| for a cross section of an optical fibre the area within which the refractive index everywhere (excluding any index dip) exceeds that of the innermost homogeneous cladding by a given fraction of the difference between the maximum of the refractive index of the core and the refractive index of the innermost homogenous cladding
NOTE – The core area is the smallest cross-sectional area of a fibre excluding any index dip, which is contained within the locus of points where the refractive index n3 is given by :
n3 = n2 + k (n1 – n2)
n1 is maximum refractive index of the core,
n2 is refractive index of the innermost homogeneous cladding,
k is a constant (usually taken to be in the range 0 – 0.05).