Abstract
A distinctive feature of single-mode fibers having deposited cladding with an index below that of the silica substrate tube is a rapid rise in loss at long wavelengths, typically beyond 1.3 μm.1 The location of this loss edge is determined by the wavelength λ′ at which the mode effective Index β/k falls below the index ns of the substrate tube. For λ ≳ λ′ the mode is leaky due to power radiated into the substrate tube.2 Figure 1 shows a typical mode diagram and a depressed-cladding index profile. At the system operating wavelength λs say, 1.3 μm, it is necessary that β/k decrease (because V, the normalized frequency, varies as λ−1), and eventually the mode becomes leaky at λ′. Note that if Δ− = 0 (i.e., deposited-cladding index equals ns), there is no leaky-mode region, and normal mode cutoff occurs when β/k = nclad·
© 1982 Optical Society of America
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