Editing Radiation mode (section)

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 
:<math>\beta = \sqrt{n^2(a) k^2 -(l/a)^2}</math>
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 [[Fiber_optics#Principle_of_operation|core]] radius, and ''k'' is the free-space wave number, ''k'' = 2π/''λ'', where ''λ'' is the wavelength. Radiation modes correspond to [[refracted ray]]s in the terminology of [[geometric optics]].