We demonstrate a nanosecond-pulse erbium-doped fiber laser that is passively mode locked by a hollow-core photonic crystal fiber filled with few-layered graphene oxide solution. Owing to the good solution processing capability of few-layered graphene oxide, which can be filled into the core of a hollow-core photonic crystal fiber through a selective hole filling process, a graphene saturable absorber can be successfully fabricated. The output pulses obtained have a center wavelength, pulse width, and repetition rate of 1561:2 nm, 4:85 ns, and 7:68 MHz, respectively. This method provides a simple and efficient approach to integrate the graphene into the optical fiber system. [14] have been used for ultrafast lasers. The common method of integrating graphene SAs in laser cavities is to sandwich a graphene SA film between two fiber connectors with a fiber adaptor [10][11][12][13][15][16][17]. However, it is known that solubility and/or processability are the first issues for many prospective applications of graphene-based materials [19]. Although GO has a good solubility in water and many organic solvents, unfortunately carbon atoms bonded with oxygen groups are sp 3 hybridized and disrupt the sp 2 conjugation of the hexagonal graphene lattice in GO and thus destroy the linear dispersion of the Dirac electrons and influence the unique optical properties of graphene [20]. This makes GO unsuitable as a broadband SA in laser cavities for ultrafast pulse generation. Thus, a solution-phase graphene with large SA becomes crucially important because it can easily be integrated into a range of photonic systems. Furthermore, a solution-phase graphene can provide a robust method of nonlinear interaction of the guided mode, which has the potential benefit of increasing the damage threshold of the SA for high power pulse formation in laser cavity.Here, using a selective hole filling technique, we report an erbium-doped fiber laser mode locked by a hollowcore photonic crystal fiber (HC-PCF) filled with fewlayered graphene oxide (FGO) solution. The good solution processing capability of FGO makes it possible to be filled into the core of HC-PCF. The cavity comprises sections with normal and anomalous dispersion and the output pulses of 4:85 ns are obtained at the center wavelength of 1561:2 nm.Our all-fiber mode-locked ring laser setup is shown in Fig. 1. A 1:5 m heavily erbium-doped fiber (OFS EDF-80) is used as the gain medium, pumped by a 1480 nm high power laser diode through a wavelength division multiplexer coupler. A polarization controller (PC) is used to optimize the mode-locking operation while a polarization independent isolator maintains the unidirectional