Sub-milliwatt-level microresonator solitons with extended access range using an auxiliary laser

The recent demonstration of dissipative Kerr solitons in microresonators has opened a new pathway for the generation of ultrashort pulses and low-noise frequency combs with gigahertz to terahertz repetition rates, enabling applications in frequency metrology, astronomy, optical coherent communications, and laser-based ranging.A main challenge for soliton generation, in particular in ultra-high-Q resonators, is the sudden change in circulating intracavity power during the onset of soliton generation.This sudden power change requires precise control of the seed laser frequency and power or fast control of the resonator temperature.Here, we report a robust and simple way to increase the soliton access window by using an auxiliary laser that passively stabilizes intracavity power.In our experiments with fused silica resonators, we are able to extend the access range of microresonator solitons by two orders of magnitude, which enables soliton generation by slow and manual tuning of the pump laser into resonance and at unprecedented low power levels.Importantly, this scheme eliminates the sudden change in circulating power ("soliton step") during transition into the soliton regime.Both single-and multi-soliton mode-locked states are generated in a 1.3-mmdiameter fused silica microrod resonator with a free spectral range of ∼50.6 GHz, at a 1554 nm pump wavelength at threshold powers <3 mW.Moreover, with a smaller 230-μm-diameter microrod, we demonstrate soliton generation at 780 μW threshold power.The passive enhancement of the soliton access range paves the way for robust and low-threshold microcomb systems and has the potential to be a practical tool for soliton microcomb generation.