Optical pumping magnetometer cell

Abstract

The present invention relates to a cell for an optical pumping magnetometer, and includes a housing for containing a gas, and a circuit for exciting a plasma within the housing, the circuit comprising one or more elements (11) made of a conductive material arranged to apply a discharge to the gas contained in the housing. The conductance of the excitation circuit is less than 8000 Siemens. This conductance is defined as the sum of the conductances of each element made of the conductive material, where the conductance of an element made of the conductive material corresponds to the conductivity of the conductive material, weighted by the ratio of the volume of the conductive material of the element to the square of the distance between the element and the center of the cell.

Description

[Technical Field]

[0001] The field of this invention is the field of optical pumping magnetometers. [Background technology]

[0002] Optical pumping magnetometers use an atomic gas, typically metastable helium or alkaline gas, confined within a cell as a sensing element. These magnetometers can be configured in various ways and can measure magnetic fields by utilizing the following three processes, performed sequentially or simultaneously: 1) By using a polarized light source, usually a laser, it is possible to create atomic states characterized by specific orientations and arrangements of atomic spins. This process is known in the art as "optical pumping." 2) These atomic states change under the influence of a magnetic field, particularly under the Zeeman effect, which corresponds to the shift in energy levels as a function of the magnetic field to which the atom is exposed. 3) The optical properties of the atomic medium then undergo changes depending on the state of the atoms. Subsequently, optical measurements, such as light absorption measurements, can be used to identify the Zeeman shifts experienced by the atoms and estimate the measured magnetic field in which the cell is located.

[0003] The sensitivity (also known as intrinsic low noise) achievable with such optical pumping magnetometers is astonishing and significantly superior to most other magnetic measurement techniques (fluxgate, Hall effect, magnetoresistance, etc.). Only SQUID magnetometers have similar noise levels, but they require cryogenic cooling of the sensing elements, and these elements include components that must be superconducting for their operation, thus limiting their practical applicability.

[0004] Measuring magnetic fields is useful for a variety of applications, particularly for characterizing electric currents circulating within the human body, enabling the understanding and diagnosis of various conditions in the brain and heart. When used in this way for measurements in the human body, it is advantageous to have a high-density network of magnetometers that provides good spatial resolution, rather than a single magnetometer. This requires that the lateral size of the magnetometers be sufficiently small (commonly called "miniature" magnetometers). Since these biomagnetic imaging techniques involve imaging the inside of the human body where the magnetic source is usually 1 to several centimeters from the body surface, magnetometers of a similar size to this distance are ideal, specifically those with a lateral size of about 3 mm to 3 cm.

[0005] Network-compatible centimeter-class lateral-size optical pumping alkaline magnetometers are now achieving sensitivities close to 10 fT / √Hz. This sensitivity can be understood as the "inherent noise" that the magnetometer's operation adds to the measured signal. This arises from noise in the light used to measure various phenomena, particularly atomic states, and in the best case, exhibits quantum fluctuations also known as "photon noise" or "optical shot noise."

[0006] In the case of optically pumped metastable helium magnetometers, a sensitivity on the order of 50 fT / √Hz has been reported in the following paper [1]. Such sensitivity is not as good as that of alkaline magnetometers. However, optically pumped metastable helium magnetometers have many other practical advantages. In particular, they do not require heating to operate and have a wider bandwidth. [Disclosure of the Invention] [Problems that the invention aims to solve]

[0007] The objective of this invention is to reduce the intrinsic noise of an optically pumped metastable helium magnetometer.

[0008] For this purpose, the present invention - A housing that contains gas, - We propose a cell for an optical pumping magnetometer comprising a circuit for exciting plasma within a housing, the circuit comprising one or more elements of a conductive material positioned relative to the housing to enable the application of a discharge to a gas contained within the housing.

[0009] The conductance of the excitation circuit is defined as the sum of the conductances of one or more elements made of a conductive material, where the conductance of an element made of a conductive material corresponds to the conductivity of the conductive material, weighted by the ratio of the volume of the conductive material of the element to the square of the distance between the element and the center of the cell. In the present invention, the conductance of the excitation circuit is less than 8000 Siemens, preferably less than 5000 Siemens, and more preferably less than 1500 Siemens.

[0010] Some preferred but non-limiting aspects of this cell are as follows: - The typical dimensions of the enclosure are 3mm to 3cm, preferably 5mm to 2cm. - The excitation circuit is capacitively coupled to the cell, and consists of two electrodes, with one or more elements made of conductive material. Each electrode consists of a ring of conductive material surrounding the housing. Each electrode consists of a deposit of conductive material on the housing. Each electrode consists of a printed conductive material on a dielectric film mounted on the housing. - Dielectric films are flexible. - The excitation circuit is inductively coupled to the housing and consists of a coil in which a conductive material element or multiple elements are wound in multiple loops around the cell. - A coil is a solenoid. - The coil is the helix of a helical resonator.

[0011] The present invention also relates to an optical pumping magnetometer comprising a cell according to the present invention, and a magnetoencephalography helmet comprising a plurality of magnetometers according to the present invention. [Brief explanation of the drawing]

[0012] Other aspects, objects, advantages, and features of the present invention will become more apparent from the following detailed description of its preferred embodiments, given by way of non-limiting example and made with reference to the accompanying drawings.

[0013] [Figure 1] FIG. 1 shows a schematic diagram of a magnetometer incorporating a cell according to the present invention. [Figure 2] FIG. 2 is an example of electrodes deposited on a flexible dielectric film in a fork pattern.

Mode for Carrying Out the Invention

[0014] Unlike an optical pumping alkali magnetometer, an optical pumping metastable helium magnetometer is based on an atomic state that is not the ground state (1 1 S0 in the spectroscopic notation of helium-4), i.e., an excited state, in this case a metastable triplet state (denoted as 2 3 S1 in helium-4). For the magnetometer to function, this state must be satisfied (there must be a sufficient number of atoms in this state). This is typically achieved using a discharge operating in the high frequency range, typically between 1 MHz and 100 MHz. This discharge generates a plasma in the cell, and high-energy collisions between atoms and electrons in the plasma excite some of the atoms to the metastable state. Various strategies can be used to initiate and maintain this plasma discharge. · A capacitively coupled discharge circuit consisting of two electrodes placed on both sides of the gas cell, or · An inductively coupled discharge circuit consisting of a solenoid arranged such that the magnetic flux generated by the circuit is partially applied to the gas in the cell. One variant consists of a helical resonator with only one end connected to the generator and facing the ground plane. At a specific frequency, this resonator behaves like a solenoid and has advantages in terms of impedance matching to the generator.

[0015] For measurement applications in the human body, the magnetometer preferably is small. Ideally, components of the magnetometer other than the sensing element (notably capacitive or inductive elements enabling plasma ignition and maintenance) should not take up much space and should be placed in the immediate vicinity of this sensing element, such that the overall dimensions of the sensor should be only slightly larger than the dimensions of the sensing element it contains. In this regard, capacitive electrodes are as advantageous as inductive circuits including a coil with a diameter only slightly larger than the diameter of the helium cell.

[0016] Research on the optimal discharge regime for obtaining high-amplitude magnetometer signals was conducted and published in the paper by J. Rutkowski [2], and relates to cells with representative dimensions of 5 mm and 1 cm and electrodes consisting of fragments of copper tape coated with an adhesive on the back side.

[0017] This document describes other capacitive electrodes, notably electrodes consisting of two (or more) rings surrounding the cell. These electrodes are mentioned in the paper by Marie-Constance Corsi [3], where they are described in terms of "thicker" wires than those used to connect copper tape (0.1 mm) coated with an adhesive on the back side. The patent application [4] related to this research discloses wires with a diameter of 315 microns.

[0018] In addition to obtaining a strong magnetometer signal, it is also desirable to limit the intrinsic noise of the magnetometer to its inevitable components, notably photon noise that occurs when other technical noises (such as laser current noise) can be ignored. Since photon noise can be calculated, the applicant was able to confirm that there is still technical noise of unknown nature limiting the total noise of the sensor. The applicant suspected that this technical noise was related to the electrodes, which is now proven as explained below.

[0019] The applicant was able to demonstrate that, in the case of sensors based on the optical pumping of metastable helium, conventionally used copper tape electrodes with adhesive on the back (35-100 microns thick) and previously used thick wire loop electrodes (315 or 500 microns thick) generate significant magnetic noise that becomes the limiting noise for the optimized sensor.

[0020] Furthermore, the applicant found that by significantly limiting the amount of conductive material constituting the means (electrodes or inductive discharge circuits) that excite the plasma discharge located near the measurement cell, it is possible to greatly reduce the contribution of this noise and thereby achieve a more favorable sensitivity level than before.

[0021] Based on this observation, the applicant proposes a cell for an optical pumping magnetometer with a circuit optimized for generating long-lived, high-density metastable atoms by exciting a plasma in a housing containing atomic gas by discharge, without introducing additional technical noise from the discharge or its excitation means.

[0022] Therefore, the present invention relates to a cell for an optical pumping magnetometer, comprising a housing for containing a gas and a circuit for exciting a plasma within the housing.

[0023] The magnetometer is preferably a small magnetometer, in which case the housing has a typical dimension between 3 mm and 3 cm, preferably between 5 mm and 2 cm (diameter for a sphere, side length for a cube, and diameter and height for a cylinder).

[0024] The gas may also be helium-4.

[0025] The circuit for exciting the plasma within the enclosure comprises one or more conductive material elements positioned relative to the enclosure to enable the application of a discharge to the gas contained within the enclosure. This circuit is powered, for example, by an RF high-frequency generator in the range of 10-100 MHz.

[0026] In this invention, the amount of conductive material placed near the sensing element is limited so that the conductance of the excitation circuit is less than 8000 Siemens, preferably less than 5000 Siemens, and more preferably less than 1500 Siemens. By reducing this conductance to a value of less than 8000 Siemens, it becomes possible to exceed the best sensitivity previously obtained with a miniature helium magnetometer.

[0027] In the context of the present invention, the conductance Q of this excitation circuit total Q is defined as the sum of the conductances Q of each element of one or more conductive materials. In other words, if the excitation circuit has an element of a single conductive material, its conductance Q is defined as the sum of the conductances Q of each element of one or more conductive materials. total Q corresponds to the conductance of the conductive material element. Furthermore, if the excitation circuit has elements made of multiple conductive materials, its conductance Q total This corresponds to the sum of the individual conductances of each element made of conductive material.

[0028] The conductance of an element made of a conductive material is given by the volume V(m³) of the conductive material of the element. 3 ) corresponds to the conductivity σ (Siemens / m) of the conductive material, weighted by the ratio of the ratio of the distance a(m) between the element and the center of the cell, i.e., the conductivity represented by Equation 1 below. Therefore, when the excitation circuit comprises elements made of multiple conductive materials (for example, two electrodes), its conductance Q total This corresponds to the sum of the conductance Q of various elements.

[0029]

number

[0030] In one possible embodiment, the excitation circuit is capacitively coupled to the cell and consists of two electrodes, with one or more elements made of conductive material.

[0031] These electrodes may each consist of a ring of conductive material surrounding a housing having conductance in accordance with the present invention, while following the arrangement disclosed in [3].

[0032] In the first modified example, each electrode may be composed of a deposit of conductive material on the housing.

[0033] In a second modification, each electrode may consist of a print of conductive material on a dielectric film attached to the housing, for example, by adhesive. The dielectric film is preferably a flexible film.

[0034] In another possible embodiment, the excitation circuit is inductively coupled to the housing, and the elements or multiple elements of conductive material consist of coils wound in multiple loops around the cell.

[0035] The coil may be a solenoid or form the helix of a helical resonator.

[0036] The present invention extends to optical pumping magnetometers equipped with the aforementioned cells, and to magnetoencephalography helmets equipped with multiple magnetometers according to the present invention.

[0037] Referring to Figure 1, such an optical pumping magnetometer has a cell comprising a housing 1 filled with an atomic gas such as helium-4, which is exposed to an ambient magnetic field B0, and three components are defined by projecting this magnetic field onto orthogonal coordinate axes.

[0038] For example, such a magnetometer can be based on a housing filled with high-purity helium-4. The typical dimensions of this housing (diameter for a sphere, sides for a cube, and diameter and height for a cylinder) may be between 3 mm and 3 cm. The housing is filled with helium at a pressure determined by its size, which is typically 10 torr for a 1 cm cylindrical cell.

[0039] The cell is illuminated by an optical pumping source 2, which can emit an optical beam F toward cell 1, for example, a laser beam tuned to a pumping wavelength (this beam is also called the pump beam). The pumping wavelength is set to the atomic transition line, for example, the D0 line at 1083 nm in the case of helium-4.

[0040] The magnetometer also includes a circuit for exciting the plasma within the housing, comprising one or more conductive elements as described above. This excitation circuit is coupled to the high-frequency generator 4 and the overvoltage coil 5.

[0041] In one possible embodiment, electrodes are positioned on the outer surface of the housing and capacitively ignite and maintain a plasma discharge in helium gas. This discharge creates a state used for magnetic measurement. 3 The S1 state is occupied.

[0042] As shown in Figure 2, these electrodes 11 can be manufactured, for example, by a flexible electronics process, which consists of lithography of tracks (e.g., a 35-micron thick layer of copper) of a conductive material 12 that forms a fork-shaped pattern. The teeth of the fork can be dimensioned to match the outer surface of the cylindrical housing over its entire height. These teeth are arranged on a rectangular portion 13 of a flexible dielectric film, which may have dimensions of 11 mm × 6 mm.

[0043] Alternatively, these electrodes can be manufactured by depositing a thin layer of conductive material (e.g., a 1-micron thick copper layer) onto the outer surface of the housing using a physical vapor deposition process, such as vapor deposition or cathode sputtering. In this case, the two electrodes are separated before deposition using a masking element such as adhesive tape. The thin layers can also be reconnected, for example, by an ultrasonic micro-welding process or by welding to a silver lacquer droplet.

[0044] In all cases, these elements are typically connected to a high-frequency generator 4 in the range of 10 to 100 MHz. To reduce power reflection caused by excessive impedance mismatch, it is advantageous to provide an impedance matching circuit between these electrodes and the high-frequency generator. This circuit is made up of an overvoltage coil 5, for example, two air-core coils with an inductance of several tens of μH. With such impedance matching, a discharge can be ignited with a power of about 200 mW and maintained with a power of about 10 mW.

[0045] When the discharge is ignited, a significant number of atoms in the cell are excited to the 3 S1 state, whereby magnetic measurements can be performed in various configurations well known in the art and reported, for example, in reference [5].

[0046] One possible operating mode of the magnetometer is to pass a collimated laser beam linearly polarized by the polarizer 3 and tuned to the D0 line of helium-4 through the cell. This laser beam is emitted from a laser 2 with excellent amplitude stability, characterized by relative intensity noise (RIN) dominated by photon noise at an output of about 1 mW. Also, the laser beam needs to have extremely low phase noise. Among laser technologies that meet this requirement, fiber lasers and lasers can achieve the best measurement performance.

[0047] This beam is photodetected after passing through the cell. Furthermore, a high-frequency magnetic field, for example, with a frequency of 40 kHz, is applied to the cell in a direction perpendicular to the polarization direction of the light. This generates a 40 kHz signal in the photodetection spectrum, allowing measurement of the magnetic field component parallel to the direction of the applied high-frequency magnetic field in the range of tens of nT around zero magnetic field. Synchronized detection of this signal makes it possible to measure the magnetic field component. The sensor's inherent noise corresponds to the noise obtained in the synchronous detection output, and its sources are diverse. In the case described here, this noise is limited by the photon noise of the laser light used to generate the light, and when converted to magnetic field units, it corresponds to a value between 20 and 30 fT / √Hz, which is favorable to the noise level obtained previously.

[0048] Therefore, this magnetometer may include a parametric resonant excitation circuit, which includes a high-frequency generator 8, which powers a Helmholtz coil 7 with orthogonal axes surrounding the cell to generate a parametric resonant excitation magnetic field (also called a high-frequency magnetic field). The magnetometer also includes a parametric resonance detection device 6 configured to measure the absorption of a light beam by an atomic gas, and a photodetector 10 arranged to receive the light beam that has passed through the cell and supply a photodetection signal to the parametric resonance detection device 6.

[0049] The magnetometer may also be equipped with a closed-loop magnetometer control system to keep the sensing element constantly exposed to a total magnetic field of zero. This control system is coupled to the detection device 6 and includes a regulator 9 that injects current into the Helmholtz coil 7 to generate a compensating magnetic field Bc, ensuring that the total Bc + B0 is always kept zero. Alternatively, the magnetometer can be operated in open-loop mode without compensating for the ambient magnetic field.

[0050] Since noise is related to the conductance of the conductive elements in the excitation circuit, it should be noted that electrodes made from materials with lower conductivity than copper or aluminum, such as transition metals like Nb or Mo, alloys like CuAl, AgCu, or AuAg, or semiconductors like Si or Ge, can be used, even though they have a larger volume than the electrodes described below.

[0051] In another possible embodiment, the inductive ignition uses, for example, a solenoid or helical resonator with a diameter of 3 cm or less, which is fabricated using one of the following: Typically, copper wire with a diameter of less than 300 microns, or A thicker wire made from a material with lower conductivity than copper or aluminum, especially in the case of capacitive electrodes, using the materials exemplified above.

[0052] Comparative Example

[0053] Example 1a: Conventional Art In this embodiment, the cells described in [2] (100 mm³ cylindrical cells with a diameter of 5 mm and a height of 5 mm) and electrodes in the form of 4*4 mm copper tape pieces (manufacturer 3M, part number 3313, thickness 35 microns) with adhesive applied to the back surface are used, and the electrodes are connected to the RF generator by wires welded to the copper elements. The amount Q ignores the weld metal. total This is 10,700 Siemens. The unexplained noise component resulting from measurements in a low magnetic field configuration is approximately 120 fT / √Hz.

[0054] Example 1b: By prior art In this embodiment, a larger cell (a cylinder with a diameter of 1 cm and a length of 1 cm) is used, similar to the cell described in [2], but larger than that of Example 1a. The electrodes are 1*0.9 cm copper tape pieces (manufactured by 3M, part number 3313, thickness 35 microns) with adhesive applied to the back surface, each bonded to the outer surface of the cell, and the electrodes are connected to the RF generator by wires welded to the copper elements. The amount Q negligible is the weld metal. totalThe value is 13,000 Siemens. The unexplained noise component resulting from measurements in a low magnetic field configuration is approximately 100 fT / √Hz.

[0055] Example 2 (Prior Art) In this embodiment, ring electrodes as described in [3] and [4] are used, each consisting of a 1 cm diameter loop made of copper wire with a diameter of 315 microns. Quantity Q total The value is 11,700 Siemens. The unexplained noise component resulting from measurements in a low magnetic field configuration is approximately 77 fT / √Hz.

[0056] Example 3 (According to the present invention) In this embodiment, a 1-micron thick thin film electrode (formed by copper deposition) is used, deposited by vapor deposition using masking that gives a 1 × 1 cm rectangular shape to the side of a cylindrical cell. These electrodes are connected to an RF generator by wires micro-welded with ultrasound. The weld metal is ignored. total This is 920 Siemens. The unexplained noise is less than 19 fT / √Hz, and because its power is lower than the power of the photon noise in the experiment (30 fT / √Hz), it is accompanied by high uncertainty.

[0057] Example 4 (According to the present invention) In this embodiment, a thin electrode fabricated on a flexible polymer such as Kapton by a lithography process well known in the field of flexible electronics is used, with a track width of 150 microns and a copper thickness of 18 microns. total This is 1170 Siemens. The unexplained noise is 16 fT / √Hz, and its low power compared to the experimental photon noise (25 fT / √Hz) results in high uncertainty.

[0058] Example 5 (According to the present invention) In this embodiment, the same electrodes as in Example 2 are used, but they are manufactured from 100-micron wire. Quantity Q totalThe value is 1200 Siemens. The unexplained noise component resulting from measurements in a low magnetic field configuration is approximately 25 fT / √Hz.

[0059] Example 6 (According to the present invention) In this embodiment, an induction ignition system is used, consisting of an induction coil made by winding a 150-micron wire six times around a cell with a diameter of 2 cm. Quantity Q total The value is 7900 Siemens. The unexplained noise component resulting from measurements in a low magnetic field configuration is approximately 32 fT / √Hz.

[0060] References [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. [2] J. Rutkowski, "Study and realization of a miniature isotropic helium magnetometer," University of Franche-Comté, 2014. [3] M.-C. Corsi, "Helium-4 optically pumped magnetometers: development and proof of concept in magnetocardiography and magnetoencephalography," PhD dissertation, University of Grenoble-Alpes, 2015. Accessed April 20, 2021. [Online]. Available at: https: / / tel.archives-ouvertes.fr / tel-01233850 [4] French Patent No. 3 035 769 A1 [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, June 2021, doi: 10.1063 / 5.0047124.

Claims

1. A gas enclosure (1) and A cell for an optical pumping magnetometer, comprising: a circuit for exciting plasma within the housing, the circuit comprising one or more conductive material elements (11) arranged to enable the application of a discharge to the gas contained in the housing, The cell is characterized in that the conductance of the excitation circuit is defined as the sum of the conductances of one or more elements made of a conductive material, wherein the conductance of the elements made of the conductive material corresponds to the conductivity of the conductive material, weighted by the ratio of the volume of the conductive material of the element to the square of the distance between the element and the center of the cell, and the sum of these values ​​is less than 8000 Siemens.

2. The cell according to claim 1, wherein the conductance of the excitation circuit is less than 5,000 Siemens, preferably less than 1,500 Siemens.

3. The cell according to claim 1, wherein the typical dimensions of the housing are 3 mm to 3 cm, preferably 5 mm to 2 cm.

4. The excitation circuit is capacitively coupled to the cell, and the element or plurality of elements made of a conductive material consist of two electrodes, according to any one of claims 1 to 3.

5. The cell according to claim 4, wherein each of the electrodes is a ring of conductive material surrounding the housing.

6. The cell according to claim 4, wherein each of the electrodes is made of a deposit of conductive material on the housing.

7. The cell according to claim 4, wherein each of the electrodes consists of a printed conductive material (12) on a dielectric film (13) attached to the housing.

8. The cell according to claim 7, wherein the dielectric film is flexible.

9. The cell according to any one of claims 1 to 3, wherein the excitation circuit is inductively coupled to the housing, and the element or a plurality of elements made of a conductive material consists of a coil wound in a plurality of loops around the cell.

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

11. The cell according to claim 9, wherein the coil is the helix of a helical resonator.

12. An optical pumping magnetometer comprising the cell according to any one of claims 1 to 11.

13. A magnetoencephalography helmet comprising a magnetometer according to one of the claims 12.

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