Fig. 1. Route map of microcombs.

Fig. 2. Typical spectral coverage of SMCs on various material platforms using different approaches.

Fig. 3. Experimental demonstration of stable temporal solitons in a high-QMgF2 microresonator using the frequency-scanning method. (a) Experimental setup for stable temporal soliton generation. (b) Optical transmission power trace when the pump scans over a resonance. The discrete steps in the red-detuned regime (green shading) indicate existence of cavity solitons. (c) Optical spectral evolution while the pump sweeps in the blue-detuned regime. (d) Optical spectra for SMCs with 1, 2, and 5 solitons. OSA, optical spectrum analyzer; ESA, electrical spectrum analyzer; PD, photodetector; LO, local oscillator; FPC, fiber polarization controller; EDFA, erbium-doped fiber amplifier. Images are adapted with permission from Ref. 10.

Fig. 4. SMC generation based on power-kicking scheme. (a) Experimental setup of power-kicking scheme. The microresonator is pumped by an external cavity diode laser that is amplified and modulated by an AOM and an EOM. (b) Typical triangular shape of the transmission power while the pump sweeps across a resonance. (c)–(e) Soliton steps of Si3N4 microcavities with repetition rates of 38, 70, and 190 GHz, respectively. (f) Measured optical spectrum of single SMC covering over 2/3 octave bandwidth. AFG, arbitrary function generator; EDFA, erbium-doped fiber amplifier; FPC, fiber polarization controller; OM, optical modulator; RF, radio frequency; TLF, tapered-lensed fiber. Images are adapted with permission from Ref. 12.

Fig. 5. Schematic and timing sequences of the power-kicking scheme. (a) Setup used to bring very short-lived soliton states to a steady state, including two modulators to adjust the pump power. (b) Timing sequences of the pump scanning, the fast and slow power modulation, and the converted light power. (c) Initial timing of the fast modulation with respect to the thermal triangle and slow power modulation. (d) The fast power modulation induced soliton steps. (e) Combined effect of the fast and slow modulation. Images are adapted with permission from Ref. 63.

Fig. 6. SMC generation based on thermal-tuning method. (a) Experimental setup of thermally controlled SMC generation in a Si3N4 microresonator. (b) Transmission optical power trace of the generated microcomb. Steps marked by arrows indicate transitions between different multisoliton states. Images are adapted with permission from Ref. 14.

Fig. 7. SMC generation by the auxiliary-laser-based method. (a) Experimental setup. (b) Schematic of the counter-coupled auxiliary-laser-assisted thermal response control method. (c) The pump and auxiliary laser counter-balance thermal influences on the microcavity. (d) Optical spectrum of single SMC. Images are adapted with permission from Refs. 16 and 66.

Fig. 8. Bichromatic SMC generation in a LiNbO3 microresonator. (a) Schematic for influences of optical Kerr and photorefraction effects. The inset is the measured optical power traces when a pump sweeps across a resonance from the red-detuned side. (b) Comb power trace versus scanning time when the pump slowly sweeps forward and backward in the red-detuned regime. (c) Comb power trace versus scanning time when the pump rapidly sweeps from the red- to blue-detuned regimes. The power spikes are caused by the relatively slower response speed of the photorefraction effect. (d) and (e) Optical spectra of SMCs in near-infrared and visible bands, respectively. Images are adapted with permission from Ref. 31.

Fig. 9. (a) Scheme of the forward frequency-tuning method. (b) 200 overlaid experimental traces of the output comb light in the pump forward tuning, revealing the formation of a predominant soliton number of N=6. (c) Scheme of the laser forward and backward tuning. (d) Experimental traces of the forward tuning (in yellow) and backward tuning (in white) for soliton switching and deterministic single SMC generation. (e) Measured absolute soliton existing range of a Si3N4 microring. The lower boundary δL presents staircase pattern that can be stably accessed step by step using backward-scanning method. (f) Optical spectrum of single SMC in a 100-GHz Si3N4 microresonator. Images are adapted with permission from Ref. 69.

Fig. 10. Deterministic single SMC generation using thermal-tuning method.¹⁵ (a) Power traces when just decreasing the operation temperature. (b) Power traces using forward and backward operation temperature tuning method. (c) Optical spectra for soliton number of 4, 3, 2, and 1 in a 49-GHz high-index doped silica glass microring. Images are adapted with permission from Ref. 15.

Fig. 11. Self-injection locking and spectral narrowing of a multifrequency laser diode coupled to an MgF2 ultrahigh-Q WGM microresonator. (a) Experimental setup. (b)–(d) Spectrum and (e)–(g) the corresponding beat note signal for the free-running multifrequency diode laser, laser stabilized by the microcavity, and single SMC in the self-injection locking regime, respectively.⁴⁵

Fig. 12. Principle and experimental scheme for SMC generation driven by optical pulses.⁷⁶ (a) For the CW-driven case, solitons propagate with a resonantly enhanced CW background. (b) For the pulse-driven case, pump pulses with repetition rate frep periodically drive the solitons. (c) Resonator transmission trace as the central driving mode scans across a resonance for an optimized repetition rate. (d) Contour plot of the resonator transmission showing soliton steps can exist for a wide (100 kHz) spanning interval of frep. Images are adapted with permission from Ref. 76.

Fig. 13. (I) Soliton crystals in a silica disk resonator.⁴⁶ Left panel: measured (in black) and simulated (in color) optical spectra. Right panel: schematic depictions of the corresponding soliton distribution in the resonator with major ticks indicating (expected) soliton location and minor ticks indicating peaks of extended background wave due to mode crossing. (II) Soliton crystals in a high-index doped silica glass microring.¹⁹ Left panel: measured (in red) and simulated (blue solid circles) optical spectra. Right panel: simulated temporal traces exhibiting (expected) soliton distributions of the corresponding soliton crystals. Images are adapted with permission from Refs. 19 and 46.

Fig. 14. Stokes soliton in a high-Q silica microdisk.¹⁷ (a) Stokes soliton (red) is overlapped with primary soliton (blue) in time and space, which introduces maximum Raman gain. Stokes soliton is trapped by optical potential well induced by Kerr effect, which locks the repetition rate to the primary soliton. (b) Measured FSRs of different mode families versus wavelength of a 3-mm silica microdisk cavity. (c) Beating RF spectra of isolated Stokes and primary soliton, indicating the repetition rate of Stokes soliton is locked to primary comb. (d) Measured optical spectrum of Stokes soliton. The inset shows the high-resolution spectrum of the overlapping range, which confirms that the Stokes soliton is formed in a different mode family. Images are adapted with permission from Ref. 17.

Fig. 15. Breather soliton in a Si3N4 microresonator. (a) Schematic of soliton “breathing” behavior in a microcavity. (b) Recorded power trace of breather solitons. (c) Operating regimes of microcombs. Breathers are generated at relatively small detuning and high pump power through three steps (illustrated by I, II, and III). (d) Simulated transmission power trace. States 1 to 4 correspond to the primary comb, unstable MI, breather solitons, and stationary soliton state, respectively. (e) Averaged spectrum of the breather soliton in a Si3N4 microresonator. (f) RF spectrum of breather soliton. Images are adapted with permission from Refs. 20 and 51.

Fig. 16. Intermode breather solitons in microcavities. (a) Simulated intracavity power trace over the laser detuning in the absence of intermode interactions. The intermode breather soliton exists in the region where stationary soliton is expected (orange area). (b) Simulated power trace based on the coupled LLEs, showing a hysteretic power transition (gray area) and an oscillatory behavior (orange area). (c), (d) Measured optical spectra for intermode breather solitons in (c) an MgF2 crystalline microresonator and (d) a SiN microring, which exhibits spikes that result from intermode interactions. Images are adapted with permission from Ref. 85.

Fig. 17. Heteronuclear soliton molecule generation using two discrete pumps. (a) Principle of bound solitons where attractive force and repulsive force are balanced. (b) Calculated repulsive force versus the temporal separation of solitons. (c) The experimental setup for soliton molecule generation. (d) Measured transmission power trace while the pumps sweep across a cavity resonance. The red-shaded area is the comb power of the major pump, while the comb power of the minor pump is indicated by the blue-shaded area. (e) Optical spectrum of soliton molecules of two bound solitons, which corresponds to a linear superposition of optical spectra of the major soliton (f) and minor soliton (g). Images are adapted with permission from Ref. 22.

Fig. 18. Laser cavity solitons. (a) Principle of cavity soliton formation. The microresonator is nested into a gain fiber cavity. (b) Mode relationship of the nonlinear microresonator and gain fiber cavity. (c) Typical optical spectrum of laser cavity soliton, which includes two equidistant solitons per round-trip. Images are adapted with permission from Ref. 21.

Fig. 19. Frequency comb generation in normal-dispersion microcavities. (a) Experiment setup using a semiconductor laser self-injection locked to an MgF2 WGM resonator, wherein the spectral envelope shows three distinct maxima. (b) Numerically simulated envelope of intracavity optical pulses in terms of normalized amplitude (blue) and the pulse formed by only a limited number of modes with no pump frequency included (red).¹⁰¹ (c) Example comb spectrum spanning more than 200 nm obtained in a SiN microring (inset: an optical micrograph of the microring). (d) Square optical pulses directly generated under special conditions at high pump power.¹⁰² (e) Spectrum for the mode-locked state using dual-coupled SiN microrings in normal-dispersion regime.⁹⁷ Insets: microscope image of microrings (upper left) and transmission spectra versus heater power showing the resonances can be selectively split (upper right). (f) Comb intensity noise corresponding to (e) measurement by an electrical spectrum analyzer (top) and autocorrelation of the transform-limited pulse after line-by-line shaping (bottom).

Fig. 20. DS generation in a normal-dispersion SiN microring using (a) and (b) the thermal-tuning method²³ and (c)–(e) second-harmonic-assisted approach.⁹³ (a) Drop-port power transmission when one mode is pumped. (b) Comb spectra (left panel) and intensity noise (right panel) corresponding to different stages in (a). (c) Experimental setup for the second-harmonic-assisted comb generation. Inset: microscope image of the microring with second-harmonic radiation. (d) Transition curves of the through port (top) and drop port (bottom) when the pump laser scans across the resonance from shorter to longer wavelengths. (e) Reconstructed waveforms at through port (top) and drop port (bottom), showing bright and dark pulses, respectively (inst. freq.: instantaneous frequency). Images (a) and (b) are adapted with permission from Ref. 23 and images (c)–(e) are adapted with permission from Ref. 104.

Fig. 21. Application areas of SMCs.