Method of measuring cutoff wavelength

A method of measuring a cutoff wavelength of a high order mode of a test fiber, the method comprising:

• a first step of measuring power spectrum P1(λ) [dB] of light output from a light source;
• a second step of measuring power spectrum P2(λ) [dB] of light emitted from one end of the test fiber when light output from the light source is made incident on the other end of the test fiber placed in a form allowing the curvature to vary in the longitudinal direction;
• a third step of obtaining difference spectrum P(λ) representing the difference between the power spectrum P2(λ) and the power spectrum P1(λ);
• and
• fourth step of obtaining the cutoff wavelength of the high order mode of the test fiber on the basis of the difference spectrum P(λ), wherein
• at the second step, the form allowing the curvature to vary in the longitudinal direction is spiral and has a minimum curvature and a maximum curvature such that the oscillatory wave form of a difference spectrum P(λ)min shifts, by half a cycle or more relative to the oscillatory wave form of a difference spectrum P(λ)max, in a wavelength range for measuring the power spectrum P2(λ), whereas the difference spectrum P(λ)min is obtainable when the whole length of the test fiber is wound with the minimum curvature and the difference spectrum P(λ)max is obtainable when the whole length of the test fiber is wound with the maximum curvature.