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Fig. 9


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Geometrical construction relating the effective group index ng-eff to the effective index neff of the fundamental mode of a fiber, with the dependence in normalized spatial frequency λc/λ. A double-slope line is drawn from where the tangent to the effective index curve crosses the ordinate axis, and the group index ng(λc/λ0) equates DS0(λc/λ0), as shown for λc/λ0 = 0.8, i.e., for λ0 = 1.25 λc. As in Figure 7, one easily sees that ng-eff is about equal to the refractive index of the core n1, in the practical domain of use of a single-mode fiber, i.e., λc < λ < 1.5 λc, or 0.67 < λc/λ < 1.

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