Fig. 1. Resonator characterization. (a) Top-view photograph of the AlGaAsOI microresonator. The radius of the microresonator is 144μm, corresponding to 91 GHz FSR. Scale bar: 100μm. (b) Normalized transmission spectrum of a typical resonance at 1552 nm. Lorentzian fitting reveals intrinsic Q factors about 2.36 × 10⁶. (c) Scanning-electron-microscopy image showing the cross section of the microresonator. The AlGaAs core is highlighted in red, and silica forms the substrate and cladding. Scale bar: 200 nm. (d) Calculated dispersion of the TE0 mode in AlGaAsOI microresonators with respect to the width of the core. The thickness of the core is set as 400 nm. The inset shows the corresponding TE0 mode profile. (e) Measured TE0 mode family dispersion. Dint=ωμ−ωo−D1μ=D2μ2/2+O(μ3), where ωμ is the resonant frequency of the μth mode. The index of the mode that is pumped is set to 0, and D1 is the FSR at pump wavelength. Parabolic fitting (red) shows D2/2π=−1.63MHz. (f) Zoom-in spectrum of the doublet resonances at μ=0 indicated in (e). The frequency is plotted relative to the fitting curve. (g) Pseudo-color plot of the relative frequency shifting of the coupled mode pairs from the ideal resonant frequency without the AMX effect, as a function of temperature.

Fig. 2. Dynamics of mode-locked microcombs. (a) Normalized transmitted total power (blue) and comb power (red). The laser is scanned from the blue side to the red side of the mode. Blue and red shadings indicate CW state and microcomb state, respectively. (b) Typical optical spectra of microcombs at different stages as indicated in (a). Red dashed lines denote the simulated spectral envelope. (c) Simulated intracavity waveforms corresponding to the spectra in (b). (d) Optical spectra of microcombs with spacing from 1 to 4 FSRs (top to bottom). Inset: intensity noise of the 1-FSR microcomb (resolution bandwidth: 100 kHz). The noise floor of the measurement system is also plotted for comparison.

Fig. 3. Power-efficient mode-locked Kerr comb generation. (a) Schematics of different microcomb generation schemes. (b) Comb spectrum at the flat step [shading area in upper panel (c)] under the on-chip pump power of <930μW. (c) Measured comb power with respect to the frequency tuning. As the increasing of the pump power, the comb existence area could be extended from 0.3 GHz (upper panel) to 1.7 GHz (mid-panel) and then 11 GHz (lower panel). (d) Wide tuning of the dark-pulse microcomb for 97.5 GHz, which is over an entire FSR. (e) Experimental setup of an on-chip semiconductor laser pumped scheme and (f) comb spectrum. The right panel exhibits that dark-pulse spectrum can be repeatedly accessed with the laser current switched on and off, showing the “turnkey” behavior. All the spectra are of 2-FSR (∼180GHz) frequency spacing. OSA, optical spectrum analyzer.

Fig. 4. Widely tunable frequency-chirped microcomb. (a) Both (i) bright soliton and (ii) dark pulse could act as a parallel frequency chirping source, in which the frequency modulation of the pump laser is transduced to each comb line. The stimulated Raman effects and higher-order dispersion would result in wavelength-dependent chirping copies, which could be mitigated in the efficient dark-pulse scheme with relatively low intracavity power. (b) Experimental setup of the parallel chirping source. WSS, wavelength-selective switch; WM, wavelength meter. (c) Measured time-frequency maps of the pump line (left panel) and the channel-10 sideband (right panel), with the pump laser chirping at 5 GHz (blue) and 10 GHz (red), respectively. (d) Frequency excursion of each channel at 10 GHz frequency chirping. (e) Experimental setup of the fast frequency modulation. An SSB modulator is employed as the chirping pump. The modulated comb lines are then charaterized by a heterodyne measurement. SSB, single-sideband modulator; OSC, oscilloscope. (f) Time-frequency maps of 5-GHz chirping pump and a comb line with modulation frequency of 400 kHz.

Fig. 5. Coherence of mode-locked frequency comb. (a) Experimental setup. PC, polarization controller; NF, notch filter; EDFA, erbium-doped fiber amplifier; BPF, bandpass filter; AOM, acousto-optic modulator; PD, photodetector; ESA, electric spectrum analyzer; OSC, oscilloscope. (b) Optical spectra of dark-pulse comb after NF. The range of telecommunication C-band is also indicated. (c) Left panel: measured RIN of comb teeth indicated in (b). Right panel: RIN at 10 MHz offset frequencies of all comb teeth within C-band. (d) Left panel: measured SSB frequency noise of comb teeth indicated in (b). Right panel: fundamental linewidth of all comb teeth within C-band.

Fig. 6. Long-term stability of the microcomb. The optical spectra of the microcomb are continuously recorded for over 7 h, and the total comb power (red) and power of the 10th comb line (blue) are plotted, showing 1.25 and 1 dB variations, respectively.