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Journal of Physics D Applied Physics

The influence of temperature and modulated magnetic field on the transmission intensity of atomic magnetometer

Yan Yin, Binquan Zhou, Kaifeng Yin, Yaxiang Wang, Junjian Tang, Mao Ye, Xiaolin Ning, Bangcheng Han

Published Aug 2, 2021• 18 citations
Publisher

Abstract

Abstract The single-beam miniaturized atomic magnetometer with modulated magnetic field is one of the most capable approaches to biomagnetic measurements. The transmission intensity of the pumping beam is critical to the sensitivity of the magnetometer, which is affected by the density of alkali metal and the polarization of atomic spin. In this study, we investigated into the influence of three variables: the temperature of atomic vapour; the amplitude of the modulated magnetic field; the frequency of modulated magnetic field on the transmission intensity of pumping beam. We have defined their relationship into a function, and the calculated values through theoretical analysis have a high degree of fit with the measured values of numerous experiments. It is discovered that the transmission intensity decreases with the increase in temperature, and this effect is modified by the modulation index. A compact magnetometer is developed as a proof of concept based on single-beam scheme. The volume of this magnetometer is 10 cm 3 , and its dual axis sensitivity are both 30 fT/Hz 1 / 2 . Based on this relationship we find that two major improvements are achieved by separating the DC and AC components of transmission intensity: 1. Realizing closed-loop temperature control for atomic vapour which improved the stability of the magnetometer. 2. Achieving closed-loop dual axis magnetic measurements under a single-beam scheme, which extends the magnetometer’s scope of application.

DOI
10.1088/1361-6463/ac19e3

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Abstract The single-beam miniaturized atomic magnetometer with modulated magnetic field is one of the most capable approaches to biomagnetic measurements. The transmission intensity of the pumping beam is critical to the sensitivity of the magnetometer, which is affected by the density of alkali metal and the polarization of atomic spin. In this study, we investigated into the influence of three variables: the temperature of atomic vapour; the amplitude of the modulated magnetic field; the frequency of modulated magnetic field on the transmission intensity of pumping beam. We have defined their relationship into a function, and the calculated values through theoretical analysis have a high degree of fit with the measured values of numerous experiments. It is discovered that the transmission intensity decreases with the increase in temperature, and this effect is modified by the modulation index. A compact magnetometer is developed as a proof of concept based on single-beam scheme. The volume of this magnetometer is 10 cm 3 , and its dual axis sensitivity are both 30 fT/Hz 1 / 2 . Based on this relationship we find that two major improvements are achieved by separating the DC and AC components of transmission intensity: 1. Realizing closed-loop temperature control for atomic vapour which improved the stability of the magnetometer. 2. Achieving closed-loop dual axis magnetic measurements under a single-beam scheme, which extends the magnetometer’s scope of application.
A single-beam spin-exchange relaxation-free atomic magnetometer is ultra-sensitive in the zero field, which has great potential for the detection of a magnetoencephalogram. The addition of a modulated magnetic field is an important approach to achieve high sensitivity for devices of this kind. In this study, we discovered that the amplitude and frequency of the modulated magnetic field (modulation index 0–3) both influence the light absorption. We defined this effect into a function by combining theoretical analysis and the results of experiments. It is discovered that the transmission intensity decreases with an increase in the modulation index. This effect is weakened under the application of a high modulation index. In addition, the transmission intensity and bias magnetic field no longer follow a strict Lorentz curve, while a high degree of fit can be achieved by applying the numerical solution of the Bloch function. A compact magnetometer with a volume of 10 cm3 and a sensitivity of 20 fT/Hz is developed based on the single beam scheme for the proof of concept. Our study is crucial in two aspects: (1) Obtaining high sensitivity through a short measurement period and (2) alignment of the scale factor of the individual magnetometer in a detection array, which further pave the way for improvement in a magnetometer’s performance under a variety of optics platforms.
Abstract A single-beam spin-exchange relaxation-free (SERF) atomic magnetometer can extract vector magnetic field information by detecting the transmission intensity of a resonant circularly polarized pumping beam, which depends sensitively on the atomic density of the alkali metal. We present a novel scheme to determine atomic density based on zero-field magnetic resonance. The resonance linewidth under different transverse DC magnetic fields is fitted by means of a quadratic function. The atomic density can be extracted from the quadratic coefficients of the fitted function. The experimental results indicate that the deviation of measured density is less than two times compared with the theoretical values between 378 K and 403 K. Furthermore, the influence of modulation field on resonance linewidth is investigated experimentally and theoretically. A miniature single-beam SERF atomic magnetometer with a sensor head volume of only 16.2 cm 3 and the measuring sensitivity of 40 fT/ Hz 1/2 has been achieved. These results should be beneficial for guiding the development of a chip-scale atomic magnetometer with high sensitivity and spatial resolution for bio-magnetic field imaging applications.
Abstract The comprehensive influence of the amplitude and frequency of the modulated magnetic field and the magnitude of the bias magnetic field on the performance of an atomic magnetometer have been investigated. Under different magnetic fields, the combined action of the spin precession signal caused by a high-amplitude magnetic field and the influence of magnetic field on relaxation makes the time domain output signal and the amplitude of the first to fourth harmonics show different characteristics, which cannot be explained by the classical analytical calculation solution. By considering the influence of the magnetic field on the transverse relaxation, a more complete model is constructed to explain the phenomenon with a numerical solution, and the overall fit is 93.26%. Based on the single beam and magnetic field modulation scheme, a compact magnetometer is constructed for verification, with a volume of 56.7 cm 3 and a sensitivity of 30 fT/Hz 1/2 .
Atomic magnetometers operated in spin-exchange relaxation-free (SERF) have the potential to replace superconducting quantum interference devices (SQUIDs) in the field of biomagnetism. Here, we demonstrate the dual-axis closed-loop control of a single-beam SERF magnetometer that allows sensing of two orthogonal magnetic fields simultaneously and independently with suppressed cross-talk effect, achieved by applying a rotating modulation field and using of the negative feedback. Operating in the dual-axis closed-loop mode with a miniature 4 × 4 × 4 mm 87Rb cell, we achieve magnetic field sensitivity of 20 fT/Hz1/2 and a bandwidth of 1.8 kHz. In addition to the experimental study of the sensitivity and bandwidth, we perform a theoretical analysis to the dual-axis response and develop an effective decoupling method. The implementation of closed-loop control enables the magnetometer to be robust against the disturbance of the ambient field, which could enlarge the dynamic range, enhance the systematic stability, and suppress the cross-talk effect. Such a simple, yet high-performance SERF magnetometer could be suitable as a promising candidate in the field of biomagnetism, allowing for a denser sensor array and opening up new possibilities for the noninvasive biomagnetic imaging.
Atomic magnetometers are widely acknowledged as one of the most sensitive devices for magnetic field measurement. Our study presents a novel method that enables bandwidth expansion of single-beam magnetometers in the spin-exchange relaxation-free regime. This method bases on increasing the modulation amplitude of magnetic field combining with negative feedback with proportional gain. Through our method, the magnetometer’s total relaxation rate is moderately increased by elevating modulation amplitude and frequency, and correspondingly, the detection bandwidth experiences dramatic improvement from 230 Hz to 1380 Hz. The application of negative feedback with proportional gain further increases this bandwidth to higher than 4 kHz, which is over 17 times of its original value. The sensitivity of the magnetometer under this condition is measured as 25 fT/Hz1/2\text {Hz}^{\text {1/2}}Hz1/2for open-loop scheme and 35 fT/Hz1/2\text {Hz}^{\text {1/2}}Hz1/2for closed-loop scheme. Comparatively, the sensitivity measured on conventional conditions in our experiment is 12 fT/Hz1/2\text {Hz}^{\text {1/2}}Hz1/2. This approach offers a profitable trade-off that extraordinary expansion of detection bandwidth can be acquired with the price of a little sensitivity loss. Our method is advantageous for miniaturization of atomic magnetometers as no extra hardware is added, which is of special interest for applications including biomagnetic measurement where large detection bandwidth is sometimes in need.
We demonstrate a single-beam atomic magnetometer (AM) capable of measuring a three-axis magnetic field with high-sensitivity, achieved by applying a small DC offset field and a high frequency modulation field. To satisfy the miniaturization demand of AMs, an elliptically polarized light detuned by 50 GHz from the resonance transition center is employed. The circularly polarized component is used to polarize the alkali-metal atoms, while the linearly polarized light is used to detect the dynamics of the polarized spin under a magnetic field. Based on theoretical analysis, parameters that significantly affect the performance are optimized, and a sensitivity of 20 fT/Hz 1/2 in x -axis, 25 fT/Hz 1/2 in y -axis, 30 fT/Hz 1/2 in z -axis is achieved with a miniature 4 × 4 × 4 mm 87 Rb vapor cell. Moreover, we also verify that the operation principle of AMs can be used to null background magnetic fields in-situ with isotropic compensation resolution of 6.7 pT, which provides an effectively precise method for zeroing ambient magnetic field. The high-sensitivity operating of an elliptically-polarized-laser-based magnetometer provides prospective futures for constructing a compact, low-cost AM, which is particularly applicable for non-invasive bio-magnetic imaging such as array-based magnetoencephalography (MEG) and magnetocardiography (MCG).