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Optics & Optoelectronic Technology

Laser Pumping Cs Atom High-Resolution Magnetometer of Theory Research Based on Hyperfine Structure

Caiping Zheng

Published Jan 1, 2010• 0 citations
OpenAlex

Abstract

Laser pumping magnetometers using alkali vapors as sensing medium can reach very high sensitivities in magnetic fields covering the range from the earth magnetic field to biological magnetic fields. In this paper, the theoretical analysis and system design of laser pumping cesium magnetometer, cesium atomic energy level formed hyperfine structure with the I-J coupling are discussed. The hyperfine structure is further split into Zeeman sublevels for the effects of magnetic field. The electrons transition is made by using laser pumping and RF magnetic field in the hyperfine structure. The results of magne-to-optical double resonance is obtained. The accurate value of the external magnetic field could be measured by the resonant frequency.

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Journal of the Optical Society of America B•Apr 2, 2015•20 citations
Laser pumping magnetometers using alkali vapors as sensing medium can reach very high sensitivities in magnetic fields covering the range from the earth magnetic field to biological magnetic fields. In this paper, the theoretical analysis and system design of laser pumping cesium magnetometer, cesium atomic energy level formed hyperfine structure with the I-J coupling are discussed. The hyperfine structure is further split into Zeeman sublevels for the effects of magnetic field. The electrons transition is made by using laser pumping and RF magnetic field in the hyperfine structure. The results of magne-to-optical double resonance is obtained. The accurate value of the external magnetic field could be measured by the resonant frequency.
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.
In order to availably solve the crosstalk of the traditionally optically pumped magnetometer,in which an oscillating magnetic field drives transitions between Zeeman magnetic sublevels,an all-optical cesium(Cs)atomic magnetometer based on Bell-Bloom structure has been investigated and demonstrated experimentally in this paper.When there is an external magnetic field,by analyzing the Larmor precession spectrum of the polarized ground state atoms induced by a modulated pump laser,the atomic magnetometer with high sensitivity is achieved.First,we present the operating principle of the magnetometer with amplitude modulation and set up the experimental system by means of a three-layer magnetic shield.Then in the case of 100 nT field by optimizing the experimental parameters,i.e.,the frequency and intensity of the pump laser,we have obtained the atomic precession spectrum with line width(full width at half maximum)of 22 Hz and signal noise ratio(SNR)of 3.2×104 per unit hertz band width,and then have evaluated the magnetometer with an intrinsic sensitivity of 100fT/Hz1/2 and spatial resolution smaller than 1.2cm,while the experimentally closed loop test shows that the cesium atomic magnetometer would have the sensitivity of 120fT/Hz1/2 and response time of 300 ms.
In order to study the effect of the geomagnetic field on cesium magnetometer sensitivity,the Breit-Rabi equation is used to analyze the Zeeman splitting of the F g = 4 level of Cs atoms. The emphasis is on the analysis of the nonlinear Zeeman effect on the resonant hyperfine spectrum of the all-optical atomic magnetometer in the geomagnetic field which is about 0.05 mT. The results show that the effect would cause the line-width to increase from 20 Hz to 53 Hz,and the amplitude to decrease by relatively 50%. If any accessional noise is not taken into account,our cesium magnetometer sensitivity will deteriorate from 0.3 pT / Hz1 /2to 1.6 pT / Hz1 /2. That showed a technical bottleneck of improving atomic magnetometer sensitivity in the geomagnetic field. A new technique of pumping from F g = 4 to F e = 4 is proposed to break through this bottleneck.
The evolution of the hyperfine structure magnetic sublevel populations of 87 Rb and 133 Cs atoms in resonant linearly polarized laser fields is theoretically analyzed. Analytical expressions are obtained for stationary populations of the magnetic sublevels of the ground state 5 2 S 1/2 F g = 1, F f = 2 in 87 Rb and 6 2 S 1/2 F g = 3, F f = 4 in 133 Cs under optical pumping at the F g ↔ F e = F g and F f ↔ F e = F f transitions of the D 2 lines ( n 2 S 1/2 n 2 P 3/2 ) versus the initial populations. The characteristic population stabilization time are obtained by numerical solution for the density matrix equations, depending on the intensity of the laser fields and the detuning of the optical resonance. It is shown that the alternation of the polarization direction of the laser field at the transition F f ↔ F e = F f almost completely transfers atoms to the lower sublevel of the "clock" M1 transition F g M = 0 ↔ F f M = 0 in rubidium and cesium frequency standards, which proportionally increases the signal of the recording system.
The D1 line spectrally selective pumping process in Doppler-broadened cesium is analyzed by solving the optical Bloch equations. The process, described by a three-level model with the A scheme, shows that the saturation intensity of broadened atoms is three orders of magnitude larger than that of resting atoms. The 丨Fg = 3） →丨Fe = 4） resonance pumping can result in the ground state丨Fg = 4, mF = 4） sublevel having a maximum population of 0.157 and the population difference would be about 0.01 in two adjacent magnetic sublevels of the hyperfine （HF） state Fg = 4. To enhance the anisotropy in the ground state, we suggest employing dichromatic optical HF pumping by adding a laser to excite D1 line 丨Fg = 4） → 丨Fe = 3） transition, in which the cesium magnetometer sensitivity increases by half a magnitude and is unaffected by the nonlinear Zeeman effect even in Earth＇s average magnetic field.
Most optically pumped magnetometers based on alkali atom vapor cells and pumped by a single narrow-band laser suffer from a loss of signal since atoms become trapped in the ground hyperfine states that are not coupled to the laser beam. This can be counteracted by additional optical repumping of these ground-state levels. We study hyperfine repumping using cesium vapor cells with partly overlapped ground-state splitting due to their nitrogen buffer-gas filling. We implement two methods of repumping and compare them to the conventional case of F=4 pumping: F=3 repumping with an additional repumper laser and combined pumping/repumping in the light-narrowing mode, where a single high-power laser is tuned near the F=3 transitions. All these modes are investigated for two different methods of spin-phase synchronization: the Mx and intensity modulation methods. For both methods, any kind of repumping results in a clear improvement of the magnetometer sensitivity compared to no repumping, but in the Mx mode it is more pronounced (about 50 versus 200 fT/√Hz for a 50 mm3 Cs vapor cell). The mechanisms responsible for the distinct results in the different working modes are discussed.