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Optics Express

Bell–Bloom magnetometer based on square optical path

Weipeng Li, Yue Chang, Xiaonan Zhang, Wenhao Luo, Xiaogang Wei, Renfu Yang

Published Jul 18, 2025• 0 citations
Publisher

Abstract

We present a single-beam Bell-Bloom magnetometer using a compact, square optical path in a vapor cell with intensity modulation of the light beam. This design achieves dead-zone-free three-axis magnetic field measurements and, additionally, can determine the magnitude and direction of the magnetic field gradient. Experimental results demonstrate the dead-zone-free operation, with sensitivity in each axis scaling with the effective optical path length along that direction. Strong magnetic field gradients induce distinct resonance peak splitting, whereby the splitting width is proportional to the gradient magnitude. By applying a reference gradient, the system can infer the direction of an external gradient, effectively functioning as a dual-axis gradiometer. Rotating the vapor cell allows optimization of the optical path to minimize gradient-induced measurement distortions by aligning it perpendicular to the gradient direction.

DOI
10.1364/oe.566844

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We present a single-beam Bell-Bloom magnetometer using a compact, square optical path in a vapor cell with intensity modulation of the light beam. This design achieves dead-zone-free three-axis magnetic field measurements and, additionally, can determine the magnitude and direction of the magnetic field gradient. Experimental results demonstrate the dead-zone-free operation, with sensitivity in each axis scaling with the effective optical path length along that direction. Strong magnetic field gradients induce distinct resonance peak splitting, whereby the splitting width is proportional to the gradient magnitude. By applying a reference gradient, the system can infer the direction of an external gradient, effectively functioning as a dual-axis gradiometer. Rotating the vapor cell allows optimization of the optical path to minimize gradient-induced measurement distortions by aligning it perpendicular to the gradient direction.
The detection dead zone is an important systematic limitation in scalar atomic magnetometers, constraining their practical utility. In this work, we demonstrate a sensitive dead zone free scalar magnetometer by integrating previously established techniques into a FID magnetometer based on Bell Bloom optical pumping. The dead zone is eliminated by inserting a reflecting mirror within a multipass cavity assisted atomic cell, which folds the optical beam into orthogonal paths. Our analysis reveals that interregion cross talk is essential for interpreting the resulted experimental signals. The sensor exhibits oscillation signal amplitude variations within a factor of three across all orientations in three dimensional space, and a field sensitivity better than 80 fT/Hz^(1/2) over the full space. We further characterize the heading error in the geomagnetic field range, where experimental results agree with theoretical predictions within 0.7 nT. Additionally, the sensor can operate in a closed loop mode by feeding back the real time frequency extracted from the FID signal to modulate the pump beam, we characterize the magnetic field slew rate of the sensor. This work paves the way towards a sensitive vector FID magnetometer.
We demonstrate an all-optical (4)He atomic magnetometer experimental scheme based on an original Bell-Bloom configuration. A single intensity-modulated linearly polarized laser beam is used both for generating spin polarization within a single (4)He vapor and probing the spin precessing under a static magnetic field. The transmitted light signal from the vapor is then phase-sensitively detected at the modulation frequency and its harmonics, which lead to the atomic magnetic resonance signals. Based on this structure, a liquid crystal is added in our magnetometer system and constitutes a polarization rotator. By controlling the voltage applied on the liquid crystal, the light linear polarization vector can be kept perpendicular with the ambient magnetic field direction, which in turn provides the maximum resonance signal amplitude. Moreover, the system exhibits a magnetic-field noise floor of about 2pT/√Hz, which is not degraded due to the presence of the liquid crystal and varying magnetic field direction. The experiment results prove that our method can eliminate the dead-zone effect, improve the system spatial isotropy, and thus be suitable in mobile applications.
High-precision Mz/Mx atomic magnetometers have “dead zones”, where the magnetic-field information will be lost because the light–atom interaction vanishes at specific field angles. To suppress dead zones, we propose a scheme with polarization-gradient optical pumping field by using counter-propagating lin⊥lin beams. We found through experiments that the lin⊥lin scheme eliminates dead zones in the transverse plane of the probe beam propagation (minimum ∼40% normalized signal) but leaves two noncanonical magic-angle-like directions (≈30 ∘ and 150 ∘ ) in the longitudinal plane, which is distinct from the standard tensor magic angle (≈54 ∘ and 126 ∘ ). These observations are quantitatively explained using a multipole-based model formulated in terms of an effective, spatially averaged density matrix, which is the first time that the spatial average density matrix has been used for energy state analysis in atomic magnetometers. Under the probing of a linearly polarized light, the measured signal is determined by the field-angle-dependent projection of the atomic longitudinal alignment moment ( m 2,0 ) onto the B 0 -defined quantization axis. Compared with other dead-zone elimination schemes, our scheme avoids polarization modulation and multi-cell architectures, showing potential for improving the spatial response range of compact atomic magnetometry.
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.
We report an all-optical atomic vector magnetometer using dual Bell-Bloom optical pumping beams in a Rb vapor cell. This vector magnetometer consists of two orthogonal optical pumping beams, with amplitude modulations at 85Rb^{85}\mathrm{Rb}85Rb and 87Rb^{87}\mathrm{Rb}87Rb Larmor frequencies, respectively. We simultaneously detect atomic signals excited by these two pumping beams using a single probe beam in the third direction, and extract the field orientation information using the phase delays between the modulated atomic signals and the driving beams. By adding a Herriott cavity inside the vapor cell, we improve the magnetometer sensitivity. We study the performance of this vector magnetometer in a magnetic field ranging from 100 mG to 500 mG, and demonstrate a field angle sensitivity better than 10 $\ensuremath{\mu}\mathrm{rad}/{\mathrm{Hz}}^{1/2}$ above 10 Hz.
A multi-beam atomic sensor with three orthogonal helium vapor cells to guarantee the availability of4He magnetometer under different directions of magnetic field is proposed and tested, which demonstrates experimentally that dead zones of magnetic field measurement can be eliminated. Furthermore, with the special optical design, the multi-beam atomic sensor is also used for laser frequency stabilization simultaneously, which shows the possibility of reducing heading errors caused by the light shift effect.