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IEEE Transactions on Instrumentation and Measurement

Effect of Optical Nonorthogonality on Dynamic Response in Dual-Beam Atomic Magnetometer

Bozheng Xing, Siran Li, Xiujie Fang, Hongtao Lu, Danyue Ma

Published Nov 25, 2024• 7 citations
Publisher

Abstract

In this article, we analyze the amplitude-frequency and phase-frequency responses of a spin-exchange relaxation-free (SERF) atomic magnetometer with the nonorthogonal angle between the pump and probe beam. The optical nonorthogonality leads to a change in the photon spin direction of circularly polarized pump light, which decides the polarization direction of the atomic ensemble, causing significant differences in dynamic responses to the magnetic field. We theoretically analyze relationships between dynamic response solutions of the Bloch equation with different pump lights and magnetic field directions based on which the equations are linearized. The equations are solved with the Fourier transform method, and three-axis dynamic responses are obtained under the influences of optical nonorthogonality and residual static magnetic field. In the experiment, we verify the linear superposition characteristics between different response components. According to the results, we propose a novel method to quickly calibrate the nonorthogonal angle, which could reduce the optical nonorthogonal angle by 20 times, reaching 0.54°, and effectively enhance the dynamic magnetic field response signal in atomic magnetometers.

DOI
10.1109/tim.2024.3502819

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In this article, we analyze the amplitude-frequency and phase-frequency responses of a spin-exchange relaxation-free (SERF) atomic magnetometer with the nonorthogonal angle between the pump and probe beam. The optical nonorthogonality leads to a change in the photon spin direction of circularly polarized pump light, which decides the polarization direction of the atomic ensemble, causing significant differences in dynamic responses to the magnetic field. We theoretically analyze relationships between dynamic response solutions of the Bloch equation with different pump lights and magnetic field directions based on which the equations are linearized. The equations are solved with the Fourier transform method, and three-axis dynamic responses are obtained under the influences of optical nonorthogonality and residual static magnetic field. In the experiment, we verify the linear superposition characteristics between different response components. According to the results, we propose a novel method to quickly calibrate the nonorthogonal angle, which could reduce the optical nonorthogonal angle by 20 times, reaching 0.54°, and effectively enhance the dynamic magnetic field response signal in atomic magnetometers.
A novel method to determine the spin polarization and its distribution for a spin-exchange relaxation-free (SERF) atomic magnetometer are proposed in this paper. The Larmor precession frequency is measured using the fast Fourier transform (FFT) analysis result of the transient response of the magnetometer under the excitation of a series of the step-changed magnetic field. The spin polarization is extracted from the slow-down factor of the precession frequency. The variation of the spin polarization with different pump light intensities and the distribution of the spin polarization along the direction of the pump light are analyzed, and the experimental results are well consistent with the theoretical predictions. The expanded uncertainties of the experimental results vary from 2% to 11% with the increase of the spin polarization and the reason for this variation is analyzed. The advantage of this method is that it allows the accurate measurement of the spin polarization in the pump beam direction within the SERF regime operating state, and it is unaffected by the large optical depth of the vapor cell.
The light shift in a spin-exchange relaxation-free (SERF) atomic magnetometer (AM) generated by detuning of circularly polarized pump light, results in deviations of orthogonality measurement and attenuation of magnetometer response. In this paper, we propose and systematically investigate a dual-axis method to suppress light shift based on a self-developed single-beam compact SERF AM capable of measuring magnetic field components along two orthogonal sensitive axes. By sequentially observing the non-sensitive axis response and applying a magnetic field along the pump light direction, the virtual magnetic field induced by light shift is compensated and is rigorously demonstrated as a dominant component in this direction. Experimental results show a significant reduction in crosstalk between the dual sensitive axes, improving accuracy in resonant frequency determination of87Rb atoms and orthogonality evaluation of triaxial coils. Furthermore, the performance of dual-axis measurement in the SERF AM is tested, showing a more notable enhancement than that of single-axis measurement.
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Abstract The frequency-response and dynamics of a dual-axis spin-exchange-relaxation-free (SERF) atomic magnetometer are investigated by means of transfer function analysis. The frequency-response at different bias magnetic fields is tested to demonstrate the effect of the residual magnetic field. The resonance frequency of alkali atoms and magnetic linewidth can be obtained simultaneously through our theoretical model. The coefficient of determination of the fitting results is superior to 0.995 with 95% confidence bounds. Additionally, step responses are applied to analyze the dynamics of the control system and the effect of imperfections. Finally, a noise-limited magnetic field resolution of 15 f T H z − 1 has been achieved for our dual-axis SERF atomic magnetometer through magnetic field optimization.
Maintaining a near-zero magnetic field condition is a requisite for atomic magnetometer under the spin-exchange relaxation-free (SERF) regime. Therefore, the precise determination of the residual magnetic fields is of great significance. Herein, we proposed an in-situ measurement method of residual magnetic fields in a dual-beam atomic magnetometer based on a transient response of the transverse polarization along the probe beam direction after switching off the pump beam. Proceeding from the Bloch equation, we established a complete evolution expression of the polarization vector under any static residual magnetic field with the pump beam switched off. On this basis, we analyzed the transient response of the transverse polarization by numerical simulation under different situations of residual magnetic fields. Besides, we also considered two possible conditions when the residual magnetic fields are relatively large or small, respectively. Accordingly, we presented an efficient and convenient estimation method of the residual magnetic fields based on certain time characteristics of the transient response. We verified the feasibility of our method by experiment and achieved a triaxial residual magnetic fields measurement within 0.5 s. This method can realize an in situ and real-time measurement of the residual magnetic fields in a fast and convenient way. Furthermore, this method can evaluate the effectiveness of different magnetic field compensation methods without introducing any modulation or extra devices.
Abstract Spin-exchange relaxation-free (SERF) co-magnetometers have promising applications in both inertial navigation and fundamental physics experiments. However, the fluctuation in the spin polarization caused by the probe beam has a non-negligible influence on the co-magnetometer signal. In this paper, a theoretical model containing three parameters of the probe beam is established by extending the coupled Bloch equation. Based on this model, the influence of probe power density on the transient and steady-state response of the SERF co-magnetometer is analyzed. According to the transient response model, a new measurement method for transverse optical pumping of the probe beam is proposed. Then, for the steady-state response model, a steady-state error suppression method is suggested by adjusting the degree of circular polarization of the probe beam. Eventually, the suppression method is used to refine the SERF co-magnetometer, achieving a suppression rate of 70.31% in transverse electron spin polarization fluctuations, thus improving the co-magnetometer to a stability of 0.0079∘ h−1. To our knowledge, this is better than what has been reported so far.