Broadband nonlinear multimode interference for mode-locking modulation

Multimode fibers are a promising platform for broadband nonlinear fiber optics. A carefully designed single mode fiber (SMF)—grade-index multimode fiber (GIMF)—SMF structure was proposed as a broadband nonlinear optical modulator for generating mode locked fiber lasers in this paper. This GIMF-based structure exhibits obvious saturable absorption properties in 1.5- and 2 μ m regions, with modulation depths of 9.6% and 3.9%, respectively. By employing it in Er- and Tm-doped fiber lasers, the mode-locking pulses with a pulse duration/repetition rate of ∼520 fs/28.86 MHz at 1560.5 nm, and a pulse duration/repetition rate of ∼490 fs/27.40 MHz at 1983 nm were achieved. The maximum average power reaches up to 13 mW and 60 mW. These results indicate that the GIMF-based structure possesses broadband modulation capability, serving as a high-quality pulse modulator.