Numerical Study Of The Passively Mode-locked Self-Similar Erbium-Doped Fluoride Fiber Laser

Mid-infrared 2.8μm ultrashort pulse laser plays an important role in precision machining,biomedical,scientific research,national defense,and other fields.Currently,2.8μm ultrashort pulse laser is mainly realized based on traditional mode-locking technology.However,the traditional mode-locked soliton laser can only withstand the maximum nonlinear phase shift ofπ.Excessive nonlinear phase shift not only causes pulse instability or even splitting,but also limits the pulse energy.Self-similar mode-locked fiber lasers can withstand nonlinear phase shifts up to 10π,so they have great potential in obtaining high energy mode-locked pulsed lasers.In this paper,using germanium rods as dispersion compensation devices,the related numerical models of self-similar mode-locked erbium-doped fluoride fiber lasers based on nonlinear polarization rotation(NPR)and saturable absorber(SA)are established respectively.Moreover,the effect of relevant parameters on the output characteristics of the mode-locked laser is investigated.The results of this study can provide an important reference for the experimental acquisition of high energy self-similar mode-locked pulsed lasers at 2.8μm.The details of the work in this paper are as follows:(1)The theoretical models of the self-similar fiber laser based on passively mode-locking at 2.8μm.Based on Maxwell’s equations,the coupled nonlinear Schr?dinger equation is derived to describe the evolution of the pulse transmission in the cavity,and the basic principle of the symmetric split-step Fourier method is introduced.At the same time,the algorithm is used to numerically solve the coupled nonlinear Schr?dinger equation.Then,the numerical models of self-similar erbium-doped fluoride fiber lasers based on NPR and SA are built,respectively,and the NPR and SA mode-locked principles are described in turn.(2)Numerical study of NPR mode-locked self-similar erbium-doped fluoride fiber laser is conducted.In the simulation,a germanium rod is used as the dispersion compensation device.When the net dispersion of the cavity is 0.0067 ps~2,the parabolic pulse with a pulse width of 6.6 ps,a peak power of 115 W,a pulse energy of0.76 n J,and a 3 d B spectral width of 16 nm is obtained.Then,the effects of parameters such as net dispersion in the cavity,small-signal gain and gain saturation energy of gain fiber,as well as linear phase bias on the output characteristics of mode-locked pulses are investigated.The simulation results show that when the net cavity dispersion and the linear phase bias are properly set,the higher the small-signal gain coefficient and gain saturation energy of the gain fiber,the more beneficial it is to obtain high peak power and high energy mode-locked pulses.(3)Numerical study of SA mode-locked self-similar erbium-doped fluoride fiber laser is conducted.The net dispersion in the cavity can be controlled by adjusting the length of the germanium rod.When the net dispersion of the cavity is 0.03 ps~2,the parabolic pulse with a pulse width of 19.7 ps,a peak power of 630 W,a pulse energy of 12.4 n J,and a 3 d B spectral width of 16 nm is obtained.Besides,to further optimize the output characteristics of the laser,the generation and evolution of the pulse in the cavity are analyzed.In addition,the effects of small-signal gain coefficient,gain saturation energy of gain fiber,as well as modulation depth and saturation power of SA on the output characteristics of laser mode-locked pulses are also studied.The simulation results show that within a reasonable range for obtaining self-similar mode-locked pulses,the higher the small-signal gain coefficient and gain saturation energy of the gain fiber,the more beneficial it is to obtain self-similar mode-locked pulses with high peak power and high pulse energy.Moreover,the higher the modulation depth and saturation power of SA,the more beneficial it is to obtain self-similar mode-locked pulses with high peak power and narrow pulse width.