Dissipative Coherent Structures and Satellite Comb Generation in Dispersion-Periodic Kerr Microresonators

Recent developments of integrated photonic platforms have opened unique possibilities for on-chip generation of coherent and broadband frequency combs, both in normal and anomalous dispersion regimes via the generation of coherent nonlinear structures [1]. The potential role played by quasi-phase matching in dispersion engineering for this has remained an open question. As it happens, Si3N4 microresonator frequency combs typically employ waveguides with a large cross sections to reduce propagation loss and to reach anomalous dispersion. Coupling between transverse spatial modes can interfere with stable microcomb generation, so a narrow tapered mode ‘suppression’ section can be added to the microresonator in order to alleviate this issue [2] (Fig. 1(a)). In our experiment, we found that this tapered waveguide section provided precisely this quasi-phase matching between higher order of dispersion, leading to the emergence of Faraday Instability (FI) [3]. When combined with synchronous pulse-driving in a microresonator having normal dispersion on average [4], we observe FI combined with switching wave formation to generate ‘satellite’ switching wave microcombs.

Recent developments of integrated photonic platforms have opened unique possibilities for on-chip generation of coherent and broadband frequency combs, both in normal and anomalous dispersion regimes via the generation of coherent nonlinear structures [1]. The potential role played by quasi-phase matching in dispersion engineering for this has remained an open question. As it happens, Si3N4 microresonator frequency combs typically employ waveguides with a large cross sections to reduce propagation loss and to reach anomalous dispersion. Coupling between transverse spatial modes can interfere with stable microcomb generation, so a narrow tapered mode ‘suppression’ section can be added to the microresonator in order to alleviate this issue [2] (Fig. 1(a)). In our experiment, we found that this tapered waveguide section provided precisely this quasi-phase matching between higher order of dispersion, leading to the emergence of Faraday Instability (FI) [3]. When combined with synchronous pulse-driving in a microresonator having normal dispersion on average [4], we observe FI combined with switching wave formation to generate ‘satellite’ switching wave microcombs. Fig. 1

Faraday instability-assisted broadband coherent structure generation in dispersion modulated microresonator. (a) Microscope image of the Si₃N₄ racetrack microresonator having 15 GHz free spectral range. The inset shows the mode stripping section that filters higher-order transverse modes. (b) The variation of the integrated dispersion through the tapered waveguide. (c) Spectrum of pulse-driven switching wave microcomb and the emergence of FI-driven satellite structures.