The design of an optical switch based on a directional coupler with tapered velocity and tapered coupling is investigated. The physical principle is based on power transfer by the evolution of a single normal mode. We consider cases where the coupling coefficient and guide mismatch variation are determined by optimized shape functions related to rectangle, raised cosine, Hamming, and Blackman window functions. Following simplified analysis by coupled mode theory, the physical parameters of the device are determined and the predicted performance is verified by the method of lines. Significant improvement in sidelobe levels and coupling length can be obtained by careful design.
Ary Syahriar, Vitor Marino Schneider, and S. Al-Bader, "The Design of Mode Evolution Couplers," J. Lightwave Technol. 16, 1907- (1998)