NOTE 1 – For a plane wave of wavelength λ, it is related thus to the refractive index n:
N=n−λ dn dλ MathType@MTEF@5@5@+=feaagCart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbbjxAHXgarqqtubsr4rNCHbGeaGqipG0dh9qqWrVepG0dbbL8F4rqqrVepeea0xe9LqFf0xc9q8qqaqFn0lXdHiVcFbIOFHK8Feea0dXdar=Jb9hs0dXdHuk9fr=xfr=xfrpeWZqaaeqabiGaciGacaqadmaadaqaaqaaaOqaaiaad6eacqGH9aqpcaWGUbGaeyOeI0Iaeq4UdW2aaSaaaeaacaqGKbGaamOBaaqaaiaabsgacqaH7oaBaaaaaa@3F3F@
where λ is the wavelength.
NOTE 2 – Each mode has its own group index.