Performance of a dual-mode-single-mode waveguide coupler as a modal filter.

Coupling characteristics of a dual-mode-single-mode evanescent field coupler are obtained and its performance as a first-higher-order-mode filter is studied. It is observed that the performance of such a device as a modal filter depends critically on the relative powers and phases of the two modes of the dual-mode waveguide at its input. In particular it is shown that if the fractional power in the fundamental mode of the dual-mode waveguide is large, then the power filtered in the single-mode waveguide depends strongly on the phase difference between the two modes, and it is not a good measure of the power in the first higher mode of the dual-mode waveguide.