Three-axis optically pumped magnetometer for gradiometric measurement

Abstract

A three-axis vector optically pumped magnetometer includes a cell filled with an atomic gas subjected to an ambient magnetic field the projection of which on three rectangular coordinate axes defines three components thereof, and a photodetector arranged to receive a probe beam that passed through the cell. The photodetector includes a plurality of measurement units arranged in a plane transverse to a direction of propagation of the probe beam, the measurement units each providing a photodetection signal. The magnetometer further comprises a processing unit configured to determine, for each measurement unit and from the photodetection signal, a measurement associated with the measurement unit of each of the three components of the ambient magnetic field; calculate at least one difference between the measurements, associated with different measurement units, of a component of the magnetic field; and deliver a gradiometric measurement signal including the at least one difference calculated.

Claims

1. A three-axis vector optically pumped magnetometer, comprising: a cell filled with an atomic gas subjected to an ambient magnetic field, a projection of which on three rectangular coordinate axes defines three components thereof, a photodetector arranged so as to receive a probe beam that passed through the cell, wherein the photodetector comprises a plurality of measurement units arranged in a plane transverse to a direction of propagation of the probe beam, the measurement units each providing a photodetection signal, and a processing unit coupled to the photodetector and being configured to: determine, for each measurement unit and from the photodetection signal provided by the measurement unit, a measurement, associated with the measurement unit, of each of the three components of the ambient magnetic field; and calculate at least one difference between the measurements, associated with different measurement units, of a component of the magnetic field and deliver a gradiometric measurement signal comprising the at least one difference calculated.

2. The magnetometer according to claim 1, wherein the measurement units of the photodetector are arranged in array form and are at least 3*3 in number.

3. The magnetometer according to claim 1, wherein the photodetector is a photodiode array.

4. The magnetometer according to claim 1, wherein the photodetector is a charge coupled detector.

5. The magnetometer according to claim 4, wherein each measurement unit of the photodetector is formed by a plurality of pixels close to the charge coupled detector, the photodetection signal of each measurement unit corresponding to an average of the photodetection signals individually delivered by the pixels of the measurement unit.

6. The magnetometer according to claim 1, further comprising: a parametric resonance excitation circuit configured to induce in the cell a radio frequency magnetic field having two components each oscillating at its own oscillation frequency, an optical pumping source arranged to emit towards the cell a pump beam tuned to an atomic transition, the probe beam being identical to or distinct from the pump beam, and a polarisation device configured to impart a linear polarisation to the pump beam, wherein, to determine a measurement, associated with each measurement unit, of each of the three components of the ambient magnetic field, the processing unit is configured to perform synchronous detection of the photodetection signal provided by the measurement unit at a harmonic of each of the oscillation frequencies and at an inter-harmonic of the oscillation frequencies.

7. The magnetometer according to claim 1, further comprising: a parametric resonance excitation circuit configured to induce in the cell a radio frequency magnetic field having three components each oscillating at its own oscillation frequency, an optical pumping source arranged to emit towards the cell a pump beam tuned to an atomic transition, the probe beam being identical to or distinct from the pump beam, and a polarisation device configured to impart an elliptical polarisation to the pump beam, wherein, to determine a measurement, associated with each measurement unit, of each of the three components of the ambient magnetic field, the processing unit is configured to perform synchronous detection of the photodetection signal provided by the measurement unit at a harmonic of each of the oscillation frequencies.

8. The magnetometer according to claim 1, wherein the three components of the ambient magnetic field comprise two components, B.sub.x and B.sub.y, transverse to the direction of propagation of the probe beam and one component, B.sub.z, longitudinal to the direction of propagation of the probe beam, and the gradiometric measurement signal comprises: transverse gradients of the B.sub.z component, ∂B.sub.z/∂x and ∂B.sub.z/∂.sub.y, longitudinal gradients of the B.sub.x and B.sub.y components, ∂B.sub.x/∂x and ∂B.sub.y/∂.sub.y, and a transverse gradient of the B.sub.x component and a transverse gradient of the B.sub.y component, ∂B.sub.x/∂y and ∂B.sub.y/∂x.

9. A method for measuring a spatial gradient of a magnetic field by means of a three-axis vector optically pumped magnetometer which comprises: a cell filled with an atomic gas subjected to an ambient magnetic field, a projection of which onto three rectangular coordinate axes defines three components thereof, a photodetector arranged so as to receive a probe beam that passed through the cell, the photodetector comprising a plurality of measurement units arranged in a plane transverse to a direction of propagation of the probe beam, the measurement units each providing a photodetection signal, and a processing unit coupled to the photodetector, said method comprising the following steps implemented by the processing unit, of: determining, for each measurement unit and from the photodetection signal provided by the measurement unit, a measurement, associated with the measurement unit, of each of the three components of the ambient magnetic field; and calculating at least one difference between the measurements, associated with different measurement units, of a component of the magnetic field and providing a gradiometric measurement signal comprising the at least one difference calculated.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further aspects, purposes, advantages and characteristics of the invention will become clearer from the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the appended drawings in which:

(2) FIG. 1 is a diagram of a magnetometer in accordance with the invention.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

(3) With reference to FIG. 1, the invention relates to an optically pumped magnetometer which comprises a cell 1 filled with an atomic gas, for example helium-4 or an alkaline gas, subjected to an ambient magnetic field B.sub.0, the projection of which on three rectangular coordinate axes defines three components thereof. The ambient magnetic field can thus be resolved into three components B.sub.x, B.sub.y and B.sub.z, each along one of the magnetometer's measurement axes x, y and z.

(4) The magnetometer according to the invention is a three-axis vector magnetometer, i.e. capable of measuring each of the three components B.sub.x, B.sub.y and B.sub.z of the ambient magnetic field. As will be described below, the magnetometer according to the invention is also capable of measuring a spatial gradient of the derivative of the ambient magnetic field.

(5) The cell 1 is lighted by an optical pumping source 2 capable of emitting towards the cell 1 a light beam, for example a laser beam, tuned to a pumping wavelength (this beam is thus also referred to as a pump beam). The pumping wavelength is tuned to an atomic transition line, for example to the D.sub.0 line at 1083 nm in the case of helium-4. The light beam is polarised by means of a polarisation device 3 inserted between the optical pumping source and the cell or directly integrated into the optical pumping source.

(6) In the case where the sensitive element is helium-4, the magnetometer further includes a high frequency (HF) discharge system, comprising a HF generator 4 and booster coils 5, to bring the atoms of the atomic gas into an energised state where they are able to undergo the atomic transition when lighted by the pump beam, typically in the 2.sup.3S.sub.1 metastable state.

(7) The light beam used for pumping (i.e. preparation of the atomic states necessary for a measurement of the magnetic field by optical pumping) can also be used as a probe beam for the detection of the pumping-induced atomic states, indeed its absorption by the atoms carrying information on the value of the components of the static magnetic field to which the sensitive element is subjected. FIG. 1 illustrates such an exemplary embodiment with a propagation of the (pump and probe) beam oriented along the direction {right arrow over (z)}.

(8) Alternatively, it is possible to use a probe beam separate from the pump beam. In such a case, it is useful to shift the frequency of the probe light relative to the frequency of the atomic transition used so as not to induce residual pumping with the probe beam. This shift means that an absorption measurement is of low efficiency and that it is therefore preferable to carry out a birefringence measurement, known as a polarimetric measurement.

(9) In general, no known optically pumped magnetometer design allows measurement of the derivative of the component of the magnetic field oriented along the direction of propagation of the probe beam. In the architectures set forth in the introduction which are based on orientation optical pumping, the longitudinal component of the magnetic field (B.sub.z, i.e. that parallel to the direction of propagation of the pump beam) cannot be measured. Therefore none of the B.sub.z gradients can be measured. Furthermore, the gradients along the direction of propagation of the probe beam cannot be measured either.

(10) The invention disclosed here consists in measuring the three components of the ambient magnetic field and their variations along the two axes transverse to the propagation of the probe light. For this, the invention suggests to provide a three-axis magnetometer with a photodetector that includes a plurality of measurement units arranged in a plane transverse to the direction of propagation of the probe beam, for example a 2D array of photodiodes or a charge coupled device (CCD) type imager. This magnetometer can thus measure the three components of the magnetic field, determine their spatial dependence on the two directions transverse to the direction of propagation of the probe beam, and derive the entirety of the components of the magnetic field tensor for reasons set out below.

(11) The three-axis vector magnetometer allows the three components of the magnetic field to be measured concomitantly, namely the B.sub.x and B.sub.y components, both transverse to the direction of propagation of the probe beam, and the B.sub.z component, longitudinal to the direction of propagation of the probe beam. The use of the photodetector makes it possible to obtain the spatial distribution of these three components of the magnetic field along the two directions orthogonal to the direction of propagation of the probe beam by measuring the signal collected independently on each measurement unit of the photodetector. This makes it possible to obtain six components of the magnetic field gradient tensor by differential measurement: ∂B.sub.z/∂x and ∂B.sub.z/∂y, transverse gradients of the B.sub.z component of the magnetic field, ∂B.sub.x/∂x and ∂B.sub.y/∂y, longitudinal gradients of the B.sub.x and B.sub.y components of the magnetic field, ∂B.sub.x/∂y and ∂B.sub.y/∂x, a transverse gradient of the B.sub.x component and a transverse gradient of the B.sub.y component.

(12) Since the magnetisation of the sensitive element used for the measurement is very low, the volume over which the magnetic field measurement is performed (the cell including the sensitive element) does not contain significant magnetic field sources (ferromagnetic materials or electric currents). Therefore, Maxwell equations governing the behaviour of the magnetic field in this medium are expressed as follows:
{right arrow over (rot)}{right arrow over (B)}=0  (1)
div{right arrow over (B)}=0  (2)

(13) These two equations lead to dependencies between some components of the magnetic field gradient tensor.

(14) Equation (1) leads to:

(15) $\begin{array}{cc}\frac{\partial B_y}{\partial z}=\frac{\partial B_z}{\partial y}& \left(3\right)\\ \frac{\partial B_x}{\partial z}=\frac{\partial B_z}{\partial x}& \left(4\right)\\ \frac{\partial B_y}{\partial x}=\frac{\partial B_x}{\partial y}& \left(5\right)\end{array}$

(16) Equation (2) leads to:

(17) $\begin{array}{cc}\frac{\partial B_z}{\partial z}=-\left(\frac{\partial B_x}{\partial x}+\frac{\partial B_y}{\partial y}\right)& \left(6\right)\end{array}$

(18) Equations (3), (4), (5) and (6) show that only five components of the magnetic field gradient tensor are independent. Thus, the measurement of five components allows the complete characterization of this tensor. The magnetometer according to the invention makes it possible to measure six components of the gradient tensor and thus to be able to deduce the last three which it is not possible to measure physically by virtue of the properties derived from Maxwell equations (equations (3), (4) and (6)).

(19) Returning to FIG. 1, the laser beam (which in this example serves both as pump and probe) can be conveyed by an optical fibre at the end of which it diverges at an angle depending on the numerical aperture of the fibre used. This diverging beam is collimated before passing through the cell, for example by using a converging lens. At the outlet of this assembly allowing collimation, the beam passes through the polarisation device 3 giving a specific polarisation to the light before it passes through the cell, for example linear or elliptical polarisation as will be described later.

(20) After passing through the cell, the laser beam passes through a so-called projection lens which allows the expansion (diverging lens) or focusing (converging lens) the collimated beam over an area similar to that of the sensitive surface area of the photodetector used.

(21) As seen previously, the photodetector 10 comprises a plurality of measurement units arranged in a plane transverse to the direction of propagation of the probe beam. Each of the measurement units provides a photodetection signal to a processing unit 6 coupled to the photodetector 10.

(22) The processing unit 6 is configured to determine, for each measurement unit and from the photodetection signal provided by the measurement unit, a measurement, associated with the measurement unit, of each of the three components of the ambient magnetic field. The processing unit is further configured to calculate at least one difference between the measurements, associated with different measurement units, of a component of the magnetic field and to deliver a gradiometric measurement signal comprising the at least one difference calculated. The processing unit is preferably configured to calculate the six above-mentioned gradients.

(23) The magnetometer can also comprise a closed loop feedback control system of the magnetometer for constantly subjecting the sensitive element to a total magnetic field of zero. The feedback control system comprises a regulator 9 coupled to the processing unit 6 and which injects current into Helmholtz coils 7 of orthogonal axes surrounding the cell 1 in order to generate a compensating magnetic field Bc such that the sum Bc+B.sub.0 is maintained at zero at all times.

(24) Alternatively, the magnetometer can be operated in an open loop, without compensation for the ambient field.

(25) The measurement units of the photodetector 10 are typically arranged in array form and are preferably at least 3*3 in number. The measurement of the different gradients is obtained by making the difference between the measurements of the components of the magnetic field resulting from photodetection of the different measurement units. This difference can be made between the measurements from all the measurement units, thus allowing with at least 3*3 measurement units to provide a reconfigurable base in length and direction for the gradiometric measurement of the three components of the magnetic field.

(26) The photodetector 10 can be a photodiode array. Alternatively, the photodetector 10 can be a charge coupled detector CCD. In this alternative, a measurement unit of the photodetector 10 can be formed by a plurality of pixels close to the CCD detector, the photodetection signal of a measurement unit corresponding to the average of the photodetection signals individually delivered from the pixels of the measurement unit. The number of close pixels can depend on the resolution of the CCD detector and the characteristics of the cell containing the sensitive element, especially the pressure of the gas contained therein. It is not helpful if the surface area of the artificial pixel (pool of close pixels) is smaller than the projection of the diffusion volume of the atoms onto the plane of the detector, which is the limit of spatial resolution achievable using an atomic vapour as the sensitive element. In the case of helium-4 metastable at room temperature and a pressure of 10 Torr, this surface area is about 4 mm.sup.2.

(27) Sensitivities of each measurement unit can be different depending on their position on the photodetector, due to the inhomogeneous distribution of the light intensity within the laser beam in the plane perpendicular to the propagation of the probe beam (the light intensity distribution is usually Gaussian in this plane). So, in one possible embodiment, a correction for these differences in sensitivity is implemented by the processing unit 6.

(28) In a preferential embodiment, the magnetometer according to the invention is a parametric resonance magnetometer. It thus comprises a parametric resonance excitation circuit which includes a radio frequency generator 8 which feeds the Helmholtz coils 7 having orthogonal axes surrounding the cell 1 in order to generate a magnetic field for exciting the parametric resonances, also referred to as the radio frequency field. The cell 1 and coils 7 assembly is placed within a passive magnetic shield made of μ-metal in order to isolate it from the ambient magnetic fields (Earth's field and electromagnetic disturbances), which are too intense to respect the physical condition γB<<Γ necessary for the appearance of parametric resonances.

(29) In a first alternative, the radio frequency field has two components each oscillating at its own oscillation frequency, for example a first component along the z axis at the angular frequency ω and a second component along the γ axis at the angular frequency Ω. In such a case, the polarisation device 3 is configured to give the pump beam a linear polarisation in a direction perpendicular to the direction of the two components of the radio frequency magnetic field (that is along the x axis). And to determine a measurement, associated with a measurement unit, of each of the three components of the ambient magnetic field, the processing unit 6 is configured to perform a synchronous detection of the photodetection signal provided by the measurement unit at a harmonic of each of the oscillation frequencies (ω and Ω) and at an inter-harmonic of said oscillation frequencies (ω±Ω).

(30) In a second alternative, it is also possible to use a three-axis optically pumped magnetometer architecture based on an elliptical polarisation of the pumping light and the application of three radio frequency fields as set forth in patent application EP 3 524 990 A1. In this alternative, the parametric resonance excitation circuit is thus configured such that it induces in the cell a radio frequency magnetic field having three components each oscillating at its own oscillation frequency. The polarisation device is configured in such a way as to give the pump beam an elliptical polarisation, the circularly polarised component of which is directed along the axis of propagation of the pump beam z and the linearly polarised component (half-major-axis of the ellipse) is directed along the x direction. And to determine a measurement, associated with a measurement unit, of each of the three components of the ambient magnetic field, the processing unit 6 is configured to perform a synchronous detection of the photodetection signal provided by the measurement unit at a harmonic of each of the oscillation frequencies.

(31) The invention is not limited to a parametric resonance magnetometer but extends to any other optically pumped magnetometer architecture allowing a measurement of the three components of the magnetic field, such as the one set forth in the article by X. Qiu et al, Applied Physics Letters 116, 034001 (2020), entitled “Three-axis atomic magnetometer for nuclear magnetic resonance gyroscopes”. This architecture, more complex than those described above, is based on the use of two atomic species, typically an optically pumped alkali and a noble gas present in the same cell. Two of the field components are measured via the n=1 parametric resonance of the alkali. The third component is obtained from the precession frequency of the noble gas.

(32) The invention, which allows gradients to be measured on very short bases, finds application not only in the medical field (for example magnetoencephalography and magnetocardiography), but also in magneto-relaxometry methods for magnetic particles and for imaging ferromagnetic parts (for example of steel) for non-destructive testing (for example locating faults such as incipient cracking).

(33) The invention is not limited to the magnetometer as described above but also extends to a method for measuring a spatial gradient of a magnetic field by means of such a magnetometer, this method comprising especially determining, for each measurement unit and from the photodetection signal supplied by the measurement unit of a measurement, associated with the measurement unit, of each of the three components of the ambient magnetic field, calculating at least one difference between the measurements, associated with different measurement units, of a component of the magnetic field and providing a gradiometric measurement signal comprising the at least one difference calculated.