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Journal of Jilin University

Research of Magnetic Resonances Optical Detection Method in Optically Pumped Magnetometer

Zhang Zhen-yua

Published Jan 1, 2011• 0 citations
OpenAlex

Abstract

In the optically pumped magnetometer,the signal to noise ratio is very low when use the method of detecting Zeeman energy level transition directly.However,the detected signal to noise ratio can be greatly improved by using the technique of magnetic resonances optical detection which can transform the microwave quantum detection to the photon detection,because the energy of photon is much greater than that of microwave quantum.The method of magnetic resonances optical detection in the OPM(Optically Pumped Magnetometer) has been studied,the principles and experimental devices are discussed which includes the method of detection of cells transparency,detection of orthogonal beams,spin exchange collision and the method of pump beam modulation.The characteristics of these methods are analyzed and compared,the application and complexity are considered,and all these will provide the basis for the design of the optically pumped magnetometer.

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In the optically pumped magnetometer,the signal to noise ratio is very low when use the method of detecting Zeeman energy level transition directly.However,the detected signal to noise ratio can be greatly improved by using the technique of magnetic resonances optical detection which can transform the microwave quantum detection to the photon detection,because the energy of photon is much greater than that of microwave quantum.The method of magnetic resonances optical detection in the OPM(Optically Pumped Magnetometer) has been studied,the principles and experimental devices are discussed which includes the method of detection of cells transparency,detection of orthogonal beams,spin exchange collision and the method of pump beam modulation.The characteristics of these methods are analyzed and compared,the application and complexity are considered,and all these will provide the basis for the design of the optically pumped magnetometer.
Rubidium laser optically pumped magnetometers (OPMs) are widely used magnetic sensors based on the Zeeman effect, laser pumping, and magnetic resonance principles. They measure the magnetic field by measuring the magnetic resonance signal passing through a rubidium atomic gas cell. The quality of the magnetic resonance signal is a necessary condition for a magnetometer to achieve high sensitivity. In this research, to obtain the best magnetic resonance signal of rubidium laser OPMs in the Earth’s magnetic field intensity, the experiment system of rubidium laser OPMs is built with a rubidium atomic gas cell as the core component. The linewidth and amplitude ratio (LAR) of magnetic resonance signals is utilized as the optimization objective function. The magnetic resonance signals of the magnetometer experiment system are experimentally measured for different laser frequencies, radio frequency (RF) intensities, laser powers, and atomic gas cell temperatures in a background magnetic field of 50,765 nT. The experimental results indicate that optimizing these parameters can reduce the LAR by one order of magnitude. This shows that the optimal parameter combination can effectively improve the sensitivity of the magnetometer. The sensitivity defined using the noise spectral density measured under optimal experimental parameters is 1.5 pT/Hz1/2@1 Hz. This work will provide key technical support for rubidium laser OPMs’ product development.
The optically pumped magnetometer (OPM) is a class of atomic devices that rely on the measurement of Larmor precession of an atomic spin ensemble in a magnetic field. They need the specific frequency light to complete the pumping and detection process. OPMs typically work under the Earth's magnetic field or a limited field. When operating in near-zero field, they can realize the spin-exchange relaxation-free (SERF) regime, thereby promoting a substantial increase in sensitivity. At present, OPMs have been widely used in magnetoencephalography, magnetocardiography, geomagnetic detection, and inertia measurement (comagnetometer). There are a large number of researchers engaged in the study of relevant mechanisms, devices, technologies, and applications. More importantly, supported by MEMS technology and micro/nano optics, OPMs have the potential to move toward chip-scale sensors. This special issue contains some latest developments in OPMs and related areas. We hope the reprint can provide valuable references for researchers in the related field.
Rb optically pumped magnetometer (OPM) based on electron paramagnetic resonance (EPR) show advantages to measure the nuclear magnetization and have succeeded in fundamental physics and rotation sensing, etc . The magnetometry sensitivity is a key performance of these Rb OPMs which should be improved. In this study, a high sensitivity Rb OPM is demonstrated theoretically and experimentally. To improve the sensitivity, acousto-optic modulation based on balanced detection is applied to suppress the probe noises. Compared with the conventional optical rotation detection for this OPM configuration, the probe noise shows a significant suppression especially in low frequencies. Eventually, a simultaneous dual-axis transverse measurement with 30 fT/Hz 1/2 sensitivity is achieved in a 200 Hz bandwidth and a 250nT linear working range. In addition, we utilize a closed-loop feedback to improve the stability and enlarge the transverse measurement range to 10µT order of magnitude while maintain the open-loop performances. A quasi-static magnetic field measurement can also be achieved in the longitudinal direction in the closed-loop mode. This OPM can serve for the nuclear magnetization measurement with a high sensitivity especially in environments with a large magnitude of the external magnetic field.
Traditional practice of suppressing low-frequency noise in atomic magnetometers is mainly through the application of magnetic field modulation, while magnetic fields of this kind introduce extra transverse relaxation, which results in a decrease of sensitivity. In this study, a novel method of measuring relaxation rate induced by spin-destruction, wall collisions and modulation magnetic fields is proposed. The optical pumping model is studied both in optical-thin and optical-thick vapor cell through the analysis of total relaxation, thus the optical pumping rate can be separated from total relaxation by analyzing the optical depth. Furthermore, based on the analysis of optical pumping rate and relaxation rate, the polarization of the electron ensemble was evaluated. A compact magnetometer with single-beam configuration is developed for the proof of concept, and the experimental results show good agreements with theoretical calculation. Our method provides a simple and feasible approach to in-situ measurement of transverse relaxation rate and optical pumping rate with physical structure of the device untouched, which is especially advantageous for designing and optimizing compact atomic magnetometers.
Optically pumped magnetometers (OPMs) have demonstrated significant potential in weak magnetic field detection due to their high sensitivity. In this study, we developed an Mz-type optically pumped rubidium magnetometer using a paraffin-coated anti-relaxation vapor cell. The system optimization and performance characterization were conducted inside a magnetic shield. Specifically, the pump light intensity and radio-frequency (RF) magnetic field were jointly optimized by using the linewidth-amplitude ratio as the core metric. Based on the frequency-domain noise spectrum, the sensitivity in open-loop mode was measured to be approximately 30.8pT/Hz 1/2 . Furthermore, a closed-loop feedback locking technique was applied, reducing the measured noise floor under the tested conditions and improving the sensitivity to 22.9pT/Hz 1/2 , with a measured −3dB bandwidth of 123 Hz. The dynamic characteristics were evaluated via magnetic-field step response, showing that the system could track magnetic-field changes stably under closed-loop operation. Finally, by using tri-axial modulation and frequency-domain demodulation, we overcame the scalar measurement limitation of traditional Mz magnetometers. This work realizes vector magnetic field detection and provides a technical basis for applications such as geomagnetic navigation and magnetic anomaly detection.
We compare the performance of two methods for the synchronization of the atomic spins in optically pumped magnetometers: intensity modulation of the pump light and the classical M(x) method using B(1) field modulation. Both techniques use the same set-up and measure the resulting features of the light after passing a micro-fabricated Cs cell. The intensity-modulated pumping shows several advantages: better noise-limited magnetic field sensitivity, misalignment between pumping and spin synchronization is excluded, and magnetometer arrays without any cross-talk can be easily set up.