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IEEE International Conference on Electronic Measurement & Instruments

Analysis of Polarization Parameters of Co-Magnetometers Based on Elliptically Polarized Light Absorption Model

Lele Ma, Z X Liu, Xinhui Liu, Lihong Duan, Haoying Pang, Jixi Lu

Published Aug 22, 2025• 0 citations
Publisher

Abstract

The spin-exchange relaxation-free (SERF) comagnetometers, owing to their ultra-high sensitivity, are widely applied in fields such as inertial and magnetic field measurements, frontier physics research, etc. This technology relies on pump light to achieve the spin polarization of alkali metal atoms. During this process, the pump light is absorbed and thus attenuated by the alkali metal atoms. In this study, an absorption decay model for elliptically polarized light was solved by the fourth-order Runge-Kutta method, breaking through the applicability limitations of conventional analytical solutions for arbitrary polarized light. The effects of pump light polarization states and power on optical pumping rate, electron polarization, and measurement sensitivity were further investigated. It was found that there is a contradiction in achieving low polarization gradient and high scale coefficient by adjusting light intensity, increasing the pump light intensity can reduce the polarization gradient but will decrease the scale coefficient. Elliptically polarized light can realize low polarization gradient while enhancing the magnetic field response. This research establishes a theoretical foundation for optimizing the selection of pump light parameters in quantum sensors.

DOI
10.1109/icemi66537.2025.11306351

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The spin-exchange relaxation-free (SERF) comagnetometers, owing to their ultra-high sensitivity, are widely applied in fields such as inertial and magnetic field measurements, frontier physics research, etc. This technology relies on pump light to achieve the spin polarization of alkali metal atoms. During this process, the pump light is absorbed and thus attenuated by the alkali metal atoms. In this study, an absorption decay model for elliptically polarized light was solved by the fourth-order Runge-Kutta method, breaking through the applicability limitations of conventional analytical solutions for arbitrary polarized light. The effects of pump light polarization states and power on optical pumping rate, electron polarization, and measurement sensitivity were further investigated. It was found that there is a contradiction in achieving low polarization gradient and high scale coefficient by adjusting light intensity, increasing the pump light intensity can reduce the polarization gradient but will decrease the scale coefficient. Elliptically polarized light can realize low polarization gradient while enhancing the magnetic field response. This research establishes a theoretical foundation for optimizing the selection of pump light parameters in quantum sensors.
Ultrasensitive spin exchange relaxation-free (SERF) comagnetometers have been widely applied in various new physics experiments and show great potential for inertial navigation. However, the spatially nonuniform spin polarization of alkali metal (AM) and noble gas (NG) atoms results in an effective magnetic field gradient on the order of 100 nT/cm which is much larger than that is produced by the coils and remanence on the magnitude of 1 nT/cm, thereby reducing the suppression ability of the magnetic noise and limiting the scale factor and noise floor of the comagnetometers. In this study, we establish a model of the effective magnetic field gradient caused by uneven spin polarization and analyze the influencing factors. Our results indicate that optimizing the density ratio of hybrid AM atoms can suppress the effective magnetic field gradient. Nevertheless, the available pump light power imposes a limitation on the density ratio. To further address this issue, we propose a balanced light-assisted hybrid spin exchange optical pumping (HSEOP) method. Our experimental results confirm the effectiveness of the mentioned approach, with 67% suppression of the nuclear transverse relaxation rate and a nearly threefold increase in the scale factor. Furthermore, the optimal density ratio in experimental applications can be reduced from 200 to below 80, thereby overcoming the limitation of laser power in SERF comagnetometers and providing the possibility for wider applications. Finally, the suppression factor of magnetic noise is enhanced by 89% by optimizing the density ratio and the balanced light-assisted HSEOP method. This research sheds light on the fundamental physics experiments and potential for inertial navigation.
The magnetic field gradient within optical pumping magnetometers (OPMs) suppresses sensitivity improvement. We investigated the effects of the magnetic field gradient along the x-, y-, and z-axes on the limiting factors of magnetometers under extremely low magnetic field conditions. We modified the magnetic field gradient relaxation model such that it can be applied to atoms in the spin exchange relaxation free (SERF) regime. The gradient relaxation time and spin polarizations, combined with fast spin-exchange interaction, were determined simultaneously using the oscillating cosine magnetic field excitation and amplitude spectrum analysis method. During the experiments, we eliminated the errors caused by the temperature and pumping power, and considered different isotope spin exchange collisions in naturally abundant Rb during the data analysis to improve the fitting accuracy. The experimental results agreed well with those of theoretical calculations and confirmed the accuracy of the improved model. The contribution of the transverse magnetic field gradient to the relaxation of the magnetic field gradient cannot be ignored in the case of small static magnetic fields. Our study provides a theoretical and experimental basis for eliminating magnetic gradient relaxation in atomic sensors in the SERF region.
In the research of the interaction between light and atoms, ellipticity is an important characterization of the pump and absorption rates. In previous researches, it was generally believed that ellipticity only originated from the interaction between magnetic fields and polarized alkali metal atomic ensembles. In this article, we derive and analyze the circular dichroism based on density matrix and Maxwell equations during the interaction between magnetic fields, probe light, and alkali metal atomic ensembles in spin-exchange relaxation-free (SERF) regime. We construct an optical path for measuring ellipticity based on photo elastic modulator (PEM) to measure ellipticity. Under different experimental conditions, we compare the measurement results of ellipticity from circular dichroism effect and rotation angle from circular birefringence effect. The results indicates that the interaction between magnetic field and alkali metal atomic ensemble which is polarized by probe light is a new source of ellipticity. Then, we research the ratio of rotation angle to ellipticity, and the results indicates that ellipticity generated by the interaction between the magnetic field and the alkali metal atomic ensemble is an intrinsic characteristic, which is independent of external conditions. The confidence level of the fitting between the experimental results and the theoretical equation is 99.7%. This research can be applied to SERF magnetometers and co-magnetometers.
The ultra-sensitivity magnetometer plays an important role in fundamental physics and applications, including magnetoencephalography (MEG) and magnetocardiography (MCG). The spin-exchange relaxation-free (SERF) atomic magnetometer (AM) has great sensitivity but is limited by the relatively complicated dual-beam structure for application, leading to the development of the compact AM based on one single laser beam. Compared with the single-beam optically pumped magnetometer (OPM) detecting the absorption of the circularly polarized beam, the SERF AM based on an elliptically polarized single beam detecting the optical rotation angle of polarization has a better response, lower laser power, and no need for signal modulation or demodulation. Here, we would like to report an elliptically-polarized single-beam SERF AM enhanced by reflecting the probe beam, increasing the light-atom interaction. Moreover, unlike the previous reflecting AM using the beam splitter, we used a D-shape mirror to extract the reflected beam, avoiding power losses of both incident and reflected beam and destruction of polarization. The sensitivity of our device in the reflecting scheme reaches 5 fT/Hz1/2 at 9–60 Hz, satisfying the need to measure the human MEG/MCG or search for the fifth force. Our research paves the way for developing the compact SERF AM for multiple future applications.
We investigate the effects of pump laser power density on the hybrid optically pumped comagnetometer operated in the spin-exchange relaxation-free (SERF) regime. The analytic steady-state output model for the comagnetometer considering two alkali metal species and one nuclear species is presented for the first time. And the effects of pump laser power density on the rotation sensitivity, suppression of low-frequency magnetic noise and long-term stability of the comagnetometer are studied experimentally. The results indicate that when the product of pumping rate and density ratio of pumped atom to probed atom is equal to the spin relaxation rate of the probed atom, the maximum response and highest sensitivity of the comagnetometer are achieved. However, the suppression of low-frequency magnetic noise and long-term stability improve with the increasing of pump laser power density due to the increasing of nuclear spin polarization. Our focus is to optimize the performance of the comagnetometer for rotation sensing, but the theory and method presented here are relevant to all applications of the hybrid optical pumping technique.
Abstract Atomic spin comagnetometers based on the spin-exchange relaxation-free (SERF) effect offer significant potential for inertial sensing and support the development of long-endurance, high-precision navigation technologies. However, limited nuclear spin polarization constrains the long-term stability of these systems. Here, a flat-top optical pumping configuration is introduced for SERF comagnetometers, expanding the uniform-intensity region of the beam to approximately 62 % , which enhances atomic polarization by around 19 % and improves magnetic noise suppression by 32 % . Experimental results demonstrate notable improvements in both sensitivity and long-term stability, highlighting the effectiveness of flat-top beams in overcoming atomic polarization uniformity limitations and advancing SERF co-magnetometer performance.