We report on a variation of the usual depressed- index cladding single-mode fiber design. An index step–actually an index ring–is added to the outer portion of the cladding profile (Fig. 1). This simple modification affects the attenuation of the LP11 mode, hence the cutoff wavelength, without changing the zero dispersion wavelength or mode- field radius of the LP01 mode. Most important, the ring parameters can be chosen so that the cutoff wavelength is decreased significantly while leaving the attenuation of the fundamental mode in the 1.55-μm window largely unaffected. This differential attenuation permits single-mode operation at higher V numbers than is possible with a standard depressed-index cladding fiber and leads to enlarged design windows for several fiber types.
© 1985 Optical Society of AmericaPDF Article