Optical fibre interferometer and method for measuring a magnetic field or an electrical current based on said interferometer

Abstract

The invention relates to an optical fibre interferometer comprising a source (20), a differential phase modulator (16), a signal-processing system (900), a sensing optical fibre (73) having a Verdet constant capable of inducing a non-reciprocal magneto-optic Faraday effect, the interferometer being able to detect a difference in phase of an interferometric beam (300) formed by interference between two polarised light waves (111, 112) that have simultaneously travelled along the optical fibre (73) along a closed optical path, and to deduce therefrom, by means of dividing the phase difference by a scale factor, a value of a magnetic field or a value of an electrical current flowing in an electrical conductor (120). According to the invention, the signal-processing system (900) is suitable for measuring a variation in power contrast of one portion of the interferometric beam and to deduce, from the variation in contrast, a measurement of variation in the scale factor.

Description

Fiber optic interferometer and method for measuring magnetic field or electric current based on this interferometer Technical field

[0001] The present invention relates generally to the field of sensors based on a fiber optic interferometer. More particularly, it relates to a sensor using the magneto-optical Faraday effect to integrate a magnetic field along a closed optical path in an optical fiber.

[0002] Such a sensor finds applications in particular as an electric current sensor (or FOCS for fiber-optic current sensor) in which the optical fiber surrounds an electrical conductor or even as a magnetic field sensor.

[0003] In particular, it concerns a sensor and a method for measuring high-precision electric current, corrected for errors impacting the scale factor. Prior art

[0004] In the above field, the publication J. Blake et al. "In-Line Sagnac Interferometer Current Sensor" lEEE Transactions on Power Delivery, Vol. 11, No. 1, pages 116-121, 1996 describes a reflective (or in-line) fiber optic interferometer used as an electric current sensor via the induced magnetic field.

[0005] The publication "Highly accurate fiber-optic DC current sensor for the electro-winning industry", K. Bohnert, H. Brandie, M. Brunzel, P. Gabus and P. Guggenbach, IEEE, PCIC- 2005-14, pp. 121-128 also describes a current sensor based on a fiber optic interferometer for measuring the Faraday effect induced in a sensing optical fiber. According to this publication, the current sensor can be based on a fiber optic Sagnac interferometer configuration or a reflective fiber optic interferometer. The sensing optical fiber is arranged so as to form at least one turn around the periphery of an electrical cable carrying an electric current whose intensity is to be measured. The sensing fiber is generally circularly polarized.In the case of a Sagnac interferometer, a phase difference is measured between two waves having traveled through the detection optical fiber with the same state of circular polarization, and in mutually opposite directions. In the case of a reflective fiber optic interferometer, a phase difference is measured. between two waves having jointly traveled the optical detection fiber following a round trip, with circular polarization states orthogonal to each other, these states being reversed between the outward path and the return path upon reflection on the mirror. At rest, that is to say here in the absence of electric current or magnetic field, the optical paths traveled by the two waves are perfectly identical due to the reciprocity of the propagation of the waves in opposite directions; we also say that the two paths are perfectly reciprocal.

[0006] An electric current passing through an electrical conductor induces a magnetic field. This magnetic field generates a Faraday effect, or non-reciprocal collinear magneto-optical effect, in the optical detection fiber. More precisely, the Faraday effect is the rotational effect of optical polarization produced by a magnetic field. Thus, the plane of polarization of a linearly polarized light beam passing through a material, for example silica, placed in a magnetic field B and propagating parallel to this field, rotates by a rotation angle P which depends on the Verdet constant of the material of the optical detection fiber, denoted V, the magnetic field B and the length, d, of the optical path in the material crossed. The rotation angle P of the polarization is proportional to the circulation of the magnetic field B along the optical detection fiber.The rotation angle of the polarization depends on the direction of the magnetic field and also on the direction of propagation of the light (see also "The fiber optic gyroscope", second Edition, Hervé Lefèvre, Artech House, figure 7.1-a page 107). This rotation of an angle P of the linear polarization is interpreted as a circular birefringence. Indeed, the linear polarization can be decomposed into two opposite circular polarizations (right-handed and left-handed). By Faraday effect, these two circular polarizations, propagating in the same direction, undergo a phase difference which is observed by a rotation of the linear polarization which is the combination of these two co-propagating circular polarizations.

[0007] In the case of a current sensor based on a Sagnac interference, the two waves propagate in opposite directions, and with the same circular polarization. The Faraday effect does not modify this circular polarization, but a phase difference is created between the two waves propagating in opposite directions (see also "The fiber optic gyroscope", second Edition, Hervé Lefèvre, Artech House, figure 7.1-b page 107). This phase difference A <I»s est mesurée après recombinaison des deux ondes en sortie de l'interfé rom être, et elle est égale à :

[0008] A s = 2 VNI

[0009] where V represents the Verdet constant of the core material of the optical detection fiber, TV the integer number of turns of optical fiber wound around the electrical conductor and I the intensity of the electric current flowing through the electrical conductor.

[0010] In the case of a current sensor based on a reflective fiber optic interferometer, the measured phase difference A <I»R est proportionnelle à :

[0012] In the case of a reflecting fiber optic interferometer, the scale factor is double (4 KTV / instead of 2 VN 1} compared to a Sagnac interferometer, because the polarized waves each travel a round trip in the detection optical fiber. In addition, a reflecting fiber optic interferometer is rather less sensitive than a loop interferometer to the rotation speed of the interferometer, to vibrations as well as to thermal effects.

[0013] Such fiber optic sensors offer many advantages. They allow contactless measurement of magnetic fields or electric currents, with an accuracy of around 1%. They allow current measurements over a wide dynamic range, for example, between a minimum detectable current value of around a few micro-amperes and a maximum detectable current value of around a few thousand amperes. Finally, they are insensitive to many environmental factors.

[0014] However, it is desirable to improve the accuracy of sensors based on a fiber optic interferometer and in particular the scale factor, which defines the linearity of the sensor response as a function of the detected signal, i.e. the measured phase difference. A measurement accuracy better than 0.1% or even 0.01% is desirable.

[0015] However, these fiber optic interferometers are susceptible to other unwanted effects. These unwanted defects are likely to impact the scale factor.

[0016] On the one hand, the scale factor of a fiber optic interferometer may have a construction defect, for example due to a misalignment of an optical component. In particular, in the case where a quarter-wave plate is used to transform a linearly polarized wave into a circularly polarized wave, this plate may have an effective phase shift different from the theoretical phase shift of n / 2 of a quarter-wave plate and / or have a misalignment of its own axes with respect to the incident linear polarization.

[0017] On the other hand, the scale factor of a fiber optic interferometer can vary over time. In particular, it is known that the Verdet constant of the fiber depends on the temperature and the wavelength of the beam. The scale factor can vary due to variations in the Verdet constant of the optical fiber, induced for example by temperature variations at the sensitive part of the sensor. The scale factor can also vary due to temperature variations at the differential optical phase modulator.

[0018] Finally, other disturbances can affect interferometric measurements, such as variations in the light power of the source.

[0019] One of the aims of the invention is to propose an electric current or magnetic field sensor based on a fiber optic interferometer making it possible to measure in real time a variation or a defect in the scale factor, whatever the origin of this variation or defect. Another aim of the invention is to make it possible to correct this scale factor error in such a sensor in real time. Statement of the invention

[0020] In order to overcome the aforementioned drawbacks of the state of the art, the present disclosure proposes a fiber optic interferometer comprising a light source capable of generating a source beam, a differential phase modulator, an optical fiber-based device, a detection system, a signal processing system, the optical fiber-based device comprising a detection optical fiber having a Verdet constant capable of inducing a magneto-optical Faraday effect. non-reciprocal, the optical detection fiber being arranged in a magnetic field or forming at least one turn around an electrical conductor, the optical fiber interferometer being capable of detecting a phase difference of an interferometric beam formed by interference between two polarized light waves having simultaneously traveled the optical detection fiber along a closed optical path, the two polarized light waves being modulated by the differential phase modulator, and the optical fiber interferometer being capable of deducing therefrom, by dividing the phase difference by a scale factor, a value of the integrated magnetic field along the closed optical path or a value of an electric current passing in the electrical conductor, the scale factor being proportional to the Verdet constant of the optical fiber.

[0021] According to the invention, the signal processing system is adapted and configured to measure a variation in power contrast of a portion of the interferometric beam modulated by the differential phase modulator and to deduce from the variation in contrast a measurement of variation in the scale factor.

[0022] According to a particular aspect, the signal processing system is adapted and configured to measure a minimum power of the portion of the detected modulated interferometric beam and / or a difference between a maximum and a minimum power of the portion of the detected modulated interferometric beam.

[0023] According to another particular and advantageous aspect, the signal processing system is adapted and configured to correct the scale factor in real time as a function of the measurement of variation of this scale factor.

[0024] In the first and second embodiments, the sensing optical fiber is circularly polarized, the optical fiber-based device comprising an optical phase retarder and a reflector, the optical phase retarder being disposed at one end of the sensing optical fiber and the reflector at another end of the sensing optical fiber, the interferometer being configured such that the two polarized light waves travel along the sensing optical fiber in a round trip with two orthogonal states of circular polarization reversing by reflection on the reflector.

[0025] According to one aspect of the first embodiment, the differential phase modulator is an electro-optical birefringence modulator which comprises a single waveguide capable of guiding two orthogonal linear polarization states along two perpendicular axes; the birefringence modulator differentially modulates the phases of the two orthogonal linear polarizations. The interferometer also comprises a polarizer disposed between the light source and the electro-optical birefringence modulator, the polarizer being oriented at 45 degrees to the axes of the electro-optical birefringence modulator, and one end of the electro-optical birefringence modulator is connected to the optical fiber-based device.

[0026] According to one aspect of the second embodiment, the interferometer comprises a Y-junction splitter arranged between the light source and the differential phase modulator comprising two waveguides capable of guiding two beams of the same linear polarization and each comprising a phase modulator preferably connected in a push-pull configuration, i.e. with an opposite sign. The optical fiber-based device comprises a polarization-maintaining optical fiber section and another polarization-maintaining optical fiber section, the optical fiber section, and respectively, the other optical fiber section, each being connected, on the one hand, to one of the two waveguides of the differential phase modulator and, on the other hand, to a polarization coupler-splitter, the other optical fiber section being oriented so as to rotate a linear polarization by 90 degrees.

[0027] According to a third embodiment, the detection optical fiber is circularly polarized, the optical fiber-based device comprises a Y-junction splitter arranged between the light source and the differential phase modulator which comprises two waveguides capable of guiding two beams of the same linear polarization, and each comprising a phase modulator preferably connected in a push-pull configuration, i.e. with an opposite sign. The optical fiber-based device comprises a polarization-maintaining optical fiber section, an optical phase retarder, another polarization-maintaining optical fiber section, and another optical phase retarder, the optical fiber section, and respectively, the other optical fiber section, each being connected, on the one hand, to one of the two waveguides of the differential phase modulator and, on the other hand, to the optical phase retarder, respectively, to the other optical phase retarder, the optical phase retarder being arranged at one end of the detection optical fiber and the other optical phase retarder being arranged at another end of the detection optical fiber, the interferometer being configured so that the two polarized light waves travel through the detection optical fiber in opposite directions with the same circular polarization state.

[0028] According to a particular and advantageous aspect of any of these embodiments, the optical phase retarder, and / or respectively the other optical phase retarder, each form a quarter-wave plate at the wavelength of the source beam.

[0029] Advantageously, the optical phase retarder and / or respectively the other optical phase retarder, is offset so as to introduce a defect, and the signal processing system is adapted to extract from the detected interferometric signal a measurement of variation of the scale factor of the system and thus deduce therefrom a variation in temperature of the optical phase retarder, respectively of the other optical phase retarder.

[0030] The invention also relates to a method for measuring a magnetic field or electric current based on a fiber optic interferometer according to one of the embodiments described, the method comprising the following steps: emission of a source beam from a light source; separation of the source beam into two polarized light waves; differential phase modulation of the two polarized light waves; transmission of the two polarized light waves to an optical fiber-based device comprising an optical fiber so that the two polarized light waves travel simultaneously along the optical fiber along a closed optical path, the optical fiber having a Verdet constant capable of inducing a non-reciprocal magneto-optical Faraday effect, the detection optical fiber being arranged in a magnetic field or forming at least one turn around an electrical conductor;recombination of the two polarized light waves at the output of the optical fiber-based device to form an interferometric beam; detection of the interferometric beam; and processing of the detected signal to extract a measurement of a phase difference of the interferometric beam and to deduce, by division of the; phase difference by a scale factor, a value of the integrated magnetic field along the closed optical path or a value of an electric current passing in the electrical conductor.

[0031] According to the present disclosure, the signal processing is adapted and configured to measure a variation in power contrast of a portion of the interferometric beam modulated by the differential phase modulator and to deduce from the variation in contrast a measure of variation in the scale factor.

[0032] Of course, the various features, variants and embodiments of the invention may be combined with each other in various combinations to the extent that they are not incompatible or mutually exclusive. Brief description of the drawings

[0033] In addition, various other characteristics of the invention emerge from the appended description given with reference to the drawings which illustrate non-limiting embodiments of the invention and where:

[0034] Figure 1 illustrates a first embodiment of an electric current or magnetic field sensor based on a reflective fiber optic interferometer;

[0035] Figure 2 illustrates a second embodiment of an electric current or magnetic field sensor based on a reflective fiber optic interferometer;

[0036] Figure 3 illustrates a third embodiment of an electric current or magnetic field sensor based on a fiber optic Sagnac interferometer;

[0037] Figure 4 represents a theoretical power response curve of the detected interferometric signal as a function of a phase shift introduced by the signal to be measured or by the modulation added using the differential phase modulator, in the absence of defects;

[0038] Figure 5 represents a power response curve of a part of the detected interferometric signal as a function of a phase shift introduced by the signal to be measured. or by the modulation added using the modulator, in the case of a fiber optic interferometer with a scale factor defect;

[0039] Figure 6 represents a power response curve of another part of the detected interferometric signal as a function of a phase shift introduced by the signal to be measured, in the case of a fiber optic interferometer having a scale factor defect;

[0040] Figure 7 represents a power response curve resulting from the sum of the detected interferometric signals as a function of a phase shift introduced by the modulation added using the modulator, represented in Figures 5 and 6, in the case of a fiber optic interferometer having a scale factor defect;

[0041] Figure 8 is a schematic view in the form of phasors of different beams propagating in a fiber optic interferometer in a magnetic field or electric current sensor application, in the absence of the Faraday effect;

[0042] Figure 9 is a schematic view in the form of phasors of different beams propagating in the same fiber optic interferometer, in the presence of the Faraday effect;

[0043] Figure 10 is a schematic representation of an example of 8-level, 12-state modulation.

[0044] It should be noted that in these figures the structural and / or functional elements common to the different variants may have the same references. Detailed description

[0045] We will describe the main elements of an electric current or magnetic field sensor based on a fiber optic interferometer in connection with figures 1 to 3. We will detail below the differences between the first embodiment illustrated in figure 1, the second embodiment illustrated in figure 2 and the third embodiment illustrated in figure 3.

[0046] Generally, an electrical current sensor based on a fiber optic interferometer comprises a source-detector block 200, a polarizer 24, a phase differential electro-optic modulator 16, a fiber-based device 400 optical forming a closed optical path to surround an electrical conductor 120, a signal processing system 900 and an interface system (not shown). In the remainder of this document, the differential phase electro-optical modulator 16 is also referred to as a differential phase optical modulator, differential phase modulator or optical modulator.

[0047] The source-detector block 200 comprises a light source 20, a detection system 18 and a source-receiver splitter 22, called a receiver splitter. All of the electrical, analog or digital electronic components can be included in a housing.

[0048] The polarizer 24 and the optical modulator 16 are arranged in series between the source-detector block 200 and the optical fiber-based device 400.

[0049] The optical fiber-based device 400 comprises a detection optical fiber 73 capable of forming at least one turn around the electrical conductor 120. The detection optical fiber 73 is used as a fiber sensitive to a magneto-optical effect. For this purpose, the detection optical fiber 73 is chosen to have a Verdet constant, V. For example, the detection optical fiber 73 has a silica core, which has a Verdet constant of the order of approximately 0.6 rad T -1 m 1at the wavelength of 1550nm for silica. Advantageously, the detection optical fiber 73 is a twisted or twisted polarization-maintaining fiber, also called a SPUN fiber. Such a SPUN fiber makes it possible to maintain the circular polarization of the light. Alternatively, the detection optical fiber 73 is a linear polarization-maintaining fiber. However, such a configuration is much less efficient.

[0050] The light source 20 generates a source beam 100. The source-receiver splitter 22 transmits the source beam 100, for example via a section of optical fiber 23, to the polarizer 24. The polarizer 24 receives the source beam and transmits a linearly polarized source beam 110 towards the optical modulator 16.

[0051] Figure 1 schematically represents a current sensor based on a fiber optic interferometer according to a first embodiment. The polarizer 24 and the phase differential electro-optical modulator 16 are arranged and configured to receive the linearly polarized source beam 110 and separate it into two waves 101, 102 linearly polarized according to two orthogonal polarization states. More precisely, a polarization splitter is formed by orienting the natural axes of the polarizer 24 at 45 degrees from the natural axes of the phase differential electro-optic modulator 16. The phase differential electro-optic modulator 16 is for example formed of an electro-optic modulator integrated on a lithium niobate substrate and formed for example by titanium diffusion. The integrated optical circuit 34 comprises a single waveguide formed for example by titanium diffusion in a lithium niobate substrate. The waveguide advantageously makes it possible to guide the two transverse linear polarization states. The electrodes of the phase modulator 16 are arranged along the sides of the waveguide. The waveguide of the integrated optical circuit 34 is birefringent.The combination of the polarizer 24 and the phase differential electro-optical modulator 16 thus makes it possible to separate the polarized source beam 110 linearly into a first polarized single-mode wave 101 and a second orthogonally polarized wave 102, which propagate on the same single-mode waveguide of the phase differential electro-optical modulator 16. For example, in the following, the first linearly polarized single-mode wave 101 TE and the second linearly polarized single-mode wave 102 TM are considered.

[0052] The differential phase electro-optic modulator 16 having a different efficiency depending on the polarization, it generates a phase shift of the two waves and will allow the phase to be modulated. The end of the differential phase electro-optic modulator 16 is here directly connected to the device 400 based on optical fiber.

[0053] The fiber optic device 400 here comprises a linear polarization-maintaining optical fiber section 74, an optical phase retarder 42 (hereinafter optical retarder), and a reflector 26. The optical fiber section 74 and the sensing optical fiber 73 are arranged in series and connected together by the optical retarder 42. The reflector 26 is arranged at another end, or distal end, of the sensing optical fiber 73. The fiber optic device 400 remains primarily outside the housing of the source-receiver block 200.

[0054] In the forward direction, the first TE-polarized single-mode wave 101 and the second TM-polarized wave 102 are injected at a proximal end of the linear polarization-maintaining optical fiber section 74. The optical retarder 42 is arranged between a distal end of the polarization-maintaining optical fiber section 74 and a proximal end of the detection optical fiber 73. The optical retarder 42 is, for example, a quarter-wave plate. Alternatively, the optical retarder 42 consists of an optical fiber with an elliptical core whose length and orientation are determined to induce a quarter-wave phase shift.

[0055] The first TE polarized single-mode wave 101 and the second TM polarized single-mode wave 102 are injected into the linear polarization-maintaining optical fiber section 74. Advantageously, the natural axes of the polarization-maintaining optical fiber 74 are aligned with the TE and TM polarization axes. The polarization-maintaining optical fiber section 74 transports the first polarized single-mode wave 101 and the second polarized single-mode wave 102, while maintaining the linear polarization of each of these waves. This polarization-maintaining optical fiber section 74 serves to offset the sensitive part of the sensor constituted by the optical fiber 73 relative to the housing of the source-detector block 200. The polarization-maintaining optical fiber section 74 has a length of between 1 m and 10 km, for example approximately 400 m. The polarization-maintaining optical fiber section 74 is optional.

[0056] The optical retarder 42 receives the first linearly polarized single-mode wave TE, denoted 101, and forms a first circularly polarized single-mode wave 111 right. Similarly, the optical retarder 42 receives the second linearly polarized single-mode wave TM, denoted 102, and forms a second circularly polarized single-mode wave 112 left. The two circularly polarized waves 111 and 112 are injected simultaneously into the detection optical fiber 73. The detection optical fiber 73 is used as a fiber sensitive to a magneto-optical rotary effect. Advantageously, the detection optical fiber 73 is a polarization-maintaining fiber twisted or twisted at the fibering, also called SPUN fiber. Such a fiber makes it possible to maintain the circular polarization of the light.In this way, the first circularly polarized single-mode wave 111 propagates in the detection optical fiber 73 while remaining right circularly polarized all along the detection optical fiber 73 to the reflector 26. In the same way, the second circularly polarized single-mode wave 112 propagates. propagates in the detection optical fiber 73 while remaining left circularly polarized all along the fiber 73 to the reflector 26.

[0057] The reflector 26 is for example a free space mirror. Alternatively, the distal end of the detection optical fiber e 73 is metallized to form the reflector 26.

[0058] After a first passage through the detection optical fiber 73, the two orthogonal circularly polarized single-mode waves 111, 112 are reflected on the mirror 26. Upon reflection on the mirror 26, the polarization states of the two single-mode waves are reversed. Upon reflection on the reflector 26, the first right circularly polarized single-mode wave 111 changes its direction of propagation and polarization to form a first left circularly polarized wave 111. And conversely, upon reflection on the reflector 26, the second left circularly polarized single-mode wave 112 changes its direction of propagation and polarization to form a second right circularly polarized wave 112. The two circularly polarized reflected waves 111, 112 propagate in the return direction in the detection optical fiber 73 while retaining their respective circular polarization.In the return direction, the optical retarder 42 receives the first left circularly polarized wave 111 and transforms it into a first linearly polarized wave TM. Similarly, in the return direction, the optical retarder 42 receives the second right circularly polarized wave 112 and transforms it into a second linearly polarized single-mode wave TE.

[0059] In the return direction, the first linearly polarized single-mode wave TM and the second linearly polarized single-mode wave TE are transmitted directly to the phase differential electro-optical modulator 16 and then to the polarizer 24.

[0060] In the return direction, the differential phase modulator and the polarizer recombine the first linearly polarized single-mode wave TM and the second linearly polarized single-mode wave TE to form a linearly polarized interferometric beam 300.

[0061] In all embodiments, the source-receiver splitter 22 guides the interferometric beam 300 to the photodetector 18. The detector 18 receives the interferometric beam and generates a detected signal 80.

[0062] In the second embodiment, illustrated in FIG. 2, the polarization splitter comprises a spatial splitter, for example consisting of a Y junction, denoted 15, which separates the linearly polarized source beam 110, for example TE, into a first TE-polarized single-mode wave 101 and a second TE-polarized single-mode wave 103. The polarizer 24 is advantageously a waveguide polarizer formed by the common branch of the Y junction, denoted 15. In this case, the first TE-polarized single-mode wave 101 and the second TE-polarized single-mode wave 103 each propagate on a single-mode waveguide separate from the phase differential electro-optic modulator 16, for example made of lithium niobate on an optical integrated circuit. Advantageously, the Y junction and the phase differential electro-optic modulator 16 are manufactured on the same optical integrated circuit 14.In this case, the waveguide is preferably formed by proton exchange, so as to guide a single polarization state on the common branch of the Y junction.

[0063] In the second embodiment, the optical fiber-based device 400 further comprises an optical fiber section 71 and another optical fiber section 72 each connected, on the one hand, to an output waveguide of the phase differential electro-optical modulator 16 and, on the other hand, to a polarization coupler-separator 27.

[0064] In the forward direction, at the output of the phase differential electro-optical modulator 16, the first single-mode wave 101 propagates in the polarization-maintaining optical fiber section 71. The second single-mode wave 103 propagates in the other polarization-maintaining optical fiber section 72. The other optical fiber section 72 is oriented so as to rotate the linear polarization of the second single-mode wave 103 by 90 degrees, which thus becomes a second linearly polarized single-mode wave 102 with a polarization orthogonal to the first single-mode wave 101. The polarization coupler-splitter 27 recombines the first single-mode wave 101 and the second single-mode wave 102, of orthogonal linear polarizations on the same single-mode waveguide. The polarization coupler-splitter 27 is connected to a proximal end of the linear polarization-maintaining optical fiber section 74.

[0065] The propagation of the first single-mode wave 101 and the second single-mode wave 102 following a round trip through the optical retarder 42, the detection optical fiber 73 and the reflection on the reflector 26 is analogous to that described in connection with the first embodiment.

[0066] In the return direction, after reflection on the reflector and transmission via the detection optical fiber 73, the optical retarder 42 and the fiber section 74, the first linearly polarized single-mode wave TM is transmitted via the other optical fiber section 72 which rotates the linear polarization of the first single-mode wave by 90 degrees, which thus becomes a first linearly polarized single-mode wave TE. The second linearly polarized single-mode wave TE is transmitted by the polarization-maintaining optical fiber section 71. The Y-junction 15 recombines the first linearly polarized single-mode wave TE and the second linearly polarized single-mode wave TE to form the linearly polarized interferometric beam 300, here TE.

[0067] In the first and second embodiments, the first single-mode wave 101 is linearly polarized according to a first polarization state and the second single-mode wave 102 is linearly polarized according to a second polarization state transverse to the first polarization state, propagating on a single waveguide, either directly at the output of the phase differential electro-optical modulator 16, or at the output of the polarization coupler-separator 27.

[0068] In the first and second embodiments, the phase differential electro-optic modulator 16 modulates the phase of the polarization of the first linearly polarized single-mode wave 101 TE with respect to the phase of the second linearly polarized single-mode wave 102 TM. For this purpose, the phase differential electro-optic modulator 16 applies a phase shift (Dm(t)) modulated periodically with a modulation period T.

[0069] Figure 3 represents an electric current or magnetic field sensor based on a fiber optic Sagnac interferometer, according to a third embodiment. We find the elements common to the first two embodiments: the light source 20 generating a source beam 100, a fiber-based device 400 optical system forming a closed optical path to encircle an electrical conductor 120, a detection system 18 and a signal processing system 900. In this third embodiment, the Sagnac interferometer, called loop, uses a closed optical path, generally between the two ends of a fiber optic coil 73. An optical component, for example consisting of a Y junction denoted 15, separates the source beam 100 into two separate waves which travel along the closed optical path in mutually opposite directions and the same optical component recombines the two waves at the output of the fiber optic coil. In a loop fiber optic interferometer, the two separate waves use the same polarization state on the closed optical path. At rest, the optical paths traveled by the two waves are perfectly reciprocal.The polarizer 24, the Y junction denoted 15 and the phase differential modulator 16 are arranged and operate in a manner analogous to the same elements described in connection with Figure 2.

[0070] In the third embodiment, the optical fiber-based device 400 comprises the detection optical fiber 73 connected at each of its ends to an optical phase retarder, denoted respectively 32 and 33. The optical fiber-based device 400 further comprises a polarization-maintaining optical fiber section 71 and another polarization-maintaining optical fiber section 72. The optical fiber section 71 is connected, on the one hand, to an output waveguide of the differential phase optical modulator 16 and, on the other hand, to the optical retarder 32. The other optical fiber section 72 is connected, on the one hand, to another output waveguide of the differential phase optical modulator 16 and, on the other hand, to the optical retarder 33. The detection optical fiber 73 is wound so as to form at least one turn around the electrical conductor 120, in an application to a current sensor.

[0071] The optical phase retarder 32, respectively 33, is for example a quarter-wave plate.

[0072] In the third embodiment, in the forward direction, at the output of the phase differential optical modulator 16, the first single-mode wave 101 propagates in the polarization-maintaining optical fiber section 71 and the second single-mode wave 102 propagates in the other polarization-maintaining optical fiber section 72. The first single-mode wave 101 and the second single-mode wave 102 are parallel linear polarizations, for example here TE. The optical retarder 32, respectively 33, receives the first single-mode wave 101, respectively the second single-mode wave 102, and transforms it into a first single-mode wave of circular polarization 111 right, respectively a second single-mode wave of circular polarization 112 right. The first single-mode wave of circular polarization 111 right, respectively a second single-mode wave of circular polarization 112 straight travel through the detection optical fiber 73 in mutually opposite directions. At the output of the detection optical fiber 73, the optical retarder 32, respectively 33, receives the second circularly polarized single-mode wave 112 straight, respectively the first circularly polarized single-mode wave 111 straight, and transforms it into a second linearly polarized single-mode wave TE, respectively a first linearly polarized single-mode wave TE. As in the second embodiment, the optical phase differential modulator 16 and the Y junction 15 recombine the first linearly polarized single-mode wave TE and the second linearly polarized single-mode wave TE to form a linearly polarized interferometric beam 300.

[0073] In the return direction, in the three embodiments described above, after a travel time AT, the phase differential optical modulator 16 receives the first linearly polarized single-mode wave and the second linearly polarized single-mode wave. The phase differential optical modulator 16 modulates the phase of the polarization of the first polarized wave with respect to the phase of the second polarized single-mode wave. Thus, the phase differential optical modulator 16 forms a first output wave and, respectively, a second output wave, having a modulated phase difference ACDm(t) = (Dm(t) - (Dmft-Ar). Here, T represents the round-trip travel time of each wave in the optical fiber device 400.

[0074] In the first embodiment, the travel time AT to be considered for the phase modulation ACDm(t) is the travel time of the first wave or the second wave for a round trip in the polarization-maintaining fiber section 74 and the detection optical fiber 73.

[0075] In the second embodiment, the travel time AT to be considered for the phase modulation ACDm(t) is the travel time of the first wave or the second wave for a passage in the optical fiber sections 71 and 72 and for a round trip in the polarization-maintaining fiber section 74 and the detection optical fiber 73.

[0076] In the third embodiment, the travel time T to be considered for the phase modulation ACDm(t) is the travel time of the first wave or the second wave for a passage in an optical fiber section 71, an optical fiber section 72 and for a single trip in the detection optical fiber 73.

[0077] In the examples illustrated in Figures 1 to 3, a digital signal processing system is used. Alternatively, the signal processing system may be entirely analog.

[0078] In Figures 1 to 3, the signal processing system 900 comprises for example an analog-to-digital converter 19, a digital processor 30, for example of the DSP (Digital Signal Processor), FPGA (Field Programmable Gate Array) or ASIC (Application Specific Integrated Circuit) type, and a digital-to-analog converter 31. The digital processor 30 makes it possible to extract a signal from a parameter to be measured 90, for example the magnetic field or the intensity of an electric current, on a digital output. The digital-to-analog converter 31 makes it possible to apply a modulation voltage 60 to the electrodes of the phase differential optical modulator 16. The results of the measured parameter(s) 90 are for example displayed on a human-machine interface.

[0079] The signal processing system 900 applies any of the known modulation-demodulation schemes to extract a phase difference measurement, which is then transformed into an electrical current intensity. A phase shift modulation (D m (t) is obtained by applying a modulated 60 V electrical voltage m (t) on the electrodes of the phase differential modulator 16. In the case of digital modulation, the modulation voltage comprises M modulation states where M is an integer, for example equal to 4, 6, 8 or 12. However, there are also analog modulation / demodulation schemes that a person skilled in the art will adapt without difficulty.

[0080] The modulation of the phase shift (Dm(t) is periodic with a modulation period T such that T / 2 = 1 / (2. F p) = AT where Fp represents the natural frequency of the fiber optic device 400. The detection system 18 acquires the power of the interferometric beam at the output of the interferometer according to the M modulation states. The signal processing system digitizes the detected interferometric beam and demodulates the detected signal.

[0081] In the application to an electric current sensor, the optical detection fiber 73 is wound to form a coil around an electrical conductor 120 in which an electric current having an intensity / flows. The axis of the coil of the optical detection fiber 73 coincides with the longitudinal axis of the electrical conductor 120. N denotes the number of turns of the optical detection fiber 73 closed around the electrical conductor 120. The electric current induces a magnetic field / ? along circular field lines around the circumference of the electrical conductor 120.

[0082] As described above, it is known that the interferometric phase shift A(|) introduced by the magneto-optic Faraday effect between the two circularly polarized waves at the output of the detection fiber 73 after a round trip in a reflection interferometry, is related to the intensity / of the electric current by the following equation, where V represents the Verdet constant of the detection optical fiber 73, when the interferometric system is perfect. [Math. 1]

[0083] In this case, the scale factor is equal to 4VN. The Verdet constant of silica is equal to approximately 0.6 rad T -1 m 1 at the wavelength of 1550nm. N represents the number of turns of the sensitive optical fiber, N being between 1 and 100000 and generally less than 20000.

[0084] As described above, in the case of a Sagnac interferometer, the scale factor is equal to 2VN.

[0085] An in-line or loop fiber optic interferometer comprising a detection optical fiber 73 thus makes it possible to measure a magnetic field or an electric current.

[0086] Figure 4 represents the response of the interferometer in terms of measured power as a function of the phase shift Acp(t) introduced by the phase differential modulator 16, in the case of a perfect interferometer. In theory, the phase shift is perfectly modulated between a maximum value, noted PO, and a minimum value Pmin, zero.

[0087] However, errors can affect the response of such an interferometer.

[0088] For example, in the first embodiment, the angle between the polarizer 24 and the phase differential optical modulator 16 is denoted 0. In general, as indicated above, 0 is adjusted to approximately 45 degrees. In the second embodiment, the angle 0 represents, for example, an imbalance between the two branches of the Y junction 15, which does not separate the source wave 50-50 between the two branches.

[0089] The modulation amplitude, noted Pmodulation, of the detected power is expressed as the difference between the maximum power, noted Pmax, and the minimum power, noted Pmin.

[0090] When the interferometer is perfect, we obtain, as illustrated in Figure 4: [Math 2]

[0091] However, the interferometric system may have a defect, for example a misalignment between the proper axes of the fibers 73 and 74, a defect in the optical retarder 42, which is not exactly a quarter-wave plate at the wavelength used or even a residual birefringence. In this case, assuming that VNI is small, the measured phase shift becomes A(|) = 4VNI / cos(2y) where y represents the angle of the defect considered, for example due to a misalignment of the fiber 74 or a defect in the quarter-wave plate 42. Here, an angle is considered small when it is less than approximately 0.1rad.

[0092] Figure 5 represents a part of the detected interferometric signal corresponding to the interferences between the two main waves and to the interferences between the two waves doubly coupled by a defect y, these waves having traveled through the interferometer as a function of a phase shift Acp(t) modulated as a function of the measurement of current or magnetic fields or as a function of the phase shift introduced by the modulation thanks to the modulator and in the presence of a defect y. We note Po the power of the source. Instead of reaching the maximum value Po, the maximum power of this part of the detected signal is limited to the value Po.cos 2 (y).

[0093] However, the defect also induces interference between other parasitic waves with simple coupling, which contribute to another part of the detected signal. As illustrated in Figure 6, it is shown that the amplitude of this other part of the interferometric signal as a function of a phase shift Acp(t) due to the signal to be measured has a maximum power equal to Po.sin 2 (y). Phase modulation through the modulator has no effect on these signals since they are decorrelated from the main and dual signals of Figure 5.

[0094] More precisely, when the interferometer is imperfect, we obtain, as illustrated in Figure 7, the sum of these two interference signals corresponding to a detected interferometric signal which is modulated between a value Pmin, equal to Po.sin 2 (y) and a maximum value Pmax. More precisely, Figure 7 represents the power as a function of the modulation applied to the phase modulator. [Math 3]

[0095] We observe that the interferometer is no longer perfectly contrasted, the minimum power Pmin not being zero and the modulation amplitude being reduced, which degrades the scale factor of the interferometric system.

[0096] In the case of a Sagnac interferometer, this technical problem of degradation of the scale factor also occurs, for example, if there is a defect on each quarter-wave plate 32, 33 at the ends of the optical detection fiber 73. We observe in In this case, Pmin remains close to zero and the value of Pmax is reduced. Therefore, the Pmax-Pmin amplitude decreases compared to a Sagnac interferometer using perfect components.

[0097] These undesirable effects may be due to parasitic couplings in the optical fiber interferometer. They may originate from thermal instabilities in the detection optical fiber 73 due to a defect in the orientation or optical delay of the optical retarder 42, or respectively of each of the optical retarders 32 and 33.

[0098] A graphical representation of the phase shift measurement error related to this degradation of the scale factor is illustrated in connection with Figures 8 and 9. We now detail the propagation of polarized waves in the online fiber optic interferometer in terms of phase vector in connection with Figure 8, that is to say in the case of an imperfect but resting interferometer, in the absence of magneto-optical Faraday effect.

[0099] At the output of the phase differential optical modulator 16, in the return direction, let us denote AH22 the first polarized single-mode wave, that is to say the main wave passing TE polarized (or in “1”) in the phase differential optical modulator 16 in the forward direction, right circular polarized (in “1”) in the detection optical fiber 73 in the forward direction and left circular polarized (in “2”) in the detection optical fiber 73 in the return direction and TM polarized (in “2”) in the phase differential optical modulator 16 in the return direction.At the output of the phase differential optical modulator 16, in the return direction, let us note A2211 the second polarized single-mode wave, that is to say the main wave passing TM polarized (at “2”) in the phase differential optical modulator 16 in the forward direction, left circular polarized (at “2”) in the detection optical fiber 73 in the forward direction then right circular polarized (at “1”) in the detection optical fiber 73 in the return direction and TE polarized (at “1”) in the phase differential optical modulator 16 in the return direction.

[0100] The recombination of the first wave An22 and the second wave A2211 forms the main interferometric signal.

[0101] However, in case of misalignment of the interferometer, there are 6 other parasitic waves: two waves resulting from double couplings (third and fourth wave) and four other waves resulting from single couplings (fifth to eighth wave).

[0102] The third wave A is the passing wave polarized TE (at "1") in the phase differential optical modulator 16 in the forward direction, left circular polarized (at "2") in the detection optical fiber 73 in the forward direction, right circular polarized (at "1") in the detection optical fiber 73 in the return direction and polarized TM (at "2") in the phase differential optical modulator 16 in the return direction.

[0103] The fourth wave A2121 is the passing wave polarized TM (at “2”) in the phase differential optical modulator 16 in the forward direction, right circular polarized (at “1”) in the detection optical fiber 73 in the forward direction, left circular polarized (at “2”) in the detection optical fiber 73 in the return direction and polarized TE (at “1”) in the phase differential optical modulator 16 in the return direction.

[0104] The third wave A and the fourth wave A2121 are coherent with the first wave An22 and the second wave A2211 because they see exactly the same phase shifts. In addition, the third wave A1212 and the fourth wave A2121 are modulated in a similar way to the main signal, it is these two waves that create a scale factor problem.

[0105] There is also a fifth wave, denoted A i which is the passing wave polarized TE (at “1”) in the modulator in the forward direction, left circular polarized (at “2”) in the detection optical fiber 73 in the forward direction, right circular polarized (at “1”) in the detection optical fiber 73 in the return direction and polarized TE (at “1”) in the phase differential optical modulator 16 in the return direction.

[0106] The sixth wave, denoted A2122, is the TM polarized passing wave (at “2”) in the modulator in the forward direction, right circular polarized (at “1”) in the detection optical fiber 73 in the forward direction, left circular polarized (at “2”) in the detection optical fiber 73 in the return direction, and TM polarized (at “2”) in the phase differential optical modulator 16 in the return direction.

[0107] The seventh wave, noted Aim, is the wave passing polarized TE (at "1") in the modulator in the forward direction, right circular polarized (at "1") in the optical fiber detection fiber 73 in the forward direction, left circular polarized (at “2”) in the detection optical fiber 73 in the return direction and TE polarized (at “1”) in the phase differential optical modulator 16 in the return direction.

[0108] The eighth wave, denoted A2212, is the TM polarized passing wave (at “2”) in the modulator in the forward direction, left circular polarized (at “2”) in the detection optical fiber 73 in the forward direction, right circular polarized (at “1”) in the detection optical fiber 73 in the return direction and TM polarized (at “2”) in the phase differential optical modulator 16 in the return direction.

[0109] The fifth wave Aun and the seventh wave A i are coherent with each other. The sixth wave A2122 and the eighth wave A2212 are also coherent with each other. On the other hand, the sixth wave and the eighth wave are not coherent with the fifth wave and the seventh wave. This is because we are using a broad spectral band source, i.e. one with a limited coherence length. Paths 1 and 2 have a difference in optical path length greater than the coherence length of the source.

[0110] The fifth through eighth waves do not create a scale factor problem. However, these waves can tell us about the proportion of waves that create a scale factor problem. Indeed, by conservation of energy, they are the complements of the waves used for measurement.

[0111] Figure 8 represents the first to fourth waves, in the form of phase vectors or phasors, in the absence of the magneto-optical Faraday effect. The first wave A1122 and the second wave A2211 have exactly the same amplitude. The third wave A1212 and the fourth wave A2121 have exactly the same amplitude, but their amplitude is much lower than that of the first and second waves, since the third wave A1212 and the fourth wave A2121 result from a defect or parasitic coupling of the interferometer. Consequently, the resultant 131 of the combination of the first wave A1122 and the third wave A1212 is of reduced amplitude compared to the amplitude of the first wave and parallel to this first wave. The resultant 124 of the combination of the second wave A2211 and the fourth wave A2121 is of reduced amplitude compared to the amplitude of the second wave, and parallel to this second wave.

[0112] Figure 9 represents the first to fourth waves in the form of phasors, in the presence of the magneto-optical Faraday effect. The magneto-optical Faraday effect induces a phase shift equal to A <I»F / 2 sur la première onde A1122 et un déphasage égal à -A<I»F / 2 sur la deuxième onde A2211. Simultanément, l'effet Faraday magnéto-optique induit aussi un déphasage égal à A<I»F / 2 sur la troisième onde A1212 et un déphasage égal à -A F / 2 sur la quatrième onde A2121. La résultante 131 de la combinaison de la première onde A1122 et de la troisième onde A1212 présente une amplitude et un angle de phase modifiés par rapport à la première onde A1122. De manière analogue, la résultante 124 de la combinaison de la deuxième onde A2211 et de la quatrième onde A2121 présente une amplitude et un angle de phase modifiés par rapport à la deuxième onde A2211. La figure 9 illustre schématiquement l'erreur introduite par les troisième et quatrième ondes sur la mesure de déphasage magnéto-optique.It is also observed that the amplitude of the resulting waves is reduced compared to the amplitude of the main waves.

[0113] This magneto-optical phase shift measurement error induces a defect in the scale factor. As seen above, in a perfect reflection interferometer, the scale factor is equal to 4VN. In a reflection interferometer with defects, the scale factor is modified and becomes 4VN / cos(2y). In a Sagnac interferometer, the scale factor is modified in a similar way and becomes 2VN / cos(2y).

[0114] According to the present disclosure, the power modulation amplitude of the detected interferometric beam, i.e. Pmax-Pmin or Pmodulated, is proportional to cos 2 (2y), where y represents the angle of the defect impacting the scale factor, and the minimum power of the detected interferometric beam, Pmin, is proportional to sin 2 (2y).

[0115] From a measurement of the power modulation amplitude of the detected interferometric beam and / or a measurement of the minimum power of the detected interferometric beam, a measurement of the angle of an impacting scale factor defect is deduced.

[0116] In particular, in a reflection interferometer, the detection that the minimum power Pmin is non-zero makes it possible to highlight a scale factor defect. For example, the power arriving at the photodetector is approximately P=30pW at maximum power. In the presence of a fault such as cos(2y)=99.5%, the minimum power is equal to Pmin=P*sin 2 (2y), or 1% of P, i.e. approximately 0.3 pW. The power modulation amplitude Pmax-Pmin is then equal to 99% of P, or 29.7 pW. In a loop interferometer (3 ème embodiment), there is no variation of the minimum but only of the amplitude Pmax -Pmin.

[0117] More generally, a measurement of variation of the minimum power of the detected interferometric beam or a measurement of variation of the amplitude of power modulation of the detected interferometric beam makes it possible to measure a variation of the scale factor as a function of time for an online fiber optic interferometer.

[0118] In practice, there are different ways to measure the minimum power Pmin and the power modulation amplitude of the detected interferometric beam.

[0119] We will now detail an interferometric system with an example of phase modulation and demodulation of the detected signal, adapted to measure a signal representative of an interferometer misalignment, this misalignment being likely to degrade the scale factor.

[0120] In the field of in-line fiber optic interferometric systems, various modulation and demodulation techniques are known. This modulation is obtained by applying a modulated electrical voltage Vm(t) between the electrodes of the phase differential modulator 16 to modulate the phase difference ACDm(t) of the measured interferometric signal. This modulation allows biasing which increases the sensitivity of the interferometric system, particularly for low amplitude magnetic field or electric current measurements.

[0121] In particular, it is known to apply a so-called 2-state modulation, by modulating the modulation voltage Vm in square between two step values, so as to produce a modulation of the phase difference on two levels, for example of ACDb(t) = ± n / 2 , called biasing phase difference, at the natural frequency Fp of the optical fiber-based device 400. The natural frequency Fp is defined in such a way that T / 2 = 1 / (2. Fp) = AT where T represents the period of the square modulation. Thus, the modulation half-period T / 2 corresponds to the travel time AT of each single-mode wave in the optical fiber-based device 400. The signal processing system Tl digitizes the detected interferometric beam and demodulates the detected signal at the natural frequency Fp by sampling two power measurements on each modulation period and assigning a negative sign to a first level and a positive sign to the next level.

[0122] In order to extend and linearize the response dynamics of an interferometric system, it is also known to apply a feedback signal.

[0123] In the remainder of this document, level (or modulation level) means the asymptotic value of the different modulated phase difference values ​​A <I» m for each modulation step. Modulation states are the different measured power values ​​P corresponding to the modulation levels that follow one another over each modulation period. Over a modulation period, several states can use the same modulation level.

[0124] In the context of the present disclosure, we consider the case of a digital modulation with at least 4 states, to enable the measurement of, on the one hand, the magneto-optical Faraday phase shift, and on the other hand, the minimum power and / or the power modulation amplitude (PMax-Pmin).

[0125] In the above field, patent FR2654827_A1 proposes to apply a so-called 4-state modulation voltage which generates 4 successive modulation levels on each modulation period T equal to 2AT, and to sample 4 power measurements on each modulation period. In 4-state modulation, it is also possible to extract a signal, called Vn or Vpi, modulated at 2Fp. The signal Vn represents the transfer function of the phase differential modulator, that is to say the ratio between the voltage applied Vm to the modulator and the phase shift (Dm induced, with Vn / n = Vm / (Dm. However, this signal Vn fluctuates with the environment, for example with the temperature of the phase differential modulator 16.

[0126] In the field of fiber optic interferometric systems, patent EP2OO5113_B1 describes a so-called 6-state modulation, based on 4 levels of biasing phase difference. This 6-state modulation can be broken down into a superposition of a first modulation of the phase shift (Dm(t) of ±n / 2 at the frequency own Fp and a second modulation of the phase shift (Dm(t) at ±alpha / 2 to 3*Fp. We sample 6 power measurements on each modulation period.

[0127] Patent EP2OO5113_B1 also describes the use of an 8-state, 8-level modulation over a total period T equal to 4AT. According to this conventional 8-state modulation, the modulation is performed first on 4 high states corresponding to ± (alpha + beta) and then on 4 other low states corresponding to ± (alpha - beta). The output power P is sampled in 8 measurements Pi corresponding to the 8 states i = 1, ..., 8 per modulation period.

[0128] Finally, patent application FR 3095053A1 describes a system based on modulation comprising at least eight modulation levels per modulation period T and on the acquisition of at least 12 power measurements of the interferometric beam detected per modulation period. The signal processing extracts a signal representative of the quantity to be measured which is equal to a sum of the power measurements of the interferometric beam acquired per modulation period, each measurement being assigned a sign +1 or -1.

[0129] Advantageously, the interferometer comprises a feedback system adapted to control the measurement of the signal representative of the quantity to be measured, on the basis of a transfer function signal of the differential phase modulator and / or the transfer function signal of the detection system.

[0130] In the example detailed here, we use an 8-level, 12-state modulation / demodulation scheme, based on that of patent application FR 3 095 053. The modulation scheme is illustrated in Figure 10. We recall that the voltage and phase modulations are homothetic because we are at the natural frequency Fp. More precisely, we are based on an 8-level modulation of A(D m (t) and 12 states of the corresponding detected power P(t).

[0131] The modulation is defined in the following Table I, where a (also noted alpha) represents the classical modulation from n and P (also noted beta) the overmodulation added to modulate on 8 levels: [Table 1]

[0132] According to a particular and advantageous aspect, the demodulation of the 12 states is defined in the following Table 2, which shows the sign or coefficient applied to the measurement of each of the 12 power states to deduce the different signals. More precisely, as indicated in the first line of Table 2, a signal representative of the magnetic field or the electric current is calculated, corresponding to the magneto-optical Faraday phase shift. As indicated in the second line of Table 2, optionally, a signal representative of the signal Vn (or Vpi), i.e. representative of the transfer function of the differential optical phase modulator, is calculated. As indicated in the third line of Table 2, according to one aspect of the present disclosure, a signal representative of the modulated power is calculated. Optionally, as indicated in the fourth line of Table 2, a signal representative of the average power is calculated.The measurement of average power makes it possible to differentiate defects or variations in scale factor from variations in source power. [Table 2]

[0133] This provides a modulation-demodulation method that is perfectly suited to simultaneously measuring, at each modulation period, a measurement of electric current or magnetic field and a measurement of the modulated power. Optionally, a measurement of the average power and / or a measurement of the signal Vn (or Vpi) representative of the transfer function of the phase differential modulator are also calculated.

[0134] We first calculate the response of the interferometer in the ideal or defect-free case, for example shown in Figure 1. We then show how a scale factor defect y affects the different measurements, in open loop and, respectively, in closed loop.

[0135] It is assumed that the light source 20 emits an unpolarized source beam 100. The source beam 100 has a total power Po = Ao 2with Ao the amplitude of the source wave. The input-output polarizer 24 is for example a polarizing fiber, also called a PZ fiber, whose axes are perfectly aligned at 45 degrees to the birefringence axes of the optical modulator 16 at the input-output 25 of the integrated optical circuit 34. It is assumed that the linear polarization-maintaining optical fiber 74 is perfect, i.e. without coupling defects between transverse polarization modes. The linear polarization-maintaining optical fiber 74 is also called an offset fiber. It is assumed that the quarter-wave plate 42 is perfect and oriented at 45 degrees relative to the natural axes of the linear polarization-maintaining optical fiber 74. The polarization-maintaining detection optical fiber 73, twisted or twisted at the fiber drawing, is also called a SPUN fiber or sensor fiber because it is the sensitive part of the magnetic field or electric current sensor.It is assumed that the sensor fiber (spun) 73 perfectly preserves the right circular polarization and, respectively, the left circular polarization. We note cps the phase shift introduced by the magnetic field magneto-optical phase shift.

[0136] At the output of the polarizer 24 (PZ fiber), the power of the light beam is reduced by half because the light source 20 is unpolarized.

[0137] The power Pi at the output of the polarizing fiber 24 is therefore equal to: [Math 4]

[0138] The amplitude of the wave at the output of the polarizer 24 (at the input of the phase differential modulator) is therefore Ao / / ^ 2 along the polarization axis of the PZ 74 fiber and zero on the other axis because we are considering here a perfect polarizer 24.

[0139] We note 0 the angle between the polarization axis of the PZ fiber 24 with the TE axis of the phase differential modulator 16. We can now project the linearly polarized beam onto the TE and TM axes of the phase differential modulator 16. We obtain the following amplitudes: [Math 5

[0140] Each polarized wave propagates perfectly in the interferometer except for the coupling between the phase differential modulator 16 and the linear polarization maintaining fiber 74. We note y the angle between the TE axis of the phase differential modulator 16 and the fast axis or TE axis of the PM fiber 74.

[0141] Result of closed-loop demodulation

[0142] In all cases, it is assumed that the voltage Vpi is well controlled by the standard demodulation because the demodulated states are similar to those of a standard Sagnac loop fiber optic interferometer in all cases. Only the noise is different here from that of a Sagnac loop fiber optic interferometer.

[0143] The calculations are applied directly for each modulation state (states 1 to 12 for example) with the appropriate sign indicated on the first line of Table 2. The magneto-optical phase shift cps is considered small and constant over a modulation period 1 / Fp.

[0144] It follows that the power of the modulated / demodulated current is expressed as follows (in the absence of fault y): [Math 6]

[0145] In a closed loop, we control so that: [Math Where cp r represents the feedback achieved using the modulator.

[0146] In the presence of a defect y affecting the scale factor, the response of the interferometer is modified. For example, we consider a misalignment of an angle y of the PM 74 fiber at the output of the phase differential modulator 16. In this case, we control in a closed loop according to the following relationship between cp r and cps.

[0147] If cp s is small and cp r is small (i.e. of the order of a few percent of 2Pi), the above expression is modified as follows:

[0148] Result of demodulation of current closed loop modulated power

[0149] The calculations are applied directly for each of the 12 modulation states with the appropriate sign indicated in 3 ème line of Table 2.

[0150] In the absence of fault y, the modulated / demodulated power in closed current loop is written as follows: [Math 10] ^puissa this mod uiéedé mod sill( — ) [0151) 2

[0152] In the presence of a fault affecting the scale factor, the modulated / demodulated power in closed current loop is modified and is written as follows:

[0153] It follows that the modulation amplitude Pmodulated = Pmax-Pmin is equal to the modulation amplitude in the absence of a fault multiplied by cos 2 (2y).

[0154] If y is small and 0 is close to 45 degrees, and if <p s is small and <p r is small (that is to say the order of a few percent of 2Pi), we deduce: . We can therefore deduce 4*cp s which is (pr*cos(2y).

[0155] This modulation and demodulation method allows the measurement of the fault y through the modulated power. Indeed, the scale factor (FE) depends only on y. The modulated power depends on P, a, Ao, 0, and y. However, P and a are controlled by the control of the voltage Vpi. On the other hand, Ao and 0 are relative to the total power that can be calculated.

[0156] It is thus possible to measure the y coefficient precisely and in real time. This measurement makes it possible to compensate for the scale factor error in real time, at each modulation period.

[0157] Particularly advantageously, from the real-time measurement of the scale factor defect, the interferometric system is adapted to compensate or correct the variations of the scale factor as a function of time and in real time.

[0158] Variations in scale factor are for example due to misalignments of the fibers and the quarter-wave plate which can vary in temperature.

[0159] Result of Vpi demodulation in current closed loop

[0160] We apply the calculations directly for each of the 12 modulation states with the appropriate sign indicated in the 2nd line of Table 2 by creating a Vpi error for example for state 1: na becomes (na).(l+s) with E small and so on for each of the other states.

[0161] In the absence of fault y, the demodulated VK power is written as follows:

[0162] In the presence of a defect affecting the scale factor, and if <p s is small and <p r is small (i.e. of the order of a few percent of 2Pi), the above expression is modified and is written as follows: P[pi demodulated^ F PI = .0. s [Math 13j 4

[0163] When the interferometer is controlled in closed loop on the voltage Vpi, the voltage Vpi is adjusted so that Pvpidemodulated is zero and therefore so that E is zero. We also have control of the parameters a and P by this parameter because the power of the voltage Vpi is calculated in real time at each modulation period, every 1 / Fp. However, the control of the Vpi makes it possible to know precisely the values ​​of a and p.

[0164] Result of closed-loop average power demodulation (++++)

[0165] The average power is demodulated by applying to the 12 modulation states the appropriate signs indicated in 4 ème line of Table 2.

[0166] In the absence of fault y, the average demodulated power is expressed as follows:

[0167] In the presence of a fault affecting the scale factor, the expression for the average power is modified and is written as follows:

[0168] It is assumed here that the fault y has been determined from the modulation of the modulated power, as indicated in the paragraph result of demodulation of the modulated power in closed loop of current. The demodulation of the average power makes it possible, for example, to determine the variations of the power of the source A0 as a function of time and 9.

[0169] The above calculations apply to a 12-state modulation for a fault-free current sensor and for a coupling fault of the offset PM fiber on the phase differential modulator. Those skilled in the art will appreciate that calculations similar also apply to quarter-wave plate defects, or to a birefringence defect, or to any other defect of the interferometer.

[0170] The method described above also makes it possible to deduce a measurement of the temperature of the optical retarder 42, for example a quarter-wave plate.

[0171] Indeed, the voltage Vpi is a good measure of the temperature at the level of the integrated optical circuit 34, which therefore makes it possible to control the temperature of the integrated optical circuit 34 and the electrical box 35.

[0172] To this end, the quarter-wave plate is deliberately shifted to create a significant but still small y defect, between 0.1% and a few %, for example 5%. A variation in temperature at the quarter-wave plate causes a variation in y. Particularly advantageously, the quarter-wave plate is shifted sufficiently to make the evolution monotonous and linear as a function of temperature variations, which is thus easy to measure.

[0173] Measuring the variations in y provides information on the temperature at the quarter-wave plate. By learning and calibrating, it is thus possible to compensate for defects and even variations in the Verdet constant of the optical detection fiber 73.

[0174] Of course, various other modifications may be made to the invention within the scope of the appended claims.

Claims

Claims: Optical fiber interferometer comprising a light source (20) capable of generating a source beam (100), a differential phase modulator (16), an optical fiber-based device (400), a detection system (18), a signal processing system (900), the optical fiber-based device (400) comprising a detection optical fiber (73) having a Verdet constant capable of inducing a non-reciprocal magneto-optical Faraday effect, the detection optical fiber (73) being disposed in a magnetic field or forming at least one turn around an electrical conductor (120), the optical fiber interferometer being capable of detecting a phase difference of an interferometric beam (300) formed by interference between two polarized light waves (111, 112) having simultaneously traveled the detection optical fiber (73) along a closed optical path, the two polarized light waves (111,112) being modulated by the differential phase modulator, and to deduce, by dividing the phase difference by a scale factor, a value of the magnetic field integrated along the closed optical path or a value of an electric current passing through the electrical conductor (120), the scale factor being proportional to the Verdet constant of the optical fiber (73),characterized in that the signal processing system (900) is adapted and configured to measure a power contrast variation of a portion of the interferometric beam modulated by the differential phase modulator and to deduce from the contrast variation a measure of the scale factor variation. A fiber optic interferometer according to claim 1, wherein the signal processing system (900) is adapted and configured to measure a minimum power of the detected portion of the modulated interferometric beam and / or a difference between a maximum and a minimum power of the detected portion of the modulated interferometric beam. A fiber optic interferometer according to claim 1 or 2, wherein the signal processing system (900) is adapted and configured to correct the scale factor in real time based on the measured scale factor variation. Optical fiber interferometer according to any one of claims 1 to 3 wherein the detection optical fiber (73) is circularly polarization maintained, the optical fiber-based device (400) comprising an optical phase retarder (42) and a reflector (26), the optical phase retarder (42) being disposed at one end of the detection optical fiber (73) and the reflector (26) at the other end of the detection optical fiber (73), the interferometer being configured such that the two polarized light waves (111, 112) travel along the detection optical fiber (73) in a round trip with two orthogonal circularly polarized states reversing by reflection on the reflector (26).Optical fiber interferometer according to claim 4 in which the differential phase modulator (16) is an electro-optical birefringence modulator comprising a single waveguide capable of guiding two orthogonal states of linear polarizations oriented along two perpendicular axes, the interferometer comprising a polarizer (24) disposed between the light source (20) and the electro-optical birefringence modulator (16), the polarizer being oriented at 45 degrees from the axes of the electro-optical birefringence modulator (16), and in which one end of the electro-optical birefringence modulator is connected to the optical fiber-based device (400).optical fiber interferometer according to claim 4 comprising a Y-junction splitter disposed between the light source (20) and the differential phase modulator (16), wherein the differential phase modulator (16) comprises two waveguides capable of guiding two beams of the same linear polarization and each having a phase modulator, and wherein the optical fiber-based device (400) comprises a polarization-maintaining optical fiber section (71) and another polarization-maintaining optical fiber section (72), the optical fiber section (71), and respectively, the other optical fiber section (72), each being connected, on the one hand, to one of the two waveguides of the differential phase modulator (16) and, on the other hand, to a polarization coupler-splitter (27), the other optical fiber section (72) being oriented so as to rotate a linear polarization by 90 degrees. An optical fiber interferometer according to any one of claims 1 to 3, wherein the detection optical fiber (73) is circularly polarization-maintaining, the optical fiber-based device (400) comprising a Y-junction separator disposed between the light source (20) and the differential phase modulator (16), wherein the differential phase modulator (16) comprises two waveguides capable of guiding two beams of the same linear polarization, and wherein the optical fiber-based device (400) comprises a polarization-maintaining optical fiber section (71), an optical phase retarder (32), and another optical fiber section. (72) polarization-maintaining, and another optical phase retarder (33), the optical fiber section (71), and respectively, the other optical fiber section (72), each being connected, on the one hand, to one of the two waveguides of the differential phase modulator (16) and, on the other hand, to the optical phase retarder (32), respectively, to the other optical phase retarder (33), the optical phase retarder (32) being disposed at one end of the detection optical fiber (73) and the other optical phase retarder (33) being disposed at the other end of the detection optical fiber (73), the interferometer being configured so that the two polarized light waves (111, 112) travel along the detection optical fiber (73) in opposite directions with the same circular polarization state. Fiber optic interferometer according to any one of claims 4 to 7, wherein the optical phase retarder (42, 32), and / or respectively the other optical phase retarder (33), each form a quarter-wave plate at a wavelength of the source beam (100). Fiber optic interferometer according to claim 8, wherein the optical phase retarder (42, 32) and / or respectively the other optical phase retarder (33) is shifted so as to introduce a defect, and wherein the signal processing system (900) is adapted to extract from the detected interferometric signal a measure of the variation in the system's scale factor and deduce therefrom a temperature variation of the optical phase retarder (42, 32), respectively of the other optical phase retarder (33). Method for measuring magnetic field or electric current based on an optical fiber interferometer according to any one of claims 1 to 9, the method comprising the following steps: emission of a source beam (100) from a light source (20), separation of the source beam into two polarized light waves; differential phase modulation of the two polarized light waves;transmission of the two polarized light waves to an optical fiber-based device comprising a detection optical fiber (73) such that the two polarized light waves (111, 112) travel simultaneously along the optical fiber (73) in a closed optical path, the detection optical fiber (73) having a Verdet constant capable of inducing a non-reciprocal magneto-optical Faraday effect, the detection optical fiber (73) being disposed in a magnetic field or forming at least one turn around an electrical conductor (120), recombination of the two polarized light waves (111, 112) at the output of the optical fiber-based device (400) to form an interferometric beam (300), detection of the interferometric beam (300);and processing of the detected signal to extract a measurement of a phase difference of the interferometric beam (300) and to deduce, by dividing the phase difference by a scale factor, a value of the magnetic field integrated along the closed optical path or a value of an electric current passing through the electrical conductor (120), characterized in that: the signal processing is adapted and configured to measure a power contrast variation of a part of the interferometric beam modulated by the differential phase modulator and to deduce from the contrast variation a measure of the scale factor variation.;

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