Generation of multiple near-visible comb lines in an AlN microring via χ ⁽²⁾ and χ ⁽³⁾ optical nonlinearities

On-chip frequency upconversion of a near-infrared (NIR) Kerr comb in a χ⁽²⁾ and χ⁽³⁾ system provides a convenient route to extending the comb spectra into the visible band. Yet to date, only limited visible or near-visible comb lines have been obtained using this scheme. In this work, we demonstrate the generation of multiple near-visible comb lines based on spectral translation from a broadband NIR Kerr comb. This physical process is implemented in an aluminum nitride (AlN)-on-sapphire microring, where we achieve a wideband frequency upconversion by incorporating the phase-mismatched fundamental and first-order near-visible modes. Upon tuning the pump into the resonance with sufficient power, we attain a broadband NIR Kerr comb and 153 corresponding near-visible comb lines in 720-840 nm with a reasonable efficiency over 4.1 × 10^-5%. The wideband frequency upconversion can be adapted to on-chip frequency stabilization of self-referenced microcombs, as required for precision optical clocks and frequency metrology.