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Chinese Physics B

Indirect pumping bell–bloom magnetometer

Meng-Bing Wang, Da-Fa Zhao, Guiying Zhang, Kai-Feng Zhao

Published Sep 1, 2017• 3 citations
Publisher

Abstract

When the ground state hyperfine splitting of alkali metal vapor atoms is well optically resolved, the spin coherence on one hyperfine sublevel can be generated directly or indirectly by pumping the same or the other sublevel respectively. We experimentally studied the pump power dependence of the field response of a Bell-Bloom magnetometer for the two pumping schemes in a paraffin coated 87 Rb vapor cell. We find that although the maximum field response is achieved by combining the two pumping schemes, indirect pumping alone can do nearly as good while being much simpler to operate. We have achieved a sensitivity of 100 fT/Hz 1/2 with a single indirect pump at room temperature.

DOI
10.1088/1674-1056/26/10/100701

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When the ground state hyperfine splitting of alkali metal vapor atoms is well optically resolved, the spin coherence on one hyperfine sublevel can be generated directly or indirectly by pumping the same or the other sublevel respectively. We experimentally studied the pump power dependence of the field response of a Bell-Bloom magnetometer for the two pumping schemes in a paraffin coated 87 Rb vapor cell. We find that although the maximum field response is achieved by combining the two pumping schemes, indirect pumping alone can do nearly as good while being much simpler to operate. We have achieved a sensitivity of 100 fT/Hz 1/2 with a single indirect pump at room temperature.
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.
We report an all-optical atomic vector magnetometer using dual Bell-Bloom optical pumping beams in a Rb vapor cell. This vector magnetometer consists of two orthogonal optical pumping beams, with amplitude modulations at 85Rb^{85}\mathrm{Rb}85Rb and 87Rb^{87}\mathrm{Rb}87Rb Larmor frequencies, respectively. We simultaneously detect atomic signals excited by these two pumping beams using a single probe beam in the third direction, and extract the field orientation information using the phase delays between the modulated atomic signals and the driving beams. By adding a Herriott cavity inside the vapor cell, we improve the magnetometer sensitivity. We study the performance of this vector magnetometer in a magnetic field ranging from 100 mG to 500 mG, and demonstrate a field angle sensitivity better than 10 $\ensuremath{\mu}\mathrm{rad}/{\mathrm{Hz}}^{1/2}$ above 10 Hz.
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.
We investigated the operation of an all-optical rubidium-87 atomic magnetometer with amplitude-modulated light. To study the suppression of spin-exchange relaxation, three schemes of pumping were implemented with room-temperature and heated paraffin coated vacuum cells. Efficient pumping and accumulation of atoms in the F=2 ground state were obtained. However, the sought-for narrowing of the resonance lines has not been achieved. A theoretical analysis of the polarization degree is presented to illustrate the absence of light narrowing due to radiation trapping at high temperature.
For precision atomic magnetometry, inert buffer gas is included in alkali-metal vapor cells to significantly broaden hyperfine transitions, which facilitates optical pumping and reduces diffusive relaxation, while also providing non-radiative excited state quenching. We show low-buffer gas pressure (below 50 Torr) alkali vapor cells with resolved hyperfine manifolds can also yield high-performance magnetometers. For high polarization in 87^{87}87Rb, we optically pump F=2F=2F=2 states with narrow linewidth σ+σ_+σ+​ light, while tuning a probe beam to depopulate F=1F=1F=1 states (Δν=6.8Δν= 6.8Δν=6.8 GHz from F=2F=2F=2). The probe tuning then also provides F=2F=2F=2 detection with high optical rotation and low probe broadening; we demonstrate top-bottom gradiometry, within a single 25 Torr, 0.5 cc cell, that yields an Earth's field free-precession magnetometer sensitivity of 18 fT/Hz\sqrt{\text{Hz}}Hz​ with a 1 kHz bandwidth, as well as RF magnetometer sensitivity of 12 fT/Hz\sqrt{\text{Hz}}Hz​ in a small band about 110 kHz.