Loss Increases in Multimode Rectangular Infrared Waveguides Due to Helical Deformations

The coupling coefficients for all TE _pq and TM _pq modes of multimode rectangular waveguides, caused by elementary rotations about the three axes of symmetry, are calculated in the microwave approximation. The modes b coupled to a common mode a by these independent rotations form three nonoverlapping families. Furthermore if a deformation consisting of no more than two of these rotations is considered, none of the modes b are coupled to each other by that deformation. These properties lead, in the case of mild deformations, to the formulation of a multimode coupling theory which shows that the loss increase due to such deformations can then be viewed as the result of multiple two-mode coupling. Explicit formulas are derived for the loss increase of TE _p0 due to mild circular bending. Qualitative features of twists and helical deformations are also brought out.