JPH04315073A - 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:Lights respectively linearly polarized are guided to first and second magnetic optical elements 22a, 22b mutually different in Verdet constant along the direction of a magnetic field to be measured. The planes of polarization of the polarized lights passing through the elements 22a, 22b are rotated by magnetic optical effect. The lights emitted from these magnetic optical elements 22a, 22b are guided to an analyzer 23 permitting the lights linearly polarized in a specific direction to transmit and the intensities of the transmitted lights are respectively detected by photodetectors 28a, 28b. On the basis of the detected values, the signal from the magnetic optical element larger in Verdet constant is used within a low magnetic field intensity range and the signal of the magnetic optical element lower 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. 3, 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. 4 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) When P0 is the light intensity when the magnetic field is 0, and P is the light intensity after passing through the analyzer, P is expressed by the following equation (2). P=P0 (1+sin2θ)……(2)

0006
] The relationship between these θ and P is shown in FIG. As is clear from Figure 3, 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 as θ《45°,
The linearity is extremely good. However, as the rotation angle increases, the nonlinearity error E expressed by the following equation (4)
becomes larger. E=(2θ−Sin2θ)/2θ……(4) For example,
The nonlinearity error is within 1% when θ≦7°.

Therefore, in order to measure low magnetic fields (low currents) with high precision in conventional optical magnetic field sensors, a magneto-optical element with a large Verdet constant is used, so non-linear errors are large 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 it is an object of the present invention to provide an optical magnetic field sensor that has a wide dynamic range and can measure magnetic field strength with high precision from low magnetic fields to high magnetic fields with a single device. The purpose is to

0010

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 means that rotates due to the magneto-optic effect of a magnetic field, an analyzer that transmits linearly polarized light in a specific direction emitted from the magneto-optical means, and a detection means that detects the intensity of the light that has passed through the analyzer. , a magneto-optical field sensor that measures the intensity of the magnetic field based on the detected value, the magneto-optical means having first and second magneto-optical elements having different Verdet constants, and the detection means having a first magneto-optical element having different Verdet constants. 1st and 2nd
the first polarized light is guided to each magneto-optical element, and the two lights emitted from each magneto-optical element are guided to the first and second detection elements, respectively, via the analyzer. An optical magnetic field sensor is provided.

[0011]

[Operation] In this invention, two magneto-optical elements with different Verdet constants are used, and the polarized light passing through them is detected by a different detection element, so measurements can be carried out appropriately depending on the magnetic field strength to be measured. Can be done. Therefore, magnetic field strength can be measured with high precision from low magnetic fields to high magnetic fields.

0012

[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. The light from the light emitting element 27 is guided to the lens 25 by the optical fiber 26, and is made into parallel light or condensed and sent to the polarizing beam splitter 21.
is incident on the The polarizing beam splitter 21 separates the incident light into two orthogonal linearly polarized lights. One of the separated lights is reflected by the reflection mirror 24 and guided to the magneto-optical element 22a, and the other is directly guided to the magneto-optical element 22b. These magneto-optical elements 22a and 22b are made of materials having different Verdet constants, and the direction of light passing through them coincides with the direction of the magnetic field H to be measured.

The Faraday rotation angles θA and θB in these magneto-optical elements 22a and 22b are calculated by setting the Verdet constant of the element 22a to VA and the optical path length to lA, and
When the Verdet constant of b is VB and its optical path length is IB, they can be expressed by the following (5) and (6), respectively. θA = VA ・H・lA ......(5) θB = VB
・H・LB……(6)

The two lights whose polarization planes have been rotated in this way are light linearly polarized in a specific direction (for example, at an angle of 45° with respect to the polarization direction of two orthogonal lights incident on each magneto-optical element). light) is guided to an analyzer 23 that transmits the light. The intensities PA and PB of the light transmitted through the analyzer 23 are expressed by the following equations (7) and (
8). PA =P0A(1+sin2θA)……(7)P
B = P0B (1-sin2θB)……(8)

0
[016] The light passing through the analyzer 23 is reflected by a reflection mirror 34, and is connected to lenses 35a, 35b and an optical fiber 36a, respectively.
, 36b to the light receiving elements 28a, 28b. These light receiving elements 28a and 28b are composed of, for example, photoelectric conversion elements, output the detected light intensity as a current signal, and calculate the magnetic field intensity based on the value.

When a material with a large Verdet constant (for example, rare earth iron garnet crystal) is used as the magneto-optical element 22a, and a material with a small Verdet constant (for example, lead glass) is used as the element 22b, the magnetic field strength and the analyzer can be The relationship with the intensity of the transmitted light is shown in FIG.

In order to measure 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+2θA),
The range that can be approximated by P0B (1-2θB),
In other words, it is necessary to use a range with good linearity. 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 FIG. 2, when measuring the low magnetic field (low current) region a, the light from the magneto-optical element 22a is used to measure the high magnetic field (large current) region b.
When measuring, light from the magneto-optical element 22b 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, two magneto-optical elements with different Verdet constants are used, and the polarized light that passes through them is detected by a different detection element, so it is possible to perform measurements appropriately depending on the magnetic field strength to be measured. can.

[0020] Depending on the range of magnetic field strength to be measured,
The material (Verdet constant) and optical path length of the magneto-optical element can be arbitrarily determined. The magneto-optical elements used are not limited to lead glass and rare earth iron garnet crystals as mentioned above.
Any combination of materials may be used as long as they have different Verdet constants and therefore different Faraday rotation angles. An example of a material with an extremely small Verdet constant is quartz, and an example of a material with a relatively large Verdet constant is a bismuth germanate single crystal. Note that the sensor of the present invention can also be applied to a photovoltage sensor to which the Pockels effect is applied.

[0021]

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

FIG. 3 is a diagram for explaining the principle of a conventional optical magnetic field sensor.

FIG. 4 is a schematic configuration diagram of a conventional optical magnetic field sensor.

FIG. 5 is a graph diagram showing the relationship between Faraday rotation angle and detected light intensity in a conventional sensor.

[Explanation of symbols]

21; Polarizing beam splitter, 22a, 22b; Magneto-optical element, 23; Analyzer, 24, 34; Reflection mirror, 25
, 35a, 35b; lens; 26, 36a, 36b; optical fiber; 27; light emitting element; 28a, 28b; light receiving element.

Claims (1)

[Claims] 1. A magneto-optic means 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 means, and a detection means that detects the intensity of the light that has passed through the analyzer, and that measures the strength of the magnetic field based on the detected value. In the sensor, the magneto-optical means has first and second magnets having different Verdet constants.
The detecting means has a first and second detecting element, and guides the first polarized light to each magneto-optical element to detect two lights emitted from each magneto-optical element. An optical magnetic field sensor, characterized in that the optical magnetic field sensor is guided through the analyzer to the first and second detection elements, respectively.