Fig. 1: Schematic of the ring resonator with an interferometric back-coupling architecture.
From: Efficient Kerr soliton comb generation in micro-resonator with interferometric back-coupling
a The ring is embedded in a cavity with twice its length. The pump-resonance detuning, δ1, is in general nonzero. Therefore, at the coupling points 1 and 2, there is a fraction of the field that is in- or out-coupled, while the remaining fraction is reflected. The interference of fields is adjusted by tuning (with a microheater) the length of the feedback section by Δ. Material propagation losses are very low, both in the ring and in the feedback sections, with intrinsic Q-factors exceeding 106. b Evolution of the frequency comb spectrum at the ring, feedback and output sections. If the pump fields coming out of the ring and feedback sections have similar amplitude and opposite phases, extreme pump depletion occurs at the device output. The amount of circulating pump power in the ring, which is necessary to sustain the solitons, can be controlled by modifying the coupling coefficients.
