Fig. 1. Schematic diagram of optical frequency comb (OFC) based on microcavity structure^[24]

Fig. 2. Several kinds of Kerr OFCs. (a) An octave-spanning frequency comb generated in a silica microtoroid ^[23]; (b) Spectrum of solitons comb generated in Si₃N₄ microtoroid^[47]; (c) Spectrum of solitons comb generated in silica^[48]

Fig. 3. Visible light OFCs are generated using second harmonic generation, four-wave mixing and sum-frequency effects. (a) Generate simultaneous near IR and visible wavelength frequency comb lines; (b) The effective index of the fundamental TE mode in near IR spectral range and the third-order TE mode in the visible spectral range; (c) The group velocity dispersion (GVD) of the fundamental TE mode^[50]; (d) The effective index of TM₀₀ mode in near IR spectral range and TM₁₀、TM₀₀ mode in the visible spectral range; (e) Normalized modal overlap; (f) Generation of IR、second harmonics and near-visible microcombs ^[51]

Fig. 4. (a) The second-order dispersion D₂/(2π) for the fundamental TE and TM modes; (b) The effective index for the fundamental TE、TM modes at the pump frequency and higher-order modes at the third-harmonic frequency; (c) Generation of IR microcombs; (d) Generation of third harmonics and green light comb ^[55]

Fig. 5. (a) Scanning electron microscope image of the deformed microtoroid resonator; (b) Chaos-assisted dynamical tunneling from WGMs to chaotic states; (c) Nonlinear frequency conversion from infrared combs to visible wavelengths^[61]

Fig. 6. Wedge angle modulation of microdisk cavity dispersion to generate visible OFCs. (a) Illustration of the mode profile for the fundamental TM₁₀ mode inside the cavity; (b) Dispersion characteristics of the TM₁₀, TM₂₀, TM₃₀ modes(Inset: simulated mode profiles of the TM₁₀, TM₂₀ and TM₃₀ modes); (c) Variation of the dispersion when changing the thickness of the disk at different wedge angles; (d) Generation of near-visible Kerr OFCs ^[67]

Fig. 7. Visible OFCs generation by higher-order mode TE₁₀. (a) Voltage pattern used to modulate the on-chip heater; (b) Evolution of comb power corresponding to the pattern in Fig.(a); (c) Comb spectrum of four-soliton state; (d)-(f) Optical spectra corresponding to point A (minicomb formation), B (chaotic state), and C (soliton state) in Fig.(b); (g)-(i) RF noise corresponding to Fig.(d)-(f)^[67]

Fig. 8. OFC generated with axial (vertical) modes ^[75]. (a) Schematic of FWM with the fundamental azimuthal mode family; (b) Schematic of FWM with the axial mode family; (c) Relationship between OFC generated with aixal modes and collection positions

Fig. 9. Visible OFCs generated by supermode with anomalous dispersion. (a) Structure diagram of coupled double ring microcavity and field distribution of symmetric mode and antisymmetric mode; (b) Dispersion of antisymmetric mode (red line) and symmetric mode (blue line); (c) Residual frequency detuning; (d) OFC generation in symmetric and anit-symmetric supermodes^[86]

Fig. 10. TM and TE modes are coupled to form supermodes, generating visible frequency combs. (a) Effective refractive index under different thicknesses; (b) Dispersion curves of supermodes with different thicknesses; (c) Optical spectrum of a soliton generated^[87]; (d) Dispersion curves of supermodes^[88]

Fig. 11. Spectrum of octave-spanning dissipative Kerr soliton^[20]

Fig. 12. Visible OFCs generated by stacked coupled supermodes. (a) A representative AlGaN hybrid waveguide structure and the field amplitude profile of super-mode; (b)-(e) Dispersion curves of supermodes in different width w、height h_f1、height h_g、heighth_f2; (f) Simulated frequency-comb spectrum^[91]

Fig. 13. Visible OFCs generated by AMXs. (a) Integrated dispersion of supermode; (b) Generated frequency comb when the mode indicated in (a) is pumped; (c) Simulation of the integrated dispersion; (d) Primary comb line generation when the mode indicated in (c) is pumped^[92]