Improved Extinction Ratio in Ultra Short Directional Couplers Using Asymmetric Structures

Directional couplers are critical components in photonic integrated circuits used in optical communication systems. The conventional directional couplers with laterally arranged waveguides can not achieve very short coupling lengths due to extreme sensitivity to fabrication variations and limitations to produce uniformly very narrow gap layers.1) Vertical directional couplers, however, can obtain a short coupling length that is smaller than 100 μm.2) The difficulty of separating the two vertically coupled waveguides into two distinct inputs and outputs limits the application of these devices.3) Recently, a novel fused vertical coupler (FVC) with a very short coupling length of 62 μm was demonstrated, that can solve this problem.4) To use FVC in large switching fabrics, it should have a low crosstalk. Ultra short directional couplers have an inherent limitation in their extinction ratio due to nonorthogonality of individual waveguide modes.5) In this paper, we analyze various means to improve the extinction ratio using slight asymmetry in the structure. We also study the fabrication tolerance needed to maintain these improved characteriatics. Since vertical coupling through the ridge structure defined by etch-stopping techniques is much less sensitive to the ridge waveguide width and sidewall smoothness than the planar waveguide couplers,6) it is expected that these slight asymmetric structures can be better realized in fused vertical coupler configuration.