Vertically tapered polymer waveguide mode size transformer for improved fiber coupling

Abstract

We describe a novel vertical taper structure fabricated at the ends of polymer optical waveguide devices to improve the coupling be- tween channel waveguides and single mode fibers. The taper smoothly converts a highly elliptical waveguide mode into a large and more circu- lar mode for low loss coupling and relaxed fiber alignment tolerances. A vertical taper 0.5 to 2 mm in length is made in the upper cladding to reduce its thickness from a few micrometers to zero. The taper is sub- sequently covered by an upper cladding. The new upper cladding has an index higher than that of the previous upper cladding but slightly lower than that of waveguide core. In the taper, the channel waveguide mode gradually loses confinement by the upper cladding so that the mode size grows larger as light propagates toward the end of the device, whereas the confinement by the lower cladding and the lateral confinement are not significantly affected. The waveguide mode grows upward away from the lossy ground electrode and substrate commonly found in many poly- mer devices; therefore, no trade-off between mode size and propagation loss is involved. Two special but simple reactive ion etching techniques, shadow masked etching and etching with a tapered photoresist mask, are developed to make the vertical taper. Mode expansion and a 1.8 dB reduction in coupling loss per tapered end are demonstrated experimen- tally. The performance of the mode size transformer is found to be in- sensitive to both waveguide width and polarization. © 2000 Society of Photo- Optical Instrumentation Engineers. (S0091-3286(00)01206-X)