Vertically tapered polymer waveguide mode size transformer for improved fiber coupling

We describe a novel vertical taper structure fabricated at the ends of poly mer optical waveguide devices to improve the coupling between channel waveg uides and single mode fibers. The taper smoothly converts a highly elliptic al waveguide mode into a large and more circular mode for low loss coupling and relaxed fiber alignment tolerances. A vertical taper 0.5 to 2 mm in le ngth is made in the upper cladding to reduce its thickness from a few micro meters to zero. The taper is subsequently covered by an upper cladding. The new upper cladding has an index higher than that of the previous upper cla dding but slightly lower than that of waveguide core. In the taper, the cha nnel waveguide mode gradually loses confinement by the upper cladding so th at the mode size grows larger as light propagates toward the end of the dev ice, whereas the confinement by the lower cladding and the lateral confinem ent are not significantly affected. The waveguide mode grows upward away fr om the lossy ground electrode and substrate commonly found in many polymer devices; therefore, no trade-off between mode size and propagation loss is involved. Two special but simple reactive ion etching techniques, shadow ma sked 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 experimentally. The performance of th e mode size transformer is found to be insensitive to both waveguide width and polarization. (C) 2000 Society of Photo-Optical instrumentation Enginee rs.