Fig. 1. Si3N4 cavity used for comb generation. The effective index’s real and imaginary parts are governed by the Kramers–Kronig relations.

Fig. 2. Spectral loss as large as 300 dB/cm with an FWHM of 200 GHz is added to the primary comb lines closest to the pump. A mode-locked Kerr comb can be generated in (a), associated with a cavity soliton in (b). The inset in (a) shows the attenuation profile of loss peak of 300 dB/cm. The ripples on the pulse pedestal are caused by beating between dispersive waves.

Fig. 3. Different comb generation processes are seen as the loss bandwidth increases, with the loss of 300 dB/cm at the nearest primary comb lines. The corresponding loss profiles are shown in red lines. The comb is not mode-locked anymore for a loss bandwidth of 600 GHz, while comb generation is completely stopped for a loss bandwidth of 1 THz.

Fig. 4. Mode-locked Kerr combs are obtained as 1 THz wide loss of 300 dB/cm moves away from the pump, beyond the nearest primary comb lines. This holds even if the loss is at the second nearest primary comb line.

Fig. 5. Initial cavity dynamics with 300 dB/cm, 1 THz wide loss located at (b) 1562 and (c) 1608 nm. Compared to the case with no loss in (a), comb generation is enhanced in (b) and (c).

Fig. 6. Initial average intracavity power is analyzed when the loss is located at different frequencies. The peaks of the oscillating curves are extracted and plotted in the inset.

Fig. 7. Three different architectures of comb-based on-chip spectroscopy systems.