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Frontiers in Optics

Sensitivity Comparison of Mx and Frequency Modulated Bell-Bloom Cs Magnetometers in a Micro-Fabricated Cell

Ricardo Jiménez-Martínez, Clark Griffith, Svenja A. Knappe, John E. Kitching

Published Jan 1, 2008• 1 citation
Publisher

Abstract

We compare the sensitivity of two optically pumped atomic magnetometers, Mx and frequency-modulated Bell-Bloom magnetometer implemented in a single pump-probe beam configuration and using a microfabricated vapor cell filled with 133Cs and N2 buffer gas.
DOI
10.1364/fio.2008.jwa13

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We compare the sensitivity performance of two optically pumped atomic magnetometers based on the Mx and frequency-modulated bell-bloom configurations. The Cs magnetometers are implemented using a millimeter-scale microfabricated vapor cell. It is found that the Bell-Bloom magnetometer yields sensitivities similar to those of the Mx magnetometer when operated under equally optimized conditions. The Bell-Bloom approach offers several attractive features over the Mx approach for chip-scale magnetometer instrumentation.
The fabrication and performance of a miniature optically pumped atomic magnetometer constructed with microfabricated components are discussed. This device measures the spin precession frequency of Rb87 atoms to determine the magnetic field by use of the Mx technique. It has a demonstrated sensitivity to magnetic fields of 5pT∕Hz1∕2 for a bandwidth from 1to100Hz, nearly an order of magnitude improvement over our previous chip-scale magnetometer. The 3dB bandwidth has also been increased to 1kHz by reconfiguring the miniature vapor cell heater.
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 demonstrate a way of operating an optically pumped magnetometer with miniaturized cesium cell using the light-narrowing effect. The magnetometer setup shows improvement of shot-noise-limited sensitivity (42fT/Hz42\phantom{\rule{0.16em}{0ex}}\mathrm{fT}/\sqrt{\mathrm{Hz}}42fT/Hz​ in a cell of only 9.3mm39.3\phantom{\rule{0.16em}{0ex}}{\mathrm{mm}}^{3}9.3mm3 volume) due to the suppression of spin-exchange relaxation to a large extent and the use of a strikingly increased fraction of alkali-metal atoms for signal generation, working even in $\ensuremath{\mu}\mathrm{T}$ magnetic fields, by using only a single high-intensity laser beam both for pumping and probing of atomic spins.
We compare the performance of two methods for the synchronization of the atomic spins in optically pumped magnetometers: intensity modulation of the pump light and the classical M(x) method using B(1) field modulation. Both techniques use the same set-up and measure the resulting features of the light after passing a micro-fabricated Cs cell. The intensity-modulated pumping shows several advantages: better noise-limited magnetic field sensitivity, misalignment between pumping and spin synchronization is excluded, and magnetometer arrays without any cross-talk can be easily set up.
We describe an optically pumped 87Rb magnetometer with 5 fT/Hz(1/2) sensitivity when operated in the spin-exchange relaxation free (SERF) regime. The magnetometer uses a microfabricated vapor cell consisting of a cavity etched in a 1 mm thick silicon wafer with anodically bonded Pyrex windows. The measurement volume of the magnetometer is 1 mm3, defined by the overlap region of a circularly polarized pump laser and a linearly polarized probe laser, both operated near 795 nm. Sensitivity limitations unique to the use of microfabricated cells are discussed.