Magneto-optical element and magnetic field measurement apparatus

Abstract: The present invention relates to a magneto-optical element which is made of a bismuth-substituted rare earth iron garnet crystal of a formula (Bi X Gd Y R Z Y 3-X-Y-Z ) (Fe 5-W Ga W )O₁₂, wherein R designates an element selected among rare earth elements, wherein 0.8 ≦ X ≦ 1.3 , 0.1 ≦ Y ≦ 0.3 , 0.1 ≦ Z ≦ 1.0 and 0.3 ≦ W ≦ 0.8 . The magneto-optical element has a constant sensitivity less than ±4.0 % between -50 and +100 °C around room temperature. Further, a magnetic field measurement apparatus comprises the magneto-optical element and a light source of a central wavelength in the 0.8 µm band. A magnetic field measurement apparatus with use of the magneto-optical element is stable against temperature change for practical use.

Description: The present invention relates to a magneto-optical element which makes use of the Faraday effect and to a magnetic field measurement apparatus for detecting and measuring a magnetic field intensity with use of the magneto-optical element. Recently, a magnetic field measurement apparatus which combines a magneto-optical element of the Faraday effect with optical fibers is proposed in order to measure a magnetic field intensity optically (K. Kyuma et al., IEEE Journal of Quantum Electronics, Vol. QE-18 No. 10, 1619 (1982)). In such a magnetic field measurement apparatus as mentioned above, an electrical current is detected by measuring the magnetic field intensity generated around an electrical conductor through which the electrical current flows. Because light is used for transmission, this method is advantageous in that electromagnetic induction noise is not detected and electrical insulation is good. Therefore, the method is applicable to a transmission and distribution equipment of electric power. Fig. 1 shows the principle of a method of measuring a magnetic field by using the Faraday effect. In Fig. 1, a magneto-optical element 1 is placed in a magnetic field H. A light which has been polarized linearly by a polarizer 2 is transmitted through the magneto-optical element 1. The polarization plane is subjected to rotation by angle  in proportion to the magnetic field intensity H according to the Faraday effect. The linearly polarized light which has been subjected to rotation is transmitted to an analyzer 3 which changes the polarization direction of transmitted polarized light by 45°, and the magnitude of the Faraday rotation angle  is converted to a change in light intensity. The optical output P out in this measurement is given by the following Equations (1) and (2): P out = K (1 + sin 2), and  = CHL, wherein K is a proportional constant, L is the length (cm) of a crystal of the magneto-optical element 1 (refer Fig. 1), the Faraday rotation angle  is expressed in unit of degree and C is a sensitivity constant in unit of degree/cm·Oe (1 Oe = 10 3 / 4π . A / m which expresses the sensitivity of the magneto-optical element 1. It is proposed to arrange a plurality of magnetic field measurement apparatus with use of the above-mentioned principle at points along a transmission and distribution line to send their electrical outputs of the apparatuses to a computer wherein for example the zero phase current in the transmission and distribution line is detected with use of a reference signal obtained by adding or subtracting the waveforms of the electrical outputs and the occurrence of accidents in the transmission and distribution line is judged. As a magneto-optical sensor which can be used in such a magnetic field measurement element, a rare earth iron garnet crystal is disclosed which can be expressed by the following chemical formula (3), wherein 1.0 ≤ X ≤ 1.4 and 0.1 ≤ Y ≤ 0.7 (USP 5,075,546): Bi X Gd Y Y 3-(X+Y) Fe 5 O 12 . In the prior art rare earth iron garnet crystal, yttrium is substituted partially by bismuth in order to increase the Faraday rotation angle  or the sensitivity C, while yttrium is substituted partially by gadolinium in order to improve the temperature characteristics. As shown in Fig. 2, the temperature dependence of the sensitivity constant C of the prior art crystals is within ±2.5 % in a temperature range between -20 °C and +80 °C. However, a magnetic field measurement apparatus constructed by use of which prior art magneto-optic elements has problems practically with the temperature characteristics, the sensitivity and the precision of measurement. At present, no magneto-optical element of bismuth substituted rare earth iron garnet crystal can be used practically for a magnetic field measuring apparatus due to bad temperature characteristics. Further, if a light source of 0.8 µm band is used for a prior art bismuth substituted rare earth iron garnet crystal, the output light intensity of the magnetic measurement apparatus becomes small due to the loss of optical absorption in a crystal, and the SN ratio decreases to worsen the precision. Therefore, if a magneto-optical element which has a smaller loss by optical absorption than the prior art bismuth substitution rare earth iron garnet crystal is provided, the SN ratio can be improved and the measurement precision can be improved. Further, if a short wavelength light source of the 0.8 µm band is used, the Faraday rotation angle  becomes larger than in the case of the 1.3 µm band, so that the sensitivity constant C of the crystal increases. If the sensitivity constant C increases, the thickness of an element to be used can be decreased, and the time required for crystal growth can be shortened with use of a liquid phase epitaxy technique or a gas phase crystal growth technique. EP-A-0 186 528 discloses a magnetic material having the formula Gd 2-x1-x2 M x1 Pr x2 Bi 1 Fe 5-y1-y2 Ga y1 Al y2 O 12 wherein M may yttrium and wherein 0 < x1 ≤ 1,5, 0 < x2 ≤ 0,5, 0 ≤ y1 ≤ 1 and 0 ≤ y2 ≤ 1. The description discusses the magnetic properties of this material, noting that the presence of yttrium allows to diminish the compensation temperature beneath the ambient temperature while the presence of gallium allows to adjust magnetization and finally the presence of praseodymium allows to adjust the magnitude of magnetization and the magnitude of magnetic anisotropy. Praseodymium can adjust the magnetic field of uniaxial magnetic anisotropy at a value of between 0 and 2 x 10 5 Am -1 . EP-A-0 086 387 discloses a measuring instrument of magnetic field utilizing Faraday rotation in which a medium exhibiting Faraday rotation includes at least one thin magnetic garnet film magnetized in the propagation direction of light and having the composition of R 3-x Q x (Fe 5-y M y )O 12 . R may be Y, Q indicates at least one element selected from the group consisting of Gd, Eu, Er, Tm, Tb, Yb, Ho and Dy, while M may be Ga. It is the object of the present invention to avoid the above disadvantages of prior art magneto-optical elements and to provide a bismuth substituted rare earth ion garnet crystal having a high sensitivity constant with low temperature dependence of the sensitivity constant. An aspect of this object is to provide a novel magnetic field measurement apparatus. This object is obtained by the magneto-optical element as defined in claim 1 and by the magnetic field measuring instrument as defined in claim 10. Suitably and preferred embodiments thereof are described in the respective subclaims. The present invention solves the above-mentioned problems and provide a magneto-optical element made of a bismuth-substituted rare earth iron garnet of a chemical formula (Bi X Gd Y R Z Y 3-X-Y-Z ) (Fe 5-W Ga W )O 12 , wherein R designates an element selected among rare earth elements, 0.8 ≤ X ≤ 1.3, 0.1 ≤ Y ≤ 0.3, 0.1 ≤ Z ≤ 1.0 and 0.3 ≤ W ≤ 0.8. Further, the present invention provides a magnetic field measurement apparatus for detecting a change in magnetic field intensity wherein a magneto-optical transducer with use of the magneto-optical element is provided. An advantage of the present invention is that the magneto-optical element has a temperature dependence of the sensitivity constant of less than ±4.0 % in the temperature range between -50 and +100 °C and that the magnetic field measurement apparatus with use of the magneto-optical element is stable in the temperature range for practical use. These and other objects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, and in which: Fig. 1 is a diagram for showing the principle of magnetic field measurement with use of the Faraday effect; Fig. 2 is a graph of the temperature dependence of sensitivity constant of prior art (BiGdY)IG crystals; Fig. 3 is a graph of the temperature dependence of the sensitivity constant of magneto-optical elements according to the present invention; Fig. 4 is a graph of the temperature dependence of the sensitivity constant of a magneto-optical element which has the smallest temperature change of sensitivity constant among magneto-optical elements; Fig. 5 is a graph of comparison data on the temperature dependence of the sensitivity constant of prior art (BiGdY)IG crystals plotted against Gd substitution amount; Fig. 6 is a graph of comparison data on the temperature dependence of the sensitivity constant of prior art (BiGdY)IG crystals plotted against Gd substitution amount; Fig. 7 is a graph of comparison data on the temperature dependence of the sensitivity constant of a bismuth substituted rare earth iron garnet crystal which has the smallest temperature dependence among prior art (BiGdY)IG crystals; Fig. 8 is a diagram for illustrating the Faraday effect of bismuth substituted rare earth iron garnet crystals; Fig. 9 is a schematic diagram of an embodiment of a magnetic field measurement apparatus with use of a magneto-optical element according to the present invention; and Fig. 10 is a graph of transmission spectra of a magneto-optical element according to the present invention. An embodiment of the present invention will be explained below with reference to the drawings. Bismuth substituted rare earth iron garnet crystal of a chemical formula (Bi X Gd Y R Z Y 3-X-Y-Z ) (Fe 5-W Ga W )O 12 , wherein R designates an element selected among rare earth elements, are grown on a Ca-Mg-Zr substituted Gd 3 Ga 5 O 12 single crystal substrate with a liquid phase epitaxy technique with use of Bi 2 O 3 -PbO-B 2 O 3 flux. Table 1 shows examples, wherein the numerical values represent the composition ratios X, Y, Z and W for each element. The temperature dependence of the sensitivity constant C (Eq. (2)) of a crystal is measured for the prepared samples in a temperature range between -50 - +100 °C which is wider than ordinary temperature ranges of between -20 - +80 °C. The results are compiled to show the temperature characteristics. The samples for which a # mark is added in the column of the temperature dependence are outside the scope of the present invention. In order to decrease the temperature dependence of the sensitivity constant C and the loss by optical absorption of bismuth substituted rare earth iron garnet crystals especially when a 0.85 µm light source is used, iron is substituted partially with gallium. On the other hand, the substitution amount X, Y of bismuth and gadolinium for yttrium is controlled against the substitution amount W of iron with gallium. Further, in order to improve the lattice matching of the single crystal films grown on a substrate, yttrium is substituted with lanthanum which has an ionic radius closest to that of bismuth. Thus, a magneto-optical element of better crystallinity for magnetic field measurement apparatus can be obtained. Though lanthanum is used as an element R for crystal matching in the Examples, one or more of rare earth elements can be used as an element R in the chemical formula. In such substitution, it is better that the substitution element is a nonmagnetic element which has no effect on the saturation magnetization of bismuth substituted rare earth iron garnet crystal. Further, when a substrate of the lattice constant different from that of Ca-Mg-Zr substituted Gd 3 Ga 5 O 12 crystal substrate is used, a bismuth substituted rare earth iron garnet crystal of good temperature characteristic can be grown by substituting with one or more of rare earth elements as R in the chemical formula (4) for crystal matching. Fig. 3 displays data on temperature dependence measured with use of 0.85 µm light source for samples designated with marks * in Table 1, wherein the data is normalized with the sensitivity constant at room temperature. Among the samples, No. 9 sample (refer Table 1) of (Bi 1.3 Gd 0.1 R 0.1 Y 1.5 ) (Fe 4.4 Ga 0.6 )O 12 has the best temperature characteristic of temperature change of ±0.4 %, as shown in Fig. 4, which is not realized previously. Composition of rare earth iron garnet crystal samples sample No. Bi (X) Gd (Y) La (Z) Y (3X-Y-Z) Fe (5-W) Ga (W) temperature dependence of sensitivity constant C 1 0.5 - 1.1 1.4 4.3 0.7 # ±4.5 % 2 0.6 - 1.1 1.3 4.6 0.4 # ±4.3 % * 3 0.6 - 1.1 1.3 4.3 0.7 # ±4.0 % 4 0.8 0.2 0.9 1.1 4.7 0.3 ±1.2 % * 5 0.8 0.2 0.9 1.1 4.5 0.5 ±1.0 % * 6 0.8 0.2 0.9 1.1 4.3 0.7 ±0.8 % 7 1.0 0.2 0.8 1.0 4.4 0.6 ±0.7 % 8 1.3 0.1 0.1 1.5 4.5 0.5 ±0.6 % * 9 1.3 0.1 0.1 1.5 4.4 0.6 ±0.4 % 10 1.3 0.1 0.1 1.5 4.2 0.8 ±0.6 % * 11 0.8 0.3 1.0 0.9 4.3 0.7 ±3.5 % 12 0.8 0.4 0.9 0.9 4.3 0.7 # ±4.0 % * 13 0.8 0.5 0.9 0.8 4.3 0.7 # ±4.5 % A bismuth substituted rare earth iron garnet for a magneto-optical element according to the present invention is represented by the above-mentioned chemical formula, (Bi X Gd Y R Z Y 3-X-Y-Z ) (Fe 5-W Ga W )O 12 , wherein R designates an element selected among the rare earth elements and lanthanum is used in the embodiment as displayed in Table 1. A crystal according to the present invention has a temperature dependence of the sensitivity constant C less than ±4 % in a temperature range between -50 and +100 °C. As shown in the data of the samples without # mark compiled in Table 1, it is preferable that the composition designated by X, Y, Z and W satisfy the following relations: 0.8 ≤ X ≤ 1.3, 0.1 ≤ Y ≤ 0.3, 0.1 ≤ Z ≤ 1.0 and 0.3 ≤ W ≤ 0.8. Further, it is more preferable that the temperature dependence of the sensitivity constant C is less than ±1.2 % in a temperature range between -50 and +100 °C. Therefore, it is more preferable that the composition designated by X, Y, Z and W satisfy the following relations: 0.8 ≤ X ≤ 1.3, 0.1 ≤ Y ≤ 0.2, 0.1 ≤ Z ≤ 0.9 and 0.3 ≤ W ≤ 0.8. Further, it is still more preferable that the temperature dependence is less than ±0.6 % in a temperature range between -50 and +100 °C. Therefore, it is still more preferable that the composition designated by X, Y, Z and W satisfy the following relations: X = 1.3, Y =≤ 0.1, Z = 0.1 and 0.5 ≤ W ≤ 0.8. The best composition is (Bi 1.3 Gd 0.1 R 0.1 Y 1.5 ) (Fe 4.4 Ga 0.6 )O 12 (No. 9 sample). For comparison, Fig. 5 shows data on temperature measured with use of 0.85 µm light source for prior art samples of Bi X Gd Y Y 3-(X+Y) Fe 5 O 12 (3) wherein 1.0 ≤ X ≤ 1.4, 0.1 ≤ Y ≤ 0.7. Fig. 6 displays the temperature dependence of the sensitivity constant against Gd substitution amount obtained from the data of Fig. 5. As shown in Fig. 6, the temperature dependence of prior art crystals is ±7.0 % or less in a temperature range between -50 and +100 °C. On the contrary, the temperature range to be used practically is broaden by 50 °C for a magneto-optical element of the present invention compared with that of the prior art element. That is, the temperature dependence of the magnet-optical element of the present invention becomes more flat. That is, the temperature characteristics of the best data are improved largely from ±1.0 % or less to less than ±0.4 % in the temperature range between -50 and +100 °C if compared with the best prior art data of Fig. 7, and the sensitivity constant C of the best crystal nearly changes against the temperature in the temperature range. The improvement is found not only for a 0.85 µm light source but also for a light source of other wavelengths which transmits a garnet crystal. Further, though the above-mentioned data relate to crystals grown on a Ca-Mg-Zr substituted Gd 3 Ga 5 O 12 substrate, similar results are obtained for crystals grown on a Nd 3 Ga 5 O 12 substrate in various crystal growth conditions. Further, it is possible to grow a crystal epitaxially on the substrates with a gas phase crystal growth technique. In any of the above-mentioned crystal growth techniques, in some conditions, a magneto-optical element made of a bismuth-substituted rare earth iron garnet crystal of the chemical formula (4) of 0.8 ≤ X ≤ 1.3, 0.1 ≤ Y ≤ 0.3, 0.1 ≤ Z ≤ 1.0 and 0.3 ≤ W ≤ 0.8 may not be grown epitaxially, and a polycrystalline or a sintered sample is grown. Even such a polycrystalline or sintered magneto-optical element can be used practically though the optical absorption loss increases a little. A basic theory for obtaining good temperature characteristics is as follows: A rare earth iron garnet is a ferrimagnet, and the Faraday effect saturates at a certain magnetic field, as shown in Fig. 8. As to magnetic field measurement, a part which changes linearly against external magnetic field is used. In this case, the rotation angle e is expressed by the following Equation (5):  =  F (H/M s )L, wherein  F is saturation Faraday rotation, M s is the saturation magnetization and L is a crystal length. Therefore, the sensitivity constant C and its temperature dependence is defined by the following Equation (6): C(T) =  F (T)/M s (T). As shown in Eq. (6), the temperature dependence of the sensitivity constant C is determined by the temperature changes of  F and M s . The rate of temperature dependence of M s (T) agrees more with  F (T) for a rare earth iron garnet crystal according to the present invention, wherein gallium is used for substitution of iron and the substitution amounts X, Y of bismuth and gadolinium of yttrium are controlled. To sum up, the Faraday rotation angle e of a bismuth substitution rare earth iron garnet becomes larger by about one digit if compared with a prior art rare earth iron garnet without bismuth substitution. Therefore, the sensitivity of magneto-optical element can be increased by substituting yttrium with bismuth as much as possible. Further, by substituting yttrium with gadolinium, temperature compensation due to the substitution of yttrium with bismuth is performed in order to improve the temperature characteristics. On the other hand, iron is substituted with gallium in order to decrease the optical absorption loss of the crystal in the 0.8 µm band. On the contrary, in a prior art rare earth iron garnet crystal Bi X Gd Y Y 3-(X+Y) Fe 5 O 12 (3), iron is not substituted with gallium. Therefore, the temperature of sensitivity constant C becomes more constant for the present invention and the temperature characteristic is improved. Still further, yttrium is substituted with at least one rare earth element in order to improve the crystal matching against a substrate. Thus, the composition of each element is controlled to have good temperature characteristics. Next, Table 2 shows the data on the absorption coefficient and on the sensitivity constant of rare earth iron garnet crystals as well as those of prior art crystals, wherein K is the absorption coefficient. λ=0.8 µm band λ=1.3 µm band prior art (Bi GdY)IG K=250 cm -1 C=5.0 deg/cm·Oe K=2.5 cm -1 C=1.4 deg/cm·Oe this invention (BiGdLaY) (FeGa)G K=200 cm -1 C=11.0 deg/cm·Oe K=2.5 cm -1 C=2.4 deg/cm·Oe The data of Table 2 shows that the absorption coefficient K especially in the 0.8 µm band becomes small by the gallium substitution and that the sensitivity constant C increases according to Eq. (6) because the saturation magnetization M s decreases due to the substitution of iron with nonmagnetic ion. In a prior art rare earth iron garnet crystal of chemical formula (3) wherein iron is not substituted with gallium, about 100 µm is necessary in order to use it for a magneto-optical element. On the contrary, the film thickness can be decreased to about a half for a magneto-optical element of the present invention, and the growth time can be shortened and the productivity of crystal can be improved. Further, the optical absorption loss as a magneto-optical element is decreased because the film thickness is decreased. In a prior art rare earth iron garnet crystal of chemical formula (3) wherein iron is not substituted with gallium, the output light intensity is small and the measurement precision is bad when a light source of 0.8 µm band is used. On the contrary, the SN ratio increases and the precision of measurement is improved for a magnetic field measurement apparatus with use of a magneto-optical element according to the present invention. Further, a light-emitting element and a photosensitive element of short wave 0.8 µm band can be used which can be manufactured at a lower cost than those of longer wavelength 1.3 µm band, and a magnetic field measurement apparatus can be provided at a lower cost. Fig. 9 shows an embodiment of magnetic field measurement apparatus with use of a magneto-optical element mentioned above. A magneto-optical element 1 is a No. 9 sample of (Bi 1.3 Gd 0.1 R 0.1 Y 1.5 ) (Fe 4.4 Ga 0.6 )O 12 (refer Table 1) of thickness 43.9 µm. The numerical number 2 designates a polarizer provided near an end face of the magneto-optical element 2. The numerical number 3 designates a analyzer provided near another end face of the magneto-optical element so as to make the polarization direction of transmitted light tilted by 45° against the polarizer 2. The polarizer 2 and the analyzer 3 are made of a polarization plate of glass for a making their sizes small. A magneto-optical transducer composed of the magneto-optical element 1, the polarizer 2 and the analyzer 3 is arranged in a magnetic field (H) to be measured or in a gap of metallic core which is used to intensify the measurement magnetic field. Especially, the magneto-optical transducer including the polarizer 2 and the analyzer 3 made of glass has a compact size, so that the gap of the metallic core can be narrowed. Then, the sensitivity of the magnetic field measurement apparatus can be improved. The reference numeral 4 designates a lens which makes an incident light to the magneto-optical transducer or a transmission light from the magneto-optical transducer collimate. That is, the lens 4 provides a collimated light. The reference numeral 5 designates an optical fiber which forms an optical transmission line. The reference numeral 6 designates an optical signal generator, which is made of a light emitting diode or a laser diode of 0.8 µm band or of 1.3 µm band. Fig. 10 displays a graph of optical transmission spectra of No. 9 sample of (Bi 1.3 Gd 0.1 R 0.1 Y 1.5 ) (Fe 4.4 Ga 0.6 )O 12 . If a bismuth substituted rare earth iron garnet crystal according to the present invention is used for a magnetic field measurement apparatus as shown in Fig. 10, the transmission rate is required to be 35.0 % or more. If a light source of central wavelength of 0.8 µm band is used, the Faraday rotation angle  increases four times compared in case of a light source of 1.3 µm band. Therefore, it is found necessary that a light source of central wavelength of 0.78 to 0.90 µm, as displayed in Fig. 10 as a hatched area, is necessary for a magnetic field measurement apparatus. Then, in this embodiment, a light emitting diode of peak wavelength of µ=0.85 µm is used. The reference numeral 7 designates a detection means for detecting light after the transmission of the element 1 to convert to an electrical signal, such as Ge photodiode, Si PIN photodiode or the like. In this embodiment, a Si PIN photodiode is used because a light emitting diode of 0.85 µm. The reference numeral 8 designates a signal processor. If magnetic field is measured with this apparatus, magnetic field of 120 Oe or less is measured in a temperature range between -50 and +100 °C at a high precision of a change of measurement sensitivity of ±0.5 % or less which is never realized previously. It is also possible to use the polarizer 2 and the analyzer 3 of Gran-Thompson prism or of polarized beam splitter instead of glass polarization plate. As explained above, magnetic field intensity can be measured in a wider temperature range never realized before, without affected by temperature range around room temperature, with a good sensitivity and with a high precision, with use of a magneto-optical element and a magnetic field measurement apparatus with use of the present invention according to the present invention. Such an element and an apparatus can be manufactured at a low cost, and this is valuable for a practical use. Though the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.

Claims: A magneto-optical element made of a bismuth substituted rare earth iron garnet crystal of the chemical formula (Bi X Gd Y R Z Y 3-X-Y-Z )(Fe 5-W Ga W )O 12 , wherein R designates a rare earth element, and wherein 0.8 ≤ X ≤ 1.3, 0.1 ≤ Y ≤ 0.3, 0.1 ≤ Z ≤ 1.0 and 0.3 ≤ W ≤ 0.8. The magneto-optical element of claim 1, wherein 0.1 ≤ Y ≤ 0.2, 0.1 ≤ Z ≤ 0.9. The magneto-optical element of claim 1, wherein X = 1.3, Y = 0.1, Z = 0.1 and 0.5 ≤ W ≤ 0.8. The magneto-optical element of claim 1, wherein the crystal has the composition (Bi 1,3 Gd 0,1 R 0,1 Y 1,5 ) (Fe 4,4 Ga 0,6 )O 12 . The magneto-optical element of anyone of the preceding claims, wherein said element R is lanthanum. The magneto-optical element of anyone of the preceding claims, wherein said crystal is a single crystal. The magneto-optical element of anyone of claims 1-5, wherein said crystal is polycrystalline or a sintered material. The magneto-optical element of anyone of the preceding claims, wherein said crystal has been grown epitaxially on a single crystal garnet substrate. The magneto-optical element of claim 8, wherein said garnet crystal substrate is a Ca-Mg-Zr substituted Gd 3 Ga 5 O 12 substrate or a Nd 3 Gd 5 O 12 substrate. A magnetic field measuring instrument, comprising: (a) a magneto-optical transducer including a polarizer (2), an analyzer (3) and a magneto-optical element (1) placed between the polarizer (2) and the analyzer (3), wherein the magneto-optical element (1) is made of a rare earth garnet iron crystal and the analyzer (3) is provided so as to make the direction of transmitted light through the magneto-optical element (1) different from that of the polarizer (2); (b) a first optical transmission line (5, 4) connected via the polarizer (2) at one end of the magneto-optical element (1) of the magneto-optical transducer; (c) a light source for generating light to the first optical transmission line (5, 4); (d) a second optical transmission line (4, 5) connected via the analyzer (3) at the other end of the magneto-optical element (1) ) of the magneto-optical transducer; (e) an optical sensor for detecting the optical output from the second optical transmission line (4, 5) and converting it into an electrical signal; and (f) a signal processor for processing the electrical output signal from the optical sensor; said instrument being adapted to measure a magnetic field by placing the magneto-optical element (1) in said magnetic field and by detecting an intensity change of the output light with the signal processor when the incident light passes through the magneto-optical transducer. characterized in that, said rare earth iron garnet crystal is a bismuth substituted rare earth iron garnet crystal of the chemical formula of one of the preceding claims. The magnetic field measurement apparatus of claim 10, characterized in that said light source generates light having a peak wavelength between 0.78 and 0.90 µm.

Inventors: ITOH NOBUKI, YAMADA HIROKAZU, ITOH, NOBUKI, YAMADA, HIROKAZU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1992
Main classification: G01R33
IPC codes: H01F10, H01F1, C30B19, G02F1, G01R33