Research On The Self-pulsing Mechanism And Suppression Of High-power Narrow-linewidth Fiber Lasers

Recently,with the development of high power fiber components and high brightness diode pump lasers,the output power of high power fiber lasers achieves the new level.The applications of the lasers cover the material processing and national defense indus-try.However,optical nonlinear effects(including stimulated Brillouin scattering(SBS),stimulated Raman scattering,four waves mixing and mode instability effect,etc.)with low threshold in the fibers limit the power scaling of the single fiber lasers.Beam com-bining method provide an available approach to overcome the limitations.Yb-doped fiber laser is an appropriate high brightness sub-beam for the beam combining due primarily to the merits of high power,narrow-linewidth and all-fiber structure.For high power narrow linewidth fiber lasers,how to mitigate SBS which is the power limitation effect with lowest threshold in the fibers,is the key scientific problem.Novel narrow linewidth fiber seed sources,especially those which are based on phase modulation,are applied to mitigate SBS and increase output power.Recently,we observed a kind of random self-pulsing trains in our experiments on narrow line width,kilowatts fiber lasers based on white noise source(WNS)phase modulation.High peak power,narrow pulse width and strong randomness are the distinguishing features of these trains.The instability threatens the high power fiber lasers system seriously and limits the output power scaling.In this work,the characteristics of self-self-pulsing in high power narrow linewidth amplifier are studied theoretically and experimentally.We provide two novel method to mitigate the self-pulsing and achieve the higher output power level.The paper begins with introducing the beam combining and its strong demands on high power narrow linewidth fiber lasers.The high power narrow linewidth fiber lasers with the different novel fiber seed sources are reviewed in detail.We discuss the advances in amplifying in the fibers of broadening light seeds with the phase modulation.The self-pulsing which has been observed in various rare-earth doped fiber,mainly in fiber oscillators is reviewed and analyzed.The necessity of studying the origination of the self-pulsing effect and its suppression technology is demonstrated.The time-dependent coupled wave equations which describe the stimulated Brillouin scattering process in high-power fiber amplifiers,is deduced.The discretization numerical calculation method of the equations is analyzed,which provides a powerful theoretical tool for the analysis of the self-pulsing.The basic principle of the phase modulation is analyzed.Based on the white noise phase modulation,the fine structure of the broadening spectrum is studied.The speculation that the spectral peak greatly reduces the SBS threshold or even leads the self-pulse,is proposed.The characteristics of self-pulsing time-domain in high-power fiber amplifiers have been experimentally researched.Several reasonable explanations have been discussed.Experimental results show that the self-pulsing may come from the SBS process.Further theoretical analysis verified the previous guess.The results show that the self-pulses are the high order Stokes light pulses.Through the experiment with a long passive fiber,it is further proved that the self-pulse is high order Stokes pulses.A cascaded phase modulation for self-pulsing mitigation is proposed.The mitigating self-pulsing with the cascaded phase modulation is experimentally analyzed.The basic principles of mitigating the SBS pulses and enhancing the SBS threshold by the cascade modulation are theoretically discussed.A narrow-width high-power fiber laser based on cascaded phase modulation is studied experimentally.The power output is 2.4 kW with the line width of 0.15 nm,which provides excellent advantages in spectral beam combining.Moreover,by adopting the new mode instability suppression scheme,3.5 kW output power is achieved,with a full width at half maximum(FWHM)linewidth of 0.38 nm.When a laser output is 3 kW,the FWHM linewidth can be controlled within 0.18 nm.To the best of our knowledge,our results has been in the leading position.The cascaded phase modulation has an huge advance in nonlinear effects mitigation.A parallel phase modulation for self-pulsing mitigation is demonstrated.The princi-ple of SBS suppression by parallel phase modulation technology is analyzed theoretically.The effectiveness of SBS mitigation is studied in 2 km passive optical fiber.A high-power narrow-linewidth fiber amplifier based on parallel modulation was studied.The laser output line width is controlled by the power ratio of the two arms.Although the FWHM linewidth has increased by nearly twice during the amplification process,it still has a great advantage compared to the fiber laser oscillator seed sources and the narrow linewidth super-fluorescence seed sources.Finally,a 2.4 kW laser output was achieved with a FWHM linewidth of about 0.15 nm.Nonlinear effects such as SBS,stimulated Raman scattering and four-wave mixing were suppressed well.