JPH04315074A - Optical magnetic field sensor - Google Patents

Abstract

PURPOSE:To obtain the title sensor capable of measuring the intensity of a magnetic field over a range from a low magnetic field to a high magnetic field with high accuracy. CONSTITUTION:Two linearly polarized lights having different wavelengths are guided to magnetic optical elements 22 different in Verdet constant according to a wavelength along the direction of a magnetic field to be measured. The planes of polarization of the lights passing through the elements 22 are rotated corresponding to the wavelengths of said lights by wavelength magnetic optical effect. Two lights emitted from the magnetic optical elements 22 are guided to an analyzer 23 permitting the linearly polarized lights to transmit in a specific direction and the intensities of the lights transmitted through said analyzer are respectively detected by photodetectors 28a, 28b. On the basis of the detected values, the signal based on the light having a wavelength large in Verdet constant is used in a low magnetic field intensity range and the signal based on the light low in Verdet constant is used in a high magnetic field intensity range.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical magnetic field sensor using a magneto-optical element.

0002

[Prior art and problems to be solved by the invention] Conventionally,
Magnetic field sensors that utilize the Faraday effect of magneto-optical elements have been developed. The Faraday effect is a phenomenon in which when light passes through a magneto-optical element, when an external magnetic field is applied in the same direction as the direction in which the light travels, the plane of polarization rotates.

The principle of such a magnetic field sensor is shown in FIG. As shown in FIG. 4, incident light 2 from the light emitting section 1 becomes linearly polarized light 4 by the polarizer 3, and is guided to the magneto-optical element 5. An external magnetic field H is applied to the magneto-optical element 5 in the direction in which the light travels, and the plane of polarization is rotated by θ due to this magnetic field H. Then, the output light 7 having a polarization direction with a relative angle difference of 45 degrees from the linearly polarized light 4 is guided to the light receiving section 8 by the analyzer 6 . The light intensity detected by the light receiving section 8 is converted into, for example, a current value, and the magnitude of the magnetic field H can be determined based on that value.

FIG. 5 shows a schematic configuration of a magnetic field sensor that has been actually used in the past. Light from the light emitting element 17 is guided to the lens 15a by the optical fiber 16a, passes through the lens 15a, and enters the polarizer 11. Polarizer 11
The linearly polarized light that has passed is guided to the magneto-optical element 12, where the plane of polarization of the light is rotated according to the magnetic field H to be measured, and further guided to the analyzer 13. The light passing through the analyzer 13 is guided to the light receiving element 18 via the reflecting mirror 14, the lens 15b, and the optical fiber 16b. For convenience of installation, as shown in FIG. 4, the input fiber 16a and the output fiber 16b are
It is common to pull them out in the same direction.

The rotation of the plane of polarization due to the Faraday effect is expressed as follows: where the rotation angle is θ, the applied magnetic field is H, the Verdet constant of the magneto-optical element is V, and the length of the part of the element through which light passes is l. θ is expressed by the following equation (1).

θ=V・H・l……(1) If P0 is the light intensity when the magnetic field is 0, and P is the light intensity after passing through the analyzer, then
P is expressed by the following formula (2).

P=P0 (1+sin2θ)……(2
) The relationship between these θ and P is shown in FIG. As is clear from Figure 6, when θ is very small, equation (2) becomes the following (
It can be approximated by equation 3), the detected light intensity is proportional to θ, and the magnitude of the magnetic field can be determined based on this light intensity.

P=P0 (1+2θ) (3) Therefore, when the rotation angle is extremely small such as θ<45°, the linearity is extremely good.

However, as the rotation angle increases, the nonlinearity error E expressed by the following equation (4) increases.

E=(2θ−Sin2θ)/2θ (4) For example,
The nonlinearity error is within 1% when θ≦7°.

[0011] Therefore, in order to measure low magnetic fields (low currents) with high accuracy in conventional optical magnetic field sensors, a magneto-optical element with a large Verdet constant is used, which results in large nonlinear errors in the high magnetic field (large current) region. Conversely, in order to measure high magnetic fields with high precision, a magneto-optical element with a small Verdet constant is used, so in the low magnetic field (low current) region, the change in light intensity is very small, resulting in a poor S/N ratio. Measurement error increases.

Therefore, when measuring from low magnetic fields to high magnetic fields with high precision, a magneto-optical field sensor equipped with a magneto-optical element having a large Verdet constant and a magneto-optical field sensor equipped with a magneto-optical element having a small Verdet constant are used. This requires two sets, which increases the installation space and increases costs.

The present invention has been made in view of the above circumstances, and provides a magneto-optical sensor having a wide dynamic range and capable of measuring magnetic field strength with high precision from low magnetic fields to high magnetic fields with a single device. The purpose is to

[0014]

To achieve this object, the invention provides that linearly polarized light is guided along the direction of the magnetic field to be measured, and that the plane of polarization of the polarized light passing through it is It has a magneto-optical element that is rotated by the magneto-optic effect of a magnetic field, an analyzer that transmits linearly polarized light in a specific direction emitted from the magneto-optical element, and a detection means that detects the intensity of the light that has passed through the analyzer. , an optical magnetic field sensor that measures the strength of the magnetic field based on this detected value, wherein the magneto-optical element is formed of a material with a Verdet constant that differs depending on the wavelength of the light passing through, and the magneto-optical element has a ,
An optical magnetic field sensor is provided, characterized in that light of at least two different wavelengths is guided depending on the strength of a measured magnetic field.

[0015]

[Operation] In this invention, a magneto-optical element whose Verdet constant varies depending on the wavelength of irradiation is used, and light of different wavelengths is guided to the magneto-optical element according to the magnetic field strength to be measured. measurements can be carried out appropriately. Therefore, magnetic field strength can be measured with high precision from low magnetic fields to high magnetic fields.

[0016]

[Examples] Examples of the present invention will be described in detail below.

FIG. 1 is a schematic diagram showing a magneto-optical field sensor according to an embodiment of the present invention. Light emitting elements 27a and 27
b emits light of different wavelengths (λA, λB), and these lights are combined by the wavelength combiner 30. The combined light is guided to a lens 25 by an optical fiber 26, and is made into parallel light or condensed and sent to a wavelength demultiplexer 29.
The light is then separated into λA and λB lights again. One of the separated lights is reflected by a reflection mirror 24 and sent to a polarizer 21.
and the other directly to the polarizer 21. These 2
Light with two wavelengths passes through the polarizer 21 and becomes linearly polarized light,
It is guided to the magneto-optical element 22. The magneto-optical element 22 is made of a material with a Verdet constant that differs depending on the wavelength, and the direction of light passing through this element depends on the magnetic field H to be measured.
is consistent with the direction of

The magneto-optical element 22 has a wavelength λA and a wavelength λ
Faraday rotation angle θλA when light of B is incident
, θλB is the Verdet constant at each wavelength VλA ,
Let VλB and the optical path length be l, the following (5) and (
6).

[0019]θλA =VλA・H・l……(5)
θλB = VλB ・H・l (6) Therefore, as a result, the light of wavelength λA and the light of wavelength λB have a Faraday
The rotation angle will change. For example, the magneto-optical element 2
2 has wavelength dependence as shown in FIG. In this case, for example, λA is 850 nm and λB is 155 nm.
If it is 0 nm, θλA will be sufficiently larger than θλB.

[0020] The two lights whose polarization planes have been rotated in this way are
Light linearly polarized in a specific direction (for example, 45° to the polarization direction of two orthogonal lights incident on each magneto-optical element)
is guided to an analyzer 23 that transmits light (having an angle of .

The intensity PA of the light transmitted through this analyzer 23
, PB can be expressed by the following equations (7) and (8), respectively.

PA=P0A(1+sin2θλA)……(7)
PB=P0B(1+sin2θλB)……(8)
One of the lights that passed through the analyzer 23 is reflected by a reflection mirror 34 and then guided to the wavelength multiplexer 31, and the other light is directly guided to the wavelength multiplexer 31 and is again combined into one light beam. be done. This light is guided to a wavelength demultiplexer 39 via a lens 35 and an optical fiber 36, where wavelength λA and wavelength λB
The light is separated into two. Then, the light with wavelength λA is guided to the light receiving element 28a, and the light with wavelength λB is guided to the light receiving element 28b. These light receiving elements 28a and 28b are composed of, for example, photoelectric conversion elements, and output the detected light intensity as a current signal.
Calculate the magnetic field strength based on that value.

In this case, the relationship between the magnetic field strength and the intensity of light passing through the analyzer is as shown in FIG.

[0024] In order to measure the magnetic field strength with high precision,
It is necessary to reduce the nonlinearity error, but for this purpose, the detected light intensity must be P0A(1+2θλA)
, P0B(1+2θB), that is, a range with good linearity needs to be used. Furthermore, in order to measure a low magnetic field (low current) with high precision, it is necessary to increase the change in light intensity. Therefore, as shown in Figure 2, when measuring low magnetic field (low current) region a, light with wavelength λA is used, and when measuring high magnetic field (large current) region b, light with wavelength λB is used. . That is, in region a, the signal from light receiving element 28a is used, and in region b, the signal from light receiving element 28b is used.

In this way, a magneto-optical element with a Verdet constant that differs depending on the wavelength of light is used, and the light having passed through this magneto-optical element with a different wavelength is detected by a different detection element, so that the intensity of the magnetic field to be measured can be adjusted. Appropriate measurements can be taken accordingly.

[0026] Depending on the range of magnetic field strength to be measured,
An appropriate wavelength can be selected. In addition, it is sufficient that the magneto-optical element used has wavelength dependence in the Verdet constant.
Examples of such materials include rare earth iron garnet crystals (YIG single crystal, etc.).

Note that the sensor of the present invention can also be applied to a photovoltage sensor to which the Pockels effect is applied.

[0028]

According to the present invention, measurements can be made appropriately depending on the magnetic field strength to be measured, and magnetic field strengths can be measured with high precision from low magnetic fields to high magnetic fields. Therefore, only one sensor is required to measure from low to high magnetic fields, saving installation space and
Cost reduction can be achieved.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram of a magneto-optical field sensor according to an embodiment of the present invention.

FIG. 2 is a diagram showing the relationship between the wavelength of transmitted light and the Faraday rotation angle in the magneto-optical element used in the sensor of FIG. 1;

FIG. 3 is a graph diagram showing the relationship between the magnetic field strength determined by the sensor shown in FIG. 1 and the detected light intensity.

FIG. 4 is a diagram for explaining the principle of a conventional optical magnetic field sensor.

FIG. 5 is a schematic configuration diagram of a conventional optical magnetic field sensor.

FIG. 6 is a graph diagram showing the relationship between Faraday rotation angle and detected light intensity in a conventional sensor.

[Explanation of symbols] 21; polarizer, 22; magneto-optical element, 23; analyzer, 2
4, 34; Reflection mirror, 25, 35; Lens, 26, 3
6; Optical fiber, 27a, 27b; Light emitting element, 28a,
28b; Light receiving element, 30, 31; Wavelength multiplexer, 29, 3
9; Wavelength demultiplexer.

Claims (1)

[Claims] 1. A magneto-optical element in which linearly polarized light is guided along the direction of a magnetic field to be measured, and the plane of polarization of the polarized light passing therethrough is rotated by the magneto-optic effect caused by the magnetic field; An optical magnetic field that has an analyzer that transmits linearly polarized light in a specific direction emitted from the element and a detection means that detects the intensity of the light that has passed through the analyzer, and that measures the intensity of the magnetic field based on the detected value. In the sensor, the magneto-optical element is formed of a material having a Verdet constant that differs depending on the wavelength of light passing therethrough, and light of at least two different wavelengths is guided to the magneto-optical element according to the strength of the magnetic field to be measured. An optical magnetic field sensor characterized by