Insertion loss reduction in high-speed polymer electro-optic modulators using tapered waveguide, fiber tip lenses, and modification of waveguide structures

Abstract

Optical insertion loss improvement is needed for high-speed, electrooptic polymer waveguide modulators. The insertion loss is mainly attributed to fiber end-coupling loss caused by mode mismatch between the fiber and the polymer waveguide. Three approaches of reducing end-coupling loss have been pursued: tapered waveguides, fiber tip lenses and modification of the waveguide structure. Tapered waveguides can be accomplished using Reactive Ion Etching (RIE) and shadow or gray-scale mask techniques. Experimental results have shown that the best coupling loss improvement up to 3 dB per end-coupling can be achieved by tapered waveguide. Fiber tip lens technique has been currently investigated for improving end-coupling loss. Preliminary testing results using fiber tip lenses have been encouraging showing a 1.5 - 3.5 dB improvement per end-coupling. The fiber tip lens technique provides a reliable and repeatable approach for loss reduction for high-speed polymer modulators. Modifying a highly asymmetric rib waveguide to a symmetric buried channel waveguide will greatly improve coupling efficiency.