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.
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.
We propose a three-axis closed-loop optically pumped magnetometer with high sensitivity. The closed-loop magnetometer has a three-axis sensitivity of approximately 30 fT/Hz 1/2 using two orthogonal laser beams for pumping and probing the alkali metal atoms. In the closed-loop mode, the dynamic range is improved from ±5 nT to ±150 nT. The bandwidth is increased from about 100 Hz to over 2 kHz with 10 kHz modulation fields in x - and y -axes and another 6 kHz modulation field along the z -axis. Compared with single-axis or dual-axis magnetometers, the proposed magnetometer not only provides the direction and magnitude of the magnetic field but also has high robustness in a challenging environment. The magnetometer has applications in biomagnetic measurements, magnetic resonance imaging, and fundamental physics.
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.
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 30.8 pT/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 22.9 pT/Hz^{1/2}, with a measured -3 dB 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.
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.
Optically pumped radio frequency (RF) atomic magnetometers have been shown to have an improved sensitivity over standard tuned coils for frequencies less than 50 MHz, making these RF magnetometers attractive for the detection of nuclear quadrupole resonance (NQR) and low-field nuclear magnetic resonance (NMR) signals. In an atomic magnetometer a linearly polarized probe beam measures, through Faraday rotation, the transverse atomic magnetization induced by the resonant RF signal. The resonance, or Larmor, frequency of the magnetometer is easily tuned with a small magnetic field. We construct an atomic magnetometer based on a potassium vapor cell with a measured sensitivity of 0.22 ± 0.02 fT/ √ Hz and a detection bandwidth of 334 ± 11 Hz when tuned to 423 kHz, giving a Q of over 1200. While high Q magnetometers are sensitive to weak magnetic fields, they are also sensitive to magnetic transients, such as those associated with the excitation pulses needed for magnetic resonance. The ringing created by such transients can obscure the signal of interest. This is particularly detrimental for magnetic resonance signals that decay faster than the transverse relaxation rate. We demonstrate that by feeding back part of the optical signal to orthogonal electromagnetic coils, this unwanted ringing can be quickly damped out leaving the magnetometer ready for the detection of the signal. This negative feedback as applied to the K spins is called spin-damping and can be used to relax the K atoms faster by more than two orders of magnitude. Using spin-damping we reduce the dead-time before data acquisition from 0.8 to 0.2 ms in the detection of NQR signals from ammonium nitrate, one of the so-called fertilizer bomb explosives with an NQR frequency of 423 kHz. Furthermore, we discover that spin-damping can not only be used to suppress signals, it can also be used to suppress noise. We show that it can be used to suppress both environmental noise and more fundamental quantum noise, such as photon shot and spin projection noise. The latter suppression opens the door for its use in quantum control to initialize spin systems before measurements. For phase-sensitive detection, as is often used in magnetic resonance, spin-damping suppresses signal and noise in such a way as to increase the sensitivity bandwidth of the magnetometer. We prove, both theoretically and experimentally, that it is possible to increase the magnetometer detection bandwidth by a factor of 3 or more without significant loss in sensitivity. Such gains in bandwidth for a high Q magnetometer translate into savings in detection time, particularly for substance detection where the frequency of the signal of interest may not be completely known.
The high-sensitivity three-axis detection of magnetic fields is widely used in various applications.Our study demonstrates an atomic magnetometer detecting vector magnetic field, of which the core components are one glass cell and two elliptically polarized laser beams.The light-intensity noise is suppressed by differential detection technology, so a higher sensitivity is obtained compared with our previous work.The three-axis average sensitivities of the atomic magnetometer simultaneously reached 14 fT/Hz 1/2 along the x axis, 11 fT/Hz 1/2 along the y axis, and 25 fT/Hz 1/2 along the z axis.Furthermore, the atomic magnetometer's dynamic range was further improved to 150 nT, and its bandwidth was increased to over 200 Hz using a closed-loop control system.The proposed three-axis atomic magnetometer with a large dynamic range and a wide bandwidth holds great promise for biomagnetic measurement in a more challenging environment.