Δ= n 1 2 − n 2 2 2 n 1 2 MathType@MTEF@5@5@+=feaagCart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbbjxAHXgarqqtubsr4rNCHbGeaGqipG0dh9qqWrVepG0dbbL8F4rqqrVepeea0xe9LqFf0xc9q8qqaqFn0lXdHiVcFbIOFHK8Feea0dXdar=Jb9hs0dXdHuk9fr=xfr=xfrpeWZqaaeqabiGaciGacaqadmaadaqaaqaaaOqaaiabgs5aejabg2da9maalaaabaGaamOBamaaDaaaleaacaaIXaaabaGaaGOmaaaakiabgkHiTiaad6gadaqhaaWcbaGaaGOmaaqaaiaaikdaaaaakeaacaaIYaGaamOBamaaDaaaleaacaaIXaaabaGaaGOmaaaaaaaaaa@414D@
where n1 and n2 are, respectively, the maximum in the core and the refractive index of the innermost homogeneous cladding