Realization of harmonic mode-locking in thulium-doped fiber laser using In2(SO4)3-based saturable absorber

Harmonic mode-locking (HML) is realized in a thulium-doped fiber laser (TDFL) using indium sulfate (In2(SO4)3) as a saturable absorber (SA). The In2(SO4)3 has nonlinear saturable absorption properties with the nonlinear optical modulation depth of 13.3 % and the saturation power of 0.11 kW at 1930 nm. The fundamentally mode-locked pulses operate at 1964.44 nm have the 3-dB bandwidth of 4.70 nm, the pulse width of 1.266 ps, and the pulse repetition rate of 25.37 MHz. The maximum resonance frequency of 228 MHz is achieved, which corresponds to the 9th harmonic. The supermode suppression level (SMSL) up to 63 dB is realized. The spectral and temporal characteristics of the output pulses are investigated as a function of harmonic order. The experimental results demonstrated that In2(SO4)3 can be a promising mode-locker for generating harmonic ultrafast pulses in 2 μm waveband.