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Zhongguo kexue. Wulixue Lixue Tianwenxue

Four-peak tracking autonomous omni-directional CPT atomic magnetometer

XueFeng WANG, YiCheng DENG, XiangDong LU, QiangFeng XU, ChongTai LIU

Published Sep 4, 2024• 0 citations

Abstract

Omni-directional measurement of the geomagnetic field in satellites is necessary. Coherent population trapping (CPT) atomic magnetometer is theoretically capable of omni-directional measurement of magnetic fields, but it requires a reference signal in the direction of the magnetic field. To solve this problem, a new control method was proposed in this paper to trap four CPT signals. First, the control system demodulates the CPT differential signals with n=–3, n=–2, n=+2, and n=+3 by a time-division frequency modulation. Second, these CPT differential signals are fused to obtain two new feedback signals: one feedback signal used to calibrate the central frequency between n=±2 or n=±3, and the other feedback signal used to adjust the frequency between the central frequency and the CPT signals with n=±2 or n=±3. When a change in the magnetic field direction causes CPT signals with n=±2 or ±3 to disappear, the two feedback signals remain unaffected. Therefore, this control method does not require a fluxgate magnetometer to provide the magnetic field direction for mode switching and can obtain continuous magnetic data without losing lock in the range of 0°–360°. This means that it is an autonomous omni-directional CPT atomic magnetometer.

DOI: 10.1360/sspma-2024-0248

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Omni-directional measurement of the geomagnetic field in satellites is necessary. Coherent population trapping (CPT) atomic magnetometer is theoretically capable of omni-directional measurement of magnetic fields, but it requires a reference signal in the direction of the magnetic field. To solve this problem, a new control method was proposed in this paper to trap four CPT signals. First, the control system demodulates the CPT differential signals with n=–3, n=–2, n=+2, and n=+3 by a time-division frequency modulation. Second, these CPT differential signals are fused to obtain two new feedback signals: one feedback signal used to calibrate the central frequency between n=±2 or n=±3, and the other feedback signal used to adjust the frequency between the central frequency and the CPT signals with n=±2 or n=±3. When a change in the magnetic field direction causes CPT signals with n=±2 or ±3 to disappear, the two feedback signals remain unaffected. Therefore, this control method does not require a fluxgate magnetometer to provide the magnetic field direction for mode switching and can obtain continuous magnetic data without losing lock in the range of 0°–360°. This means that it is an autonomous omni-directional CPT atomic magnetometer.

We have set up a coherent population trapping (CPT)-based magnetometer prototype with the D1 line of ⁸⁷Rb atoms. The dichromatic light field is derived from a fiber electro-optic modulator (FEOM) connected to an external cavity laser diode. A CPT resonance signal with a 516 Hz linewidth is observed. By feeding back the derivative of the resonance curve to the FEOM with a proportional integral controller, of which the voltage output is directly converted to the measured magnetic field intensity, the resonance peak is locked to the environmental magnetic field. The measurement data we have achieved are well matched with the data measured by a commercial fluxgate magnetometer within 2 nT, and the sensitivity is better than 8 pT/√Hz in a parallel B field.

Coherent population trapping(CPT)magnetometer is based on coherent population trapping coherent dark states,using an all-optical method to measure the magnetic field with high accuracy.In our experiment,we used the method of injection locking to get the two color lasers with locked phase and large frequency difference,which equald to the hyperfine splitting of the cesium atoms′ground states,and observed the CPT transmission signals.We added the longitudinal magnetic field which was along the laser propagation direction,and observed the widen CPT transmission peak and the splitting signals.For the magnetic field of medium intensity,we got the magnetic field value by measuring the splitting spacing,and for the weak region,the value could be obtained by the relation of current and magnetic field.In our experiment,we could measure the static field with several nT.

The magnetometer which is based on the coherence population trapping theory, works by exciting atoms with two modulated coherent light fields and obtaining magnetic resonance signals containing magnetic field intensity information. In view of the problem of measuring blind area in this scheme, the direction characteristics of CPT resonance signal are analyzed. A dark state coupling measurement scheme based on two groups of modulated light fields is proposed. The system performance based on this scheme is verified by the all-optical sensing experiment system. Experimental results show that the resonance signal with higher signal to noise ratio could be obtained by using the dark state coupling measurement technology. This effectively eliminates the blind area of the CPT magnetometer and avoids losing lock of system. As a result, sufficient system stability performance could be obtained in omnidirectional measurement of magnetic field.

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.

The method of operation of an atomic magnetometer based on coherent population trapping (CPT) without any requirement of radio frequency scanning is demonstrated. Using a hybrid approach comprising of polarization rotation and tailored transmission due to CPT states, the simultaneous operation of the device as atomic frequency standard as well as atomic magnetometer is investigated. The magnetometer sensitivity can be further improved by utilizing the inherent option of magnetic field assisted synchronized radio frequency scanning.

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.