Mode coupling and radiation loss of clad-type optical waveguides due to the index inhomogeneities of the core material

Abstract

Closed form approximate expressions are derived for the mode-coupling, radiation loss and losses of each guided mode in multimode clad-type dielectric slab waveguides due to the index inhomogeneities of the core material. The analysis considers the inhomogeneities of the core-index in the axial and transverse directions and includes correlation lengths of any magnitude. It allows us to conclude that there will be considerable mode-dependency for the mode-coupling and radiation loss if the axial and/or transverse correlation lengths are of the order of or greater than the wavelength. It is also found that the mode-coupling and radiation loss peak at some value of the axial correlation length, and the radiation loss can be reduced by increasing the refractive-index difference of the core and cladding materials. Finally, it is shown that if the correlation lengths are shorter than, or of the order of the wavelength, the losses of each guided mode will mainly be due to the radiation process. But, in the case where the axial correlation length is much greater than the wavelength, the scattering into other guided modes is the dominant mechanism.