Research On Mode-locked All Polarization-maintaining Yb-doped Fiber Laser Based On Nonlinear Polarization Evolution

In recent years,the technology of the fiber laser had been widely employed in various fields.Among them,the ultrashort pulse produced by the passively mode-locked fiber laser has the characteristics of the narrow temporal width,broad spectrum and high peak power,which makes it attractive for optical communication,sensor,medical treatment,material processing,etc.In this paper,the nonlinear polarization evolution based on polarization-maintaining fiber(PM-NPE)was used as a saturable absorber,and the simulation model of the normaldispersion all-PM Yb-doped fiber mode-locked laser was established.On the basis of the simulation,the experiment is carried out.The normal-dispersion all PM Yb-doped fiber mode-locked laser was designed,where the fiber laser can operate in the states of the muti-wavelength continuous wave and mode locked single pulse.The main contents of this paper are as follows:Firstly,the PM-NPE structure based on the fiber splicing is theoretically analyzed by solving the coupled nonlinear Schr?dinger equations.By comparing the saturable absorption characteristics between the transmission and reflection structures,the following conclusions are obtained: the modulation depth of the three-segment-splicing transmission structure is larger;the polarization-maintaining nonlinear polarization evolution based on the Faraday-mirror(PM-NPE-FM)structure can automatically compensate the fiber length error.By analyzing the transmission function,the optimal splicing angle and modulation depth of the two structures can be obtained respectively,it can be found that the three-segment-splicing structure is sensitive to the fiber length error.Secondly,the simulation model of the normal-dispersion all-PM Yb-doped fiber mode-locked laser based on the three-segment-splicing structure and PM-NPE-FM structure are established.The following conclusions can be obtained: the simulation model based on two PM-NPE structures can output typical normal dispersion pulse.The PM-NPE structure based on three-segment-splicing has higher pulse peak power and energy.On this basis,by tuning the group velocity dispersion and other parameters in the simulation model,the different output characteristics of fiber laser are obtained and analyzed.Finally,the experiment is carried out to prove the simulation results.The fiber laser based on three-segment-splicing structure is verified.The experimental results show that the fiber laser can operate in two states,which are muti-wavelength continuous wave and mode-locked single pulse by tuning the pump power.When the pump power is in the range of 60 mw to 300 m W,the laser can be switched between single-wavelength and dual-wavelength by tuning the polarization controller.When the pump power is greater than 300 m W,the operation with mode-locked single pulse begins.When the pump power is 600 m W,the output pulse had temporal width of about 10.95 ps,central wavelength of about 1033 nm.The central frequency of the RF spectrum is 17.65 MHz,the signal-tonoise of the pulse is more than 50 d B.The pulse energy is about 1.46 n J.The property of switchable between the dual-wavelength continuous wave and mode-locked pulse can make the fiber laser to be a very convenient and multi-purpose laser source.