For fiber-coupling purposes an effective fiber NAe² defined at the 1/e² level is more convenient than the nominal fiber NA defined by the refractive indices since normally Gaussian beams are defined by their 1/e² diameter, too.
Schäfter+Kirchhoff defines an effective fiber NA which corresponds to the divergence of the power distribution emitted by the fiber taken at the 1/e² level of the Gaussian angle distribution.
This NA value is designated the effective numerical aperture NAe². For a typical singlemode fiber the value is NAe² = 0.075.
For singlemode fibers and for polarization-maintaining fibers, the effective NAe2 slightly decrease with increasing wavelength l.