Boosting silica micro-rod Q factor to 8.28×109 for fully stabilizing a soliton microcomb

Optical microcavities with strong light confinement serve as a powerful tool for significantly enhancing both optical field intensity and photon lifetimes. They have become a cornerstone for probing fundamental physics and developing advanced devices, being widely used in versatile applications such as precision sensing, nonlinear excitation, and quantum information. To achieve high-efficiency light-matter interactions within an optical microcavity, the quality factor (Q) is one of the most fundamental and important parameters, reflecting the intracavity loss. To date, the actual Q factor of a microcavity remains far below the theoretical limit, as the specific factors affecting the losses inside microcavities are still not well quantified. In this work, based-on a silica whispering-gallery-mode microrod cavity, we quantitatively analyze the loss sources, identifying how radiation/scattering loss, absorption loss, and contaminant loss contribute to the total loss. We find that radiation/scattering loss and contaminant loss are the major factors limiting the total cavity Q in practice. Therefore, by employing two-step laser polishing and heat treatment, we achieve a Q factor of up to 8.28×10 ⁹ , which is high enough for high-performance optical stabilization. Experimentally, within a compact size, we demonstrate a fully stabilized soliton microcomb system utilizing the microcavity with optimized Q, showing phase noise suppressions over 45.2 dB at pump frequency and over 60.6 dB at repetition frequency. This work deepens the understanding of intracavity loss within microcavities and paves the way for practically improving the performance of microcavity photonics.