Analysis of influence of RF power and buffer gas pressure on sensitivity of optically pumped cesium magnetometer

A further study is conducted on two factors which respectively influence the sensitivity of optically pumped cesium magnetometer （CsOPM）. The influence of radio frequency （RF） power and the buffer gas pressure on the sensitivity is theoretically analyzed, and some properties are predicted. Based on the established measurement system and the visible Zeeman spectrum, not only is the real influence of these factors studied, but also, under our experimental condition, optimum parameters based on the measured curves are ascertained. The properties of these measured curves match the theoretical result very well. Our research attempts to provide theory reference to help magnetometer designers determine optimum parameters under certain conditions.

Magnetic field imaging with radio-frequency optically pumped magnetometers [Invited]

High-sensitivity radio-frequency optically pumped magnetometers (RF-OPMs), working without cryogenic condition, play a critical role in magnetic field imaging(MFI) at low frequencies(e.g., 100 Hz to 1 MHz). We introduce the principle of operation and recent developments of RF-OPMs and focus on reviewing the MFI applications in magnetic induction tomography, ultralow-field magnetic resonance imaging, and magnetic particle imaging. For the applications of RF-OPMs, ranging from industrial monitoring to medical imaging and security screening, the unshielded and portable RF-OPMs(and RF-OPM array)techniques are still under the further development for detecting and scanning over the target object for accomplishing the final three-dimensional imaging, and thus extremely require the abilities of active compensation of the ambient magnetic field and sensor miniaturization in the future.

A dual-axis, high-sensitivity atomic magnetometer

Atomic magnetometer （AM） operated in a spin-exchange relaxation-free （SERF） regime features superior sensitivity and non-cryogenic operation, and thus is competitive with the best superconducting quantum interference devices. Previously, SERF AM with fT/Hz1/2 level sensitivity commonly acted as a single-axis sensor. Here we demonstrate a dual-axis SERF AM capable of simultaneously and independently detecting x-and y-field components with a sensitivity of 20 fT/Hz1/2. As there is no necessity to worry about the cross-talk effects arising from field modulations, the dual-axis scheme proposed here is of particular interest to AM array and hence the biomagnetic applications.

A laser pump-re-pump atomic magnetometer

We demonstrate experimentally an atomic magnetometer based on optical pumping theory, a magnetic resonance that is induced by a radio frequency field and dependent on the magnetic field strength. Compared with the conventional method using one radiation field, which is used not only as the probe beam but also as a pump beam, the additional re-pump beam can increase remarkably the amplitude of the signal. It is shown that the amplitude of the magnetic field resonance signal can increase more than 55% by using an additional re-pump beam, which makes the sensitivity of the magnetometer higher. Finally, we investigate the relation between amplitude of the signal and re-pump laser power, and calculate the atomic population in the trapping states with rate equations.

Measurement of T wave in magnetocardiography using tunnel magnetoresistance sensor

Several critical clinical applications of magnetocardiography(MCG)involve its T wave.The T wave’s accuracy directly affects the diagnostic accuracy of MCG for ischemic heart disease and arrhythmogenic.Tunnel magnetoresistance(TMR)attracts attention as a new MCG measurement technique.However,the T waves measured by TMR are often drowned in noise.The accuracy of T waves needs to be discussed to determine the clinical value of MCG measured by TMR.This study uses an improved empirical mode decomposition(EMD)algorithm and averaging to eliminate the noise in the MCG measured by TMR.The MCG signals measured by TMR are compared with MCG measured by the optically pumped magnetometer(OPM)to judge its accuracy.Using the MCG measured by OPM as a reference,the relative errors in time and amplitude of the T wave measured by TMR are 3.4%and 1.8%,respectively.This is the first demonstration that TMR can accurately measure the time and amplitude of MCG T waves.The ability to provide reliable T wave data illustrates the significant clinical application value of TMR in MCG measurement.

VCSEL frequency stabilization for optically pumped magnetometers

Optically pumped magnetometers(OPMs)have developed rapidly in the bio-magnetic measurement field,which requires lasers with stable frequency and intensity for high sensitivity.Herein we stabilize a vertical-cavity surface-emitting laser(VCSEL)without any additional setup except for the parts of an OPM.The linewidth of the absorption spectrum as a frequency reference is broadened to 40 GHz owing to pressure broadening.To enhance performance,the VCSEL injection current and temperature are tuned simultaneously using a closed-loop control system.The experiments reveal that the VCSEL frequency stability achieves 2×10^(-7) at an average time of 1 s,and the intensity noise is 1×10^(-6)V/Hz^(1/2) at 1-100 Hz.This approach is useful for suppressing OPM noise without additional sensor probe parts.

Zero field optically pumped magnetometer with independent dual-mode operation

We propose a dual-mode optically pumped magnetometer(OPM)that can flexibly switch between single-beam modulation mode and double-beam DC mode.Based on a 4 mm×4 mm×4 mm miniaturized vapor cell,the double-beam DC mode achieves a sensitivity of 7 fT=Hz^(1/2) with probe noise below 4 fT=Hz^(1/2) and working bandwidth over 65 Hz.This mode is designed to precisely measure the noise floor of a mu-metal magnetic shield.The single-beam modulation mode(sensitivity 20 fT=Hz^(1/2))exhibits bandwidth characteristics suitable for biomagnetic measurements.Thus,our design is suitable for a miniaturized OPM with multiple functions,including magnetic-shield background noise measurement and medical imaging.