Aperture Numerical for a graded index fiber:
For graded index waveguides, the NA is a function of position on the waveguide
endface. A ray entering the endface must turn parallel the zaxis before it reaches
the limits of the graded region. As the enters ray the waveguide further from the
axis of the graded region, it clearly must come more closely parallel to the zaxis.
The NA is:
NA ( r ) = n 2 ( r ) ! n22
which decreases from maximum at r = 0 to zero where the gradient stops.