Portable intrinsic gradiometer for ultra-sensitive detection of magnetic gradient in unshielded environment

Abstract

We demonstrate a portable intrinsic scalar magnetic gradiometer composed of miniaturized cesium vapor cells and verticalcavity surface-emitting lasers (VCSELs). Two cells, with an inner dimension of 5 mm x 5 mm x 5 mm and separated by 5 cm, are driven by one VCSEL and the resulting Larmor precessions are probed by a second VCSEL through optical rotation. The off-resonant linearly polarized probe light interrogates two cells at the same time and directly reads out the amplitude difference between magnetic fields at two cell locations. The intrinsic gradiometer scheme has the advantage of avoiding added noise from combining two scalar magnetometers. We achieve better than 18 fT/cm/√Hz sensitivity in the gradient measurement. Ultra-sensitive short-baseline magnetic gradiometers can potentially play an important role in many practical applications, such as nondestructive evaluation and unexplode ordnance (UXO) detection. Another application of the gradiometer is for magnetocardiography (MCG) in an unshielded environment. Real-time MCG signals can be extracted from the raw gradiometer readings. The intrinsic gradiometer greatly simplifies the MCG setup and may lead to ubiquitous MCG measurement in the future.Over the last two decades, many breakthroughs have been achieved in atomic magnetometer research. For example, the discovery of the spin-exchange relaxation-free (SERF) phenomenon at high atomic density and low magnetic field leads to a great improvement in the magnetometer noise performance [1]. Sensitivities comparable with [2] or even outperforming [3] those of superconducting quantum interference devices (SQUIDs) have been reported with SERF magnetometers. Another example is the successful fabrication of atomic magnetometers using the technique of Micro-Electro-Mechanical Systems (MEMS) [4,5,6,7,8]. MEMS techniques enable chip-scale devices, significantly reducing size and power-consumption of atomic magnetometers. Chip-scale magnetometers can have sizes approaching 10 mm 3 and dissipate less than 200 mW. Despite all these advances, applications of atomic magnetometers are still limited. Highly sensitive SERF magnetometers require a magnetically shielded environment while chip-scale total-field magnetometers have subpar noise performances [5], although a scalar magnetometer with a sensitivity of 100 fT/√Hz has been demonstrated using a MEMS-based cesium vapor cell [9]. With bigger cells, scalar magnetometers can reach sensitivities of sub-10 fT/√Hz [10] or even sub-fT/√Hz [11]. In practical applications in an unshielded environment, the output noise of scalar magnetometers is often dominated by the background field fluctuation, instead of their fundamental sensitivities. To overcome this problem, a common solution is to set up a gradiometer system using two or more magnetometers [12,13,14]. By taking the reading difference between adjacent magnetometers, this conventional gradiometer configuration suppresses the common field fluctuations at the cost of worsening the fundamental sensitivity by at least a factor...

Scalar Magnetometry Below 100 fT/Hz1/2 in a Microfabricated Cell

“…The noise floor could be lowered by implementing a multipass vapor cell configuration, as demonstrated with larger cells [13]. An intrinsic gradiometer configuration [40] can also be used, in which the photon shot noise will be common-mode.…”

Tunnel Magnetoresistance Sensor with AC Modulation and Impedance Compensation for Ultra-Weak Magnetic Field Measurement

“…It was proved that the approach was effective and able to provide more complete cardiac magnetic information. A gradiometer was developed for weak magnetic field measurement, which could reduce the common noise [12,13]. The modulated method was utilized in an OPM system.…”

Pulsed Optically Pumped Magnetometers: Addressing Dead Time and Bandwidth for the Unshielded Magnetorelaxometry of Magnetic Nanoparticles

“…Unshielded MRX has been performed before with SQUID [ 38 ] and fluxgates [ 39 ], while OPM-MRX has been performed in a weakly shielded environment [ 26 ]. Recently, several developments of portable OPM for unshielded operation have been published [ 40 , 41 , 42 , 43 ]. It should be noted, that although not commercially available, the high bandwidth sensor presented in [ 40 ] is also a well suited candidate for unshielded MRX.…”