DE10145764C1 - Magnetic field measuring method using Faraday effect uses linearly polarized light signals passed through magnetic field sensitive measuring path in opposite directions - Google Patents

Abstract

In the method and the arrangement for measuring a magnetic field with the aid of the Faraday effect, a first and a second linearly polarized light signal (L1 or L2) pass through a magnetic field-sensitive measuring section (2) showing the Faraday effect in the opposite direction. The measuring section (2) is followed by a first or second analyzer (14 or 12) for the first or second light signal (L1 or L2). First and second analyzer angles (PSI¶1¶ and PSI¶2¶) of the first and second analyzers (14 and 12) and first and second polarization angles (PHI¶1¶ and PHI¶2¶) of the first and second second light signal (L1 or L2) coupled into the measuring section (2) are set to the intrinsic axes (x, y) of the linear birefringence in such a way that an error occurring due to the temperature dependence of the linear double calculation within the measuring section (2) for a given and non-zero Faraday angle (rho) is minimized.

Description

The invention relates to a method and an arrangement for measuring a magnetic field using the Faraday Effect, as for example from DE 44 46 425 A1 are known.

In the known method in a Faraday-Sen designated magnetic field sensitive measuring section a first and a second each linearly polarized Coupled light signal in such a way that the measuring section in one in other opposite directions and the for linear polarization of the first or second light signal used first and second polarizer at the same time as Analyzer of the second one decoupled from the measuring section or first light signal is used. By forming a measuring sig nals from the quotient of the difference and the sum of the light intensities belonging to both light signals the damping factors of the respective transmission links from Light transmitters up to the light receivers eliminated.

To compensate for a temperature dependency of the measurement signal essentially caused by the linear voltage birefringence, the polarization angles Φ ₁ , Φ _{2 of} the first or second polarizer are set in relation to a natural axis of the linear birefringence such that the relationship cos (2Φ ₁ + 2Φ ₂ ) = - 2/3 is at least approximately fulfilled. This measure ensures that the measurement signal is largely independent of temperature.

When setting the polarization angle after the first one However, the formula mentioned is such an extensive one Temperature compensation only guaranteed for Faraday angle ρ = 0 provides. In other words: for clearly different from 0  Faraday angle ρ, that is, in the actually relevant Be drive or modulation range according to this measuring method operated arrangement, the measurement signal is again through cause the temperature dependence of the linear birefringence gentle temperature sensitivity imprinted.

The invention is based on the object of a method for measuring a magnetic field using the Faraday effect specify whose temperature sensitivity in each inte relevant control or operating range is minimized. Au Furthermore, the invention is based on the task of an arrangement to specify the implementation of the procedure.

The first object is achieved according to the invention with a method with the features of claim 1.

In the method of measuring a magnetic field using the Faraday effects go through a first and a second line ar polarized light signal showing the Faraday effect magnetic field sensitive measuring path in opposite directions set direction. The measurement section is a first and a second Analyzer for the first or second light signal afterwards net, the first or second polarization angle of the first ten or second light signal coupled into the measuring section nals and the first and second analyzer angles to the Natural axes of the linear birefringence are set, that a due to the temperature dependence of the linear dop break occurring within the measuring path for a predetermined and non-zero Faraday angle is minimized.

The invention is based on the consideration that the for such a measurement method absolutely permissible errors per is based on the so-called nominal value. This As a rule, the nominal value coincides with the upper end of the measuring range rich match, so that with measured values in the upper measuring range compliance with the required fault tolerances in particular  is critical. An allowable error of 0.5% corresponds to be moved to a nominal value of, for example, 100 an absolute th error of 0.5, so that in the measuring range from 10 to 1 rela tive errors of 5% to 50% are permissible. In other words: With measured values near zero (Faraday angle ρ = 0, magnetic field free fall) is the allowed percentage error very high. To ensure that the measuring method in Ar the required error tolerance satchel, it is therefore unfavorable to compensate for the error-dependent temperature dependence of the linear dop refraction the setting of the polarization angle for the to optimize the magnetic field-free fall (Faraday angle ρ = 0).

According to the invention it is therefore provided that the optimal com compensation of those caused by linear birefringence Temperature sensitivity for the measuring range, which corresponds to either the working range or the nominal value, to ensure that the required tolerances are also a can be held.

The second task is solved with the combination of features nation of claim 4, the advantages of which are analogous from the advantages of the inventions already explained above method according to the invention.

Preferred embodiments of the method according to the invention and the device according to the invention result from the respective subclaims.

To further explain the invention, reference is made to the drawing directed. Show it:

Fig. 1 shows an arrangement according to the invention in a schemati 'principle representation,

FIGS. 2a and 2b, the orientation of the first and second polarizer relative to the eigen axes of the magnetic-field-sensitive measuring section in each case based on a diagram,

FIGS. 3 and 4 are each a graphical representation in which the error in each case angle for a particular polarization and a particular angle resulting Faraday are shown for polarizer arrangements in which the polarization angle of + or -45 ° distinguish

Fig. 5 is a table in which the optimal polarization angle is given for a given linear birefringence for different Faraday angles (working areas).

According to Fig. 1 are in a magnetic field sensitive Messstre bridge 2, a first and a second light signal L1 and L2, a coupled so that they pass through the measuring section 2 in each entge gengesetzte directions. For this purpose, a light signal L generated by a light source 4 is divided into a first and a second transmission path 8 or 10 by means of an optical coupler 6 . In the first transmission path 8 , a first polarizer 12 is arranged, which linearly polarizes the light signal L1 transmitted into it by the optical coupler 6 . In the same way, a second polarizer 14 is arranged in the second transmission path 10 , which serves for the linear polarization of the second light signal L2.

In the exemplary embodiment, the magnetic field-sensitive measuring section 2 is a fiber coil which is wound, for example, around a current-carrying conductor, so that the magnetic field generated by the current is oriented essentially parallel to the fiber axis.

The first polarizer 12 also serves as a second analyzer for the second light signal L2 emerging from the magnetically sensitive measuring section 2 . In the same way, the second polarizer 14 also serves as the first analyzer for the first light signal L1 emerging from the magnetic field-sensitive measuring section 2 .

Alternatively, it is also possible between measurements stretch and polarizer to arrange a beam splitter and the exiting light signals to supply analyzers that space Lich arranged separately from the polarizers.

The first and second light signals L1, L2 are coupled out behind the second and first polarizers 14 , 12 from the second and first transmission links 10 , 8 and a first by means of the respective decouplers 16 arranged in the first and second transmission links 8 , 10 or second light receiver 18 or 20 supplied with which the intensity of the first light signal L1 emerging from the second polarizer 14 or of the second polarizer 12 emerging second light signal L2 are measured. The corresponding intensity signals I1 and I2 are fed for further processing and analysis to an evaluation device 22 , in which, for example to compensate for the transmission properties of the transmission links 8 , 10, the quotient (I1 - I2) / (I1 + I2) from the difference and the sum of the intensity signals I1, I2 and is used as the measurement signal P for the magnetic field strength or for the flowing current. In principle, however, it is also possible to form the measurement signal in a different way from the intensity signals I1, I2, as is disclosed in more detail in the aforementioned DE 44 46 425 A1.

Referring to FIG. 2a and 2b are determining the orientation of the polarization angle Φ _1, Φ ₂ of the light signals L1 and L2 of the first polarization angle Φ _1, ie the angular position of the first polarizer 12, and the second polarization angle Φ _2, the angular position that the second polarizer 14 , ge against the mutually orthogonal natural axes x, y of the linear birefringence of the magnetic field-sensitive measuring section. These natural axes are defined through the winding axis when using a fiber coil as a magnetic field sensitive measuring section. In this case, the eigen axes are oriented perpendicular to the longitudinal axis of the fiber and parallel or perpendicular to the winding axis.

The first polarizer 12 is also an analyzer (hereinafter referred to as the second analyzer) for the second light signal L2 coupled out from the magnetic field-sensitive measuring section 2 . Its orientation is shown in dashed lines in Fig. 2b. The second analyzer angle Ψ ₂ belonging to the second analyzer for the second light signal L2 is due to the opposite light path from the first polarization angle Φ ₁ by the relationship Ψ ₂ = Π - Φ ₁ . This applies in an analogous manner to the second polarizer 14 serving as the analyzer (hereinafter referred to as the first analyzer) for the first light signal L1, the orientation of which is entered in FIG. 2a and the first analyzer angle die ₁ for the above reasons the relationship Ψ ₁ = Π - Φ ₂ he fills. In addition, the first and second polarization angles Φ ₁ and Φ ₂ each differ from the first and second th analyzer angles Ψ ₁ , Ψ ₂ by 45 °, ie Ψ ₁ - Φ ₁ = Ψ ₂ - Φ ₂ = + / - π / 4 , In other words, the polarizer and analyzer are at 45 ° to each other for each light path.

In the graph of FIG. 3 is the relative error R Darge provides as ρ for different Faraday angle and polarization angle Φ is obtained for the case is that the first and second polarization angle Φ _1, Φ ₂ of the first and second Analysatorwinkeln Ψ ₁ , Ψ ₂ different by + 45 ° each, and the measurement signal P is formed from the relationship (I1 - I2) / (I1 + I2). The relative error R is entered in percent, with the curve lines shown connecting the points in the diagram at which the relative error is the same. It is calculated as an example for a situation in which the linear birefringence angle γ, ie the phase delay between the eigenaxes x, y of the linear birefringence, is 15 ° and due to the temperature dependence of the linear birefringence in the temperature range under consideration by 10% (1, 5 °) can deviate from this. It is according to the relationship

R = 100 (P _{γ = 15 °, ρ} - P _{γ = 15 ° + 10%, ρ} ) / P _{γ = 15 °, ρ = 15 °}

normalized to the measurement signal P that results at a Fara day angle ρ = 15 ° and a linear birefringence angle γ = 15 °. It can be seen from the figure that the error relating to a nominal value of ρ = 15 ° becomes minimal for different working areas at different polarization angles in accordance with the elliptical curves a and b drawn in. In order to minimize the error at the nominal value ρ = 15 °, the polarization angles Φ _opt belonging to points A, B, C, D in the diagram are the most favorable. In order to minimize the error in the working range ρ = 35 ° related to the nominal value ρ = 15 °, the settings of the polarization angle Φ _opt belonging to E, F, G, H prove to be particularly favorable.

FIG. 4 shows the representation analogous to FIG. 3, in which case the polarization / analyzer angles differ by -45 °. The optimal polarization angles Φ _opt for ρ = 15 ° are shown in the diagram by the intersection points A ', B', C 'and D'.

The exact values are given for the nominal value ρ = 15 ° in the table according to FIG. 5. The table shows that for ρ = 0 ° an optimal first polarization angle Φ _opt, for example, is approximately 10.5 °, to which a first analyzer angle of 55.5 ° is assigned. This results in two polarization angles and a second analyzer angle of 124.5 ° and 169.5 °, respectively. These are the values known from DE 44 46 425 A1, column 10, line 13. However, if you want to optimize the relative error at ρ = 35 °, for example, the angle settings Φ _opt 14.4 ° and 59.4 ° are more favorable.

Claims (6)

1. A method for measuring a magnetic field with the aid of the Fa raday effect, in which a first and a second linearly polarized light signal (L1 or L2) pass through a magnetic field sensitive measuring section ( 2 ) showing the Faraday effect, and in which the measuring section ( 2 ) has a first and second analyzer ( 14 and 12 ) for the first and second light signals (L1 and L2) respectively, the first and second polarization angles (Φ ₁ and Φ ₂ ) of the first or second into the measuring section ( 2 ) a coupled light signal (L1 or L2) and the first and second te analyzer angles (Ψ ₁ or Ψ ₂ ) of the first or second analyzer ( 14 or 12 ) the natural axes (x, y) of the linear birefringence are set such that an error occurring due to the temperature dependence of the linear birefringence within the measuring section ( 2 ) is minimized for a given Faraday angle (ρ) other than zero. 2. The method of claim 1, wherein the first and second polarization angles Φ ₁ and Φ ₂ differ from the first and second analyzer angles Ψ ₁ and Ψ ₂ by 45 °. 3. The method according to claim 1 or 2, in which for the linear polarization of the first and second light signals (L1, L2) a first or second polarizer ( 12 or 14 ) is used, which at the same time as a second or first analyzer ( 12 or 14 ) for the first or second light signal (L1, L2) emerging from the measuring section ( 2 ). 4. Arrangement for measuring a magnetic field with the help of the Fa raday effect, in which a first and a second linearly polarized light signal (L1 or L2) pass through a magnetic field sensitive measuring path ( 2 ) showing the Faraday effect, and in which the measuring section ( 2 ) has a first and second analyzer ( 14 and 12 ) for the first and second light signals (L1 and L2) respectively, the first and second polarization angles (winkel ₁ , Φ ₂ ) of the first or second into the measuring section ( 2 ) coupled light signal (L1 or L2) and the first and second analyzer angles (Ψ ₁ or Ψ ₂ ) of the first or second analyzer ( 14 or 12 ) in such a way to the self-axes (x, y) of the linear birefringence are set such that an error occurring due to the temperature dependence of the linear birefringence within the measuring section ( 2 ) is minimized for a predetermined Faraday angle (ρ) other than zero. 5. Arrangement according to claim 4, in which the first and second polarization angles Φ ₁ and Φ ₂ differ from the first and second analyzer angles Ψ ₁ and Ψ ₂ by 45 °. 6. Arrangement according to claim 4 or 5, in which for the linear polarization of the first and second light signals (L1, L2) a first or second polarizer ( 12 or 14 ) is used, which at the same time as a second or first analyzer ( 12th or 14 ) for the first or second light signal (l1 or L2) emerging from the measuring section ( 2 ).