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Optics Letters

Femtotesla 4 He magnetometer with a multipass cell

Yang Liu, Xiang Peng, Haidong Wang, Bowen Wang, Kaiwen Yi, Dong Sheng, Hong Mei Guo

Published Sep 19, 2022• 21 citations
Publisher

Abstract

In this Letter, we propose a single-beam nonlinear magneto-optical rotation (NMOR) magnetometer with a multipass 4 He gas-discharged cell. In contrast to the single-pass cell, the multipass cell allowed laser beams to pass through the metastable-state atomic ensemble 22 times, which directly increases the optical path length and significantly enhances magneto-optical rotation in the 4 He gas sample. Based on nonlinear Faraday rotation, the 4 He magnetometer with the multipass cell demonstrates a noise floor of 9 fT/Hz 1/2 , which approaches the photon-shot noise floor limit of 6.4 fT/Hz 1/2 . In addition, the wider linewidth in metastable-state atoms realizes an NMOR 4 He magnetometer with a 3 dB bandwidth of 4.3 kHz, in contrast to the ultranarrow linewidth in the antirelaxation-coated cells or spin-exchange relaxation-free regime alkali-metal cells with buffer gas. Since the 4 He cell functions without heating or cryogenic cooling, the femtotesla sensitivity and kilohertz-bandwidth 4 He magnetometer exhibits potential in biomagnetic applications such as magnetocardiography and magnetoencephalography.

DOI
10.1364/ol.471557

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In this Letter, we propose a single-beam nonlinear magneto-optical rotation (NMOR) magnetometer with a multipass 4 He gas-discharged cell. In contrast to the single-pass cell, the multipass cell allowed laser beams to pass through the metastable-state atomic ensemble 22 times, which directly increases the optical path length and significantly enhances magneto-optical rotation in the 4 He gas sample. Based on nonlinear Faraday rotation, the 4 He magnetometer with the multipass cell demonstrates a noise floor of 9 fT/Hz 1/2 , which approaches the photon-shot noise floor limit of 6.4 fT/Hz 1/2 . In addition, the wider linewidth in metastable-state atoms realizes an NMOR 4 He magnetometer with a 3 dB bandwidth of 4.3 kHz, in contrast to the ultranarrow linewidth in the antirelaxation-coated cells or spin-exchange relaxation-free regime alkali-metal cells with buffer gas. Since the 4 He cell functions without heating or cryogenic cooling, the femtotesla sensitivity and kilohertz-bandwidth 4 He magnetometer exhibits potential in biomagnetic applications such as magnetocardiography and magnetoencephalography.
Miniaturized high-sensitivity atomic magnetometers play a crucial role in biomagnetic measurements. The utilization of multipass cells enhances the sensitivity of atomic magnetometers. This study explores the benefits of employing a multipass cell in spin-exchange relaxation-free (SERF) atomic magnetometers through theoretical and experimental analysis. A signal-to-noise ratio (SNR) model was established by accounting for the optimal detuning frequency of the probe light and low-frequency 1/f noise. This enabled the optimal number of probe light passes through the vapor cell to be determined. Additionally, compact and symmetric reflective optical paths were designed to mitigate polarization gradients in atomic magnetometers with high atomic number density. Experimental results confirmed that the magnetic field sensitivity of the triple-pass configuration (1.8 fT/Hz1/2^{1/2}1/2) was 1.89 times that of the single-pass configuration (3.4 fT/Hz1/2^{1/2}1/2), consistent with the theoretical analysis. This study provides valuable theoretical guidance for analyzing the SNR performance of SERF atomic magnetometers and demonstrates the potential of multipass cells in enhancing their magnetic field sensitivity. The utilization of multipass cells presents a viable approach toward the realization of highly sensitive and portable miniaturized SERF atomic magnetometers.
Optically pumped magnetometers (OPMs) based on the free-induction-decay (FID) configuration have recently attracted interest owing to their advantages, such as being calibration-free, easy to operate, and showing reduced light shifts induced by the pump light. Currently, alkali-metal vapor cells have been employed in most of the FID magnetometers. However, the relatively narrow linewidth of the magnetic resonance signal and nonlinear Zeeman (NLZ) effects generate bandwidth limitation and heading errors of these magnetometers in the Earth's field, respectively. In this work, we propose a 4He{}^{4}\mathrm{He}4He-based FID magnetometer, which not only has the advantages of being NLZ-free and showing a high bandwidth, but also maintains an enhanced sensitivity assisted by a multipass cell. This magnetometer demonstrates a magnetic field noise floor of 0.34pT/Hz1/20.34\phantom{\rule{0.2em}{0ex}}\mathrm{pT}/{\mathrm{Hz}}^{1/2}0.34pT/Hz1/2 with a Nyquist-limited bandwidth of 555 kHz, which opens another route for atomic magnetometry using FID signals, and exhibits potential for applications in high-frequency magnetic field detection, such as biomagnetic measurements and magnetic communications in the geomagnetic field.
Atom sensing based on Faraday rotation is an indispensable method for precision measurements, universally suitable for both hot and cold atomic systems. Here we demonstrate an all-optical magnetometer where the optical cell for Faraday rotation spectroscopy is augmented with a low finesse cavity. Unlike in previous experiments, where specifically designed multipass cells had been employed, our scheme allows to use conventional, spherical vapour cells. Spherical shaped cells have the advantage that they can be effectively coated inside with a spin relaxation suppressing layer providing long spin coherence times without addition of a buffer gas. Cavity enhancement shows in an increase in optical polarization rotation and sensitivity compared to single-pass configurations.
Abstract We demonstrate a dual-beam high-sensitivity room-temperature atomic magnetometer (AM) with a large dynamic range based on a nonlinear magneto-optical rotation (NMOR) enhancing scheme. Using an inelastic wave-mixing laser, we find significant NMOR signal enhancement compared with a single-beam scheme under the same far-detuned operation conditions. This dual-beam scheme is shown to operate well in the regime of lower laser intensity and off-resonance detuning. The best sensitivity achieved is 20 fT/Hz1/2 with a bandwidth of 2 kHz, substantially greater than common spin-exchange relaxation-free AMs. This provides a way to develop low-power, inexpensive miniaturized AMs for wide application.
Atomic spin polarization of alkali atoms in the ground-state can survive thousands of collisions with paraffin-coated cell walls. The resulting long spin-relaxation times achieved in evacuated, paraffin-coated cells enable precise measurement of atomic spin precession and energy shifts of ground-state Zeeman sublevels. In the present work, nonlinear magneto-optical rotation with frequency-modulated light (FM NMOR) is used to measure magnetic field-induced spin precession for rubidium atoms contained in a paraffin-coated cell. The magnetometric sensitivity of FM NMOR for the rubidium D2 line is studied as a function of light power, detuning, frequency-modulation amplitude, and rubidium vapor density. For a 5-cm-diameter cell at temperature T≈35 °C, the optimal shot-noise-projected magnetometric sensitivity is found to be 2×10−11 G/Hz (corresponding to a sensitivity to spin precession frequency of ≈10 μHz/Hz or a sensitivity to Zeeman sublevel shifts of ≈4×10−20 eV/Hz).
Abstract We propose a signal- and sensitivity-enhanced spin-exchange-relaxation-free atomic magnetometer utilizing a multi-pass cell with a highly spatially homogeneous optical pumping scheme. The relationship between the magnetometer’s performance and the number of probe beam passes is measured. The optical rotation angle exhibits a linear correlation with the number of passes, achieving 89 mrad nT−1 for a quadruple-pass cell. However, the sensitivity does not increase linearly when further increasing the optical length due to the noise limit. This work provides a method to study the enhancement performance of the magnetic response and sensitivity in atomic magnetometers by using multi-pass cells.