Geometric optimization of vapor cell stem for enhanced nuclear spin coherence in SERF co-magnetometers

SERF (spin-exchange relaxation-free) co-magnetometers show great promise for miniaturized inertial sensing. However, the vapor cell-the core sensing component-is constrained by the stem structure inherently introduced during fabrication, which exacerbates gradient-induced relaxation via restricted diffusion. To investigate this, a Monte Carlo random walk model is developed, incorporating magnetic field gradients arising from non-uniform electron spin polarization to evaluate transverse nuclear spin relaxation in vapor cells with fine stem structures. Simulations show that the transverse relaxation time strongly depends on vapor cell size, stem length, and filling port cross-sectional area. Smaller vapor cells yield longer theoretical relaxation limits. Additionally, for a fixed vapor cell size and gradient, trimming the stem length and enlarging the filling port cross-sectional area can extend the transverse relaxation time. These findings are experimentally validated using vapor cells with varying filling port areas and laser-trimmed stem length. Experimental results show that, from 185 °C to 195 °C, vapor cells with a filling port cross-sectional area of 0.095±0.005mm² exhibit a transverse nuclear spin relaxation rate approximately 5–6 times higher than those with an area of 1.762±0.022mm² in a 3 mm cubic vapor cell. Moreover, when the stem length is reduced from approximately 4.05 mm to 1.24 mm, a notable enhancement in the suppression efficiency of transverse nuclear spin relaxation is observed, rising from 35% to 49% as the operational temperature increases within the 185 °C–195 °C range. These observations provide clear design references for optimizing vapor cell structures in SERF-based inertial sensing systems, with the potential to improve long-term operational stability and measurement precision in atomic spin gyroscopes.