Abstract

The use of any form of evanescent coupler to couple between a high index waveguide system and a low index waveguide system requires the introduction of a buffer layer to shield the evanescent coupling region from the high index substrate. For example to use a tapered velocity coupler to couple between a high index indiffused electro-optical waveguide and a low index single mode glass fiber as shown in Fig. 1, requires that the power in the electrooptical waveguide be lifted off the substrate over a low index buffer layer before the evanescent coupling region is introduced. A buffer layer with an index lower than the effective index of the fiber mode is necessary to prevent phase matched evanescent coupling between the guided mode of the fiber and the substrate modes of the electrooptical crystal.

© 1976 Optical Society of America