The operation of tapered velocity couplers is analyzed from the point of view of mode conversion between local-normal modes. An approximate coupled mode representation of the local-normal modes is used with a step transition model to estimate analytically power transfer. This yields an upper limit for the maximum permissible variation in mode synchronism across the coupler to achieve satisfactory coupler operation. The analysis has also been used to demonstrate the importance of obtaining a large total change in mode synchronism to minimize coupler length.
A. F. Milton and W. K. Burns, "Tapered Velocity Couplers for Integrated Optics: Design," Appl. Opt. 14, 1207-1212 (1975)