Methods for passive fiber chip coupling of integrated optical devices

A useful technique for high precision passive coupling of single mode optical fibers to integrated optical devices is crucial for cost effective packaging especially in multiport devices like switches (N x N) and other WDM components. These devices were fabricated on two different material bases, silicon on insulator (SOI) and polymers. In both cases the waveguides are based on the oversized rib waveguide concept and utilize silicon as a substrate. Two possible fabrication processes for this passive fiber chip coupling IN or ON silicon will be presented and compared. The first approach involves a technology similar to flip chip fabrication using a sub- and superstrate, that allows separate processing of v-grooves for fiber alignment and the integrated optical devices. The self aligned mounting of the chip is achieved by a v-shaped rib-groove combination created by wet chemical etching, where the rib is the exact negative of the groove so that the flip chip is put on precisely defined crystal planes rather than on sensitive edges, which would be the case when using rectangular alignment ribs. The second approach utilizes the same chip for waveguides and fiber alignment structures which makes it possible to define both in the same lithographic step and thereby eliminating any vertical displacement. Processing difficulties arise primarily from completely different processing requirements of fiber aligning v-grooves and integrated waveguides. The need to define patterns of the size of only several microns (mum) in the proximity to deep grooves makes the use of an electrophoretic photoresist necessary that is deposited via galvanic means on the extremely nonplanar surface. Both processes allow for fiber chip alignment precisions in the sub-mum range which was also experimentally verified with coupling losses as low as 0.7 dB per end-face. The fabrication processes along with experimental and theoretical results will be presented.