SPOT SIZE EXPANSION FOR LASER-TO-FIBER COUPLING USING AN INTEGRATED MULTIMODE COUPLER

Efficient coupling of light from a semiconductor waveguide to an optical fiber is difficult to achieve due to the inherent differences in the spot sizes. The problem is particularly severe in the vertical dimension, i.e., perpendicular to the layers in the semiconductor waveguide, since the spot sizes typically differ by more than a factor of 5. This difference can be corrected by using a waveguide taper, but this is difficult to implement in the vertical direction. In this paper we propose to use a uniform, untapered intermediate coupler structure which consists of a multimode waveguide; this structure has potential for integrating with the semiconductor waveguide. The basic design rules for this coupler are worked out using coupled mode theory, and the detailed designs are based on direct calculation of the mode profiles in the coupler. We examine a range of realistic examples and show that the butt coupling efficiency from a typical semiconductor laser to an optical fiber can be improved by up to 5 dB. The performance dependence on fabrication tolerances is also studied.