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Fourth Seminar on Novel Optoelectronic Detection Technology and Application

An omnidirectional measurement technology of CPT magnetometer based on coupling of the dark state

Xiao Nan Zhang, Jun Kou, Jie Li, ZONG KANG WEI, Zhang Ren

Published Feb 20, 2018• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.2315410

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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.
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.
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.
The combination of unmanned aerial vehicles and atomic magnetometers can be used for detection applications such as mineral resource exploration, environmental protection, and earthquake monitoring, as well as the detection of sunken ships and unexploded ordnance. A dark-resonance atomic magnetometer offers the significant advantages of a fully optical probe and omnidirectional measurement with no dead zones, making it an ideal choice for airborne applications on unmanned aerial vehicles. Enhancing the sensitivity of such atomic magnetometers is an essential task. In this study, we sought to enhance the sensitivity of a dark-state resonance atomic magnetometer. Initially, through theoretical analysis, we compared the excitation effects of coherent population trapping (CPT) resonance on the D1 and D2 transitions of 133Cs thermal vapor. The results indicate that excitation via the D1 line yields an increase in resonance contrast and a reduction in linewidth when compared with excitation through the D2 line, aligning with theoretical predictions. Subsequently, considering the impact of various quantum system parameters on sensitivity, as well as their interdependent characteristics, two experimental setups were developed for empirical investigation. One setup focused on parameter optimization experiments, where we compared the linewidth and contrast of CPT resonances excited by both D1 and D2 transitions; this led to an optimization of atomic cell size, buffer gas pressure, and operating temperature, resulting in an ideal parameter range. The second setup was employed to validate these optimized parameters using a coupled dark-state atom magnetometer experiment, achieving approximately a 10-fold improvement in sensitivity.
A pocket coherent population trapping (CPT) atomic magnetometer scheme that uses a vertical cavity surface emitting laser as a light source is proposed and experimentally investigated. Using the differential detecting magneto–optic rotation effect, a CPT spectrum with the background canceled and a high signal-to-noise ratio is obtained. The experimental results reveal that the sensitivity of the proposed scheme can be improved by half an order, and the ability to detect weak magnetic fields is extended one-fold. Therefore, the proposed scheme is suited to realize a pocket-size CPT magnetometer.
This paper reports experimental investigations of the spectral response of synchronous coherent population trapping (CPT) for magnetometry applications. Synchronous CPT is demonstrated using resonant two-photon excitation with a modulated bichromatic laser field in the 87RbD1^{87}\mathrm{Rb} {D}_{1}87RbD1​ manifold. Spectra containing multiple magnetic resonances are produced due to two-photon excitations created by various frequency components of the modulated beam. Theoretical modeling based on the atomic density-matrix equations is used to simulate the spectral response of synchronous CPT. Our results show potential advantages of synchronous CPT over synchronous optical pumping for magnetic field measurement. Magnetic resonance signals produced by synchronous CPT can be used in eliminating the dead-zone problem and also in determining the magnetic field direction.