Research On Time-domain Instability And Its Suppression In High-power Continuous-wave Fiber Lasers

High power Continuous wave(CW)fiber lasers has been widely used in industrial processing,national defense and military fields due to their simple structure,strong environmental adaptability,etc.However,with the improvement of the output power,there are still some problems in fiber lasers,such as the slow steady-state process of the output power and giant pulses,continuous wave splitting into high peak power pulses and the dynamic change of the output beam profile.The above problems will lead to the deterioration of the laser output spectrum and beam quality,seriously affect the application of the lasers,and threaten the safety of the systems.In this paper,time-domain instability and its suppression in High-power Continuous-wave fiber lasers are studied,and the main research contents are as follows:The instability of the transient response in high power fiber lasers and its suppression method is studied.Firstly,aiming at the giant pulse phenomenon in the transient response process,the overshoot effect in fiber lasers is analyzed based on the transient rate equation and the overshoot suppression technology of high-power fiber lasers based on pump modulation is proposed and verified experimentally.In the experiment,the giant pulse with the peak power of 1.4 k W is suppressed by the pump modulation method in a fiber laser with the average power of 750W.Secondly,aiming at the problem of the slow transient response process of output power,a theoretical model of thermal transient response of high-power fiber laser is established,and the influence of the laser structure parameters on the transient response speed of laser is analyzed.The problem of the slow transient response process is solved by optimizing the temperature control of the pump source,increasing the laser cavity length,and increasing the ytterbium ion doping concentration.Finally,the time required for the output power to reach 97%of the stable power is reduced from 8.5s to less than 0.1s.Stokes induced modulation instability(MI)in high-power fiber lasers is investigated.A theoretical model of the interaction between inter-modal four-wave mixing(IMFWM)and stimulated Raman scattering(SRS)in high power fiber lasers is proposed.The influence of IMFWM and SRS on the frequency shift characteristics of Stokes light is analyzed.Based on the linear stability analysis,a hypothetical model of Stokes induced MI in high-power fiber lasers is established.Fiber laser amplifiers were built,and the Stokes induced MI under the influence of IMFWM and SRS was studied experimentally.The experimental results show that the Stokes induced MI leads to the second-order Stokes sideband in the spectrum and the self-pulsing phenomenon in the time-domain.Aiming at the second-order Stokes sideband and self-pulsing phenomenon caused by MI,the length of output passive fiber was shortened,then the suppression ratio of second-order Stokes light and signal light is increased from 20d B to 50d B,while the self-pulsing phenomenon is effectively suppressed.The influence of transverse mode instability(TMI)on nonlinear effects in high-power fiber lasers is investigated.Firstly,the dynamic coupling process between the fundamental mode and the higher-order mode caused by TMI in the fiber is analyzed theoretically.Secondly,the experimental system is built to study the influence of TMI on nonlinear effects.The experimental results show that TMI can promote the occurrence of IMFWM and inhibit SRS effect.Based on principle of fiber-coiling mode control method,the TMI and SRS effects are suppressed simultaneously by increasing the coiling diameter of the fiber.The TMI threshold of the amplifier is increased from 2.5k W to more than 3.9k W,and the SRS suppression ratio is about 35d B when the output power is 3.1k W.Built on the analysis of time-domain instability of high-power fiber lasers,the optimization experiment of time-domain characteristics of high-power fiber laser was carried out,in the experiments,the spectral broadening caused by MI was suppressed,and the thresholds of TMI and SRS were improved.Finally,the output power of the amplifier is 3.15k W,the beam quality is M~2≈2.0,the 3-d B is about 1.3nm,the 20-d B spectral width is about 6.7nm,and the SRS suppression ratio is 33d B.The output laser of the system is stable in time domain,the output power fluctuation is less than 1%for a long time,and there is no laser overshoot and self-pulsing phenomenon.When the output power is 3.15k W,there is no TMI effect,and the output power is only limited by the pump power.