Dynamics And Suppression Of Self-pulsing In High-power Fiber Lasers

Fiber lasers own prominent advantages of high brightness,compact structure,better stability and easy thermal management.In recent years,fiber lasers have been widely applied in the fields of industry,national defense,medicine care,etc.At present,there commonly exists the self-pulsing phenomenon in the output laser of the continuous-wave(CW)fiber laser system,which is the strong random fluctuation of the instantaneous power observed in the CW laser output by the high-speed photo-detector.The self-pulsing effect can result in the decrease of the nonlinear effect threshold and the deterioration of the output stability,which will limit the improvement of the laser performance.However,related research about the self-pulsing,especially the generation mechanism and suppression of self-mode-locking,is relatively less.To this end,the comprehensive and systematic theoretical and experimental researches on the dynamics and suppression method of self-pulsing in Ytterbium-doped CW fiber laser have been conducted in this dissertation.This dissertation mainly consists of the following two parts: 1.The theoretical and experimental research on the generation mechanism of self-pulsing in fiber oscillator1.1 The generation and characterization of self-mode-locking intensity in fiber oscillatorFirstly in the framework of the nonlinear Schr?dinger equation as well as the coherent combining of the longitudinal modes,a theoretical model for the timing sequence of the fiber laser output under the multi-longitudinal modes operation was proposed.The analytical perturbation numerical algorithm was designed for the model to numerically investigate the dynamics,characteristics,and measurement as well as characterization methods of the self-pulsing in fiber lasers.For the first time,the physical process of the self-mode-locking generating through multi-longitudinal modes coherent combining in the fiber laser has been revealed theoretically.Last but not least,with regard to the temporal characteristics of the self-pulsing,the parameters as normalized range,normalized standard deviation and probability density distribution have been proposed for the first time to characterize self-pulsing intensity.At the same time,we argue that there is a significant difference between the measured results and the actual timing sequence of the output in the fiber laser,except that,the measured results can truly reflect the intensity of the nonlinear effect caused by the self-pulsing in the actual timing sequence.1.2 The theoretical and experimental study for the effect of the fiber oscillator's parameters on self-pulsing.The crucial parameters,i.e.cavity length,laser linewidth and output power,that have significant influence on the self-pulsing generation in the output timing sequence were numerically analyzed.The simulation results reveal that the increase of the linewidth and output power can effectively improve the output stability.Moreover,changing the cavity length of the fiber oscillator within a certain range has little effect on temporal stability of the output laser,however,it only affects the pulse duration of the self-mode-locking in the output.Based on different types of self-made fiber Bragg gratings(FBGs),the experimental set up investigating the effects of these parameters on the self-pulsing was designed and demonstrated.When the reflection bandwidth of the output coupler(OC)FBG ascended from 0.05 nm to 0.23 nm,the self-pulsing intensity reduced by over 70 %.The suppression effect that could be obtained by increasing output power would be saturated at high power output.In addition,there was little alteration in terms of the self-pulsing intensity and probability density distribution,when the cavity length reduced from 6.5 m to 1.2 m.Only the pulse duration of self-mode-locking was shortened.The experimental results were in good accordance with the theoretical analysis,which verified the feasibility of the theoretical model in this dissertation.This also provide theoretical tool and ideas for further self-pulsing suppression in fiber laser.2.The numerical simulation and experimental study of suppression of self-pulsing in fiber oscillator2.1 Self-pulsing suppression based on spectral shaping of OC-FBGAccording to the theoretical model and simulation analysis,the spectral shaping of OC-FBG has been proposed and applied in the self-pulsing suppression for the first time.Three types of OC FBGs with special reflection spectrum,which are Topflat OC-FBG,Triangle-R OC-FBG,and Chirped moire OC-FBG,were fabricated and used in a 50W-level fiber oscillator to study the suppression of self-pulsing.Compared to common OC FBGs,these spectral shaping OC FBGs all lead to considerable decrease,e.g.75 % and 99 % for Chirped moire OC-FBG,in terms of the self-pulsing intensity and strong pulses occurrence.The experimental study of suppressing the SRS effect by the self-pulsing suppression technique was conducted and the SRS effect was significantly reduced in the long-cavity fiber oscillator.By using these spectral shaping OC FBGs,the suppression amplitude over 20 dB was achieved in Stokes component of the laser output,when the maximum output power was of 66.2 W.2.2 Self-pulsing suppression in high power fiber oscillator based on externalfeedbackBy employing the theoretical model proposed in this dissertation,the self-pulsing suppression scheme of all-fiber structure based on external feedback was designed.The external feedback was introduced into the 50W-fiber-oscillator through a low-reflection FBG,which completely suppressed the sustained self-pulsing(SSP)and dramatically descending the intensity of SML pulses.The self-pulsing intensity was reduced up to 91 % when the output temporal stability was relatively poor.The experimental study of applying external-feedback self-pulsing suppression technology to high-power fiber oscillators was carried out,and the temporal stability of the output laser was effectively improved in a 500W-level 20/400?m fiber oscillator under all pumping conditions,and the suppression amplitudes of the self-pulsing intensity and strong pulses occurrence probability could reach up to 50% and 97% respectively,thereby,achieving CW high-stable high-power laser output with a maximum output power of 486 W.