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
β = n 2 ( a ) k 2 − ( l / a ) 2 {\displaystyle \beta ={\sqrt {n^{2}(a)k^{2}-(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 Guided ray Numerical aperture Cladding (fiber optics)
References This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).