Mode coupling effects of the cutoff wavelength characteristics of dispersion-shifted and dispersion-unshifted single-mode fibers

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Abstract
The authors present theoretical background and experimental results to clarify the nature of the hump in the cutoff region of different types of single-mode fibers and discuss its implications. A theoretical model in terms of the coupling between the whispering-gallery mode formed near the silica-cladding/primary-coating boundary and the first higher-order core mode is presented to explain the presence of the hump. An experimental verification of this theoretical model is presented for these fibers. In addition, the curvature sensitivity of the hump is studied for these three fiber types. The results of the study show that for both dispersion-shifted and matched-cladding, dispersion-unshifted fibers, the hump is curvature sensitive and occurs at intermediate (28 and 15 cm) bend diameters. At large (40 cm) and small (10 cm) bend diameters, the hump was not observed in the cutoff region of these two fiber types. However, in the case of the depressed-cladding fiber, the hump was found to be curvature insensitive and occurred at all bend diameters (40, 28, 15, and 10 cm) used in this study.