EP1055127A1

EP1055127A1 - Method and device for measuring a magnetic field with the aid of the faraday effect

Info

Publication number: EP1055127A1
Application number: EP99914415A
Authority: EP
Inventors: Ottmar Beierl; Thomas Bosselmann
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1998-02-12
Filing date: 1999-02-11
Publication date: 2000-11-29
Also published as: CN1290350A; CA2320037A1; US6495999B1; NO20004064L; WO1999041617A1; NO20004064D0

Abstract

The invention relates to a method for measuring a magnetic field with the aid of the Faraday effect, wherein light is injected into a Faraday element (4) arranged in a magnetic field, said light having a first linearly polarized light segment (Pa) with a first wavelength and a second non-polarized light segment (Pb) with a second wavelength differing from the first. A light signal is then decoupled from the Faraday element (4). Said light signal is optically split into a first light signal segment (LSa) with the first wavelength and a second light signal segment (LSb) with the second wavelength. A first measuring signal (Sa) is derived from the first light signal segment (LSa) and a second measuring signal (Sb) is derived from the second light signal segment (LSb), which are used to form a corrected measuring signal (S). The influences of attenuations in the transmission paths can be compensated for to a large extent even when a time constant magnetic field or a magnetic field with a time constant segment are measured.

Description

1 description

Method and device for measuring a magnetic field using the Faraday effect

The invention relates to a method and a device for measuring a magnetic field, in particular for measuring an electrical current flowing in a current conductor, using the Faraday effect.

Optical measuring devices for measuring an electrical current flowing in a current conductor using the Faraday effect are known, which are also referred to as magneto-optical current transformers. The Faraday effect is the rotation of the plane of polarization of linearly polarized light that propagates in a medium in the presence of a magnetic field. The angle of this rotation is proportional to the path integral over the magnetic field along the path covered by the light with the Verdet constant as the proportionality constant. The Verdet constant in turn depends on the material in which the light propagates, on the wavelength of the light and on other disturbing variables which influence the properties of the material, for example the temperature and the mechanical stress state. To measure the current, a Faraday element is arranged in the vicinity of the current conductor and contains an optically transparent material that shows the Faraday effect. Linearly polarized light is coupled into this Faraday element. The magnetic field generated by the electric current causes the plane of polarization of the light propagating in the Faraday element to rotate by an angle of polarization that can be evaluated by an evaluation unit as a measure of the strength of the magnetic field and thus of the strength of the electric current. In general, the Faraday element surrounds the current conductor, so that the polarized light circulates around the current conductor in a practically closed path. This is the amount of the polarization tion rotation angle in good approximation directly proportional to the current.

In an embodiment known for example from European patent 088 419, the Faraday element is designed as a solid glass ring around the current conductor. In this embodiment, the light circulates around the current conductor once.

In another embodiment known, for example, from PCT application WO 91/01501, the Faraday element is designed as part of an optical monomode fiber which surrounds the current conductor in the form of a measuring winding. The light therefore circulates the current conductor N times during a pass if N is the number of turns of the measuring winding. Two types of such magneto-optical current transformers with a measuring winding consisting of an optical fiber are known, namely the transmission type and the reflection type. With the transmission type, the light is coupled into one end of the optical fiber and coupled out again at the other end, so that the light only passes through the measuring winding once. In the case of the reflection type, on the other hand, the other end of the optical fiber is mirrored, so that the light coupled in at the first end is then reflected at this other mirrored end, the measuring winding passes through a second time in the opposite direction and is coupled out again at the first end. Due to the non-reciprocity of the Faraday effect, the plane of polarization of the light is rotated again in the same direction in the same direction during the reverse pass. The angle of rotation is thus twice as large for the same measuring winding as for the transmission type. A beam splitter is provided to separate the injected and the extracted light.

A problem with all magneto-optical current transformers is interference, which is caused, for example, by changing the damping constants in the optical transmission links. In the aforementioned magneto-optical current transformer known from European patent 088 419, the light coupled out of the Faraday element is divided in an analyzer with a Wollaston prism as a beam splitter into two linearly polarized light signals A and B with mutually perpendicular polarization planes. These two light signals A and B are transmitted to corresponding light detectors via corresponding optical transmission fibers and converted into electrical signals PA and PB. From these two signals PA and PB a Faraday rotation angle is calculated as a measurement signal in a computing unit, which corresponds to the quotient (PA - PB / PA + PB) from the difference and the sum of the two signals. A measurement signal is determined by this quotient formation, which is independent of the attenuation of the light signals A and B in the transmission path.

It is known from PCT application WO 94/24573 that the electrical signals S1 and S2 received by the receivers arranged behind the Wollaston prism are each broken down into a DC signal component D1 or D2 and an AC signal component AI or A2. For each signal S1 and S2, an intensity-standardized signal Pl or P2 is then formed as the quotient Pl = Al / Pl or P2 = A2 / D2 from its alternating signal component AI or A2 and DC signal component Dl or D2. The intensity normalization of the signals S1 and S2 can compensate for fluctuations in intensity in the transmission links provided for the corresponding light signals LSI and LS2 and differences in sensitivity in these two transmission links.

In this known method it is assumed that the attenuation changes taking place in the transmission link due to environmental influences are practically static with respect to the frequency of the alternating current to be measured. With this known method, however, a temporal change in the damping properties of the transmission link can be avoided co M cn o cn o cπ o Cπ a υ-5 d XS P aa <s: ι-id tr ¹ φ d P- P p- cn

PJ P p o φ φ P- P- Φ Φ p- J Φ p P- o P Φ Φ o p d Φ cn a O rt P- p PJ a Pι l-i n rt 3 P> n H P P p

P PJ H) tr PJ tr a 3 n P- I- ¹ a PP "ω cn rt) P. PO

Φ tr rt PJ φ XS O SO a 1-1 pj: Φ rt tu * 5Ö> P, rt Φ p- rt rt Pl d <Φ p. ^p - a Q Φ 3 SO a CΛ PJ P- P li a p- Λ Pp-- Φφ rt Λ P- p- P- P P- Λ P- p- P o P p- P a P- cn cn cn ιQ cn Φ d Hl dn P φ P d Φ cn Ό li I— 'ω Φ ts Φ rt σ \

Φ P- Φ rt cn Φ Hi Φ PJ Φ tr Φ Φ H PJ o: p- P P- cn Φ Pl »- * ιQ 3

PJ Φ i Φ Φ Φ HP Φ cn PJ J rt rt l- ¹ Φ P φ Ω Φ P er φ σ d li P 3 Φ ιQ P d tr P ω Φ O PJ ω Pl 3 d tr cn Φ

P- Φ cn rt P- φ φ P Φ φ P- cn p- Φ H p. φ Φ rt PJ li ω - \ P- d lh P- Φ P tr ¹ PP P- N cn φ P tr ¹ p- t P Φ ^ QO rt <Φ cn

Hi n P p- JP cn rt s: ü cn rt o Φ Φ P- PJ cn φ cn P Ό H Φ rt s P Φ t α φ nn φ φ p- Φ P- p- φ φ Φ li 3 n P P- P cn Φ: O Pl N P- N li 3 t ^"1 tr P ^J cn ιP d H φ 3 H PJ a p- tr φ p- d rt P- 3 Hi Hi φ PJ

Φ cn P- rt sQ 3 p- P φ cn P rt rt Φ H n O cn PJ PJ ι- (

P P- d O Φ _h Q φ rt N PJ Φ PJ CΛ p rt O rt

P tr a P tr cn d cn rt Z rt J dd ti rt Φ φ cn P- li Φ a P- a PPO Ω P tr Φ cn rt 15 tr rt s: Φ rt Φ n p- Φ tr P, OP 3 P- Φ p- cn dd rt Φ Φ Φ d P- tu * o cn P tr φ p- in tr Φ H cn Φ P. P a PP p P- P p- P Φ P-

P P- r h-. rjj ι-i rt -p pj: Φ H ω rt Φ Φ t " ¹ N rt T cn P- ¹ P cn cn to P cn O CΛ

DJ "3 φ * rt Φ PJ P 3 H φ rt o Φ J rt p- φ φ

Φ on ⁱ n cn N φ P-

P P- rt Φ P iQ p- P Φ li n P- Pl tr J d 3 a H- P- P> rt Φ P p- p- φ φ cn N t- ¹ tr rt cn PJ Φ rt p. φ cn a Φ rt ti P PJ cn d O P- xs rt n li P Φ P p- tö 3 p- Φ li Φ f J φ φ φ cn HO oa Φ P- t p- P Ω P- P Φ Φ Φ P- PJ P p- cn <! rt tr H Φ n Φ cn CΛ rt tr Ω φ Pl

Φ ≤ ι-S ii P ι- (Ω Hl O rt φ ι-i P ⁾ O rt PJ 3 r p- p- φ cn rt cn tr φ

(- • Φ rt N Φ rt CΛ tr PJ = φ P H cn Φ P tr P- P. p- φ P Ω cn rt <p-

Φ cn Φ Φ cn rt rt J rt p a Φ ι-i cn tr ιQ d p- Ω

Φ P P- rt rt iQ rt o tr P- a P P d Φ p- φ CL cn tr p- cn cn rt J P rt O ι-i Φ P J er tr rr P 3 H Φ Φ

Φ d P φ Φ O p- P- P O p- P p- p- n Φ p- Φ iQ d ρ>> e>

Pι tr rt PJ cn φ rt H rt 3 tr rt cn H P p a φ <! φ tr cn Hi P- cn J

P- Φ P- Φ rt n Φ cn H φ O Φ J d ι-io P, Φ N rt Φ ^ Q rt a φ cn P- n P Φ φ P P- WP Φ tr P apo H rt P Φ p - P- Φ P p- Φ p- cn

O rt P ^J φ d P- cn cn p. P- Φ Φ a φ irr o PP φ P- o Φ P vQ O tr rt rt Φ P p- φ n SP t- ¹ rt P Φ tr d J li n rt rt φ P ^

Φ Φ cn Φ Hi ι-i tr rt li tr Φ Φ Φ p- h ^J P Φ a PN tr cn P- ι-i

P p- - Φ rt φ p: (_ K p- Φ φ er o 3 φ o P CΛ rt P- J--

P P P: H <Ω a φ rt p- r φ P cn tr φ PJ 03 a o a d Φ

Φ n ~ J tr

P l P-iι rt Φ P S P- P- to cn rt cn Φ P P rt l-i l-i cn φ tr Φ H d

Φ PJ P P-- a

Φ P cn ιQ Φ p- ω P cn aJ P- p- P- oo Φ p φ c cnn ω a P>& P Hi sQ p- cn oa Φ cn p PJ PP p- cn S α P n p- a P- cn PJ φ P ιQ rt li Pl O Φ a PN ⁾ Ω p- PJ Φ tr s: PJ: N tr PJ Φ φ p- P- P ^J li J tQ P Φ tr <p 3 ιQ a P do tr ω ιQ π Φ Φ da 3 ^ Q φ φ PJ P φ dpo Φ N t tr P- ω p PN cn J a Φ 0 P cn C CΛΛ P P- p Φ P t- ¹ CΛ cn o Φ cn rt Hi a Φ

PJ Φ Φ 3 cn cn o PJ φ N φ M P Ό p- Φ rt Φ • d p- rt ι-i a H Φ d cn

P- rt d Φ aa P- cn P cn cn ω 03 O lh o Φ n P Φ p- OPP p £ »φ PJ φ p- Φ cn a Φ tr cn d PJ Φ 3 Sl

CΛ Φ rt cn P a CΛ Φ P ιQ PJ Hi? PJ li rt o Φ tr ¹ cn φ

H "3 P cn Φ M <φ Φ tr: O a H N ι- (H p-: P n fu P a 3 Ω o

H φ d P P H O PJ CΛ - P O p- d P> d rt n er O P- tr φ cn P p- l-i rt ιQ l-i ιQ TJ cn rt cn

P p Φ p- tr H φ a Φ PJ P cn Φ p- Φ Φ Φ φ Φ W Φ rt P rt er P- a ω rt a? a Φ Φ Φ P PJ l-i φ o rt

P- PJ li H ω PJ Φ cn P- cn P- a rt Pl p. H d ω P- P a vO Φ

C l-i P p- Φ n Φ uq cn cn rt J Φ Φ cn Pl o

Φ P P Φ Φ a w rt

3 P- Φ tr n tr i p- Φ Φ H p p- a H- ¹ Φ φ rt P- 3 p ^j : l P cn Φ H cn P- P p IPJI © rt III ι- (

5

Reference signal S _R proportional to the magnetic field and independent of the intensity of the first light source.

In this way, changes over time in the damping properties of the transmission link can also be compensated for. A DC current or DC component can also be measured with this method. However, the known measuring device is complex in terms of the electronic processing of the measurement signals and the control of the light sources and is susceptible to malfunction due to the large number of electronic components required to control the light sources.

The invention is based on the object of specifying a method for measuring a magnetic field with the aid of the Faraday effect, which enables simple compensation of the attenuation present in the transmission path. The invention is also based on the object of specifying a device for carrying out the method.

The first-mentioned object is achieved according to the invention with the features of patent claim 1.

In the method for measuring a magnetic field with the aid of the Faraday effect, according to the invention, light is coupled into a Faraday element arranged in the magnetic field, said light having a linearly polarized first light component with a first wavelength and an unpolarized second light component with a different second one Has wavelength. A light signal coupled out from the Faraday element is optically divided into a first light signal component with the first wavelength and a second light signal component with the second wavelength, a first measurement signal being derived from the first light signal component and a second measurement signal being derived from the second light signal component and for formation a corrected measurement signal can be used. 6

In this way, damping influences in the transmission links can be largely compensated even when measuring a magnetic field that is constant over time or a magnetic field with a component that is constant over time. The method according to the invention thus enables the precise and of the

Damping properties of the transmission path largely independent measurement of a direct current or a current with a direct component.

In particular, the corrected measurement signal is formed by simply dividing the first by the second measurement signal.

In a preferred embodiment of the method, the linear polarization of the first light component is generated in a polarizing optical fiber which is connected upstream to the Faraday element in a transmission path. This enables a particularly simple and lossless arrangement, since the polarizing optical fiber also contributes to the transmission of the light over a partial length of the entire transmission path.

In particular, light is coupled into the transmission path to the Faraday element, which has a first component with the first wavelength and a second component with the second wavelength, which are transmitted together in the polarizing optical fiber to the Faraday element.

The second object is achieved according to the invention with a device with the features of claim 5.

According to the invention, the device for measuring a magnetic field using the Faraday effect comprises a Faraday element and a device for generating light upstream of the Faraday element, which has a linearly polarized first light component with a first wavelength and an unpolarized second light component with a different second wavelength. In addition, one of the Faraday 7

Element-downstream device for receiving and analyzing the light signal coupled out of the Faraday element is provided, which has a wavelength-selective beam splitter for splitting this light signal into a first light signal component with the first wavelength and a second light signal component with the second wavelength, and a receiving unit for forming one contains the first light signal portion of the first measurement signal and a second measurement signal derived from the second light signal portion, and contains an evaluation unit for forming a corrected measurement signal from the first and second measurement signals.

In particular, the evaluation device comprises a dividing device for dividing the first by the second measurement signal.

In a preferred embodiment, a polarizing optical fiber connected upstream of this in a transmission path to the Faraday element is provided for linear polarization of the first light component.

In particular, a light source is used to generate light, which has a first component with the first wavelength and a second component with the second wavelength.

In a further preferred embodiment, the polarizing optical fiber is provided as part of the transmission path for the joint transmission of the first and the second portion to the Faraday element.

To further explain the invention, reference is made to the exemplary embodiments of the drawing. Show it:

1 shows an exemplary embodiment of the invention in a schematic illustration, 8th

2 shows a diagram in which the transmission properties of a polarizing optical fiber preferably provided for use in a device according to the invention for different modes are plotted against the wavelength,

3 and 4 each show a further advantageous exemplary embodiment of the invention, likewise in a schematic illustration.

According to FIG. 1, a current conductor 2 is surrounded by a Faraday element 4, which is shown in the exemplary embodiment as a measuring winding constructed from a single-mode fiber. The input of the Faraday element 4 is coupled via a plug-in connection 6 to an optical transmission path formed by an optical fiber 8, which is connected via a Y-coupler 9 to two mutually independent light sources 12a and 12b of a light source arrangement 12. The first light source 12a generates light with a first component La and with a first wavelength λa and the second light source 12b generates a second component Lb with a second wavelength λb. The wavelengths λa, λb of the components La and Lb are different from one another. The components La and Lb are unpolarized and are simultaneously transmitted to the Faraday element 4 via the common optical fiber 8. Alternatively, it is also possible to control the light sources 12a, 12b in such a way that the portions La, Lb are coupled into the transmission path alternately in time.

At the end of the transmission path, ie in front of the input of the Faraday element 4, a polarizing optical fiber 10 is arranged in the transmission path. This polarizing optical fiber 10 thus forms at least part of the total transmission path from the light source arrangement 12 to the Faraday element 4. In this polarizing optical fiber 10, a linearly polarized first light component Pa is generated from the first component La, while the component Lb with respect to its polarization state im remains essentially unaffected and is coupled into the Faraday element 4 as an unpolarized second light component Pb.

Linearly polarized first light component Pa and unpolarized second light component Pb pass through the Faraday element 4 and are coupled at the output thereof via a plug-in or splice connection 14 into a transmission path 16 which does not influence the polarization state and is formed, for example, by an optical fiber with low birefringence laid without curvature. At the end of this transmission link 16, a converter unit 20 is connected. Instead of a polarization-neutral optical fiber, a so-called strongly birefringent polarization-maintaining fiber can also be used, with which linearly polarized light, which is polarized parallel to one of the two mutually orthogonal polarization axes of the optical fiber, is transmitted while maintaining its polarization state.

The linearly polarized first light component Pa experiences a rotation of its polarization plane in the Faraday element as a function of the magnetic field generated by the electric current I _A and is coupled out of the Faraday element 4 as the first light signal component LSa. The polarization state of the second unpolarized light component Pb coupled into the Faraday element 4 is not influenced by the Faraday element 4 and is coupled out of the latter as an unpolarized second light signal component LSb. The first and the second light signal components LSa and LSb are optically separated from one another in the converter unit 20 in a wavelength-selective beam splitter 22.

A component LSα polarized in a predetermined polarization plane is generated from the first light signal component LSa in an analyzer 23. The analyzer 23 is preferably at 45 ° to the plane of polarization of the linearly polarized first coupled into the Faraday element 4 10

Light portion Pa arranged. If a polarization-maintaining optical fiber is used in the transmission link 16, one of its two polarization axes must coincide with the orientation of the analyzer axis of the analyzer 23.

The component LSα and the second light signal component LSb are each converted in an optoelectric converter 24a or 24b into a first or a second electrical measurement signal Sa or Sb. The first and second electrical measurement signals Sa and Sb, which may be corrected at the output of the optoelectric converter 24a and 24b, are corrected according to the sensitivity of the converter 24a and 24b for the different wavelengths λa, λb. which forms a corrected measurement signal S by dividing the first measurement signal Sa by the second measurement signal according to the equation S = Sa / Sb. This corrected measurement signal S is largely independent of the damping properties of the optical components contained in the transmission path, it being assumed that the change in damping resulting from environmental influences is approximately the same at the wavelengths λa and λb.

The polarization rotation angle α can be determined directly from the corrected measurement signal S even when only one analyzer 23 is used, since there is a fixed relationship between the intensity of the linearly polarized first light component Pa and the intensity of the unpolarized second light component Pb, which is due to the light sources used 12a and 12b is predetermined. The second measurement signal Sb is thus proportional to the intensity Ia of the first light signal component Lsa, Sb = k »Ia, where k is a proportionality factor that depends on the intensities of the light components Pa and Pb. The first measurement signal Sa measured behind the analyzer 23 is then proportional to the square of the cosine of the angle difference between the polarization rotation angle α and the orientation α ^{Λ of} the analyzer 23 and the intensity of the first light 11 signal component, Sa = p «cos ² (α-oc)« Ia, where p is another proportionality factor. The corrected measurement signal S = (p / k) «cos ² (α-α) thus enables the polarization rotation angle α to be determined directly.

In a further embodiment of the exemplary embodiment, detectors can also be assigned to the light sources 12a and 12b, which detect possible relative intensity fluctuations and are used to determine the correct proportionality factor k.

The properties of the polarizing fiber 10 are explained in more detail with reference to FIG. 2, in which the attenuation loss D of an approximately 1.7 m long polarizing fiber, as can be obtained, for example, from 3M under the designation FS-PZ-4616 the wavelength λ is plotted. From a technical point of view, such a polarizing fiber is a highly degenerate, highly birefringent optical fiber, in which at a certain wavelength one of the two modes is attenuated much more than the other. With a sufficiently long fiber length, the damping ratio is so large that the fiber acts as a polarizer. The diagram shows that at a wavelength λa of approximately 770 n the damping for the fast mode f is more than a thousand times as great as the damping for the slow mode s. The fast mode f is thus filtered out of a light coupled into the polarizing optical fiber 10 with the wavelength λa, so that only the linearly polarized slow mode s is coupled out at the output of the polarizing optical fiber 10.

For the wavelength λa there is now at least one wavelength λb which, with the same length of the polarizing optical fibers 10, also has a practically negligible attenuation for both modes f and s. Although the attenuation for the slow mode s is also more than 1000 dB less for the fast mode f at the wavelength λb, the latter contributes 12 of the present length of the optical fiber 10 does not matter, since the damping of the fast mode f is practically negligible with less than 0.2 dB. An unpolarized light with the wavelength λb coupled into the polarized optical fiber 10 is thus coupled out at the output of the optical fiber as unpolarized light.

In the embodiment according to FIG. 3, the analyzer 23 is arranged in front of the wavelength-selective beam splitter 22. As shown in the exemplary embodiment of the figure, this can be arranged directly in front of the wavelength-selective beam splitter 22. In this case, a polarization-neutral or polarization-maintaining transmission link 16 is required between the Faraday element 4 and the beam splitter 22. Alternatively, however, the analyzer 23 can be arranged directly behind the Faraday element 4. In this case, the transmission path 16 to the converter unit 20 can be set up with a normal standard optical fiber. In addition, the wavelength-selective filter 22 can also be arranged directly behind the Faraday element 4, so that the converter unit only consists of the optoelectric converters and the electrical components assigned to them.

According to FIG. 4, a polarizing optical fiber 102 is provided in the transmission path from the Faraday element 4 to the wavelength-selective beam splitter 22, which is identical in construction to the optical fiber 10 and differs from it only in the orientation of its polarization axes. This optical fiber 102 then acts as an analyzer and replaces the analyzer 23 according to FIGS. 1 and 3.

Claims

13 claims

1. Method for measuring a magnetic field using the Faraday effect, in which light is coupled into a Faraday element (4) arranged in the magnetic field, which has a linearly polarized first light component (Pa) with a first wavelength and an unpolarized second one Has a light component (Pb) with a different second wavelength, a light signal decoupled from the Faraday element (4) optically divided into a first light signal component (LSa) with the first wavelength and a second light signal component (LSb) with the second wavelength and a first measurement signal (Sa) is derived from the first light signal component (LSa) and a second measurement signal (Sb) is derived from the second light signal component (LSb) and is used to form a corrected measurement signal S.

2. The method according to claim 1, wherein the corrected measurement signal S is formed by dividing the first measurement signal (Sa) by the second measurement signal (Sb).

3. The method according to claim 1 or 2, wherein the linear polarization of the first light component (Pa) is generated in a polarizing optical fiber (10) which is connected upstream in a transmission path to the Faraday element (4).

4. The method according to claim 3, in which light is coupled into the transmission path to the Faraday element (4), which has a first component (La) with the first wavelength and a second component with the second wavelength (Lb), which together in the polarizing optical fiber (10) to the Faraday element (4) are transmitted.

5. Device for measuring a magnetic field using the

Faraday effect, with a Faraday element (4) and a device (10) upstream of the Faraday element (4) for detecting 14 produce light which has a linearly polarized first light component (Pa) with a first wavelength and an unpolarized second light component (Pb) with a different second wavelength, and with a device (20) connected downstream of the Faraday element (4) ) for receiving and analyzing the light signal coupled out from the Faraday element, which has a wavelength-selective beam splitter (22) for splitting this light signal into a first light signal component (LSa) with the first wavelength and a second light signal component (LSb) with the second wavelength (24a, 24b) for forming a first measurement signal (Sa) derived from the first light signal component (LSa) and a second measurement signal (Sb) derived from the second light signal component (LSb) and an evaluation unit (26) for forming a corrected measurement signal ( S) from the first and second measurement signals (Sa and Sb).

6. Device according to claim 5, in which the evaluation device (26) is provided for dividing the first by the second measurement signal.

7. Device according to claim 6 or 7, in which a polarizing optical fiber (10) connected upstream of this is provided in a transmission path to the Faraday element (4) for linear polarization of the first light component (La).

8. Device according to claim 7, with a light source (12) for generating light, which has a first portion (La) with the first wavelength and a second portion (Lb) with the second wavelength.

9. Device according to claim 8, in which the polarizing optical fiber (10) is provided as part of the transmission path for the joint transmission of the first and the second portion (La, Lb) to the Faraday element (4).