| High-power narrow-linewidth fiber lasers have important applications in the fields of gravitational wave detection,lidar,terahertz generation,and optical parametric oscillation.In practical applications,high-power narrow-linewidth linearly-polarized fiber lasers are more favored due to the advantages of higher coherence,higher detection sensitivity,and higher conversion efficiency.However,compared with wide-spectrum fiber lasers and narrow-linewidth randomly-polarized fiber lasers,narrow-linewidth linearly-polarized fiber lasers face stronger nonlinear effects among which the SBS effect is the primary limiting factor for the power scaling.With the adoption of many methods to suppressing SBS effects,after the output power of the narrow-linewidth linearly polarized fiber laser achieves a certain breakthrough,the mode instability effect becomes the primary limiting factor for obtaining high beam quality.Therefore,to promote further improvement of high power and high beam quality narrow linewidth fiber laser with linear polarization,SBS effect and mode instability effect are two primary problems to be solved.In this paper,with the high-power narrow-linewidth linearly-polarized fiber laser taken as the research object and the high-power high-beam-quality output as the research goal,systematic theoretical and experimental research focusing on the key technical issues is carried out:1.Comprehensively considering the suppression of SBS effect and mode instability effect,the optimization of high-power and high-beam-quality narrow-linewidth fiber laser with linear polarization is analyzed in detail.Based on the SBS dynamics model,the influence of fiber parameters,fiber type and system parameters on the SBS threshold is systematically analyzed.At the same time,the evolution of time-frequency characteristics in the process of pulse amplification is analyzed for the case of pulsed output.All of the work provides theoretical guidance for the optimized design of high-power narrow-linewidth continuous-wave(CW)or pulsed fiber lasers with linear polarization.Meanwhile,based on the semi-analytical model on mode instability effect,theoretical guidance for the optimal design of high-beam-quality narrow-linewidth fiber lasers with linear polarization is provided.2.The theoretical and experimental research on the system of high-power narrow-linewidth linear-polarization continuous-wave fiber laser is systematically carried out.Firstly,the ability of different types of conventional large-mode-area(LMA)high-concentration polarization maintaining(PM)fibers to achieve high power and high brightness output of single-frequency linearly polarized fiber laser is comparatively investigated.Furthermore,the feasibility of applying stress gradient to the conventional LMA PM fiber to suppress the SBS effect is demonstrated,and an output power of 414 W is realized with linear polarization degree of >99%,which is the highest output power of the reported all-fiber single-frequency fiber lasers with linear polarization and near-diffraction-limited beam quality by far.Further,the high-power single-frequency linearly polarized fiber laser based on LMA tapered high-concentration PM fiber is systematically studied,and an output power of 510 W is realized,which is the highest output power of the reported all-fiber single-frequency fiber lasers with linear polarization up to now.At the same time,the feasibility of domestic long tapered fiber for high-power narrow-linewidth output is studied for the first time.The optimized path of domestic long tapered fiber as well as narrow-linewidth fiber laser system based on it is pointed out.The feasibility of using random lasers to obtain high-power narrow-linewidth linearly-polarized fiber lasers was studied for the first time and the effects of seed linewidth and spectral morphology on spectral broadening effects and mode instability thresholds were comparatively investigated.As a result,an output power of 442 W with the linewidth of 0.28 nm and linear polarization degree of 94.2%is realized.This work explores a new path to realize high-power and high-beam-quality fiber lasers narrow linewidth and linear-polarization.3.The theoretical and experimental research on the high-peak-power and high-average-power narrow-linewidth linear-polarization pulsed fiber laser is systematically carried out.The high-peak-power narrow-linewidth linearly-polarized fiber laser based on conventional LMA highly-doped PM active fiber is designed and structured.The effects of time-domain characteristics and frequency-domain characteristics of the seed pulse laser on the time-frequency evolution characteristics and SBS threshold characteristics in the power scaling process are analyzed.The feasibility of applying stress gradient to the conventional LMA highly-doped PM active fiber for increasing the SBS threshold is demonstrated.The advantages of LMA tapered highly-doped PM fiber in suppressing SBS effect and spectral broadening effect are demonstrated and analyzed.As a result,the pulsed laser with the pulse duration of 4 ns,the peak power of 60.54 k W,the linewidth of 1972.97 MHz and the polarization extinction ratio of > 12 d B as well as the pulsed laser with the pulse duration of 3.8 ns,the peak power of 29.97 k W,the linewidth of 283.75 MHz and the polarization extinction ratio of > 14 d B is realized respectively,which represents the highest peak powers of the reported all-fiber 2 GHz-level as well as 300 MHz-level narrow-linewidth linear-polarization pulsed fiber lasers up to now.Furthermore,based on conventional LMA active fiber,the pulsed laser with the pulse duration of 4 ns,the linewidth of203.6 MHz,the average power of 466 W and the polarization degree of ?90% is realized,which is the highest average output power of the reported all-fiber narrow-linewidth linear-polarization ns pulsed fiber lasers at present.4.Detailed theoretical and experimental studies have been carried out around high-power linearly-polarized dual-frequency quasi-continuous fiber laser and multi-frequency pulsed fiber lasers carrying RF signals.Firstly,the generation principle and time-frequency characteristics of the double-frequency quasi-continuous fiber laser carrying low-noise RF signal are theoretically analyzed,and corresponding generating device and power amplifying system are designed to realize 434 W output.At the same time,the generation principle and time-frequency characteristics of multi-frequency pulsed fiber laser carrying low-noise RF signal are theoretically analyzed,and corresponding generating device and power amplification system are designed to realize328 W output.Both research results represent the highest output power of the same type of fiber laser at present,opening up a new space for the applications of dual-frequency/multi-frequency fiber laser. |
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