Reseatch On Narrow Linewidth And Linearly Polarized Nanosecond Pused Fiber Laser

Narrow-linewidth linearly polarized nanosecond fiber laser has the characteristics of high peak power,high coherence and good stability,which has been widely used in radar,nonlinear frequency conversion,laser coherent synthesis and other fields.It is an important research focus in the field of fiber laser.Faced with the obstacles of non-linear effects and other adverse factors,there are still many challenges to achieve high peak power narrow-linewidth linearly polarized laser in the optical fiber and accomplish the measurement of related indicators.Therefore,this article carries out the following research works:Firstly,the research background and significance of this subject are explained.The related researches and development trends of narrow-linewidth linearly polarized fiber laser are investigated and summarized in detail.Then,the theoretical research on the narrow-linewidth pulsed fiber laser is carried out.The working principle of the ytterbium-doped fiber laser is analyzed.Based on the stimulated radiation area of ytterbium ions,the advantages and disadvantages of two different pumping methods with 915 nm and 976 nm are analyzed.Then,the gain expression of the ytterbium-doped fiber amplifier is derived based on the rate equations.And through the numerical simulation in MATLAB,the characteristics of the gain,pump power threshold and optimal fiber length of the ytterbium-doped fiber amplifier are discussed.The mechanisms of stimulated Raman scattering and stimulated Brillouin scattering are analyzed aiming at the nonlinear effects in narrow-linewidth pulsed fiber laser.Finally,the theoretical model of narrow-linewidth double clad fiber amplifier is established and the influences of pump mode and gain fiber length on SBS are analyzed.The further experimental researches are carried out based on the theoretical analysis.Firstly,the implementation methods of the narrow-linewidth pulsed fiber laser are analyzed.Then,an extra-cavity pulse modulation system is designed.The optical pulsed signal that is basically consistent with the modulation signal is obtained.The problem of pulse shape distortion in the process of modulation is solved.For the research of narrow-linewidth pulsed lasers,we analyze the time-bandwidth product and simulate the effect of amplitude modulation on the laser spectrum.Then the linewidth of a 50 ns pulsed laser obtained by intensity modulation is measured based on the delay self-heterodyne and data processing.The measurement result is 9.7 kHz which breaks through the limitation of time-bandwidth product on the resolution of pulsed laser.Finally,the amplification of narrow-linewidth linearly polarized pulsed laser is studied.A five stage polarization maintaining amplification system is designed,which including a two-stage core pumping amplification system and a three-stage cladding pumping amplification system.Finally,a laser output with an average power of 21.3 W and a peak power of 267 kW is obtained.At the maximum output power,the intensity ratio of signal laser to pump laser is more than 30 dB.The beam quality of pulsed laser is M_x~2=1.28,M_y~2=1.23,which is close to the beam diffraction limit.