Elliptical polarisation magnetometer with two radiofrequency field components for parametric resonance detection in absorption
11579220 · 2023-02-14
Assignee
Inventors
Cpc classification
International classification
Abstract
A parametric resonance magnetometer is provided comprising a cell filled with an atomic gas; an optical pumping source arranged to emit a light beam in a direction of the cell; a polarization device configured so that by the effect of the light beam, the atomic gas simultaneously acquires a state aligned according to an alignment direction and a state oriented according to an orientation direction; a parametric resonance excitation source configured to generate a radiofrequency magnetic field in the cell; and a device to detect parametric resonances and to measure an absorption of the light beam by the atomic gas. The parametric resonance excitation source is configured so that the radiofrequency magnetic field consists of two components orthogonal to one another, each oscillating at its natural oscillation frequency. The two components include a component longitudinal to the orientation direction and a component longitudinal to the alignment direction.
Claims
1. A parametric resonance magnetometer, comprising: a cell filled with an atomic gas; an optical pumping source configured to emit a light beam in a direction of the cell; a polarization device configured to polarize the light beam so that by an effect of the light beam, the atomic gas simultaneously acquires a state aligned according to an alignment direction and a state oriented according to an orientation direction; a parametric resonance excitation source configured to generate a radiofrequency magnetic field in the cell; a parametric resonance device configured to detect parametric resonances and configured to measure an absorption of the light beam by the atomic gas, wherein the parametric resonance excitation source is further configured so that the radiofrequency magnetic field generated in the cell consists of two components orthogonal to one another, each oscillating at a corresponding oscillation frequency, wherein the light beam crosses the cell according to a direction of propagation, the polarization device being configured so that the alignment direction is orthogonal to the direction of propagation of the light beam and the orientation direction is longitudinal to the direction of propagation of the light beam, and the parametric resonance excitation source is further configured so that said two components comprise a component longitudinal to the orientation direction and a component longitudinal to the alignment direction.
2. The magnetometer according to claim 1, wherein the oscillation frequency of the component longitudinal to the alignment direction is higher than the oscillation frequency of the component longitudinal to the orientation direction.
3. The magnetometer according to claim 2, wherein 0.1<γB.sub.1/ω<1.2 and 0.1<γB.sub.2/Ω<1.2, with B.sub.1 being an amplitude of the component longitudinal to the alignment direction, ω/2π being the oscillation frequency of the component longitudinal to the alignment direction, B.sub.2 being an amplitude of the component longitudinal to the orientation direction, Ω/2π being the oscillation frequency of the component longitudinal to the orientation direction, and γ being a gyromagnetic ratio of the atomic gas.
4. The magnetometer according to claim 1, further comprising a photodetector configured to receive the light beam having crossed the cell and output a photodetection signal to the parametric resonance detection device.
5. The magnetometer according to claim 4, wherein the parametric resonance detection device is further configured to perform a synchronous detection of the photodetection signal at a corresponding harmonic of the oscillation frequency of each of the two components and at an inter-harmonic of the oscillation frequencies of the two components.
6. The magnetometer according to claim 1, wherein the polarization device is further configured to confer an elliptical polarization on the light beam.
7. The magnetometer according to claim 6, wherein the polarization device comprises a linear polarizer having a transmission axis followed by a quarter-wave plate having neutral axes, one of which is parallel to the alignment direction.
8. The magnetometer according to claim 7, wherein the transmission axis of the linear polarizer is rotated by an angle between 21° and 31° with respect to one of the neutral axes of the quarter-wave plate that is parallel to the alignment direction.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Other aspects, objects, advantages and features of the invention will become more apparent upon reading the following detailed description of preferred embodiments thereof, provided as a non-limiting example, and made with reference to the appended drawings wherein:
(2)
DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS
(3) Referring to
(4) The cell is illuminated by an optical pumping source 2 adapted to emit in the direction of the cell 1 a light beam F, for example a laser beam, tuned to a pumping wavelength (thus, this beam is also referred to as pump beam). The pumping wavelength is aligned, or tuned, on the atomic transition line, for example on the line D.sub.0 at 1083 nm in the case of helium-4.
(5) Using a pump beam tuned on the atomic transition used for optical pumping allows getting rid of the undesirable effects of the phenomenon known as “light-shift”, in particular so-called vector “light-shift” that appears when the atoms are illuminated by a circularly polarised light non-resonant with an atomic transition and also so-called tensor “light-shift” that results in coupling the alignment and the orientation.
(6) In the case where the sensitive element is helium-4, the magnetometer 10 further includes a high-frequency (HF) discharge system, comprising a HF generator 4 and overvoltage coils 5, to set the atoms of the atomic gas in an energised state where they could undergo an atomic transition when they are illuminated by the laser beam, typically in the metastable state 2.sup.3S.sub.1.
(7) In the context here described, the light beam F used for pumping (i.e. the preparation of the atomic states necessary for a measurement of the magnetic field) is also used as a probe beam for the detection of the atomic states induced by pumping, its absorption by the atoms actually carrying information on the value of the components of the static magnetic field to which the cell is subjected. The beam F propagates according to a direction of propagation which is oriented according to the direction {right arrow over (z)} in
(8) The magnetometer also comprises a parametric resonance excitation circuit which includes a radiofrequency generator 8 which supplies Helmholtz coils 7 with orthogonal axes which surround the cell in order to generate a parametric resonance excitation magnetic field, also referred to as radiofrequency magnetic field. Moreover, the magnetometer comprises a parametric resonance detection device 6 configured so as to measure the absorption of the light beam by the atomic gas and a photodetector 10 arranged so as to receive the light beam having crossed the cell and to output a photodetection signal to said parametric resonance detection device 6.
(9) The cell 1 and coils 7 set is placed within a passive magnetic shield made of a μ-metal in order to isolate it from ambient magnetic fields (terrestrial field and electromagnetic disturbances), which are too strong to meet the physical condition γB.sub.0<<Γ necessary for the apparition of the parametric resonances where γ refers to the gyromagnetic ratio of the atoms of the atomic gas and Γ the relaxation rate of the atoms of the atomic gas.
(10) The magnetometer may also comprise a closed-loop servo-control system of the magnetometer to constantly subject the sensitive element to a zero total magnetic field. The servo-control system comprises a regulator 9 coupled to the detection device 6 and which injects a current in the Helmholtz coils 7 in order to generate a compensation magnetic field Bc such that the sum Bc+B.sub.0 is permanently maintained at zero.
(11) Alternatively, the magnetometer may be operated in open loop, without ambient field compensation.
(12) The light beam F is polarised by means of a device 3 for polarising the light beam. This device 3, interposed between the optical pumping source 2 and the cell 1 or directly integrated to the optical pumping source, allows conferring a very specific polarisation on the light before it crosses the cell.
(13) In one embodiment, the light beam is a laser beam conveyed by an optical fibre at the end of which it diverges with an angle depending on the numerical aperture of the used fibre. This divergent beam is collimated before crossing the cell, for example using a convergent lens. At the outlet of this set enabling the collimation, the beam passes through the polarisation device 3. After passage in the cell, the laser beam passes through a so-called projection lens which allows expanding (divergent lens) or focusing (convergent lens) the collimated beam on the photodetector 10.
(14) In the context here described, the polarisation device 3 is more particularly configured so that, by the effect of the light beam, the atomic gas simultaneously acquires a state aligned according to an alignment direction and a state oriented according to an orientation direction.
(15) Thus, the polarisation device 3 may be configured so as to simultaneously confer a linear polarisation and a circular polarisation on the light beam, the beam emitted in the direction of the cell being therefore elliptically polarised.
(16) An effect of such an elliptical polarisation is the preparation of the atomic states according to an orientation and alignment resulting from both a linear polarisation and a circular polarisation. Thus, the state acquired by the atoms of the atomic gas in the cell is both aligned (according to an alignment direction) and oriented (according to an orientation direction), proportionally to the degree of ellipticity of the polarisation. More particularly, the state of the atoms is divided into an orientation along the direction of propagation of the light beam and an alignment directed mainly along the vertical axis, orthogonal to the direction of propagation of the pump beam (the alignment including a small proportion longitudinal to the direction of propagation of the pump due to the pumping with a light partially circularly polarised on an atomic state with a total kinetic moment greater than or equal to 1). Thus, the orientation direction is longitudinal to the direction of propagation (i.e. according to the direction {right arrow over (z)}) whereas the alignment direction is orthogonal to the direction of propagation (according to the direction {right arrow over (x)} in the reference frame of
(17) Such a polarisation device 3 conferring an elliptical polarisation on the light beam may comprise a linear polariser and a quarter-wave plate whose neutral axes are rotated by an angle different from 45° with respect to the transmission axis of the linear polariser. More particularly, one of neutral axes (slow or rapid) of the quarter-wave plate is parallel to the alignment direction (i.e. the direction {right arrow over (x)}). The linear polariser is placed before the quarter-wave plate on the optical path. Thus, the polarisation device 3 confers an elliptical polarisation characterised by a semi-major axis of the ellipse which lies aligned with that of the neutral axes of the quarter-wave plate which forms the smallest angle with the transmission axis of the linear polariser place before the quarter-wave plate. For example, a configuration for which the semi-major axis of the ellipse lies aligned with the alignment direction is selected.
(18) Preferably, the transmission axis of the linear polariser is rotated by an angle comprised between 21° and 31°, for example 26°, with respect to that of the neutral axes of the quarter-wave plate that is parallel to the alignment direction. Thus, the semi-major axis of the ellipse lies aligned with the alignment direction. Moreover, the selection of this angle range is justified by a +/−5° inaccuracy on setting of the 26° value in miniature configurations where this angle is set by bonding. The inventors were able to check up in laboratory configurations, where this angle could be set accurately, that this angle range allows obtaining signals with a particularly favourable sensitivity according to criteria that will be detailed hereinafter.
(19) Alternatively, the polarisation device 3 may be configured to alternately confer a linear polarisation and then a circular polarisation. Such a polariser 3 may comprise a linear polariser and an electrically-controllable delay plate. This plate is modulated with a square wave so that, during the half-periods where its output level is high (denoted SH), it behaves like a quarter-wave plate, while during the low-level half-periods (denoted SL), it behaves like a delay-less plate or like a half-wave plate. Thus, during the half-periods SH, the beam emitted in the direction of the cell is circularly polarised and the atomic states in the cell are oriented. And during the half-periods SL, the beam emitted in the direction of the cell is linearly polarised and the atomic states in the cell are aligned. The dynamics of the cells being dominated by a relaxation time in the range of 1 ms, by adopting a more rapid SL/SH modulation, the atoms are placed in a superimposition of the aligned and oriented states. Hence, this is not a successive passage through these two states but rather the concomitance of the two properties.
(20) In contrast with the magnetometer described in the application EP 3 524 990 which requires a radiofrequency magnetic field including three components, the magnetometer here described requires only two. Thus, the parametric resonance excitation source is configured so that the radiofrequency magnetic field generated in the cell by means of the Helmholtz coils 7 has two components orthogonal to one another and each oscillating at its natural oscillation frequency. More particularly, said components comprise a component longitudinal to the orientation direction (according to {right arrow over (z)}) and a component longitudinal to the alignment direction (according to {right arrow over (x)}).
(21) Thus, a first component B.sub.1{right arrow over (x)} cos(ωt) with a frequency ω/2π is applied according to the main direction of the atomic alignment and a second component B.sub.2{right arrow over (z)} cos(Ωt) with a frequency Ω/2π is applied according to the orientation direction of the atoms (i.e. direction of propagation of the pumping beam). The first component controls the evolution of the atomic orientation (and of the alignment residue directed according to this orientation direction) and enables the measurement of the component of the magnetic field parallel to the main direction of the atomic alignment. The second component controls the evolution of the transverse atomic alignment and enables the measurement of the component of the magnetic field parallel to the direction of propagation of the pump beam.
(22) Preferably, the frequency ω/2π of the first component is higher than the frequency Ω/2π of the second component so that the dynamics of the atomic states optically pumped by each component of the radiofrequency field are time-decoupled. Thus, the dynamics in the rapid component at the frequency ω/2π must be “too” rapid for the slow component at the frequency Ω/2π in order not to affect the dynamics of the spins in the latter. In turn, these dynamics in the slow component must be “too” rapid in comparison with the natural dynamics of the spins (relaxation rate Γ) and that in the static field B.sub.0 to be measured (Larmor frequency f.sub.L=γB.sub.0).
(23) Furthermore, the rapid RF field component (ω) is to be applied according to the axis {right arrow over (x)} (direction of the semi-major axis of the ellipse) and the slow component (Ω) is to be directed according to the axis {right arrow over (z)}, the direction of propagation of the laser beam. The inventors were actually able to check that, should the RF field components have to be reversed, the sensitivities at the obtained isotropic operating point would be lower.
(24) Preferably, the ratios γB.sub.1/ω and γB.sub.2/Ω (with γ the gyromagnetic ratio of the atomic gas, for example γ=2π×28 rad/nT.Math.s for the state 2.sup.3S.sub.1 of helium-4) which control the sensitivities of the measurement of the magnetic field are set such that 0.1<γB.sub.1/ω<1.2 and 0.1<γB.sub.2/Ω<1.2. In such a manner, the existence of an operating point where the sensitivity of each measurement axis represents between 30% and 37% of the sum of the sensitivity on the three axes, for angles from 21° to 31°, is ensured.
(25) The parametric resonance detection device 6 configured so as to measure the absorption of the light beam by the atomic gas measures the light transmitted by the atoms on the elliptical base, i.e. only with a photodetector and possibly collimation optics at the outlet of the cell, and so without the addition of intermediate optical elements allowing separating the linear and circular bases of light between the cell containing the sensitive element and the photodetector as is the case for the magnetometer described in the application EP 3 524 990.
(26) To determine a measurement of each of the three components of the ambient magnetic field, the parametric resonance detection device 6 is configured so that a synchronous detection of the photodetection signal at a harmonic of each of the oscillation frequencies (ω and Ω) and at an inter-harmonic of said oscillation frequencies (ω±Ω). A demodulation of the photodetection signal at the frequency ω enables a measurement of the component of the magnetic field parallel to the main direction of alignment of the atoms (B.sub.x), a demodulation at the frequency Ω enables a measurement of the component of the magnetic field parallel to the direction of propagation of the pump beam (B.sub.z). The last component of the magnetic field (B.sub.y), orthogonal to the direction of application of the two components of the radiofrequency field, is obtained by demodulating the photodetection signal at one of the inter-harmonics ω±Ω.
(27) In one embodiment, the angle between the natural axis of the quarter-wave plate parallel to the direction {right arrow over (x)} and the transmission axis of the linear polariser is set at 26°, the frequency ω/2π is set at 40 kHz, the frequency Ω/2π is set at 15 kHz. The ratios γB.sub.1/ω and γB.sub.2/Ω are respectively set at 0.97 and 0.76, corresponding to amplitudes of the components of the radiofrequency field B.sub.1=1385 nT and B.sub.2=407 nT.
(28) In another embodiment, the angle between the natural axis of the quarter-wave plate parallel to the direction {right arrow over (x)} and the transmission axis of the linear polariser is set at 26°, the frequency ω/2π is set at 40 kHz, the frequency Ω/2π is set at 9 kHz. The ratios γB.sub.1/ω and γB.sub.2/Ω are respectively set at 0.76 and 0.99, corresponding to amplitudes of the components of the radiofrequency field B.sub.1=1085 nT and B.sub.2=530 nT. In this embodiment, the sensitivity at each axis is slightly lower than is the case in the previous example but it is perfectly isotropic.