Reflection means such as a mirror are provided on the output end of an optical fiber, and the input signal light and control light are returned to the optical fiber. Although the zero-dispersion wavelength of the optical fiber fluctuates in the longitudinal direction, if the length is relatively short, it is possible to manufacture a high yield of optical fibers, which monotonically changes the zero-dispersion wavelength. Therefore, a relatively short optical fiber with a monotonic zero-dispersion change can be used. Since the zero-dispersion change is monotonic and the optical fiber is short, the amount of change in the zero-dispersion wavelength is small and the bandwidth becomes broader when the control light is set at the position of the average zero-dispersion wavelength. Additionally, although the length of the optical fiber is short, the operating length is twice as long and thus the generation efficiency does not degrade.