Dispersion-flattened concentric structure for microcomb bandwidth broadening in GaP-OI resonators

Abstract

We propose and theoretically investigate the coupled concentric ring resonators on a thickness-constrained GaP on insulator (GaP-OI) integrated photonic platform. Achieving anomalous dispersion is fulfilled by mode hybridization in the coupled structure on a 200 nm thick GaP-OI resonator which originally only exhibits normal dispersion for the fundamental mode. The anomalous dispersion profile for the anti-symmetric mode is flattened and broadened in favor of Kerr frequency comb generation by optimizing the waveguide width and the coupling gap size synergistically. We show the flexibility of this design methodology by simultaneously flattening the dispersion profile while anchoring the dispersion peak location at 1550 nm. The optimized design has a flat anomalous dispersion span of 460 nm with a small peak of 160 ps/km/nm, 1.69 times lower than a traditional rectangular waveguide. The engineered dispersion profile enables a broadband Kerr frequency comb generation that has a 3 dB bandwidth of 67 nm and a 20 dB bandwidth of over 250 nm at both 1550 and 1650 nm pump wavelengths. The proposed design proves useful to achieve broad and flat anomalous dispersion on thickness-constrained materials, paving the way towards low-loss GaP-OI frequency comb resonators.