Theoretical And Experimental Study On Nanosecond Pulsed Fiber Raman Amplifier

High power,high beam quality narrow-linewidth pulsed fiber lasers have wide applications in nonlinear frequency generation,LIDAR,quantum information and remote sensing.However,lasers directly obtained by the stimulated emission of rare earth ion only accounts for a smaller part of the whole spectral range,some special wavelengths can not be obtained in this way.Fiber Raman lasers / amplifiers can effectively expand the spectrum range of Fiber lasers / amplifiers,which attracted comprehensive attention by researchers.Nevertheless,the nonlinear dynamics process is very complex in the narrow-linewidth nanosecond Raman amplifier due to the interaction of many nonlinear effects,such as stimulated Raman scattering,stimulated Brillouin scattering,self-phase modulation and cross-phase modulation.And the relevant research is lacked at present.In this paper,we make theoretical and experimental research on narrow-linewidth nanosecond fiber Raman amplifier.The main contents are as follows:We firstly built the general theoretical model for the dynamics of narrow-linewidth pulsed fiber Raman amplifier,and then deduced the coupling amplitude equation among signal laser,pump laser and Brillouin laser with stimulated Raman scattering,stimulated Brillouin scattering,self-phase modulation and cross-phase modulation in the singlemode fiber taken into account.Secondly,a numerical simulation model is built and carried out based on the parallelizable bidirectional finite difference time-domain method.The nonlinear interaction among stimulated Raman scattering,stimulated Brillouin scattering and self-phase modulation in Raman amplifiers is revealed by numerical calculation.The impacts of factors,such as pulse duration of pump laser,power of seed laser and length of fiber are analysised by employing the theoretical model,which can provide an effective reference for the design of the experimental scheme.Thirdly,based on the simulation results,we designed the Raman pump source and carried out the relevant experiment research.We also make optimized the design of double pass amplification system and achieve high magnification rate of week seed.Based on the scheme of laser diode(LD)current modulation and amplified spontaneous emission(ASE)source intensity modulation,a pulse source at 1064 nm that is tunable in duration between 4 ns and 40 ns is obtained with 800~1500W of peak power.A pulsed laser was designed with peak power/pulse energy of 87.1 kW/476 ?J based on an all-fiber-integrated MOPA laser system,which was successfully used in the field of laser ultrasonic.Experiments of narrow-linewidth nanosecond Raman fiber amplifier are demonstrated at last.The output characteristics of narrow-linewidth nanosecond Raman fiber amplifier are studied by changing the repetition rates and pulse widths of pump source,as well as the length of the fiber.The theory and experiment are in good agreement.A pulse source at 1120 nm that is tunable in duration between 3.2 ns and 25 ns is obtained with a polarization extinction ratio(PER)of >20 dB.The maximum peak power is ~427W,which is the highest level that ever reported in china.