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Physical Review Applied

Faraday-Rotation Atomic Magnetometer Using Triple-Chromatic Laser Beam

Yi Zhang, Yuan Tian, Songsong Li, Jie‐Hua Chen, Sihong Gu

Published Jul 24, 2019• 13 citations
Publisher

Abstract

A Faraday-rotation atomic magnetometer typically employs a resonant, circularly polarized pump beam and an off-resonant, linearly polarized probe beam. Using two light beams is an obstacle to miniaturization, though. This study develops a single-beam scheme that converts a linearly polarized three-color beam into a circularly polarized carrier with orthogonally linearly polarized side bands. In a magnetic field, the carrier prepares anisotropic atoms, which then symmetrically rotate the polarizations of the side bands in opposite directions. This approach should enable miniature devices, and the sensing technologies that depend on them.

DOI
10.1103/physrevapplied.12.011004

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A Faraday-rotation atomic magnetometer typically employs a resonant, circularly polarized pump beam and an off-resonant, linearly polarized probe beam. Using two light beams is an obstacle to miniaturization, though. This study develops a single-beam scheme that converts a linearly polarized three-color beam into a circularly polarized carrier with orthogonally linearly polarized side bands. In a magnetic field, the carrier prepares anisotropic atoms, which then symmetrically rotate the polarizations of the side bands in opposite directions. This approach should enable miniature devices, and the sensing technologies that depend on them.
We propose a scheme for a Bell–Bloom atomic magnetometer based on a single beam. In our approach, the light is periodically modulated synchronously with Larmor precession. As a result, the light that interacts with atoms is alternately polychromatic and monochromatic. When the light is polychromatic, the ±1st sidebands of the light are optically resonant with the atoms, and the atoms are polarized by pumping. When the light is monochromatic, a differential detection technique extracts the Faraday-rotation signal of the light, and the polarization of the light is probed. We demonstrate an experimental implementation of the Bell–Bloom magnetometer that achieves a sensitivity for magnetic-field detection of 0.25 pT/Hz1/2 at 1 Hz. Our scheme is suitable for developing compact, miniaturized magnetometers.
We describe a dual-axis atomic spin comagnetometer for rotation sensing using a single laser beam. A circularly polarized laser is used to pump the co-located atomic ensembles of alkali metal and noble gas and probe their gyroscopic precession at the same time, which is different from the traditional two-beam orthogonal pump-probe configuration. Combined with magnetic field modulations perpendicular to the laser propagating direction, the comagnetometer could measure dual-axis rotation rates simultaneously. This system is demonstrated theoretically and experimentally. A rotation sensitivity of 5.1 × 10–7 rad s−1 Hz−1∕2 and a bias instability of 0.016 °/h have been achieved. The theory and method presented here shed light on the way to miniaturize the comagnetometer with high precision for practical applications.
This paper reports the development and characterization of an atomic magnetometer with a microfabricatedRb atomic vapour cell as the sensing element. The measurement principle is based on the Faraday rotation of linearly polarized probe light, which interacts resonantly with atomic spin states in the presence of a magnetic field. In the chip-scale vapour cell (dimensions: 8.0 mm × 12.0 mm × 1.5 mm), the atomic vapour is confined to a micromachined optical cavity of 3.5 µL with an optical path length of 0.5 mm. Optical interrogation is performed using a single laser beam tuned to the D1 absorption resonance of ⁸⁵Rb. The rotation arises due to magnetically induced circular birefringence in the hot atomic vapour. A bench model of the magnetometer is demonstrated over a measurement range of ±50 µT with a maximum uncertainty of 800 nT in an unshielded environment. The magneto-optic rotation (MOR) response in the laboratory was investigated and is observed to vary from 5×10³ rad/T for near zero magnetic field to 3.2×10³ rad/T for ambient geomagnetic field.
We report a rubidium atomic magnetometer based on pump-probe nonlinear magneto-optical rotation. The rubidium vapor cell is placed in a five-layer magnetic shield with inner coils that can generate uniform magnetic fields along the direction of pump beam, and the cell is also placed in the center of a Helmholtz coil that can generate an oscillating magnetic field perpendicular to the direction of pump beam. The atoms are optically pumped by circularly polarized pump beam along a constant magnetic field in a period of time, then the pump beam is turned off and a /2 pulse of oscillating magnetic field for 87Rb atoms is applied. After the above process, the individual atomic magnetic moments become phase coherent, resulting in a transverse magnetization vector precessing at the Larmor frequency in the magnetic field. The linearly polarized probing beam is perpendicular to the direction of magnetic field, and can be seen as a superposition of the left and right circularly polarized light. Because of the different absorptions and dispersions of the left and right circularly polarized light by rubidium atoms, the polarization direction of probing beam rotates when probing beam passes through rubidium vapor cell. The rotation of the polarization is subsequently converted into an electric signal through a polarizing beam splitter. Finally, the decay signal related to the transverse magnetization vector is measured. The Larmor frequency proportional to magnetic field is obtained by the Fourier transform of the decay signal. The value of magnetic field is calculated from the formula:B=(2/) f, where and f are the gyromagnetic ratio and Larmor frequency, respectively. In order to measure the magnetic field in a wide range, the tracking lock mode is proposed and tested. The atomic magnetometer can track the magnetic field jump of 1000 nT or 10000 nT, indicating that the atomic magnetometer has strong locking ability and can be easily locked after start-up. The main performances in different magnetic fields are tested. The results show that the measurement range of the atomic magnetometer is from 100 nT to 100000 nT, the extreme sensitivity is 0.2 pT/Hz1/2, and the magnetic field resolution is 0.1 pT. The transverse relaxation times of the transverse magnetization vector in different magnetic fields are obtained, and the relaxation time decreases with the increase of the magnetic field. When the measurement range is from 5000 nT to 100000 nT, the magnetic field sampling rate of the atomic magnetometer can be adjusted in a range from 1 Hz to 1000 Hz. The atomic magnetometer in high sampling rate can measure weak alternating magnetic field at low frequency. This paper provides an important reference for developing the atomic magnetometer with large measurement range, high sensitivity and high sampling rate.
In recent years, atomic magnetometers (AMs) have demonstrated sub-femtotesla sensitivities and have emerged as potential replacements for superconducting quantum interference devices (SQUIDs). In an AM, a circularly polarized pump laser aligns the electron spins in a cloud of alkali metal vapor. Changes occur in the optical properties of the vapor when its collective moment interacts with an external magnetic field, resulting in a measurable polarization rotation of a linearly polarized probe beam. We present a unique pump/probe scheme in which a pump beam tuned to the87Rb D1 line (795 nm) and a probe beam tuned to the D2 line (780 nm) share a single optical axis. A dichroic waveplate is used to circularly polarize the pump beam while leaving the probe beam linearly polarized. Based upon this scheme, we have developed a small profile, fiber-coupled AM. By independently optimizing pump/probe power and detuning, we are able to achieve intrinsic single-channel sensitivity of <; 5 fT/√Hz, which is suitable for demanding applications such as magnetoencephalography (MEG). Furthermore, through straightforward adaptations of the current design, it should be possible to create dense, reconfigurable arrays of AMs for whole-head MEG.
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.