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IEEE Sensors Applications Symposium (SAS)

Broadband Ultra-Sensitive Adiabatic Magnetometer

Igor Savukov, Young Jin Kim

Published Aug 23, 2021• 1 citation
Publisher

Abstract

We introduce a new ultra-sensitive adiabatic magnetometer that has a broad bandwidth and can operate in the presence of magnetic fields and gradients. It follows conceptually typical implementations of atomic magnetometers based on alkali-metal vapor cells and lasers for optical pumping and optical Faraday effect detection, while its unique feature is a measurement of an oscillating magnetic field along the probe beam direction at frequencies lower than the resonant frequency, proportional to a static magnetic field along the pump beam direction. The bandwidth of the adiabatic magnetometer scales as the strength of the field along the pump beam. From our theoretical studies it is expected that the adiabatic magnetometer can reach 1 fT sensitivity with a bandwidth of 10 kHz, which any type of atomic magnetometers cannot achieve. Among anticipated various applications of this adiabatic magnetometer are biomagnetic sensing, nuclear magnetic resonance detection, and alkali-metal density measurements. We experimentally conducted alkali-metal density measurements, as an example of applications.

DOI
10.1109/sas51076.2021.9530142

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We introduce a new ultra-sensitive adiabatic magnetometer that has a broad bandwidth and can operate in the presence of magnetic fields and gradients. It follows conceptually typical implementations of atomic magnetometers based on alkali-metal vapor cells and lasers for optical pumping and optical Faraday effect detection, while its unique feature is a measurement of an oscillating magnetic field along the probe beam direction at frequencies lower than the resonant frequency, proportional to a static magnetic field along the pump beam direction. The bandwidth of the adiabatic magnetometer scales as the strength of the field along the pump beam. From our theoretical studies it is expected that the adiabatic magnetometer can reach 1 fT sensitivity with a bandwidth of 10 kHz, which any type of atomic magnetometers cannot achieve. Among anticipated various applications of this adiabatic magnetometer are biomagnetic sensing, nuclear magnetic resonance detection, and alkali-metal density measurements. We experimentally conducted alkali-metal density measurements, as an example of applications.
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.
Advances in Optical and Photonic Devices 330 integration into microfabricated packages.Such packages will be not only inexpensive and easy to batch produce they will also have lower power consumption, light weight and unimpeded mobility (Knappe et al., 2006).Sensitive magnetic field measurements, for which AMs can be used, are important owing to many existing and potential applications.Magnetometers have been in wide use, for example, in geology, military, biomagnetism, space magnetic field measurements.AMs have been used because they are both relatively sensitive compared to conventional inexpensive magnetometers, such as fluxgates, and more convenient compared to SQUIDs that require cryogenic cooling.For a long time low-T c SQUIDs had been by far the most sensitive magnetometers at low frequency.However, this has changed with the demonstration with AMs of record 0.5 fT/Hz 1/2 sensitivity (Kominis et al., 2003).High-sensitivity AMs are based on spin-polarized atomic gases or vapors, and AM research is closely related to the atomic physics subfield dedicated to the investigation of spin interactions in such media.This subfield includes the research on optical pumping and related topics such as atomic clocks, masers, hyperpolarized gases, spin-based spectroscopy, and some others.A comprehensive review of optical pumping experiments before 1972 and theory of optical pumping and various relaxation mechanisms are given by William Happer (Happer, 1972).This theory, which still stands the test of time, includes the formulation of density matrix equations, which can be directly applied to the analysis of magnetic resonances of vapors on which atomic magnetometers are based.Atomic magnetometers find many applications as other magnetometers, but they are most useful when high sensitivity and non-cryogenic operation are required.For example, AMs have been developed for submarine detection, and many other military applications are possible on similar principles.Some applications of Rb-vapor or 3 He magnetometers in geophysics and space physics were mentioned as early as in 1961 by Bloom (Bloom, 1961) who did some pioneering work on the high-sensitivity measurements of magnetic fields and theory for the operation of atomic magnetometers.Theoretical analysis of early atomic magnetometers was also done by Dehmelt (Dehmelt, 1957) and by Bell and Bloom (Bell & Bloom, 1957).Although the history of atomic magnetometers is quite fascinating and they played an important role in applications where simpler and less expensive methods did not provide sufficient sensitivity, the atomic magnetometers of the past were much less sensitive than the modern ones.Most exciting results appeared only recently with the various demonstrations of performance of high-density AMs, which we would like to focus on in this chapter. How to referenceIn order to correctly reference this scholarly work, feel free to copy and paste the following: Igor Savukov (2010).Ultra-Sensitive Optical Atomic Magnetometers and Their Applications, Advances in Optical and Photonic Devices, Ki Young Kim (Ed.),
We demonstrated a scalar atomic magnetometer using a microfabricated Cs vapor cell. The atomic spin precession is driven by an amplitude-modulated circularly-polarized pump laser resonant on the D1 transition of Cs atoms and detected by an off-resonant linearly-polarized probe laser using a balanced polarimeter setup. Under a magnetic field with amplitude in the Earth's magnetic field range, the magnetometer in the gradiometer mode can reach sensitivities below 150fT/Hz, which shows that the magnetometer by itself can achieve sub-100fT/Hz sensitivities. In addition to its high sensitivity, the magnetometer has a bandwidth close to 1 kHz due to the broad magnetic resonance inside the small vapor cell. Our experiment suggests the feasibility of a portable low-power and high-performance magnetometer which can be operated in the Earth's magnetic field. Such a device will greatly reduce the restrictions on the operating environment and expand the range of applications for atomic magnetometers, such as detection of nuclear magnetic resonance in low magnetic fields.
Spin-exchange relaxation free alkali-metal magnetometers typically operate in the regime of high optical density, presenting challenges for simple and efficient optical pumping and detection. We describe a high-sensitivity Rb magnetometer using a single elliptically polarized off-resonant laser beam. Circular component of the light creates relatively uniform spin polarization while the linear component is used to measure optical rotation generated by the atoms. Modulation of the atomic spin direction with an oscillating magnetic field shifts the detected signal to high frequencies. Using a fiber-coupled distributed feedback laser, we achieve magnetic field sensitivity of 7 fT/Hz7\text{ }\text{fT}/\sqrt{\text{Hz}}7 fT/Hz​ with a miniature $5\ifmmode\times\else\texttimes\fi{}5\ifmmode\times\else\texttimes\fi{}5\text{ }\text{mm}\text{ }\text{Rb}$ vapor cell.
We demonstrate an ultra-high sensitivity rf optical magnetometer based on a compact 5×5×8 mm3 Rubidium-87 vapor cell in a mu-metal shielded environment. The rf field is detected via a Faraday rotation effect on the linear polarization of a probe beam, and the resonance frequency is tuned via Zeeman effect by applying a static bias magnetic field to the cell. Sensitivities down to 3 fT/√Hz are achieved between 10 kHz and 100 kHz, which is a performance comparable to that of prior larger cells. An increase in the sensitivity with frequency and temperature has been observed.
We demonstrated a scalar atomic magnetometer using a microfabricated Cs vapor cell. The atomic spin precession is driven by an amplitude-modulated circularly-polarized pump laser resonant on the D1 transition of Cs atoms and detected by an off-resonant linearly-polarized probe laser using a balanced polarimeter setup. Under a magnetic field with amplitude in the Earth's magnetic field range, the magnetometer in the gradiometer mode can reach sensitivities below 150fT/√Hz, which shows that the magnetometer by itself can achieve sub-100fT/√Hz sensitivities. In addition to its high sensitivity, the magnetometer has a bandwidth close to 1 kHz due to the broad magnetic resonance inside the small vapor cell. Our experiment suggests the feasibility of a portable low-power and high-performance magnetometer which can be operated in the Earth's magnetic field. Such a device will greatly reduce the restrictions on the operating environment and expand the range of applications for atomic magnetometers, such as detection of nuclear magnetic resonance in low magnetic fields