High Repetition Rate Harmonic Mode Locking With Good Quality At 2?m Laser Cavity

Mode-locked fiber lasers operating at 2 ?m band have attracted large amount of attention owing to its widespread applications in the fields of optical communication,medicine,generation of mid-IR,LIDAR,and so on.In this paper,the fundamental and harmonic mode-locked Tm-doped fiber lasers in the 2 ?m wavelength region were theoretically and experimentally studied and can be divided into the following three parts:(1)Based on the Nonlinear Schr?dinger Equation,the numerical model of 2 ?m passively mode-locked Tm-doped fiber laser using nonlinear polarization rotation(NPR)was established.The model was numerically solved adopting the typical Split-step Fourier method.The temporal evolution of fundamental soliton with the pump power was studied.In order to maximize the pulse energy of fundamental soliton,the optimized fiber length of Tm-doped fiber and single mode fiber SMF28 e were 31 m and 5 m,respectively.(2)A fundamental mode-locked Tm-doped fiber laser based on NPR was built up.The total cavity length is 36 m corresponding to a total cavity dispersion of-2.9 ps2.At the pump power of 1.232 W,stable fundamental solitons were obtained giving a repetition rate of 5.8 MHz which matched well with cavity length.The center wavelength and FWHM were 1992.7 nm and 2.02 nm,respectively.The pulse duration was measured to be 2.2 ps.The maximum output power was 10 m W,corresponding to pulse energy of 1.72 n J.While a hybrid mode-locked TDF laser based on NPR structure and CNT saturable absorber was built up in order to reduce the cavity length while make the mode-locking self-starting.Stable self-started mode-locked pulses with pulse duration of 1.4 ps and a fundamental repetition rate of 22.81 MHz were obtained.The center wavelength was 1983.2 nm with a FWHM of 3.42 nm.(3)A harmonic mode-locked Tm-doped fiber laser was modified by introducing a piece of 20 m length dispersion compensation fiber into the scheme mentioned above.This fiber aimed at reducing the soliton splitting threshold thus producing harmonic mode-locked pulses.Stable fundamental mode-locked pulses with a repetition rate of 4.22 MHz were obtained first at the pump power of 750 m W.With the increased pump power,the harmonic mode-locked pulses with increased order were achieved.At the available pump power of 3.2 W,largest repetition rate of 14.5 GHz(3436th)with a measured SSR of 19 d B,was obtained,which is also the current highest level from mode-locked fiber lasers in the 2 ?m wavelength region.The obtained pulse duration and wavelength were 1.3 ps and 1982.3 nm,respectively.