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