Research Of Hybrid Mode-Locked Tm-Doped Fiber Laser Based On NALM And GCM Saturable Absorbers

Passive mode-locked Tm-doped fiber lasers have wide applications in communications and medical fields due to their extremely narrow pulse width and wide spectrum.The passive mode-locked Tm-doped fiber laser based on the real saturable absorber can self-starting,but its performance is often limited by the characteristic parameters of materials.The passive mode-locked Tm-doped fiber laser based on the equivalent saturable absorber has a higher pulse energy output,but the output pulse is unstable due to the influence of the enviroment.Therefore,a passive mode-locked Tm-doped fiber laser that combines a true saturable absorber and an equivalent saturable absorber as a mode-locked device,the output pulse is not only high in energy but also very stable.In this thesis,a series of theoretical and experimental studies were carried out in a Tm-doped fiber laser using a Nonlinear Amplified Loop Mirror(NALM)and a Graphene Covered Microfiber(GCM)saturable absorber.Firstly,according to the theoretical basis of mode-locked Tm-doped fiber laser,a system scheme based on NALM and GCM saturable absorber hybrid mode-locked Tm-doped fiber laser was designed.Then,a hybrid mode-locked Tm-doped fiber laser based on NALM and GCM saturable absorbers and a mode-locked Tm-doped fiber laser based on a single saturable absorber(NALM/GCM)were designed according to the system scheme.In the case where the pump power is adjusted and the polarization controller is not,the hybrid mode-locked Tm-doped fiber laser based on NALM and GCM saturable absorbers can obtain Q-switched mode-locked pulses and mode-locked pulses.The wavelength,3 dB bandwidth,pulse energy,and signal-to-noise ratio are 1946 nm,2.3nm,8.49 nJ,and 53 dB,respectively.The mode-locked Tm-doped fiber laser based on the NALM saturable absorber have a Q-switched-like mode-locked pulse output and a noise-like mode-locked pulse output by adjusting the polarization controller and increasing the pump power.In the Q-switched-like mode-locking state,the repetition frequency and pulse width of the envelope do not change with the variation of pump power.The mode-locked spectrum also has a large modulation envelope,indicating the instability of this transition state.In the noise-like mode-locked state,the center wavelength of the mode-locked pulse is approximately 1989 nm,3 dB bandwidth is19 nm,and pulse energy is 3.13 nJ.It has a higher 3 dB bandwidth and higher pulseenergy than traditional mode-locked pulses.A smooth spectrum with a double peak and a signal-to-noise ratio of 43 dB further demonstrate that the mode-locked pulse is a noise-like mode-locked pulse.In the case where the pump power is adjusted and the polarization controller is not.The mode-locked Tm-doped laser based on GCM saturable absorber can obtain Q-switched mode-locked pulses and continuous mode-locked pulses.The wavelength,3 dB bandwidth,pulse energy,and signal-to-noise ratio are 1987 nm,8.31 nm,2.6 nJ,and 40 dB,respectively.Finally,through the comparative analysis of the results of three kinds of saturable absorber fiber lasers and the related literatures,it is concluded that the output of the Tm-doped fiber laser based on NALM and GCM saturable absorbers has more states of output and higher mode-locked pulse energy,higher stability and higher signal-to-noise ratio.