Optical waveguide mode-combiners for fundamental and first-order modes, based on multimode interference (MMI) couplers are presented. These devices convert a fundamental mode into a transversal first-order mode and combine it in lossless fashion with a second fundamental mode. They can separate zero- and first-order modes in a common waveguide and allow the splitting and combining of zero- and first-order modes with nonuniform power splitting ratios. Realizations in InGaAsP/InP are demonstrated. These new components have successfully been integrated into all-optical switches and were found to have advantageous characteristics in all-optical devices.
Juerg Leuthold, Juerg Eckner, Emil Gamper, Pierre A. Besse, and Hans Melchior, "Multimode Interference Couplers for the Conversion and Combining of Zero- and First-Order Modes," J. Lightwave Technol. 16, 1228- (1998)