Optically pumped magnetometer cell

Abstract

The invention relates to a cell for an optically pumped magnetometer, including: - an enclosure containing a gas; and - a circuit for exciting a plasma in the enclosure, which comprises one or more elements (11) made of an electrically conductive material arranged so as to apply an electrical discharge to the gas enclosed in the enclosure. A conductance of the excitation circuit is less than 8000 Siemens. This conductance is defined as the sum of the conductance of each of the element or elements made of electrically conductive material wherein the conductance of an element made of electrically conductive material corresponds to the conductivity of the electrically conductive material weighted by the ratio between the volume of electrically conductive material of the element and the square of the distance between the element and the centre of the cell.

Description

[0001] Optically pumped magnetometer cell

[0002] TECHNICAL FIELD

[0003] The field of the invention is that of optically pumped magnetometers.

[0004] PRIOR ART

[0005] Optically pumped magnetometers use atomic gases confined in a cell, typically metastable helium or alkali gases, as the sensing element. These magnetometers, which can take different configurations, allow the magnetic field to be traced by exploiting the following three processes which take place either sequentially or concomitantly:

[0006] 1) The use of polarized light sources, typically lasers, makes it possible to prepare atomic states characterized by a certain orientation or alignment of their spins. This process is called "optical pumping" in the field.

[0007] 2) These atomic states evolve under the effect of the magnetic field, in particular under the Zeeman effect which corresponds to shifts in energy levels depending on the magnetic field to which the atoms are subjected.

[0008] 3) The optical properties of the atomic medium then undergo modifications which depend on the state of the atoms. We can thus, by an optical measurement, for example by an optical absorption measurement, go back to the Zeeman shift undergone, and deduce from it a measurement of the magnetic field in which the cell is immersed.

[0009] The sensitivity, also called low intrinsic noise, achievable with such optically pumped magnetometers is remarkable and significantly more favorable than that of most other magnetic measurement technologies (fluxgate, Hall effect, magnetoresistance, etc.). Only the SQUID type magnetometer has a similar noise, but it requires cryogenic cooling of the sensitive element, which contains elements that need to be superconducting for its operation, which restricts its practical application.

[0010] The measurement of magnetic fields is useful for various applications, including the characterization of electric currents circulating in the human body, allowing, for example, the understanding and diagnosis of various pathologies of the brain and heart. In such a use for measurements on the human body, it is advantageous to have not a single magnetometer, but a dense network of magnetometers allowing good spatial resolution to be obtained. This requires that the magnetometers be of a sufficiently small lateral size (it is then customary to speak of "miniature" magnetometers). Since these biomagnetic imaging techniques involve imaging the interior of the human body where the magnetic sources are typically at a distance of one to a few centimeters from the surface of the body, the ideal is to have magnetometers of a size similar to this distance, thus typically having a lateral size of between 3 mm and 3 cm.

[0011] Optically pumped magnetometers of alkalis with centimeter lateral size and compatible with networking currently achieve sensitivities close to 10 fT / sqrt(Hz). This sensitivity can be understood as the "intrinsic noise" that the operation of the magnetometer adds to the measured signal. It comes from various phenomena, notably the noise of the light used to measure atomic states, which, in the best case, presents quantum fluctuations also called "photon noise" or "optical shot noise".

[0012] In the case of optically pumped metastable helium magnetometers, sensitivities of the order of 50 fT / sqrt(Hz) have been reported in the paper [1] listed below. Such sensitivity is not as favorable as the case of alkali magnetometers. However, optically pumped metastable helium magnetometers have many other practical advantages. In particular, they do not require heating to operate. They also have a significantly wider bandwidth.

[0013] STATEMENT OF THE INVENTION

[0014] The invention aims to reduce the intrinsic noise of optically pumped magnetometers of metastable helium.

[0015] To this end, the invention proposes a cell for an optically pumped magnetometer, comprising: an enclosure containing a gas; and a circuit for exciting a plasma in the enclosure, which comprises one or more elements made of electrically conductive material arranged relative to the enclosure so as to allow the application of an electrical discharge to the gas contained in the enclosure.

[0016] A conductance of the excitation circuit is defined as the sum of the conductance of each of the element(s) made of electrically conductive material where the conductance of an element made of electrically conductive material corresponds to the conductivity of the electrically conductive material weighted by the ratio between the volume of electrically conductive material of said element and the square of the distance between said element and the center of the cell. According to the invention, the conductance of the excitation circuit is less than 8000 Siemens, preferably less than 5000 Siemens, even more preferably less than 1500 Siemens.

[0017] Some preferred but non-limiting aspects of this cell are as follows: the enclosure has a characteristic dimension of between 3 mm and 3 cm, preferably between 5 mm and 2 cm; the excitation circuit is capacitively coupled to the cell, the element(s) made of electrically conductive material consisting of two electrodes; each of the electrodes consists of a ring of electrically conductive material which surrounds the enclosure; each of the electrodes consists of a deposit of electrically conductive material on the enclosure; each of the electrodes consists of an impression of electrically conductive material on a dielectric film applied to the enclosure; the dielectric film is flexible; the excitation circuit is inductively coupled to the enclosure, the element(s) made of electrically conductive material consisting of a coil wound in several loops around the cell; the coil is a solenoid;the coil is a helix of a helical resonator. The invention also relates to an optically pumped magnetometer comprising a cell according to the invention as well as a magnetoencephalography helmet, comprising a plurality of magnetometers according to the invention.;

[0018] BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Other aspects, aims, advantages and characteristics of the invention will appear better on reading 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: Figure 1 a diagram of a magnetometer integrating a cell according to the invention; Figure 2 is an example of an electrode deposited on a flexible dielectric film in a fork pattern.

[0020] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0021] Unlike optically pumped alkali magnetometers, optically pumped metastable helium magnetometers are based on an atomic state that is not the ground state (the 1 So in spectroscopic notation for helium-4), but an excited state, in this case the metastable triplet state (noted 2 3 If for helium-4). For the magnetometer to work, this state must be populated. This is usually achieved using an electrical discharge operating in the high-frequency range, typically between 1 MHz and 100 MHz. This discharge creates a plasma inside the cell in which high-energy collisions between atoms and electrons excite some of the atoms to the metastable state. Different strategies for initiating and maintaining this plasma discharge can be used:

[0022] • A capacitively coupled discharge circuit, consisting of two electrodes placed on either side of the gas cell; or

[0023] • An inductively coupled discharge circuit, consisting of a solenoid located so that the magnetic flux it creates is partly applied to the gas inside the cell. A variant consists of a helical resonator connected to a generator at only one of its ends and facing a ground plane. For certain frequencies, this resonator behaves like a solenoid, with advantages in terms of impedance matching to the generator.

[0024] For measurement applications on the human body, it is preferable for the magnetometer to be of small size. Ideally, this requires that the elements of the magnetometer other than the sensitive element (in particular the capacitive or inductive element which enables the ignition and maintenance of the plasma) be compact and that they can be located in the immediate vicinity of this sensitive element, so that the overall size of the sensor is only very slightly greater than the size of the sensitive element it contains. Capacitive electrodes are interesting in this sense, as well as an inductive circuit comprising a coil with a diameter very slightly greater than that of the helium cell.

[0025] A study on the optimal discharge regimes to obtain a high amplitude magnetometry signal was carried out and is published in the thesis of J. Rutkowski [2], for cells of 5 mm and 1 cm characteristic dimension and electrodes consisting of fragments of copper tape.

[0026] Other capacitive electrodes are mentioned in the literature, including electrodes consisting of two (or more) rings surrounding the cell. These electrodes are mentioned in particular in the thesis of Marie-Constance Corsi [3], which mentions a wire "thicker" than that used to connect the copper tape (which is 0.1 mm). The patent application [4] associated with this work discloses a wire diameter of 315 microns.

[0027] In addition to obtaining a strong magnetometric signal, it is also desirable that the intrinsic noise of the magnetometer be limited to its unavoidable components, in particular photon noise, which occurs when other technical noise (current noise in the laser, for example) becomes negligible. However, since the photon noise can be calculated, the Applicant was able to observe that there were still technical noises, of an unknown nature, which limited the total noise of the sensor. The Applicant suspected that this technical noise was linked to the electrodes, which is now proven as will be described below.The Applicant was thus able to show that in the case of sensors based on the optical pumping of metastable helium, the copper tape electrodes traditionally used (thickness between 35 and 100 microns) as well as the thick wire loop electrodes also used in the past (thickness of 315 or 500 microns) introduce significant magnetic noise, which, for optimized sensors, becomes the limiting noise of the sensor.

[0028] It has also been found that it is possible to significantly reduce this noise contribution, thus achieving more favourable sensitivity levels than previously, by drastically limiting the quantity of conductive material which constitutes the means of excitation of the plasma discharge (electrodes or inductive discharge circuit) placed near the measuring cell.

[0029] Based on this observation, the Applicant proposes a cell for an optically pumped magnetometer equipped with a circuit for exciting a plasma in an enclosure containing an atomic gas by means of an electrical discharge, a circuit optimized to create high densities of metastable atoms, with long lifetimes, without creating additional technical noise due to this discharge or its excitation means.

[0030] The invention thus relates to a cell for an optically pumped magnetometer, which comprises an enclosure containing a gas and a circuit for exciting a plasma in the enclosure.

[0031] The magnetometer is preferably a miniature magnetometer, the enclosure then having a characteristic dimension (diameter if it is a sphere, side if it is a cube, diameter and height if it is a cylinder) between 3 mm and 3 cm, preferably between 5 mm and 2 cm.

[0032] The gas may be helium-4.

[0033] The circuit for exciting a plasma in the enclosure comprises one or more elements made of electrically conductive material arranged relative to the enclosure so as to allow the application of an electrical discharge to the gas contained in the enclosure. This circuit is powered by an RF radiofrequency generator, for example in the 10-100 MHz range. According to the invention, the quantity of conductive material placed near the sensitive element is limited so that the conductance of the excitation circuit is less than 8000 Siemens, preferably less than 5000 Siemens, even more preferably less than 1500 Siemens. By reducing this conductance to values ​​less than 8000 Siemens, it becomes possible to exceed the best sensitivities previously obtained on miniature helium magnetometers.

[0034] In the context of the invention, this conductance Qtotai of the excitation circuit is defined as the sum of the conductance Q of each of the element(s) made of electrically conductive material. In other words, when the excitation circuit has a single element made of electrically conductive material, its conductance Qtotai corresponds to that of the element made of electrically conductive material. And when the excitation circuit has several elements made of electrically conductive material, its conductance Qtotai corresponds to the sum of the individual conductances of each of the elements made of electrically conductive material.

[0035] The conductance of an element made of electrically conductive material corresponding to the conductivity a of the electrically conductive material (in Siemens / m) weighted by the ratio between the volume V of electrically conductive material of said element (in m 3 ) and the square of the distance a (in m) between said element and the center of the cell, or Q = a * Thus, when the excitation circuit comprises several elements in electrically conductive material (two electrodes for example), its conductance Qtotai corresponds to the sum of the conductance Q of the different elements.

[0036] In one possible embodiment, the excitation circuit is capacitively coupled to the cell, the element(s) made of electrically conductive material consisting of two electrodes.

[0037] Each of the electrodes may consist of a ring of electrically conductive material which surrounds the enclosure, according to the arrangement disclosed in [3] but with a conductance in accordance with the invention.

[0038] In a first variant, each of the electrodes may consist of a deposit of electrically conductive material on the enclosure. In a second variant, each of the electrodes may consist of a print of electrically conductive material on a dielectric film, this film being attached, for example by gluing, to the enclosure. The dielectric film is preferably a flexible film.

[0039] In another possible embodiment, the excitation circuit is inductively coupled to the enclosure, the element(s) of electrically conductive material consisting of a coil wound in several loops around the cell.

[0040] The coil may be a solenoid or may constitute the helix of a helical resonator.

[0041] The invention extends to an optically pumped magnetometer comprising a cell as previously described as well as to a magnetoencephalography helmet, comprising a plurality of magnetometers according to the invention.

[0042] With reference to Figure 1, such an optically pumped magnetometer is equipped with a cell which comprises an enclosure 1 filled with an atomic gas, for example helium-4, subjected to an ambient magnetic field BO whose projection onto three rectangular coordinate axes defines three components.

[0043] For example, such a magnetometer can be based on a chamber filled with high-purity helium-4. This chamber can have a characteristic dimension between 3 mm and 3 cm (diameter if it is a sphere, side if it is a cube, diameter and height if it is a cylinder). It is filled with helium, at a pressure depending on its size, which typically can be 10 Torr for a 1 cm cylindrical cell.

[0044] The cell is illuminated by an optical pump source 2 capable of emitting a beam of light F, for example a laser beam, tuned to a pump wavelength (this beam is thus also referred to as a pump beam) towards the cell 1. The pump wavelength is set to an atomic transition line, for example to the DO line at 1083 nm in the case of helium-4.

[0045] The magnetometer also comprises a plasma excitation circuit in the enclosure which comprises one or more conductive elements as previously described. This excitation circuit is coupled to an HF generator 4 and to overvoltage coils.

[0046] 5.

[0047] In one possible embodiment, electrodes are arranged on the outer surface of the enclosure in order to capacitively ignite and maintain a plasma discharge in the helium gas. This discharge makes it possible to populate state 2 3 Sq which is the one used for magnetic measurement.

[0048] As shown in Figure 2, these electrodes 11 can for example be produced by a flexible electronics process, consisting of the lithography of a track of conductive material 12 (for example a copper layer 35 microns thick) by forming a pattern, for example in the shape of a fork. The teeth of the fork can be sized to fit the outer surface of a cylindrical enclosure over its entire height. These teeth are arranged on a rectangular part 13 of a flexible dielectric film, this rectangular part being able to have dimensions of 11 mm x 6 mm.

[0049] Alternatively, these electrodes can be made by depositing a thin layer of conductive material on the outer surface of the enclosure (for example, a 1 micron thick copper layer) using a physical vapor deposition process, such as evaporation or sputtering. In this case, separation between the two electrodes is provided before depositing by using masking elements, such as tape. Reconnection of the thin layers is also provided and can be achieved, for example, by ultrasonic micro-welding processes, or by welding onto a drop of silver lacquer.

[0050] In all these cases, the elements are connected to the radiofrequency field generator 4, typically in the 10-100 MHz range. It is advantageous to have an impedance matching circuit between these electrodes and the RF generator in order to reduce power reflections caused by too strong an impedance mismatch. This circuit is implemented by means of the surge coils 5, for example two air core coils, with an inductance of a few tens of pH. Such an impedance matching can make it possible to ignite the discharge with powers of the order of 200 mW and maintain it with powers of the order of 10 mW. Once the discharge is ignited, a substantial population of atoms in the cell is brought to state 2 3 Sq which allows a magnetic measurement to be carried out according to different configurations well known in the field, and reported for example in the article [5],

[0051] One of the possible operating modes of the magnetometer consists of crossing the cell with a collimated laser beam, linearly polarized by means of a polarizer 3 and tuned to the DO line of helium-4. This beam comes from a laser 2 with excellent amplitude stability, characterized by a relative noise RIN ("Relative Intensity Noise") dominated by photon noise at powers of the order of 1 mW. It must also have very low phase noise. Among the laser technologies meeting this need, fiber lasers as well as lasers allow achieving the best measurement performances.

[0052] This beam is photodetected after passing through the cell. In addition, a radiofrequency field, at a frequency of e.g. 40 kHz, is applied to the cell in a direction orthogonal to that of the polarization of the light. A signal then appears in the photodetection spectrum at 40 kHz which, in a range of a few tens of nT around the zero field, allows a measurement of the component of the magnetic field parallel to the direction of the applied radiofrequency field. Synchronous detection of this signal allows a component of the magnetic field to be measured. The intrinsic noise of the sensor then corresponds to the noise obtained at the synchronous detection output, the sources of which can be diverse.In the case described here, this noise is limited by the photon noise of the laser light used to produce the light, and it corresponds in magnetic units to values ​​between 20 and 30 fî / sqrt(Hz), which is more advantageous than the noise levels obtained previously.

[0053] The magnetometer can thus comprise a parametric resonance excitation circuit which comprises a radiofrequency generator 8 which powers Helmholtz coils 7 with orthogonal axes which surround the cell in order to generate a magnetic field for exciting the parametric resonances, also referred to as a radiofrequency magnetic field. The magnetometer further comprises a parametric resonance detection device 6 configured to measure the absorption of the light beam by the atomic gas and a photodetector 10 arranged to receive the light beam having passed through the cell and deliver a photodetection signal to said parametric resonance detection device 6.

[0054] The magnetometer may also comprise a closed-loop servo system for the magnetometer to constantly subject the sensitive element to a zero total magnetic field. The servo system comprises a regulator 9 coupled to the detection device 6 and which injects a current into the Helmholtz coils 7 in order to generate a compensation magnetic field Bc such that the sum Bc+BO is kept at zero permanently. Alternatively, the magnetometer may be operated in open loop, without compensation for the ambient field.

[0055] It should be noted that since noise is linked to the conductance of the conductive elements of the excitation circuit, it is possible to use electrodes with larger volumes than the electrodes described below but made from materials that are less good electrical conductors than copper and aluminum, for example transition metals such as Nb and Mo, alloys such as CuAI, AgCu, AuAg, or semiconductors such as Si or Ge.

[0056] In another possible embodiment, an inductive ignition can be achieved using a solenoid or a helical resonator with a diameter less than or equal to 3 cm for example, and achieved:

[0057] ° either with a copper wire with a diameter typically less than 300 microns;

[0058] ° either with a thicker wire, made of a material with lower conductivity than that of copper or aluminum, in particular made of the materials listed above for the case of capacitive electrodes.

[0059] Comparative examples

[0060] • Example 1: according to the prior art

[0061] This example uses the cell described in [2] (100 mm cell 3 in the form of a cylinder 5 mm in diameter and height), electrodes in the form of fragments of copper tape (from the manufacturer 3M reference 3313, 35 microns thick) of 4*4 mm each glued to the outer surface of the cell, and connected to the RF generator by wires soldered to these copper elements. The quantity Qtotai, neglecting the metal of the solders, is worth 10700 Siemens. The unincluded noise component resulting from measurements in low-field measurement configuration is of the order of 120 fT / sqrt(Hz).

[0062] • Example lb: according to the prior art

[0063] This example uses a larger cell (cylinder 1 cm in diameter and 1 cm long), also described in [2]. The electrodes are fragments of copper tape (from the manufacturer 3M reference 3313, 35 microns thick) of 1 x 0.9 cm each glued to the outer surface of the cell, and connected to the RF generator by wires soldered to these copper elements. The quantity Qtotai, neglecting the metal of the solders, is 13000 Siemens. The unincluded noise component resulting from measurements in low-field measurement configuration is of the order of 100 fî / sqrt(Hz).

[0064] • Example 2 (prior state of the art)

[0065] This example uses ring electrodes such as those described in [3] and [4], each consisting of a 1 cm diameter loop made of 315 micron diameter copper wire. The quantity Qtotai is 11700 Siemens. The unaccounted noise component resulting from measurements in the low-field measurement configuration is of the order of 77 fî / sqrt(Hz).

[0066] • Example 3 (according to the invention)

[0067] This example uses thin-film electrodes (formed by a copper deposit) with a thickness of 1 micron, deposited by evaporation, with a masking allowing it to have the shape of a rectangle of 1 x 1 cm on the lateral faces of a cylindrical cell. These electrodes are connected to the RF generator by an ultrasonically micro-welded wire. The quantity Qtotai, neglecting the metal of the welds, is 920 Siemens. The unexplained noise is less than 19 fî / sqrt(Hz), with a high uncertainty due to its lower power than that of the photon noise during the experiment (30 fî / sqrt(Hz)).

[0068] • Example 4 (according to the invention)

[0069] This example uses thin electrodes made on a flexible polymer such as Kapton using lithography processes well known in the field of flexible electronics, with a track width of 150 microns and a copper thickness of 18 microns. The quantity Qtotai is 1170 Siemens. The unexplained noise is 16 fî / sqrt(Hz), with a high uncertainty due to its low power compared to that of the photon noise during the experiment (25 fT / sqrt(Hz)).

[0070] • Example 5 (according to the invention)

[0071] This example uses electrodes like those in Example 2 but made with 100 micron wire. The quantity Qtotai is 1200 Siemens. The unaccounted noise component resulting from measurements in the low-field measurement configuration is of the order of 25 fî / sqrt(Hz).

[0072] • Example 6 (according to the invention)

[0073] This example uses an inductive ignition method consisting of an inductive coil with a 150 micron wire wrapped 6 times around the cell with a diameter of 2 cm. The quantity Qtotai is 7900 Siemens. The unaccounted noise component resulting from measurements in a low-field measurement configuration is of the order of 32 fî / sqrt(Hz).

[0074] References

[0075] [1] W. Fourcault et al., “Helium-4 magnetometers for room-temperature biomedical imaging: toward collective operation and photon-noise limited sensitivity,” Opt. Express, vol. 29, no. 10, pp. 14467-14475, May 2021, doi: 10.1364 / OE.420031.

[0076] [2] J. Rutkowski, "Study and realization of a miniature isotropic helium magnetometer," University of Franche Comté, 2014.

[0077] [3] M.-C. Corsi, "Optically pumped magnetometers with Helium 4: development and proof of concept in magnetocardiography and magnetoencephalography," PhD, Université Grenoble Alpes, 2015. Accessed: Apr. 20, 2021. [Online], Available: https: / / tel.archives-ouvertes.fr / tel-01233850

[0078] [4] FR 3 035 769 Al

[0079] [5] G. Le Gal, L.-L. Rouve, and A. Palacios-Laloy, “Parametric resonance magnetometer based on elliptically polarized light yielding three-axis measurement with isotropic sensitivity,” Appl. Phys. Lett., vol. 118, no. 25, p. 254001, Jun. 2021, doi: 10.1063 / 5.0047124.

Claims

CLAIMS 1. Cell for an optically pumped magnetometer, comprising: an enclosure (1) containing a gas; and a circuit for exciting a plasma in the enclosure, which comprises one or more elements made of electrically conductive material (11) arranged so as to apply an electric discharge to the gas contained in the enclosure, characterized in that a conductance of the excitation circuit, defined as the sum of the conductance of each of the element(s) made of electrically conductive material where the conductance of an element made of electrically conductive material corresponds to the conductivity of the electrically conductive material weighted by the ratio between the volume of electrically conductive material of said element and the square of the distance between said element and the center of the cell, is less than 8000 Siemens.

2. Cell according to claim 1, wherein the conductance of the excitation circuit is less than 5000 Siemens, preferably less than 1500 Siemens.

3. Cell according to claim 1, in which the enclosure has a characteristic dimension of between 3 mm and 3 cm, preferably of between 5 mm and 2 cm.

4. Cell according to one of claims 1 to 3, in which the excitation circuit is capacitively coupled to the cell, the element(s) made of electrically conductive material consisting of two electrodes.

5. Cell according to claim 4, in which each of the electrodes consists of a ring of electrically conductive material which surrounds the enclosure.

6. Cell according to claim 4, in which each of the electrodes consists of a deposit of electrically conductive material on the enclosure.

7. Cell according to claim 4, in which each of the electrodes consists of an impression of electrically conductive material (12) on a dielectric film (13) applied to the enclosure.

8. Cell according to claim 7, in which the dielectric film is flexible.

9. Cell according to one of claims 1 to 3, in which the excitation circuit is inductively coupled to the enclosure, the element(s) made of electrically conductive material consisting of a coil wound in several loops around the cell.

10. A cell according to claim 9, wherein the coil is a solenoid.

11. Cell according to claim 9, in which the coil is a helix of a helical resonator.

12. Optically pumped magnetometer comprising a cell according to one of claims 1 to 11.

13. Magnetoencephalography helmet, comprising a plurality of magnetometers according to claim 12.

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