Abstract
Optically-pumped magnetometers (OPM) based on parametric resonance allow real-time tri-axial measurement of very small magnetic fields with a single optical access to the gas cell. Most of these magnetometers rely on circularly polarized pumping light. We focus here on the ones relying on linearly polarized light, yielding atomic alignment. For these magnetometers we investigate three second order effects which appear in the usual regimes of operation, so to clarify if they translate to metrological problems like systematic errors or increased noise. The first of these effects is the breakdown of the three-step approach when the optical beam has a large intensity. The second one is the breakdown of the rotating wave approximation when the frequencies of the RF fields are not much larger than the rates of other atomic processes. The third one is the tensor light-shift which appears when the light is slightly detuned from resonance. This work should help to clarify the accuracy reachable with OPM, which is an important question notably for medical imaging applications.
Highlights
These last years, optically pumped magnetometers (OPM) operating in very low magnetic fields have reached excellent sensitivities [1,2,3]
6 Discussion and conclusion We have studied above three effects which are disregarded in the usual analysis of parametric resonance magnetometers
For optimizing the signal-to noise ratio of parametric resonance magnetometers (PRM) it is usual to raise the pump intensity well above the limits of the three-step approach. This is a situation which has been broadly addressed in the field of non-linear magneto-optical rotation (NMOR) [51]
Summary
These last years, optically pumped magnetometers (OPM) operating in very low magnetic fields have reached excellent sensitivities [1,2,3]. For metastable helium-4 [22,23,24], this configuration is advantageous, showing better sensitivities in a multi-axis configuration, and no vector light-shifts [25] Both for orientation and alignment-based PRM two approximations are usual for obtaining closed-form expressions for the photodetection signals:. Linearly-polarized light detuned from an optical transition causes tensor light-shifts (TLS) [30,31,32] It is not clear if such TLS could translate into systematic errors or increased noise for an alignment-based magnetometer operating in very low magnetic fields. The whole setup is enclosed in a three-layer cylindrical magnetic shielding made of mumetal, of 12 cm inner diameter
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