Maxwell Fisheye Lens As A Waveguide Crossing For Integrated Photonics

Abstract

Integrated silicon (Si) photonics represents one of the key technologies for developing compact high speed optical systems for computing and telecommunications. In such systems, electric buses are replaced with integrated Si waveguides which transport light across the chip. In order to implement high density networks, it is inevitable that waveguides will need to be crossed to transport information across orthogonal directions. However, when two or more waveguides cross, light is scattered due to the abrupt change in the modal index resulting in losses of up to 40 percent. This loss occurs to both the environment as well as the overlapped waveguide, causing cross-talk into the other channel resulting in false signals. Current Si based waveguide crosses require either a large footprint or are limited in the number of waveguides that can be crossed simultaneously. In this work, we develop integrated gradient index elements based on the Maxwell Fisheye (MFE) to provide low-loss and massively parallel optical waveguide crossings. To realize a crossing, waveguides which are modal index matched to the MFE are coupled across the lens wherein the output of one waveguide is imaged to the input of its partner on the opposite side. Based on this methodology, we present full-wave modeling of the device demonstrating a 0.1 dB loss (97.7% transmission) per crossing for an overall waveguide cross footprint of 28.26 square microns, among the most efficient designs to date. We also propose how this device can be realized using smoothly tapered Si waveguides to provide the required 2D gradient refractive index profile.