JPH0337584A - Magnetic field sensor - Google Patents

Abstract

PURPOSE:To enable detection of a magnetic intensity stable in a wide temperature range by making a phase difference between two axes, X and Y, of a 1/2 wavelength plate at a room temperature larger than pi(180 deg.) when a temperature coefficient of a Faraday effect element is negative. CONSTITUTION:Light 1 introduced from an optical fiber is transmitted through a polarizers 2, a 1/2 wavelength plate 7, a Faraday effect element 4 and an analyzer 5 sequentially. In this case, a rotary polarization element is replaced with the 1/2 wavelength plate 7 and when a temperature coefficient of the Faraday effect element is negative, a phase difference between two axes, X and Y, is made larger than pi in the 1/2 wavelength plate 7 at a room temperature and to the contrary, when the temperature coefficient of the Faraday effect element is positive, the phase difference between the two axes is made smaller than pi. This enables correction of a sensor as a whole in such a direction that the temperature coefficient of the Faraday effect element is down to zero thereby permitting detection of an intensity of a magnetic field stable in a wide temperature range.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a magnetic field sensor, and more particularly to a magnetic field sensor whose measurement accuracy can be improved by correcting the temperature characteristics of its Faraday effect element.

Inverse 11 Rie (- The first conventional example is a magnetic field sensor that determines the magnetic field strength by sequentially transmitting light guided by an optical fiber through a polarizer, a Faraday effect element, and an analyzer and detecting the intensity of the transmitted light. As is known, in this case the analyzer is arranged at a rotation of 45 degrees with respect to the optical axis relative to the polarizer so as to obtain an output proportional to the magnetic field strength. Optical rotation ability (the ability to rotate the plane of polarized light due to the Faraday effect when a magnetic field is applied) or natural optical rotation ability (a special asymmetry in the molecular structure or crystal structure even when no magnetic field is applied) For example, Zn5e does not have the ability to cause the plane of polarization to rotate when linearly polarized light passes through a material.
If polycrystalline material is used, the intensity P of the transmitted light is given by the following equation.

P-Po(1+5in2θ)-P. (l-1-sin2Ve・11・Il)
- (1) (However, P... Intensity of transmitted light: Po - - Intensity of transmitted light when the magnetic field is set to zero: θ Faraday rotation angle: Ve... Verdet constant of 2nSe: ■... Magnetic field strength : C...7 Length of Alladay effect element: Note that [
θ−Veill]. ) In formula (1), the term 5in2Veill is n]
If we put rm=sin2Ve・■・0. n...(2)
'' is the degree of modulation, that is, the rate of change in light intensity.

However, it is generally known that the help constant of a Faraday effect element is affected by temperature. For example,
The modulation depth at room temperature (20°C) is m. , when the modulation degree when the temperature changes by ΔT'O from room temperature or 20° C. is defined as ml, the rate of change of the modulation degree n] is given by the following equation.

(ml mo)/mo...(3) (However,
ml + mo” temperature (20+ΔT)'C!, 20°C
The degree of modulation when
)/m. (%) represents each change. The solid line a slopes to the upper right, and since the rate of change in modulation degree increases as the temperature rises, Z
It can be seen that the N5E Faraday effect element has a positive temperature coefficient.

As described above, according to the first conventional example, since the modulation rate changes in response to temperature changes, that is, the temperature coefficient is not zero, there is the trouble of requiring separate correction work after that. Also, as mentioned above, the work of rotating the analyzer by π/4 and arranging it is also relatively troublesome.

For this reason, as a second conventional example, as shown in FIG. Some analyzers are arranged so that the analyzer can be arranged without any rotation. In the figure, light 1 guided by an optical fiber passes through a polarizer 2, an optical rotator 3, a Faraday effect element 4, and an analyzer 5 in order. In this case, the optical rotator 3 is made of a BSO (hismuth silicon oxide; for example, Bi+2SiO+z) single crystal, and the Faraday effect element 4 is made of Zn5e polycrystal as described above.

In this case, the BSO single crystal has not only a natural optical rotation ability but also a magnetic optical rotation ability, and since the natural optical rotation ability is also affected by temperature, its modulation degree Ill and its modulation degree change rate are respectively calculated as described in (2) above. , is given as follows with reference to equation (3).

m= 5in2(Ve+・L +Ve2・Q2)H(
4) (Ill+mo)/m.

=(++ (k, ・ΔT-Bruise, 2 in 10 ・ΔT-Buddha 2)/ (Ve+aL +Vez IL)l/
(1ΔT in 12) (5) (However, ml + no”’temperature (20+ΔT)0C,
Modulation degree at 20°C; Ve, , Ve2...
4nSe, Verdet constant of BSO; Q+, Q2
-4nSe, Length of BSO: 1 to 2 ・Temperature coefficient of Berbe constant Ve, , Ve2 (i.e. change per unit temperature): ・Temperature coefficient of natural rotation power of BSO:
ΔT...Temperature change from 20°C) This is illustrated by the broken line in Figure 1, which shows that the temperature coefficient of the rate of change in modulation index is the temperature characteristic of the natural rotation power of BSO. The temperature coefficient changes from positive to negative due to the influence of , and it is also approaching zero, so it can be seen that the temperature characteristics are being corrected.

However, when adopting the BSO optical rotator 3 as in the second conventional example, the wavelength of the transmitted light is, for example, 850-87
At 0 nm, the angle of rotation per 1 mm length of BSO is 10
, 5 degrees, in order to obtain an optical rotation angle of π/4 (-45 degrees), a crystal thickness (length) of 45/lO, 5-4.3 mm is required. However, in recent years there has been a strong demand for miniaturization of magnetic sensors, and the above-mentioned thickness has the disadvantage that it cannot meet this demand for miniaturization.

Therefore, as a third conventional example, as another method for obtaining the optical rotation angle of π/4, as shown in FIG. It is known to use a half-wave plate 6 made of aluminum and utilize the birefringence of quartz crystal. This quartz half-wave plate only needs to have a thickness of about 1 mm, and can be made as thin as about t74 in the case of BSO. The reason why the thickness is approximately 1 mm is as follows. The optical path difference (2) for 1/2 wavelength is given by the following equation.

1-, t/2(ne-no) = 85072 (1,5+7-1.538)-1722
2 (nm) -17,222 (μm) ・ (6)
(However, I... optical path difference: λ... wavelength of transmitted light, in this case 850 nm; n,, n. - refractive index for ordinary and extraordinary rays of crystal at room temperature, in this case 1538.1.517 respectively) ) Therefore, in theory, a crystal thickness of 17.222 (71 m) is good, but since this thickness would be difficult to polish, two crystal plates with thicknesses dl and d2 should be placed together. A wave plate equivalent to one crystal plate with a thickness equal to the difference in thickness (d2d,) or the above optical path difference I is made by overlapping the optical axes so that they are perpendicular and canceling the linear birefringence. obtain. Currently, commercially available products with a thickness of approximately 1 mm, such as those obtained by laminating and bonding as described above, are often used.

By the way, the half-wave plate gives a phase difference δ (ideally 2δ-π) between the X and Y axes of the light that passes through it, and the phase difference δ is given by the following formula. .

δ-(nc-no)(d2-d, )/λ・2yr
−(7) (However, δ ・phase difference; 'O+ n,...
-Refractive index of crystal for ordinary and extraordinary rays; d2. d
,...Thickness of each plate of laminated crystal plate: λ...
Wavelength of transmitted light:) Therefore, in order to correct the temperature coefficient of the rate of change in modulation degree, in other words, the temperature coefficient of the Faraday effect element (ZnSe), in the case of the second conventional example, the help constant of the BSO rotator and In the case of the third conventional example, it is necessary to focus on the phase difference δ of the 1/2 wavelength plate.

Problems to be Solved However, in the third conventional example, the temperature characteristics of the phase difference δ of the half-wave plate or the modulation rate change (m,
At present, the influence of mo)/mo on temperature characteristics has not yet been investigated.

The purpose of the magnetic field sensor of the present invention is to solve the above-mentioned conventional problems, and its configuration includes a polarizer, a Faraday effect element, and an analyzer along the path of light guided by an optical fiber. arranged sequentially, and between the Faraday effect element and either the polarizer or the analyzer,
In a magnetic field sensor in which a crystal half-wave plate is further arranged, if the temperature coefficient of the Faraday effect element is negative, the phase difference between the X and Y axes of the half-wave plate at room temperature is π
(i.e., 180 degrees) and if the temperature coefficient of the Faraday effect element is positive, the phase difference between the X and Y axes of the half-wave plate at room temperature is made smaller than π (i.e., 180 degrees). It is characterized by

Embodiment FIG. 4 shows an embodiment of the magnetic field sensor according to the present invention.
FIG. 3 is a perspective view showing a schematic configuration of the magnetic field sensor using a /2 wavelength plate, in which the same parts as in FIG. 3 are denoted by the same reference numerals.

In the figure, 7 is a 1/2 wavelength plate made by laminating two crystal plates together, but as will be described later, it has a different thickness from the 1/2 wavelength plate 6 in FIG.

The light 1 guided by the optical fiber sequentially passes through a polarizer 2, a 1/2 wavelength plate 7, a Faraday effect element 4, and an analyzer 5. Note that the 1/2 wavelength plate 7 may be placed between the Faraday effect element 4 and the analyzer 5.

In this case, the intensity P of the transmitted light exiting the analyzer 5 is given by the following equation.

P-P+/2(1+(cos2δ・coS2θ-5in
2δ・5in2θ))...(8) (However, P...Intensity of transmitted light exiting analyzer 5: Pl・
...Intensity of incident light; δ...Phase difference of l/2 wave plate:
(θ...Faraday rotation angle) Here, assuming that the half-wave plate 6 has a phase difference of exactly π, it becomes δ - π/2, so the above equation (8) can be rewritten as the following equation. .

P= P+/2(1-5in2θ)・(9) In this formula,
The term 5in2θ represents the modulation degree.

Next, assuming that no magnetic field acts on the device shown in FIG. 4, Faraday rotation does not occur and therefore "θ-0" is obtained, so the above equation (8) can be replaced with the following equation.

Pde-P+/2(1+ CO52δ) ・(
10) (However, P6o, no magnetic field acts on the device, and no light or 1
(Intensity of transmitted light when subjected to modulation effect only by the /2 wavelength plate) P, which is determined by this equation (10). The value of represents the direct current component (DC) intensity of the transmitted light, and therefore the modulation degree m of the apparent I is as follows.

m-(P P, +c)/Pdc = (CO32δ0CO32θ-5in2δ6sin2
θ)/(1+cos2δ) ・(11) Here,
If the modulation degree at room temperature or 20°C is +112o, and the modulation degree at room temperature or 90°C is n19°, then
The modulation degree change rate is given by the following equation.

(ms.-m2o)/mzo...(12) Calculation example Next, as a Faraday effect element, the Fara T rotation is not affected by the degree at all.Ideal temperature property 11-1 (i.e., the temperature coefficient is 70) Assuming that a Faraday effect element of
Calculate and see if the phase difference δ between the two axes is affected by the temperature characteristics alone. The phase difference δ is as follows from the above equation (7).

δ−(n, no)(d2d+)/λ・2yr
−(7) In this formula (7), the setting thickness d2 of the /2 wavelength plate
d, (-optical path difference I) is given as follows from the above equation (6).

d2-d, = I-λ/2(n, no) −
(13) Here, as a specific example, 730 nm (0,
The calculation will be performed using a wavelength of 73 μm) as an example. At this time, the refractive index n of the crystal for ordinary rays and extraordinary rays in the same formula.

, no values are as follows.

20℃ 90℃ Mouth. 1.519 1.5
．． 411524no-1, 5391, 5386011 Furthermore, it is assumed that the Faraday rotation angle of the Faraday effect element 4 is constant at 2° (-π/90), for example.

The calculation procedure is based on the above refractive indexes n0 and n.

2 Based on the value and formula (13), from formula (7), 20°C and 90
Determine each phase difference δ at °C, then use equation (11) to determine the modulation degree m2. and m9. are determined respectively, and finally, the modulation degree change rate (ms.-mzo)/mzo is determined using equation (12).

In this case, attention will be paid to the set thickness d2d1 of the half-wave plate 7 as an element for correcting the temperature characteristic of the modulation rate change rate. Since one type of the above modulation rate of change can be obtained for one type of set thickness d2d, the modulation rate of change is determined for each set thickness d2-d, and the modulation rate of change as a whole is calculated. It is sufficient to consider whether or not it is possible to correct the temperature characteristics of . Note that the set thickness d2-d is calculated as follows from the above equation (13) at room temperature (ie, 20° C.).

d2-dl-λ/2(n, no) -〇,73/2(1,519-1,539)-36,5
(μm) Therefore, the set thicknesses d2 and d+ are varied within a range of 10 to 50 μm with approximately 36.5 μm as the center.

FIG. 5 shows the results of this calculation. In the figure, the X-axis is the change in the set thickness of the 1/2-wave plate (+L+ -cl+) (μm), and the Y-axis is the rate of change in the modulation degree (m, o m2o)/m2
It represents o (%) change, and the following can be said from the same figure.

■ When the set thickness of the 1/2 wavelength plate is at position p in the figure, that is, 36.5 μm for a wavelength of 730 μm, when light with a wavelength of 730 μm actually passes through, the modulation rate of change of the entire sensor will change. The value (ie, temperature coefficient) is approximately zero, indicating stable temperature characteristics.

■ When the set thickness is at position q in the diagram, that is, approximately 49μIn for a wavelength of 780μIn, the actual wavelength 7
When light of 30 μm is transmitted, the temperature coefficient becomes +05%.

■ When the set thickness is about 46 μm for the position r in the figure, that is, for a wavelength of 830 μn, the actual wavelength is 73 μm.
When light of 0 μm is transmitted, the temperature coefficient becomes +2.4%.

■ The larger the set thickness, the larger the value of the rate of change in modulation degree (temperature coefficient) of the entire sensor. Also, the set thickness is wavelength 7
The temperature coefficient becomes positive as the thickness becomes larger than the thickness for light with a wavelength of 730 μm, and becomes negative as the thickness becomes smaller than the thickness for light with a wavelength of 730 μm.

The relationship between the above set thickness and the sign of the temperature coefficient is schematically shown in FIGS. 6 and 7 for easy understanding. In addition, Fig. 6 (B) to (D
) and FIGS. 7(B) to (I)), the X-axis represents temperature, and the Y-axis represents changes in modulation degree change rate.

As shown in FIG. 6 (I3), if the temperature coefficient of the 7 Alladay effect element is actually positive (upwards to the right), the thickness of the 1/2 wavelength plate 7 should be changed as shown in FIG. 6 (A). , wavelength 730μm
Setting thickness E for light. It is better to set it smaller than (-365 μm). Then, as shown in the same figure (C), the temperature coefficient of the entire sensor when assuming that a Faraday effect element with ideal temperature characteristics that does not receive Faraday rotation or n1 As explained in the section (2), it becomes negative (toward the bottom right).Therefore, the temperature characteristics in the same figure (B) are corrected by the temperature characteristics in the same figure (C), and the actual temperature characteristics of the entire sensor are as shown in the same figure. As shown in (D), it becomes approximately zero over a predetermined temperature range, indicating that good temperature 5 correction is achieved.

Next, as shown in Figure 7 (B), if the temperature coefficient of the Faraday effect element is actually negative (lower right),
), the thickness t of the 1/2 wavelength plate 7 is set to the above-mentioned set thickness t. (=36.5 μm) or more.

Then, as shown in the same figure (C), assuming that a Faraday effect element with ideal temperature characteristics is used, the temperature coefficient of the entire sensor becomes positive (toward the top right), as also explained in section (■) above. Therefore, the temperature characteristic in the same figure (B) is the same as that in the same figure (C
), and it can be seen that the overall temperature characteristic is similarly well corrected as shown in FIG.

Experimental example Next, FIG. 8 shows an actual experimental example.

In the figure, light emitted from a light emitting diode 11 is guided by an optical fiber 12, and is passed through a polarizer 2 and a 1/2 wavelength plate 7.
, a Faraday effect element 4 made of Zn5e, and an analyzer 5 in order. The transmitted light is further guided by an optical fiber 13 to a light receiving diode 14, and the converted current is passed through an amplifier 15 to an AC amplifier 1.
6. The AC and DC components are amplified by the DC amplifier 17, further divided by the divider 18, and outputted from an output 19 as a ratio AC/DC of the AC and DC components. on the other hand,
A U-shaped iron core 22 wound with a coil 21 connected to an AC power source 20 is arranged to surround the Faraday effect element 4 and apply a magnetic field to the element 4. These are housed in a constant temperature bath 23 so that the above-mentioned temperature change from 20°C to 90°C can be performed accurately.

In this case, the wavelength of the light from the light emitting diode 11 is 850.
The Faraday effect element 4 is made of Zn5e and has a length of 12 mm, and the set thickness of the 172 wavelength plate 6 (d2-
dt) was set at two different thicknesses, 46° 3 μm and 48.3 μm, which correspond normally to light wavelengths of 830 nm and 87 Onm, respectively.

The results are shown in FIG. 9, in which the X axis represents temperature and the Y axis represents changes in modulation rate.

This depends on the set thickness of the 1/2 wavelength plate 7 or the wavelength 830
In the case of wavelength 870 (d2-d, = 46.3 μm), line C shows the change rate at 90°C, and the change rate is +1.6%. , 3μm)
was as shown by line d, and the rate of change was +3.4% at 900C. Therefore, it can be seen that by appropriately changing the set thickness (d2dt) for an actual light wavelength of 850 nm, the value of the modulation rate change (temperature coefficient) can be changed.

Here, when comparing Figure 9 with Figures 6 and 7, we can see that the 1/2 wavelength plate is actually used at a wavelength of 850 μm.
If it has a set thickness for
It seems to be as shown by the broken line e in the figure, so by reducing the set thickness, it is possible to correct the actual temperature characteristics so that the temperature coefficient of the entire sensor becomes negative (towards the bottom right), and vice versa. It was found that by increasing the set thickness, it was possible to correct the movement in the positive direction (upward to the right), proving that the above calculation was correct.

Effects of the Invention As described above, the magnetic field sensor according to the present invention uses a 1/2 fi long plate in place of the optical polarizer, and when the temperature coefficient of the 7 Alladay effect element is negative, the magnetic field sensor at room temperature If the phase difference between the X and Y axes of the two-wave plate is made larger than π, and conversely, the temperature coefficient of the 7 Alladay effect element is positive, then the phase difference between the two axes is made smaller than π, so the following There are some advantages shown in this.

■ As a whole sensor, the temperature coefficient of the Faraday effect element can be corrected in the direction of zero (that is, the rate of change of modulation degree or zero), and the magnetic field strength can be stably detected over a wide temperature range, improving detection accuracy. can be improved.

■ The thickness of the 172 wavelength plate is approximately 1. Since it is as small as about mm, the overall thickness of the sensor can be reduced and the sensor can be miniaturized.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the rate of change in the modulation degree of the sensor and the temperature in a first conventional example and a second conventional example of a magnetic field sensor. FIG. 2 is a schematic configuration of the second conventional example of a magnetic field sensor. 9. FIG. 3 is a perspective view showing a schematic configuration of a third conventional example of a magnetic field sensor. FIG. 4 is an embodiment of a magnetic field sensor according to the present invention. 1
FIG. 5 is a perspective view showing the schematic structure of the magnetic field sensor using a /2 wavelength plate. FIG. Figure 6 (A) is a schematic configuration diagram when the temperature coefficient of the Faraday effect element of the magnetic field sensor of the present invention is negative and the thickness of the half-wave plate is smaller than the normal thickness. Figure (B) shows the temperature characteristics of the Faraday effect element alone, and Figure 6 (C) shows the ideal temperature characteristics (i.e. temperature coefficient or cello).
Figure 6 (D) is a temperature characteristic diagram of the sensor when using the Faraday effect element having the above-mentioned 1/2 length plate, and the temperature characteristics of Figures 6 (B) and (C) are at the bottom. Temperature characteristic diagram, Figure 7 (A) is a schematic configuration diagram when the temperature coefficient of the Faraday effect element of the magnetic field sensor of the present invention is positive and the thickness of the 1/2 wavelength plate 0 is larger than the normal thickness.
Figures (B) to (D) show the results of the sixth test under the conditions of Figure 7 (A), respectively.
Figures corresponding to Figures (B) to (D), Figure 8 is a diagram showing a schematic configuration of an experimental example of an embodiment of the magnetic field sensor of the present invention, Figure 9 is a diagram showing the modulation rate of change and temperature of the magnetic field sensor of the present invention. FIG. 2 is a diagram illustrating the relationship between 1...Light 2...Polarizer 3...BSO π/4 optical rotator 4...Faraday effect element 5...Analyzer 6.7
- Half-wave plate made by ZnS e 11... Light emitting diode 12, 13... Optical fiber 14... Light receiving diode 15 to 17 - Amplifier 18... Divider 21... Coil 22... Iron core 23... Constant temperature bath.

Claims (1)

[Claims] Along the path of the light guided by the optical fiber, a polarizer,
In a magnetic field sensor, a Faraday effect element and an analyzer are sequentially arranged, and a quartz half-wave plate is further arranged between the Faraday effect element and either the polarizer or the analyzer. , when the temperature coefficient of the Faraday effect element is negative, the phase difference between the X and Y axes of the half-wave plate at room temperature is larger than π (i.e., 180 degrees), and the temperature coefficient of the Faraday effect element is positive. In this case, the phase difference between the X and Y axes of the half-wave plate at room temperature is π (i.e., 18
A magnetic field sensor characterized by being smaller than 0 degrees).