In2S3-based saturable absorber for passively harmonic mode-locking in 2 μm region

Abstract There are few reports on the mode-locking operation of indium sulfide (In2S3) in fiber lasers. In this paper, we experimentally demonstrate a harmonic mode-locked fiber laser in 2-μm region by taking advantage of an In2S3-based saturable absorber (SA). The In2S3-based SA has nonlinear optical modulation depth of 20.8 % and saturation power of 0.14 kW at 1930 nm. The fundamentally mode-locked solitons at 1938.59 nm with the 3-dB bandwidth of 5.04 nm, pulse width of 1.16 ps, and pulse repetition rate of 41.73 MHz are generated. The maximum harmonic-resonance frequency is 459 MHz (11th harmonic), which is the highest operating frequency that has been demonstrated in In2S3-based mode-locked fiber lasers as far as we know. The spectral and temporal characteristics of the output pulses are investigated versus harmonic order. The results prove that In2S3 possesses the characteristics of mode-lockers and could be a promising optical switch for ultrafast pulses generation.