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

Optical microcavities with strong light confinement are powerful tools for significantly enhancing light-matter interactions. Currently, the actual Q factor of a microcavity remains far below the theoretical limit, as specific factors affecting losses inside microcavities are not yet fully quantified. Here, using silica whispering-gallery-mode microrod cavities, we quantitatively identify how radiation loss, scattering loss, and contaminant loss contribute to the total loss, finding that after fine fabrication, contaminant loss is the major factor limiting the total cavity Q. By employing two-step laser polishing and heat treatment, we achieve a Q factor of up to 8.28 × 109. Accordingly, we demonstrate a fully stabilized soliton microcomb system using the Q-enhanced microcavity, achieving phase noise suppression of over 45.2 dB at the pump frequency and over 60.6 dB at the repetition frequency. This work deepens the understanding of intracavity loss and may pave the way for practically improving the performance of microcavity photonics.