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Applied Optics and Photonics China (AOPC): Optoelectronics and Nanophotonics

Effect of cell temperature on the bias magnetic sensitivity of an atomic spin co-magnetometer

Wenfeng Fan, Lihong Duan, Jiong Huang, Weijia Zhang, Wei Quan

Published Nov 5, 2020• 3 citations
Publisher

Abstract

The atomic spin co-magnetometer operating in the spin-exchange relaxation-free regime is a promising rotation sensor for its ultrahigh sensitivity to inertial forces. However, it was found that the bias magnetic sensitivity (BMS) of the comagnetometer was much larger than theoretically expected, making magnetic noise a dominant error. This paper reports on the experimental results of the effect of cell temperature on the BMS. It is found that the BMS increases rapidly as the cell temperature rises. This is attributed to the polarization gradient of atomic spins induced by the absorption of the pump beam by the atoms. The experiments and theory reveal that a high-power laser source for pumping is essential for the suppression of BMS and the improvement of bandwidth in the co-magnetometer.

DOI
10.1117/12.2575681

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The atomic spin co-magnetometer operating in the spin-exchange relaxation-free regime is a promising rotation sensor for its ultrahigh sensitivity to inertial forces. However, it was found that the bias magnetic sensitivity (BMS) of the comagnetometer was much larger than theoretically expected, making magnetic noise a dominant error. This paper reports on the experimental results of the effect of cell temperature on the BMS. It is found that the BMS increases rapidly as the cell temperature rises. This is attributed to the polarization gradient of atomic spins induced by the absorption of the pump beam by the atoms. The experiments and theory reveal that a high-power laser source for pumping is essential for the suppression of BMS and the improvement of bandwidth in the co-magnetometer.
Abstract The chip-scale hybrid optical pumping spin-exchange relaxation-free (SERF) atomic magnetometer with a single-beam arrangement has prominent applications in biomagnetic measurements because of its outstanding features, including ultrahigh sensitivity, an enhanced signal-to-noise ratio, homogeneous spin polarization and a much simpler optical configuration than other devices. In this work, a miniaturized single-beam hybrid optical pumping SERF atomic magnetometer based on a microfabricated atomic vapor cell is demonstrated. Although the optically thin Cs atoms are spin-polarized, the dense Rb atoms determine the experimental results. The enhanced signal strength and narrowed resonance linewidth are experimentally proven, which shows the superiority of the proposed magnetometer scheme. By using a differential detection scheme, we effectively suppress optical noise with an approximate five-fold improvement. Moreover, the cell temperature markedly affects the performance of the magnetometer. We systematically investigate the effects of temperature on the magnetometer parameters. The theoretical basis for these effects is explained in detail. The developed miniaturized magnetometer has an optimal magnetic sensitivity of 20 fT/Hz 1/2 . The presented work provides a foundation for the chip-scale integration of ultrahighly sensitive quantum magnetometers that can be used for forward-looking magnetocardiography (MCG) and magnetoencephalography (MEG) applications.
Heating the alkali metal vapor cell to obtain high atomic density is a prerequisite for realizing the high-precision measurement for spin-exchange relaxation-free (SERF) co-magnetometer. However, the thermo-magnetic and thermo-optical coupling noise caused by the temperature diffusion from the alkali vapor cell is one of the main factors restricting the co-magnetometer performance at present. Here the influence of heating on the SERF co-magnetometer was comprehensively studied, and the thermal error model of the system was established. A novel heat insulation structure for the co-magnetometer was designed, and the effectiveness was verified through monitoring of characteristic temperature points. Compared to the conventional oven support, the average maximum temperature difference within 12 h of the optical path and magnetic shielding systems (OPMSS) decreased by 52%, the bias instability of the co-magnetometer decreased by 24%, and the noise of the co-magnetometer at 1 Hz decreased by 33%. The experimental results demonstrate that this method reduces the temperature fluctuation of the OPMSS, effectively suppresses the thermal coupling noise, and thus improves the performance of the SERF co-magnetometer. Moreover, this method is also applicable to other quantum sensors based on alkali metal vapor cell.
A spin-exchange relaxation-free comagnetometer is a promising high-precision gyroscope as its inertial measurement sensitivity can rival that of existing technologies. The magnetic field error is one of the main factors that limits the long-term stability of the comagnetometer. The steady-state response and frequency response models of a K-Rb-21Ne comagnetometer are modified by considering the spin-exchange rate between the electron spins and the nuclear spins. The modified models are used to redefine the bias magnetic sensitivity of the K-Rb-21Ne comagnetometer and the suppression method of low-frequency magnetic field error is proposed. We demonstrate the magnetic drift suppression method proposed in the article by optimizing the cell temperature and pump laser power density. This study is helpful to improve the rotation measurement performance of the K-Rb-21Ne comagnetometer.
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.
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.
The cell temperature working point optimization of the spin-exchange relaxation-free (SERF) co-magnetometer is studied theoretically and experimentally in this article. Based on the steady-state solution of the Bloch equations, the steady-state response model of the K-Rb- 21 Ne SERF co-magnetometer output signal with cell temperature is established in this paper. And combined with the model, a method to find the optimal working point of the cell temperature that incorporates the pump laser intensity is proposed. The scale factor of the co-magnetometer under different pump laser intensities and cell temperatures is obtained experimentally, and the long-term stability of the co-magnetometer at the different cell temperatures with corresponding pump laser intensities is measured. The results show that the bias instability of the co-magnetometer is reduced from 0.0311 deg/h to 0.0169 deg/h by obtaining the optimal working point of the cell temperature, which verifies the validity and accuracy of the theoretical derivation and the proposed method.