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Journal of Physics Conference Series

Effect of optical polarization error on the magnetic resonance signal of NMOR atomic magnetometer

Mengnan Tian, Yanchao Chai, Zhenglong Lu, Wei Quan, Liwei Jiang

Published Feb 1, 2025• 0 citations
Publisher

Abstract

Abstract In NMOR magnetometers, we choose the linear polarized light to pump atoms, and the polarization state is the key factor affecting the evolution of alkali metal atoms. However, polarization errors are introduced due to the refraction of the glass surfaces of the cell and the imperfection of optical devices, which causes the actual laser light to be elliptically polarized. In this paper, the optical ellipticity for this system is accurately measured by the π transition principle first. Then, the magnetic resonance signals under different ellipticity are investigated theoretically and experimentally, and the atomic polarization state is visualized. The results show that the optical ellipticity error introduced by the system will reduce the measurement accuracy and sensitivity of the system, and it is meaningful to use high extinction ratio optical devices and introduce the quarter-wave plate to offset the circularly polarized components.

DOI
10.1088/1742-6596/2964/1/012077

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Abstract In NMOR magnetometers, we choose the linear polarized light to pump atoms, and the polarization state is the key factor affecting the evolution of alkali metal atoms. However, polarization errors are introduced due to the refraction of the glass surfaces of the cell and the imperfection of optical devices, which causes the actual laser light to be elliptically polarized. In this paper, the optical ellipticity for this system is accurately measured by the π transition principle first. Then, the magnetic resonance signals under different ellipticity are investigated theoretically and experimentally, and the atomic polarization state is visualized. The results show that the optical ellipticity error introduced by the system will reduce the measurement accuracy and sensitivity of the system, and it is meaningful to use high extinction ratio optical devices and introduce the quarter-wave plate to offset the circularly polarized components.
Optically pumped alkali metal atoms currently provide a sensitive solution for magnetic microscopic measurements. As the most practicable plan, Faraday rotation of linearly polarized light is extensively used in spin polarization measurements of alkali metal atoms. In some cases, near-resonant Faraday rotation is applied to improve the sensitivity. However, the near-resonant linearly polarized probe light is elliptically polarized after passing through optically pumped alkali metal vapor. The ellipticity of transmitted near-resonant probe light is numerically calculated and experimentally measured. In addition, we also analyze the negative impact of elliptical polarization on Faraday rotation measurements. From our theoretical estimate and experimental results, the elliptical polarization forms an inevitable error in spin polarization measurements.
The nonlinear magneto-optical rotation (NMOR) atomic magnetometer is a highly sensitive total-field device with significant applications in fundamental physics, geophysics, and biomedical monitoring. The atomic polarization, which is optically induced through resonant light pumping, plays a pivotal role in determining the sensitivity and stability of the magnetometer. In this paper, refined analysis and representation of atomic polarization in NMOR atomic magnetometers are presented through the establishment of a three-dimensional polarization distribution model based on the rate equation. The polarization characteristics of the single-beam pump and repump modes are compared, and the scale factors of both modes are experimentally measured. Both simulation and experimental results demonstrate that the proposed model accurately captures the spatial distribution of optically induced polarization, which is essential for improving the performance and extending the application potential of NMOR magnetometers in high-sensitivity optical sensing systems.
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.
Abstract Polarization moments play a crucial role in measuring magnetic fields for nonlinear magneto‐optical rotation (NMOR) atomic magnetometers. However, it is challenging to distinguish between each polarization moment and evaluate its effect on the magnetic resonance response signal in an alkali vapor cell with buffer gas. To address this issue, a method is proposed to identify different polarization moments through the frequency shift of the magnetic resonance response signal. The proportion of each polarization moment is determined, and it is demonstrated that the magnetic resonance response signal is affected by the hexadecapole moment, resulting in a frequency shift and a decrease in signal amplitude. To mitigate this effect, an approach is investigated to manipulate the polarization moments by flipping the phase of the pump light. Ultimately, a 15.19% increase in response amplitude is achieved in the simulated geomagnetic environment within the magnetic shield barrel. The theory and method presented here provide strong support for the study of the polarization moments in an alkali vapor cell with buffer gas, which potentially enhance the performance of NMOR atomic magnetometers.
An all-optical atomic spin magnetometer using a hybrid cell of K and Rb atoms is demonstrated in this paper. Via deriving and solving the Bloch equations, we established the models for the polarization rate of Rb and the magnetometer’s output signal, respectively. In order to achieve the maximum sensitivity, we carried out research on the influence of the pump power on the polarization rate and magnetometer’s output signal. The magnetometer’s sensitivity was found to be proportional to the polarization rate of Rb, which was confined by the pump power. In a certain context, theoretical predictions indicated that the larger the pump power was, the higher magnetometer’s sensitivity that could be realized. However, the theoretical estimate deviated from the experimental results. An analysis of the errors between the actual value and the theoretical value showed that the different molar ratio between the two species of alkali-metal atoms was the primary reason causing the deviation. By comparing the experimental measurement result with simulation result, we found the difference in the molar ratio to be approximate 4 times. After various parameters had been optimized, a sensitivity of 18 fT/Hz 1/2 was achieved by analyzing the signal-to-noise ratio.
Spin-exchange relaxation free alkali-metal magnetometers typically operate in the regime of high optical density, presenting challenges for simple and efficient optical pumping and detection. We describe a high-sensitivity Rb magnetometer using a single elliptically polarized off-resonant laser beam. Circular component of the light creates relatively uniform spin polarization while the linear component is used to measure optical rotation generated by the atoms. Modulation of the atomic spin direction with an oscillating magnetic field shifts the detected signal to high frequencies. Using a fiber-coupled distributed feedback laser, we achieve magnetic field sensitivity of 7 fT/Hz7\text{ }\text{fT}/\sqrt{\text{Hz}}7 fT/Hz​ with a miniature $5\ifmmode\times\else\texttimes\fi{}5\ifmmode\times\else\texttimes\fi{}5\text{ }\text{mm}\text{ }\text{Rb}$ vapor cell.