A fiber laser system including a laser pumping source, first and second wavelength reflectors, first and second gain fibers, and first and second long wavelength reflectors is provided. The laser pumping source is adapted to emit a pumping beam. The first wavelength reflector is coupled to the laser pumping source. The first gain fiber is coupled between the first and the second wavelength reflectors. The first long wavelength reflector is coupled between the first gain fiber and the second wavelength reflector. The second long wavelength reflector is coupled between the first long wavelength reflector and the second wavelength reflector. The second gain fiber is coupled between the first and the second long wavelength reflectors. The diameter of the core of the first gain fiber is greater than the diameter of the core of the second gain fiber.