Observation of interband Kelly sidebands in coupled-ring soliton microcombs

Kelly sidebands are a special type of dispersive wave that appear in mode-locked systems and they have recently been observed by pulsed excitation in integrated microcombs. Here, Kelly sidebands are generated by continuous-wave excitation in a partially coupled racetrack-resonator microcomb. The coupled-racetrack system supports two optical bands so that, in contrast to earlier studies, the soliton and Kelly sideband reside in distinct bands. The resulting interband excitation of the Kelly sidebands relaxes power requirements and continuous-wave sideband excitation is demonstrated. Tuning of sideband spectral position under pulsed excitation is also studied. Numerical simulation and the experiment show that the sidebands rely upon symmetry breaking caused by partial coupling of the two-ring system. More generally, multiband systems provide a new way to engineer Kelly sidebands for spectral broadening of microcombs.