In this paper, we propose a tapered graded-index (GI) core polymer optical waveguide with only 300-micrometer length for applying to a very short light path such as optical VIA and optical pin. The tapered GI core polymer waveguides are actually fabricated utilizing the imprint method. We theoretically and experimentally demonstrate that tapered GI core polymer waveguides exhibit lower loss (1 dB or more) than tapered step-index (SI) core waveguides.
In recent years, the data traffic in datacenters has grown rapidly due to the deployment of cloud services. In order to support this growth, optical interconnection technologies are gradually deployed and approaching to short-reach regions in the vicinity of LSI chips. Hence, a low loss very short optical path that perpendicularly passes through printed circuit boards (PCBs) or interposers are required. The optical VIA in PCBs and optical pin in optical transceivers are the examples. In such a short optical path, a tapered waveguide structure has been reported. However, the excess loss due to the scattering at the core-cladding interface and the increase in the divergence angle of the output light would be problems in the current SI core waveguide based optical VIA and optical pin.
Therefore, we focus on GI core waveguides in this paper, because GI core waveguides confine the propagating modes strongly to the center of the core. In addition, the short GI-cores play a role of GRIN (convex) lenses, as well. So, the output NA from tapered GI core waveguides is optimized by adjusting the waveguide parameters.