Soliton formation in microresonators provides an important way to generate chip-scale frequency combs [1]. Generating a smooth and phase coherent comb spectrum is critical for many applications, e.g., optical arbitrary waveform generation, spectroscopy and optical communications. For this purpose, single cavity soliton generation is favored, compared to multi-soliton combs. However, the random nature of the soliton generation process usually leads to multiple-solitons instead of a single soliton [1]. To generate single solitons, many experimental methods have been proposed and used, including: fast and precise control of the laser scan speed [1, 2], backward tuning [3], active capture [4], and phase/amplitude modulation of the pump [5]. Most of these techniques require relatively complicated active control; it is desirable to discover passive mechanisms that can induce single soliton generation.

Here we introduce such a passive mechanism arising from mode-interaction that induces single soliton generation deterministically. Furthermore, our numerical results reveal that the interplay between the soliton and the mode-interaction induced Cherenkov radiation (CR, also termed as dispersive wave) greatly affects the stability of a multi-soliton state [7].

Soliton generation dynamics are distinct in these two devices. When scanning the laser across the resonance, we find that the soliton step comprising a single soliton is always observed in Device 1 (Fig. 1(b)). After stabilizing the soliton, we find there is a strong mode-interaction induced CR in the spectrum (Fig. 1(c)). In contrast, multiple solitons are typically generated in Device 2; an example of such a multi-soliton comb is shown in Fig. 1(d). When we generate single solitons by backward tuning [3], the single soliton comb in Device 2 does not have a strong CR in the mode-interaction region. These data suggest that mode-interaction induced CR can be responsible for the single soliton generation.

To confirm that and further unveil the role of mode-interaction, we conducted numerical simulations based on the Lugiato-Lefever equation [6]. The mode-interaction term is included in the model by adding additional phase shift to the mode-interaction perturbed mode (see ref. [7] and its Supplementary Material for more information on the simulation model and method).

Figure 2(a) shows an example illustrating that with sufficient mode-interaction strength, only a single cavity soliton emerges when the detuning is scanned. The simulated soliton steps in 50 scans further show that a single soliton is always generated (Fig. 2(b)). To further reveal the role of mode-interaction induced Cherenkov radiation, we seed the simulation with two solitons. With this mode-interaction, the two solitons become unstable during propagation (Fig. 2(c)). Then one of the soliton annihilates (see the arrow in Fig. 2(c)), and the other one regains stability. Further analyses of the power in the Cherenkov radiation shows that transient power shed into the CR is stronger when two solitons (Pt in Fig. 2(d)) are present than the steady power in CR when a single soliton is generated (Ps in Fig. 2(d)). Thus, a single soliton tends to be generated, in order to reduce the nonlinear loss into CR.

In conclusion, we have shown that a strong mode-interaction can induce single soliton generation in microresonators. The results show that mode-interaction may not always be detrimental to soliton generation. The revealed mechanism also gives insights into how CR affects the soliton stability in dissipative systems.