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Optics Express

Simultaneously improving the sensitivity and absolute accuracy of CPT magnetometer

Shangqing Liang, Guoqing Yang, Yunfei Xu, Qiang Lin, Zhiheng Liu, Zhengxiang Chen

Published Mar 17, 2014• 29 citations
Publisher

Abstract

A new method to improve the sensitivity and absolute accuracy simultaneously for coherent population trapping (CPT) magnetometer based on the differential detection method is presented. Two modulated optical beams with orthogonal circular polarizations are applied, in one of which two magnetic resonances are excited simultaneously by modulating a 3.4GHz microwave with Larmor frequency. When a microwave frequency shift is introduced, the difference in the power transmitted through the cell in each beam shows a low noise resonance. The sensitivity of 2pT/Hz @ 10Hz is achieved. Meanwhile, the absolute accuracy of ± 0.5nT within the magnetic field ranging from 20000nT to 100000nT is realized.

DOI
10.1364/oe.22.006837

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A new method to improve the sensitivity and absolute accuracy simultaneously for coherent population trapping (CPT) magnetometer based on the differential detection method is presented. Two modulated optical beams with orthogonal circular polarizations are applied, in one of which two magnetic resonances are excited simultaneously by modulating a 3.4GHz microwave with Larmor frequency. When a microwave frequency shift is introduced, the difference in the power transmitted through the cell in each beam shows a low noise resonance. The sensitivity of 2pT/Hz @ 10Hz is achieved. Meanwhile, the absolute accuracy of ± 0.5nT within the magnetic field ranging from 20000nT to 100000nT is realized.
We report a method to enhance the sensitivity of coherent population trapping (CPT) magnetometers using a combination of left-handed and right-handed circularly polarized light phase-delay detection and a differential detection scheme. The approach can achieve a four third-fold enhancement of the CPT dispersion signal slope and a three-fold reduction in noises. The proposed method experimentally exhibits a four third-fold magnetic field resolution enhancement in CPT open-loop measurements, and the differential method could achieve a sensitivity of 1 pT/Hz at 10 Hz and a sensitivity of 0.4 pT/Hz at 50–100 Hz in the CPT closed-loop measurement, which is a four-fold sensitivity enhancement compared to the single-transmitted CPT magnetometer.
We demonstrate the excitation and low-noise differential detection of a coherent population trapping (CPT) resonance with two modulated optical fields with orthogonal circular polarizations. When a microwave phase delay of lambda/4 is introduced in the optical path of one of the fields, the difference in the power transmitted through the cell in each polarization shows a narrow, dispersive resonance. The differential detection allows a high degree of suppression of laser-induced noise and will enable nearly shot-noise-limited operation of atomic frequency references and magnetometers based on CPT.
We demonstrate a suppression of the light shift in a Coherent-Population-Trapping (CPT) atomic magnetometer by using linearly polarized light and a differential measurement between magnetic resonances. The radio frequency that creates the optical sidebands for CPT quickly switches between two magnetic sensitive transitions, and the magnetic field is extrapolated from the difference of the center frequencies of the magnetic resonances. Light shifts and common drifts like collisional shifts can be suppressed through careful choice of measured resonances, and we show the light shift reduction by more than a factor of 20 compared to excitation with circularly-polarized light. Various limitations of the method are discussed.
A fully automated magnetometer suitable for long lasting measurement under stable and controllable experimental conditions has been implemented. The device is based on Coherent Population Trapping (CPT) effect produced by a multi-frequency excitation. It has an all optical sensor connected only by an optical fiber to the laser head. This allows truly remote sensing and minimization of the field perturbation. The CPT resonance is observed when a frequency comb, generated by a diode laser current modulation, excites a sample of Cs atoms confined in a cell with 2 Torr of buffer gas. In differential sensor configuration, allowing for common-mode noise cancellation, the sensitivity of 45pT/square root of Hz is obtained.
A glass lithium vapor cell is an atomic sensor operating at high temperature. The longitudinal magnetic-field strength is measured by coherent population trapping (CPT) in ground-state hyperfine levels. Two linearly polarized light beams are polarized orthogonally to each other. The difference of optical frequencies is tuned to generate a dark state. Additionally, a few light modes of equidistant frequency spacing are generated by modulating the difference frequency. The symmetry of the light modes and the energy levels enables simultaneous measurement of the hyperfine splitting frequency of the ground state and the magnetic-field strength. Tuning the optical frequency difference to the central CPT resonance, a sensitivity of 15 p T / H z is achieved. The time response and the sensitivity of the CPT magnetometry are studied by changing the cell temperature, the laser power, the frequency separation of the light modes, and the Zeeman frequency.
We have set up a coherent population trapping (CPT)-based atomic magnetometer experimental system worked with (87)Rb atoms, and with this system experimentally investigated the behaviour of a CPT magnetometer prototype proposed by us, which worked with the atomic energy level modulation scheme. Through systematical comparison with generally used microwave frequency modulation scheme, the experimental results reveal that the energy level modulation scheme not only extends the detection range, but also improves the sensitivity of magnetic field detection.