DE102022201690A1 - Sensor unit for measuring magnetic fields - Google Patents

Abstract

The invention relates to a sensor unit (100) for the unshielded measurement of weak magnetic fields, comprising: two magnetometers (300, 400a, 400b) which are arranged at a fixed distance from one another, each magnetometer having a sensor medium (101, 102, 304) and to is set up to detect a magnetic field strength at a measurement location by reading out a spin resonance in the sensor medium that is dependent on the magnetic field strength, the sensor unit also having at least one excitation light source (120) for radiating light (124) into the sensor media (101, 102) of the Having magnetometer, the sensor unit further having at least one signal processing unit (406) for determining a magnetic field gradient (100, 200, 300, 400) as the difference of the output signals of the two magnetometers.

Description

The present invention relates to a sensor unit for measuring magnetic fields based on magnetic resonances.

Background of the Invention

Various sensor technologies are known for measuring very small magnetic field strengths. One of the most sensitive sensor types are SQUID magnetometers (Superconducting QUantum Interference Device), which are based on superconducting components and with which individual magnetic flux quanta or fields can be resolved down to the range of a few picotesla (pT). The disadvantage of SQUID sensors, however, is the need for cooling below the superconducting transition temperature, which for most superconducting ones is below 63 Kelvin, i.e. below the temperature of liquid nitrogen. In addition, the measuring range of these sensors is limited and cannot work with large interference fields, so that measurements are only possible in specially magnetically shielded rooms.

Likewise, for example, from the DE 10 2018 220 234 A1 or the DE 10 2018 214 617 A1 known quantum-based magnetic sensors based on nitrogen vacancy centers, which are based on the detection of spin resonances in magnetic fields.

Disclosure of Invention

According to the invention, a sensor unit for measuring magnetic fields with the features of patent claim 1 is proposed. Advantageous configurations are the subject of the dependent claims and the following description.

In particular, a sensor unit is proposed which has two magnetometers which are arranged at a fixed distance from one another, each magnetometer having a sensor medium and being set up to measure a magnetic field strength at a measurement location by reading out a spin resonance in the sensor medium which is dependent on the magnetic field strength detect, wherein the sensor unit further comprises at least one excitation light source for radiating light into the sensor media of the magnetometer and at least one signal processing unit for determining a magnetic field gradient as the difference between the output signals of the two magnetometers.

By using two fixedly spaced magnetometers, which can then be arranged so that one magnetometer is further away from a weak magnetic field source than the second magnetometer, the magnetic field gradient approximates the field emanating from the weak source while much stronger ones Background fields are eliminated. This eliminates the need for magnetic shielding, making magnetic field measurement possible in everyday environments. Accordingly, the sensor unit is particularly suitable for the unshielded measurement of weak magnetic fields.

In particular, a sensor unit is proposed as a possible embodiment, in which each magnetometer has a nitrogen vacancy center magnetometer, the sensor medium having a diamond crystal or a section of a diamond crystal with nitrogen vacancy centers, and the sensor unit further having at least one microwave source for Generating a resonant field in the sensor medium and having at least one photodetector for detecting resonance-dependent fluorescent light from the sensor medium. Nitrogen vacancy center magnetometers offer very high sensitivity on the one hand, but are also easy to implement in a compact design, so that small sensor heads are created. Expensive cooling, such as with SQUID sensors, is not necessary. In addition, inherently vector magnetic field measurements can be made with NV vacancy centers.

The sensor medium can include a thin diamond plate, for example, since a large volume is not required for the fluorescence measurement. A sensor can thus be designed to be even more compact and also more cost-effective. Alternatively or additionally, the sensor medium of the two magnetometers can also each comprise a section of the same diamond crystal. In addition to compactness, this design also ensures that the two sections used as the sensor medium have essentially the same properties and thus also show the same optical properties when reading out the spin resonance, so that no sources of error arise when the gradiometer calculates the difference.

It is also possible for a diamond crystal used as a sensor medium to be designed in such a way that it serves as a waveguide for conducting the light from the excitation light source to the measurement locations of the two magnetometers. This means that feed lines can also be saved.

The same excitation light source and/or the same microwave source are preferably used in all variants for the two magnetometers. In this way, fluctuations and noise components from these elements are automatically compensated by the Differential signal formation eliminated (common noise rejection).

In addition, the sensor unit can include a microwave resonator in the area of the sensor medium, which is tuned to the spin resonance of the sensor medium. With a microwave resonator, a homogeneous microwave field can be generated in the relevant points of the sensor medium, while the microwave source can also be arranged remotely and fed into the resonators of the magnetometers via suitable connections.

In order to enable vectorial magnetic field measurements, the sensor unit can also include a device for generating a substantially homogeneous bias magnetic field in the area of the sensor medium. Since the NV center in the diamond has four possible configurations, each of which is visible in the fluorescence spectrum as its own frequency splitting, the magnetic field direction and strength can be extracted for each crystal direction, so that the source of the magnetic field can also be localized. In order to generate such a bias magnetic field, a Helmholtz coil arrangement can be used, for example, within which at least the sensor medium or the sensor heads of the two magnetometers are located. A particularly homogeneous field can be generated with Helmholtz coils.

The distance between the two magnetometers, i.e. between the measuring points on the sensor media, can range from millimeters to centimeters and ultimately also depends on the strength of the magnetic field to be measured. As an example, a distance can be chosen that is between 1 and 30 millimeters, preferably between 5 and 20 millimeters.

Further advantages and refinements of the invention result from the description and the attached drawing.

The invention is shown schematically in the drawing using exemplary embodiments and is described below with reference to the drawing.

character list

• 1 shows a schematic overview of a sensor unit according to an exemplary embodiment of the invention;
• 2 shows magnetic field strengths when measuring brain magnetic fields at different distances from the head;
• 3 12 shows a sensor head that can be used as part of a sensor unit according to an exemplary embodiment; and
• 4 shows a sensor unit with two of the in 3 illustrated sensor heads.

Embodiments of the invention

In order to measure very small magnetic field strengths, quantum-based or optically pumped magnetometers are particularly suitable as sensors, with diamond NV magnetometers being described in the following embodiments. However, it is also conceivable to integrate other magnetometers with comparable properties into the sensor unit.

The magnetometers described here by way of example use optically pumped and/or optically detected magnetic resonances (ODRM). This exploits the fact that the energy levels of certain spin states of unpaired electrons split under the influence of an external magnetic field, the so-called Zeeman effect. The splitting of the energy levels results in changed transitions during relaxation from excited states, which 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 the absorption of light. The magnetic field strength can then in turn be inferred from the measured optical parameters.

Diamond NV magnetometers are based on reading out magnetic resonances from special defect centers in diamond, in particular nitrogen vacancies (NV, nitrogen vacancy), which occur as impurities in the carbon lattice of diamond and can also be introduced in a targeted manner. If the NV center is optically excited in the normal state without an existing magnetic field, e.g. by irradiating a pump laser beam with a suitable wavelength (in this case in the green wavelength range, e.g. at 532 nm for off-resonance excitation), the electrons are converted from the triplet ground state into the excited triplet state and relax while emitting fluorescence light in the red wavelength range at 637 nm. Since the probability of non-spin-conserving transitions from the spin state with the spin quantum number m _s =±1 is higher, continuous excitation pumping ensures that the NV centers are mostly hyperpolarized in the spin state m _s =0.

There is an energy difference between the m _s = 0 and m _s =±1 spin states in the ground state, which in this case is about 2.87 GHz. Shine So if, in addition to the optical excitation, microwave radiation is also injected into the diamond, the red fluorescence collapses at this resonance frequency of 2.87 GHz, since the spin-polarized electrons are caused by the microwave field from m _s = 0 to m _s =±1 - Ground state can be raised and from there excited by the pumping light into the m _s =±1 excited state. From there, however, mainly non-radiative transitions and weak infrared fluorescence transitions via the singlet state occur, while the fluorescence in the red range is absent.

If an external magnetic field is present, the so-called Zeeman effect causes the otherwise identical m _s =±1 triplet levels to split into energetically equidistant Zeeman levels. Plotting the fluorescence against a frequency spectrum of the microwave excitation shows two dips in the fluorescence spectrum, the frequency spacing of which is linearly proportional to the magnetic field strength of the external magnetic field. The magnetic field sensitivity is primarily defined by the minimum resolvable frequency shift and can reach up to 1 pTA/Hz. Since the NV center in single-crystal diamond has four options for arranging itself in the crystal lattice, the presence of a directed magnetic field means that the NV centers present in the crystal react differently to the external magnetic field, depending on their position in the crystal. In the ideal case, four pairs of fluorescence minima can appear in the spectrum, from the shape and position of which both the magnetic field strength as amount and the direction of the external magnetic field can be clearly determined.

The NV center magnetometer offers a number of advantages for the application at hand. In addition to the very high sensitivity already mentioned, a large measuring range (> 1 Tesla) can also be covered. The underlying Zeeman effect is linearly dependent on the existing magnetic field and also shows no degradation since the measurement is based on quantum mechanical states. In addition, an NV center magnetometer offers the possibility of determining external magnetic fields vectorially based on the different orientations present in the diamond lattice.

There are also alternative ways to electrically read out the magnetic spin resonance in diamond. Charge carriers are detected that have been raised into the conduction band of diamond by two-photon ionization of the NV centers. If such a method is used to read out the resonance effects, the parts for detecting the fluorescence light in the previous examples are not required and are replaced by suitable photocurrent detectors on the diamond. Apart from that, however, the method for measuring magnetic fields can be transferred accordingly and used in all embodiments with NV center magnetometers.

In order to be usable in an everyday environment, magnetic fields that do not come from the desired weak source should be eliminated from the measurement as far as possible. Especially in the automotive sector, comparatively high and hardly shielded magnetic fields arise in the background, which are in the range of about 10 ^-6 to 10 ^-9 Tesla (nanotesla). In addition, the earth's magnetic field is in the range of 10 ^-5 Tesla (a few microtesla). In contrast, biomagnetic fields, which are to serve as an example here, are in the range of 10 ^-12 Tesla (Picotesla) or even lower.

According to exemplary embodiments, the elimination of the background magnetic fields can be achieved by a gradiometer arrangement during the magnetic field measurement. Basically, sensor units are referred to as gradiometers, which are able to record not only the field strength but also the gradient of the field.

At least two individual magnetometers can be used for this purpose, which are arranged at spatially different locations. As an example, the following is linked to 1 described a sensor assembly using two NV center magnetometers in a gradiometer array.

In this case, a diamond 101, 102 with nitrogen defects can initially be present as the sensor medium for each magnetometer. Optical excitation of the NV centers in diamond is achieved by a suitable light source 120 such as a pump laser. A Nd:YAG laser in the green range with a frequency of 532 nm is suitable here to excite the respective transitions. The light 124 of the pump laser 120 can be radiated into the diamonds via suitable optical elements 128 such as mirrors, beam splitters, lenses and optionally via fiber optic elements 122 . A single light source 120 is used here for both diamonds 101, 102, from which the excitation light is guided into the respective diamond via an optical fiber in each case. In addition, the excitation light can be radiated in continuously by the laser or in a pulsed manner, so that, for example, time windows for interference-free fluorescence light measurement are kept free.

In addition, a microwave source 140 can be present in the sensor unit, which is able to generate an electromagnetic field via a Bandwidth that covers the desired resonant frequency in the sensor medium, ie in the area of the NV centers in the diamonds 101, 102 to generate. The same microwave source 140 can preferably be used for both sensors or diamonds. The resulting fluorescence light 125 can in turn be guided via the fiber optics and detected by a photodetector 130, which is sensitive in the range of the fluorescence wavelength; alternatively, one or more photodetectors could also be arranged directly on the sensor diamonds, which receive the fluorescent light. Here, too, suitable reflecting or semi-transparent mirrors, filters, beam splitters, lenses for beam shaping, coupling lenses for fiber optics or for the diamond crystals, and other optical elements 126, 128 on the optical path of the fluorescent light 125 can be used. The signal from the photodetector 130 can then be further processed by various elements, such as a preamplifier 132 or a processing unit 136 for signal evaluation.

The elements of the sensor unit are shown only schematically in the figure; it is understood that, for example, the microwave source is designed and arranged so that the RF field is generated in the sensor diamonds. Other elements such as microwave resonators or housing components are also not expressly shown here.

The distance d between the two diamonds or sensor heads of a gradiometer unit corresponds to the distance between the locations at which magnetic field measurements are carried out at the same time. As long as the distance between the measurement locations is relatively small, it can be assumed that the strength of the additional background magnetic field B _env is approximately the same at both locations. In contrast, the weak magnetic field of interest will decrease significantly with increasing distance from the magnetic field source. Thus, by placing two sensors at different distances from the source, e.g attenuates with the square of the distance, the largest magnetic field change is detected by the sensor diamond in the vicinity of the source. For this purpose, for example, two magnetometers can be arranged one above the other in an axial gradiometer configuration.

$${\text{B}}_{\text{2sens}}={\text{B}}_2+{\text{B}}_{\text{env}}\text{,}$$

$${\text{B}}_{\text{grad}}={\text{B}}_{1\text{sens}}-{\text{B}}_{\text{2sens}}={\text{B}}_{\text{1}}-{\text{B}}_{\text{2}}\approx {\text{B}}_1$$

wobei B_1sens die gemessene Feldstärke am ersten Sensor ist, der sich näher an der interessierenden Magnetfeldquelle befindet, wobei B₁ der Anteil der interessierenden schwachen Magnetfeldstärke am Ort des ersten Sensors ist, wobei B_2sens die gemessene Feldstärke am zweiten, weiter entfernten Sensor ist, wobei B₂ der Anteil der interessierenden schwachen Magnetfeldstärke am Ort des zweiten Sensors ist, und wobei B_grad der resultierende Gradient der Magnetfeldstärke ist.It therefore applies to an axial gradiometer with a first and a second magnetic field sensor: $${\text{B}}_{\text{1sens}}={\text{B}}_1+{\text{B}}_{\text{maybe}},$$

$${\text{B}}_{\text{2sens}}={\text{B}}_2+{\text{B}}_{\text{maybe}}\text{,}$$

$${\text{B}}_{\text{Degree}}={\text{B}}_{1\text{sens}}-{\text{B}}_{\text{2sens}}={\text{B}}_{\text{1}}-{\text{<figure-callout id="2" label="B" filenames="DE102022201690A1\_0005.png" state="{{state}}">B</figure-callout>}}_{\text{2}}\approx {\text{B}}_1$$

where B _1sens is the measured field strength at the first sensor, which is closer to the magnetic field source of interest, where B ₁ is the fraction of the weak magnetic field strength of interest at the location of the first sensor, where B _2sens is the measured field strength at the second, more distant sensor, where B ₂ is the fraction of the weak magnetic field strength of interest at the location of the second sensor, and where B _grad is the resultant magnetic field strength gradient.

Since the small magnetic field B ₂ at the second sensor, which is further away from the source, is many times smaller than at the first sensor, the gradient B _grad of the total magnetic field at this point corresponds approximately to the magnetic field B ₁ of interest at the first, closer sensor head, while the approximately uniform background fields B _env are eliminated.

The distance d between the sensor heads within a gradiometer unit, ie in this case the distance between the two sensor diamonds, can be in the range from mm to a few cm, for example between 0.5 cm and 2 cm. For values of this magnitude, good results could be achieved, for example, for the measurement of biomagnetic fields that are in the picotesla range. However, as long as sufficient independence of the signals from the background fields is achieved, larger or smaller distances or other gradiometer configurations, such as sensors arranged side by side, can also be used. If the distance between the individual sensor heads is too great, the background field can no longer be identical at both locations, particularly in the case of local background fields (electrical lines, etc.); if the distance between the sensor heads is too small, the difference between the field strengths of the magnetic field to be measured between the two locations can be too small. The selection of the appropriate distance between the two sensors of a gradiometer unit is therefore preferably also dependent on the type and alignment of the magnetic field source of interest and on the expected field strength. The distance between the sensor unit and the magnetic field source can also be in the mm to cm range.

2 shows exemplary magnetic field strengths of biomagnetic brain magnetic fields in comparison, which occur at different distances from the skull surface. These very weak magnetic fields are induced by the currents that flow in the brain neurons during neuronal activity and therefore offer similar effects to an electroenze phalogram Evidence of various mental or health conditions. The x-axis shows the depth of the actual source of the magnetic field, i.e. the depth of the neuronal activity in the brain that generates the magnetic field, while the logarithmic y-axis shows the maximum magnetic field strength in femtotesla (fT).

The lower curve 210 shows the magnetic field strength that occurs 3 cm from the skull surface and ranges between approximately 10 ^-13 Tesla (approximately 100 fT) and 5*10 ^-15 Tesla (approximately 5 fT) depending on the depth of brain activity. In contrast, the upper curve 220 shows magnetic field strengths at a distance of about 6 mm from the scalp ranging from about 10 ^-12 Tesla (about 1 pT) to about 10 ^-14 Tesla (about 10 fT). It is thus evident over the entire range that the measurable field strengths of the brain's magnetic field between a distance of 6 mm and 3 cm from the scalp already drop at least fivefold, so that the compact sensor units according to the present invention, which, for example, are not like SQID Magnetometers rely on complex nitrogen cooling, enable significantly more sensitive measurements.

Instead of two sensor diamonds as shown in 1 were shown, even a single diamond with NV centers, having at least some spatial extent along one direction, could be used to enable a gradiometer configuration. For this purpose, the diamond is excited at at least two points and the resulting fluorescent light is read out by at least two suitably arranged photodetectors. A diamond waveguide with NV centers can also be used for this purpose, for example, so that the excitation light can propagate along the gradiometer axis in the waveguide and reach both readout locations. The crystal does not have to be very large perpendicularly to it, so that thin diamond plates, for example, are conceivable as sensor heads.

If the optimal distance between the sensor heads for a gradiometer configuration is larger than the possible or desired extension of a single diamond crystal, two diamonds with identical properties can preferably be used. The optical properties are influenced, for example, by the type and number of NV centers or defect density and possible mechanical stresses in the diamond crystal. In order to achieve properties that are as homogeneous as possible, two pieces can then be cut from a larger diamond crystal, for example. In particular, synthetically produced diamonds are suitable, which can be synthesized, for example, by means of chemical vapor deposition (CVD) or in the high-pressure, high-temperature process (HPHT, high pressure, high temperature).

3 shows an example of a possible embodiment of a single sensor head 300 of a gradiometer unit, which is formed from two of these sensor heads, in an oblique view. In this case, excitation light from an excitation light source is coupled into an optical fiber 301 via an optical fiber coupler. The fiber is held by a fiber mount 305, which can also be used to stabilize the entire system. The embodiment shown is therefore only to be understood as an example.

The end of the optical fiber 301 may be bonded to a gradient refractive index (GRIN) lens 303 using an optical adhesive. This GRIN fiber composite can then in turn be glued to a diamond 304 with the aid of a transparent adhesive layer. The diamond 304 has a first side 304a and a second side 304b, with the GRIN fiber composite adhered to at least a portion of the diamond surface on the first side 304a. Optionally, an optical filter layer can be used between the surfaces of the GRIN lens 303 and the diamond 304, so that the fluorescent light emitted by the at least one NV center of the diamond cannot escape unhindered from the corresponding surface of the diamond 304, but in the direction of the second side 304b is reflected back. A photodiode system 308 having a first photodiode 309 and a second photodiode 310 is arranged on this second side 304b, ie below the diamond 304 in the figure. The photodiodes can be designed in such a way that the first photodiode 309 can detect the emitted fluorescence but blocks the wavelength of the excitation light, while the second photodiode 310 blocks the fluorescence emitted by the at least one NV center and only allows the excitation light to pass. For example, this can be achieved by optical filters, e.g., in the form of coatings.

With the help of appropriate signal processing, e.g. a logarithmic circuit, a balanced measurement signal can be generated from the two measured signals of the two photodiodes 309 and 310, which already takes into account fluctuations in the excitation light. In addition, the sensor head 300 has a microwave resonator structure 306 in order to homogeneously distribute the microwaves generated by a suitable microwave source over the volume of the diamond 304 . In this case, the microwaves can be introduced into the microwave resonator structure via a high-frequency plug 307 . The resonator structure 306 is tuned to the frequency of the electron spin resonances.

4 shows an overview of a gradiometer unit, which has two identical ones, as in 3 having sensor heads shown. In this example, the two sensor heads 400a, 400b are mounted at a defined distance from one another on a mechanical support plate. A different attachment or arrangement is also possible for the sensor heads and can be selected depending on the later application, the required distance between the sensor heads or the type of gradiometer configuration. Both sensor heads 400a and 400b are preferably supplied by the same excitation light source 420, it being possible for the excitation light to be guided, for example, through a fiber splitter 421 to the optical fibers of both sensor heads. Likewise, both sensor heads can be supplied by the same microwave source 440, with a microwave splitter 441 being used accordingly.

In order to determine the location of a magnetic field source, i.e. to enable vector magnetometry, an additional bias magnetic field 150 can be generated, which is identical at both measurement locations of the gradiometer (i.e. in the area of the sensor heads or sensor diamonds). The field gradient is to be determined in three axes. The direction in which the magnetic field gradient is maximum indicates the location of the magnetic field source. The bias magnetic field is indicated here by a single coil 450, within which the two sensitive sensor heads 400a and 400b are located. For example, the sensor heads can be used in a Helmholtz coil arrangement, which generates a bias magnetic field that is as homogeneous as possible in the area of the two sensor heads and is supplied by a suitable power source 451 (e.g. a battery).

If the same microwave source and the same light source is used for both sensor diamonds or sensor heads, noise components from these sources (noise from the excitation light or frequency and amplitude noise from the microwave source) also occur on both sensor heads, so that these simultaneously occurring noise components are automatically eliminated by the gradiometer arrangement . This reduces the demands on the noise characteristics of these components. For example, instead of a high-precision, expensive and temperature-controlled crystal oscillator, a low-cost standard oscillator with higher phase and amplitude noise can then be used for a microwave source. Also for the bias magnetic field, the use of a single field source and current source preferably ensures that the same noise components are to be expected at both sensors, which in turn are eliminated by the gradient formation.

In order to further suppress the sensor noise of the gradiometer, a modulation method can be used, for example, in which the microwave frequency or the amplitude of the microwaves is modulated. In this case, a modulation of the fluorescence is generated by modulating the microwave frequency with a specific modulation depth and modulation frequency. By demodulating the detected fluorescence signal, a signal that is linearly dependent on the magnetic field is then obtained in the range of the magnetic resonance frequency. Alternatively or additionally, the light intensity of the excitation light can also be modulated.

The fluorescence signals from the two sensor heads in a gradiometer are then further processed in a suitable manner. In order to obtain the field gradient, a difference in the signals can be formed, as already described, which can be achieved, for example, by a simple or logarithmic differential amplifier. The signal can also be fed to a post-amplifier, an AD converter and/or frequency filters, for example. The pre-processed difference signal obtained, which represents the weak magnetic field of interest, can then be evaluated for further purposes.

In addition or as an alternative to a gradiometer configuration, lock-in detection can be used for a magnetometer in order to eliminate the unwanted background fields from the measurement. For this purpose, for example, the excitation light or the microwave source can be modulated and a lock-in amplifier (134 in 1 ) may be provided in the area of the signal processing of the sensor unit 100, which essentially acts as a narrow band pass filter for the resonance signal of interest.

Such a highly sensitive sensor based on a gradiometric arrangement of two magnetometers can be used in a wide variety of areas.

In this way, for example, the brain magnetic fields already mentioned above can be measured by arranging one or more sensor units in the vicinity of a user's head. For example, the sensor units may be integrated into head-worn mounts or headgear, or may be integrated into elements that are close to the head, such as a headrest in a vehicle or a headboard of a bed.

With the measurement of brain magnetic fields, various states of a user can be recorded, such as tiredness or stress, but emergencies or neurological problems can also be detected cal diseases such as epilepsy, stroke, Parkinson's disease and others can be detected. In addition, the measurement of the brain's magnetic fields can be used, as with brain current measurements, to control various devices (e.g. prostheses, vehicles) by recording and evaluating the neural activity of an imaginary command or involuntary reflexes and patterns.

Other biomagnetic fields can also be measured in a corresponding manner. Small electrical currents are also present in muscle cells and in nerve activity throughout the body, which again induce a weak magnetic field. This means that magnetic field measurements on the body can be used, for example, to monitor the activity of the heart muscle. It can also be used to record nerve signals in any part of the body outside of the brain, which can then be evaluated and implemented to control prostheses, for example.

In further embodiments, a highly sensitive magnetic field sensor as presented here can be used to generally measure small electrical currents (e.g. in lines or on technical components), which also induce a magnetic field. With this type of sensor, for example, the current intensity can be determined indirectly. In other embodiments, a sensor unit can be used to monitor electrical switchgear.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 102018220234 A1 [0003]
• DE 102018214617 A1 [0003]

Claims (10)

Sensor unit (100) for measuring magnetic fields, comprising: two magnetometers (300, 400a, 400b) arranged at a fixed distance from one another, each magnetometer having a sensor medium (101, 102, 304) and being set up to measure a magnetic field strength at a measurement location by reading out one of the magnetic field strengths to detect dependent spin resonance in the sensor medium, at least one excitation light source (120) for radiating light (124) into the sensor media (101, 102) of the magnetometer, at least one signal processing unit (406) for determining a magnetic field gradient (100, 200, 300, 400) as the difference between the output signals of the two magnetometers. sensor unit claim 1 , wherein each of the magnetometers (101, 102, 400a, 400b) comprises a nitrogen vacancy center magnetometer, wherein the sensing medium comprises a diamond crystal (101, 102) or a portion of a diamond crystal having nitrogen vacancy centers, and wherein the Sensor unit further comprises at least one microwave source (140) for generating a resonant field in the sensor medium and at least one photodetector (130) for detecting resonance-dependent fluorescent light (125) from the sensor medium. sensor unit claim 2 , wherein the sensor medium comprises a thin diamond plate. Sensor unit according to one of claims 2 or 3 , wherein the sensing medium of the two magnetometers each comprises a portion of the same diamond crystal. Sensor unit according to one of claims 2 until 4 , wherein the diamond crystal is designed in such a way that it serves as a waveguide for guiding the light from the excitation light source (120, 420) to the measuring locations of the two magnetometers. Sensor unit according to one of claims 2 until 5 , wherein the two magnetometers are assigned the same excitation light source (120, 420) and/or the same microwave source (140, 440). Sensor unit according to one of claims 2 until 6 , wherein the sensor unit has a microwave resonator (306) in the area of the sensor medium, which is tuned to the spin resonance of the sensor medium. Sensor unit according to one of the preceding claims, further comprising a device (450) for generating a substantially homogeneous bias magnetic field (150) in the region of the sensor medium. sensor unit claim 8 , wherein the means (450) for generating the bias magnetic field comprises a Helmholtz coil arrangement, wherein at least the sensor medium of the two magnetometers is arranged within the Helmholtz coil arrangement. Sensor unit according to one of the preceding claims, wherein the distance (d) between the two magnetometers (101, 102, 400a, 400b) is between 1 and 30 millimeters, preferably between 5 and 20 millimeters.

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