Polarization-maintaining Mode-locked Figure-of-9 Erbium-doped Fiber Laser With Low Repetition Rate

Low-repetition-rate pulse has been widely used in the fields of lidar,micromachining,microimaging and biomedicine.With the development of unmanned aerial vehicles,autonomous driving and telemedicine,the stability of low-repetition-rate pulse becomes particularly important in practical applications.The fiber laser with all-polarization-maintaining structure can significantly improve the stability of the system.Much larger cavity length is required to achieve low-repetition-rate mode-locked pulse in fiber laser with all-polarization-maintaining structure.However,fiber nonlinearities will become strong with the increasing of the fiber length and thus deteriorate pulse quality severely,which makes it more difficult to obtain mode-locked pulse with low repetition rate in the all-polarization-maintaining fiber laser.The appearance of figure-of-8 and figure-of-9 cavity bring opportunities to all-polarization-maintaining fiber lasers with low repetition rate.This paper studies the effect of cavity length on mode-locked pulse and the mode-locked performance of the figure-of-9 cavity under different parameters.An all-polarization-maintaining mode-locked figure-of-9 erbium-doped fiber laser with low repetition rate is designed based on the research conclusions.The research content and experimental results are as follows:(1)This paper builds a typical passively mode-locked fiber laser model with low repetition rate.This model iteratively solve the nonlinear Schrodinger equation by the fourth-order Runge-Kutta method with high accuracy to simulate the transmission of the pulse in the cavity.The influence of cavity length on the mode-locked pulse is studied from the nonlinear perspective.This provides an important reference for the experimental design of this paper and the reference bandwidth of the filter corresponding to different cavity lengths.(2)According to the mode-locked mechanism of the all-polarization-maintaining figure-of-9 fiber laser,the transmittance function is derived.The influence of the coupler ratio,gain magnification,phase shifter and cavity length on the mode-locked performance of the all-polarization-maintaining figure-of-9 fiber laser are studied.It is found that the theoretical value of the optimal coupler ratio is 50:50 and the figure-of-9 fiber laser with long cavity is easy to enter the anti-saturation absorption region.(3)Based on simulation results,the cavity length and filter bandwidth of the actual laser cavity are preset.Then,we experimentally build an all-polarization-maintaining mode-locked figure-of-9 erbium-doped fiber laser with low repetition rate based on the research results of figure-of-9 fiber laser,which can generate the mode-locked pulse at a center wavelength of 1550 nm,with a repetition rate of 2.15 MHz and a single pulse energy of up to 11.68 nJ.Stability evaluation shows that the RMS value of the output power error of the laser is 0.45%.