Device and method for measuring weak magnetic field

Abstract

The invention discloses a device and a method for measuring a weak magnetic field. The device comprises a laser, a first prism, a second prism, a Helmholtz coil, a magneto-optical crystal, a diaphragm, a photomultiplier and an oscilloscope, when a light source is changed into linearly polarized light after passing through the first prism, the linearly polarized light passes through the magneto-optical crystal in the magnetic field, the direction of the polarized light is rotated, the polarized light has weak emergent light intensity after passing through the second prism orthogonal to the first prism, after the diaphragm shields stray light, emergent light spots irradiate the cathode of the photomultiplier, and the photomultiplier amplifies optical signals and converts the optical signals into electric signals at the same time; and the direct-current voltage value is displayed on the oscilloscope. According to the method, in the measurement process of the direct-current low-intensity magnetic field, the Faraday optical rotation effect is utilized, the intensity of emergent light passing through the two orthogonal polarizers is measured, the size of the magnetic field to be measured is obtained, the method is simple, and the measurement precision is high.

Description

Technical Field

[0001] The present invention relates to the technical field of magnetic field detection, and in particular to a device and method for measuring weak magnetic fields. Background Art

[0002] Magnetic field measurement technology plays a very important role in many fields such as medical devices, materials science, space research, and geological exploration. However, during the measurement process, the magnetic field signal is often submerged in a large amount of noise due to many interference factors and limited precision of the measuring instrument. Therefore, the accurate measurement of the magnetic field, especially the measurement of weak magnetic fields, is difficult. Currently, commonly used measurement methods include the Hall effect method, the fluxgate method, the optical rotation effect method, etc., and the corresponding measuring instruments include teslameters, fluxmeters, etc. These instruments are generally applicable to situations with relatively strong magnetic fields, and have deficiencies such as being greatly affected by temperature and relatively low sensitivity. Of course, there is also a superconducting quantum interference magnetometer with very high measurement precision, but its process is complex, it needs to operate at low temperature with liquid nitrogen, and it has not been widely used. Summary of the Invention

[0003] Object of the Invention: The object of the present invention is to provide a device and method for measuring weak magnetic fields, which obtain the magnitude of the magnetic field to be measured by using the relationship between the outgoing light intensity after two orthogonal polarizers and the magnetic field, make up for the defect of the existing low precision in weak field measurement, and are applicable to the measurement of uniform magnetic fields and complex non-uniform magnetic fields at the same time.

[0004] Technical Solution: A device for measuring weak magnetic fields includes:

[0005] A laser as a light source;

[0006] A first prism and a second prism, which together form a pair of prisms with orthogonal polarization planes; among them, the first prism is used as a polarizer, and the second prism is used as an analyzer;

[0007] A Helmholtz coil for forming a uniform magnetic field in the central region of the magneto-optical crystal;

[0008] A magneto-optical crystal placed at the center of the Helmholtz coil for rotating the polarization direction of the linearly polarized light passing through it;

[0009] A diaphragm for blocking stray light so that the outgoing light spot irradiates the cathode of the photomultiplier tube;

[0010] A photomultiplier tube for amplifying the optical signal and converting it into an electrical signal;

[0011] An oscilloscope for displaying the DC voltage value output by the photomultiplier tube;

[0012] When the light source becomes linearly polarized light after passing through the first prism, passes through the magneto-optical crystal in the magnetic field, the direction of the polarized light rotates, and there is a weak outgoing light intensity after passing through the second prism. After the diaphragm blocks the stray light, the outgoing light spot irradiates the cathode of the photomultiplier tube. While amplifying the optical signal, the photomultiplier tube converts it into an electrical signal, and a DC voltage value is displayed on the oscilloscope.

[0013] Furthermore, both the first prism and the second prism are selected as Glan-Taylor prisms.

[0014] Furthermore, the diaphragm is selected as a stray light eliminating diaphragm for eliminating stray light.

[0015] Furthermore, after the Helmholtz coil is energized, a uniform magnetic field is formed in the central region of the magneto-optical crystal.

[0016] A method for measuring a weak magnetic field realizes the measurement of the weak magnetic field through the device for measuring a weak magnetic field in any one of the above, and includes the following steps:

[0017] S1, energize the Helmholtz coil;

[0018] S2, the laser works, and read the DC voltage value displayed on the oscilloscope;

[0019] S3, according to the relationship that the light intensity is proportional to the square of the magnetic field, combined with the magnetic field calibration, the magnitude of the magnetic field at any light intensity can be obtained;

[0020] S4, perform weighted non-linear quadratic fitting on the obtained data points to fit the curve of the voltage and the magnetic induction intensity at the center of the coil; according to the fitting result, obtain the magnetic induction intensity corresponding to any DC voltage.

[0021] Compared with the prior art, the remarkable effects of the present invention are as follows:

[0022] The present invention obtains the magnitude of the magnetic field to be measured by using the relationship between the outgoing light intensity after two orthogonal polarizers and the magnetic field. Compared with the traditional method of obtaining the magnetic field by measuring the deflection angle of the optically active crystal, the measurement method is simple and has high precision, and can realize the measurement of weak DC magnetic fields of 0.1 mT to 2 mT. The measurement limit of 0.1 mT is about twice the magnetic field; at the same time, the present invention is not only applicable to the measurement of uniform magnetic fields, but also applicable to complex non-uniform magnetic fields. Brief Description of the Drawings

[0023] Figure 1 is a schematic diagram of the device of the present invention;

[0024] Figure 2 is a schematic diagram of weighted non-linear fitting of magnetic field strength and voltage. Detailed Embodiments

[0025] The present invention will be further described in detail below in conjunction with the accompanying drawings of the specification and specific embodiments.

[0026] As Figure 1 shown, a device for measuring weak magnetic fields includes a laser emitter 1, a first prism 2, a Helmholtz coil 3, a magneto-optical crystal 4, a second prism 5, a diaphragm 6, a photomultiplier tube 7, and an oscilloscope 8.

[0027] In this embodiment, the laser emitter 1 selects a helium-neon laser (DH-HN250) as the light source. The magneto-optical crystal 4 is placed at the center of the Helmholtz coil 3, and the first prism 2 and the second prism 5 are respectively placed before and after the Helmholtz coil 3. The first prism 2 and the second prism 5 form a pair of prism groups with orthogonal polarization planes (that is, the polarization directions of the two are 90 degrees different), and the extinction ratio of the two prisms is less than 10 -5 ; The first prism 2 is used as a polarizer, and the second prism 5 is used as an analyzer. After the Helmholtz coil 3 is energized, a uniform magnetic field is formed in the central region of the magneto-optical crystal 4, and the intensity of the magnetic field can be controlled by adjusting the current. The magneto-optical crystal 4 is placed at the center of the Helmholtz coil to ensure that the magnetic field acts uniformly on the crystal.

[0028] The light source becomes linearly polarized light after passing through the first prism 2. Passing through the magneto-optical crystal 4 in the magnetic field, the polarization direction of the polarized light rotates. Therefore, a weak light intensity will pass through the second prism 5. After blocking the stray light with the diaphragm 6, the outgoing light spot is irradiated onto the cathode of the photomultiplier tube 7. The photomultiplier tube 7 amplifies the light signal and converts it into an electrical signal, and displays the DC voltage value on the oscilloscope 8.

[0029] In this embodiment, both the first prism 2 and the second prism 5 select Glan-Taylor prisms;

[0030] The diaphragm 5 selects a stray light elimination diaphragm, which is used to eliminate stray light and ensure that only the light passing through the magneto-optical crystal and the second prism can reach the photomultiplier tube 7. This can reduce the background noise and improve the measurement accuracy.

[0031] The photomultiplier tube 7 is a high-sensitivity light detector that can convert a weak light signal into an electrical signal and amplify it. In this embodiment, the photomultiplier tube is used to detect the weak light intensity change passing through the second prism and convert it into an electrical signal.

[0032] The oscilloscope 8 is used to display the electrical signal output by the photomultiplier tube. Since the rotation angle of the polarized light passing through the magneto-optical crystal is very small, and the light intensity change passing through the second prism is also very weak, the DC voltage value displayed on the oscilloscope 8 can reflect the degree of rotation of the polarized light.

[0033] The Faraday rotation effect refers to the phenomenon that when a plane-polarized light beam passes through a magneto-optical medium placed in a magnetic field, the polarization plane of the plane-polarized light rotates along with the magnetic field parallel to the light direction. The rotation angle of the polarization plane is linearly proportional to the component of the magnetic field in the direction of light wave propagation, and this angle is called the Faraday rotation angle θ. Research shows that the Faraday rotation angle satisfies the following form,

[0034] θ = VBL (1)

[0035] where B is the magnetic induction intensity, L is the distance that light propagates in the magneto-optical material, and V is the Verdet constant. Here, taking terbium gallium garnet (TGG) as an example of the magneto-optical crystal, it is a type of magneto-optical crystal with a relatively large Verdet constant.

[0036] A method for measuring weak magnetic fields includes the following steps:

[0037] Step 1: Place the magneto-optical crystal 4 at the center of the Helmholtz coil 3. After passing a current through the Helmholtz coil 3, a uniform magnetic field is formed in the central region of the magneto-optical crystal 4.

[0038] Step 2: The laser beam becomes linearly polarized light after passing through the Glan prism 1 and then passes through the magneto-optical crystal in the magnetic field, and the polarization direction rotates. Therefore, a weak light intensity passes through the Glan prism 2. After blocking the stray light with a diaphragm, the outgoing light spot is irradiated onto the cathode of the photomultiplier tube, which amplifies the light signal and converts it into an electrical signal at the same time, and a DC voltage value is displayed on the oscilloscope. This DC voltage value is proportional to the outgoing light intensity. When the Faraday rotation angle θ is small, the outgoing light intensity passing through the analyzer can be approximated as:

[0039]

[0040] Combining formulas (1) and (2), the relationship that the light intensity is proportional to the square of the magnetic induction intensity is obtained:

[0041] I∝B 2 (3)

[0042] where I represents the outgoing light intensity after passing through the Glan prism 2, I0 represents the outgoing light intensity after passing through the Glan prism 1, and B represents the magnetic induction intensity at the center of the magneto-optical crystal.

[0043] Step 3: By changing the excitation current of the Helmholtz coil, the corresponding DC voltage (corresponding to the outgoing light intensity) is measured, and the average value and variance of the measured values corresponding to each excitation current are calculated, as shown in Table 1. Using two Helmholtz coils with a radius of 0.1 m, a spacing exactly equal to the radius, and 500 turns, when the excitation current is 500 mA, the magnetic field at the center of the two coils is 2.25 mT. The relationship between the DC voltage and the magnetic induction intensity at the center is calibrated, and thus the scatter plot of the DC voltage and the magnetic induction intensity at the center is obtained, as Figure 2as shown

[0044] Table 1 Variation of DC Voltage with Excitation Current

[0045]

[0046] Step 4, in order to better fit the curve of DC voltage and magnetic induction intensity at the center of the coil, perform weighted non-linear quadratic fitting on the obtained data points. According to the fact that the DC voltage is proportional to the square of the magnetic induction intensity, the non-linear quadratic fitting formula is U = aB 2 + b, where U is the measured DC voltage value, a and b are constants, and the weight ω is set to one over the square of the variance, that is

[0047]

[0048] σ i is the measurement variance of the i-th group of DC voltage values, that is, the larger the measurement variance, the smaller the weight, and the weight coefficient satisfies the normalization formula as follows:

[0049]

[0050] According to the measurement data in Table 1, i max = 11; finally, the obtained fitting curve is as shown Figure 2 by the red curve in, and the fitting relationship is:

[0051] U = 1.59(8)B 2 + 7.20(17)(6)

[0052] where the values in parentheses represent the uncertainty, and the constant 7.20 is due to the incomplete extinction of the two Glan prisms, the dark current of the photomultiplier tube, and the stray light in the environment. According to Figure 2 the curve obtained by fitting, the magnetic induction intensity corresponding to any DC voltage obtained by the photomultiplier tube can be measured.

Claims

1. A device for measuring weak magnetic fields, characterized in that, Comprising: A laser (1) as a light source; A first prism (2) and a second prism (5), which together form a pair of prisms with orthogonal polarization planes; wherein, the first prism (2) is used as a polarizer and the second prism (5) is used as an analyzer; A Helmholtz coil (3) for forming a uniform magnetic field in the central region of the magneto-optic crystal (4); A magneto-optic crystal (4) placed at the center of the Helmholtz coil (3) for rotating the polarization direction of the linearly polarized light passing through it; A diaphragm (6) for blocking stray light so that the outgoing light spot irradiates the cathode of the photomultiplier tube (7); A photomultiplier tube (7) for amplifying the optical signal and converting it into an electrical signal; An oscilloscope (8) for displaying the DC voltage value output by the photomultiplier tube (7); When the light source becomes linearly polarized light after passing through the first prism (2), passes through the magneto-optic crystal (4) in the magnetic field, the polarization direction of the polarized light rotates, and there is a weak outgoing light intensity after passing through the second prism (5). After the diaphragm (6) blocks the stray light, the outgoing light spot irradiates the cathode of the photomultiplier tube (7). The photomultiplier tube (7) amplifies the optical signal and converts it into an electrical signal at the same time, and the DC voltage value is displayed on the oscilloscope (8).

2. The device for measuring a weak magnetic field according to claim 1, wherein Both the first prism (2) and the second prism (5) are selected as Glan-Taylor prisms.

3. The device for measuring a weak magnetic field according to claim 1, characterized in that, The diaphragm (6) is selected as a stray light eliminating diaphragm for eliminating stray light.

4. The device for measuring a weak magnetic field according to claim 1, wherein, After the Helmholtz coil (3) is energized, a uniform magnetic field is formed in the central region of the magneto-optic crystal (4).

5. A method for measuring a weak magnetic field, which realizes the measurement of the weak magnetic field through the device for measuring a weak magnetic field according to any one of claims 1-4, characterized in that, Including the following steps: S1, energize the Helmholtz coil (3); S2, the laser (1) operates, and read the DC voltage value displayed on the oscilloscope (8); S3, according to the relationship that the light intensity is proportional to the square of the magnetic field, and combined with the magnetic field calibration, the magnitude of the magnetic field at any light intensity can be obtained; S4, perform weighted non-linear quadratic fitting on the obtained data points to fit the curve of the voltage and the magnetic induction intensity at the center of the coil; according to the fitting result, obtain the magnetic induction intensity corresponding to any DC voltage.

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