For an optical fiber , an unbound mode . ( 188 ) 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 .