Investigation On Dynamics Of Stimulated Raman Scattering In High-power Fiber Lasers

High-power fiber lasers have been widely applied in many fields,such as industrial processing,earth science and military defense,due to their outstanding advantages.The nonlinear effect is closely interrelated to the time-frequency properties in fiber lasers.On the one hand,the nonlinear effect will have a significant effect on the time-frequency properties of fiber lasers,which restricts the applications of fiber lasers in some specific fields.On the other hand,the time-frequency property of a fiber laser is the direct characterization of nonlinear effect,and the nonlinear effect will show different behaviors for fiber lasers with different time-frequency properties.In the thesis,investigation on dynamics of stimulated Raman scattering(SRS)in high-power fiber lasers is presented to systematically study the interactions between the time-frequency properties and the nonlinear effect in fiber lasers,and the main work is as follows:1.A genenral method has been proposed to describe the evolution the time-frequency properties in fiber lasers.Through the analysis of the connections and differences between the power amplification and nonlinear propagation processes,the time-frequency properties of fiber lasers could be obtained through their temporal or spectral evolution processes.Based on the spectral evolution process in fiber amplifiers,the simulation results show the influences of the time-frequency properties of seed source and the structural parameters of amplifier stage on the SRS threshold in fiber amplifiers,which reveals the physical origins determining the SRS threshold in fiber amplifiers.2.The meachisms of the pulsation in multi-longitudinal mode fiber oscillators and filtered superfluorescent fiber sources have been illustrated through their temporal evolution processes.The simulation results reveal that,for multi-longitudinal mode fiber oscillators,the intensity-dependent nonlinear effect plays an important role on the onset of these strong temporal fluctuations.Besides,the sustained self-pulsing,the self-mode locking and the turbulence-like pulsing are all the intrinsic properties of multi-longitudinal mode fiber oscillators.For superfluorescent fiber sources,the pump power will not affect the temporal properties.Besides,the filtering process will enhance the correlation among the turbulence-like pulsing,which makes the duration of strong temporal fluctuations longer in filtered superfluorescent fiber sources.3.The spectral evolution processes have been presented for different seed sources,including fiber oscillators,superfluorescent fiber sources and phase-modulated single-frequency fiber lasers,which have illustrated the influences of their structural parameters on the SRS effect in the fiber amplifiers.Both the time-frequency properties of the seed sources and the output properties of the corresponding fiber amplifiers are studied quantitatively.Among the three seed sources mentioned above,the phase-modulated single-frequency fiber lasers have advantages of maintaining linewidth and higher SRS threshold in terms of high-power fiber amplifier.4.The spectral evolution model incorporating the SRS effect has been established for the large-mode-area(LMA)fiber amplifiers,based on the spectral evolution processes of different transverse modes.The simulation results reveal the mechanism behind the SRS-induced mode distortion in high-power LMA fiber amplifier.5.Quantitative analysis has been conducted to study the time-frequency properties of Raman-gain-based single-frequency Raman fiber amplifiers and random distributed feedback fiber lasers based on the temporal evolution processes.For single-frequency Raman fiber amplifiers,the gain dynamics and spectral evolution properties in different pump manners have been analyzed,which reveals the physical mechanism behind the influence of the pump manners on the spectral evolution properties.As for random fiber lasers,the influences of the cavity structures and the temporal property of the pump source on the time-frequency properties have been studied.Combining the simulation results of the two Raman-gain-based fiber lasers indicate that applying the temporal stable pump source is beneficial to improve the property of fiber lasers.