JPH11281722A - Optical fiber sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an unexpensive optical fiber sensor with superior linearity in low magnetic field and temperature characteristics. SOLUTION: At least, a light modulation part comprising a polarizer 11, a magnetooptic element 12, and an analyzer 14, a light incidence part 101 for introducing the light transferred from a light source through an incidence optical fiber 103, a light out-going part 102 wherein the light modulated at the light modulation part is taken out, an outgoing optical fiber 104 which guides the signal light to a light-receiving element, and a signal processing circuit where the signal obtained with the light-receiving element is processed, are provided. In addition, the magnetooptic element 12 is entirely non-saturated by a permanent magnet 13. With a magnetooptic element 12 made into a single magnetic domain by the permanent magnet 12, problem of drop in the linearity due to immobile domain wall in a low magnetic field. In addition, the ratio between Faraday rotation angle and biased magnetic field is almost constant in an operating temperature 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 or current measuring sensor using a magneto-optical element.

[0002]

2. Description of the Related Art Conventionally, an optical magnetic field (current) sensor has been already put into practical use as a current measuring device for electric power because of its electrical insulation property and resistance to electromagnetic induction noise (for example, NAtion).
al Technical Report Vol. 2 (199
2)). Also, to improve the measurement accuracy of optical fiber sensors,
It has been shown that a sensor having good linearity can be realized by receiving diffracted light caused by a domain structure of a garnet, which is a magneto-optical element, to a high order (for example, Japanese Patent Application Laid-Open No. 7-209397, Japan Society of Applied Magnetics). Magazine Vol.N
o. 2, PP. 209-212 (1995) and IEEE
E, Trans. Magn. Vol. 31, No. 6
(1995)). Further, in order to improve the temperature characteristics, a method of applying a DC bias magnetic field in parallel or obliquely to the measurement magnetic field of the optical fiber sensor has been studied (for example, see Japanese Patent Application Laid-Open No. HEI 2 (1990) -210).
-293680).

Further, a high-sensitivity magnetic field sensor in which a light source / light receiving element and a magneto-optical element are integrated, and an optical fiber sensor in which light is incident from a light source through an optical fiber have been devised. (For example, J. APPl. Phys., Vol. 53, No. 1)
1, PP. 8263-8265 (1982), U.S. Patent Publication 4,516,073).

[0004]

When a garnet material, which is a ferromagnetic material, is used for a magneto-optical element, there are generally the following problems to realize an optical fiber sensor having good linearity even in a low magnetic field. That is, the magnetic domain of the garnet cannot move at a very small low magnetic field due to the domain wall coercive force. As a result, the magnetic domain cannot move even at a low magnetic field even if the magnetic field changes, so that the linearity deteriorates. As a method for solving this problem, a DC magnetic field is applied to the magneto-optical element from a direction orthogonal to the measurement magnetic field, and a known AC magnetic field is also applied in the direction of the specific measurement magnetic field to achieve high sensitivity even in a very low magnetic field. Methods for measuring magnetic fields have been devised. (For example, J.A.
ppl. Phys. , Vol. 53, No. 11, p
p. 8263-8265 (1982)). However, in these conventional examples, the light source, the light receiving element, and the magneto-optical element are integrated, so that it is necessary to connect an electric line such as a power supply and a signal line to the sensor head. There is a problem that electrical insulation of the processing unit cannot be maintained.

FIG. 7 shows a configuration of an optical fiber sensor for explaining a problem of another conventional example (for example, US Pat. No. 4,516,073). Also in an optical fiber sensor that transmits light using an optical fiber, a known AC magnetic field is applied to the light modulator 75 by the optical sensor (coil 702).
Therefore, an electric connection to the sensor unit is required. Also, U.S. Patent Publications 4,516,073 and J.P. A
ppl. Phys. , Vol. 53, No. 11, P
P. 8263-8265 (1982), the fiber for light incidence and the fiber for light emission are the same (720 in the figure), and an expensive optical splitter 75 is required in the signal processing unit. There is a problem of becoming. further,
Semiconductor laser 71 whose oscillation becomes unstable due to reflected return light
As a light source, there is a problem that an extremely expensive optical isolator 72 is required to block reflected return light. Further, in the above conventional example, the temperature characteristics of the conventional optical fiber sensor were not considered. For this reason, there has been a problem that a configuration in which a known modulation current does not flow through the coil has poor temperature characteristics and is not suitable for an optical fiber sensor used outdoors or the like.

[0006]

In order to solve the above-mentioned problems, the present invention provides an optical modulator comprising at least a polarizer, a magneto-optical element and an analyzer, and light transmitted from a light source through a first optical fiber. A light incident part for entering the light modulating part, and a signal light modulated by the light modulating part.
A light emitting portion for guiding the optical fiber, a permanent magnet for applying a DC bias magnetic field to the magneto-optical element, a light receiving element for receiving light transmitted through the second optical fiber, An optical fiber sensor comprising a signal processing circuit for processing a signal obtained by the element, wherein the magneto-optical element is made of a garnet material, and the magneto-optical element is magnetically saturated by the DC bias magnetic field. It is desirable that the ratio θ _F / Hb between the Faraday rotation angle θ _F per unit length of the magneto-optical element and the DC bias magnetic field Hb be substantially constant over the operating temperature range. The use of an optical fiber sensor characterized by the following. More preferably, the DC bias magnetic field Hb
Is to use an optical fiber sensor whose direction is substantially orthogonal to the direction of the ratio measurement magnetic field Hm.

The present invention also provides an optical fiber having a desired shape, a substrate having an optical fiber fixing groove for fixing the optical fiber, and an optical path in which a polarizer, a magneto-optical element, and an analyzer comprise the optical fiber. It is inserted in the middle, and more preferably, an optical fiber sensor characterized in that the desired shape is a U-shaped optical fiber having a flat bottom surface. More preferably, the magneto-optical material is made of a Bi-substituted garnet, and more preferably, an optical fiber sensor in which the permanent magnet is an Sm-Co-based magnet, or the permanent magnet is an oxide-based magnet. It is preferable to use an optical fiber sensor characterized by the following:
An optical fiber sensor characterized in that it is substantially closed in a space other than the space where the C bias magnetic field is applied.

[0008]

Embodiments of the present invention will be described below.

The present invention relates to an optical fiber for incidence, an optical fiber for emission, a light source, a light receiving element, a signal processing circuit, an analyzer,
Magneto-optical element, a light modulating section comprising an analyzer, and a light incident section for introducing light into the light modulating section, a light emitting section for introducing the modulated light modulated by the light modulating section into an output optical fiber, It consists of a permanent magnet for applying a DC bias magnetic field to the magneto-optical element. The use of an optical fiber for light incidence and an optical fiber for emission makes an expensive optical branching device indispensable in the case where a single optical fiber conventionally serves as both an optical fiber for incidence and an optical fiber for emission. Becomes unnecessary. Further, an optical isolator for preventing reflected return light to the light source, which is required when a semiconductor laser light source is used, becomes unnecessary.

The principle of operation when a single magnetic domain is formed using a DC bias magnetic field and the magneto-optical element is applied to an optical fiber sensor will be described below with reference to FIG. The magnitude of the DC bias magnetic field is larger than the saturation magnetic field of the garnet, which is a magneto-optical element, to form a single magnetic domain structure by applying a magnetic field Hb. The magnitude of the saturation magnetization at this time is Ms. In order to make the following description easy to understand, it is assumed that the direction of light transmission and the direction of the measured magnetic field Hm are parallel, and the measured magnetic field and the bias magnetic field Hb are perpendicular. In practice, the magnetic field to be measured and the bias magnetic field need not be perpendicular to each other, but if they are perpendicular to each other in a range of about 90 ° ± 10 °, the magnetic field and the current value can be easily converted. . . FIG. 2A is a diagram schematically showing the above conditions. When a measured magnetic field Hm is applied, D
The magnetization Ms rotates in a direction in which the vector of the C bias magnetic field Hb and the vector of the measured magnetic field Hm are combined (see FIG. 2B). Since the rotation of the magnetization is not affected by the domain wall coercive force, the magnetization rotates in response to an external magnetic field even from a low magnetic field. As a result, an optical fiber sensor having good linearity can be realized even in a low magnetic field.
The direction of the magnetization Ms is determined by the DC bias magnetic field Hb and the measured magnetic field H.
The direction is the direction of the composite vector H of m. Assuming that the accuracy of the magnetic field H combined with the light transmission direction is γ, cos γ is represented by the following (Equation 1).

[0011]

(Equation 1)

The Faraday rotation angle θ in the light transmission direction is represented by the following (Equation 2), where t is the thickness of the crystal.

[0013]

(Equation 2)

[0014] Here theta _F the direction of magnetization light transmission direction coincides, a Faraday rotation angle per unit length when the magneto-optical element is completely saturated.

In general, when the transmission polarization planes of the polarizer and the analyzer are rotated by 45 degrees with each other and the Faraday rotation angle of the magneto-optical element of a single magnetic domain is θ (when an optical bias is applied), the transmitted light Is expressed by the following (Equation 3).

[0016]

(Equation 3)

Here, Po is the maximum value of the transmitted light intensity.
The sensitivity of the optical fiber sensor is a value obtained by dividing the change in transmitted light power by the measured magnetic field Hm. Therefore, output light proportional to SIN (2θ) is obtained as fiber output light. When the value of 2IN is small, SIN (2θ) can be approximated to 2θ, so that the sensitivity S of the sensor is eventually expressed by the following (Equation 4).

[0018]

(Equation 4)

Here, θ _F is a function of temperature inherent to the magneto-optical material, and Hm is a temperature characteristic of a magnetic field generated by the bias permanent magnet. The operating condition of the sensor is usually Hm≪Hb
Therefore, if the magneto-optical element and the permanent magnet for generating the DC bias are selected so that the value of θ _F / Hb becomes constant in the operating temperature range of the optical fiber sensor, light having good temperature characteristics can be obtained. A fiber sensor can be realized.

As a material of the permanent magnet, a permanent magnet of an oxide type (for example, ferrite) having a large temperature change is suitable for a magneto-optical element having a large temperature change of θ _{F, and} a magneto-optical element having a small temperature change of θ _F is suitable. An Sm-Co-based permanent magnet with a small temperature change in the generated magnetic field is suitable for the element.

By making the magnetic circuit formed by the permanent magnet as closed as possible except for the portion to which the bias magnetic field is applied, the effect of the external magnetic field is reduced, and the demagnetizing effect of reducing the magnetic field generated as the permanent magnet (irreversible reduction). Magnetism) is reduced, and a long-term stable optical fiber sensor can be realized.

Hereinafter, the present invention will be further described with reference to specific embodiments of the present invention. In the following description, for the sake of simplicity, the case where the direction of the DC bias magnetic field and the direction of the measurement magnetic field are orthogonal to each other will be described. However, the bias magnetic field completely saturates the magneto-optical element. If it is a system utilizing the rotation of the direction of magnetization by a magnetic field, it functions as an optical fiber sensor.

(Embodiment 1) A first embodiment of the present invention will be described with reference to FIG. In the following description, a Y ₃ Fe ₅ O ₁₂ (hereinafter abbreviated as YIG) crystal grown by using the floating zone method is cut into a desired size and used as the magneto-optical element 12. The light that has propagated through the incident optical fiber 103 from the LED light source having a wavelength of 1.3 μm
The light is converted into parallel light through a light incident portion including the lens holder 17 and the ferrule 18. The parallel light passes through the polarizer 11, and linearly polarized light is extracted. The linearly polarized light passes through the magneto-optical element 12 and undergoes rotation of the polarization plane represented by (Equation 2).

Here, the magneto-optical element 12 is a permanent magnet 13
, The light is not diffracted by the domain wall. The light whose polarization plane has been rotated can be observed as a change in light intensity by transmitting through the analyzer 14. The intensity-modulated light is bent at 90 degrees accuracy by a mirror, passes through a light emitting unit 102 such as a lens, and is focused on an emitting optical fiber 104. The light transmitted through the emission optical fiber is photoelectrically converted by a photodetector, further amplified and signal-processed by a signal processing circuit, and converted into a magnetic field intensity or a current intensity. Since the input optical fiber and the output optical fiber are completely separated, it is not necessary to use an effective optical splitter. Further, even when a semiconductor laser light source is used as the light source, the signal light does not return to the light source, so that an expensive optical isolator is not required. FIG. 3 shows the results of evaluating the linearity of the optical fiber sensor by applying a magnetic field using the optical fiber sensor of FIG. When the optical fiber line sensor of the present invention is used, even in a low magnetic field of 0.1 to 10 Oe, non-error (deviation from linearity) shows good linearity of 1% or less.

For comparison, there is shown a result of producing an optical fiber sensor similar to that shown in FIG. 1 in the case where there is no DC bias magnetic field, and evaluating the linearity thereof. In the magnetic field range of 0.1 to 10 Oe, the non-error was reduced by as much as 20% at the maximum, and the linearity was particularly poor at low magnetic fields.

Next, FIG. 4 shows an example in which an optical fiber current sensor is constructed using the sensor head having the construction shown in FIG. Magnetic core (soft material) with a gap around electric wire 43
43 is arranged. Directional silicon steel was used as a material for the magnetic core. The light emitted from the light source 45 passes through the incident optical fiber 403 and enters the sensor head 41 having the same configuration as in FIG. The light whose intensity has been modulated by the sensor head 41 passes through the light emitting section, passes through the emitting optical fiber 404, and is
Is photoelectrically converted. The electric signal is amplified and signal-processed by a signal processing circuit to obtain a modulated signal proportional to the magnetic field generated in the gap between the magnetic cores 42. Since the magnetic field generated in the magnetic core gap is proportional to the current flowing through the electric wire, the value of the current flowing through the electric wire can be known. The optical fiber sensor of this configuration is
By using the input optical fiber and the output optical fiber, the optical branching device and the semiconductor, which are indispensable when the sensor is constituted by a single optical fiber that serves both as the input optical fiber and the output optical fiber in the past, There is no need for an optical isolator for blocking reflected return light to the light source, which is required when a laser light source is used. When the linearity was evaluated by variously changing the value of the current flowing through the electric wire 43, a good linearity with a non-error of ± 1% or less was obtained at 0.5 to 1000A.

As a comparison, using a normal bias magnetic field,
When a sensor head employing a parallel optical system was used, large non-error characteristics of -20% to + 2% were exhibited in the above range, and the reduction of non-error on the low magnetic field side was particularly large.

(Embodiment 2) A second embodiment of the present invention will be described with reference to FIG.

In this embodiment, the magnetic circuit formed by the permanent magnet 53 for generating a DC bias magnetic field is a magneto-optical element 5
2 is substantially closed except for a gap for applying a bias magnetic field. Incident optical fiber 503, light incident section 501, polarizer 51, magneto-optical element 52, analyzer 5
4, mirror 55, light emitting portion 502, optical fiber for emission 5
04 has the same configuration as in the first embodiment.

After leaving the sensor head in FIG. 2 at about 80 degrees for about half a year, the sensor head was returned to room temperature and its sensitivity was measured. The change was ± 0.5% or less, which was consistent within the measurement accuracy.

As a comparison, a similar experiment was conducted using the sensor head having the configuration shown in FIG.
0.5%, a slight increase. This is due to the irreversible demagnetization of the permanent magnet for DC bias (the generated magnetic field becomes weaker). As a method of reducing the irreversible demagnetization, a sensor with good long-term stability can be realized by using a permanent magnet that has been subjected to initial demagnetization by annealing in addition to the shape of the permanent magnet.

(Embodiment 3) A third embodiment of the present invention will be described with reference to FIG. Here, an example of an integrated optical fiber sensor in which polarization, a magneto-optical element, an analyzer, an optical fiber, a permanent magnet for bias, and the like are integrated in a hybrid manner on a substrate will be described. As the material for the substrate, any material may be used as long as it is an insulating material such as glass, ceramics, and resin. In particular, a glass epoxy substrate having high insulating properties and excellent workability was used. The substrate 69 used had a hole in which a permanent magnet 601 for bias was previously installed. An optical fiber fixing groove 68 is formed on the substrate 69 using a rotary blade saw. Next, an optical fiber core wire having ferrules 603 attached to both ends is prepared. A part of the coating of the optical fiber is removed to make the optical fiber portion visible, and the optical fiber is bonded and fixed to the optical fiber fixing groove. In this state, three optical element insertion grooves 67 are formed again using the rotary blade saw. At this time, the optical fiber is also cut at the same time. A polarizer 63, a magneto-optical element 64, and an analyzer 64 are provided in these grooves. Finally, a permanent magnet for bias 60 made of a ferrite magnet
2 are installed to complete the optical fiber sensor head. The temperature characteristic of the magnetic field generated by this ferrite magnet is-
0.2% / ° C. The direction of the non-measurement magnetic field is the longitudinal direction of the optical fiber, and the direction of the bias magnetic field is a direction perpendicular to the direction as shown in FIG.

As a magneto-optical element, LPE was grown on a substitution-type GGG substrate with a large amount of Ga substituted (BiR) _3.
(FeGa) ₅ O ₁₂ was used. Crystal thickness 500-800
μm growth. Thereafter, the following two processes were performed to reduce magnetic anisotropy. The obtained crystal substrate was completely removed by polishing, etching or the like. Then, the crystal was cut into a desired size. Furthermore, this chip crystal is 900-1
Annealing was performed at 050 ° C. for several days to several hundred days, and a crystal chip (about 1 × 1 × 0.5 mm) was used as the magneto-optical element 64. Temperature change rate of the absolute value of the magneto-optical element 64 was measured by fully saturate theta _F is -0.2% / ° C., it was consistent with the temperature characteristics of the bias magnetic field ferrite magnet 601 is generated.

The linearity of the obtained sensor head is -0.0.
From 1 to 50 Oe, non-error ± 2%, showing good results.
Further, when the temperature characteristic of this sensor was measured, it was found that -20 to +8
Excellent temperature characteristics of ± 2% or less in a temperature range of 0 ° C.

In this embodiment, the present invention has been described in connection with the case where the optical fiber has a linear shape. Similarly, the optical fiber has a U-shape having a flat bottom surface as disclosed in JP-A-8-219825. Of course, it can also be used for optical fiber sensors.

As described above, the present invention has the following effects. (1) By using an optical fiber sensor configuration having a light incident portion and a light emitting portion, an optical fiber sensor can be realized without using expensive optical components (optical isolators and optical splitters). Furthermore, the sensor can be easily installed in the core gap around the electric wire.

(2) The non-linearity of a low magnetic field due to light diffraction and domain wall coercive force due to the magnetic domain structure of the magneto-optical element is eliminated, and particularly a minute magnetic field and a minute current can be observed with high accuracy.

(3) By designing the ratio of the Faraday rotation angle θ _F per unit length to the ratio θ _F / Hb of the DC bias magnetic field Hb to be substantially constant in the operating temperature range, an optical fiber having good temperature characteristics can be obtained. A sensor can be realized.

(4) By making the magnetic circuit formed by the DC bias magnetic field applying magnet almost completely closed except for the part to which the bias magnetic field is applied, the influence of the external magnetic field is small and the long-term stability is excellent. Optical fiber sensor can be realized.

[0040]

As described above, according to the present invention, an optical fiber sensor having excellent linearity in a low magnetic field and excellent temperature characteristics can be provided at low cost, and its industrial value is high.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an optical fiber sensor according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating the principle of the optical fiber sensor according to the present invention.

FIG. 3 is a graph showing a non-error measurement example according to the first embodiment of the present invention and a non-error measurement example according to a conventional example.

FIG. 4 is a diagram showing the configuration of the entire optical fiber sensor of the present invention.

FIG. 5 is a diagram showing a configuration of an optical fiber sensor according to a second embodiment of the present invention.

FIG. 6 is a diagram showing a configuration of an optical fiber sensor according to a third embodiment of the present invention.

FIG. 7 is a diagram showing a schematic configuration of a conventional optical fiber sensor.

[Explanation of symbols]

 101,501,608 Light incidence part 102,502,609 Light emission part 13,53,605,701 Permanent magnet 11,51,63 Polarizer 12,52,64 Magneto-optical element 14,54,65 Analyzer 15,55 Mirror 103, 403, 503 Incident optical fiber 104, 404, 504 Outgoing optical fiber 16 Lens 17 Holder 18 Ferrule 41 Sensor head 42 Magnetic core 43 Electric wire 45 Light source 46 Light receiving element 47 Signal processing circuit 61 Optical fiber strand 62 Optical fiber core 68 Optical fiber fixing groove 69 Substrate 603 Ferrule 31 Light source 71 Laser light source 72 Optical isolator 75 Splitter 79 Light receiving element 720 Incident / emitted optical fiber 702 Coil 703 Reflector mirror 75 Light modulator

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor: Nori Ishizuka 1006 Kazuma Kadoma, Kadoma City, Osaka Inside Matsushita Electric Industrial Co., Ltd.

Claims (9)

[Claims] An optical modulator including at least a polarizer, a magneto-optical element, and an analyzer; a light incident unit configured to input light transmitted from a light source via a first optical fiber to the optical modulator; A light emitting unit for transmitting the signal light modulated by the light modulating unit through the second optical fiber and extracting the signal light; a permanent magnet for applying a DC bias magnetic field to the magneto-optical element; and the second optical fiber. A light-receiving element for receiving the transmitted light; a signal processing circuit for processing a signal obtained by the light-receiving element; the magneto-optical element is made of a garnet material; and the magneto-optical element is magnetically driven by the DC bias magnetic field. An optical fiber sensor characterized by being substantially saturated. 2. The method according to claim 1, wherein the ratio θ _F / Hb of the Faraday rotation angle θ _F per unit length of the magneto-optical element to the DC bias magnetic field Hb is substantially constant over the operating temperature range. Optical fiber sensor. 3. The optical fiber sensor according to claim 1, wherein the direction of the DC bias magnetic field Hb is substantially orthogonal to the direction of the ratio measurement magnetic field Hm within a range of 90 degrees ± 10 degrees. 4. An optical fiber having a desired shape, a substrate having an optical fiber fixing groove for fixing the optical fiber, and a polarizer, a magneto-optical element, and an analyzer are inserted in an optical path including the optical fiber. The optical fiber sensor according to claim 1, wherein the optical fiber sensor is used. 5. The optical fiber sensor according to claim 1, wherein the desired shape is a U-shaped optical fiber having a flat bottom. 6. The optical fiber sensor according to claim 1, wherein the magneto-optical material is made of a Bi-substituted garnet. 7. The optical fiber sensor according to claim 1, wherein the permanent magnet is an Sm—Co magnet. 8. The optical fiber sensor according to claim 1, wherein the permanent magnet is an oxide magnet. 9. The optical fiber sensor according to claim 7, wherein the magnetic circuit formed by the permanent magnet is substantially closed except for a space for applying a DC bias magnetic field.