Magnetometer unit having magnetic field modulation for direction-sensitive measurement of magnetic field, and gradiometer unit having magnetometer unit
By generating a biased magnetic field with time-varying components in the sensing medium and using a gradient meter circuit, combined with NV center magnetic resonance in diamond crystals, the problem of insufficient determination of the direction of the magnetic field in the prior art is solved, and high-precision measurement and direction determination of the weak magnetic field in daily environments are achieved.
Patent Information
- Application Number
- CN202380078210.X
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Priority Date
- 2022-11-09
- Filing Date
- 2023-10-25
- Publication Date
- 2025-07-08
AI Technical Summary
In the prior art, when measuring very small magnetic field strengths, especially in daily environments, it is difficult to accurately determine the direction of the magnetic field, especially in the presence of a background magnetic field, it is impossible to distinguish the direction of the magnetic field to be measured from the static biased magnetic field.
Using a biased magnetic field with a time-vacancies, a gradient meter circuit composed of at least two magnetometer units is combined with a biased magnetic field circuit, a nitrogen vacancy center (NV center) in a diamond crystal is used to perform magnetic resonance measurement, using the Zeeman effect to detect the magnetic field intensity and direction, and the common mode noise is eliminated through differential signals.
It realizes unshielded measurement of weak magnetic fields in daily environments, improves the certainty and measurement accuracy of the magnetic field direction, and can accurately distinguish the magnetic field direction in the presence of a background magnetic field, and is suitable for safety-related applications such as the automobile industry.
Smart Images
Figure CN120283169A_ABST
Abstract
Description
Field of the Invention
[0001] The present invention relates to a magnetometer unit for measuring a magnetic field, and a gradiometer unit having at least two such magnetometer units. Background Art
[0002] For measuring very small magnetic field strengths, optically pumped or quantum sensors based on NV centers in diamond are particularly suitable as sensors. A magnetometer is described in DE 10 2022 204 526.2 which utilizes optically pumped and optically detected magnetic resonance (ODMR). Here, it is utilized that the energy levels of specific spin states of unpaired electrons split under the influence of an external magnetic field, the so-called Zeeman effect. Due to the splitting of the energy levels, changing transitions occur during relaxation from the excited state, and these transitions can then be measured, for example, by optical excitation and frequency-dependent detection of the resulting fluorescence radiation, or by observing optical properties such as light absorption. Then, the magnetic field strength can be inferred from the measured optical parameters. Summary of the Invention
[0003] According to the present invention, there is provided a magnetometer unit for measuring a magnetic field having the features of claim 1, and a gradiometer unit having at least two such magnetometer units. Advantageous configurations are the subject matter of the dependent claims and the following description.
[0004] Specifically, there is provided a magnetometer unit for measuring a magnetic field, having: a sensing medium exposed to the magnetic field to be measured; an excitation light source for emitting light into the sensing medium; a microwave source for generating an electromagnetic field in the sensing medium; and a facility for generating a bias magnetic field in the region of the sensing medium. The magnetometer unit is configured to detect the magnetic field strength and the magnetic field direction of the magnetic field to be measured by reading spin resonance in the sensing medium that is related to the magnetic field strength and the magnetic field direction. For example, a photodetector or a photocurrent detector for detecting fluorescence light from the sensing medium can be used for performing the reading.
[0005] In one configuration, the sensing medium has a diamond crystal or a section of a diamond crystal with nitrogen-vacancy centers (NVs). The diamond NV magnetometer is based on reading the magnetic resonance of specific defect centers in diamond, particularly nitrogen vacancies, which occur as impurities in the carbon lattice of diamond and can also be introduced purposefully. If an NV center is optically excited in the ground state, for example, by emitting a pump laser beam with a suitable wavelength (in this case in the green wavelength range, e.g., 532 nm for non-resonant excitation), the electron is lifted from the triplet ground state to the excited triplet state, and in the case of emitting fluorescence light, it relaxes in the red wavelength range of 650 - 800 nm (637 nm = zero phonon line). Since the probability of non-spin-conserving transitions from the spin states with spin quantum number m s = ±1 is greater, continuous excitation pumping causes the majority of NV centers to be hyperpolarized in the spin state with m s = 0.
[0006] There is an energy difference between the spin states with ms = 0 and ms = ±1 in the ground state, and in this case, this energy difference is approximately 2.87 GHz. Thus, if in addition to optical excitation, microwave radiation is also emitted into the diamond, then at this 2.87 GHz resonance frequency, a decrease in red fluorescence occurs because the spin-polarized electrons are lifted from the ground state with ms = 0 to the ground state with ms = ±1 by the microwave field, and from there they are excited to the excited state with ms = ±1 by the pump light. However, from there, non-radiative transitions and weak infrared fluorescence transitions mainly occur via the singlet state, while the fluorescence in the red region weakens.
[0007] If there is an external (to be measured) magnetic field at this time, due to the so-called Zeeman effect, the originally degenerate triplet energy levels with m s = ±1 split into equi-energetic Zeeman energy levels. When the fluorescence is plotted relative to the frequency spectrum of the microwave excitation, two dips are shown in the fluorescence spectrum, and the frequency separation of these dips is proportional to the magnetic field strength of the external magnetic field. Here, the magnetic field sensitivity is mainly defined by the minimum resolvable frequency shift and can reach 1 pT / √Hz or lower. Since there are four possible arrangements of NV centers in the lattice of single-crystal diamond, when there is an additional bias magnetic field, the NV centers present in the crystal respond to the external magnetic field with different intensities depending on their positions in the crystal. Thus, in an ideal case, four pairs of fluorescence minima can appear in the spectrum, from whose shapes and mutual positions, the magnetic field strength as the magnitude and the direction of the external magnetic field can be determined unambiguously.
[0008] For magnetic field measurement to be possible, the magnetometer unit thus has facilities for generating a bias magnetic field in the region of the sensing medium. Here, this can be a Helmholtz coil arrangement, in which at least the sensing medium is arranged inside the Helmholtz coil arrangement. It can also be other facilities, such as a simple coil, an elongated coil, a permanent magnet solution, such as a permanent magnet solution in the form of a Halbach array, etc. To additionally be able to determine the vectorial (direction-related) magnetic information, the direction of the bias magnetic field needs to be defined here.
[0009] It has been shown that applying a constant bias magnetic field can lead to ambiguity in direction determination in certain situations, i.e., it is not possible to distinguish between the direction along or against the applied static bias magnetic field. Therefore, it is proposed here that the bias magnetic field has a constant component and a component that varies in time. By adding a variable component to the static bias magnetic field at this time, the direction of the magnetic field vector to be measured can be distinguished. Thus, the determination of the magnetic field direction can be ensured.
[0010] At least the component that varies in time can be very simply generated by a change in the current through one or more coils, for example, by means of a modifiable current driver.
[0011] In one configuration, the amplitude of the component that varies in time is less than 50% of the amplitude of the constant component, especially at most 10% or at most 20% or at most 30%. Therefore, the bias magnetic field especially does not undergo a polarity reversal, and further, especially is mainly determined by the static component.
[0012] In one configuration, the component that varies in time follows a periodic function, especially a sine function. This facilitates analytical evaluation, especially when the fundamental frequency of the component that varies in time is additionally at least 10 Hz and / or at most 10 kHz.
[0013] When using a gradiometer circuit consisting of at least two magnetometer units, one magnetometer unit always has a greater distance to the magnetic field source than the other magnetometer unit. Through this gradiometer circuit, i.e., basically (vectorially) subtracting the measured quantity, the magnetic field gradient approximately corresponds to the field from a weak source, while much stronger background fields (which are basically the same in the two magnetometer units) are eliminated. In this way, magnetic shielding is not required, enabling magnetic field measurement in a daily environment. Correspondingly, the present invention is especially suitable for unshielded measurement of weak magnetic fields. Technical details of a gradiometer solution that can also be applied within the scope of the present invention are disclosed in DE 102022201690.4, and it should be included here.
[0014] In one configuration, the sensing media of the magnetometer units of the gradiometer unit may each include a section of the same diamond crystal. In this structural arrangement, in addition to compactness, it is ensured that the two sections used as sensing media basically have the same characteristics, and thus also exhibit the same optical characteristics when reading spin resonance, so that no error sources are generated when the gradiometer performs differencing. In one configuration, the same excitation light source and / or the same microwave source are used for the magnetometer units of the gradiometer unit. In this way, the fluctuation and noise components from these elements are automatically eliminated by forming a differential signal (common mode noise rejection).
[0015] Further advantages and configurations of the present invention are derived from the description and the drawings.
[0016] The present invention is schematically illustrated by way of embodiments in the drawings and is described hereinafter with reference to the drawings. Description of the Drawings
[0017] Figure 1 The main components of an NV center magnetometer as available within the scope of the present invention are shown in the form of a schematic block diagram.
[0018] Figure 2 Two magnetic fields that cannot be distinguished without using a static bias magnetic field according to the present invention are schematically shown.
[0019] Figure 3 The variation curves of the measurement signals for magnetic fields pointing in opposite directions during measurement according to an embodiment of the present invention are shown. Detailed Description of the Invention
[0020] Figure 1 The main components of an NV center magnetometer according to one configuration are schematically shown. Here, first, there is a diamond 110 having a nitrogen vacancy (NV) as the sensing medium. Optical excitation of the NV center can be achieved by a suitable light source 120, such as a pump laser. Here, for example, a frequency-doubled Nd:YAG laser or a semiconductor laser in the green range of about 510 - 532 nm, such as 532 nm for non-resonant excitation, is suitable. Alternatively, an LED within a suitable wavelength range can also be used. Depending on the arrangement, the light from the light source 120 can be emitted into the diamond 110 via suitable optical elements 122, such as mirrors, beam splitters, focusing optics such as lenses, and, if necessary, via fiber optic elements. In addition, the excitation light can be emitted continuously or pulsed by the laser, so that, for example, a time window is left for undisturbed fluorescence light measurement.
[0021] In addition, the magnetometer may include a microwave source 150 that is capable of generating an electromagnetic field in a sensing medium, i.e., in the region of the NV centers of the diamond 110, over a bandwidth covering the desired resonance frequency. A microwave resonator structure may be used to evenly distribute the generated microwaves over the volume of the measurement region in the diamond. Here, the resonator structure or the microwave source 150 is preferably tuned to the frequency of electron spin resonance. To enable a vector magnetometer, an additional bias magnetic field is generated by means of the facility 140. This makes the measurement essentially vectorial. For this purpose, different spatial directions in the crystal structure are used. For example, Helmholtz coils are suitable for generating such a magnetic field, in which a substantially uniform magnetic field can be generated within a limited region by means of coil pairs. It may also be other facilities 140, such as simple coils, elongated coils, permanent magnet solutions, such as permanent magnet solutions in the form of Halbach arrays, etc.
[0022] The fluorescence light 112 generated from the diamond 110 can again be guided via suitable optical elements 134, such as optical filters, beam splitters, lenses, and / or fiber optic elements, to the first photodetector 130, which is sensitive at least in the range of the fluorescence wavelength. The first photodetector 130 may also be arranged adjacent to the diamond 110. The second photodetector 132 is arranged such that it can detect at least a part of the excitation light of the light source 120, which part can be decoupled, for example, by means of a beam splitter, a filter, or a partially transmissive element. This detector signal 132 of the excitation light can be used as a reference signal to eliminate the background signal and highlight the resonance signal of interest, for example, by modulating the excitation light with a lock-in amplifier. Additionally or alternatively, this reference signal can be used to account for the fluctuations of the excitation light. Accordingly, a corresponding circuit 160, such as a preamplifier, a logarithmic amplifier, a lock-in amplifier, a signal filter, or other circuits, is provided to obtain the signals of the first and second photodetectors and to preprocess the signals in a suitable manner for further analysis and evaluation. Finally, the preprocessed fluorescence signal can be analyzed and evaluated by a signal processing unit 170, such as using a suitable microcontroller or processor, to obtain the desired parameters of the detected magnetic field from the signal, especially the magnetic field strength and the direction of the magnetic field.
[0023] It is understood that such a device may also have other units not shown, such as a communication unit or an interface for outputting the measurement results. Such a device can also be advantageously integrated into an ASIC or an FPGA.
[0024] NV center magnetometers offer several advantages for current applications. In addition to the already mentioned very high sensitivity, a larger measuring range can also be covered (>1 Tesla). The underlying Zeeman effect is linearly dependent on the current magnetic field and, since the measurement is based on quantum mechanical states, does not show degeneration. In addition, NV center magnetometers offer the possibility of vector determination of the external magnetic field based on the different orientations present in the diamond lattice.
[0025] Alternatively, the magnetic spin resonance in diamond can also be read electrically. Here, the charge carriers that are lifted into the conductor band of the diamond by two-photon ionization of the NV center are detected. If this method is used to read the resonance effect, the part for detecting the fluorescent light in the above example is no longer necessary, but is replaced by a suitable photocurrent detector on the diamond. However, in addition to this, the method for magnetic field measurement can be converted accordingly and applied to all embodiments using NV center magnetometers.
[0026] To enable use in everyday environments, magnetic fields not coming from the expected weak sources should be eliminated from the measurement as much as possible, especially in the case of 10 -5 The geomagnetic field in the Tesla (several μTesla) range.
[0027] In the magnetic field measurement according to the exemplary embodiment, the elimination of the background magnetic field can be achieved by shielding or by a gradiometer arrangement. In principle, a magnetometer unit that can detect field strength as well as field gradient is called a gradiometer. For this purpose, at least two single magnetometer units arranged at different spatial positions can be used, and their signals are operated on each other.
[0028] exist Figure 2 An example of a magnetic field BM to be measured is given in which these magnetic fields cannot be distinguished when a static bias magnetic field B0 is used conventionally. That is, in principle, in such an NV magnetometer unit, it is impossible to distinguish between magnetic fields that are parallel and antiparallel to the bias magnetic field. Although such a distinction can still be made based on the magnetic field strength if the composite field B0+BM is smaller than the bias magnetic field B0 (because the field to be measured must be antiparallel in this way). However, if the composite field B0+BM is larger than the bias magnetic field B0, it is impossible to distinguish whether it is a "small" (smaller than B0) magnetic field parallel to the field to be measured BM or a "large" (larger than B0) magnetic field antiparallel to the field to be measured BM.
[0029] Therefore, if the measured composite field B0+BM (right arrow) is greater than B0 (left arrow) and parallel or antiparallel to B0, the field to be measured BM (middle arrow) can be either also oriented parallel to the bias magnetic field B0 and less than B0 ( Figure 2in the upper diagram), or is oriented antiparallel to the bias magnetic field B0 and is greater than B0 ( Figure 2 in the lower diagram).
[0030] This can be solved by a time-varying component in the bias magnetic field, where the entire bias magnetic field B(t) is then composed of a constant component B0 and a time-varying component Bmod:
[0031]
[0032] Because in this way, the orientation can be determined unambiguously based on the sign of the time variation (plus for parallel and minus for antiparallel) or based on the phase of the measured signal BM + B(t) relative to the generated modulation phase as shown in Figure 3 the measured signal BM + B(t) is plotted against time Figure 3 as shown in
[0033] 301 indicates the measured curve without modulation, where the two above-mentioned cases cannot be distinguished. 302 and 303 indicate the measured curves with modulation, which are phase-shifted by 180° from each other according to the direction of the resultant field compared to the time-varying component. If the field to be measured is less than B0 and parallel, the phase of the measured field corresponds to the phase of the modulation. If the field to be measured is greater than B0 and antiparallel, the phase of the measured field is phase-shifted by 180° from the phase of the modulation.
[0034] The bias magnetic field component can be generated, for example, by a corresponding current in a coil arrangement. Correspondingly, the current for generating this bias magnetic field can be composed, for example, of:
[0035]
[0036] The amplitude of the slowly varying magnetic field component can be relatively small, for example, only corresponding to a small component of the static magnetic field, and the frequency can be in the range of 10 Hz to 10 kHz, for example.
[0037] Instead of a static current I0 for generating the static magnetic field component B0 by means of a coil, the static magnetic field component B0 can also be generated at least in part by an arrangement of permanent magnets.
[0038] Therefore, with the concept proposed here, it can be reliably identified whether the magnetic field to be measured may have exceeded a certain amplitude and indicates another direction. Therefore, this can also be used for unambiguous credibility verification. This may be a necessary prerequisite especially in safety-related applications of magnetic field sensors, for example, in the automotive industry.
Claims
1. A magnetometer unit for measuring a magnetic field, comprising: A sensing medium (110) exposed to the magnetic field to be measured, An excitation light source (120) for emitting light (124) into the sensing medium (110), A microwave source (150) for generating an electromagnetic field in the sensing medium (110), A facility (140) for generating a bias magnetic field in a region of the sensing medium (100), Among them, The magnetometer unit is configured to detect the magnetic field strength and magnetic field direction of the magnetic field to be measured by reading spin resonance in the sensing medium (110) related to the magnetic field strength and magnetic field direction, Wherein, the facility (140) for generating the bias magnetic field is configured to generate a bias magnetic field having a constant component and a component varying in time.
2. The magnetometer unit according to claim 1, wherein, The amplitude of the component varying in time is less than 50%.
3. The magnetometer unit according to claim 1 or 2, wherein, The component varying in time follows a periodic function, in particular a sine function.
4. The magnetometer unit according to claim 3, wherein, The fundamental frequency of the component varying in time is at least 10 Hz and / or at most 10 kHz.
5. The magnetometer unit according to any one of the above claims, having a diamond crystal or a section of a diamond crystal having a nitrogen-vacancy center as the sensing medium (110).
6. A gradiometer unit having at least two magnetometer units according to any one of the above claims and a signal processing unit (170), the signal processing unit being configured to obtain a magnetic field gradient from the magnetic field strength and magnetic field direction of the magnetic field to be measured detected by the at least two magnetometer units.
7. The gradiometer unit according to claim 6, wherein, The sensing media (110) of the at least two magnetometer units each have a section of the same diamond crystal.
8. The gradiometer unit according to claim 6 or 7, wherein, The at least two magnetometer units share the same excitation light source (120) and / or the same microwave source (150).