Radiation mode explained
For an optical fiber or waveguide, a radiation mode or unbound mode is a mode which is not confined by the fiber core. Such a mode has fields that are transversely oscillatory everywhere external to the waveguide, and exists even at the limit of zero wavelength.
Specifically, a radiation mode is one for which
\beta=\sqrt{n2(a)k2-(l/a)2}
where
β is the imaginary part of the axial
propagation constant, integer
l is the azimuthal index of the mode,
n(
r) is the
refractive index at radius
r,
a is the core radius, and
k is the free-space wave number,
k = 2π/
λ, where
λ is the wavelength. Radiation modes correspond to refracted rays in the terminology of
geometric optics.
See also
