Patent Application
20200150196
This application claims priority from French Patent Application No. 1860319 filed on Nov. 8, 2018. The content of this application is incorporated herein by reference in its entirety.
The field of the invention is that of optical pumping magnetometers, and more particularly that of Hanle effect magnetometers.
Optical pumping magnetometers use atomic gases confined in a cell, typically metastable helium or alkaline gases, as a sensitive element. These magnetometers, which can have different configurations, make it possible to move through the magnetic field by exploiting the three following processes that take place either sequentially or at the same time:
1) The use of sources of polarised light, typically lasers, makes it possible to prepare atomic states that are characterised by a certain orientation or alignment of their spins. This process receives the name “optical pumping” in the field.
2) These atomic states change under the effect of the magnetic field, in particular under the Zeeman effect which corresponds to offsets of the energy levels according to the magnetic field to which the atoms are subjected.
3) The optical properties of the atomic medium then undergo modifications that depend on the state of the atoms. It is thus possible through an optical measurement, for example via an optical absorption measurement, to go through the subjected Zeeman offset and to deduce therefrom a measurement of the magnetic field wherein the cell is plunged.
According to the various possible configurations of the existing optical pumping magnetometers, we distinguish a measurement of the module, also called norm, of the magnetic field for scalar magnetometers, or a determination of the various components of the magnetic field at the location of the cell for vector magnetometers.
In order to take a vector measurement of the magnetic field with a large bandwidth, there are two well-known configurations: the first is the so-called “Hanle effect” and the second which receives the name of “parametric resonance magnetometer”. These configurations are described in particular in the article by J. Dupont-Roc, “Détermination par des méthodes optiques des trois composantes d'un champ magnétique trés faible,” Revue de Physique Appliquée, vol. 5, no. 6, pp. 853-864, 1970. They operate at very low exterior magnetic field values, inducing a Zeeman offset that is lower than the rate of relaxation of the Zeeman sublevels of the atom, which for the case of helium sets a limit around 100 nano Tesla, which is 500 times less intense than the Earth's magnetic field.
When a weak static transverse magnetic field is applied to the cell and swept around zero, the atoms are subjected to a precession movement and the number of photons absorbed, coming from the optical pumping laser, undergoes resonant variations (Hanle effect). Similar resonances, called parametric resonances, are observed when a radiofrequency magnetic field is applied. In these conditions, the magnetic moment of each atom undergoes resonant oscillations at multiple frequencies of that of the radiofrequency field. Measuring the amplitude of these oscillations makes it possible to move up to the module of the component of the magnetic field collinear to the radiofrequency field.
The Hanle effect magnetometer has for disadvantages being sensitive to the low-frequency noise of the probe laser and to be based, for a measurement of the state of orientation of the atoms, on at least two optical accesses orthogonal to the measuring cell which makes it bulky and complex to carry out. Such a magnetometer, based on a circular polarisation pumping and a polarmetric measurement via the Faraday effect, is for example described in J. C. Allred, R. N. Lyman, T. W. Kornack, and M. V. Romalis, “High-Sensitivity Atomic Magnetometer Unaffected by Spin-Exchange Relaxation,” Phys. Rev. Lett., vol. 89, no. 13, p. 130801, September 2002.
The parametric resonance magnetometer makes it possible to take measurements of several components of a magnetic field with a single optical access using the radiofrequency field or fields used to carry out the frequency modulation of the parametric resonances. This magnetometer however has the following disadvantages:
It is sought in general to have a magnetometer that has an optical configuration that is as simple as possible while still offering a signal-to-noise ratio that is as substantial as possible. This is in particular the case in applications of the magnetometer wherein measuring the magnetic field is used to deduce the position of the field sources (currents or magnetic materials) and wherein a signal-to-noise ratio that is too weak is likely to induce great uncertainties as to the magnitude of these sources or the location thereof.
To this effect, the invention proposes a magnetometer comprising a cell intended to be filled with an atomic gas, an optical source and a detector. The optical source is configured to illuminate the cell with a light that has:
The detector comprises a polarisation analyser configured to take a differential measurement of the right circular polarisation and of the left circular polarisation of the probe contribution that has passed through the cell.
Certain preferred but not limiting aspects of this magnetometer are as follows:
Other aspects, purposes, advantages and characteristics of the invention shall appear better when reading the following detailed description of preferred embodiments of the latter, given by way of a non-limiting example, and in reference to the accompanying drawings wherein:
In reference to
In the case where the sensitive element is helium-4, the magnetometer 10 moreover comprises a high-frequency (HF) discharge system, comprising a HF 4 generator and overvoltage coils 5, to bring the atoms of the atomic gas in an energised state where they are able to be subjected to an atomic transition, typically in the metastable state 23S1.
The magnetometer can also include a closed loop control system of the magnetometer in order to constantly subject the cell to a zero total magnetic field. The control system comprises a regulator 7 coupled to the electronic processing that injects a current into Helmholtz coils 8 of orthogonal axes that surround the cell 1 in order to generate a magnetic field of compensation Bc such that the sum Bc+B0 is constantly maintained at zero. Alternatively, the magnetometer can be operated in an open loop, without compensation for the ambient field.
Within the framework of the invention, the optical source is configured to illuminate the cell with a light that has both a pump contribution and a probe contribution, with these two contributions having a linear polarisation and collinear or orthogonal polarisation directions.
Under the effect of the pump contribution, the atoms of the atomic gas undergo an atomic transition. The pump contribution can to this effect be tuned in wavelength at the centre of an atomic transition line, for example on the line D0 at 1083 nm in the case of helium-4, even be slightly offset from such a centre as shall be described in what follows in order to provide both the role of a pump and a role of a probe. The term wavelength tuned at the centre of a line typically means that the wavelength is separated from the centre of the line by at most half of the width at mid-height of the line, i.e. at ±0.85 Ghz from the centre for the line D0 of helium.
The probe contribution undergoes variations in polarisation during the passing through of the cell. It can to this effect be tuned in wavelength to the maximum of the imaginary portion of the Voigt profile of an atomic transition line (which may or may not be the same as that used for pumping) or be sufficiently separated from the centre of the atomic line in order to not also pump the atoms, by being for this typically separated from the centre of the line by at least one quarter of the width at mid-height of the line.
When the line used for the probing is different from that used for the pumping, the sensor and probe contributions are then tuned on two relatively separated wavelengths. In such a case, an optical spectral filtering element of the light can be inserted between the cell and the detector in order to suppress the pump contribution so that it does not contribute any noise on the detection.
In a possible embodiment, which is the one shown in
The pump beam Fp is polarised by means of a polarisation device 3 inserted between the pumping element 2 and the cell 1 or directly integrated into the pumping element 2. Within the framework of the invention, the pump beam Fp is polarised linearly, at least partially, which induces so-called “aligned” atomic states in the cell 1, the axis of alignment being fixed by the direction of the electric field of the light used for the pumping. In what follows, a reference trihedral xyz is considered with the X axis aligned on the direction of linear polarisation of the pump beam. The term partially polarised beam means a beam which is partially polarised linearly according to the X axis and partially depolarised, with its Stokes parameters being such that S1+S2+S3≤S0 as is presented in chapter 8 of the book “Optics” by Eugene Hecht, Addison Wesley 2002.
The probe beam Fs is polarised linearly by means of a polarisation device 11 inserted between the probing element 9 and the cell 1 or directly integrated into the probing element 9. Its direction of polarisation is according to the X axis (collinear to the direction of polarisation of the pump beam) or according to the Y axis (orthogonal to the direction of polarisation of the pump beam). The probe beam Fs propagates according to the Z axis of the trihedral xyz.
In
Alternatively, the directions of propagation of the pump and probe beams are collinear in such a way that the magnetometer has only a single optical access.
In another possible embodiment with a single optical access, the optical source is configured to emit in the direction of the cell a single beam polarised linearly, with this beam playing both the role of the pump and of the probe by carrying both the pump contribution and the probe contribution. To do this, this beam is tuned in wavelength between the centre of an atomic line and the maximum of the imaginary portion of the Voigt profile of the atomic line in order create an adequate compromise between effectiveness of the role of the pump and effectiveness of the role of the probe, by being for example at 563.9 MHz from the centre of the line D0 for helium 4.
According to the invention, the detector 6 comprises a polarisation analyser configured to take a differential measurement of the right circular polarisation σ+ and of the left circular polarisation σ− of the probe contribution that has passed through the cell. The detector 6 thus delivers, as a signal carrying information relative to the state of alignment of the atoms of the atomic gas in the cell, a signal representative of the difference in the intensities of the right and left circular polarisations. This signal depends solely on the component of the magnetic field according to the Z axis and has, as shown in
The polarisation analyser can be configured to carry out a temporal separation of the right and left circular polarisations of the probe contribution that has passed through the cell, for example using a photoelastic modulator. A photodetector thus detects alternatively in time each one of these polarisations.
Alternatively, the polarisation analyser can be configured to carry out a spatial separation of the right and left circular polarisations of the probe contribution that has passed through the cell. To this effect, the polarisation analyser can comprise a quarter-wave plate, a polarisation separator able to separate over a first and a second path the right circular polarisation and the left circular polarisation of the probe contribution that has passed through the cell and a photodetector on each one of the first and second paths.
The magnetometer according to the invention can moreover include a modulator of the probe contribution. The probe contribution can thus be amplitude modulated, in polarisation, even in wavelength in a degraded mode of implementation of an amplitude modulation. The modulation frequency can be sufficiently high, for example of about 30 kHz, in order to overcome the problems of low-frequency noise of the laser that supplies the probe contribution, without however losing signal amplitude. The modulator of the probe contribution can also be a polarisation modulator, for example a photo-acoustic modulator, arranged between the measurement cell and the detector.
The invention makes it possible to provide a magnetometer allowing for the measurement of a component of the ambient magnetic field that has a simplified optical access (a single window) and does not have the loss of the signal-to-noise ratio that is proper to parametric resonance magnetometers. The pump contribution, which is linearly polarised and does not undergo any modification in polarisation, is invisible in the signal coming from the differential measurement.
The operating principle described hereinabove for the measurement of the field according to the Z axis can be enlarged to the measurement of several components of the magnetic field (i.e. measurement according to several axes) with a measured component fixed by the direction of propagation of a dedicated probe contribution. In what follows, the configuration described hereinabove is retained, pumping along the X axis and measuring along the Z axis.
If it is desired to measure the Y axis in addition to the Z axis, a probe contribution that is linearly polarised along the X axis or the Y axis is added and which propagates according to the Y axis.
If it is desired to measure the X axis in addition to the Z axis, a probe contribution is added that propagate according to the X axis. It is then necessary to add a pumping component of the system along a different axis, either by using a single pump contribution that is partially polarised, or by having recourse to another pump contribution.
If it is desired to take a measurement along the three axes, it is necessary to have three probe contributions each propagating in a different direction of space (x, y and z), and an optical pumping that has components according to at least two different axes. A simple way consists in using a total of three beams, where each beam carries out both the role of a pump and of a probe, and of which two of them share the same axis of polarisation.
The invention also relates to a method for measuring a magnetic field using the magnetometer such as described hereinabove. This method in particular comprises:
| Number | Date | Country | Kind |
|---|---|---|---|
| 1860319 | Nov 2018 | FR | national |
