Towards a compact soliton microcomb fully referenced on atomic reference

In this work, we propose a simple architecture for achieving an atomic-referenced fully stabilized soliton comb based on MgF₂ microresonators. In the scheme, we directly utilize a laser as the pump source, with its frequency (fp) locked to the optical frequency reference of a rubidium 5S-5D two-photon transition, and mechanically control the resonator's length with a piezoelectric ceramic (PZT) to generate solitons. With the thermal compensation from a resonance close to the soliton mode, we can easily maintain the soliton state and then successfully phase-lock the soliton's repetition frequency (frep) to a radio frequency (RF) reference by PZT. This method described allows for no coupling between fp and frep. Unlike previous solutions, our implementation does not require any AOM or EOM optoelectronic devices, auxiliary lasers, or optical frequency phase-locking loops and a decoupled strategy for locking parameters, which typically increase the system's complexity and reduce its compactness. Our results confirm that the stability of a comb line, positioned approximately 0.66 THz from the pump source, aligns with the stability of the Rb optical reference, achieving a remarkable precision of approximately 4 Hz over 100 seconds. Moreover, we examined the frequency repeatability of the comb line over six days, achieving a frequency standard deviation of about 10 kHz, which marks the highest level ever reported for atom-reference soliton microcombs. Our approach offers a low-power, compact alternative for fully stabilizing soliton microcombs, providing a more practical and efficient option compared to conventional methods.