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Magnetic-field measurements with amplitude-modulated light and intensity correlation technique (Conference Presentation)

G. S. Pati, Kevin Heesh, Bruce Barrios, Renu Tripathi, Anthony W. Yu, Michael A. Krainak

Published Mar 4, 2019• 0 citations
Publisher

Abstract

Resonant excitation of an atomic medium with amplitude modulated (AM) light can produce synchronous optical pumping effects. Nonlinear magneto-optical rotation (NMOR) is produced with AM light when the optical pumping rate is synchronous with the Larmor frequency. Traditionally, magnetic field measurements have been performed using lock-in detection of the NMOR signal with the transmitted light. Instead of NMOR, we have studied intensity correlation between two orthogonally polarized components of the AM light passing through a rubidium vapor cell, for magnetic field measurements. We have used a pure 87Rb cell (1 cm in diameter and 2 cm in length) filled with Ne buffer gas for this study. Intensity correlation is performed by recording the intensity data from two fast photodetectors with a high bandwidth digitizer board. We have measured the dependence of the zero-delay intensity correlation on the magnetic field. We have also studied the dependence of the correlation width on the light intensity and cell temperature. Results obtained from the theoretical model are also compared with the experiment. The results suggest intensity correlation can be used as a viable technique for improving the performance of magnetic field measurements with AM light.

DOI
10.1117/12.2507057

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Resonant excitation of an atomic medium with amplitude modulated (AM) light can produce synchronous optical pumping effects. Nonlinear magneto-optical rotation (NMOR) is produced with AM light when the optical pumping rate is synchronous with the Larmor frequency. Traditionally, magnetic field measurements have been performed using lock-in detection of the NMOR signal with the transmitted light. Instead of NMOR, we have studied intensity correlation between two orthogonally polarized components of the AM light passing through a rubidium vapor cell, for magnetic field measurements. We have used a pure 87Rb cell (1 cm in diameter and 2 cm in length) filled with Ne buffer gas for this study. Intensity correlation is performed by recording the intensity data from two fast photodetectors with a high bandwidth digitizer board. We have measured the dependence of the zero-delay intensity correlation on the magnetic field. We have also studied the dependence of the correlation width on the light intensity and cell temperature. Results obtained from the theoretical model are also compared with the experiment. The results suggest intensity correlation can be used as a viable technique for improving the performance of magnetic field measurements with AM light.
A self-oscillating magnetometer based on nonlinear magneto-optical rotation using amplitude-modulated pump light and unmodulated probe light (AM-NMOR) in 87Rb has been constructed and tested towards a goal of airborne detection of magnetic anomalies. In AM-NMOR, stroboscopic optical pumping via amplitude modulation of the pump beam creates alignment of the ground electronic state of the rubidium atoms. The Larmor precession causes an ac rotation of the polarization of a separate probe beam; the polarization rotation frequency provides a measure of the magnetic field. An anti-relaxation coating on the walls of the atomic vapor cell results in a long lifetime of 56 ms for the alignment, which enables precise measurement of the precession frequency. Light is delivered to the magnetometer by polarization-maintaining optical fibers. Tests of the sensitivity include directly measuring the beat frequency between the magnetometer and a commercial instrument and measurements of Earth's field under magnetically quiet conditions, indicating a sensitivity of at least 5 pT/νHz. Rotating the sensor indicates a heading error of less than 1 nT, limited in part by residual magnetism of the sensor.
Larmor precession of laser-polarized atoms contained in antirelaxation-coated cells, detected via nonlinear magneto-optical rotation (NMOR), is a promising technique for a new generation of ultrasensitive atomic magnetometers. For magnetic fields directed along the light propagation direction, resonances in NMOR appear when linearly polarized light is frequency or amplitude modulated at twice the Larmor frequency. Because the frequency of these resonances depends on the magnitude but not the direction of the field, they are useful for scalar magnetometry. Additional NMOR resonances at the Larmor frequency appear when the magnetic field is tilted away from the light propagation direction in the plane defined by the light propagation and polarization vectors. These resonances, studied both experimentally and with a density matrix calculation in the present work, offer a convenient method of achieving additional information about a direction of the magnetic field.
We experimentally demonstrate that a squeezed probe optical field can improve the sensitivity of magnetic field measurements based on nonlinear magneto-optical rotation (NMOR) with an amplitude-modulated pump when compared to a coherent probe field under identical conditions. To realize an all-atomic magnetometer prototype, we utilize a nonlinear atomic interaction, known as polarization self-rotation, to produce a squeezed probe field. An independent pump field, amplitude-modulated at the Larmor frequency of the bias magnetic field, allows us to extend the range of most sensitive NMOR measurements to sub-Gauss magnetic fields. While the overall sensitivity of the magnetometer is rather low ( > 250 p T / H z ), we clearly observe a 15 % sensitivity improvement when the squeezed probe is used. Our observations confirm the recently reported quantum enhancement in a modulated atomic magnetometer [ Phys. Rev. Lett. 127 , 193601 ( 2021 ) PRLTAO 0031-9007 10.1103/PhysRevLett.127.193601 ].
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.
The detection of nonlinear magneto-optical rotation (NMOR) of polarized light through alkali atomic vapor is a highly sensitive technique for measuring magnetic fields. We demonstrate that when using frequency modulated light to excite the NMOR resonance, it is possible to cause the system to self-oscillate. The NMOR signal is not a simple replica of the sine wave modulation of the light, but rather contains many higher harmonics of the modulation frequency, and we implement two ways of processing the signal to recover the fundamental modulation frequency in the feedback loop and induce self-oscillation. Self-oscillation simplifies and reduces the power consumption of the electronics required to operate a magnetometer, making the NMOR technique attractive for commercialized magnetic sensors.
A magnetometric technique is demonstrated that may be suitable for precision measurements of fields ranging from the submicrogauss level to above the earth field. It is based on resonant nonlinear magneto-optical rotation caused by atoms contained in a vapor cell with antirelaxation wall coating. Linearly polarized, frequency-modulated laser light is used for optical pumping and probing. If the time-dependent optical rotation is measured at the first harmonic of the modulation frequency, ultra-narrow (\ensuremath{\sim} a few hertz) resonances are observed at near-zero magnetic fields, and at fields where the Larmor frequency coincides with half the light modulation frequency. Upon optimization, the sensitivity of the technique is expected to exceed ${10}^{\ensuremath{-}11} G/\sqrt{\mathrm{Hz}}.$