TECHNICAL FIELD

The field of the invention is that of magnetic gradiometry, namely the measurement of a spatial gradient of a magnetic field. The invention relates in particular to the performance of a gradiometric measurement by means of an optically pumped magnetometer.

PRIOR ART

A measurement of the magnetic field gradient is usually carried out by subtracting the signals of several spatially distributed magnetometers. Such a measurement allows the rejection of common mode noise between the different measurement points and thus improvement of the sensitivity level of the magnetometric measurement. Gradiometry thus provides complementary information to magnetometry, especially as the spatial dependence of the measurements is different, which makes it possible to locate the sources of the magnetic field.

A gradiometric measurement can also be carried out by means of SQUID sensors, which have the capacity to intrinsically perform the magnetic field difference between two measurement zones. Indeed SQUID sensors measure the magnetic flux passing through a metal turn that includes superconducting junctions. By arranging two turns connected to each other so that the fluxes passing through each of them induce currents in opposite directions, the resulting current measured is directly proportional to the difference in magnetic fluxes through the turns.

SQUID sensors, on the other hand, allow only one component of the magnetic field, that orthogonal to the plane of the coil, to be measured. Two types of gradiometers can thus be made with SQUID sensors:longitudinal gradiometers where the two turns are in two parallel planes and separated by a distance b called the gradiometer base. Such a configuration allows the measurement of the magnetic field gradient in the direction longitudinal to the magnetic field (∂Bz/∂z);transverse gradiometers where the two turns are in the same plane and separated by a base b, the distance between the two centres of each turn. This configuration allows the gradient of a component of the magnetic field to be measured with respect to one of the directions transverse to the component (∂Bz/∂x ou ∂Bz/∂y).

One drawback of these gradiometers is that once the sensor is made, the base b as well as the direction of measurement of the gradient (transverse or longitudinal) are set. Furthermore, SQUID sensors require cryogenic cooling to operate and are usually placed inside magnetically shielded enclosures to dispense with external magnetic fields that could interfere with measurements. The size of these enclosures has to be large enough to contain the cryogenic vessel, which results in strong constraints on the cost and architecture of the buildings containing these devices, which constraints hinder their spreading.

An alternative to the SQUID sensor is the optically pumped magnetometer which uses an atomic gas confined in a cell, typically metastable helium or alkali gases, as the sensitive element. Such a magnetometer, which can take different configurations, allows the magnetic field to be deduced by exploiting the following three processes which take place either sequentially or concomitantly:

1) The use of polarised light sources, typically lasers, allows the preparation of atomic states characterised by some orientation or alignment of their spins. This process is called “optical pumping” in this field.

2) These atomic states change under the effect of the magnetic field, especially under the Zeeman effect, which corresponds to shifts in the energy levels as a function of 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. An optical measurement, for example an optical absorption measurement, can be used to deduce the Zeeman shift undergone and to derive a measurement of the magnetic field in which the cell is immersed.

There are two main categories of optically pumped magnetometers depending on the type of pumping made.

In the most common category, pumping is carried out with a pumping beam emitting circularly polarised light and the atomic gas acquires a so-called oriented state characterised by a non-zero average value of its magnetic moment along an axis, which happens to be that of the pumping beam propagation.

In the other category, pumping is carried out with a pumping beam emitting linear polarisation light and the atomic gas acquires a so-called aligned state characterised by a zero value of its magnetic moment, but by a non-zero value of a quantity of the type (3 Fz2−F2) where F is the total angular momentum and Fzthe angular momentum along the direction of polarisation of the pumping beam.

The characterisation of the atomic states (step3above) can be carried out according to at least two ways:by measuring the absorption of a beam tuned (or very close) to the atomic transition under consideration (the beam used for pumping or a so-called “probe” beam with the same polarisation as the beam used for pumping);by using a so-called “probe” beam with linear polarisation and shifted in wavelength with respect to the atomic transition under consideration. Depending on the type of atomic polarisation (orientation or alignment), this beam undergoes a modification of its polarisation (rotation of the polarisation plane in the case of orientation, creation of a circularly polarised component in the case of alignment) which can be measured by separating two of the polarisation states of the beam (the two 45° polarisations in the case of orientation, the two circular polarisations in the case of alignment) and then photodetecting them in such a way as to identify the increase in one of the polarisations relative to the other. This is referred to as polarimetric measurement.

There are two physical effects that allow a vector measurement of the magnetic field. These are the Hanle effect and parametric resonances.

The first effect is mainly used in orientation magnetometers and has different alternatives to be probed, mainly in polarimetry, through a Faraday rotation measurement or a modulation of the probe beam polarisation. Its main advantage is that it allows an all-optical measurement of one or two components of the magnetic field, without requiring the application of radio frequency (RF) magnetic field(s). However, Hanle orientation configurations suffer from a lack of compactness, requiring several laser beams (pump and probe) propagating orthogonally to each other.

Parametric resonances are used in orientation and alignment magnetometers, mainly in absorption. The difference with the Hanle effect lies in the application of radio frequency magnetic fields to the cell containing the sensitive element, which allow linear dependencies of some frequency components of the photodetection signal on the components of the magnetic field to be shown. The advantage of this type of architecture is that a measurement of one or more components of the magnetic field can be made using a single laser beam acting as a pump and probe, thus enabling more compact magnetometers to be made. The use of this effect is often preferred for medical applications such as magnetoencephalography or magnetocardiography, which require a high density of sensors in the zone under study.

Gradiometry work with optically pumped magnetometers using a spatially resolved photodetector has been carried out by the team of M. V. Romalis at Princeton University (Kominis et al.,Nature Vol422, 2003). The architecture used relies on the Hanle effect to measure a single component of the magnetic field (the one perpendicular to the direction of propagation of the pump beam and the probe beam). Atoms, pumped in orientation, are probed by modulated Faraday rotation. The signal is collected on a segmented photodiode in the direction longitudinal to the measured component of the magnetic field. In this way it is possible to measure the longitudinal gradient of the component of the magnetic field (∂By/∂y).

Further work made subsequently by the same team makes use of an array of 16*16 photodiodes to measure the longitudinal gradient of the component of the magnetic field (∂By/∂y) as well as a gradient transverse to this component (∂By/∂z) while having a reconfigurable base.

More recently, a similar architecture to those aforementioned has been adapted to a magnetoencephalography measurement (Zhang et al., AIP Advances 8, 125028, 2018). The only difference is that the signal is analysed on the two linear polarisation bases instead of using a modulation of the probe beam polarisation to obtain the measurement signal. An array of 2*2 photodiodes is used, not allowing a reconfigurable base.

Other work using a spatially resolved photodetector has been made by P. Schwindt's team in Albuquerque (Colombo et al., Optics Express Vol 24, No 14, 2016). The architecture set forth in this publication is based on the principle of parametric resonances, allowing propagation of the pump and probe beams along the same direction and measurement of one of the two components of the magnetic field transverse to the direction of propagation of the laser beams in this case. The pump and probe beams are split into 8 different beams (4 pump beams and 4 probe beams) by diffraction, each copropagating pump/probe pair being sent to the 4 corners of a square cell containing the sensitive element. An array of 2*2 photodiodes is used to collect information on the 4 probe beams. This architecture allows the measurement of the longitudinal gradient of the component of the magnetic field (∂By/∂y) as well as a transverse gradient to the same component (∂By/∂x) with a fixed base in both directions.

All of the aforementioned work has been carried out by measuring a single component of the magnetic field. These architectures can easily be extended to a measurement of a second component of the magnetic field, the one perpendicular to the direction of propagation of the pump beam. In the case of Hanle configurations, this requires the addition of a second probe beam propagating perpendicular to the pump beam and the first probe beam. In the case of the parametric resonance architecture, this requires the addition of a second radio frequency magnetic field orthogonal to the direction of propagation of the probe beam and the direction of the first radio frequency field. It is not possible to obtain the third and final component of the magnetic field in orientation-pumped Hanle configurations. In the case of orientation-pumped parametric resonances, it is possible to obtain it by a second-order effect but with a much lower sensitivity than in the other two axes.

From the gradient measurement point of view, these architectures allow at best to measure only four components of the gradient tensor (measurement of two components of the magnetic field in two directions) out of the nine it includes.

DISCLOSURE OF THE INVENTION

One purpose of the invention is to provide an optically pumped magnetometer capable of measuring all the components of the magnetic field tensor. To this end, the invention relates to a three-axis vector optically pumped magnetometer, comprising: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,a photodetector arranged to receive a probe beam that passed through the cell, anda processing unit coupled to the photodetector.

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.

The processing unit 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; andcalculate 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.

Some preferred but non-limiting aspects of the magnetometer are as follows:

the measurement units of the photodetector are arranged in array form and are at least 3*3 in number;

the photodetector is an array of photodiodes;

the photodetector is a charge coupled detector;

a measurement unit of the photodetector is formed by a plurality of pixels close to the charge coupled detector, the photodetection signal of a measurement unit corresponding to the average of the photodetection signals individually delivered by the pixels of the measurement unit;

it further comprises:a parametric resonance excitation circuit configured so that it induces 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,a polarisation device configured to impart a linear polarisation to the pump beam,

and, to determine a measurement, associated with a 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 said oscillation frequencies:

it further comprises:a parametric resonance excitation circuit configured so that it induces 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,a polarisation device configured to impart an elliptical polarisation to the pump beam,

and, to determine a measurement, associated with a 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.

The three components of the ambient magnetic field comprise two components, Bxand By, transverse to the direction of propagation of the probe beam and one component, Bz, longitudinal to the direction of propagation of the probe beam and the gradiometric measurement signal comprises:the transverse gradients of the Bzcomponent, ∂Bz/∂x and ∂Bz/∂y,∂,the longitudinal gradients of the Bxand Bycomponents, ∂Bx/∂x and ∂By/∂y,a transverse gradient of the Bxcomponent and a transverse gradient of the Bycomponent, ∂Bx/∂y et ∂By/∂x.

