Cutoff wavelength measuring method and optical communication system

A method for measuring a higher-order mode cutoff wavelength of an optical fiber, comprising:

• a first step of joining a multimode fiber to a first end of an optical fiber under test whose higher-order mode cutoff wavelength is to be measured, allowing light to propagate from the multimode fiber to the optical fiber under test, measuring an intensity of light exiting from a second end of the optical fiber under test after propagating through the optical fiber under test, and determining a first power spectrum on the basis of the measured intensity of light;
• a second step of joining the multimode fiber to a first end of a reference fiber having bending loss higher than that of the optical fiber under test in a predetermined wavelength range, allowing light to propagate from the multimode fiber to the reference fiber, measuring an intensity of light exiting from a second end of the reference fiber after propagating through the reference fiber, and determining a second power spectrum on the basis of the measured intensity of light;
• a third step of determining a difference spectrum by subtracting the second power spectrum from the first power spectrum;
• and
• a fourth step of determining a higher-order mode cutoff wavelength of the optical fiber under test on the basis of a shape of the difference spectrum.