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Measurement Science and Technology

Lower magnetic field measurement limit of the coupled dark state magnetometer

Michaela Ellmeier, Alexander Betzler, Christoph Amtmann, Andreas Pollinger, Christian Hagen, I. Jernej, Martín Agú, Werner Magnes, Laurentius Windholz, M. K. Dougherty, Patrick Brown, Roland Lammegger

Published Jul 22, 2024• 6 citations
Publisher

Abstract

Abstract The Coupled Dark State Magnetometer (CDSM) is an optically pumped magnetometer. For the Jupiter Icy Moons Explorer mission, the CDSM and two fluxgate magnetometers are combined in the J-MAG instrument to measure the static and low frequency magnetic field in the Jupiter system. During certain calibration manoeuvres, the CDSM has to be able to measure magnetic field strengths down to 100 nT with an accuracy of 0.2 nT ( 1 σ ). At such low magnetic fields, the CDSM’s operational parameters must be carefully selected to obtain narrow resonance structures. Otherwise, the coupled dark state resonances, used for the magnetic field detection in different instrument modes, overlap and result in a systematic error. The overlap of the resonances and therefore the systematic error mainly depends on the resonance line width and the selected modulation frequencies for the detection of the resonances. We show that a line width of less than 200 Hz and selecting a modulation frequency of about the resonance line width are beneficial at magnetic field strengths B < 1.5 μ T. In this paper we demonstrate that with the found instrument settings the CDSM is able to measure magnetic field strengths below 100 nT with a systematic error less than 0.2 nT resulting from the overlap of the resonances.

DOI
10.1088/1361-6501/ad6623

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The ability to measure the magnetic fields around celestial bodies enables various insights into processes and phenomena deep within such bodies.The use of magnetometry in space missions dates back to the 1950s, and a number of different technologies have been used to measure these fields, ranging from fluxgates to Anisotropic Magneto Resistance (AMR) sensors.Optically-pumped magnetometers (OPMs) represent the state-of-the-art in magnetic field sensitivity. 1 Such sensors rely upon the interaction of laser light to spin polarise a warm vapour of neutral atoms.Detecting the precession of these spins, again with laser light, is a direct way to measure a magnetic field and is relatable to fundamental constants.Improvements in quantum technology within the last decade have led to the miniaturisation of these sensors.We have now reached the point where compact, lightweight, and low power devices offer greater sensitivity to magnetic fields than any other technology.In this work, we identify the following four key space application areas in which OPMs may offer significant advantages over existing technologies: Environments in which high sensitivity is needed at low magnetic fields (10s of nT), full vector read-out with high directional sensitivity and reduced SWaP compared to helium magnetometers, miniaturised and lightweight scalar magnetometers to complement vector fluxgate measurements, vector or scalar sensors that need high (40 dB) gradiometer performance.This work identifies OPM configurations for each area and makes a first estimate of size, weight and power requirements for each.By enabling improvements in magnetic field sensitivity, this represents an exciting prospect for precision magnetic field measurements while also saving mass and reducing complexity.

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Abstract The Coupled Dark State Magnetometer (CDSM) is an optically pumped magnetometer. For the Jupiter Icy Moons Explorer mission, the CDSM and two fluxgate magnetometers are combined in the J-MAG instrument to measure the static and low frequency magnetic field in the Jupiter system. During certain calibration manoeuvres, the CDSM has to be able to measure magnetic field strengths down to 100 nT with an accuracy of 0.2 nT ( 1 σ ). At such low magnetic fields, the CDSM’s operational parameters must be carefully selected to obtain narrow resonance structures. Otherwise, the coupled dark state resonances, used for the magnetic field detection in different instrument modes, overlap and result in a systematic error. The overlap of the resonances and therefore the systematic error mainly depends on the resonance line width and the selected modulation frequencies for the detection of the resonances. We show that a line width of less than 200 Hz and selecting a modulation frequency of about the resonance line width are beneficial at magnetic field strengths B < 1.5 μ T. In this paper we demonstrate that with the found instrument settings the CDSM is able to measure magnetic field strengths below 100 nT with a systematic error less than 0.2 nT resulting from the overlap of the resonances.
The Coupled Dark State Magnetometer (CDSM) is a new type of scalar magnetometer, which is based on two-photon spectroscopy of free alkali atoms. CDSM measurements do not depend on the sensor temperature. Furthermore, it has no dead zones, no moving parts and no excitation coils at the sensor. This paper deals with the control loops which are developed for tracking the magnetic field dependent resonances and for avoiding drifts of the carrier frequency of the laser diode used as excitation light source. It is the first time that a magnetometer of this type is operated in a control loop. The implementation of several functional blocks in a Field Programmable Gate Array (FPGA) is a first step towards miniaturization for a future space application. The magnetic field magnitude is measured by a frequency. Therefore, the Direct Digital Synthesis (DDS) principle is used for accurate signal generation. A noise floor measurement has shown no 1/f dependence near DC and 70 pT/√Hz up to the corner frequency of 3 Hz for which the CDSM is designed.
The Coupled Dark State Magnetometer (CDSM) is an optically-pumped absolute scalar magnetometer which is based on two-photon spectroscopy of free alkali atoms. Several magnetic field dependent resonances occur in the presence of an external magnetic field, reaching their maximal strengths at different angles between the magnetic field direction and the reference axis of the sensor defined by the optical path of the laser excitation field. Dependent on this angle the strongest available resonances are selected. This enables an omni-directional dead zone free measurement. The paper discusses the measurement principle, describes the technical realization of the resonance switching, investigates the influence of this transition on the magnetic field measurement and proposes a method to detect the angle of the magnetic field direction in relation to the optical axis. The results presented within this paper show that dead zone free magnetic field measurements are possible with the CDSM without the need of a complex sensor design, e.g. additional moving parts or excitation coils.
We report on detailed experimental setup and the results of an enhanced acousto-optics modulator (AOM) setup for observation of a dark state magnetometer. A Λ-type dark state based on D1 line of 87Rb:F=2→F′=1 Zeeman sub-levels with neon (Ne) buffer gas was created using a single laser and a scanning acousto-optics modulator. The technical challenges in using this method and how to overcome these difficulties are discussed, and we report on the observation of a dark state resonance with linewidth of 168 Hz and a detectable magnetic field of 9 nT. This method offers many advantages, including the creation of mutually coherent beams outside an external cavity diode laser (ECDL), where the beams are equally affected by external perturbations to the ECDL. Only factors related to the AOM dictates the difference between the two beams.
Rubidium laser optically pumped magnetometers (OPMs) are widely used magnetic sensors based on the Zeeman effect, laser pumping, and magnetic resonance principles. They measure the magnetic field by measuring the magnetic resonance signal passing through a rubidium atomic gas cell. The quality of the magnetic resonance signal is a necessary condition for a magnetometer to achieve high sensitivity. In this research, to obtain the best magnetic resonance signal of rubidium laser OPMs in the Earth’s magnetic field intensity, the experiment system of rubidium laser OPMs is built with a rubidium atomic gas cell as the core component. The linewidth and amplitude ratio (LAR) of magnetic resonance signals is utilized as the optimization objective function. The magnetic resonance signals of the magnetometer experiment system are experimentally measured for different laser frequencies, radio frequency (RF) intensities, laser powers, and atomic gas cell temperatures in a background magnetic field of 50,765 nT. The experimental results indicate that optimizing these parameters can reduce the LAR by one order of magnitude. This shows that the optimal parameter combination can effectively improve the sensitivity of the magnetometer. The sensitivity defined using the noise spectral density measured under optimal experimental parameters is 1.5 pT/Hz1/2@1 Hz. This work will provide key technical support for rubidium laser OPMs’ product development.
Summary form only given, as follows.The 30 kHz linewidth and extremely steep dispersion of a coherent dark resonance in a caesium vapor cell allows measurements of magnetic fields to about 10 nT in 0.1 s integration time, limited by stray fields. The narrow resonance should make possible a compact frequency standard. In a preliminary experiment the authors used a coherent dark resonance in a 2 cm long caesium vapor cell to measure magnetic fields with about 10-5 uncertainty. Two diode lasers (852 nm wavelength) were phase-locked onto each other with a 9.2 GHz frequency offset, corresponding to the frequency ground state separation of the two caesium levels. The difference frequency is derived from a stable tunable rf source. Absorption spectrum of the caesium coherent population trapping resonance for linear laser polarizations is illustrated; it is split into seven components by a magnetic field parallel to the laser polarization direction. The width of the individual lines is only 130 kHz due to power broadening. The steep slope of the resonance could be used to lock the frequency of the 9.2 GHz rf source. This would provide a compact frequency reference.
The ability to measure the magnetic fields around celestial bodies enables various insights into processes and phenomena deep within such bodies.The use of magnetometry in space missions dates back to the 1950s, and a number of different technologies have been used to measure these fields, ranging from fluxgates to Anisotropic Magneto Resistance (AMR) sensors.Optically-pumped magnetometers (OPMs) represent the state-of-the-art in magnetic field sensitivity. 1 Such sensors rely upon the interaction of laser light to spin polarise a warm vapour of neutral atoms.Detecting the precession of these spins, again with laser light, is a direct way to measure a magnetic field and is relatable to fundamental constants.Improvements in quantum technology within the last decade have led to the miniaturisation of these sensors.We have now reached the point where compact, lightweight, and low power devices offer greater sensitivity to magnetic fields than any other technology.In this work, we identify the following four key space application areas in which OPMs may offer significant advantages over existing technologies: Environments in which high sensitivity is needed at low magnetic fields (10s of nT), full vector read-out with high directional sensitivity and reduced SWaP compared to helium magnetometers, miniaturised and lightweight scalar magnetometers to complement vector fluxgate measurements, vector or scalar sensors that need high (40 dB) gradiometer performance.This work identifies OPM configurations for each area and makes a first estimate of size, weight and power requirements for each.By enabling improvements in magnetic field sensitivity, this represents an exciting prospect for precision magnetic field measurements while also saving mass and reducing complexity.