Study On Nonlinear Effect And Transverse Mode Instability In High Power Thulium-doped Fiber Laser

The output power of rare earth doped fiber laser is increasing,reaching the average kilowatt level in both continuous wave and pulse systems.Transverse mode instability(TMI)effect is an important factor limiting the near diffraction limit power of high-power fiber laser.The average output power of fiber laser system is also limited by many other factors,such as stimulated Raman scattering(SRS),stimulated Brillouin scattering(SBS)and so on.In this paper,TMI threshold and its influencing factors in2?m-band thulium-doped fiber lasers(TDFL)are studied,which can provide an effective reference for the experimental exploration of high-power TDFL and the further improvement of its power.The main research contents are as follows:(1)The theoretical and simulation study on two main nonlinear effects that affect the power enhancement of high power fiber laser is carried out.The characteristics and thresholds of SBS and SRS are described and analyzed.The simulation results show that the threshold of SBS increases gradually with the fiber core diameter and the signal bandwidth,and the threshold of SRS also increases gradually with the fiber core diameter.In addition,the simulation results show that SBS has a lower threshold than SRS,which has a more significant effect on the power improvement of fiber laser.(2)After transferring and simplifying the theoretical model of Hansen et al.from Technical University of Denmark,from the optical fiber heat conduction equation,rate equation and wave equation,the process of establishing and solving the semi analytical numerical model of TMI is analyzed theoretically.Two expressions of TMI threshold induced by quantum noise and relative intensity noise are obtained.This three-dimensional rate equation model is used to study the transverse mode instability threshold in TDFL,which improves the accuracy and calculation speed.(3)The nonlinear coupling coefficient of 2?m-band high-power thulium-doped fiber laser is obtained by using semi-analytical theoretical model and MATLAB software,and the influence of fiber core radius and fiber coolant convection coefficient on the nonlinear coupling coefficient x(?)is analyzed.On the other hand,the influence of fiber core radius,pump wavelength,working wavelength and relative intensity noise on the TMI threshold of 2?m-band TDFL is studied.Compared with1?m-band ytterbium doped fiber laser,TDFL has higher threshold.The simulation results show that TMI threshold is negatively correlated with fiber core radius and signal wavelength,but positively correlated with pump wavelength.Theoretically,the TMI threshold can be further improved by reducing the fiber core radius,shifting the signal wavelength to the shorter wavelength or shifting the pump wavelength to the longer wavelength.In addition,reducing the relative intensity noise can also improve the TMI threshold to a certain extent,and the relationship curve between them is approximately logarithmic.(4)A high-power thulium doped fiber laser with MOPA structure was built experimentally.A linear cavity single longitudinal mode TDFL was used as seed source,and its output power is 80 m W.After three-stage amplification,the system with output power 11.7W is obtained,and the slope efficiency of the system is 23%.