Fig. 1. Timeline of the microcomb technology development. The four columns from left to right are developments of the application, the design, the principle, and the fabrication, respectively. Abbreviations: DCS, dual-comb spectroscopy; OCT, optical coherence tomography; MIR, mid-infrared; FSO, free-space optical communication; PhCR, photonic crystal ring; SIL, self-injection locked; Q, quality factor; M, million. References: optical clock^[42]; microwave processor^[43]; optical communication^[37]; microwave generator^[44]; dual-comb spectroscopy^[36]; self-referencing^[45]; optical frequency synthesizer^[46]; ranging^[35]; astrocomb^[47]; OCT^[48]; integrated optical clock^[34]; parallel LiDAR^[49]; MIR spectroscopy^[50]; optical computing^[51]; FSO^[52]; highly integrated system^[53]; chaotic LiDAR^[54]; random number generator^[55]; parallel coherent source^[56]; waveguide structure^[57]; inter-mode crossing^[58]; efficient coupled ring^[59,60]; concentric microring^[61]; battery-operated generation^[62]; pump recycling^[63]; width-chirped microring^[64]; PhCR^[65]; pulse pump^[66]; super efficiency^[67]; multimodality microring^[68]; inverse design^[69]; observation of microcomb^[70]; noise source^[71]; bright soliton microcomb^[11]; dark pulse microcomb^[72]; dispersion wave^[73]; quiet point^[74]; synchronization^[75]; laser cavity soliton^[76]; SIL turnkey generation^[77]; photonic molecule^[78]; quantum^[79]; Kerr-induced synchronization^[80]; hybrid cavity microcomb^[81]; ${\mathrm{SiO}}_2$^[82]; ${\mathrm{MgF}}_2$^[14]; Hydex^[15]; Si^[16]; SiN^[17]; AlN^[18]; diamond microcavity^[19]; ${\mathrm{Ta}}_2{\mathrm{O}}_5$ microcavity^[20]; AlGaAs microcavity^[21]; ${\mathrm{Ge}}_{23}{\mathrm{Sb}}_7{\mathrm{S}}_{70}$^[22]; LNOI microcavity^[23]; Damascene process^[83]; GaPOI^[84]; 4H-SiC^[25]; $\mathit{Q}>100$ M^[85]; heterogeneous integration^[86]; 6-inch mass production^[87]; ${\text{LiTaO}}_3$^[88].

Fig. 2. The basic principles of microcombs^[11,89,90].

Fig. 3. Dynamic evolution of mode-locked soliton formation in a microresonator^[11,92] (a) Numerical simulation results. Average intracavity power trace during the laser scan. The colored region shows the existing area of different comb states: soliton state (green), breather solitons with time-variable envelop (yellow), and solitons cannot exist (red). (b) Experimental observation of soliton steps, Top: pump power transmission of a silica microresonator as the pump tunes, exhibiting multiple steps indicative of soliton formation. Middle: imaging of soliton formation corresponding to the scan where the $x$-axis is time, and the $y$-axis is time in a frame that rotates with the solitons. Bottom: soliton intensity patterns measured at four moments, mapped onto the microcavity coordinate frame.

Fig. 4. Numerical simulation of the microcomb evolution. (a) The evolution process of intracavity optical spectra (top), pulse shapes (middle), and total powers (bottom). (b) Optical spectra under different states marked in (a). (c) Pulse shapes under different states marked in (a).

Fig. 5. Methods of frequency sweeping and pumping optimization. (a) Fast frequency scanning method^[127,137]. (b) Thermal tuning of the resonant frequency^[137]. (c) Power kicking method^[124,143]. (d) Bi-directional scanning method^[122].

Fig. 6. Principle and experimental scheme of pulse pump method. (a) Concept and comparison of the pulse pumping with traditional methods^[129]. (b) Experimental setup, simulation results, and experimental results of pulse pumping microcombs^[129].

Fig. 7. Auxiliary laser cooling for soliton generation. (a) Concept and principle of the auxiliary cooling methods. (b) Experimental setup and simulation results of auxiliary laser thermal balance^[132,135].

Fig. 8. Illustration of self-injection microcombs. (a) Concept and principle of self-injection locking and turnkey operation^[77]. (b) Self-injection locking of a laser diode to a bulk cavity for microcomb generation^[145]. (c) Self-injection locking of a laser diode chip to an integrated SiN microcavity chip for microcomb generation^[138]. (d) Bright soliton generated by self-injection locking^[86]. (e) The dark pulse is generated by self-injection locking^[140]. (f) Suppressed frequency noise^[140].

Fig. 9. Optical comb wavelength ranges and Q factors of different resonators based on various material platforms. Values of wavelength ranges and Q factors are taken from (top to bottom, left to right): ${\mathrm{MgF}}_2$^[147], ${\mathrm{CaF}}_2$^[148], ${\mathrm{BaF}}_2$^[149], ${\mathrm{SrF}}_2$^[150], ${\mathrm{MgF}}_2$^[147], ${\mathrm{SiO}}_2$^[70], ${\mathrm{MgF}}_2$^[11], ${\mathrm{SiO}}_2$^[135], ${\mathrm{MgF}}_2$^[151], ${\mathrm{SiO}}_2$^[117], ${\mathrm{SiO}}_2$^[148], ${\mathrm{SiO}}_2$^[148], ${\mathrm{Si}}_3{\mathrm{N}}_4$^[152], ${\mathrm{Si}}_3{\mathrm{N}}_4$^[153], Ge^[88], ${\text{LiTaO}}_3$^[88], SiC^[154], ${\mathrm{Ta}}_2{\mathrm{O}}_5$^[155], ${\mathrm{Si}}_3{\mathrm{N}}_4$^[156], AlGaAs^[157], Hydex^[17], Hydex^[158], ${\mathrm{Si}}_3{\mathrm{N}}_4$^[122], GaN^[19], diamond^[19], ${\text{LiNbO}}_3$^[23], AlN^[18], ${\mathrm{Si}}_3{\mathrm{N}}_4$^[159], ${\mathrm{Si}}_3{\mathrm{N}}_4$^[17], Si^[160], ${\text{LiNbO}}_3$^[161], GaP^[162], and GaP^[162].

Fig. 10. Metal fluoride microcavities. (a) ${\mathrm{MgF}}_2$^[147]. (b) ${\mathrm{CaF}}_2$^[171]. (c) Q factor measurement of the ${\mathrm{BaF}}_2$ cavity^[149]. (d) Generated bright soliton microcomb in a ${\mathrm{MgF}}_2$ cavity^[44].

Fig. 11. Silica microcavities. (a) Silica toroid microcavity^[82]. (b) Wideband microcomb generated in silica microcavity^[70]. (c) Bright soliton microcomb generated in silica microcavity^[174]. (d) Silica microsphere cavity^[175]. (e) Silica microrod cavity^[176]. (f) Silica wedge microresonator^[177].

Fig. 12. Hydex microcavity^[15].

Fig. 13. Silicon nitride microcavity. (a) Massively produced silicon nitride chips^[87,140]. (b) Thin silicon nitride microcavity^[140]. (c) Thick silicon nitride microcavity^[185].

Fig. 14. Chalcogenide platforms. (a) Integrated chalcogenide microcavity^[170]. (b) Chalcogenide on silica^[195]. (c) Chalcogenide on silicon photonics^[196].

Fig. 15. Group IV semiconductor microcavities. (a) Silicon microring with PIN junction^[160]. (b) Mode-locked microcomb in silicon microrings^[123]. (c) High-quality germanium microcavity^[165]. (d) High-quality SiGe microring cavity^[197].

Fig. 16. III–V compound semiconductor platforms. (a) AlGaAsOI microcavities^[21,205]. (b) GaP microcavity. (c) AlN microcavity. (d) GaN microcavity^[168]. (e) Comb generation with the second-order nonlinearity in AlN^[115].

Fig. 17. Electro-optical crystal platforms. (a) ${\text{LiNbO}}_3$ microcavities^[23]. (b) ${\text{LiNbO}}_3$ microcavity with electrodes (top) and the tuned repetition rate of the generated microcomb with the applied microwave field (bottom) ^[207]. (c) Self-starting solitons in ${\text{LiNbO}}_3$ microcavities^[104]. (d) Visible comb generation in ${\text{LiNbO}}_3$ microcavities with the second-order nonlinearity^[104]. (e) ${\text{LiTaO}}_3$ microcavities^[88]. (f) Comparison of the dispersion profiles of ${\text{LiNbO}}_3$ (right) and ${\text{LiTaO}}_3$ (left) microcavities^[88].

Fig. 18. Fabrication process of crystalline microcavities. (a) Cutting and shaping process^[215]. (b) The photo of a diamond tool^[216]. (c) Polishing with abrasive particles^[217].

Fig. 19. Melting and solidification processes. (a) The fabrication process of microtoroid cavity^[82]. (b) Preparation process of microsphere cavities^[222].

Fig. 20. Fabrication processes of thick silicon nitride microcavities. (a) The subtractive process^[187]. (b) The Damascene process^[214].

Fig. 21. Fabrication processes of AlGaAsOI microcavities^{[21,157,169,205]}.

Fig. 22. Recent advances in packaging and integration technologies for microcombs. (a) Hybrid integrated amplifiers with microcavities for microcomb generation^[62]. (b) Direct butt-coupling of the laser diode with the microcavity chip for self-injection locking^[138]. (c) Photonics wire bonging^[226]. (d) Packaged microcomb module with a laser diode butt-coupled with a microcavity chip^[77]. (e) The integrated Er-doped amplifier chip^[227]. (f) The schematic (left) and the packaged (right) Er-doped laser chip^[228]. (g) Single-chip microcomb generators^[86].

Fig. 23. Microcombs under different dispersion conditions. (a) Bright soliton under anomalous dispersion^[11]. (b) Dark pulse under normal dispersion^[72]. (c) Dispersion wave due to high-order dispersion^[73]. (d) Quartic soliton supported by fourth dispersion^[64]. (e) Bright pulse under near-zero dispersion^[252].

Fig. 24. Different dispersion engineering methods. (a) Concentric microring^[61]. (b) Coupled rings for single-point dispersion changing^[78]. (c) Coupled rings for wideband dispersion changing^[68]. (d) PhCR for single-point dispersion changing^[65]. (e) PhCR for multiple-point dispersion changing^[265].

Fig. 25. Schemes for high-efficiency microcombs. (a) The comparison between the soliton and the dark pulse^[277]. (b) The pulse pumped soliton^[130]. (c) Pump recycling for high efficiency with dual rings^[63]. (d) Interferometric back-coupling for high-efficiency solitons^[278]. (e) Interferometric back-coupling for high-efficiency dark pulses^[279]. (f) Dispersion engineering for high-efficiency solitons^[67]. (g) Soliton laser for high-efficiency microcombs. (h) The electrically empowered microcomb laser^[81].

Fig. 26. Noise sources and suppression methods. (a) Different noise sources for the intrinsic linewidth of comb lines^[288]. (b) The influence of thermal noise^[289]. (c) Intrinsic (top) and effective (bottom) linewidth distribution of comb lines^[288]. (d) Self-injection locking for high-coherence lasers^[140]. (e) Reduced thermal-optical effect under low temperature^[290]. (f) Suppressed thermal noise with laser cooling^[289]. (g) Low-noise soliton microcomb operating at the quiet point^[176]. Top: spectrum; bottom: phase noise of the repetition rate. (h) KIS for reducing the repetition rate fluctuation^[80].

Fig. 27. Microcomb-based frequency standard. The stabilization of the microcomb could be achieved by locking (a) one optical mode and repetition frequency^[67], (b) two separate optical modes^[42], and (c) repetition frequency and CEO frequency via the f-2f technique^[304]. First demonstrations of octave-spanning microcomb on (d) SiN^[225], (e) AlN^[206], and (f) 4H-SiC^[305] platforms. The highly integrated (g) optical clock^[34] and (h) optical frequency synthesizer^[46] based on dual-comb frequency clockwork.

Fig. 28. Microcomb-based spectroscopy. The direct frequency comb spectroscopy (DFCS) utilizes a single frequency comb for molecular fingerprint recognition, which can measure (a) atomic transition^[311] and (b) gas phase^[312]. (c) The plasmonic-enhanced DFCS systems^[313]. Dual-frequency spectroscopy (DCS) is enabled by a pair of microcombs, which can be generated simultaneously by (d) separately pumping^[36], (e) counterpropagating stimulation, and (f) single-pump driving^[314]. (g) The microcomb densification via iDFG scheme^[50].

Fig. 29. Microcomb based LiDAR. Two kinds of LiDAR schemes, ToF and FMCW, are implemented in microcomb-based LiDAR systems. (a) A pair of separated microcombs^[35] and (b) a pair of counter-propagating microcombs^[323] are employed for ToF schemes. (c) Microcomb-based dispersive interferometry^[324] for accurate ranging under long distances. (d) Principle of parallel FMCW LiDAR^[49].

Fig. 30. Microcomb-based OCT. (a) Chaotic microcomb^[48] and (b) soliton-microcomb^[330] -based OCT setups and their scan results.

Fig. 31. Microphotonic astrocomb. (a) Silica microcomb applied to Keck II telescope^[335]. (b) SiN soliton microcomb adapted to calibrate GIANO-B high-resolution spectrometer^[336].

Fig. 32. Microcomb-based optical computing. (a) A SiN soliton microcomb combined with a phase-change material attached to an on-chip waveguide array for tensor core operation^[345]. (b) A time-stretch strategy for both the convolution layer and fully connected layer in the optical neural network^[51]. (c) Silicon-photonic-assisted highly integrated optical computing processor based on a microcomb^[346].

Fig. 33. Microcomb-based microwave processing. (a) Reconfigurable filter^[358]. (b) Channelizer^[359]. (c) Beam former^[360]. (d) Hilbert transformer^[361]. (e) Differentiator^[362]. (f) Integrator^[363]. (g) Arbitrary waveform generator^[364].

Fig. 34. Microcomb-based chaotic applications. (a) Schematic of chaotic microcomb and its applications. (b) Chaotic heterodyne LiDAR^[371]. (c) Chaotic direct-detection LiDAR for interference-free^[54]. (d) Chaotic microcomb-based decision maker^[55].

Fig. 35. Highly integrated microcomb-based on-chip systems. (a) Schematic of microcomb-driven silicon photonic systems^[53]. (b) Integrated chromatic dispersion compensator^[375]. (c) 32-channel microcomb-enabled silicon photonic transmitter^[376].

Table 1. Summary of Different Mode-Locked Soliton States of Microcombs

Table 2. Summary of Different Excitation Methods of Mode-Locked Microcombs

Table 3. Linear and Nonlinear Optical Properties of Various Materials for Nonlinear Photonics

Table 4. Integrated Solutions for Microcomb Generation

Table 5. Comb State in Different Dispersion Conditions in an Experiment

Table 6. Design Methods for Dispersion Engineering

Table 7. Conversion Efficiency of Mode-Locked Microcomb

Table 8. Comparative Analysis of Parallel Data Transmission in Microcomb Systems

Table 9. Low-Noise Radio Frequency Generation Based on Microcombs