Study On All Polarization-maintaining Mode-locked Erbium-doped Fiber Laser

Polarization maintaining(PM)fiber has a high birefringence effect and can maintain the polarization state of the light transmitted in the core.Compared with ordinary fiber lasers,all polarization maintaining mode-locked fiber lasers have the advantage of resisting external environmental interference,higher stability.And can be applied in unstable environments such as vehicles and airplanes,have broad application prospects.In all PM lasers,the pulse width obtained by the active mode locking structure is difficult to compress to the fs level,the damage threshold of the material saturable absorber is lower,and the mode-locking performance over time will decline.The nonlinear loop mirror mode-locking technology can overcome many shortcomings and can be better used in all PM lasers.However,the current research on the pulse evolution characteristics in the PM nonlinear amplifying-loop mirror(NALM)structure is still insufficient.Therefore,this thesis using the all PM figure-eight cavity and figure-nine cavity mode-locking structure to carry out the optimization study of the all PM mode-locked erbium-doped fiber laser,and output pulse characteristics were studied.The specific research contents are as following:1.An all PM figure-eight cavity mode-locked fiber laser with no phase shifter was designed,the resonator mode-locking threshold is 620 m W,and the noise-like mode-locked pulse was obtained.Through the numerical simulation between the nonlinear phase shift difference and the transmittance in the resonant cavity,the phase shift amount of the ?/2phase shifter was selected,and the mode-locking threshold of the resonant cavity was reduced from 620 m W to 70 m W.Compare to the previous study,that the mode-locking threshold of the all PM figure-eight cavity laser was reduced by at least three times.2.The pulse output characteristics of the PM figure-eight cavity laser were studied.A self-starting 1560 nm conventional soliton mode locking was obtained at a low power of 70 m W,the pulse width is 863.8 fs.In the process of increasing the pump power,the conversion of soliton pulses to noise-like pulses was completed in PM figure-eight cavities for the first time.The maximum energy of the rectangular noise pulse is 17.94 n J.In a polarizationmaintaining structure,the output can be converted from ultra-fast pulses to high-energy pulses only by adjusting the pump power.Experimental analysis proves that the designed polarization-maintaining laser has high stability in two mode-locked states.3.Designed an all polarization-maintaining figure-nine cavity structure,the characteristics of the mode-locked pulse with gradually compressed cavity length were studied.The shorter the cavity length used,the narrower the pulse width,the higher the average output power and the higher the conversion efficiency.With a cavity length of 18 m,a self-starting conventional soliton mode locking was realized at a pump power of 120 m W,and after compressed the pulse width is 494.6 fs,the pulse energy is 0.05 n J.Increasing the pump power can obtain noise-like pulses,the maximum output power is 73.9 m W,and the corresponding pulse energy is 6.66 n J,which realizes the increase of pulse energy.The characteristics of the two mode-locked pulses were analyzed,which proved the high stability of the resonator.