RU2638918C1 - Sensor of magnetic field based on brillouin scattering - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: magnetic field sensor contains a magneto-optical cell with a single-crystal film of garnet ferrite on a nonmagnetic substrate and a modulating coil. In addition, the device comprises first and second Fabry-Perot interferometers placed along a common optical axis. A magneto-optical cell with garnet ferrite film is placed between the Fabry-Perot interferometers. Modulating coil contains four flat windings with forming two orthogonal pairs, wherein substrate with film of garnet ferrite is inside the coils with providing modulation of magnetization by rotating magnetic field in the plain of the film. A harmonic signal generator is used in the frequency range 10⁷-10⁹ Hz, and the registration system is designed to extract harmonics at the output of the photodetector, multiples of the generator frequency, whose amplitude is proportional to the value of the recorded constant magnetic field.

EFFECT: increase the sensor sensitivity.

8 cl, 10 dwg

Description

The invention relates to measuring equipment, namely to optical magnetometers, and can be used to measure weak magnetic fields in science and technology, including, for example, biology, medicine, non-destructive microcontrol, low-frequency communication, etc.

A large number of magneto-optical magnetic field sensors based on thin films of ferromagnetic materials are described. So, in patent US 8519323 (B2) -ELECTRIC FIELD / MAGNETIC FIELD SENSORS AND METHODS OF FABRICATING THE SAME, NAKADA MASAFUMI et al., 08/27/2013, a fiber-optic sensor is described in which a layer of magnetically sensitive medium of yttrium iron garnet directly at the end of the fiber. The necessity of fulfilling the relation between the parameters of the magneto-optical medium and the parameters of the fiber is noted. However, such a sensor using the classical method of polarization measurements is not sensitive enough to measure weak magnetic fields. Due to the small perturbation of the polarization of the reflected laser radiation, the intensity of the light transmitted through the analyzer and incident on the photodetector changes very weakly under the influence of the measured magnetic field. As a result of this, the problem arises of measuring small changes in the intensity of light against the background of optical noise inevitably present in laser radiation. Another disadvantage of this scheme is the lack of a photodetector calibration system or a balanced measurement scheme in the circuit, which leads to an additional decrease in the sensitivity of the photodetector when measuring low-frequency magnetic fields.

US 4,563,646 (A) - Optical magnetometer using a laser coupled to a magneto-optical medium, THOMSON CSF, 1/7/1986, describes an optical magnetometer for measuring the magnetic field component using the Faraday effect. A semiconductor laser emits two laser beams that are directed from opposite laser faces in opposite directions along two fiber optic cables. This optical magnetometer implements a method based on the interference of optical radiation in the laser cavity, however, the laser optical noise in this system is not filtered and not suppressed, which will be a significant limitation of the sensor sensitivity. The use of an external fiber resonator coupled to the laser cavity can lead to additional noise due to external factors acting on the fiber.

The patent FIBER-OPTICAL MEASURING DEVICE (RU 2429498 C2, FSUE RFNC-VNIIEF, September 20, 2011) describes a device for measuring the pulse current and magnetic field strength. The device comprises a source (single-mode semiconductor laser), an optical fiber supply, a magneto-optical sensor, an optical fiber output and a recorder. The magneto-optical sensor contains a series-connected connector, one coil covering the current lead, and an adapter. The coil of the magneto-optical sensor is made of single-mode fiber with one Verde constant. The technical result is the expansion of functionality, the range of measured currents and magnetic field strength, reducing the error of current measurements. This device is simple and versatile, but it is designed for remote measurement of magnetic fields of extremely high intensity (up to 1 kT) and cannot be used to measure fields of less than 1 mT.

The invention “MAGNETIC FIELD SENSOR WITH OPTICALLY SENSITIVE DEVICE AND METHOD FOR MEASURING A MAGNETIC FIELD” (US 8125644 (B2), CROWE; RAYTHEON COMPANY, 02.28.2012) describes an optically sensitive device with a material that changes size in response to a change in the magnetic field . The magnetosensitive element is placed in the optical resonator with high quality factor. Resonance leads to an increase in the magneto-optical effect and, therefore, sensitivity is proportional to the quality factor of the resonator. The disadvantage of this scheme is that the magnetically sensitive element introduces losses into the resonator, while reducing the quality factor, worsening the signal-to-noise ratio.

Known fiber optic sensor magnetometer based on the Faraday effect (US 7106919 (B2) - Magneto-optical sensing employing phase-shifted transmission bragg gratings, Kochergin V., etc., 09/12/2006). The design uses a fiber magneto-optical phase modulator and planar Bragg gratings, which form a Fabry-Perot resonator around the phase modulation region. If the wavelength of the incident light coincides with the mode of the Fabry-Perrot resonator, then the effect of rotation of the polarization in the fiber under the influence of a magnetic field will be significantly enhanced.

In this device, the Fabry-Perot interferometer is used to enhance the magneto-optical effect due to the multiple passage of light through the film, however, the optical noise of the laser is not suppressed, but remains at the same level. Absorption in the magnetically sensitive layer will be the limiting factor of this gain. A similar effect could be obtained if a magnetosensitive element of a larger thickness were used instead of an interferometer.

The closest to the purpose is the sensor described in US patent 8000767 (B2) - Magneto-optical apparatus and method for the spatially-resolved detection of weak magnetic fields, EDEN JG; GAO JU; BOARD OF TRUSTEES OF THE UNIVERSITY OF ILLINOIS, 08.16.2011. The magnetoplasmonic magnetic field sensor contains a magneto-optical cell with a YIG film and a modulation winding connected to an electric signal generator, an optical path of the registration system, including a narrow-band single-mode laser, a photodetector connected to a computer registration system. The magnetic field of the subject is determined by measuring the angle of rotation of the plane of polarization of light reflected from the YIG film doped with bismuth, a few microns thick. A sensor based on the magneto-optical Kerr effect uses the transition region between two magnetization states of a magneto-optical film to detect magnetic fields of less than 100 pT.

The disadvantage of this device is that the measurement is carried out by the polarization method, therefore, optical noise of laser radiation is added to the useful signal without attenuation. Moreover, the magnetization modulation method used in this device leads to limitations of the upper frequency range of about 1 kHz.

The present invention is directed to solving the problem of increasing the sensitivity of a magneto-optical magnetic field sensor.

The patented magnetic field sensor contains a magneto-optical cell with a single-crystal film of ferrite garnet on a non-magnetic substrate and a modulation coil connected to a signal generator, an optical path of the registration system, including a narrow-band single-mode laser, a photodetector connected to a computer registration system.

The difference is as follows.

The first and second Fabry-Perot interferometers placed along a common optical axis are additionally introduced into the device, a magneto-optical cell with a garnet-ferrite film is placed between the Fabry-Perot interferometers.

The modulation coil contains four flat windings with the formation of two orthogonal pairs, the substrate with a garnet-ferrite film being located inside the said coil providing modulation of magnetization by a rotating magnetic field in the film plane, and the said flat windings are installed so that the central part of the film is open to a common optical axis.

A harmonic signal generator was used in the frequency range 10 ⁷ -10 ⁹ Hz, and the registration system is configured to extract harmonics that are multiples of the frequency of the said generator at the output of the photodetector, whose amplitude is proportional to the magnitude of the recorded constant magnetic field.

The sensor can be characterized in that the single-crystal film of ferrite garnet with the (111) orientation is formed by liquid-phase epitaxy on a single-crystal gadolinium-gallium garnet substrate and has a thickness of 0.5-2.0 μm and a diameter of 1-5 mm, as well as the fact that a single-crystal film of ferrite garnet has the composition Re _3-x Bi _x Fe _5-y , Me _y O ₁₂ , where Re is a rare-earth element, Me is an ion replacing iron; 0 <x <2; 0.3 <y <0.7.

The sensor can be characterized by the fact that the single-crystal film of ferrite garnet has the composition (BiGd) ₃ (FeSc) ₅ O ₁₂ , and also by the fact that the distance between the windings in each pair is chosen to be less than or equal to half the width of the winding with the formation of a gap for the passage of radiation, not exceeding 0.5 mm.

The sensor can be characterized, in addition, by the fact that the edges of the peripheral part of the ferrite garnet film from the center to the periphery are rounded so that they have a shape close to the generatrix of the ellipse in cross section.

The sensor can also be characterized by the fact that a lattice of a metal and / or dielectric layer with a slit width of 0.1 to 0.4 of the lattice period is formed on the surface of a single-crystal ferrite garnet film, and in addition, the modulation frequency range magnetization by a rotating magnetic field in the plane of the film is from 50 to 300 MHz.

The technical result is an increase in the sensitivity of the sensor due to the selective suppression of optical noise in the interferometer and the possibility of using spatial filtering of the useful signal, which can dramatically increase the signal-to-noise ratio.

The invention is based on its own experimental studies, described, in particular, in the publications of P.M. Vetoshko, A.K. Zvezdin, V.A. Skidanov, I.I. Serum, I.M. Serum, V.I. Belotelov. Influence of the profile of a disk magnetic element on the saturation field and the noise of a magnetomodulating magnetic field sensor / Letters in ZhTF, 2015, Volume 41, no. 9, pp. 103-110; as well as P.M. Vetoshko, N.A. Gusev, D.A. Chepurnova, E.V. Samoilova, I.I. Serum, I.M. Serum, A.K. Zvezdin, A.A. Korotaeva, V.I. Belotelov. Measurements of rat magnetocardiograms using a magnetomodulating magnetic field sensor based on ferrite garnet films / Letters in ZhTF, 2016, Volume 42, no. 16, pp. 64-71.

As a rule, the design of magnetometers with optical reading uses a balanced method for recording small changes in light intensity. The intensity difference is measured at two photodetectors. A signal modulated by a magnetic field enters one photodetector, and a reference signal without modulation or (for example, in the case of polarization measurements) modulated in antiphase with respect to the first one enters a signal modulated by a magnetic field. If A is the intensity of the light emerging from the laser, and n is the noise intensity in the optical beam, then for the difference of the signals at the photodetector, the relation

Δ = mA + n (m + 1),

where m is the modulation coefficient of light by a magnetic field.

It can be seen from this formula that for m << 1, the signal-to-noise ratio can be significantly lower compared to the input signal. Placing a magneto-optical cell inside a Fabry-Perot resonator, as described in the patents discussed above, only leads to an increase in the effective parameter m. However, an increase in the signal-to-noise ratio in this case is not as effective as in the patented case.

Due to the use of frequency and spatial filtering of the signal by Fabry-Perot resonators before and after magneto-optical interaction, considered as Mandelstam-Brillouin scattering by magnons, the magnitude of the signal at the photodetector will be described by the following formula:

Δ = mA + n (m / Q ₁ +1) / Q ₂ ,

where Q _1,2 is the quality factor of the Fabry-Perot resonators. From this follows the indicated possibility of a sharp increase in the signal-to-noise ratio and, accordingly, the sensitivity of the magnetic field sensor.

The invention is illustrated in the figures, where:

FIG. 1 is a block diagram of a sensor;

FIG. 2 - topology of a magneto-optical cell, side view;

FIG. 3 - topology of a magneto-optical cell, plan view;

FIG. 4 is a sectional view of a film on a substrate;

FIG. 5 is a diagram of a magneto-optical modulator based on the equatorial Kerr effect;

in FIG. 6 is a diagram of a magneto-optical modulator based on the meridional Kerr effect;

in FIG. 7-10 - frequency dependence of the intensity of the output signal from an external magnetic field.

The device (Fig. 1) contains a magneto-optical cell 10, including a thin magnetic film 12 of single crystal ferrite garnet with orientation (111), which is formed by liquid phase epitaxy on a single crystal substrate 14 of gadolinium-gallium garnet and has a thickness of 0.5-2, 0 microns and a diameter of 1-5 mm.

The substrate with the film 12 is surrounded by a system of 16 inductors connected to a high frequency (HF) harmonic signal generator 18. The generator 18 operates in the frequency range 10 ⁷ -10 ⁹ Hz. The coil system 16 and the generator 18 provide the creation of a rotating magnetic field that modulates the magnetization of the film 12.

The optical path of the registration system contains a narrow-band single-mode laser 20, the first 22 and second 24 Fabry-Perot interferometers, a photodetector 26, and a computer registration and control system 30. The magneto-optical cell 10 is placed between the interferometers 22 and 24.

The principle of operation of the magnetoplasmonic magnetic field sensor is based on the Brillouin spectroscopy scheme. In FIG. 1, positions a , b, c, and d show the sequential conversion of the spectrum of laser radiation during passage through the optical path.

The optical beam 40 generated by the narrowband single-mode laser 20 (pos. A), is directed to the Fabry-Perot interferometer 22 which is configured so that its transmission line coincides with a maximum in the spectrum of the laser radiation. After passing through the interferometer 22, there is a significant narrowing of the spectrum of optical radiation by an amount determined by the quality factor of the interferometer, which can reach 10 ⁶ (pos. B). Next, the optical beam 40 is incident on the film 12 of a ferrite garnet, the magnetization of which is modulated with a frequency of the order of 10 ⁸ Hz. The magnetization is modulated by a system of 16 coils, on the windings of 17 of which a harmonic signal is supplied from the high-frequency generator 18.

As a result of the magneto-optical effect, the laser beam 40 is modulated, therefore, two side components appear in its spectrum, which are spaced from the central maximum by an amount equal to the frequency of the signal coming from the generator 18 (pos. C). Due to the low efficiency of the magneto-optical interaction, these spectral components are much weaker than the fundamental frequency and cannot be detected directly by the photodetector 26. Therefore, a second Fabry-Perot interferometer 24 is used to isolate them. The interferometer 24 is tuned so that the maximum transmittance corresponds to the side spectral component, and the intermode distance is equal to twice the frequency of the signal of the RF generator 18. Due to this tuning, the central spectral component is substantially suppressed. Since its frequency falls at a minimum transmittance, the suppression is equal to the quality factor of the interferometer 24 and can reach 10 ⁶ . In the spectrum of the optical radiation transmitted through the interferometer 24, the central component is smaller or close in magnitude to the lateral components (pos. G). This radiation is detected by the photodetector 26. In the detection process, the principle of optical heterodyning is used, so the signal of the photodetector 26 is a harmonic function with a fundamental frequency equal to the frequency of the RF generator 18 and an amplitude equal to the magnitude of the side components of the optical signal (pos. G).

In the presence of a detectable constant magnetic field in the spectrum of the optical signal and, therefore, in the signal of the photodetector 26 there are harmonics that are multiples of the frequency of the generator 18 and proportional to the magnitude of the constant magnetic field. Thus, the measurement of weak magnetic fields is achieved by detecting and measuring the magnitude of these harmonics in the spectrum of the signal of the photodetector 26.

The wave number of magnons substantially depends on the parameters of the film. The parameters of the film 12 and the modulating windings 17 of the system 16, as well as the frequency of the current in the windings 17, determine the wave number of excited magnons and, therefore, determine the angles at which Mandelstam-Brillouin scattering occurs. The harmonics of optical radiation that arise after the beam 40 passes through the magneto-optical cell 10 will generally be separated in space from each other and from the transmitted beam at the fundamental frequency of the optical radiation. To simplify the circuit, it is advisable to use such a configuration of modulating windings 17, which provides the wave numbers of magnons, equal to or close to zero. In this case, scattering becomes equivalent to light modulation.

In FIG. 2, 3 shows the structure of modulating windings 17. Two pairs of windings 171, 172 orthogonal to each other are used. The windings 17 have a cross-sectional shape close to rectangular. To achieve uniform magnetization of the film 12, the distance h between the windings in each pair is chosen so as to be less than or equal to half the smaller internal size s of the winding, i.e. h≤0.5s. Thus, between the windings 171 and 172, a gap 121 is formed for the passage of radiation. The dimension g of the gap 121 does not exceed 0.5 mm.

To reduce magnetic noise, the edges 122 of the magnetic film 12 are rounded from the center to the periphery. The edges are processed so that the cross section 123 of the film 12 is close to half the ellipse (see Fig. 3). The achievement of such a form in practice is possible due to its approximation by steps 124, as shown by the dashed line, by layer-by-layer lithography of the edges of the film.

Light can be modulated in a magneto-optical cell by modulating the polarization of light — the Faraday effect or the meridional Kerr effect, and also by modulating the light intensity — the equatorial Kerr effect and the meridional intensity magneto-optical effect. In FIG. 5 is a diagram of a magneto-optical modulator based on polarization effects, and FIG. 6 - on the intensity meridional magneto-optical effect.

In the case of using the Faraday effect (as well as the meridional Kerr effect), the garnet-ferrite film 12 is located at an angle α to the optical beam 40 (Fig. 5). A polarizer 51 is located in front of cell 10, and an analyzer 52 is located after the cell. An equatorial Kerr effect also requires oblique incidence, but the polarizer 51 and analyzer 52 are not used. In the case of the meridional intensity effect, the ferrite-garnet film 12 is located orthogonal to the optical beam 40. A layer 60 is applied to the surface of the film 12, which is a periodically structured metal or dielectric film (Fig. 6), in which case the polarizer and analyzer are not used.

Layer 60 is a periodic system of slots, the width of which is from 0.1 to 0.4 of the period. So, at a laser wavelength of 780 nm and a magnetic film thickness of 1.9 μm, the thickness of the metal layer 60 is 80 nm, the lattice period is 335 nm, and the slit width is 120 nm. This embodiment of layer 60 allows a significant enhancement of the meridional intensity magneto-optical effect (see V.I. Belotelov et al. Plasmon-mediated magneto-optical transparency / Nature Communications, 2013, V. 4, 2128) and, accordingly, increase the sensitivity of the sensor.

The sensor works as follows.

In the absence of an external (measured) magnetic field, light is modulated with a frequency f equal to the current frequency in the modulating windings 17 connected to the RF generator 18. In the presence of a measured magnetic field, harmonics corresponding to doubled, quadrupled, etc. appear in the spectrum of the optical signal. frequency (2f, 4f, 6f, ...) of the modulating signal of the generator 18. The harmonic amplitude at twice the frequency 2f is proportional to the magnitude of the measured field.

In FIG. 7 shows the transmission spectrum of the first Fabry-Perot resonator 22 (pos. 71), the laser radiation spectrum (pos. 72), and the radiation spectrum of the radiation transmitted through the first resonator 22 (pos. 73). The resonator 22 must be tuned so that one of its transmission maximums is at the wavelength of the laser radiation 20, and the nearest minimum corresponds to the line 2f.

In FIG. Figure 8 shows the transmission spectrum of the second Fabry-Perot resonator 24 (key 74), the spectrum of the radiation transmitted through the magneto-optical cell (key 75), and the spectrum of the radiation transmitted through the second resonator (key 76). As a result of passing through the second resonator, the amplitude of the useful signal (peak 2f in Fig. 8) becomes dominant.

The graphs in FIG. 9, 10 relate to an embodiment of cell 10 using a meridional intensity effect. A layer 60 of a periodically structured metal or dielectric film is formed on the surface of the garnet-garnet film 12. In the absence of an external (measured) field, light is modulated at a frequency of 2f, where f is the frequency of the current in the modulation winding. In the presence of a measured field in the spectrum of the optical signal, harmonics appear corresponding to the frequencies f, 3f, 5f, etc. The harmonics amplitudes at frequencies f and 3f are equal to each other and are proportional to the magnitude of the measured field. In FIG. 9 shows the transmission spectrum of the first Fabry-Perot resonator 22 (dashed line 81), the radiation spectrum of laser 20 (dashed line 82), and the spectrum of radiation transmitted through the first resonator (solid line 83). The first Fabry-Perot resonator must be tuned so that one of its transmission maxima is at the wavelength of the laser radiation, and the nearest minimum corresponds to the line f.

In FIG. 10 shows the transmission spectrum of the second Fabry-Perot resonator (dashed line 84), the spectrum of the radiation transmitted through the magneto-optical cell (dashed line 85), and the spectrum of the radiation transmitted through the second resonator (solid line 86). As a result of passing through the second resonator, the amplitude of the useful signal (peaks f and 3f in Fig. 10) becomes dominant. It is most preferable to use peak 3f, since after detection in the electrical signal at a frequency of 3f there will be no interference created by the magnetization field oscillating with frequency f.

Thus, the experimental data show that increasing the sensitivity of the sensor due to the selective suppression of optical noise in the interferometer using frequency filtering of the useful signal can sharply increase the signal-to-noise ratio up to the level of thermal magnetization fluctuations. Estimates show that since the amplitude of thermal magnons corresponds to fluctuations of the magnetic field at a level of 1 fT, the sensitivity of the sensor should be expected at a level of 100 fT and lower, in the frequency band of 1 Hz.

Claims (13)

1. A magnetic field sensor containing a magneto-optical cell with a single-crystal film of ferrite garnet on a non-magnetic substrate and a modulation coil connected to a signal generator, an optical path of the registration system, including a narrow-band single-mode laser, a photodetector connected to a computer registration system, characterized in that the first and second Fabry-Perot interferometers placed along a common optical axis are additionally introduced into the device, a magneto-optical cell with a ferrite garnet film is placed between the said Fabry-Perot interferometers, the modulation coil contains four flat windings with the formation of two orthogonal pairs, and the substrate with a garnet-ferrite film is located inside the coil with modulation of magnetization by a rotating magnetic field in the film plane, and the said flat windings are installed so that the central part of the film is open to a common optical axis, in this case, a harmonic signal generator was used in the frequency range 10 ⁷ -10 ⁹ Hz, and the registration system is configured to extract harmonics that are multiples of the frequency of the said generator at the output of the photodetector, whose amplitude is proportional to the magnitude of the recorded constant magnetic field. 2. The sensor according to claim 1, characterized in that the single-crystal film of ferrite garnet with the (111) orientation is formed by liquid phase epitaxy on a single crystal gadolinium-gallium garnet substrate and has a thickness of 0.5-2.0 μm and a diameter of 1-5 mm . 3. The sensor according to claim 1, characterized in that the single crystal ferrite garnet film has the composition Re _3-x Bi _x Fe _5-y Me _y O ₁₂ , where Re is a rare-earth element, Me is an ion replacing iron; 0 <x <2; 0.3 <y <0.7. 4. The sensor according to claim 1, characterized in that the single crystal ferrite garnet film has the composition (BiGd) ₃ (FeSc) ₅ O ₁₂ . 5. The sensor according to claim 1, characterized in that the distance between the windings in each pair is selected to be less than or equal to half the width of the winding with the formation of a gap for the passage of radiation not exceeding 0.5 mm. 6. The sensor according to claim 1, characterized in that the edges of the peripheral part of the ferrite garnet film from the center to the periphery are rounded so that in cross section they have a shape close to the generatrix half of the ellipse. 7. The sensor according to claim 1, characterized in that a lattice of a metal and / or dielectric layer with a slit width of 0.1 to 0.4 of the lattice period is formed on the surface of a single-crystal ferrite-garnet film. 8. The sensor according to claim 1, characterized in that the frequency range of the modulation of magnetization by a rotating magnetic field in the film plane is from 50 to 300 MHz.

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