| At present,the output power of high-power near-single-mode fiber lasers has reached the kilowatt level,which are widely used in industrial and military fields.The beam quality and spectral characteristics of high-power lasers are the technical bottlenecks in their application development,and still need to be further improved.The improvement of beam quality for a high-power fiber laser is mainly achieved by mode control technology and cladding light stripping technology.At present,mode control is mainly realized by fiber coiling,but the accuracy of the conventional fiber-coiling mode control model and the flexibility of the common fiber coiling methods need to be improved.Cladding light stripping is mainly achieved by chemical etching,but there are some problems such as poor stripping uniformity,local overheating of etched fibers,so it is difficult to achieve high-power cladding stripping.In high-power fiber lasers,the deterioration of the spectral characteristics is mainly caused by nonlinear effects such as stimulated Raman scattering(SRS) effect and inter-modal four-wave mixing(IMFWM) effect.However,the theoretical and experimental researches on the suppression method of SRS and IMFWM for high-power fiber lasers have not been perfected.This paper studies the above issues,and the main research content includes the following parts:In order to improve the beam quality of high-power fiber lasers,firstly,aiming at the model error of conventional fiber-coiling mode control,the theoretical model of fiber-coiling mode control technology based on beam propagation method is proposed.The beam propagation method is applied to estimate the bend loss of each mode,and the theoretical research of fiber-coiling mode control technology is optimized,which provides theoretical guidance for the optimization of fiber coiling method and the determination of fiber coiling parameters.Then,aiming at the problem that the cylinder coiling method is difficult to operate and the mode control ability of water-cooled-plate coiling method is weaker in the common fiber coiling methods,the direct-liquid-cooling fiber-coiling mode control mtehod is proposed,which can flexibly change fiber coiling parameters.The cylindrical direct-liquid-cooling fiber-coiling method not only has the mode control ability of the cylinder coiling method,but also ensures the operability of fiber coiling.Finally,the master oscillator power amplifier(MOPA)was constructed using the cylindrical direct-liquid-cooling fiber-coiling method,and the high-efficiency k W-level mode control experiment on the 20/400 fiber was realized.By reducing the bending radius of the gain fiber,the beam quality was reduced from M_x~2=1.377,M_y~2=1.518toM_x~2=1.132,M_y~2=1.21.In order to avoid the influence of high-power cladding light on the beam quality and ensure the stability of the fiber laser system,firstly,aiming at the lack of theoretical analysis for chemical-etching cladding light stripping,the theoretical model of etched-fiber stripping is proposed to determine the relationship between etched-fiber parameters and cladding light attenuation.Ten etched fiber samples with different surface roughness and cladding diameter were fabricated to carry out the theoretical model experimental verification.The consistency between the measurement results of the fiber samples and the simulation results prove the accuracy of the theoretical model.Then,aiming at the problem of poor stripping uniformity and local overheating of etched fibers,the segmented corrosion method is proposed,and a two-segment etched fiber was fabricated to carry out the experimental verification.The cladding light attenuation of the two-segment etched fiber was 20 dB with the power handling capability up to 670 W,and the temperature distribution was uniform without any hot spot formation.For the tandem pumping light at 1018 nm,the segmented corrosion method was applied to achieve 1.1 kW tandem pumping light stripping.The cladding light attenuation was 18 dB and the temperature rise was <20oC.In order to optimize the spectral characteristics of high-power fiber lasers,firstly,aiming at the problem of output spectrum deterioration caused by SRS effect,the theoretical model of SRS effect for multimode fiber lasers is established,and the SRS suppression method based on laser structural parameter optimization is improved.By optimizing the laser structural parameters,the SRS effect was effectively suppressed,and the Raman power ratio was reduced from 14% to 0.4%.Then,aiming at the problem of output spectrum deterioration caused by IMFWM effect,the calculation model of the phase-matching frequency shift and coherence length is proposed to determine the relationship between laser parameters and the generation of IMFWM products,thereby providing theoretical guidance for optimizing the laser parameters to suppress the IMFWM products.Considering the limitations of the laser parameter optimization approach,the fiber-coiling IMFWM suppression method is proposed.The IMFWM peaks at 1108 and 1071.6 nm in the output spectrum were removed by reducing the bending radius of the gain fiber,and the height difference of the IMFWM peak compared to the main peak was>50 dB.Based on the analysis results of beam quality control technology and non-linear effect suppression technology for high-power fiber lasers,a high-power bidirectional-pumped fiber laser system based on a MOPA configuration was designed and built to achieve the output power of 3 kW with the M~2 factor of<1.7.The 3-dB spectral width is about 1.7 nm,the 13-d B spectral width is about 5.7 nm,and the SRS and IMFWM peaks differ from the main peak height by>20 dB.The experimental results verify the accuracy of theoretical analysis of beam quality control and non-linear effect suppression.While achieving high-power laser output,the beam quality and spectral characteristics of the fiber laser system are effectively improved. |
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