The Bell-Bloom-type optically-pumped FID Rubidium atomic magnetometer with a multi-passing probe beam and two counter-propagating pump beams

Abstract

The Bell-Bloom-type optically pumped atomic magnetometers are well suited for weak geomagnetic field detection. However, conventional single-beam pumping introduces an atomic spin polarization gradient, which limits the measurement accuracy and sensitivity. To address this issue, this paper proposes and experimentally demonstrates a Bell-Bloom-type rubidium FID magnetometer scheme integrating orthogonally polarized counter-propagating pumping and multi-pass probe detection. This design homogenizes the atomic spin polarization distribution and suppresses light shifts and power broadening effects induced by the pump beam. Meanwhile, the five-pass probe configuration significantly enhances the signal amplitude. Experimental results reveal that, compared with the traditional single-beam pumping and single-pass detection scheme, the proposed magnetometer achieves a remarkable improvement in magnetic field measurement accuracy, and the magnetic field sensitivity is improved from 18.9 pT/\sqrt{Hz} to 3.1 pT/\sqrt{Hz}. This work provides an effective technical approach and reference for optimizing the performance of atomic magnetometers and extending their applications in integrated arrays.

Figures (9)

• Figure 1: (a) Schematic of the Bell-Bloom FID magnetometer in the laboratory frame. The atomic ensemble is placed in a static magnetic field $B_0$ along the z-axis to be measured. Pump light incident along the x-direction spin-polarizes the atoms; probe light incident along the y-direction detects the precession signal of atomic spins in the magnetic field $B_0$; (b) Schematic of pump-probe timing control. During the pump phase of duration $T_{\text{pump}}$, transverse spin polarization is generated. The pump light is then switched off, and the probe light is turned on to monitor the FID signal for a duration $T_{\text{probe}}$.
• Figure 2: Principle of spin polarization
• Figure 3: Schematic comparison between single‑beam and counter‑propagating orthogonally circularly polarized pumping. (a) In conventional single‑beam pumping, resonant absorption causes exponential intensity attenuation, resulting in a large spin polarization gradient; (b) In the counter‑propagating scheme, orthogonally polarized $\sigma^+$ and $\sigma^-$ beams propagate oppositely and collinearly. Their superposition compensates for absorption, homogenizes the pump intensity, and suppresses the axial spin polarization gradient, improving spatial uniformity.
• Figure 4: Schematic of the experimental setup. ISO: Isolator; AOM: Acousto-optic modulator; PFM: Polarization-maintaining fiber; BE: Beam expander; $\lambda/4$: Quarter-wave plate; $\lambda/2$: Half-wave plate; P: Polarizer; DPD: Differential photodetector; DAQ: Data acquisition card.
• Figure 5: Configuration of the multi-pass cell. (a) Multi-reflection cavity inside the gas cell; (b) Multi-reflection outside the gas cell.