Single-mode coupling between fibers and indiffused waveguides

We have investigated two approaches for single-mode coupling between fibers and channel waveguides: evanescent field and end fire. In the pursuit of an evanescent fiber to film coupler, we have investigated an all adiabatic mode-liftoff structure which is needed to combat the mode-sinking problem in coupling between two waveguide-material systems with vastly different indices of refraction. Experimentally, we observed high loss in the mode-liftoff structure. Using local normal mode-coupling theory and surface-scattering theory, we have had good success in predicting the observed performance. This suggests that evanescent coupling from a LiNbO3indiffused waveguide to a low-index waveguide such as a fused-silica single-mode fiber is not feasible with state-of-the-art LiNbO3surface roughness. In the pursuit of an alternate approach, we have demonstrated a flip-chip technique suitable for positioning a number of single-mode fibers and channel waveguides for end-fire coupling which uses preferentially etched grooves in silicon for fiber positioning.