FID Magnetometer Based on Paraffin-Coated Planar Reflective Multipass Cells

We demonstrate a paraffin-coated planar reflective multipass vapor cell for compact optical atomic magnetometry. The cell has an internal volume of $12\times 12\times 8{\mathrm{m}\mathrm{m}}^3$ and supports 20 optical passes with a total transmittance exceeding $65\mathrm{\%}$, while maintaining a longitudinal spin-relaxation time of ${}^{87}\mathrm{R}\mathrm{b}$ longer than $1\mathrm{s}$. The planar geometry provides spatially separated input and output beams, enabling compact optical integration. A single-cell free-induction-decay (FID) magnetometer reaches $10\mathrm{p}\mathrm{T}\mathrm{/}\sqrt{\mathrm{H}\mathrm{z}}$ in the geomagnetic-field range, presently limited by current-source noise in the field coils. A two-cell differential configuration achieves a sensitivity of $\sim 28\mathrm{f}\mathrm{T}\mathrm{/}\sqrt{\mathrm{H}\mathrm{z}}$ over the $1$–$15\mathrm{H}\mathrm{z}$ band for bias fields of $0.3$–$0.7\mathrm{G}$. These results establish that paraffin-coated planar multipass cells offer high optical depth, long coherence times, and an integration-friendly platform for ultrasensitive magnetometry.