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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Amplitude Dependence on Probe Frequency Detuning in Faraday-rotation Alkali Atomic Magnetometers

Yucheng Yang, Jingbiao Chen, Xiang Peng, Hong Mei Guo

Published Apr 1, 2019• 2 citations
Publisher

Abstract

In this article, the dependence of the magnetic resonance signal amplitude on the frequency detuning of probe light in Faraday-rotation alkali atomic magnetometers is quantitatively studied. The sensitivity of atomic magnetometers can be improved by proper probe frequency detuning because of larger Signal-to-Noise ratio (SNR).
DOI
10.1109/fcs.2019.8856087

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Abstract An atomic magnetometer (AM) is very sensitive to magnetic fields and useful in many applications. AM works by optical interrogation of orientation of alkali-atom spins in a vapor cell. Since the sensitivity characterizes the performance of AM, it should be optimized. We investigate the effect of the gain of a balanced photodetector on the sensitivity of an RF atomic magnetometer setup. While the amplitude of RF signal (BRF) varies between 0.1-10 nT, the frequency is fixed at 100 kHz. The gain of the balanced photodetector varies between 103 - 107 V/A. the noise density and AM signal are measured to calculate the signal-to-noise ratios (SNR) and the AM sensitivity. The results show that noise density increases with the gain value. The AM signal increases with both BRF as well as the gain value. SNR increases with BRF while the AM sensitivity is independent on BRF. When the gain is increased from 103 - 105 V/A, SNR increases and reaches a maximum while the AM sensitivity decreases and reaches a minimum value of 0.4 pT/√Hz. However, SNR and the AM sensitivity are almost unchanged for the gain value of 105 - 107 V/A. Therefore, the gain of balanced photodetector in our AM setup should be set sufficiently high, at least 105 V/A.
Alkali atomic magnetometer is a good choice for magnetic testing devices with very-high sensitivity. Many parameters could affect the sensitivity of the atomic magnetometer. In this paper we described a Cs atomic magnetometer operating at normal temperature. A circularly polarized pumping light is used to polarize Cs atoms, while a linear polarized probing light is used to test the magneto-caused polarization rotation. During the experiment, we found the temperature is a very important parameter for the signal-to-noise ratio(SNR) enhancement. On the one hand, the higher temperature makes the SNR higher, which is good for sensitivity. On the other hand, the higher temperature causes more frequent collisions between the Cs atoms and the Cs atom and container wall, so that the polarization is easy to be broken, which reduces its sensitivity. So there must be a proper temperature for the high-sensitivity atomic magnetometer. We measured the magnetometer linewidth and sensitivity in the experiment. At optimum temperature the magnetometer achieves magnetic field sensitivity of 3.2pT/Hz1/2.
The existence of an approximately uniform and unsaturated electron spin polarization distribution within a high-density alkali-metal vapor is considered of great importance for significantly improving the response amplitude and sensitivity properties of an atomic magnetometer. However, when a high-density alkali-metal vapor is formed, the optical depth is much larger than the value of one, resulting in the electron spin polarization gradient. In this work, it was demonstrated from both numerical simulations and experimental points of view, that by replacing the resonant pumping light with two off-resonant pumping light sources, the signal amplitude of the magnetometer can be doubled. By using this approach, the electron spin polarization gradient can be significantly suppressed and the sensitivity can be improved by more than 10%. The proposed scheme is generally applicable to various optical pumping high-density alkali-metal vapor systems, where a uniform electron spin polarization distribution is required, such as optical pumping co-magnetometers and atomic gyroscopes.
We propose a far off-resonance laser frequency stabilization method that can accurately adjust the frequency lock points based on the Faraday rotation spectroscopy. The atomic magnetometer based on the spin-exchange relaxation-free (SERF) theory needs to stabilize the frequency of the probe laser on the detuning of several gigahertz (GHz) away from the resonance of the alkali metal atom, to reduce the absorption of the probe light by the alkali metal vapor cell. We propose a laser frequency stabilization method that can accurately adjust the frequency lock points using an acousto-optic modulator based on Faraday rotation spectroscopy. We reveal the principles of the method and simulate the new Faraday rotation spectra. Besides, we study the effect of the amount of frequency shift of the acousto - optic modulator and the temperature of the alkali metal vapor cell on the frequency lock points of the spectra, and give the formula for calculating the frequency point. Our proposed laser frequency stabilization method can stabilize the laser frequency on the detuning of several gigahertz (GHz) away from the resonance of the alkali metal atom and can adjust the frequency lock points quickly and accurately. This method can be used in atomic magnetometer, degenerate Raman sideband cooling (DRSC) and two photon excitation of Rydberg states.