Radiating leaky-mode losses in single-mode lightguides with depressed-index claddings

Cutoff characteristics are calculated for the fundamental mode in a single-mode double-clad lightguide structure whose refractive index in the inner cladding is less than the index of the outer cladding. Results of this study indicate how to choose the proper depressed cladding width and depth in order to reduce long-wavelength losses that have been observed in experimental MCVD fibers with fluorine-doped phosphosilicate claddings.