Propagation Characteristics of Double-Mode Fibers

This paper describes transmission characteristics of double-mode fiberguides which can have larger diameters than single-mode fibers without sacrificing bandwidth. Wide bandwidths are achieved in double-mode fibers because there is a wavelength at which the group delays are the same for their two propagating modes. The normalized frequency parameter, V, can be used to determine the number of modes in a fiberguide. When V Vco = 2.41 Vl + (2/g), then only one mode,1 designated LP(01),2 propagates in a guide with a graded refractive-index profile described by n = no Vl -- 2A{r/a) H . Since V = (2-ua/X) V2n An, it is evident that the core diameter, 2a, and the core-to-cladding index difference, An = n(0) -- n(a), may be varied over a wide range while still maintaining singlemode operation. However, when Vis significantly smaller than the cut-off value, VCO} the field propagating in the fiber becomes loosely guided and transmission losses increase due to bending and cladding imperfections. To minimize material dispersion effects, bending losses, and splicing difficulties, single-mode fibers should be designed with V ~ Vco at a 1061