Vectorial, optically pumped magnetometer with high sensitivity

Abstract

The invention relates to a vectorial, optically pumped magnetometer (OPM) with high sensitivity, in particular an OPM which defines its sensitive direction independently of the position of the external (Earth's) magnetic field. The object of the present invention, to provide a vectorial, optically pumped magnetometer (OPM) whose sensitive direction is self-defined independently of the position of the external (Earth's) magnetic field and which can be used unshielded in the Earth's magnetic field with high bandwidth and simultaneously very high sensitivity in any spatial direction, is achieved by the magnetometer comprising an OPM sensor head (3) surrounded by an external magnetic field (4) and a light source (5) for generating pump light and probe light, wherein the light from the light source (5) passes through a converging lens (6), a polarizing filter (7), a quarter-wave plate (8), a Wollaston prism (9), a wedge prism (10), a deflecting prism (11), and a half-wave plate (12) to generate probe light as pump light through an alkali-vapor-filled OPM cell (13) with a thermally insulated housing (14).which is directed from the OPM cell (13) via a deflecting prism (11) onto photodiodes (16) for detection, wherein the OPM sensor head (3) is located in the interior (2) of a permanent magnet (1) and the interior (2) encloses a statically homogeneous magnetic field in the form of a constant holding field (15), which is perpendicular to the axis of rotation of the permanent magnet (1) and is superimposed on the external magnetic field (4), wherein the magnetic flux density of the holding field (15) is at least ten times greater than that of the external magnetic field (4) and the OPM sensor head (3) measures the magnitude of the vector sum of holding field (15) and external magnetic field (4).

Description

[0001] The invention relates to a vectorial, optically pumped magnetometer (OPM) with a high sensitivity, in particular an OPM which defines its sensitive direction independently of the position of the external (earth) magnetic field.

[0002] A magnetometer is a sensor device for measuring magnetic flux densities.

[0003] Magnetic flux densities are measured in the unit Tesla (T), and typical measuring ranges of magnetometers are in the range of approximately 10 -15 T to 10 T.

[0004] Because of this wide range of values, different measuring methods are used under the term “magnetometer”.

[0005] The most common magnetometers are: - Hall sensors - Förster probes or saturation core magnetometers - fixed and rotating coils - optically pumped magnetometers - SQUIDs - Proton magnetometer - Magnetometer based on Bose-Einstein condensates - Kerr magnetometer - Faraday magnetometer (https: / / de.wikipedia.org / wiki / Magnetometer)

[0006] Superconducting quantum interference devices (SQUIDs) were invented more than 50 years ago. These are highly sensitive vector magnetometers, meaning they measure the component of the magnetic field perpendicular to their surface. By using three SQUIDs arranged perpendicular to each other, the complete magnetic field vector can be determined.

[0007] Since their invention, many applications for SQUIDs have opened up. As early as 1980, the use of SQUIDs in geosciences was discussed in a workshop (Weinstock and Overton 1981 SQUID Applications to Geophysics (Society of Exploration Geophysicists).

[0008] Manufacturing and cooling technologies, electronics, and other components of the SQUID system have improved significantly in recent years. SQUIDs are now more sensitive and effective, as well as robust and reliable, making them suitable for geophysical measurements. In recent years, SQUIDs have developed into a mature technology for use in geoscientific applications. (Stolz R, Schmelz M, Zakosarenko V, Foley CP, Tanabe K, Xie X, Fagaly R (2021b) Superconducting sensors and methods in geophysical applications. Supercond Sci Technol. https: / / doi.org / 10.1088 / 1361-6668 / abd7ce)

[0009] Optically pumped magnetometers (OPMs) are integrated optical arrays in the form of highly sensitive quantum sensors that can detect tiny magnetic fields by spectroscopically measuring the effect of an ambient magnetic field on an atomic vapor.

[0010] At its core is a transparent cell filled with an alkali vapor, such as an atomic vapor of the elements rubidium, cesium, potassium, or metastable helium. A change in the magnetic field surrounding the cell affects the optical properties of the vapor atoms, which are detected with laser light.

[0011] OPMs exist in numerous, sometimes very different implementations, which can, however, be divided into two categories: Type I OPMs are those without magnetic shielding or active compensation of the Earth's magnetic field. Their sensitivity depends, among other things, on the cell volume and the alkali vapor used and can reach up to 200 fT / √Hz for commercially available sensors [Oelsner G, Ijsselsteijn R, Scholtes T, Krüger A, Schultze V, Seyffert G, Werner G, Jäger M, Chwala A and Stolz R 2022 Integrated Optically Pumped Magnetometer for Measurements within Earth's Magnetic Field Phys. Rev. Appl. 17 024034]. Without special additional measures, this sensor type is a scalar magnetometer that only measures the magnitude of the magnetic field. Depending on the relative orientation of the sensor to the magnetic field, there are zones with reduced or even zero sensitivity (dead zones) as well as systematic measurement errors (directional errors).

[0012] Type II are zero-field OPMs. In the so-called SERF mode (for Spin Exchange Relaxation Free), they exhibit significantly higher sensitivity than the OPMs from 1. (Twinleaf microSERF: <30 fT / √Hz [webpage: www.twinleaf.com / vector / microSERF], QuSpin QZFM Gen-3: <15 fT / √Hz [webpage: www.quspin.com / productsqzfm / ]). However, they can only be operated in extremely weak magnetic fields (typically < 100 nT), making them unsuitable for direct measurement of the Earth's magnetic field, which can range from 25 µT to 70 µT depending on the region.

[0013] To the extent that the use of OPMs instead of SQUIDs is possible, it offers numerous advantages for the user: • No complex cooling using cryogenic liquids, which must be handled by trained personnel. • Lower operating costs, in particular the costs for cooling media and their logistics are eliminated. • Lower weight and potentially more compact design, making the systems more portable and opening up new fields of application.

[0014] In the case of geomagnetic applications, however, the vector measuring SQUIDs cannot easily be replaced by OPMs.

[0015] As previously explained, only Type I OPMs are suitable for this purpose. Unlike SQUIDs, these are scalar magnetometers. This results in the following disadvantages compared to SQUIDs: 1. Information about the components of the magnetic field vector is lost, which may be essential for some applications. 2. The magnetometer is only sensitive along a preferred direction, which is defined by the vector of the Earth's magnetic field. Variations in the magnetic field perpendicular to this direction are significantly suppressed by the scalar nature of the measurement, as explained below.

[0016] The magnetic field to be measured is decomposed into a static component B0→ and two significantly weaker, time-varying variations B∥→ and B⊥→ parallel or perpendicular to B0→. The Larmor frequency measured by the OPM is proportional to the magnetic field magnitude: fL=Y2π⋅|B0→+B∥→+B⊥→|≈Y2π⋅|B0→+B∥→|+γ2π⋅|B⊥22B0|

[0017] From the above equation it follows that the variation B∥→ directly into the Larmor frequency, whereas B⊥→ only occurs as a quadratic disturbance term, which for the case assumed here |B⊥→|<<|B0→| can be neglected.

[0018] In order to adequately replace SQUIDs with OPMs in geomagnetic applications, an OPM-based sensor concept must be developed that meets the following requirements: • A1: All three vector components of the (Earth) magnetic field must be able to be measured with the same sensitivity. • A2: The white noise of each measured vector component should be less than 100 fT√Hz / . • A3: The measurement bandwidth of the sensor should be greater than 500 Hz in each vector component.

[0019] The previous approaches for vectorial measuring OPMs can be divided into three categories • K1: OPM without magnetic field manipulation • K2: OPM with magnetic field manipulation • K3: OPM in a stationary holding field, which is significantly stronger than the magnetic field to be measured, which determines its sensitive axis (preferred direction).

[0020] In category K1, the Type I OPM is extended to allow the determination of not only the magnitude of the magnetic field but also the direction of the magnetic field vector [Fairweather AJ and Usher MJ 1972 A vector rubidium magnetometer J. Phys. E. 5 986-90; Patton B, Zhivun E, Hovde DC and Budker D 2014 All-optical vector atomic magnetometer Phys. Rev. Lett. 113 28-30; Sun WM, Huang Q, Huang ZJ, Wang PW and Zhang JH 2017 All-Optical Vector Cesium Magnetometer Chinese Phys. Lett. 34 058501]. This can be achieved, for example, by using multiple lasers that irradiate the alkali vapor from different directions. However, this approach does not solve the problem that this type of magnetometer is only sensitive in the direction of the Earth's magnetic field, ie requirement A1 is not met.

[0021] As examples of approaches in category K2, two approaches are presented: The first variant uses a three-axis coil system to actively compensate the external magnetic field at the sensor to zero [Seltzer SJ and Romalis M V. 2004 Unshielded three-axis vector operation of a spinexchange-relaxation-free atomic magnetometer Appl. Phys. Lett. 85 4804-6; Bertrand F, Jager T, Boness A, Fourcault W, Le Gal G, Palacios-Laloy A, Paulet J and Léger JM 2021 A 4He vector zero-field optically pumped magnetometer operated in the Earth-field Rev. Sci. Instrum. 92]. The magnetic field vector can then be calculated from the three coil currents. Although this approach offers the same sensitivity in all three spatial directions, it has several disadvantages. The required coil currents must be extremely low-noise and, according to the current state of the art, significantly dominate the sensor noise, resulting in a loss of sensitivity. Thus, this approach does not meet requirement A2.Furthermore, the technical requirements for the power sources (thermal drift) and the coil system (mechanical expansion) with regard to long-term stability are enormous.

[0022] In another approach, the magnetic field is modulated by a small amount in the three spatial directions using coils with a given frequency and the corresponding components are measured by demodulation as a projection onto the external (Earth) magnetic field [Gravrand O, Khokhlov A, Le Mouël JL and Léger JM 2001 On the calibration of a vectorial 4He pumped magnetometer Earth, Planets Sp. 53 949-58; Alexandrov EB, Balabas M V., Kulyasov VN, Ivanov AE, Pazgalev AS, Rasson JL, Vershovski AK and Yakobson NN 2004 Three-component variometer based on a scalar potassium sensor Meas. Sci. Technol. 15 918-22; Vershovskii AK, Balabas M V., Ivanov A É, Kulyasov VN, Pazgalev AS and Aleksandrov EB 2006 Fast three-component magnetometer-variometer based on a cesium sensor Tech. Phys. 51 112-7].

[0023] The main disadvantage of this technical solution is that it is again only sensitive in the direction of the external magnetic field, thus violating requirement A1. Another disadvantage is that the usable bandwidth of the sensor is significantly reduced due to the modulation method, so that requirement A3 usually cannot be met either.

[0024] A known approach belonging to category K3 [Affolderbach C, Stähler M, Knappe S and Wynands R 2002 An all-optical, high-sensitivity magnetic gradiometer Appl. Phys. B Lasers Opt. 75 605-12] is used in which an OPM is operated in a magnetically shielded environment and converted into a vectorial magnetometer using a weak holding field (< 1 µT) generated by an electromagnet. The purpose of the holding field is to define the sensitive axis of the magnetometer. For this purpose, the holding field – analogous to the discussion on equation (1) – must be significantly stronger than the magnetic background field to be measured. This approach cannot simply be used for measurements in the Earth's magnetic field. In this case, a holding field of > 500 µT (corresponding to approximately 10 times the Earth's magnetic field) would have to be generated. Since the technical requirements for the coil system and the power sources regardingIf noise and long-term stability increase proportionally to the magnetic field amplitude, they would be considerably higher than in the magnetic field compensation methods of category K2 and would thus also violate requirement A2.

[0025] WO 2021045953 A1 discloses, as a further representative of category K3, a magnetic field measuring system comprising the following: an optically pumped magnetometer (OPM) having a vapor cell comprising a housing and a vapor of atoms arranged in the housing, a light source, a detector, and a magnetic field generator; and a processor coupled to the OPM and configured to perform actions including: • Directing a light beam from the light source through the vapor cell of the OPM; • Applying an RF excitation to the atoms to cause the spins of the vapor atoms to precess; • Measuring a frequency of precession by observing the light beam after passing through the vapor cell; • Directing a light beam from the light source through the vapor cell, applying a magnetic field through the vapor cell along the axis, applying an RF excitation to the atoms to cause the spins of the atoms of the vapor to precession, and measuring a frequency of the precession in the applied magnetic field by observing the light beam after passing through the vapor cell for each of a plurality of different axes relative to the vapor cell; • Determine the magnitude and components of an ambient background magnetic field along the axes using the measured frequencies; • and applying a magnetic field based on the components around the vapor cell to counteract the ambient background magnetic field to facilitate operation of the OPM in a spin exchange relaxation free (SERF) mode.

[0026] The disadvantage of these technical solutions is that active modulation with small amplitudes only allows for measurement of projections onto the Earth's magnetic field, which leads to significant losses in the sensor's sensitivity in all three spatial directions. When compensating for the external magnetic field using appropriate field coils, the required currents must be extremely low-noise and, according to the current state of the art, significantly dominate the sensor noise, resulting in a loss of sensitivity.

[0027] In addition, the approach based on the holding field generated by an electromagnet cannot simply be applied to measurements in the unshielded Earth's magnetic field. In this case, a holding field of > 500 µT (equivalent to approximately 10 times the Earth's magnetic field) would have to be generated. Since the noise of the coil currents increases with field strength, it can be assumed, based on the current state of the art, that this current noise is significantly higher than the intrinsic noise of the OPM.

[0028] Optically pumped magnetometers measure only the magnitude of the Earth’s magnetic field vector based on the Larmor frequency → fL=γ2π⋅|B→|. This leads to two crucial disadvantages compared to fluxgates and superconducting quantum interference detectors (SQUID), namely - Vector information is not available, which makes the detection and location of magnetic sources significantly more difficult and causes a reduced information content, e.g. for inversion calculations and magnetic interpretation, and - the OPM is only sensitive in the preferred direction, ie only projections onto the Earth's magnetic field are measured: fL=γ2π⋅|B0→+B∥→+B⊥→|≈γ2π⋅|B0→+B∥→|+γ2π⋅|B⊥22B0|

[0029] Since the direction of the Earth's magnetic field is determined externally, the sensitive direction of the OPM is also determined.

[0030] In general, it can be stated that all known methods that do not manipulate the magnetic field at the location of the sensor are characterized by the fact that the magnitude of the magnetic field is measured with a high sensitivity and additionally angle information is obtained, the accuracy of which, however, is not sufficient to determine all three vector components of the magnetic field with a comparable sensitivity.

[0031] With the methods known so far, it is not possible to generate a holding field of > 500 µT with simultaneously very low noise, which is below the typical sensitivity of an OPM < 100.

[0032] DE 10 2010 020 863 A1 discloses an arrangement for noise reduction in optical magnetometers, wherein the object of reducing the influence of the noise of the pump light source and, with a corresponding circuit design, of the magnetic field on the noise-limited magnetic field resolution of optical magnetometers and approximating the intrinsic, shot-noise-limited value is achieved by irradiating with light from the same light source a second alkali vapor cell in a branch parallel to the magnetometer, wherein this second alkali vapor cell is exposed to a magnetic field different from that detected by the first alkali vapor cell,which is also followed by a photodetector and the amplified signals of the first photodetector in the magnetometer branch and the second photodetector are immediately fed to an electronic difference former and only then is its output signal fed to the phase detector associated with this magnetometer (M↓1↓), which is connected to the frequency generator which applies the defined adjustable alternating magnetic field (B↓1↓1↑(1)↑) to the alkali vapor cell belonging to the magnetometer branch.

[0033] However, this optical magnetometer has the disadvantage that its sensitive axis cannot be freely selected. Furthermore, directional errors or dead zones can occur.

[0034] The object of the present invention is to provide a vectorial, optically pumped magnetometer (OPM) (= a vectorially measuring magnetic field sensor) with a high sensitivity, which defines its sensitive direction independently of the position of the external (earth) magnetic field itself, so that the sensitive axis can be freely selected, and at the same time allows further intrinsic disadvantages, such as directional errors or dead zones, to be avoided.

[0035] The technical solution is also intended to serve the realization of a mobile vectorial OPM, which can be used unshielded in the Earth's magnetic field and has a high bandwidth as well as a very high sensitivity in any spatial direction.

[0036] According to the invention, this object is achieved by the features of the first patent claim. Further advantageous embodiments of the invention are specified in the subordinate patent claims.

[0037] The essence of the invention is that an optically pumped, vectorially measuring magnetometer (OPM) (= a vectorially measuring magnetic field sensor) is provided, in which the sensitive direction is defined by embedding the OPM in a very low-noise constant holding field, which is generated by permanent magnets, particularly advantageously in the form of a Halbach cylinder, and whose magnetic flux density is at least ten times greater than that of the Earth's magnetic field.

[0038] This vector-measuring magnetometer measures changes in the external magnetic field as a projection along this holding field. A 3D or 2D OPM vector magnetometer is realized using an ensemble of three or two orthogonally aligned arrangements of OPM and holding field.

[0039] This provides 3D or 2D OPM vector magnetometers, each of which can measure a component of the Earth's magnetic field along a fixed, sensitive axis with high sensitivity.

[0040] This is achieved by embedding the respective OPM in a low-noise, constant magnetic holding field of high field strength.

[0041] It is particularly advantageous to place the OPMs in a strong, static holding field whose magnetic flux density is at least 10 times the magnetic flux density of the Earth's magnetic field, so that a vectorial measurement of the temporal variation of the Earth's magnetic field is possible without magnetic shielding.

[0042] The respective holding field is generated by a special arrangement of permanent magnets so that it is sufficiently low-noise not to dominate the noise of the OPM, in which the holding field is generated in such a way that it is sufficiently spatially homogeneous within the arrangement so as not to impair the sensitivity of the OPM, and decays very quickly outside the arrangement to enable an arrangement of several units at a short distance without mutual interference.

[0043] Furthermore, the permanent magnets are mechanically adjusted so that the holding field at the desired operating point has a low temperature dependence.

[0044] When arranging these OPMs, they are placed in a strong holding field with > 10 times the Earth's magnetic field, so that no magnetic shielding is necessary for measurements in the Earth's magnetic field.

[0045] The holding field is generated > 500 µT to compensate for the preferred direction determined by the Earth's magnetic field

[0046] This results in the generation of a vectorial preferred direction of the optically pumped magnetometer, which is comparable to the previously mentioned point 3 (OPM in stationary holding field >> background field → specifies preferred direction).

[0047] The holding field will be generated by using solid-state magnets to achieve low noise and thus high magnetic field resolution in the preferred direction.

[0048] In particular, the holding field can be generated particularly preferably by a Halbach cylinder, as this results in the following advantages: 1. Halbach cylinders generate magnetic fields with excellent relative homogeneity across their total volume. This allows for a compact design with a holding field sufficiently homogeneous to prevent magnetic field gradients from impairing the sensitivity of the OPM. 2. Halbach cylinders have low stray fields in the outer chamber. In multi-channel applications, this is essential to prevent magnetic field gradients from impairing the sensitivity of neighboring channels.

[0049] Furthermore, the magnets used in the Halbach cylinder can be made from specially manufactured magnetic materials (alloys) that have the following advantageous properties: 1. The permanent magnetic moment is set to a suitable value so that the holding field is not significantly stronger than 10 times the Earth's magnetic field. 2. The permanent magnetic moment exhibits a negligible temperature dependence at the desired operating point.

[0050] 1. addresses the problem that parasitic effects, such as temperature-related fluctuations or spatial inhomogeneity of the magnetic field, scale linearly with the magnetic field strength. A holding field that is too strong (e.g., > 1 mT) would therefore be counterproductive.

[0051] The use of magnetic materials with negligible temperature dependence, as mentioned in 2., is essential, as otherwise there is a risk that temperature fluctuations will be interpreted as a supposed magnetic signal. This latter parasitic effect can be further reduced by actively controlling the temperature of the Halbach cylinder to the operating point with the lowest temperature dependence.

[0052] By using a 3-axis or 2-axis vector OPM, consisting of three or two systems of Halbach cylinder and OPM, a 3D or 2D OPM vector magnetometer is created that has low noise and thus a high magnetic field resolution along the three (two) freely selected preferred directions.

[0053] This provided technical solution in the form of an optically pumped magnetometer (OPM) (= a vectorially measuring magnetic field sensor) exhibits a high sensitivity, whereby the OPM defines its sensitive direction independently of the position of the external (earth) magnetic field, so that the sensitive axis can be freely selected, while at the same time allowing further intrinsic disadvantages, such as directional errors or dead zones, to be avoided.

[0054] At the same time, this provided technical solution can be used for the realization of a mobile vectorial OPM, which can be used unshielded in the Earth's magnetic field and has a high bandwidth as well as a very high sensitivity in any spatial direction.

[0055] The advantage of using such vectorially measuring magnetic field sensors in geophysics (in magnetics and electromagnetics (EM) or transient electromagnetics (TEM)) is that compatibility with the Earth's magnetic field can be generated, so that vectorially measuring OPMs can replace previously established sensors such as SQUIDs (very sensitive, but requires cryogenic cooling and is therefore rather large and heavy) or fluxgates (light and small, but usually not sufficiently sensitive) in the future.

[0056] By using vectorially measuring magnetic field sensors, in contrast to the SQUIDs used previously, the previously mandatory cooling can be dispensed with, so that the use of vectorially measuring magnetic field sensors is accompanied by a very significant weight reduction compared to previous measuring arrangements, so that, for example, applications with very sensitive sensors on small mobile platforms (drones) are possible, which with the existing technology required transport / measurement by aircraft or powerful towing vehicles with trailers.

[0057] Furthermore, unlike other known methods, zeroing the magnetic field at the sensor eliminates the need for active feedback loops with very high vertical resolution and bandwidth. This eliminates problems with control speed and temporal variability. While known methods using small modulations of the magnetic field obtain vector information, they still measure the projection onto the Earth's magnetic field vector.

[0058] When using three orthogonally aligned arrangements as a 3D vector magnetometer, the otherwise typical dead zones of the OPM (solid angle regions with greatly reduced or completely disappearing sensitivity) are eliminated.

[0059] Thus, this technical solution for geomagnetics is a very good alternative to SQUIDs, since the 3D vector magnetometers with vector optically pumped magnetometers (OPMs) have sufficient sensitivity and bandwidth, do not require cooling or auto-resets, and do not have dead zones, making them much easier to handle than the previously known SQUIDs or OPMs.

[0060] This enables miniaturization for drone applications (airborne application), instead of using SQUIDs and other sensors (e.g. on aircraft) in magnetic and electromagnetic methods in mineral exploration, especially for deep exploration.

[0061] In addition to this airborne application, the invention also includes other applications of this technical solution, such as biomagnetism measurements, directed sensors for magnetocardiography and magnetoencephalography.

[0062] The method according to the invention is explained in more detail below with reference to the figures and the exemplary embodiment, without being limited to these. In the following: Fig. 1: a schematic representation of the structure of an embodiment of an OPM sensor head, Fig. 2: a schematic representation of the OPM sensor head according to Fig. 1 in the holding field of the Halbach cylinder and Fig. 3: a representation of the transmission spectrum of the vectorial OPM according to Fig. 1 in LSD-Mz operating mode, Fig. 4: a schematic representation of the vectorial measuring principle in an embodiment of the vectorial optically pumped magnetometer Fig. 2, Fig. 5: a representation of the temperature dependence of the holding field when measuring according to the principle of Fig. 2 and Fig. 6: a representation of the noise spectrum in a measurement according to the principle of Fig. 1. Example 1

[0063] The embodiment describes a single-axis vectorial OPM.

[0064] A Halbach cylinder is chosen to generate the holding field. The OPM sensor head (3) is located, as shown in Fig. 2, in the interior space (2) of the Halbach cylinder (1). The interior space (2) is characterized by a static, homogeneous magnetic field, called the holding field, which runs perpendicular to the cylinder's rotation axis and overlaps with the external (earth) magnetic field (4). The holding field is extremely weak outside the Halbach cylinder (1) and can be neglected compared to the external magnetic field (4).

[0065] In this arrangement, the OPM sensor head (3) measures the amount of the vectorial sum of the holding field and the external magnetic field (4), in which changes in the external magnetic field can be detected by the magnetometer as a projection along the holding field (15) onto photodiodes (16), wherein the measured and detected data are processed and evaluated by a processor which is coupled to the OPM sensor head (3) and the photodiodes (16) in a data and information-conducting manner (not shown). Fig. 2).

[0066] A possible embodiment of an OPM sensor head (3) is shown in Fig. 1. To generate the pump and probe light, a (same) light source (5) is used, which approximately represents a point light source.

[0067] The wavelength emitted by the light source corresponds to a suitable optical transition of the alkali vapor used. Furthermore, the light should exhibit as little noise as possible in terms of wavelength, intensity, and polarization state. For this reason, a laser diode specified for optical spectroscopy with an optical output power of approximately 100 mW is used, whose wavelength can be finely tuned around the range of approximately 895 nm.

[0068] The laser diode is equipped with integrated optics so that it acts as a virtual point light source.

[0069] The light emitted by the light source (5) is then collimated by a suitable converging lens (6) and linearly polarized with a rotatable linear polarization filter (7).

[0070] The task of the linear polarization filter is to eliminate potential fluctuations in the polarization state of the light by transforming them into intensity fluctuations.

[0071] The polarization plane of the polarization filter (7) is selected to correspond to that of the light source (5), i.e., the polarization filter (7) is rotated until the transmission is maximum. The collimated light beam then passes through a combination of a rotatably mounted λ / 2 plate (8), also called a half-wave plate, and a Wollaston prism (9).

[0072] By rotating the half-wave plate (8), the polarization plane of the light can be modified. During subsequent transmission through the Wollaston prism (9), the light beam is split into two perpendicular, linearly polarized partial beams. The intensity ratio of the partial beams can be adjusted by rotating the half-wave plate (8).

[0073] To achieve optimal performance, the half-wave plate (8) should be rotated so that both partial beams have approximately the same intensity.

[0074] Since the two partial beams leave the Wollaston prism (9) at a defined angle, they are subsequently parallelized by a pair of wedge prisms (10).

[0075] In the embodiment shown here, two deflection prisms (11) are used to create a U-shaped beam path. This has the advantage that the electrical connections of the light source (5) and photodiodes (16) are located next to each other on the same side. Deflecting the beam path is also necessary because the OPM is operated in LSD-Mz mode. This operating mode requires the light beam to pass through the OPM cell (13) approximately parallel to the holding field (15).

[0076] Before the pair of partial beams passes through the OPM cell (13), it passes through a rotatably mounted λ / 4 plate (12), also called a quarter-wave plate. The goal is to modify the polarization state of the two partial beams by rotating the quarter-wave plate (12) so that both partial beams are circularly polarized as precisely as possible.

[0077] Both partial beams then pass through the OPM cell (13), which is housed in a thermally insulated housing (14) with suitable optical windows. The OPM cell is heated to the desired temperature (typically between 90°C and 120°C) by a heater. The B1 coil required to generate the B1 field is housed in the housing (14) and aligned so that the B1 field is applied perpendicular to the holding field.

[0078] After transmission through the OPM cell (13), both partial beams are directed by a deflection prism (11) onto a photodiode (16) and detected there. Further signal processing by transimpedance amplifier, A / D conversion, and B1 frequency feedback can be found in the literature (Oelsner G, Ijsselsteijn R, Scholtes T, Krüger A, Schultze V, Seyffert G, Werner G, Jäger M, Chwala A and Stolz R 2022 Integrated Optically Pumped Magnetometer for Measurements within Earth's Magnetic Field Phys. Rev. Appl. 17 024034).

[0079] Before commissioning the vector OPM, the wavelength of the light source (5) should first be optimized to achieve the best sensitivity. In the case of the laser diode used here, the wavelength can be varied by varying the current and / or the temperature of the laser diode. One way to optimize the wavelength is to record the signal from the photodiodes (16) while varying the B1 frequency in small increments around the Larmor frequency. The transmission spectrum of a single-channel vector OPM thus obtained is shown in Fig. 3. The wavelength is considered to be optimally adjusted when the increase in the difference signal of the transmission spectrum is maximum.

[0080] For the actual measurement, the vector OPM is operated in a feedback mode. For this purpose, the frequency of the B1 field is fed back using a PID controller so that the difference signal of the transmission spectrum is controlled to zero. In this case, the frequency f of the B1 field corresponds to the Larmor frequency, and the magnetic field B at the OPM is calculated as: B=2πγ⋅f≈nT3.5 Hz⋅f

[0081] The basic proof of the vectorial measuring principle is in Fig. 4. For this purpose, magnetic test fields parallel and perpendicular to the holding field are applied using a multi-axis Helmholtz coil. As can be seen from the figure, the measured Larmor frequency curve corresponds to the theoretical prediction according to equation (2), i.e., the component parallel to the holding field exhibits a linear curve, whereas the perpendicular component corresponds to a strongly suppressed, quadratic disturbance term.

[0082] In Fig. Figure 5 shows the measured temperature dependence of the holding field. Since the holding field has a flat maximum at approximately 40°C, its temperature dependence around this operating point is negligible.

[0083] Based on the Fig. 6 shows that the white noise of the vector OPM is about 50 50 fT / Hz The sensitivity of the vector OPM is thus approximately equal to that of a liquid nitrogen cooled HTS-SQUID (e.g. 30 fT / Hz in Hato T, Tsukamoto A, Adachi S, Oshikubo Y, Watanabe H, Ishikawa H, Sugisaki M, Arai E and Tanabe K 2013 Development of HTS-SQUID magnetometer system with high slew rate for exploration of mineral resources Supercond. Sci. Technol. 26 115003 and is about two orders of magnitude better than a fluxgate magnetometer [Bartington Instruments Mag-13 Brochure https: / / bartingtondownloads.com / wp-content / uploads / DS3143.pdf].

[0084] The advantage of such an OPM is that it defines its sensitive direction independently of the position of the external (earth) magnetic field. Example 2

[0085] The embodiment describes an arrangement of two or three vectorial OPMs (two- or three-axis system)

[0086] An arrangement of two or three vectorial OPMs is achieved by constructing two or three systems, each in the form of an optically pumped, single-axis, vectorially measuring magnetometer according to embodiment 1, orthogonally to one another, so that a two- or three-axis system is formed (not shown in the figures).

[0087] The advantage of using three orthogonally aligned vectorial OPMs, each in the form of an optically pumped, single-axis, vectorially measuring magnetometer, which are arranged to form a 3D vector magnetometer, is that the otherwise typical dead zones of the OPM (solid angle regions with greatly reduced or completely disappearing sensitivity) are eliminated, so that this arrangement of three vectorial OPMs represents a very good alternative to the previously used SQUIDs for geomagnetics. Reference symbol 1 permanent magnet / Halbach cylinder 2 Interior 3 OPM sensor head 4 external magnetic field 5 Light source 6 Converging lens 7 polarizing filters 8 λ / 2 plate (quarter-wave plate) 9 Wollaston prism 10 wedge prisms 11 deflection prisms 12 half-wave plate 13 OPM cell 14 housings 16 photodiodes

Claims

[1] Vectorial OPM in the form of an optically pumped, single-axis, vectorial measuring magnetometer comprising: - an OPM sensor head (3) which is surrounded by an external magnetic field (4), and - a light source (5) for generating pump light and sample light, in which the light of the light source (5) passes through a converging lens (6), a polarizing filter (7), a quarter-wave plate (8), a Wollaston prism (9), a wedge prism (10), a deflection prism (11), a half-wave plate (12) in order to generate the sample light as pump light through an alkali vapor-filled OPM cell (13) with a thermally insulated housing (14), which light is incident from the OPM cell (13) via a deflection prism (11) on photodiodes (16) for detection, and - a processor coupled to the OPM sensor head (3) and the photodiodes (16) in a data and information-conducting manner, wherein the wavelength emitted by the light source (5) corresponds to a suitable optical transition of the alkali vapor used in the OPM cell (13), characterized by , that - the OPM sensor head (3) is located in the interior (2) of a permanent magnet (1), - the interior space (2) encloses a statically homogeneous magnetic field in the form of a constant holding field (15) which runs perpendicular to the axis of rotation of the permanent magnet (1) and overlaps with the external magnetic field (4), wherein the magnetic flux density of the holding field (15) is at least ten times greater than that of the external magnetic field (4), so that the holding field (15) is generated at > 500 µT and the sensitive direction of the optically pumped, single-axis, vectorially measuring magnetometer is defined by embedding the OPM sensor head (3) in the interior (2) of the permanent magnet (1), and wherein the OPM sensor head (3) measures the amount of the vectorial sum of the holding field (15) and the external magnetic field (4), in which changes in an external magnetic field can be detected as a projection along the holding field (15) onto photodiodes (16) by the optically pumped, uniaxial, vectorially measuring magnetometer, and the measured and detected data are processed and evaluated by the processor. [2] Vectorial OPM according to claim 1, characterized by that the permanent magnet (1) is a Halbach cylinder. [3] Vectorial OPM according to claim 1 or 2, characterized bythat a laser diode specified for optical spectroscopy is used as the light source (5), which is equipped with integrated optics so that it acts as a virtual point light source. [4] Vectorial OPM according to claim 3, characterized by that the laser diode has an optical output power of 100 mW and its wavelength can be fine-tuned to 895 nm. [5] Vectorial OPM according to claim 1 or 2, characterized by that the polarization plane of the polarization filter (7) is selected so that it corresponds to that of the light source (5) by rotating the polarization filter (7) until the transmission is maximum. [6] Vectorial OPM according to claim 1 or 2, characterized by that the half-wave plate (8) is rotatably mounted, whereby the polarization plane of the light can be modified by the rotation of the half-wave plate (8). [7] Vectorial OPM according to claim 1 or 2, characterized bythat the light beam is split into two partial beams which are linearly polarized perpendicular to one another by the Wollaston prism (9), whereby the intensity ratio of the partial beams can be adjusted by rotating the half-wave plate (8). [8] Vectorial OPM according to claim 7, characterized by that the two partial beams leave the Wollaston prism (9) at a defined angle, wherein they are subsequently parallelized by a pair of wedge prisms (10) so that the light beam runs parallel to the holding field (15) through the OPM cell (13), wherein before the pair of partial beams passes through the OPM cell (13), it passes through a rotatably mounted quarter-wave plate (12) in order to modify the polarization state of the two partial beams by rotating the quarter-wave plate (12) so that both partial beams are circularly polarized as exactly as possible in opposite directions. [9] Vectorial OPM according to claim 8, characterized bythat two deflection prisms (11) are used to obtain a U-shaped beam path, wherein the electrical connections of the light source (5) and photodiodes (16) are located next to each other on the same side. [10] Vectorial OPM according to claim 8, characterized by that the two partial beams subsequently pass through the OPM cell (13), wherein the OPM cell is regulated by a heater to a temperature between 90°C and 120°C and a B1 coil required to generate a B1 field is accommodated in the housing (14) and aligned such that the B1 field is applied perpendicular to the holding field (15). [11] Arrangement of two or three vectorial OPMs in the form of an optically pumped, single-axis, vectorially measuring magnetometer according to one or more of claims 1 to 10, in which two or three vectorial OPMs in the form of an optically pumped, single-axis, vectorially measuring magnetometer are constructed orthogonally to one another.

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