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.© 1979 Optical Society of America.