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Applied Optics

Role of the probe beam in a radio-frequency atomic magnetometer

Patrick Bevington, Jenelle Rajroop, R. Gartman, Witold Chałupczak

Published Jun 5, 2020• 3 citations
Publisher

Abstract

We explore the role of the linearly polarized probe beam in the radio-frequency atomic magnetometer. Two regimes of coupling of the linearly polarized light to the atomic ensemble, near and off-resonant, are demonstrated and discussed. Our studies cover three types of interaction between the atomic spin system and the linearly polarized beam, i.e., absorptive production of spin polarization, dispersive spin state readout, and nonlinear spin couplings. The work is performed with the outlook of optimization and simplification of magnetometer operation. Operation of a magnetometer with a single beam, generating and probing the atomic orientation, is presented. This single-beam configuration, which combines optical pumping, nonlinear spin couplings, and spin-exchange collisions, creates the option for a portable device with simple instrumentation.

DOI
10.1364/ao.396941

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We explore the role of the linearly polarized probe beam in the radio-frequency atomic magnetometer. Two regimes of coupling of the linearly polarized light to the atomic ensemble, near and off-resonant, are demonstrated and discussed. Our studies cover three types of interaction between the atomic spin system and the linearly polarized beam, i.e., absorptive production of spin polarization, dispersive spin state readout, and nonlinear spin couplings. The work is performed with the outlook of optimization and simplification of magnetometer operation. Operation of a magnetometer with a single beam, generating and probing the atomic orientation, is presented. This single-beam configuration, which combines optical pumping, nonlinear spin couplings, and spin-exchange collisions, creates the option for a portable device with simple instrumentation.
Traditionally, atomic spin orientation is achieved by the transfer of angular momentum from polarized light to an atomic system. We demonstrate the mechanism of orientation generation in room-temperature caesium vapors that combines three elements: optical pumping, nonlinear spin dynamics, and spin-exchange collisions. Through the variation of the spin-exchange relaxation rate, the transition between an aligned and an oriented atomic sample is presented. The observation is performed by monitoring the atomic radio-frequency spectra. The measurement configuration discussed paves the way to simple and robust radio-frequency atomic magnetometers that are based on a single low-power laser diode that approach the performance of multilaser pump-probe systems.
In this paper, we present an analysis of the amplitude variations of the opto-magnetic resonance absorption signals obtained in a single-beam magnetometer driven by radially or azimuthally polarized light (RPL/APL). It is shown that optically polarized atoms driven by cylindrical vector beams obtained only the alignment of atomic multipole moments but not the orientation, which is in good agreement with our simulation and experimental results. In comparison with the plane polarized pump light fields, cylindrical vector beams with much more complete electric vector polarization distribution in the transverse plane, make it unlikely to create the "emptying state " (no-atom populated) among the ground-state Zeeman sublevels for any possible orientation of the applied static magnetic field. These characteristics of the RPL/APL lead to generally smaller atomic population difference and lower response intensity of the transmitted signal. The tensor decomposition of atomic polarized states and the evolution of atomic multipole moments with the sweeping radio frequency (RF) field offer the way to show the magnetic orientation sensitivity of the radially or azimuthally polarized probe light, which possess similar profiles as that of the linearly polarized light, only with a constant phase lag of about π/2 and obvious amplitude differences.
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.
An atomic magnetometer is a sensor which is used to measure a magnetic field through its interaction with the atomic sample. Significant research into atomic magnetometry has led to the development of very sensitive atomic sensors capable of matching the sensitivity of the most sensitive magnetometers, superconducting quantum interference devices (SQUIDs). Because SQUIDS require cryogenics to operate, atomic magnetometers provide a sensitive, yet low-cost alternative. They have found use in many areas such as medicine, security, explosives detection and fundamental physics research. One of the primary factors influencing sensitivity is the detuning of the probe beam from the resonant transitions of the atomic ground state. A caesium room temperature radio-frequency (rf) magnetometer is constructed and used to investigate the influence of the probe beam detuning on the magnetometer signal of the F = 3 and F = 4 ground states. The results of probing near and far from resonance revealed an off-resonant regime and two absorptive regimes. In the off-resonant regime, the atomic spins are unperturbed by the probe beam; it is a quantum non-demolition (QND) interaction. The two absorptive regimes, found when the probe beam is in the vicinity of either the 62S1 2 F = 3 → 6 2P3 2 F 0 = 2,3,4 or the 62S1 2 F = 4 → 6 2P3 2 F 0 = 3,4,5 transitions, is characterised as a non-QND interaction in which the probe beam influences the measurement. The sensitivity of the rf magnetometer is determined to be ≈ 1.98 fT/ √ Hz. In addition, the exploration of the relationship between the signal to noise ratio (SNR) and probe beam detuning revealed that the SNR is constant with detuning but the larger the detuning, the higher the probe beam power needs to be to reach the optimum SNR.
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.