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Geometrical construction to derive the effective group index ng-eff(λ/λc) of the fundamental mode (solid line curve) from its effective phase index neff(λ/λc) (dashed line curve). The tangent to neff(λ/λc) crosses the ordinate axis at the value of the corresponding effective group index ng-eff(λ/λc), as shown for λ = 1.25 λc. One sees easily 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; above 1.5 λc, the mode starts to widen a lot and the curvature loss increases drastically.
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