Saturable absorber based on a hybrid structure of graded-index multimode fiber and graphene for a passive mode-locked erbium-doped fiber laser

A saturable absorber based on a hybrid structure of graded-index multimode fiber and graphene is proposed and demonstrated for passive mode-locked fiber laser pulse generation. The center wavelength of the output pulses is 1564 nm, with a 3 dB bandwidth of 3.52 nm and pulse duration of 634 fs at a fundamental repetition rate of 18.72 MHz, with a signal-to-noise ratio of 71 dB. Compared with the saturable absorber based on a pure graded-index multimode fiber structure, the pulse duration is compressed by 253 fs, and the signal-to-noise ratio is increased by 10 dB. The hybrid structured saturable absorber has the advantages of simple structure, convenient operation, and good robustness, which makes it attractive in the generation of ultrafast pulses.