.In this case,the numbers of the incident channels are a lot and the narrow linewidth requirement them difficult to sustain high output power,which cause serious problems such as difficult to control and poor stability.To achieve 10 k W combined beam with high efficiency and high beam quality in a simple and effective method,a deep and systematic research on the spectral beam combination technology based on multilayer dichroic mirrors（DM）has been conducted in this doctoral dissertation.The main research contents are organized as follow.In order to fabricate the dichroic mirrors with high threshold and high efficiency,a multilayer coating structure has been conducted according to the laser-induced damage mechanisms of optical thin films.Zr O₂ and Si O₂ thin films,which are respectively used as a high reflectivity index media and a low reflectivity index media,are deposited in an alternating fashion on the fused silica substrate.In this case,the dichroic mirrors are capable of sustaining20 MW/cm² average power density in theory,meeting the requirements of the high power SBC system.Then,via combining the Fresnel formula and scalar scattering theory,we develop an analytical model of the combining efficiency and investigate the relationship between deposition technology and combining efficiency.The simulation results show that the surface roughness should be less than 1.8 nm and the thickness error should be limited to within0.25 nm so as to meet the requirements of high efficiency.To solve the combined beam quality deterioration problem induced by scattering,the analytical relation between the surface roughness and combined beam quality is derived.The results demonstrate that the deterioration of beam quality is positively correlated with the surface roughness,regardless of combination of single mode or multimode laser beams.Moreover,the laser-induced thermal lens effect is another important factor that attributes to the deterioration of the combined beam quality.In this regards,the physical mechanism analysis for its formation and development is discussed in detail based on the heat conduction theory.On this basis,the DM’s temperature rise and thermal deformation with different incident power and irradiation time is simulated using Comsol Multiphysics,which provides some theoretical guidance for the design of optimization system.Furthermore,an air-cooling device and a passive compensation system based on Ca F₂ windows are integrated into the SBC system so as to overcome or at least minimize the severe problem of thermal lensing and optimize the combined beam quality.On the basis of theoretical analysis,we demonstrate a spectral beam combining scheme of two 6 k W-level,broad-linewidth multimode laser beams using dichroic mirrors as the combining element.10.25 k W combined output power is achieved with an efficiency of 97%and the optimized combined beam quality is twice that of the individual channel.Then,the feasibility of the SBC system extending to high power and multi-channels is analyzed both in theory and experiment,which points the direction for the further development and optimization.Besides,a test of damage effect of the combined output laser on the metallic and nonmetallic targets is made in order to evaluate its implementing value in engineering field.The results show that the combined beam could destroy conventional material in a short time and has a good application prospect in laser processing.Finally,the DM-based 10 kW-level SBC scheme succeeds in spectrally combining high power multimode fiber lasers.Despite the broad emission spectrum of the single channel,it not only obtains a maximum power beyond 10 k W,but remarkably simplifies the structure of the system.Compared to the traditional method,this scheme becomes more concise,safer and more stable,which offers a creative method to achieve the 10 kW laser source.