Efficient optical waveguide coupler

A new method for high-efficiency interconnection of two planar optical waveguides is described. This technique is based on mode-matched coupling and is particularly useful when the difference between the refractive indices of the individual guides is large. The method consists of etching a deep vertical step in the highindex waveguide followed by the deposition of low-index waveguide on the etched section so that the two guides remain connected end to end. A theoretical treatment for the design of the coupler and for determination of fabricational tolerences is given. Couplers were fabricted using high-index waveguides like GaAs and LiNbO(3) and low-index waveguides made up of glass or solution-deposited (polyurethane or photoresist) films on SiO(2) substrate. It is shown that by controlling the junction profile, efficiencies close to theoretical values can be obtained. Some applications of such a coupler are given.