Research On Large Mode Field Short Fiber Amplification Technology Of High Power Narrow Linewidth Fiber Laser

At present,the high beam quality and high stability of 10,000-watt fiber laser output are usually realized by power combining methods such as coherent combining and spectral combining.Both the spectral synthesis technology and the coherent synthesis technology have very high requirements on the spectral width of the output laser.The use of new technical means to suppress nonlinear effects to compress the spectral linewidth of lasers,expand the band of narrow-linewidth fiber lasers,and continue to increase the output power of lasers is still the main direction for the development of narrow-linewidth fiber lasers in the future.This paper studies the short fiber amplification technology based on large mode field fiber to achieve the narrow linewidth output of high power fiber laser.The theory of short-gain fiber oscillator spectral broadening is studied,and a hundred-watt oscillator system based on 20/400μm fiber is designed and built.The influence of the length of the resonant cavity on the output spectral linewidth of the seed source is experimentally studied.In the experiment,when the cavity length is 11 m,the spectral linewidth increases linearly with the output power,and the broadening speed is 0.0115 nm/W;and when the cavity length is 3 m,the broadening begins when the output power reaches 30W.The speed is 0.0018 nm/W.The results show that,compared with long gain fibers,although the use of short gain fibers reduces the light-to-light conversion efficiency,under the same output power,the use of short gain fibers can effectively suppress spectral broadening.The theory of modulation instability(MI)induced by cross-phase modulation(XPM)in high power amplifiers is studied,and a model of spectral broadening induced by MI induced by XPM is established.The results show that the power threshold of the spectrum broadening caused by the MI effect increases with the amount of sideband frequency shift.Increasing the core diameter can reduce the MI peak gain coefficient and reduce the frequency shift range of the gain spectrum,thereby effectively suppressing the sideband frequency shift and slowing down the spectral broadening speed.Compared with the theoretical model of multiple four-wave mixing(FWM)spectral broadening,the spectral broadening model induced by XPM-induced MI can explain the secondary peak phenomenon of the laser spectral broadening in the experiment.Aiming at the problem of low laser light-optical efficiency caused by short gain fiber,a narrow linewidth fiber laser structure with short gain fiber oscillation-amplification co-pumped(PSOA)is proposed.Based on the laser rate equations,a theoretical model of the PSOA structure is established,and the steady-state power distribution of the PSOA structure is simulated and analyzed.A narrow linewidth fiber laser experimental system with a PSOA structure was built.The results show that the light-to-light conversion efficiency of the PSOA structure is higher than that of the short-fiber non-co-pumped main oscillation power amplification(MOPA)structure,and it has an effect on amplified spontaneous emission(ASE).And stimulated Raman scattering(SRS)effect is better.The effect of temperature on the output spectrum is studied.Lowering the temperature of the seed source and amplifier can effectively suppress the ASE effect and the SRS effect.Finally,a 3.1 k W laser output was obtained in the 1050 nm short-wavelength band,the 3 d B output spectral linewidth was 0.22nm,the spectral signal-to-noise ratio was 45.5 d B,and the beam quality factorM~2=1.33.As far as we know,this is a high-power fiber laser with the narrowest linewidth of 3 k W in the1050 nm band reported so far.