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Optics Express

Analysis and suppression of the misalignment error for the pumping laser in the atomic comagnetometer

Jiong Huang, Wenfeng Fan, Zhuo Wang, Linlin Yuan, Kai Zhang, Hongyu Pei, Haoying Pang, Wei Quan

Published Feb 7, 2022• 30 citations
Publisher

Abstract

The misalignment error of the pumping laser in the atomic comagnetometer (ACM) dramatically diminishes the efficiency of the optical pumping process (characterized by the polarization of the hybrid atomic spin ensembles containing electron spins and nuclear spins) and deteriorates the performance of the ACM (characterized by the Allan standard deviation). In this work, a steady-state response model considering the misalignment error of the pumping laser is established and an in-situ evaluation method for this error is proposed. Based on the evaluation method, the influence of this misalignment error on the pumping efficiency and the performance of the ACM is quantitatively analyzed. Furthermore, a pumping laser alignment method based on the second harmonic of a single-beam magnetometer is then proposed, whose effectiveness is verified by experiments. The experimental results show that compared to the original ACM with the severely misaligned pumping laser, the polarization of the hybrid atomic spin ensembles of the ACM with the pumping laser aligned by the proposed method is increased by about 19%, and the corresponding Allan variance at 100s is reduced by about 40%.

DOI
10.1364/oe.451042

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IEEE Sensors Journal•Oct 26, 2021•14 citations
The misalignment error of the pumping laser in the atomic comagnetometer (ACM) dramatically diminishes the efficiency of the optical pumping process (characterized by the polarization of the hybrid atomic spin ensembles containing electron spins and nuclear spins) and deteriorates the performance of the ACM (characterized by the Allan standard deviation). In this work, a steady-state response model considering the misalignment error of the pumping laser is established and an in-situ evaluation method for this error is proposed. Based on the evaluation method, the influence of this misalignment error on the pumping efficiency and the performance of the ACM is quantitatively analyzed. Furthermore, a pumping laser alignment method based on the second harmonic of a single-beam magnetometer is then proposed, whose effectiveness is verified by experiments. The experimental results show that compared to the original ACM with the severely misaligned pumping laser, the polarization of the hybrid atomic spin ensembles of the ACM with the pumping laser aligned by the proposed method is increased by about 19%, and the corresponding Allan variance at 100s is reduced by about 40%.
Ultrasensitive atomic comagnetometers (ACMs) are invaluable tools for both inertial measurement and fundamental physics research. However, the misalignment between the pump light and the main magnetic field affects the polarization characteristics of nuclear spins of the noble gas atoms, making it a significant source of noise. In this article, an atomic magnetometer response model with the optical-magnetic misalignment error is established, and the mechanism of the influence of optical-magnetic misalignment on nuclear spin polarization is analyzed. An in situ evaluation method and a suppression method of optical-magnetic misalignment error are proposed, and their effectiveness is verified through experiments with varying pump light incidence directions. The experimental results indicate that after optical-magnetic alignment using the proposed method, the spin polarization of the coupled atomic ensemble is enhanced by 20.7%, and the longitudinal relaxation rate of the nuclear spin is reduced by 6.9%. Additionally, the amplitude-frequency response (AFR) of the x-axis magnetic field is reduced by 33.2% at 1 Hz and by 53.1% at frequencies below 0.002 Hz. This indicates an improvement in the transverse magnetic field error suppression capability of the ACMs.
This paper investigates the laser polarization error in the optical rotation detection system (ORDS) of an atomic comagnetometer (ACM), which will seriously degrade the long-term performance of the ORDS. We first establish an optical transmission model of the ORDS by using Jones matrix concerning the optical imperfection of polarizers. Then, we analyze the polarization error based on this model and propose a novel error suppression method. Finally, we experimentally test the long-term performance of the ORDS and the ACM before and after the polarization error suppression to verify the effectiveness of the proposed method. The experimental results show that the long-term performance of the ORDS and the ACM can be improved by approximately 3.4 times with the proposed polarization error suppression method.
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.
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.
In atomic spin sensors based on optical pumping, pump beam instabilities can drive atomic spin polarization fluctuations, referred to here as pump-induced spin polarization noise, which may limit sensitivity and long-term stability. Because the local pump field inside the cell can differ from that inferred from external optical diagnostics, it remains difficult to directly quantify the actual in situ spin polarization noise and to evaluate and suppress its contribution to the sensor noise floor. Here, we propose an in situ decoupling method based on spin dynamics, in which a large longitudinal detuning field suppresses magnetic field and rotation responses, while a transverse projection field converts pump-induced fluctuations in longitudinal spin polarization into a measurable transverse spin signal. Demonstrated in a K-Rb-21Ne spin exchange relaxation free (SERF) comagnetometer, the proposed method suppresses the magnetic-field response by approximately 180-fold and enhances the pump-power-noise response by approximately 380-fold, thereby enabling effective separation and evaluation of pump-induced spin polarization noise. The method employs the atomic ensemble as an in situ probe of the spin dynamic response to the local pump optical field. It enables quantitative identification of pump fluctuation associated noise and direct evaluation of optical field stability under actual operating conditions, thereby providing an experimental basis for optical field stabilization and noise suppression assessment in SERF comagnetometers and related alkali atomic spin sensors.
The light shift generated by the circularly polarized light leads to an error in the spin-exchange relaxation-free (SERF) comagnetometer, since it turns the fluctuations of the pumping light frequency and intensity into a drift in the SERF comagnetometer. Moreover, the light shift is a source of cross-talk effect between the two sensitive axes. Firstly, the relationship between the vapor cell temperature and the total light shift felt by Rb atoms in the K-Rb hybrid pumping SERF comagnetometer is formulated and simulated. It is found that the total light shift is the result of the collision mixing of K and Rb ac Stark shifts and the increase of cell temperature makes the light shift of K felt by Rb dominate the total light shift. Furthermore, different from the method of adjusting the frequency of laser, a new method that suppresses the total light shift in hybrid pumping by adjusting the cell temperature is proposed and experimentally verified. The total light shift at 165.7°C is -0.00152 nT experimentally, which is 3 orders of magnitude smaller than that at 185.0°C, indicating that it can be suppressed effectively by adjusting the cell temperature. The method does not require any extra hardware or sacrifice the pumping efficiency and can be applied to suppress the error induced by the pumping light and the cross-talk effect between the axes. In addition, the relationship between total relaxation rate and cell temperature is briefly discussed.