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

A magnetometer based on coherent population beating

Li Liu, Yigen Wang, Xiao-Na Zhao, Yu-Xin Zhuang, Zhong Wang

Published Apr 1, 2015• 0 citations
Publisher

Abstract

We proposed a novel magnetic field measurement method extended from the CPT method. It is based on the coherent population beating (CPB) phenomenon. CPB occurs in a typical three-level system, when the frequency difference of the two pump laser fields have a detuning from the ground states splitting, and the CPB oscillation frequency is equal to the detuning. We are able to detect the beat frequency shift with the external magnetic field changing via digital processing, thus we can acquire the Zeeman frequency shift and then calculate external magnetic field intensity accurately.

DOI
10.1109/fcs.2015.7138848

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We proposed a novel magnetic field measurement method extended from the CPT method. It is based on the coherent population beating (CPB) phenomenon. CPB occurs in a typical three-level system, when the frequency difference of the two pump laser fields have a detuning from the ground states splitting, and the CPB oscillation frequency is equal to the detuning. We are able to detect the beat frequency shift with the external magnetic field changing via digital processing, thus we can acquire the Zeeman frequency shift and then calculate external magnetic field intensity accurately.
We demonstrated an atomic clock based on coherent population beating (CPB). CPB occurs in typical Λ system, which enables us to obtain the beat frequency of the radio frequency for laser modulation and the hyperfine splitting. The beat frequency and its fluctuation is detected and accurately measured through digitized processing for compensating the shift of the radio frequency. It enables us to achieve a standard frequency which directly reference to the hyperfine transition. In our preliminary experiment an Allan Deviation of 3e-12 at 1000s is observed. CPB atomic clock eliminates the phase locking loop, and can be easily digitized and integrated.
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.
Coherent population beating (CPB) phenomenon occurs in a typical three-level system. When the frequency difference between two coherent pumping laser fields has a certain detuning from the ground-state hyperfine splitting, the excited state population will experience a transient oscillation before reaching equilibrium, and the oscillation frequency is equal to the detuning. The CPB phenomenon enables us to directly obtain the beat frequency between the measured radio frequency (RF) signal and the atomic transition frequency. Then we can get the standard frequency by compensating the beat frequency to the RF. We propose a scheme to implement atomic clock based on the CPB phenomenon in 2009, and the scheme has been implemented. When this effect is used to achieve an atomic clock, the frequency stability is directly related to the amplitude and SNR (signal to noise ratio) of the CPB signal. Influence of the ground-state hyperfine sublevels' coherence on CPB signal is theoretically simulated and experimentally investigated in this paper. A formula of the CPB signal is derived by using the semi-classical model of the interaction of atoms with light, and the theoretical simulation is done using the formula obtained. In the experiment two coherent pumping laser fields are used to interact with 87Rb atoms. A CPB process includes the coherence build-up and the CPB stimulation. The coherence of the ground-state hyperfine sublevels is achieved by controlling the pumping time of the coherent laser fields that are resonant to the ground-state hyperfine sublevels. With this method, the relationship between CPB signal and coherence of the ground-state hyperfine sublevels can be observed. Result shows that the amplitude of CPB signal is proportional to the ground-state hyperfine sublevels' coherence. The hign quality CPB signal can be achieved when the CPB stimulation is started with a pure coherent population trapping (CPT) state. In the CPB process, the coherence build-up rate is approximately equal to the coherence decay rate. So a 50% duty cycle square wave can be used to modulate the RF, and the period of the square wave had better be twice of the decay time of the ground-state hyperfine sublevels' coherence. To improve the SNR of CPB signal and the stability of atomic frequency standard, the ground-state hyperfine sublevels' coherence must be built up, improved, and maintained before the CPB stimulation. The feasibility of applying CPB phenomenon to the weak magnetic field measurement and other applications is also discussed in this paper.
An accurate frequency control method and atomic clock based on the coherent population beating (CPB) phenomenon is implemented. In this scheme, the frequency difference of an rf and an atomic transition frequency can be digitally obtained by measuring the CPB oscillation frequency. The frequency measurement resolution of several milli-hertz can be achieved by using a 10 MHz oven controlled crystal oscillator as the reference. The expression of the Allan deviation of the CPB clock is theoretically deduced and it is revealed that the Allan deviation is inversely proportional to the signal-to-noise ratio and proportional to the line-width of coherent population trapping spectrum. We also approve that the CPB atomic clock has a large toleration of the drift of the local oscillator. In our CPB experimental system, a frequency instability of 3.0 × 10 −12 at 1000s is observed. The important feature of high frequency measurement resolution of the CPB method may also be used in magnetometers, atomic spectroscopy, and other related research.
The purpose of this communication is to summarize and compare the results obtained under the project "New all-optical systems and methods for magnetic and electromagnetic field measurement", financed by EC (contract G6RD-CT-2001-000642). The main aim of the project is investigation of the potential of different approaches based on the effect of Coherent Population Trapping (CPT) for magnetic field measurement. The investigated approaches are: (i) CPT prepared by coupling pairs of non-degenerate Zeeman sublevels belonging to the two ground-state hyperfine components of alkali atoms by means of polychromatic laser field; (ii) CPT prepared by coupling degenerate Zeeman sublevels belonging to a single ground-state hyperfine component of alkali atoms by means of single-frequency laser field of appropriate polarization at zero magnetic field (MF); and (iii) polychromatic laser field coupls pairs of non-degenerate Zeeman sublevels belonging to a single ground-state hyperfine level of alkali atoms.
In the Λ three-level system, we will make use of semi-classical density matrix method to analyze cesium (Cs) hyperfine structure of coherent population trapping (CPT) phenomenon. Coherent population trapping has been widely used in electromagnetically induced transparency, non-inversion laser, optical storage and ultra-slow speed of light fields, then strong-coupling light and weak detecting light form coherent laser light to change the hyperfine structure energy levels of particles number distribution, which impacts on laser pumping magnetic resonance. In this paper, we will discuss the theoretical analysis and system design of laser pumping cesium magnetometer, cesium atomic energy level formed hyperfine structure with the I-J coupling, the hyperfine structure has been further split into Zeeman sublevels for the effects of magnetic field. To use laser pump and RF magnetic field make electrons transition in the Zeeman sublevels to produce the results of magneto-optical double resonance and on this basis we analyze the influence of laser pumping magnetic resonance based on coherent population trapping.