Fig.3 Equi-loss curve of one repeater section for single-mode optical fiber

Equi-loss curve of one repeater section for single-mode optical fiber

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    Optimum single-mode fiber (SMF) parameters were studied extensively to make it possible to construct a transmission line operating at a 1.3 μm wavelength. It was found that spot size Wo and effective cutoff wavelength λce provided a more suitable combination than the conventional combinations of core radius a and relative refractive-index difference Δ, or λce and Δ. Figure 3 shows the situation. It is noted that various refractive-index profiles are acceptable as well as the ideal rectangular profile for single-mode operation. Typical and actual probable profiles in view of fabrication methods are shown as Types I to IV in the inset of Fig. 3. Here, I is a profile with an ideal rectangular shape, II has a center dip, III has a tail, and IV has a hump and a tail. When the parameters were defined by the combination of a and Δ, the equi-loss contours were completely different for each profile type. The situation can be tolerated to some degree for the λce and Δ combination, but the equi-loss contours are still different for each profile as shown in Fig. 3(a). While, with λce and Wo, as shown in Fig. 3(b), the equi-loss contours are almost all the same, and also the optimum parameter range is represented by a rectangle. Thus, this combination is found to be the most appropriate for specifying the unique fiber transmission characteristics for different index profiles. The optimum parameter ranges were found to be 1100 nm≤λce≤1280 nm and Wo=5±0.5 μm, for SMF with zero dispersion at 1.3 μm wavelength.

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