Extended Data Fig. 1: Experimental evidence suggesting an NB soliton crystal.

Intensity spectrum of a probable fifth harmonic frequency comb, where the intermode spacing equals 5 FSRs. The soliton crystal regime is suggested by the smooth bell-shaped envelope of the spectrum. In the Fig. 3 of the main manuscript, we have shown an experimental and theoretical characterization of a multisoliton state comprised of two co-propagating NB solitons in a single roundtrip. A special case of multisolitonic states, where all of the solitons within one roundtrip are equidistant, is called a soliton crystal¹⁵. In the frequency domain, these waveforms correspond to a harmonic frequency comb whose spacing between adjacent comb modes is equal to an integer multiple of the FSR: N × FSR, where N is the number of solitons in the soliton crystal. The coherence of the state is suggested by the high suppression ratio of the fundamental modes that fall beneath the noise floor, leaving only the harmonic equidistant modes. Furthermore, the modes form a smooth bell-shaped spectral envelope that indicates the soliton nature of the state. The high frequency of the intermode beatnote (around 68 GHz) lies well above the cutoff frequency of our optical detector, thus prohibiting SWIFTS characterization to truly assess the coherence of the state. This begs for the future use of another coherent technique to study soliton crystal dynamics in active ring resonators.