radiation mode
For an optical fiber, an unbound mode.
Note: In an optical fiber, a radiation mode is one having fields that are transversely oscillatory everywhere external to the waveguide, and which exists even at the limit of zero wavelength. Specifically, a radiation mode is one for which 
where 
is the imaginary part (phase term) of the axial propagation constant, integer 
is the azimuthal index of the mode, n(a) is the refractive index, where a is the core radius, and k is the free-space wave number, k = 2
/
, where is the wavelength. Radiation modes correspond to refracted rays in the terminology of geometric optics. Synonym unbound mode.