Mode Conversation Caused by Diameter Changes of a Round Dielectric Waveguide

A theory of mode conversion and radiation losses of a guided mode in a dielectric slab was described in Ref. 1. The power conversion to spurious guided modes as well as to the continuum of unguided radiation modes was assumed to be caused by deviations from perfect straightness of the air-dielectric interface of the slab. The model of the dielectric slab waveguide was chosen for its simplicity. Even though the dielectric slab exhibits all the relevant features of mode conversion caused by surface roughness and allows one to draw conclusions as to the order of magnitude of the losses suffered by guided modes in dielectric waveguides of other geometries, it is desirable to report the calculations for a round dielectric rod. The results of calculations for the dielectric rod are directly applicable to light transmission along optical fibers. Furthermore, we wanted to test the predictions of the theory at microwave frequencies where a controlled 3217 3218 THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER