Nondestructive measurement of core radius, numerical aperture, and cutoff wavelength for single-mode fibers.

A method is proposed for determining the core diameter, numerical aperture, and cutoff wavelength of single-mode fibers using the end separation loss measured at two light wavelengths. A spacer with adequate end separation is used to reduce the fluctuation in connection losses caused by multiple Fresnel reflections between the fiber end faces and by small lateral offsets. The cutoff wavelengths of four 2-m long fibers measured by this method agreed with those in which abrupt changes in the near-field patterns were observed.