Demonstration of Inverse Designed Broadband Cavity-Waveguide Couplers

The ability to control cavity-waveguide coupling over a wide wavelength span is necessary in a number of applications such as frequency conversion and frequency comb generation. However, achieving a specified target coupling at multiple wavelengths using the conventional approach of evanescent coupling is an outstanding challenge. In our approach, we instead treat the cavity-waveguide coupling region as a 2-input, 2-output port device and optimize the structure to produce the desired coupling spectrum. This inverse design method enables us to specify any coupling spectrum as our optimization objective, and the fabrication-constrained optimization produces coupler structures that are fully fabricable with standard lithography processes. We experimentally demonstrate our ability to control the coupling spectrum between a straight waveguide and racetrack resonator on 220 nm SOI while maintaining a Q factor of about 30,000. Our progress on efficient coupling over an octave span is an important step for integrated nonlinear photonics.