Title: Nonsolitonic Kerr Combs
Description: Kerr frequency combs enable compact and robust platforms for applications such as data communications and spectroscopy. Initial demonstrations used dissipative soliton combs operating in the anomalous group velocity dispersion (GVD) regime to illustrate the promising capabilities of Kerr-comb-based technologies. However, many real-world applications, such as wavelength division multiplexing and LiDAR, benefit from higher comb-line powers that are inherently inaccessible to Kerr frequency combs. Nevertheless, nonsolitonic Kerr combs operating in the normal GVD regime offer promise as a platform for the integration of such applications due to their ability to easily access high pump-to-comb conversion efficiencies and spectral profiles with slower power falloffs. Unlike a dissipative Kerr soliton which is a single cycle periodic pattern on top of a homogenous background, a nonsolitonic Kerr comb arises through the interlocking of two switching waves of opposite polarity, each of which connects the two homogenous state solutions of the bistable cavity response.

Here, we present a systematic approach for turn-key generation of Kerr combs in the normal GVD regime. We use a coupled ring geometry to induce and control avoided mode crossings for the generation of low-noise frequency combs with conversion efficiencies of up to 57%. Moreover, we explore the regime of synchronization for these nonsolitonic Kerr combs. Synchronization is a universal mechanism of coupled nonlinear oscillators that manifests itself as the spontaneous appearance of order within nature's tendency for disorder and chaos. It is a process by which the natural rhythms of interacting oscillators adjust to a common frequency and produce a mutually phase-locked state. In the realm of Kerr frequency combs, synchronization allows for the repetition rates, or equivalently the comb-line spacings, of individual Kerr combs to become identical. We reveal the universality of Kerr comb synchronization by synchronizing two nonsolitonic Kerr combs and thereby extending the scope of synchronization beyond the soliton regime.

Furthermore, we introduce a method to overcome the inherently low output power of Kerr combs while maintaining the high conversion efficiency of a normal GVD Kerr comb. We demonstrate efficient comb-line power enhancement by coherently combining two nonsolitonic Kerr combs via on-chip synchronization. Our on-chip synchronization design removes the requirement for dispersion compensation of the coupling signal while increasing the overall stability of the system. Our techniques of comb generation, synchronization, and coherent combining enable nonsolitonic Kerr combs as a platform to achieve a fully integrated, high-power Kerr comb source.