Study On Linewidth Control Technology In High Power Narrow Linewidth Fiber Laser Amplifier

High power fiber amplifiers with narrow linewidth and near diffraction limited(NDL)beam quality have been found wide applications,such as coherent beam combining(CBC),spectral beam combining(SBC),gravitational wave detection(GVD),nonlinear frequency conversion.However,stimulated Brillouin scattering(SBS)effect is one of the primary limitations for power scaling of such types of laser sources.This paper focuses on the linewidth control of high-power narrow-linewidth amplifiers.It aims to achieve high-power,high-time coherent fiber laser output by optimizing the narrow linewidth spectral generation method while increasing the laser output power while taking into account the linewidth control..The application background of high-power narrow-linewidth fiber laser source is investigated.The technical difficulties in improving the laser output power while narrowing the laser linewidth are pointed out.And the development status of high-power narrow-linewidth fiber laser amplifiers at home and abroad in recent years is summarized.Based on the three-wave coupling equation and the rate equation,the physical process of SBS effect suppression by sine signal modulation and white noise signal modulation is studied.The rectangular spectrum and white noise phase modulation are effective techniques for linewidth regulation.Based on the high-power large mode field polarization-maintaining fiber amplifier,the effects of the rectangular spectrum modulated by sinusoidal signal and the near-Gaussian spectrum by white noise modulation on the SBS threshold are studied.Gain competition is introduced into high-power narrow-linewidth fiber amplifiers,and the variation of SBS threshold is studied under different seed power ratios.The effect of longitudinal stress on SBS effect was studied experimentally.Considering the stimulated Brillouin effect and the mode instability effect,the highly doped polarization-maintaining gain fiber is used to effectively narrow the laser line width to achieve a kilowatt-level laser output with linewidth of 1.8 GHz.Taking the co-aperture coherent combination system as an example,the effects of white noise modulation spectrum and rectangular spectrum on the combination efficiency are compared and analyzed,which provides a guide for the selection of the spectral shape of the unit beam of the large array element coherent combination system.