All-fiber Tm-doped soliton laser oscillator with 6 nJ pulse energy based on evanescent field interaction with monoloayer graphene saturable absorber

We demonstrate an all-fiber Tm-doped soliton laser with high power by using a monolayer graphene saturable absorber (SA). Large area, uniform monolayer graphene was transferred to the surface of the side-polished fiber (SPF) to realize an in-line graphene SA that operates around 2 μm wavelength. To increase the nonlinear interaction with graphene, we applied an over-cladding onto the SPF, where enhanced optical absorption at monolayer graphene was observed. All-fiber Tm-doped mode-locked laser was built including our in-line graphene SA, which stably delivered the soliton pulses with 773 fs pulse duration. The measured 3-dB spectral bandwidth was 5.14 nm at the wavelength of 1910 nm. We obtained the maximum average output power of 115 mW at a repetition rate of 19.31 MHz. Corresponding pulse energy was estimated to be 6 nJ, which is the highest value among all-fiber Tm-doped soliton oscillators using carbon-material-based SAs.