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Applied Optics

Optical frequency dependence of the light shift effect for vector magnetometry with cesium

Janet W. Lou, Geoffrey A. Cranch

Published Jan 28, 2020• 2 citations
Publisher

Abstract

Atomic vapor magnetometers have demonstrated very high sensitivity to the magnitude of the magnetic field. Vector field measurements are possible using bias fields applied to the vapor. For remote operation, the bias field can be generated using the optical light shift (LS) effect created with an optical beam delivered through optical fiber. Here, it is shown that the optical frequency of the LS effect for an optically pumped vapor vector magnetometer causes an angular error in the measured field orientation when it is tuned near the absorption resonances, countering the benefit of higher response. This angle-shifting effect is dependent on the LS beam power. The LS response, normalized to the LS power, is higher for lower powers due to a saturation effect but also shows a dependence on the LS optical frequency. We show that it is necessary to carefully choose the LS optical frequency and power to maximize the LS effect while avoiding potential angular error.

DOI
10.1364/ao.379800

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Atomic vapor magnetometers have demonstrated very high sensitivity to the magnitude of the magnetic field. Vector field measurements are possible using bias fields applied to the vapor. For remote operation, the bias field can be generated using the optical light shift (LS) effect created with an optical beam delivered through optical fiber. Here, it is shown that the optical frequency of the LS effect for an optically pumped vapor vector magnetometer causes an angular error in the measured field orientation when it is tuned near the absorption resonances, countering the benefit of higher response. This angle-shifting effect is dependent on the LS beam power. The LS response, normalized to the LS power, is higher for lower powers due to a saturation effect but also shows a dependence on the LS optical frequency. We show that it is necessary to carefully choose the LS optical frequency and power to maximize the LS effect while avoiding potential angular error.
The optical light shift effect is an all-optical technique to create magnetic bias fields to determine the vector components of the magnetic field using a scalar optically pumped magnetometer. We show that the measurement of the induced bias fields is affected by the response pattern of the scalar magnetometer. The sensitivity of the magnetometer affects the minimum total field resolution as well as the angular resolution of the vector measurement. Three configurations of multiple-laser, optically pumped cesium vapor vector magnetometers are characterized for their spatial sensitivity patterns. Each configuration has a unique response level as a function of the relative magnetic field orientation. Thus, the formulations used for angular determination are dependent on the magnetometer configuration, and the angular resolution of the vector measurement is dependent on the relative angle between the magnetic field and the optical probe beam. The spatial dependence of the light shift signals is measured, and the equations describing their responses are presented for each configuration. Equations for determining the field angles from the bias field responses are derived.
Vector field measurement is demonstrated with an all-optical scalar atomic magnetometer using intrinsic parameters related to its scalar operation. The Bell-Bloom type atomic magnetometer measures the Larmor precession of cesium atoms through on-resonant absorption of a probe beam. While the AC component of the probe signal is used for the field magnitude measurement, the probe DC signal contains information about the polar angle, defined as the angle between the magnetic field and the probe beam. Additional polar angle information is obtained from the light-shift-induced magnetic field caused by the frequency modulation of the probe beam. With a measurement time of 100 ms, better than 0.02° sensitivity has been achieved using a commercial miniaturized sensor at the optimal sensor orientation. The angle measurement accuracy is checked against an optical encoder over the entire polar angle range of 0°–180°. Better than 1° error is recorded over most set polar angles. Azimuthal angle measurement is also exhibited with two orthogonally oriented sensors.
Vector field measurement is demonstrated with an all-optical scalar atomic magnetometer using intrinsic parameters related to its scalar operation. The Bell-Bloom type atomic magnetometer measures the Larmor precession of cesium atoms through on-resonant absorption of a probe beam. While the AC component of the probe signal is used for the field magnitude measurement, the probe DC signal contains information about the polar angle, defined as the angle between the magnetic field and the probe beam. Additional polar angle information is obtained from the light-shift-induced magnetic field caused by the frequency modulation of the probe beam. With a measurement time of 100 milliseconds, better than 0.02 degree sensitivity has been achieved using a commercial miniaturized sensor at the optimal sensor orientation. The angle measurement accuracy is checked against an optical encoder over the entire polar angle range of 0 to 180 degrees. Better than 1 degree error is recorded over most set polar angles. Azimuthal angle measurement is also exhibited with an additional orthogonally oriented sensor.
The frequency shifts of a self-oscillating magnetometer with cesium vapor are studied quantitatively by solving rate equations for cesium atoms undergoing the action of a circularly polarized pumping light beam. Particular attention is given to the frequency shifts due to the line asymmetry of the rf spectrum in the light transmitted through the vapor. The line asymmetry varies with the orientation of the light beam relative to the field to be measured, the intensity of the light, and the temperature of the absorption cell. As an effect of a buffer gas, two extreme cases are considered, i.e., the optically excited state is in the complete mixing state and in the nonmixing state. The results show the fact that the frequency shifts depend largely on the collisional mixing in the excited state and on the sense of the circular polarization of the light. In order to reduce the frequency shifts, a new type of magnetometer, called the symmetrized spectrum magnetometer, is discussed, in which two circularly polarized light beams with opposite sense are simultaneously applied to a partitioned absorption cell.
The light shift is an important error source for atomic magnetometers. It would vanish if the frequency of the circularly polarized light is tuned to the optical resonance center. For high-density alkali-metal atomic magnetometers, the vapor cell has a large optical depth near the resonance, resulting in the difficulty in obtaining the exact resonance center by measuring the optical absorption line. In order to solve this problem, we propose a method to tune the pump light to a frequency where the shift is minimum based on the magnetic field zeroing technique. In addition, we also compare our results with the fitting center of the optical absorption line based on the measurement data far detuned from the resonance. The experimental results show that our method achieves a better suppression result of the light shift.