Fig. 2. Optical spectra.

(A to C) Generated spectrum when the heaters on (A) R1, (B) R2, and (C) both rings are tuned into resonance with the pump laser at 1561.4 nm. OSA resolution, 0.01 nm. The insets are top-view IR camera images showing which of the two rings are on resonance. The combs are in the single-soliton mode-locked state. Solid red and green lines are fits to a sech² spectral envelope.(D) Dual-comb state with a single soliton in R1 and two solitons in R2. The zoomed-in inset in (C) and (D) reveals dual combs with pairs of lines closely spaced in wavelength. By changing the magnitude of the final upward voltage ramp shown in (E), one can choose the number of solitons in the final state. As an example, the state shown here has a single soliton in R1 and two solitons half a roundtrip apart in R2 (harmonic mode-locking). All spectra span 400 nm (51 THz). (E) Time domain traces of the voltage ramp used to access the mode-locked states of both the rings. The blue (green) line represents the voltage applied to the heater on R1 (R2). The red and purple lines represent the transmitted pump power and the converted comb power (excluding the pump and vertically offset for clarity), respectively. Discrete steps are seen in the pump power, as well as the comb power, characteristic of multisoliton states in the ring. The inset shows the stages of comb formation in R1, including the high-noise state (I), the multisoliton state (II and III), and the single-soliton state (IV). a.u., arbitrary units.