Research On Raman Gain Properties Of High-power Fiber Laser

Raman fiber lasers have attracted much attention owing to advantages of wavelength flexibility,high quantum efficiency and the potential of high-power scaling.However,the research on Raman gain of fiber laser is not thorough and is mainly conducted on low-power level.This thesis will study the Raman gain properties of high-power fiber laser related to spectrum,polarization and transverse mode.A theoretical model of Raman fiber laser has been conducted,which verified the occurrence of extreme frequency shift for high-power and high efficiency output.By fixing the signal wavelength and tuning the pump wavelength in the experiment,the frequency shift could be changed in the Raman fiber laser.The experimental results indicated that the fiber laser with frequency shift from 10.6 THz to 15.2 THz showed high efficiency.The extreme value of 15.2 THz has been applied in the selection of pump wavelength in Raman fiber laser centering at 1178 nm and the output power of11.8 W was obtained with the conversion efficiency of 80.2%.The effect that the polarization states have on Raman gain has been studied.A wavelength tunable linearly polarized Raman fiber laser has been demonstrated with record power,the highest output power of which is 125.3 W.An all-fiberized filter has been designed with the function of changing the polarization directions.The transmission spectra under different fiber lengths and temperatures are calculated.By using the filter,the influence that different polarization states have on the output power of Raman fiber was investigated.By optimizing the design of this filter,the second order Stokes light was suppressed in a degree.The output power of 74.5 W was obtained under the pump power of 105.3 W in a linearly polarized Raman fiber laser,which increased by 48.7%.A transverse mode switching method was proposed based on the temperature control of few-mode grating.The capability of generating high-power output of this method was verified in an ytterbium-doped fiber laser,and?17 W fiber laser in fundamental mode and LP₁₁ mode was demonstrated.Applying this method to Raman fiber laser,?20 W output was obtained both in fundamental mode and LP₁₁ mode.