2.11 GHz all-fiber harmonic mode-locked laser based on highly nonlinear hybrid multimode interference saturable absorber

This paper proposes a harmonic mode-locked fiber laser based on a novel highly nonlinear hybrid multimode interference saturable absorber (SA). The hybrid multimode interference SA is integrated by tapered graded-index multimode fiber (TA-GIMF) and Bi2Te3. The numerical simulation results show that TA-GIMF can improve the stability of nonlinear multimode interference mode-locking by filtering higher-order transverse modes. In addition, the larger evanescent-wave fields generated by TA-GIMF are more beneficial to the light–matter interaction. By depositing Bi2Te3 on TA-GIMF, the nonlinear saturation absorption properties and nonlinear coefficients in the laser cavity are further improved. Based on the hybrid multimode interference SA, the repetition rate of the fiber laser reached 2.11 GHz, corresponding to the 30th harmonic of the fundamental repetition rate of 70.41 MHz. Our results are the highest repetition rate generated based on the NL-MMI effect and provide novel insights into implementing highly integrated, high repetition-rate mode-locked fiber lasers.