Periodic coupling of waveguide modes

When two waveguide modes are coupled uniformly along the direction of propagation, power is partially transferred from one mode to the other. This transfer is only complete if the phase velocities of the modes are equal. If the phase velocities differ, complete transfer of power is still possible if a certain spatially periodic coupling is provided. The exact conditions for complete transfer for a square-wave and raised-square-wave coupling function are derived. To illustrate the theory, experimental results for coupling between the TE10 and TE20 modes in an overmoded X band guide are presented. Experimental results for a sinusoidal function are also presented. These results provide strong support for the theory of periodic-mode coupling.