Optical magnetic field sensor

Abstract: An excellent optical magnetic field sensor which can precisely measure a magnetic field strength without measurement errors caused by temperature change is provided, which comprises a light source (1), a polarizer (4), a Faraday's element (5), an analyzer (6), a first light beam-receiving element (9), a first calculation circuit (10), a second light beam-receiving element (13), a second calculation circuit (14), and a third calculation circuit (15). A S polarized light beam from the analyzer is passed through the first light beam-receiving element and an output signal V₁₁ therefrom is calculated in the first calculation circuit. A P polarized light beam from the analyzer is passed through the second light beam-receiving element and an output signal therefrom V₂₂ is calculated in the second calculation circuit. The third calculation circuit provides an output V₃ from the equation V₃=1/(α/V₁₁+β/V₂₂) when V₁₁ and V₂₂ have positive values or the equation V₃=1/(α/V₁₁-β/V₂₂) when V₁₁ and V₂₂ have negative values where α and β are constants and α+β=1. The magnetic field strength is obtained from the output V₃ of the third calculation circuit.

Description: The present invention relates to an optical magnetic field sensor, particularly an optical magnetic field sensor which can measure a magnetic field strength with high precision using Faraday's effect. Heretofore, various structures of the optical magnetic field sensor have been known which uses Faraday's effect for measuring a magnetic field strength. As an example, JP-A-61-82,179 discloses a method of measuring a magnetic field strength capable of making the measurement with high precision wherein a light beam intensity ratio of two (P and S) polarized light beams passed through a Faraday element and emitted from an analyzer is used to correct errors resulting from light beam losses at the light beam transmission paths which are not relative to the Faraday's effect. In the technique of the above JP-A-61-82,179, a light beam intensity ratio a (=J 1 /J 2 ) of a P polarized light beam intensity J 1 to a S polarized light beam intensity J 2 emitted from an analyzer is preliminarily determined at the time when a magnetic field is not applied on the optical magnetic field sensor, and a treatment of regularization of signals at the time of receiving the signals when a magnetic field is applied on the optical magnetic field sensor is performed wherein the S polarized light beam intensity J 2 is multiplied by the above ratio a, so that the errors resulting from light beam losses at the light transmission paths can be corrected. However, the technique of JP-A-61-82,179 performs merely the treatment of regularization of signals of multiplicating J 2 by the above ratio a, so that signal errors resulting from temperature could not be corrected and a problem arose in that it cannot be used as a highly precise optical magnetic field sensor suitable for use at large temperature change. US-A-4973899 discloses a current sensor using the Faraday effect in which two electrical signals are derived from two mutually perpendicular components of the polarized light beam emitted by the magneto-optic element. The two signals are processed as follows. The first electrical signal is processed by: filtering an AC component of the first electrical signal to produce a first filtered output signal; filtering a DC component of the first electrical signal to produce a second filtered output signal; and calculating a ratio S 1 of the first and second filtered output signals. The second electrical signal is processed by: filtering an AC component of the second electrical signal to produce a third filtered output signal; filtering a DC component of the second electrical signal to produce a fourth filtered output signal; and calculating a ratio S 2 of the third and fourth filtered output signals. An AC component of the current is then calculated in accordance with the calculation: (2·S 1 ·S 2 )/(S 2 -S 1 ). A DC component of the current is then calculated in accordance with the calculation: (S 2 +S 1 )/(S 2 -S 1 ). In this way, the AC and DC components of the measured current are obtained. This procedure is stated to solve various problems in Faraday effect devices. An object of the present invention is to obviate the above problem so as to provide an optical magnetic sensor which can measure a magnetic field strength with high precision with reduced or zero measurement errors resulting from temperature change. The present invention is an optical magnetic field sensor as set out in claim 1. The optical magnetic field sensor of the present invention has the structure as mentioned above of adding the third calculation circuit to the structure of an optical magnetic field sensor which is substantially the same as the prior art, so that measurement errors caused by losses in the light beam paths are excluded by the regularization (normalization) of the signals, and errors caused by temperature change are largely or entirely excluded from the signals from the first and second calculation circuits by the calculation at the third calculation circuit. Therefore, a magnetic field strength can be measured with high precision. For a better understanding of the present invention, reference is made to the accompanying drawings, in which: Fig. 1 is a schematic block diagram showing a structure of an embodiment of the optical magnetic field sensor of the present invention; Fig. 2 is a schematic block diagram showing a structure of an optical magnetic field sensor of a comparative example; Figs. 3a, 3b and 3c are graphs for the present optical magnetic field sensor showing an example of temperature characteristics relative to V 11 output change ratio, V 22 output change ratio, and V 3 output change ratio, respectively. Figs. 4a, 4b and 4c are graphs for the present optical magnetic field sensor showing another example of temperature characteristics relative to V 11 output change ratio, V 22 output change ratio, and V 3 output change ratio, respectively; and Figs. 5a, 5b and 5c are graphs for the present optical magnetic field sensor showing still another example of temperature characteristics relative to V 11 output change ratio, V 22 output change ratio, and V 3 output change ratio, respectively. Hereinafter, the present invention will be explained in more detail, with reference to embodiments. Referring to Fig. 1 showing a structure of an embodiment of the present optical magnetic field sensor, reference numeral 1 is a light source made of LED, 2 is an optical fiber forming a light beam transmission path, 3 is a rod lens which causes distributed light beams to parallel light beams, 4 is a polarizer which makes a light beam to a linear polarized light beam, 5 is a Faraday's element made of Bi 12 SiO 20 which causes the linear polarized light beam to rotate to give Faraday's effect depending on a given magnetic field strength, and 6 is an analyzer which separates a given linear polarized light beam into a S polarized light beam and a P polarized light beam. In addition, reference numeral 7 is a first rod lens for condensing parallel light beams of the analyzer 6, 8 is a first optical fiber forming a light beam transmission path, 9 is a first light beam-receiving element made of a photodiode for converting the S polarized light beam into an electric signal depending the strength thereof, 10 is a first calculation circuit for calculating and multiplicating the converted electric signal to exclude measuremental errors and the like resulting from losses in the light beam path, 11 is a second rod lens for condensing parallel light beams of the P polarized light beams separated at the analyzer 6, 12 is a second optical fiber forming a light beam transmission path, 13 is a second light beam receiving element made of a photodiode for converting the P polarized light beam into an electric signal depending on the strength thereof, and 14 is a second calculation circuit for calculating and multiplicating the converted electric signal to exclude measuremental errors and the like resulting from losses in the light beam path. The multiplication ratio in the first calculation circuit 10 is equal to that in the second calculation circuit 14. The structure just described above is substantially the same as in the prior art optical magnetic field sensor, but a characteristic feature of the present invention is the provision of a third calculation circuit 15 which is supplied with the outputs from the first and second calculation circuits 10, 14 to perform a desired calculation so as to exclude measuremental errors resulting from temperature change. That is, the third calculation circuit 15 is supplied with the output V 11 from the first calculation circuit 10 and the output V 22 from the second calculation circuit 14, the outputs V 11 and V 22 being optionally reversed and multiplicated, to calculate and obtain the output V 3 based on the equation V 3 =1/(α/V 11 -β/V 22 ), wherein α and β are constants and α+β=1. Hereinafter, the principle of operation of the present optical magnetic field sensor of the aforementioned structure at the respective calculation circuit will be explained. When the wave length of the light beam emanated from the light source 1 is taken as 850 nm, the length ℓ of an element of a single crystal Bi 12 SiO 20 is taken as ℓ=4.02 mm, and the intensity of the incident light beam from the light source 1 is taken as I 0 , the intensities I 1 , I 2 of the output light beams at the first and second light beam-receiving elements 9, 13 are expressed by the following formula: I 1 = I 0 /2{ℓ-2Δ 0 ΔTℓ-2(V e +ΔV e ΔT)Hℓ} I 2 = I 0 /2{ℓ+2Δ 0 ΔTℓ+2(V e +ΔV e ΔT)Hℓ} wherein Δ 0 : Temperature change ratio of natural light rotational ability ΔT : Temperature variation from 25°C ℓ : Length of the single crystal element V e : Verdet's constant ΔV e : Verdet's constant temperature change ratio H : Alternate current magnetic field strength In the first and second calculation circuits 10, 14, the circuits are supplied with the electric signals V 1 , V 2 from the first and second light beam-receiving elements 9, 13, respectively, the electric signal V 1 and V 2 having respective direct current components DC1 and DC2, to obtain the outputs V 11 and V 22 of the following formula based on the equations of V 11 =(V 1 -DC1)/DC1 and V 22 =(V 2 -DC2)/DC2. V 11 = -2(V e +ΔV e ΔT)Hℓ/(1-2Δ 0 ΔTℓ) V 22 = 2(V e +ΔV e ΔT)Hℓ/(1+2Δ 0 ΔTℓ) In this case, if V 3 is defined as V 3 =1/(α/V 11 -β/V 22 ) and α+β=1, the following formula (5) can be obtained. V 3 = -2(V e +ΔV e ΔT)Hℓ/{1-2(2α-1)Δ 0 ΔTℓ} Therefore, in order that the output V 3 expressed by the formula (5) is not dependent on temperature, it is only necessary to fulfill the following formula (6). V 3 (ΔT) = V 3 (ΔT=0) Accordingly, a value α of α=(1-ΔV e /2V e Δ 0 ℓ)/2 is obtained to derive values of α=0.6805 and β=0.3195 in this embodiment. Three types of optical magnetic field sensor consisting of an embodiment of the present invention of the above structure, the comparative example of the structure as shown in Fig. 2 and a prior example of JP-A-61-82179 as explained above were actually prepared to obtain a relation between the magnetic field strength and the output, output change due to change of the amount of the light beam, and output change due to temperature change, respectively. As a result, the following data as shown in the following Table 1 were obtained to ascertain that the output V 3 can be measured in proportion with the alternate current magnetic field strength H in the embodiment of the present invention without depending on output changes due to change of the amount of the light beam or loss in the light transmission path and change of temperature as compared with the comparative example and the prior example. Moreover, in the embodiment of the present invention, the temperature of the sensor was varied in a range of -20°C through 80°C, and output change ratios relative to the output at 25°C were measured to evaluate the temperature characteristic property of the present sensor. The results are shown in Figs. 3a-3c. As seen from Figs. 3a-3c, it can be understood that the output V 3 of the third calculation circuit 15 which is a characteristic feature of the present invention was not at all influenced by temperature change, even when the outputs V 11 and V 22 of the first and second calculation circuits 9, 13 were influenced by the temperature change. Though the temperature characteristic property in the above embodiment is not always 0% at the values of the constants α=0.6805 and β=0.3195 due to errors in the element length and errors resulting from increasement of the effective element length caused by oblique bonding of the elements, however, even in such a case, the temperature characteristic property can be made 0%, if the constants α, β are deductively derived from actually measured temperature characteristic properties of V 11 and V 22 and the thus obtained optimum α, β are used to the outputs treatment or regularization of the signals at the third calculation circuit 15. For instance, in case when the element length ℓ is short resulting from an error or the like and the temperature characteristic properties of V 11 and V 22 are as shown in Figs. 4a and 4b, the temperature characteristic property of the output V 3 changes as shown in Fig. 4c, if the above values are treated in the third calculation circuit 15 using the values of α=0.6805 and β=0.3195. Then, from the outputs V 11 and V 22 at -20°C and the outputs V 11 and V 22 at 80°C, the constants α, β are deductively derived as follows, namely from the formula (7). α/V 11 (-20°C)+β/V 22 (-20°C)=α/V 11 (80°C)+β/V 22 (80°C) As a result, in the embodiment wherein the element length is shorter than normal one, values of α=0.7222 and β=0.2778 were obtained which gave the temperature characteristic property V 3 as shown in Fig. 5 by the outputs treatment, showing clearly that the outputs can be treated in the third calculation circuit 15 by using the deductively derived values of α, β. As clearly apparent from the foregoing explanations, according to the present invention, the present optical magnetic field sensor can achieve measurements of a magnetic field strength with high precision, because it has further the third calculation circuit in addition to the structure of the optical magnetic field sensor having substantially the same structure as that of conventional one thereby to exclude measuremental errors caused by temperature change from the outputs obtained from the first and second calculation circuits by virtue of the determined the outputs treatment at the third calculation circuit. Moreover, measuremental errors caused by change of the amount of the light beam emanated from the light source and losses in the light beam transmission paths can be excluded because it performs calculations for regularization of the outputs at the first and second calculation circuits in the same fashion as in conventional ones. Although the present invention has been explained with specific examples and numeral values, it is of course apparent to those skilled in the art that various changes and modifications are possible.

Claims: An optical magnetic field sensor, comprising a light source (1), a polarizer (4) which changes a light beam emanated from the light source through an optical fiber transmission path (2) into a linear polarized light beam, a Faraday's element (5) which gives a Faraday's effect to the linear polarized beam depending on an applied magnetic field, an analyzer (6) which separates the linear polarized light beam given with the Faraday's effect into a S polarized light beam and a P polarized light beam, a first light beam-receiving element (9) which receives and changes the S polarized light beam into an electric signal V 1 depending on the beam intensity, a second light beam receiving element (13) which receives and changes the P polarized light beam into an electric signal V 2 depending on the beam intensity, a first calculation circuit (10) which is supplied with the electric signal V 1 having a direct current component DC1 from the first light beam-receiving element to obtain an output V 11 based on an equation V 11 =(V 1 -DC1)/DC1, a second calculation circuit (14) which is supplied with the electric signal V 2 having a direct current component DC2 from the second light beam-receiving element to obtain an output V 22 based on an equation V 22 =(V 2 -DC2)/DC2, and a third calculation circuit (15) which is supplied with the outputs V 11 and V 22 to obtain an output V 3 based on the equation V 3 =1/(α/V 11 -β/V 22 ), wherein α and β are constants and α+β=1, the output V 3 of the third calculation circuit being a decisive value for obtaining a magnetic field strength, characterized in that the values of α,β in the third calculation circuit are deductively derived from temperature characteristic properties of actually measured V 11 and V 22 in order to compensate measurement errors caused by temperature changes.

Inventors: NODA MASAKI, OKAJIMA HISAKAZU, NODA, MASAKI, OKAJIMA, HISAKAZU
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1993
Main classification: G01R33
IPC codes: G01R33