Broadband Near-Visible Frequency Comb Generation via High-Order Mode Dispersion Engineering in a Microbubble Resonator

Remarkable progress has been made on microresonator-based optical frequency combs (microcombs) in recent years, leading to widespread applications including optical communication and bioimaging. However, due to the significant normal material group-velocity dispersion (GVD) in the visible wavelength range, investigations of microcombs are primarily limited to the infrared and mid-infrared wavelength bands. In this study, we demonstrate the generation of broadband microcombs spanning ∼130 nm in the visible wavelength range, by leveraging the geometric dispersion of high-order modes confined within the thin wall of a microbubble resonator (MBR). Furthermore, we utilize the dispersive wave emission generated by the high-order dispersion to efficiently extend the comb span. Our study highlights the potential of MBRs with precisely controlled wall thickness as an additional degree of freedom for engineering GVD and extending nonlinear photonic applications into the visible and near-visible spectral range.