Multimodal locking-enabled robust and day-scale low-repetition-rate soliton microcomb for high-precision metrology.

Chip-scale soliton microcombs, particularly those operating at low, electronically detectable repetition rates (≤ 26.5 GHz), are highly promising for portable metrology. However, their practical deployment has been critically hindered by poor robustness against intracavity noise and environmental perturbations. Here, we overcome this limitation by proposing a multimodal locking architecture that actively and simultaneously stabilizes all three fundamental parameters in a soliton microcomb: the pump frequency, the cavity resonance, and the repetition rate. This architecture is confirmed both theoretically and experimentally. Implemented on a Si3N4 microresonator with an FSR of 24.96 GHz, this approach enables robust soliton generation and sustains record-long, collapse-free operation for over 48 h. More importantly, it maintains soliton robustness with exceptional resilience to environmental shocks, under temperature variations exceeding 10°C and vibration accelerations beyond ±4 g. The microcomb's metrological utility as a precise optical ruler is further validated through optical frequency calibration and frequency-sweeping-based absolute ranging demonstration. This work provides a critical solution for robust and field-deployable low-repetition-rate soliton microcombs, paving the way for their use in portable optical clocks, high-precision ranging, time-frequency transfer and spectroscopic sensing.