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Fig. 8


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Two possible geometrical constructions for relating group index ng to phase index n: (a) with the dependence in wavelength λ, i.e., the spatial period of the wave, the tangent T0(λ) to the phase index curve crosses the ordinate axis at the value ng(λ0) of the group index; (b) with the dependence in angular frequency ω, a double-slope line DS0(ω) is drawn from where the tangent T0(ω) to the phase index curve crosses the ordinate axis, and the group index ng(ω0) equates DS0(ω0).

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