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Remote magnetometry using fluorescence resonance in sodium vapor (Conference Presentation)

Renu Tripathi, Kevin Heesh, Christiane Ebongue, Lorna Caesar, G. S. Pati, Anthony W. Yu, Michael A. Krainak

Published Mar 4, 2019• 0 citations

Abstract

Measurement and monitoring of geomagnetic field in the mesosphere has many potential applications such as detection of oceanic currents, mapping of large-scale magnetic structures, and study of electric-current fluctuations in the ionosphere. Remote measurement of the geomagnetic field can be performed by optical pumping of sodium atoms in the mesosphere with an amplitude modulated (AM) light at the Larmor precession frequency, and observing the magnetic resonance in fluorescence at a ground station. In this context, we have conducted an experiment in the laboratory to demonstrate remote magnetic field measurement using fluorescence signal from a sodium cell. Sodium atoms in the cell are interrogated with AM light produced by an acousto-optic modulator (AOM), introduced in the beam path of a frequency-doubled amplified sodium laser made in our laboratory. Magnetic resonance observed in sodium cell fluorescence is studied as a function of varying magnetic field intensity. Characteristics of the magnetic resonance such as linewidth and contrast (or SNR), and their dependence on optical power density, AM duty cycle, B-field orientation and intensity are studied. Theoretical modeling and experimental measurements are also carried out to determine the sensitivity of the magnetometer for realizing remote magnetometry with mesospheric sodium atoms.

DOI: 10.1117/12.2507056

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We have demonstrated a technique for remote magnetometry using fluorescence measurements in a sodium cell. This technique utilizes two-photon resonance instead of single-photon resonance to generate magnetic resonances with synchronous modulation of two laser fields. A strong magnetic resonance with its dip matching the Larmor frequency, ΩL, is produced when a magnetic field perpendicular to the laser propagation direction is applied. A magnetic field sensitivity of 45 pT/√Hz is measured at a 35% modulation duty cycle where ΩL resonance has a maximum amplitude. We have discussed ways to further improve the sensitivity closer to the shot-noise limit by making changes to our existing setup. An additional magnetic resonance at 3ΩL is also formed due to two-photon resonance, which can be utilized in determining the magnetic field orientation. We have developed a theoretical model based on density matrix equations to study the magnetic resonances under different conditions and also for validating our experimental observations.

We report experimental on-sky observations of atomic spin precession of mesospheric sodium driven by polarization modulation of a continuous-wave laser. A magnetic resonance was remotely detected from the ground by observing the enhancement of induced fluorescence when the driving frequency approached the precession frequency of sodium in the mesosphere, between 85 and 100 km altitude. The experiment was performed at La Palma, and the uncertainty (0.2 kHz) in the measured Larmor frequency (≈260 kHz) corresponded to an error in the geomagnetic field of 0.3 mG. The results are consistent with geomagnetic field models and with the theory of light-atom interaction in the mesosphere.

We report experimental results on the first on-sky observation of atomic spin precession of mesospheric sodium driven by polarization modulation of a continuous-wave laser. The magnetic resonance was remotely detected from the ground by observing the enhancement of induced fluorescence when the driving frequency approached the precession frequency of sodium in the mesosphere, between 85 km and 100 km altitude. The experiment was performed at La Palma, and the uncertainty in the measured Larmor frequency ($\approx$260 kHz) corresponded to an error in the geomagnetic field of 0.4 mG. The results are consistent with geomagnetic field models and with the theory of light-atom interaction in the mesosphere.