Analysis of helium permeation and gas-venting dynamics in midsection-exposed hollow-core fibers

The coupled system of cold atoms and photons guided by a hollow-core fiber (HCF) in ultra-high vacuum (UHV) is widely utilized in quantum optics. Exposing the midsection of the HCF to the ambient atmosphere enhances optical accessibility, enables compact integration, and facilitates atomic near-field sensing. This study proposes a midsection-exposed dual-end venting (ME-DEV) model to predict the spatiotemporal pressure evolution within such configurations. The results indicate that an ultra-low permeability Al₂O₃ coating effectively suppresses helium permeation. Specifically, a 20-nm coating reduces the steady-state midpoint pressure of a typical HCF from 1.44×10⁻⁵ Pa to 4.3×10⁻⁸ Pa. This reduction corresponds to an estimated ⁸⁷Rb lifetime of 40.7s, representing an enhancement by a factor of approximately 333 over that in the uncoated HCF. This work provides a theoretical foundation for compact HCF-based quantum sensors.