Exploiting Ultralow Loss Multimode Waveguides for Broadband Frequency Combs

Low propagation loss in high confinement waveguides is critical for chip-based nonlinear photonics applications. Sophisticated fabrication processes which yield sub-nm roughness are generally needed to reduce scattering points at the waveguide interfaces to achieve ultralow propagation loss. Here, ultralow propagation loss is shown by shaping the mode using a highly multimode structure to reduce its overlap with the waveguide interfaces, thus relaxing the fabrication processing requirements. Microresonators with intrinsic quality factors (Q) of 31.8 +/- 4.4 million are experimentally demonstrated. Although the microresonators support ten transverse modes only the fundamental mode is excited and no higher order modes are observed when using nonlinear adiabatic bends. A record-low threshold pump power of 73 mu W for parametric oscillation is measured and a broadband, almost octave spanning single-soliton frequency comb without any signatures of higher order modes in the spectrum spanning from 1097 to 2040 nm (126 THz) is generated in the multimode microresonator. This work provides a design method that can be applied to different material platforms to achieve and use ultrahigh-Q multimode microresonators.

Automated summary

Low propagation loss in high confinement waveguides is achieved by shaping the mode using a highly multimode structure to reduce overlap with waveguide interfaces, without the need for sophisticated fabrication processes. Experimental demonstration of microresonators with intrinsic quality factors of 31.8 +/- 4.4 million is provided, showing the generation of a broadband single-soliton frequency comb without higher order modes.