Cut-off wavelength measurements for single-mode optical fibers

A new technique has been proposed for direct measurement of the cut-off wavelength, at which the first higher-order mode disappears. It uses a change of a near-field pattern of a fiber, which is excited by a variable wavelength source. The cut-off wavelength can be measured with +/-5-nm accuracy. The most suitable fiber length for precise measurement is 10-20 mm. It is found, furthermore, that the first higher-order mode under the condition near cut-off rapidly attenuates because of waveguide imperfections, in which the loss due to core-cladding boundary distortions is the most dominant.