For an optical fiber, an unbound mode.

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 .