Magnetic field measurement apparatus

A magnetic field measurement apparatus with high sensitivity and high accuracy in which a Bi-substituted rare-earth iron garnet crystal with a high and no temperature dependent sensitivity constant grown by liquid phase epitaxy is used for a magneto-optic element.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an apparatus for determining the intensity of 
magnetic fields, in which Faraday rotation is observed using a 
magneto-optic element. 
2. Description of the Prior Art 
In recent years, as an optical method for determining the intensity of 
magnetic fields, the use of the Faraday effect has been proposed by, for 
example, Kyuma et al., IEEE, QE-18, 1619 (1982). 
Methods for measuring the magnetic field intensity around a conductor 
through which current flows to thereby detect the current are advantageous 
in that an excellent insulation can be attained because of the use of 
light as a medium, and electromagnetic induction noise immunity may also 
be attained, so that the said methods can be applied to the disposition of 
power transmission. 
FIG. 1 shows the principle of the method of measuring a magnetic field 
using the Faraday effect, in which a magneto-optic element 1 is placed in 
a magnetic field H. The linearly polarized light by a polarizer 2 is 
passed through the magneto-optic element 1. The polarization plane of 
light is rotated by an angle proportional to the magnetic field intensity 
H due to the Faraday effect. The polarized light rotated by the Faraday 
effect passes through an analyzer 3 which has a direction of polarization 
different from that of the polarizer 2 by 45.degree., and the angle of 
rotation, .theta., is converted to the change in the optical power. 
Optical output power in this case is given by the following equation. 
EQU Pout=K(1+sin 2.theta.) (1) 
.theta.=CHl 
wherein Pout denotes the optical output power, K is a proportional 
constant, .theta. is the Faraday rotation angle (degrees), l represents 
the length of the magneto-optic element 1 in the direction of the 
propagation of light, and C is the sensitivity constant in units of 
degrees/cm.Oe representing the sensitivity of the magneto-optic element. 
Applications of magnetic field measurement apparatuses based on this 
principle have been proposed, such as that which detects zero-phase 
current to determine the occurrence of accidents by feeding signals from 
magnetic field measuring instruments arranged at multiple points to an 
arithmetic operation unit where the waveforms are added or subtracted to 
generate reference signals. 
A typical magneto-optic element used in such a is magnetic field 
measurement apparatus is YIG crystal which is represented by the general 
formula Y.sub.3 Fe.sub.5 O.sub.12. However, as shown in FIG. 2, the 
sensitivity constant C of YIG changes greatly with temperature, showing an 
increase as large as 16% over a temperature ranging from -20.degree. C. to 
120.degree. C. around the working temperature, resulting in the practical 
problem of great deviation in the measurement accuracy with the change of 
the ambient temperature. To eliminate this problem, a rare-earth iron 
garnet crystal, represented by the general formula Tb.sub.x Y.sub.3-x 
Fe.sub.5 O.sub.12 wherein x is limited to 0.3.ltoreq.x.ltoreq.0.9, is used 
for a magneto-optic element. An apparatus which uses this magneto-optic 
element has a remarkably improved measuring-accuracy in which the 
variation with temperature is .+-.1% over a temperature ranging from 
-25.degree. C. to 120.degree. C. 
A rare-earth iron garnet crystal substituted with Bi has a large Faraday 
effect and, when used for a magneto-optic element, improves the 
sensitivity of the magnetic field measurement apparatus. At present, the 
Bi-substituted rare-earth iron garnet crystal having a temperature 
independent sensitivity constant and good characteristics in practical 
application, has not heretofore been available. 
Because magneto-optic elements with a length of about 2 mm are required, 
rare-earth iron garnet crystals such as YIG which does not include Bi are 
made by the Flux method or the FZ method, which makes the manufacturing 
period long, causing a disadvantage in mass production of the measurement 
apparatus. Moreover, a magnetic field measurement apparatus utilizing as a 
magneto-optical element such rare-earth iron garnet crystals which do not 
include Bi must be provided with an expensive light source and 
photo-detector designed for the 1.3 .mu.m band, which makes the magnetic 
field measurement apparatus expensive. 
SUMMARY OF THE INVENTION 
The magnetic field measurement apparatus of this invention, which overcomes 
the above-discussed and numerous other disadvantages and deficiencies of 
the prior art, comprises: 
a magneto-optic transducer that has a polarizer, an analyzer, and a 
magneto-optic element made of Bi-substituted rare-earth iron garnet 
crystal which is represented by the general formula Bi.sub.x Gd.sub.y 
Y.sub.3-(x+y) Fe.sub.5 O.sub.12 (1.0&gt;x.ltoreq.1.4; 
0.1.ltoreq.y.ltoreq.0.7) and placed between the polarizer and the analyzer 
which have polarization directions different from each other; 
light transmission paths that are provided on both sides of the 
magneto-optic transducer; 
a light generating means that supplies light to the light transmitting 
path; 
a light detecting means that detects the optical output power generated by 
the incident light that has passed through the magneto-optic transducer so 
as to convert the optical output power into electric signals; and 
an electric circuit that processes the electric signals fed from the light 
detecting means, wherein the magneto-optic transducer in a magnetic field 
is placed into the magnetic field to be measured to thereby determine the 
intensity of the magnetic field. 
In a preferred embodiment, the magnetic field measurement apparatus 
comprises a plurality of magneto-optic transducers in the same number as 
that of the magnetic fields to be measured, a plurality of light detecting 
means that convert the optical output power into electric signals in the 
same number as that of the magnetic fields to be measured, and a signal 
processing circuit that adds and subtracts the electric signals received 
from each of the detecting means, wherein the magneto-optic transducers 
are placed into the magnetic fields to be measured to thereby determine 
the intensity of each of the magnetic fields. 
In a more preferred embodiment, the magneto-optic element is made of a 
crystal that is epitaxially grown on a Ca-Mg-Zr substituted Gd.sub.3 
Ga.sub.5 O.sub.12 or Nd.sub.3 Ga.sub.5 O.sub.12 substrate. 
In a preferred embodiment, the spectrum band of the light generating means 
has a peak at a wavelength in the range of 0.7 .mu.m to 0.9 .mu.m. 
Thus, the invention described herein makes possible the objectives of (1) 
providing a magnetic field measurement apparatus with high sensitivity and 
high accuracy in which a Bi-substituted rare-earth iron garnet crystal 
with a high sensitivity constant that does not substantially vary 
depending upon temperatures is used for a magneto-optic element; and (2) 
providing a magnetic field measurement apparatus for which a magneto-optic 
element is formed by liquid phase epitaxy, and an inexpensive light source 
and photodetector for the 0.8 .mu.m band are used, thereby attaining mass 
production of the said magnetic field measurement apparatus at low costs.

DETAILED DESCRIPTION OF THE INVENTION 
The magneto-optic element used in this invention is formed by a material 
that is represented by the general formula Bi.sub.x Gd.sub.y Y.sub.3-(x+y) 
Fe.sub.5 O.sub.12. The deviation of the sensitivity constant of this 
crystal with temperature is shown in FIG. 3. When the value of y is in the 
range of 0.1 to 0.7 (i.e., 0.1.ltoreq.y.ltoreq.0.7), the change is within 
.+-.1% over a temperature ranging from -20.degree. C. to 80.degree. C. 
When the magneto-optic element used in the present invention is formed by 
a material having the formula Bi.sub.1.3 Gd.sub.0.43 Y.sub.1.27 Fe.sub.5 
O.sub.12, it exhibits the the excellent characteristics with a 
temperature-dependence of as low as .+-.0.5%. The reasons therefor are as 
follows. 
Rare-earth iron garnet crystals are a ferrimagnetic material and the 
Faraday effect of which saturates at a specified magnetic field intensity 
as shown in FIG. 4. For the measurement of the magnetic field, a part of 
the characteristic curve of FIG. 4 which shows linear change in response 
to an external magnetic field is used. In this case, the angle of Faraday 
rotation, .theta. caused by the external magnetic field is given as 
follows. 
EQU .theta.=.theta..sub.F (H/Ms)l (2) 
where in .theta..sub.F denotes the specific Faraday rotation angle, Ms the 
saturation magnetization, and l the length of the crystal. 
Accordingly, the sensitivity constant C and its temperature-dependence are 
defined as 
EQU C(T)=.theta..sub.F (T)/Ms(T) (3) 
As can be seen from equation (3), the dependence of the sensitivity 
constant on temperature is determined by the changes of both .theta..sub.F 
and Ms with temperature. Differentiated with respect to temperature (T), 
both sides of equation (3) become 
##EQU1## 
As can be seen from equation (4), the difference between the variation in 
the Faraday rotation angle with temperature (i.e., 
##EQU2## 
and the change in the saturation magnetization with temperature (i.e., 
##EQU3## 
may be reduced to cause a decrease in the deviation of the sensitivity 
constant on the left side of equation (4) with temperature (i.e., 
##EQU4## 
FIG. 5 shows the change in .theta..sub.F of Bi.sub.x Gd.sub.y Y.sub.3-(x+y) 
Fe.sub.5 O.sub.12 with temperature. The measurements are normalized to 
.theta..sub.F at room temperature. The change of .theta..sub.F with 
temperature is independent of the amount y of Gd. This is because it is 
determined by x, the amount of substitution with Bi (x=1.0-1.3 in this 
case). 
FIG. 6 shows the change in Ms of Bi.sub.x Gd.sub.y Y.sub.3-(x+y) Fe.sub.5 
O.sub.12 with temperature. The change in Ms with temperature around room 
temperature is decreased with an increase in the quantity y of Gd. The 
magnitude of 
##EQU5## 
on the right side of Equation 4 can be varied by the addition of Gd and 
the magnitude of 
##EQU6## 
can be decreased according to the amount of Gd added. As shown in FIG. 3, 
the change in the sensitivity constant of Bi.sub.x Gd.sub.y Y.sub.3-(x+y) 
Fe.sub.5.sub.O.sub.12, wherein x is in the range of 1 to 1.3 and y is 
within the range of 0.1 to 0.7, (i.e., 1.0.ltoreq.x.ltoreq.1.3; 
0.1.ltoreq.y.ltoreq.0.7) with temperature, satisfies the relationship: 
##EQU7## 
As a result, the temperature-dependence of the sensitivity constant of 
materials belonging to the Bi.sub.x Y.sub.3-y Fe.sub.5 O.sub.12 group is 
improved by the addition of Gd. The result is shown in FIG. 7. As also 
shown in FIG. 3, Bi.sub.1.3 Gd.sub.0.43 Y.sub.1.27 Fe.sub.5 O.sub.12 shows 
good temperature-dependence, i.e., within .+-.0.5%. These crystals have 
been grown on a Ca-Mg-Zr substituted Gd.sub.3 Ga.sub.5 O.sub.12 substrate 
by means of liquid phase epitaxy that has good performance in means 
production. It has a sensitivity constant C that is 
1.4.degree./cm.multidot.Oe (.lambda.=1.3 .mu.m) or 
5.0.degree./cm.multidot.Oe (.lambda.=0.85 .mu.m), both of which are 
greater than that of a conventional YIG crystal. The sensitivity thereof 
is further improved by using a light source with shorter wavelengths, such 
as .lambda.=0.85 .mu.m. Consequently, a magnetic field measurement 
apparatus that has high sensitivity, high temperature stability and good 
mass-production performance can be manufactured by using the magneto-optic 
element. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
EXAMPLE 1 
FIG. 8 shows a magnetic field measurement of this invention, which 
comprises a magneto-optic transducer 100 that has a polarizer 2, an 
analyzer 3, and a magneto-optic element 1 made of Bi-substituted 
rare-earth iron garnet crystal which is represented by a general formula 
Bi.sub.x Gd.sub.y Y.sub.3-(x+y) Fe.sub.5 O.sub.12 
(1.0.ltoreq.x.ltoreq.1.4; 0.1.ltoreq.y.ltoreq.0.7) and placed between the 
polarizer and the analyzer which have polarization directions different 
from each other; 
light transmission paths 5 that are provided on both sides of the 
magneto-optic transducer 100; 
a light generating means 6 that supplies light to the light transmission 
path 5; 
a light detecting means 7 that detects the optical output power generated 
by the incident light that has passed through the magneto-optic transducer 
100 so as to convert the optical output power into electric signals; and 
an electric circuit 8 that processes the electric signals fed from the 
light detecting means 7, wherein the magneto-optic transducer 100 in a 
magnetic field is placed into the magnetic field to be measured to thereby 
determine the intensity of the magnetic field. The magneto-optic element 1 
is made by the epitaxial growth of Bi.sub.1.3 Gd.sub.0.43 Y.sub.1.27 
Fe.sub.5 O.sub.12 which is 90 .mu.m thick on a Ca-Mg-Zr substituted 
Gd.sub.3 Ga.sub.5 O.sub.12 substrate. The polarizer 2 is provided on an 
end of the magneto-optic element 1. The analyzer 3 is provided on another 
end of the magneto-optic element 1 in such a configuration so the 
direction of polarization is inclined by 45.degree. relative to the 
polarizer 2. For the polarizer 2 and the analyzer 3, a Glan-Thompson prism 
or a polarized beam splitter is used. A magneto-optic transducer 100, 
which comprises the magneto-optic element 1, the polarizer 2 and the 
analyzer 3, is placed in a magnetic field (H) to be measured. A lens 4 
collimates the light incident upon the magneto-optic transducer 100 or the 
light passed through the magneto-optic transducer 100. The light 
transmission path 5 is formed by an optical fiber. The light generating 
means 6 is constituted by a light emitting diode or laser diode that 
generates light with a 0.8 .mu.m or 1.3 .mu.m wavelength band. A light 
emitting diode which has a spectrum band with a peak at a wavelength 
.lambda. of 0.85 .mu.m is used in this example. The detecting means 7 
detects light which has passed through the element 1 and converts it to an 
electric signal. Although materials such as Ge-PD, Si-PIN-PD, etc., are 
usually used to make the detecting means 7. Si-PIN-PD is used in this 
example because a light emitting diode of 0.85 .mu.m wavelength was used 
herein. 
The intensity of a magnetic field, 150 Oe or less, was measured with such a 
magnetic field measurement apparatus, and measurement accuracy within 
.+-.1% was achieved over temperatures ranging from -20 to 80.degree. C. 
FIG. 9 shows the ambient temperature-dependence of the measurement error 
for magnetic field of 30 Oe generated by a constant alternate current. The 
variation is limited to .+-.0.5% with the change of the ambient 
temperature from -20.degree. to 80.degree. C. 
EXAMPLE 2 
FIG. 10 shows another magnetic field measurement apparatus of this 
invention, which comprises a magneto-optic transducer 9 that is composed 
of three magneto-optic transducer elements 9-a, 9-b, and 9-c each of which 
is the same as the magneto-optic transducer 100 (FIG. 8) of Example 1, 
light transmission paths 50 that are composed of three pair of light 
transmission path elements 5-a, 5-b, and 5-c, each pair of which are the 
same as the light transmission paths 5 (FIG. 8) of Example 1, a light 
generating means 60 that is composed of three light generating means 
elements 6-a, 6-b, and 6-c, each of which is the same as the light 
generating means 6 (FIG. 8) of Example 1, and a light detecting means 70 
that is composed of three light detecting means elements 7-a, 7-b, and 
7-c, each of which is the same as the light detecting means 7 (FIG. 8) of 
Example 1. This magnetic field measurement apparatus further comprises an 
arithmetic operation processing circuit 10, for the processing of electric 
signals from the light detecting means 70, which is used herein instead of 
the electric circuit 8 (FIG. 8) of Example 1. By the use of this 
apparatus, the addition and subtraction of the measurements of the 
intensities of three different magnetic fields Ha, Hb and Hc were obtained 
with great accuracy. 
As will be clear from the above description, the magnetic field measurement 
apparatus of this invention is capable of measuring a magnetic field 
intensity with high sensitivity and high accuracy independent of the 
changes in the ambient temperature, providing great advantages in 
industrial applications. 
It is understood that various other modifications will be apparent to and 
can be readily made by those skilled in the art without departing from the 
scope and spirit of this invention. Accordingly, it is not intended that 
the scope of the claims appended hereto be limited to the description as 
set forth herein, but rather that the claims be construed as encompassing 
all the features of patentable novelty that reside in the present 
invention, including all features that would be treated as equivalents 
thereof by those skilled in the art to which this invention pertains.