Fig. 1. (a) Scanning electron micrographs of a Si3N4 microring with diameter of 200 μm. Insets show the microring cross section of 1.8  μm×0.8  μm and the corresponding fundamental transverse-magnetic mode profile. (b) Experimental setup for Kerr frequency comb generation. EDFA, FPC, EOM, WDM, DWDM, FP, OSC, and ESA are erbium-doped fiber amplifier, fiber polarization controller, electro-optical modulator, wavelength-division multiplexer, dense wavelength-division multiplexer, Fabry–Perot cavity, oscilloscope, and electronic spectrum analyzer, respectively. (c) Detailed comb line spectrum of a breathing soliton measured by the FP spectrum analyzer, with two sidebands indicating the breathing frequency around 0.4 GHz. The inset shows a typical resonance of the microring, with a loaded Q of 1.5×106 according to the Lorentzian fitting (red line).

Fig. 2. Evolution of the soliton generation processes during the scanning of the pump laser detuning. (a)–(d) Typical optical spectra. Four evolution stages are (a) primary comb, (b) modulation instability comb, (c) breathing soliton, and (d) stable soliton, respectively. (e)–(h) The corresponding evolution of RF spectra. Inset: the transmission spectrum of the microring when the laser frequency is scanned across the resonance mode.

Fig. 3. Features of a breathing soliton. (a) The detailed RF spectrum of a breathing soliton state. Inset: the corresponding optical spectrum. (b) The recorded fast power evolution of a single comb line around the center (1562 nm, blue curve) and in the wings (1531 nm, green curve) of the optical spectrum of a breathing soliton. Inset: the corresponding Fourier transform spectrum.

Fig. 4. (a) Simulated evolution of the intracavity power when the laser frequency is scanned across the resonance mode. The inset shows the oscillations of the power for a fixed laser frequency in the breathing soliton state. (b) Periodic spectrum evolution of a breathing soliton state. (c) RF spectra of the initial breathing soliton state (blue line) and modulated breathing soliton state (red line). The initial breathing frequency fbr is 287 MHz, and the modulated frequency fmod is 270 MHz. The inset shows the concept of injection locking of a breathing soliton. A modulation signal with fmod is applied to the pump laser after the appearance of breathing soliton, and fbr is injection locked if fmod is within the locking range.

Fig. 5. (a) Evolution of the RF spectrum when gradually increasing the modulation power from −50 to −5  dBm. The initial breathing frequency fbr is 276 MHz (I) and the modulation frequency fmod is 281 MHz. With the increase of the modulation power, there is a competition between fbr and fmod, and other harmonics components appear (II and III). Eventually, fbr is synchronized to fmod as the modulation power is strong enough (IV). fbr returns back to the initial frequency after turning off the modulation signal (V). (b) Snapshots with different evolution stages in (a).

Fig. 6. (a) Evolution of the RF spectrum centered at 262 MHz with varied fmod. The modulation power is 0.1 mW, and fbr is synchronized to fmod when the frequency difference Δf is less than ∼15  MHz. (b) Locking ranges with varied modulation power.