Theoretical And Experimental Research On High Power Narrow Linewidth Fiber Lasers

Power combining is the effective way to scale the power of fiber lasers for the reason that the power of monolithic fiber is limited. The narrow linewidth laser is needed for both coherent beam combining and spectral beam combining. According to the requirements of power combining, nonlinear effect, depolarization and ASE of high power narrow linewidth fiber laser were theoretically and experimentally researched.The model was established, which can simultaneously analyze the different physical process in fiber laser amplifier. According to this model, the laser amplifier seeded with both FBG and modulated SF source were numerically simulated and the parameters of which were also optimized. The new methods of further narrowing the linewidth and suppressing SBS were proposed, and proved to be useful. The method for effectively controlling the deplorization in amplifying process was proposed according to the analysis of the change of beam quality and linewidth during the deplorization process, and this method was also proved to be useful. The impact on ASE imposed by the parameters of fiber amplifier was analyzed by adopting the classical model and experimental research, which would support the design for short wavelength fiber amplifier.?. The development of narrow linewidth fiber laser and its application home and abroad was introduced. The requirements of typical fiber laser power combining for sub-beam were summarized. The key physical and technical issues in narrow linewidth fiber laser were focused on.?. Firstly, based on ordinary model, the model of narrow linewidth fiber laser amplifier was established, which considered the main physical process such as laser amplifying as well as SRS, thermal effect, ASE, SBS and FWM in fiber oscillator and amplifier. And this model can be used for designing the laser amplifiers seeded with FBG as well as seeded with modulated SF source. Secondly, the impact on SBS and FWM imposed by the parameters of the laser amplifiers seeded with both FBG and modulated SF source was analyzed, which had guided the optimation to designs. Thirdly, the design had been verified by experimentally researching both of 2 solutions:a) The research on narrow linewidth laser amplifier seeded with FBG source was developed. For solving the problem of the spectral boardening using one oscillator as the seed, the new method was proposed for suppressing the spectral boardening, which was using few longitudinal mode oscillator and one stage pre-amplifier as the seed instead of just one oscillator. Linewidth 0.3 lnm ?FWHM=0.16nm?,30dB SNR, maximum power 1800W cw laser was achieved. The O-O efficiency was 85%, and the beam quality was M^x₂=1.218, M^y₂=1.185. b) The research on narrow linewidth laser amplifier seeded with modulated SF source was developed. Some satisfying results had been achieved in suppressing SBS by the experiments for amplifiers using different diameter of core. With amplifier which 20/400 ?mYb-doped fiber was used as the active fiber, linewidth 0.04nm,57dB SNR, maximum power 1228W cw laser was achieved. The O-O efficiency was 84%, and the beam quality was M^x₂=218, M^y₂=1.155. With amplifier which 25/400 umYb-doped fiber was used as the active fiber, linewidth 0.04nm,43dB SNR, maximum power 2000W cw laser was achieved. The O-O efficiency was 84%. Fourthly, according to the problems of these two solutions, the new solution which narrow linewidth amplifier seeded with two modulated SF source was proposed to suppressing the SBS and spectral boarding. With amplifier which 20/400 ?mYb-doped fiber was used as the active fiber, linewidth 0.075nm, maximum power 1200W cw laser was achieved. The O-O efficiency was 82%, backward power was 41mW,2 times less than the condition which using just one modulated SF source. At last, based on the theoretical and experimental research, the physical design for more than 5kW narrow linewidth fiber laser with linewidth less than 60 GHz was performed:based on the solution of the narrow linewidth amplifier seeded with modulated SF source, the linewidth of SF source was modulated to amolst 60GHz, and with forward pumped amplifier which 20/400 ?mYb-doped fiber was used as the active fiber, more than 5kW output can be achieved.?. The new view on the basic requirements how to maintain polarization in high power fiber laser was obtain. The good beam quality which means near diffraction limit is one of the necessary conditions for maintaining polarization in large mode fiber laser, and narrow linewidth also should be achieved for decreasing the depolarization caused by polarization mode dispersion. The results of experiments showed that the beam quality declined along with DOP, and the higher DOP, the better the beam quality. According to these results, we concluded that random polarization laser is high level mode. Because of spectral boardening in narrow linewidth amplifier seeded with FBG source, DOP was still 77% even if the beam quality was near diffraction limit, which means the intermodal dispersion caused by spectral boardening is another reason for DOP declining. Under the control of beam quality and linewidth,90% of DOP was achieved with non-PM few-mode fiber laser. Linewidth 0.04nm, M2=1.2,14.5 PER, maximum power 1000W was achieved with active polarization control.?. ASE of different center wavelength was simulated and compared using the classical model, and the results of which showed that both forward and backward ASE of 1030nm laser were much higher than that of ordinary wavelength laser. The design for amplifier was optimazed according to the simulation of 1030nm amplifier. With narrow band seed and one stage amplifier, central wavelength 1030nm, linewidth 0.072nm ?FWHM?, maximum power 1010W cw laser was achieved. The O-O efficiency was 81%, and 1030nm output power was more than 99% of total output power.