All-optical sampling needs femtosecond fiber laser as sampling pulse source. A passively mode-locked erbium-doped femtosecond fiber laser, pumped by 976 nm laser diode, using nonlinear polarization rotation mode-locking mechanism and inline polarization controllers is reported. At pump power of 176 mW, average output power is about 8.1 mW, with 29.69 MHz repetition rate and 16.8 nm spectrum width. The laser is easily mode-locked and stable. All-fiber ring cavity construction makes it compactable and convenient.