Reverse exchange method for burying proton exchanged waveguides

Reverse exchange has been used to bury proton exchanged LiNbO₃ waveguides. Following initial exchange and annealing steps, which define the guides, a second exchange reintroduces lithium, thus lowering the extraordinary refractive index at the surface. The buried guides have little optical power at the surface and a more nearly symmetrical mode than unburied guides, resulting in lower propagation losses and better fibre-to-guide coupling.