Rhythmic Soliton Interactions for Integrated Dual-Microcomb Spectroscopy

Soliton waves are sustained by a self-created index potential well, and their interactions occur when the potential well is invaded by other solitons. The interactions are important to soliton compounds in plasma, Bose-Einstein condensation, and optical systems. In optical microresonators, interactions between counterpropagating (CP) solitons can lead to Vernier frequency locking (VFL), which is invaluable for dual-comb applications. Here, we synthesize CP solitons with VFL in an integrated silicon nitride microresonator and characterize the rhythmic soliton interaction dynamics. Seconds-long mutual coherence between solitons arises passively from the interactions. The interactions also cause the solitons to oscillate periodically. This temporal motion is discerned by frequency-domain measurements of the instantaneous frequency changes and microcomb sidebands. A theory is derived to describe the sideband strength and leveraged to retrieve the motion trajectory. The complex soliton motion trajectory is further measured by optical cross-correlation using a CP soliton trimer state. We further prove that precise dual-comb spectroscopy with a single interferogram and an acquisition time of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mn>0.6</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi mathvariant="normal">μ</a:mi><a:mi mathvariant="normal">s</a:mi></a:mrow></a:math> is feasible using these solitons, but the temporal motion limits the dynamic range. Our work establishes the rhythmic soliton interaction dynamics and reveals the characteristics of VFL solitons in the silicon nitride platform, which can be used to guide the design of integrated dual-comb systems.