Fig. 3: Pathway to power-efficient DKS microcombs.

a, Set-up used to control and monitor the high-efficiency soliton initiation. The pump laser generates a comb in the main cavity, whereas the probe laser is scanned in frequency to spectrally characterize the pumped resonance. b, Evolution of the converted power and transmitted power over time measured in oscilloscope 1 (OSC1). The three tuning steps (laser frequency, auxiliary heater and input power) used to reach a high-efficiency soliton are indicated, with arrows indicating the tuning direction of each control signal. The converted power shows where a comb is generated, with modulational instability states and multi-solitons found in the first 20 s of the converted trace, whereas a single soliton state was found after transition near the end of the heater tuning step. c, Selected comb states in the initiation process, marked by dashed lines in b. The top row shows the resonance scan from the counterpropagating probe measured by the oscilloscope OSC2, with the x-axis showing frequency change and the shading indicating the blue and red sides of the pumped resonance. The beat note with the pump laser shows that the pump laser is consistently located at the blue side of the resonance throughout the initiation process. The second row shows the corresponding comb state measured in the optical spectrum analyser (OSA). The third and fourth rows show the output spectrum and the main cavity temporal power found through numerical simulations, qualitatively replicating the initiation process. d, A highlighted version of the final soliton, showing an experimental (red) and simulated on-chip output spectrum. The inset shows the measured beat note between comb lines, acquired by electro-optic downconversion and measured with a resolution bandwidth (RBW) of 100 Hz. The final measured (simulated) spectrum has 54% (48%) conversion efficiency.