JP2948677B2 - Optical magnetic field sensor - Google Patents

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 for detecting a magnetic field (or a current for generating a magnetic field) using a magneto-optical element having a Faraday rotation capability, and in particular, to reduce the temperature dependence of the detection sensitivity. And a high-sensitivity optical magnetic field sensor.

[0002]

2. Description of the Related Art FIG. 4 is a block diagram showing a general optical applied magnetic field sensor. In the figure, (1) is a light source such as an LD (laser diode) or an LED (light emitting diode),
(2) is an optical fiber for transmitting light emitted from the light source (1), (3) is a collimator lens disposed at the tip of the optical fiber (2), and (4) is through a collimator lens (3). It is a polarizer that polarizes light.

[0005] (5) is a magneto-optical element having Faraday rotation capability, and is arranged so that light passing through the polarizer (4) is incident on the element surface. Magneto-optical element (5), when the composition ratio of manganese Mn was _{x, Cd 1-X Mn X} Te ( where, 0 <x <0.7) is formed from a single crystal semiconductor material which satisfies, for this Is described, for example, in "Journal of Crystal Growth" (Vol. 52, 1981).
Years), pp. 614-618.

[0004] (6) is an analyzer into which light passing through the magneto-optical element (5) is incident, and is arranged so that its polarization direction is 45 ° relative to the polarizer (4). I have. (7)
Is a condenser lens that converges light passing through the analyzer (6),
(8) is an optical fiber that transmits light with the condenser lens (6) as an incident end, and (9) is an optical receiver (detector) that includes a light receiving element and converts light passing through the optical fiber (8) into an electric signal. It is.

Next, the operation of the general optical applied magnetic field sensor shown in FIG. 4 will be described. The light emitted from the light source (1) is guided to a collimator lens (3) via an optical fiber (2), becomes parallel light, is linearly polarized by a polarizer (4), and is magneto-optical element (5). ).

At this time, when a magnetic field H is applied in the light passing direction, the linearly polarized light rotates by a small angle θ due to the Faraday effect. And the magneto-optical element
Of the light emitted from the other end of (5), a component having a polarization angle of 45 ° passes through the analyzer (6) and passes through the condenser lens (7) and the optical fiber (8) to the optical receiver ( The light is received at 9) and measured as a signal of the light amount. This change in the amount of light
(Or current), the optical applied magnetic field sensor can detect the magnetic field H or the current. Assuming that the length of the magneto-optical element (5) is L, the rotation angle θ of the polarization plane due to the Faraday effect acting while passing through the magneto-optical element (5) is θ = L · Ve · H (where Ve: Verde constant). Where the polarizer
Since the relative angle of the polarization direction between (4) and the analyzer (6) is 45 °, the light amount I of light passing through the analyzer (6) is
If the light quantity from (4) is Io and the transmittance ρ in the magneto-optical element (5) is 1, I = Io (1 + sin2θ) / 2 = Io [1 + sin (2L · Ve · H)] / 2 Becomes

Generally, since the rotation angle θ due to the AC magnetic field H is very small, sin (2θ) / 2 = 2θ / 2 = L · Ve · H in the equation, and the AC magnetic field H changes the light amount I (ie, It can be seen that the measurement can be performed based on the change in the light signal). Further, the sensor sensitivity R at this time is given by the ratio (modulation rate) between the AC component and the DC bias component of the light amount I due to the change in the magnetic field H, and R = 2L · Ve · H... You can see that

However, since the Verdet constant Ve has a temperature characteristic, in order to prevent the temperature dependence of the sensor sensitivity R, a conventional optical applied magnetic field sensor uses an LD or LED combined with an interference filter as a light source (1). ) To generate light of a narrow band whose center wavelength λc is set to a zero-point wavelength λ _T (a wavelength at which the sensor sensitivity R does not change in temperature). Such a technology is, for example, "Appl.Phys.Lett."
(Vol. 46, No. 11, June 1, 1985) at pages 1016 to 1017.

FIGS. 5 to 7 are characteristic diagrams shown in the above literature. FIG. 5 is a wavelength characteristic of the Verde constant Ve when the composition ratio x = 0.15. FIG. 6 is a Verde constant when the composition ratio x = 0.15. Constant V
Temperature characteristics of e, FIG. 7 shows respective characteristics of zeros wavelength lambda _T and Verdet constant V _T zeros in the zero point wavelength lambda _T on the composition ratio x. Here, the unit of the Verde constant Ve is [degree / mm · T] (however, T [tesla] = 10 ⁴ G [gauss]). Each temperature shown in FIG. 5 (−60 ° C., 20 ° C., 100 ° C.)
From the wavelength characteristics in C), it can be seen that the Verdet constant Ve decreases on the long wavelength side and increases on the short wavelength side as the temperature increases. Further, each wavelength (755 nm, 775 nm, 795 nm) shown in FIG.
It can be seen from the temperature characteristics at (nm, 815 nm) that the Verdet constant Ve changes almost linearly with temperature. Furthermore,
5 and 6 that the wavelength at which the Verdet constant Ve does not depend on the temperature, that is, the zero-point wavelength λ _T that cancels the temperature change of the Verdet constant Ve, exists near 790 nm. Zero point wavelength λ _T
Is linearly changed with respect to the composition ratio x of the magneto-optical element (5) as shown in FIG. 7, also varies approximately linearly with zeros Verdet constant V _T be the composition ratio x.

Accordingly, the light source (1) and using it for example LD, coincides with the center wavelength λc is zero wavelength lambda _T, and magneto-optical element with light having a wavelength distribution of narrow band (about half-width Δλ is 1 nm) (5 ) Realizes a high-sensitivity optical magnetic field sensor with small temperature dependence. Also, light source
When an LED having a broadband wavelength distribution with a half width of about 50 nm is used as (1), an interference filter is incorporated in the optical system to narrow the band.

However, an optical applied magnetic field sensor using an LD is described in, for example, “IEEE Journal of Quantu
m Electronic) '' (Vol.QE-18, No. 10), it is known that the SN ratio (accuracy) deteriorates because of the large fluctuation of the LD light amount, that is, the DC bias component. I have. Also, it is clear that when monochromatic light is used by combining an LED that generates light with a half-value width Δλ with an interference filter, the light amount decreases and the sensor sensitivity R deteriorates.

On the other hand, when an LED without an interference filter is used to increase the light amount of the light source (1), the generated light has a wavelength distribution, so the effective Verdet constant considering the wavelength distribution is used. It is necessary to consider Ve '. The effective Verdet constant Ve 'is described, for example, in Japanese Patent Application No. 63-17872.
As described in the specification of No. 0, the Verdet constant Ve (λ) for each wavelength is weighted averaged with respect to the wavelength distribution of light, and is obtained by Ve ′ = ∫Ve (λ) dλ / Io ′. Here, Io 'is an average light amount, and when the light amount for each wavelength is Io (λ), it is expressed by Io' = ∫Io (λ) dλ. Actually, the transmittance ρ of the magneto-optical element (5)
Is not 1, so considering the transmittance ρ (λ) for each wavelength with respect to the light amount Io (λ), the transmitted light amount Iρ (λ) passing through the magneto-optical element (5) is Iρ (λ) = Io ( λ) ρ (λ). Further, the sensor sensitivity R is represented by an effective Verdet constant Ve ′,
R = Ve ′ (Iρ ′) ^1/2 ... With the average light amount Iρ ′ of the transmitted light amount passing through the magneto-optical element (5) from the light source (1).

FIG. 8 is a characteristic diagram showing a wavelength characteristic and a temperature characteristic of the Verdet constant Ve when the composition of the magneto-optical element (5) represented by Cd ₁ -x Mn _X Te is changed. In the figure, the point where the chromatic dispersion at 100 ° C. (dashed line) and the chromatic dispersion at 20 ° C. (solid line) intersect in the characteristic curve of the Verde constant Ve for each composition is the wavelength at which the Verde constant Ve does not depend on the temperature. , That is, zero wavelength λ _T1 when using monochromatic light
~ Λ _T7 .

FIG. 9 is a characteristic diagram showing a light absorption distribution for each composition of the magneto-optical element (5).
The wavelength at which light starts transmitting is the absorption edge wavelength λk in each composition. Here, the case where the temperature is 23 ° C. is shown, and although not shown, the absorption edge wavelength λk shifts to the longer wavelength side as the temperature rises.

Therefore, in order to make the temperature change of the effective Verdet constant Ve '(see the equation) substantially zero, the wavelength distribution of light should not fall on the shorter wavelength side than the absorption edge wavelength λk at the upper limit of the operating temperature range. It is necessary to combine the composition of the magneto-optical element (5) and the light source (1) so as to satisfy the following conditions.

FIG. 10 is a characteristic diagram showing the absorption edge wavelength λk and the zero-point wavelength λ _T with respect to the composition ratio x converted into light energies Ek and λ _T. Absorption edge wavelength λk and zero wavelength lambda _T, when respectively converted into energy Ek and _{E T, Ek = 1.467 + 1.296x} ... is represented by E _T = 1.419 + 0.966x ..., linear with respect to the composition ratio x Become a relationship.

FIG. 11 is a characteristic diagram showing an allowable range of the half width Δλ when an LED having a broad emission wavelength is used as the light source (1). In this case, it is assumed that the center wavelength λc of the light generated by the light source (1) coincides with the zero-point wavelength λ _T, and the light source (1) has a half-width Δλ in a hatched region for each composition ratio x.
Is used, the temperature change of the effective Verdet constant Ve 'can be made substantially zero.

[0018]

As described above, a conventional optical applied magnetic field sensor uses an LD as a light source (1) or an LED.
The light source is a monochromatic light by combining
Used as (1), center wavelength λc of generated light and magneto-optical element
The magneto-optical element is adjusted so that the zero-point wavelength λ _T of (5) matches.
Since the composition of (5) is set, there is a problem that the sensor sensitivity R is deteriorated. Moreover, the use of practical LED having a half-width Δλ as the light source (1), since the absorption edge wavelength λk the zero point wavelength lambda _T are close, the upper limit of the temperature range, the wavelength distribution of light magnetic There is a possibility that the wavelength is shorter than the absorption edge wavelength λk of the optical element (5). Therefore, the change in the emission wavelength distribution increases due to the temperature change of the absorption edge wavelength λk, the temperature change of the effective Verdet constant Ve 'increases, and the higher the temperature becomes, the more the light quantity decreases and the sensor sensitivity R deteriorates. was there.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and has been made in consideration of the above-mentioned problems.
It is an object of the present invention to obtain a high-sensitivity optical applied magnetic field sensor by minimizing the decrease in light amount by reducing (temperature dependency).

[0020]

An optical applied magnetic field sensor according to the present invention has a zero-point wavelength λ _{T at} which a Verdet constant Ve corresponding to the composition ratio x of the magneto-optical element becomes constant irrespective of temperature, and λ _T , .lamda.k the absorption edge wavelength at the upper limit of the temperature range, when the center wavelength of the light from the light source was λc, λc-λk> if it meets the [Delta] [lambda] / 2 of the relationship, the magneto-optic so that [lambda] c = lambda _T When the composition ratio x of the element and the center wavelength λc of the light source are set, and the relationship of λc−λk ≦ Δλ / 2 is satisfied, the wavelength shift amount of 5 nm to 50 nm
In consideration of dλ, the composition ratio x of the magneto-optical element and the center wavelength λc of the light source are set so that λc = λ _T + dλ.

[0021]

According to the present invention, the half width Δ of the light from the light source is obtained.
lambda may overlap the absorption edge wavelength λk, without coincide with the zero point wavelength lambda _T of the center wavelength λc and magneto-optical element of the optical center wavelength λc or zero wavelength lambda _T is shifted to overlap suppression direction Thus, the conditions of the light source or the magneto-optical element are set, and the temperature change rate of the effective Verdet constant Ve 'is reduced.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. The configuration of one embodiment of the present invention is as shown in FIG. 4, and if necessary, the light source (1) or the magneto-optical element (5)
The only difference is that the composition and the like are set in a specific range. Accordingly, the basic operation is the same as described above, and will not be described here.

FIGS. 1 to 3 are characteristic diagrams for explaining the effects of the embodiment of the present invention.
LE with center wavelength λc of 820 nm and half width Δλ of 50 nm
The case where D is used is shown. FIG. 1 shows a magneto-optical element.
The temperature change rate ΔV of the effective Verdet constant Ve ′ when the manganese composition ratio x in (5) is changed in the range of 0.037 to 0.197.
The measured values of e ′ and the modulation ratio K _R corresponding to the degree of improvement of the sensitivity R are shown. Here, the magneto-optical element (5) whose zero-point wavelength λ _T coincides with the center wavelength λ c (= 820 nm)
The composition ratio x (= 0.096) of the conventional composition ratio x ₁ is used as the conventional composition ratio x1, and the modulation factor when the magneto-optical element having the optimum composition ratio x ₂ is used is divided by the modulation ratio of the magneto-optical element having the conventional composition ratio x _1. the ratio when normalized is the modulation ratio K _R. FIG. 2 shows a composition ratio x (= 0.09) in which the zero wavelength λ _T coincides with the center wavelength λc (= 820 nm).
-20 ° C (solid line) when using the magneto-optical element (5) of 6)
FIG. 7 is a characteristic diagram showing the wavelength distribution of the Verdet constant Ve and the transmittance ρ [%] (absorption characteristics) at 60 ° C. (broken line). In the figure, Io is the light source wavelength distribution when the transmittance ρ is 1,
Is the light source wavelength distribution according to the transmittance ρ, and here, the unit of the Verdet constant Ve is [minute / cm · G]. However,
[Minute] is [degree / 60]. FIG. 3 uses the above equation,
And the modulation ratio 'temperature change rate ΔVe' of the effective Verdet constant Ve (sensitivity relative ratio) calculation results of K _R, a characteristic diagram showing a composition ratio x as a parameter, the broken line in the case of monochromatic light of wavelength 820nm FIG.

In FIG. 1, when the conventional magneto-optical element (5) having the composition ratio x ₁ (= 0.096) is used, the effective Verdet constant V
The temperature change rate ΔVe ′ of e ′ is ± 8.5%. However, when the magneto-optical element (5) having the optimum composition ratio x ₂ (= 0.12) set according to the present invention is used, the temperature change rate ΔVe ′ of the effective Verdet constant Ve ′ becomes ± 6.5%, and the conventional composition the ratio x ₁ as compared to the temperature change rate .DELTA.Ve 'is improved by about 30%. At this time, FIG. 8 shows that the zero-point wavelength λ when the composition ratio x is 0.12.
_{Since T} is about 807 nm, the center wavelength λc (= 820n) of the light source (1)
m) is shifted to a longer wavelength side by 13 nm from the zero-point wavelength λ _T (= 807 nm) determined by the composition ratio x of the magneto-optical element (5). Accordingly, assuming that the wavelength shift amount with respect to the zero-point wavelength λ _T is dλ, the center wavelength λc is represented by λc = λ _T + dλ.

In FIG. 2, with respect to the initial wavelength distribution Io (dashed line) of the light source (1), the wavelength distribution I after passing through the magneto-optical element (5) is affected by the absorption characteristics due to the transmittance ρ. Thus, it can be seen that the light amount changes as shown by a solid line at -20 ° C, and as a broken line at 60 ° C, and the light amount clearly decreases. For example, at 60 ° C., since the absorption edge wavelength λk of the magneto-optical element (5) having the composition ratio x of 0.096 is 800 nm, the light having a central wavelength λc of 820 nm and a half-value width Δλ of 50 nm is expressed by The relationship of λk ≦ Δλ / 2 holds.

Thus, the light source for which the equation holds
When (1) is used, the composition ratio x of the magneto-optical element (5) is, as apparent from FIG. 3, the composition ratio x at which the temperature change rate ΔVe ′ of the effective Verdet constant Ve ′ becomes zero in monochromatic light. ₁ (= 0.
Than 096), who often composition ratio x ₂ of manganese (> x ₁₎ the temperature change rate .DELTA.Ve 'and the rate of change ratio K _R is improved. That is, when the magneto-optical element (5) having a composition ratio x in which the zero-point wavelength λ _T is shifted to a shorter wavelength side than the center wavelength λ c is used, the change in the light amount of the light source wavelength distribution due to the absorption characteristics is small, and The sensitivity R as a magnetic field sensor is improved.

From the above, the center wavelength λc of the light source (1)
And a half value width [Delta] [lambda], and the absorption edge wavelength .lamda.k of the magneto-optical element in the upper limit of the temperature range (5), if it meets the relationship λc-λk> Δλ / 2, similar to the conventional zero point wavelength lambda _T to coincide with the center wavelength [lambda] c, [lambda] c = a composition ratio x and the light source of the magneto-optical element so that λ _T (5) (1)
When the relationship of λc−λk ≦ Δλ / 2 is satisfied as in the equation, the wavelength shift amount of 5 nm to 50 nm is set.
Considering dλ, the composition ratio x of the magneto-optical element (5) and the light source are set so that λc = λ _T + dλ.
It can be seen that it is desirable to set the center wavelength λc in (1). Thereby, when the half width Δλ of the light source wavelength distribution overlaps with the absorption edge wavelength λk at the upper limit of the use temperature,
Central wavelength λc and the zero-point wavelength lambda _T of the magneto-optical element is not coincident, the central wavelength λc or zero wavelength lambda _T is shifted to overlap suppression side.

For example, if the expression is satisfied, the magneto-optical element
The composition ratio x in (5) is obtained from the composition ratio x _{1 where} λc = λ _T, and the zero-point wavelength λ _T is equal to the wavelength shift d
It is set to be on the shorter wavelength side by λ (= 5 nm to 50 nm). Specifically, when a near-infrared LED is used as the light source (1), the composition ratio x of the magneto-optical element (5) may be set so as to satisfy 0.03 <x <0.2.

Near-infrared LED generally commercially available
Has a center wavelength λc of 800 nm to 850 nm and a half width Δλ of about 50 nm. On the other hand, when the center wavelength λc is 800 nm, the composition ratio x of the magneto-optical element (5) is optimal near x = 0.16, and when the center wavelength λc is 850 nm, x = 0.11. The neighborhood is optimal. At this time, since the zero-point wavelength λ _{T at} each composition ratio x (= 0.16, 0.11) is 790 nm and 810 nm, respectively, the wavelength shift amount dλ is 10 nm and 40 nm, respectively.
Becomes This means that the center wavelength λc of the light source (1) is set to the longer wavelength side by the wavelength shift amount dλ from the zero wavelength λ _T , or the zero wavelength λ _T is shorter by the wavelength shift amount dλ than the center wavelength λc. Means to be set on the side. In practice, the wavelength shift amount dλ is set to about 5 nm to 50 nm.

[0030]

As described above, according to the present invention, the zero point wavelength at which the Verdet constant Ve according to the composition ratio x of the magneto-optical element becomes constant irrespective of the temperature is λ _T , and the operating temperature range of the magneto-optical element is when the absorption edge wavelength at an upper limit .lamda.k, the center wavelength of the light from the light source was set to [lambda] c, when satisfying the relationship λc-λk> Δλ / 2, the composition ratio of the magneto-optical element so that [lambda] c = lambda _T When x and the center wavelength λc of the light source are set, and the relationship of λc−λk ≦ Δλ / 2 is satisfied, the wavelength shift amount of 5 nm to 50 nm
Considering dλ, the composition ratio x of the magneto-optical element and the center wavelength λc of the light source are set so that λc = λ _T + dλ, and the overlap of the half width Δλ and the absorption edge wavelength λk is suppressed. In addition, the temperature dependence of the effective Verdet constant can be reduced to minimize the decrease in the amount of light, and there is an effect that a highly sensitive optical applied magnetic field sensor can be obtained.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram illustrating a temperature change rate of an effective Verdet constant and a modulation rate ratio corresponding to a sensitivity relative ratio when a composition ratio is used as a parameter, for explaining an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a wavelength characteristic of a Verdet constant and a wavelength characteristic of transmittance for explaining one effect of the present invention.

FIG. 3 is a characteristic diagram showing a temperature change rate of an effective Verdet constant and a transmittance corresponding to a sensitivity relative ratio when a composition ratio is used as a parameter, for explaining one effect of the present invention.

FIG. 4 is a configuration diagram illustrating a general optical applied magnetic field sensor.

FIG. 5 is a characteristic diagram showing wavelength characteristics of a general Verdet constant.

FIG. 6 is a characteristic diagram showing a temperature characteristic of a general Verdet constant.

FIG. 7 is a characteristic diagram showing a zero-point wavelength with respect to a general composition ratio.

FIG. 8 is a characteristic diagram showing wavelength characteristics at general composition ratios with different Verdet constants.

FIG. 9 is a characteristic diagram showing wavelength characteristics at general composition ratios having different transmittances.

FIG. 10 is a characteristic diagram showing light energy corresponding to a general absorption edge wavelength and a zero-point wavelength when a composition ratio is used as a parameter.

FIG. 11 is a characteristic diagram showing characteristics of light energy corresponding to a general zero-point wavelength and characteristics of a half-value width allowable region when a composition ratio is used as a parameter.

[Explanation of symbols]

(1) Light source (5) Magneto-optical element H Magnetic field x Composition ratio Ve Verde constant Ve 'Effective Verde constant λ _T Zero point wavelength λk Absorption edge wavelength λc Center wavelength Δλ Half width

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Chie Nagao 8-1-1 Tsukaguchi Honmachi, Amagasaki City Inside Materials Research Laboratory, Mitsubishi Electric Corporation (72) Inventor Takao Sawada 8-1-1 Tsukaguchi Honmachi, Amagasaki City Mitsubishi (72) Inventor Noboru Mikami 8-1-1, Tsukaguchi-Honmachi, Amagasaki City Mitsubishi Electric Corporation Materials Research Laboratory (56) References JP-A-64-63882 (JP, A) JP-A-2-132389 (JP, A) JP-A-2-28573 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G01R 33/032

Claims (1)

(57) [Claims] 1. A magneto-optical element for converting a change in the intensity of a magnetic field into a change in the intensity of light using the Faraday effect, wherein Cd _{1 -X} Mn _X Te (where x is a manganese composition ratio,
0 <x <0.7), and the half-value width is Δ
In an optical applied magnetic field sensor using a light source that generates light having a wavelength characteristic of λ, a zero point wavelength at which a Verdet constant Ve according to a composition ratio x of the magneto-optical element becomes constant regardless of temperature is λ _T , .lamda.k the absorption edge wavelength at the upper limit of the operating temperature range of the optical element, when the center wavelength of the light from the light source was set to [lambda] c, if it meets the λc-λk> Δλ / 2 the relationship, the [lambda] c = lambda _T The composition ratio x of the magneto-optical element and the center wavelength λc of the light source are set so that the half-value width Δλ of the light from the light source is the operating temperature of the magneto-optical element.
When the wavelength overlaps the absorption edge wavelength λk at the upper limit of the degree range and satisfies the relationship of λc−λk ≦ Δλ / 2, the center wavelength λc of the light source and the
Without matching the zero wavelength λ _T of the magneto-optical element ,
Overlap suppression how the mind wavelength λc or the zero-point wavelength λ _T
To shift in direction, characterized in that in consideration of the wavelength shift amount d [lambda] of 5 nm to 50 nm, and setting the center wavelength [lambda] c of the composition ratio x or the light source of the magneto-optical element so that λc = λ _T + dλ Optical applied magnetic field sensor.