Patent ID: 8896822

Claim:
A method of measuring a cut-off wavelength of a silica-based fiber, the method comprising: preparing a single mode fiber as a reference fiber; preparing a measurement target fiber; estimating, at an estimation unit, a relationship between a length of the measurement target fiber and a cut-off wavelength thereof in advance based on a refractive index profile thereof, thereby predicting a predicted cut-off wavelength; adjusting, at an adjustment unit, a length of the reference fiber such that the length of the reference fiber is longer than a length of the measurement target fiber so that the length of the reference fiber is adjusted such that a difference between a logarithmic value, in common logarithm, of the length of the reference fiber and a logarithmic value, in common logarithm, of the length of the measurement target fiber is 0.5 or more at the time of measuring power of transmission light transmitted through the reference fiber and the reference fiber propagates only light of a base mode at the predicted cut-off wavelength of the measurement target fiber; measuring, at a measurement unit, wavelength dependence of power of transmission light transmitted through the reference fiber and wavelength dependence of power of transmission light transmitted through the measurement target fiber; and calculating, at a calculation unit, a cut-off wavelength of the measurement target fiber based on wavelength dependence that is represented as a ratio of the power of transmission light transmitted through the measurement target fiber to the power of transmission light transmitted through the reference fiber.