Graphene-filled hollow optical fiber saturable absorber for efficient soliton fiber laser mode-locking.

We demonstrate a novel in-line saturable absorber based on hollow optical fiber (HOF) filled with graphene composite for high power operation of mode-locked fiber laser. Evanescent field of guided mode propagating in few centimeter-long HOF interacts with the graphene/polyvinyl acetate (PVAc) composite, which enables robust and efficient nonlinear absorption leading to stable passive mode-locking. The mode-locked fiber laser generates soliton pulses with 5.9-nm spectral bandwidth and its maximum output power is measured up to 80 mW. We also observe passive harmonic mode-locking of soliton laser delivering stable pulses with a repetition rate of 506.9 MHz at 33rd harmonics.