FIBER OPTIC ACCELERATION SENSOR

Description

TECHNICAL AREA

[0001] Embodiments of the present invention generally relate to a fiber optic accelerometer, in particular for determining acceleration along an axis, and furthermore to a method for manufacturing a fiber optic accelerometer. In particular, embodiments relate to a fiber optic accelerometer system and a wind turbine. STATE OF THE ART

[0002] Measuring, monitoring, or controlling the acceleration of objects is of great importance in many areas of industrial application. Conventional acceleration measurement systems use a spring-mass system to determine acceleration and are based on detecting the displacement of an object from a rest position. In particular, in a fiber optic measurement system, the displacement of a diaphragm can be detected by measuring changes in the properties of light. To make such a fiber optic measurement system more sensitive, the spring action of the diaphragm is conventionally altered, or the diaphragm is subjected to an additional mass, which in many cases leads to an asymmetrical design of the spring-mass system.

[0003] However, existing solutions for measuring acceleration have several disadvantages. In particular, conventional fiber optic measurement systems exhibit cross-sensitivity to rotational accelerations, low stability, or low sensitivity.

[0004] Document EP 3 227 552 B1 discloses a fiber-optic accelerometer for determining acceleration along an axis, comprising an optical fiber with a fiber end face and a vibration diaphragm with a diaphragm frame and a deflection mass. The vibration diaphragm is configured to reflect at least some of the primary radiation emanating from the fiber end face.

[0005] Document DE 11 2005 003758 B4 proposes a vibration diaphragm equipped with connecting bridges for a similar sensor.

[0006] Document US 2019 / 360323 A1 discloses an acceleration sensor according to the preamble of claim 1. SUMMARY OF THE INVENTION

[0007] The invention is defined by the independent claims. Preferred embodiments are found in the dependent claims. One aspect relates to a fiber optic accelerometer for determining acceleration along an axis, comprising an optical fiber with a fiber end face, and a vibration diaphragm with a diaphragm frame, a deflection mass, and connecting bridges, wherein the vibration diaphragm is configured to reflect primary radiation emanating from the fiber end face at least partially, and wherein the connecting bridges connect the diaphragm frame and the deflection mass at at least two different axial positions.

[0008] Another aspect concerns a fiber optic acceleration sensor system with at least two fiber optic acceleration sensors according to the embodiments described herein, wherein the fiber optic acceleration sensor system is configured to determine accelerations along at least two axes.

[0009] Another aspect concerns a wind turbine with at least one fiber optic acceleration sensor according to the embodiments described herein or at least one fiber optic acceleration sensor system according to the embodiments described herein.

[0010] Another aspect relates to a method for manufacturing a fiber optic accelerometer for determining an acceleration along an axis, in particular a fiber optic accelerometer according to embodiments described herein, comprising providing a transparent substrate and selectively laser etching a vibration membrane from the transparent substrate, wherein the vibration membrane comprises a membrane frame, a deflection mass and connecting bridges, and wherein the connecting bridges connect the membrane frame and the deflection mass at at least two different axial positions. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Examples of implementation are shown in the drawings and explained in more detail in the following description: Figure 1 schematically shows a fiber optic acceleration sensor with associated beam path, according to one embodiment; Figure 2schematically shows a vibration membrane of a fiber optic acceleration sensor according to one embodiment; Figure 3 schematically shows part of a wind turbine with rotor blades and attached fiber optic acceleration sensors; Figure 4 Figure 1 schematically shows a rotor of a wind turbine with rotor blades and fiber optic acceleration sensors according to the embodiments described herein; Figure 5 schematically shows a measuring system for a fiber optic measuring system according to the embodiments described herein; Figure 6 schematically shows another measuring system according to the embodiments described herein; and Figure 7 Figure 1 shows a flowchart illustrating a method for manufacturing a fiber optic accelerometer according to the embodiments described herein.

[0012] In the drawings, identical reference numerals denote identical or functionally equivalent components or steps. For clarity, not all features of a type are marked with a reference numeral, for example, the connecting bridges (reference numeral: 125) in Figure 1 . WAYS TO IMPLEMENT THE INVENTION

[0013] The following section provides a detailed account of various embodiments of the invention, with one or more examples illustrated in the drawings. The term "or" is to be understood here specifically as "and / or".

[0014] Fiber optic sensors, which rely on acceleration-dependent changes in, for example, the light intensity or phase of light propagating through the optical fiber, have great application potential. Fiber optic acceleration measurement is particularly advantageous in areas of objects exposed to lightning strikes and therefore inaccessible to electrical measurement. Conveniently, any electrical wiring along the object is avoided, allowing for purely optical data acquisition.

[0015] For high-resolution acceleration measurements, fiber optic accelerometers are used according to the embodiments described herein, which comprise an optical fiber and a vibrating diaphragm. The vibrating diaphragm, for example, a deflection mass of the vibrating diaphragm, and a fiber end face of the optical fiber can be reflective. Thus, for example, an optical resonator of a Fabry-Perot interferometer can be constructed from the optical fiber and the vibrating diaphragm. When the fiber optic accelerometer is accelerated, the deflection mass of the vibrating diaphragm can be deflected relative to the diaphragm frame, thereby changing, in particular, the interference wavelength of the optical resonator. The acceleration can be detected via the change in the interference wavelength.

[0016] Examples of objects where acceleration measurements should be performed without electrical wiring include wind turbine rotor blades or aircraft wings. With suitable mounting techniques, distributed fiber optic accelerometers can be used to detect accelerations, such as vibrations, on such objects. For example, wind turbines are subject to complex control systems that rely on sensor data from the turbine itself, such as reliable acceleration data. Transmitting optical signals via fiber optics reduces the risk of lightning damage. Therefore, fiber optic accelerometers can be designed to allow mounting in the radially outer area of ​​a rotor blade without increasing the risk of lightning damage.

[0017] Figure 1Figure 1 schematically shows a fiber optic accelerometer 110, which can be configured as a Fabry-Perot interferometer. In particular, the fiber optic accelerometer 110 is shown in a longitudinal section along an axis 127, along which the fiber optic accelerometer 110 can measure acceleration. Furthermore, a beam path is shown to illustrate the principles of the invention. The fiber optic accelerometer 110 comprises an optical fiber 112 with a fiber end face 131 and a vibration diaphragm 103. For the sake of simplicity, the following is shown in Figure 1 Only a part of the optical fiber 112 is shown, while a fiber sheath and a fiber protective sheath are not shown for the sake of clarity.

[0018] A cavity 107 is formed between the fiber end face 131 and the vibration diaphragm 103, in particular between the fiber end face 131 and the deflection mass 123. The vibration diaphragm 103 comprises a diaphragm frame 121, connecting bridges 125, and a deflection mass 123. The diaphragm frame 121 is arranged, in particular, at least substantially stationary relative to the fiber end face 131. For example, the diaphragm frame 121 is rigidly connected to the optical fiber 112. The deflection mass 123 is connected to the diaphragm frame 121 via the connecting bridges 125. The connecting bridges can be designed as spring elements between the diaphragm frame 121 and the deflection mass 123. When the fiber optic accelerometer 110 is accelerated, the deflection mass 123 can be deflected relative to the diaphragm frame 121. In particular, the deflection mass 123 can be deflected along the axis 127.According to embodiments, the deflection mass 123 can be designed to be at least substantially cylindrical, wherein, for example, the cylinder axis of the cylindrical deflection mass 123 runs along the axis 127.

[0019] According to embodiments, the vibration diaphragm 103 has a reflective diaphragm surface. In particular, the reflective diaphragm surface is oriented perpendicular to the axis 127. In the Fig. 1 For example, an axial membrane surface 133 on one side of the vibrating membrane 103 facing the cavity 107 is configured as a reflective membrane surface. In particular, the reflective membrane surface and the fiber end surface 131 are aligned at least substantially parallel to each other. The fiber end surface 131 can be reflective. In further embodiments, another reflective surface is arranged in the beam path between the fiber end surface 131 and the vibrating membrane 103.

[0020] In this context, "reflective" or "mirror" means in particular that at least a proportion of the intensity of an incident light beam is reflected, for example a proportion of at least 0.5% of the incident light beam, in particular a proportion of at least 1% or at least 2%.

[0021] In the Figure 1Light is emitted into the fiber optic accelerometer 110 via the optical fiber 112. For example, a first incident beam 143 from the optical fiber 112 strikes the fiber end face 131. A first part of the first incident beam 143 is reflected back into the optical fiber 112 at the fiber end face 131 as the first reflected beam 145. A second part of the first incident beam 143 is transmitted at the fiber end face 131 as a second incident beam 147, specifically as primary radiation from the optical fiber 112, into the cavity 107 between the fiber end face 131 and the vibration membrane 103. The second incident beam 147 strikes the vibrating membrane 103, in particular the deflection mass 123 of the vibrating membrane 103. The second incident beam 147 is at least partially reflected by a reflecting membrane surface, in which Fig. 1For example, at the axial membrane surface 133 of the deflection mass 123 facing the cavity 107. In particular, at least part of the second incident beam 147, for example as a second reflected beam 149, is reflected back into the cavity 107 and in the direction of the optical fiber 112. A portion of the second reflected beam 149 transmitted through the fiber end face 131 can interfere with the first reflected beam 143 in the optical fiber 112. Furthermore, the light can be reflected multiple times between the fiber end face 131 and the deflection mass 123. The fiber end surface 131 of the optical fiber 112 serves in particular as a reflection surface for light from the optical fiber 112, as a light exit surface for emitting light from the optical fiber in the direction towards the vibration membrane 103, and as a light entry surface for receiving light which is reflected back into the optical fiber 112 from the cavity 107.

[0022] The interference of reflected beams, in particular the first reflected beam 145, the second reflected beam 149, or multiple reflected beams, can form an interference pattern according to the Fabry-Perot effect. This interference pattern can be used to determine the acceleration acting on the fiber optic accelerometer 110. For example, the cavity length 111 of the cavity 107 or the displacement of the displacement mass 123 can be determined, and from this, the acceleration of the fiber optic accelerometer 110 can be calculated. In further embodiments, the acceleration can be determined from the interference pattern using a mathematical function or a lookup table.

[0023] In a Fabry-Perot interferometer, as is also used in Figure 1As illustrated, a free spectral range (FSR) is provided in which a wavelength spacing can be uniquely determined. This means that no ambiguities occur in the measurement signal when taking a measurement within the free spectral range. The in Figure 1 The cavity length 111, d shown is determined by the following equation: d = λ 1 λ 2 2 n Δ λ cos θ where Δ λ = λ 2 , - λ 1 where Δλ denotes the wavelength difference between two interference minima at λ 1 and λ 2, n denotes the refractive index of the medium enclosed in the cavity, for example air, and θ denotes the angle between the surface normal of the reflection surfaces and the direction of light propagation in the optical resonator.

[0024] A phase difference between the reflected beams, for example between the first reflected beam 145, the second reflected beam 149, or multiple reflected beams, is determined by the cavity length 111. In particular, the phase difference is determined by the distance d between the fiber end face 131 and the reflecting membrane surface, for example the axial membrane surface 133. In other words, the interferometer setup is such that light can enter the cavity 107 and be reflected by the vibrating membrane 103. The reflecting membrane surface, the fiber end face 131, and the cavity 107 thus form the optical resonator, such as the optical resonator of a Fabry-Perot interferometer.The light reflected back into the optical fiber 112 exhibits an interference spectrum, in particular interference maxima or interference minima, the number or position of which depends on the cavity length 111, especially the distance d between the fiber end face 131 and the reflecting membrane surface. By analyzing the position of the interference maxima or interference minima in the reflected spectrum, a change in the cavity length 111 or, in particular, an acceleration-dependent displacement of the displacement mass 123 can be determined.

[0025] According to embodiments, the reflective membrane surface is provided on the deflection mass. In particular, the reflective membrane surface, for example the axial membrane surface 133, is oriented perpendicular to the axis 127. In particular, the reflective membrane surface and the fiber end surface 131 are aligned at least substantially parallel to each other.

[0026] According to the embodiments, the connecting bridges 125 connect the membrane frame 121 and the deflection mass 123 at at least two different axial positions, such as in the Figures 1 and 2 The term "axial" here refers specifically to the axis 127. The connecting bridges 125 can be configured as spring elements between the membrane frame 121 and the deflection mass 123. The vibration membrane 103 is configured such that the deflection mass 123 is deflected when an acceleration acts on the fiber optic accelerometer 110, for example, relative to a rest position of the deflection mass 123. In particular, the deflection mass 123 is deflected when an acceleration is directed along the axis 127 or when an acceleration includes a component directed along the axis 127.

[0027] By connecting the deflection mass 123 to the membrane frame 121, the deflection of the deflection mass 123 can, for example, be reduced at least substantially to a movement of the deflection mass 123 along the axis 127. In particular, tilting of the deflection mass 123 can be reduced. Among other things, the transverse sensitivity of the fiber optic accelerometer 110, for example in the case of rotational accelerations, can be reduced compared to known accelerometers. A further advantage can be that the mass of the deflection mass 123 extends along the axis 127 and can have a larger mass, especially without an asymmetric distribution of the mass with respect to the axis 127. For example, the fiber optic accelerometer 110 can have a higher sensitivity, especially due to the larger mass.Furthermore, the stability of the vibration membrane 103 can be increased, for example, by connecting the deflection mass 123 to the membrane frame 121 at at least two axial positions.

[0028] According to some embodiments, the connecting bridges 125 between the membrane frame 121 and the deflection mass 123 are designed as meandering arms. In particular, the connecting bridges 125 each have a first connection point 151 to the membrane frame 121 and a second connection point 153 to the deflection mass 123, wherein the first connection point 151 is offset circumferentially about the axis 127 relative to the second connection point 153. In embodiments, a meandering arm comprises, in particular, at least three arm sections, wherein at least two of the arm sections are oriented at least substantially radially and at least one arm section is oriented at least substantially circumferentially about the axis 127. "At least substantially radial" is understood, for example, to mean a deviation of a maximum of 30°, in particular a maximum of 20° or a maximum of 10°, compared to a radial direction."At least substantially in the circumferential direction" means, for example, a deviation of a maximum of 30°, in particular a maximum of 20° or a maximum of 10°, compared to a circumferential direction. For example, the connecting bridges 125 of the in . Figure 2 The depicted vibration membrane 103 is designed as meandering arms.

[0029] According to embodiments, the vibration diaphragm 103 comprises at least two, in particular at least three, connecting bridges 125 at each of the at least two axial positions. For example, the vibration diaphragm 103 comprises exactly three or exactly four connecting bridges 125 at each of the at least two axial positions. In the exemplary embodiment of the Figure 2 The vibration membrane 103 comprises three connecting bridges 125 at each of the two axial positions.

[0030] According to some embodiments, the connecting bridges 125 comprise first connecting bridges at a first axial position of the at least two axial positions and second connecting bridges at a second axial position of the at least two axial positions, wherein the first connecting bridges are arranged circumferentially offset about the axis 127 relative to the second connecting bridges. For example, respective first connection points 151 of the first connecting bridges and the second connecting bridges can be arranged circumferentially offset from one another, or respective second connection points 153 of the first connecting bridges and the second connecting bridges can be arranged circumferentially offset from one another. In the Figure 2The connecting bridges 125 are arranged circumferentially offset about the axis 127. For example, at the two axial positions, the respective connecting bridges 125, such as the respective first connection points 151 between the connecting bridges 125 and the membrane frame 121 or the respective second connection points 153 between the connecting bridges 125 and the deflection mass 123, are arranged circumferentially offset from each other by 60°. By arranging the connecting bridges 125 circumferentially offset, for example, the transverse sensitivity of the deflection of the deflection mass 123 can be reduced.

[0031] In embodiments, the axial length of the deflection mass 123 or the axial distance 155 between the at least two axial positions is at least 0.1 mm, in particular at least 0.2 mm or at least 0.5 mm, or a maximum of 5 mm, in particular a maximum of 3 mm or a maximum of 1.5 mm. According to embodiments, the vibration diaphragm 103 has a diameter transverse to the axis 127 of at least 1 mm, in particular at least 2 mm, or a maximum of 10 mm, in particular a maximum of 7 mm or a maximum of 5 mm. In embodiments, the vibration diaphragm 103 has a natural frequency of at least 5 kHz, in particular at least 10 kHz, or a maximum of 500 kHz, in particular a maximum of 200 kHz or a maximum of 100 kHz.

[0032] According to embodiments, the vibration diaphragm is configured for a mechanical displacement of the displacement mass of at least 0.25 nm / G, in particular at least 0.5 nm / G or at least 1 nm / G, or of a maximum of 40 nm / G, in particular at most 30 nm / G, at most 25 nm / G, or, for example, at most 20 nm / G. "G" here stands for the acceleration due to gravity, for example, approximately 9.81 m / s².

[0033] In some embodiments, the vibration diaphragm 103 is manufactured in one piece. In particular, the vibration diaphragm 103 is made of a single material, for example, a single transparent material. Manufacturing it from a single material can, for example, prevent stresses in the vibration diaphragm 103. According to embodiments, the vibration diaphragm 103 is made of glass, in particular quartz glass or borosilicate glass.

[0034] In embodiments, the vibration membrane 103 is produced by selective laser etching (SLE). In selective laser etching, a substrate, in particular a glass substrate made of quartz glass or borosilicate glass, is provided. The shape of the vibration membrane 103 is first imprinted into the substrate by a laser, whereby, in particular, the areas of the substrate surrounding the vibration membrane 103 are selectively modified by laser radiation, for example, chemically. The modified areas of the substrate can then be selectively etched. In particular, the modified areas of the substrate can be removed at a higher etch rate than the unmodified structure of the vibration membrane 103 imprinted into the substrate.

[0035] According to some embodiments, the vibration diaphragm 103, in particular the deflection mass 123, has a highly reflective coating. "Highly reflective" here means, in particular, that at least 50% of the incident light, for example at least 70% or at least 90%, is reflected. For example, the axial diaphragm surface 133 of the deflection mass 123 facing the cavity 107 can be coated with a highly reflective coating, as shown, for example, in Figure 1 As shown. In further embodiments, a rear membrane surface facing away from cavity 107 can be coated with a highly reflective coating. By providing a highly reflective coating, for example, a higher intensity of the reflected light can be provided.

[0036] According to embodiments, the highly reflective coating can be produced, for example, by vapor deposition of a layer of metal or a metal alloy onto the vibration diaphragm 103, in particular onto the deflection mass 123. In further embodiments, the vibration diaphragm 103 does not have a highly reflective coating. For example, light incident on the vibration diaphragm 103 can be at least partially reflected at a surface of the vibration diaphragm 103, for example at a transition between glass and air.

[0037] In embodiments, the deflection mass 123 comprises an axial membrane surface 133 facing the cavity 107 and, behind the axial membrane surface 133, a defect region for the diffuse scattering of light. The axial membrane surface 133 is designed as a reflective membrane surface. In the defect region, light transmitted through the axial membrane surface 133 can be diffusely scattered. Defects in the defect region can, for example, be inscribed into the deflection mass 123 using a laser.

[0038] According to some embodiments, the fiber optic accelerometer 110 is configured as a Fabry-Perot interferometer. The fiber end face 131 of the optical fiber 112 forms a first mirror of the Fabry-Perot interferometer, and the vibration diaphragm 103, in particular the deflection mass 123, forms a second mirror of the Fabry-Perot interferometer.

[0039] In embodiments, the optical fiber 112 is configured as an optical fiber or a polymer conductor, wherein materials such as optical polymers, polymethyl methacrylate, polycarbonate, fused silica, or ethylene tetrafluoroethylene can be used, optionally doped. In particular, the optical fiber 112 can be configured as an SMF-28 fiber.

[0040] The optical fiber 112 is held by a fiber holder 161 of the fiber optic accelerometer 110. The fiber holder 161 can be arranged in a sensor housing (not shown) of the fiber optic accelerometer 110. The sensor housing can, in particular, enclose a volume around the vibration diaphragm 103. According to one embodiment, the fiber holder 161 with the optical fiber 112, in particular with an end piece of the optical fiber 112 having the fiber end face 131, is arranged in a measuring position relative to the vibration diaphragm 103. In particular, the optical fiber 112 is attached to the fiber holder 161 by gluing or soldering.

[0041] According to some embodiments, the fiber optic accelerometer 110 or an end piece of the optical fiber 112 has at least one optical beam shaping component, for example at the fiber end face 131, to shape a light beam exiting the optical fiber 112. For example, the optical beam shaping component has at least one of the following: a gradient-index lens (GRIN lens), a prism, a spherical lens, a cylindrical lens, and any combination thereof. According to another embodiment, the optical fiber 112 can be configured as a single-mode fiber.

[0042] According to embodiments, a fiber optic acceleration sensor system is provided, comprising at least two fiber optic acceleration sensors as described herein, wherein the fiber optic acceleration sensor system is configured to determine accelerations along at least two axes. The acceleration sensor system may, in particular, comprise two or three fiber optic acceleration sensors. In embodiments, at least two of the axes of the at least two fiber optic acceleration sensors are not aligned parallel, in particular, they are aligned at least substantially orthogonally to each other. The fiber optic acceleration sensor system may, for example, be arranged in or on a rotor blade.

[0043] According to embodiments, a wind turbine is specified, comprising at least one fiber optic accelerometer or at least one fiber optic accelerometer system as described herein. In particular, a wind turbine comprises a rotor blade, wherein the at least one fiber optic accelerometer or at least one fiber optic accelerometer system is arranged in or on the rotor blade. According to embodiments, the wind turbine comprises two or three rotor blades, wherein at least one fiber optic accelerometer or at least one fiber optic accelerometer system is arranged on each of the rotor blades.One advantage may be that a fiber optic accelerometer according to the embodiments described herein has a lower lateral sensitivity to rotational accelerations, which occur particularly in rotor blades during the operation of a wind turbine.

[0044] Figure 3 shows a wind turbine 200 as an application example for a fiber optic accelerometer 110 as described herein, for example with reference to the Figures 1 and 2 The wind turbine 200 comprises a tower 202 and a nacelle 203. A rotor 500 is attached to the nacelle 203. The rotor 500 includes a hub 205 to which rotor blades 100 are attached. As in Figure 3As shown, at least one sensor unit 110 for fiber-optic acceleration measurement is provided on a rotor blade 100. The sensor unit 110 is connected to an evaluation unit 114 via an optical fiber 112. The evaluation unit 114, for example, provides a signal to a control unit 204 of the wind turbine 200 for controlling or regulating the wind turbine 200. The rotor blade 100 has a blade axis 101 along its longitudinal extent. The length of the rotor blade 100 extends from a blade flange to a blade tip, and no electrical conductors are expediently present in this area. Fiber-optic acceleration sensors, on the other hand, can be attached anywhere along the longitudinal extent of the rotor blade. For example, the fiber-optic acceleration sensor 110 can be arranged at a radial position in an outer region of the rotor blade 100.

[0045] According to some embodiments, an optical signal, such as an interference spectrum, is transmitted to the evaluation unit 114 via the optical fiber 112. Each rotor blade 100 can exhibit individual accelerations, in particular vibrations or oscillations. Therefore, according to some embodiments, at least one fiber optic acceleration sensor 110 is provided in each rotor blade 100.

[0046] To use a fiber optic accelerometer 110, such as the one used in Figure 1 As shown, it is particularly easy to provide on a rotor blade, especially in an outer radial area of ​​the rotor blade. Therefore, it is advantageous if the fiber optic accelerometer 110 has a small dimension in a cross-section perpendicular to the axis 127. A maximum dimension in a cross-section perpendicular to the axis 127 can, for example, be 10 mm or less.

[0047] Figure 4Figure 1 shows, as an application example for the fiber optic accelerometer 110, the rotor 500 of a wind turbine, on which acceleration measurements are to be performed. The rotor 500 has a hub 205 and rotor blades 100 attached to it. An accelerometer 110 is provided in at least one of the rotor blades 100. The signal from the fiber optic accelerometer 110 is transmitted via the optical fiber 112 to a distributor 501. The distributor 501 can, for example, be a field distributor where several signals from different sensors are provided. The distributor 501 can be mounted on the blade shroud of the rotor blade 100 and can be designed for connecting and disconnecting a signal cable from a sensor, for example, an optical fiber 112 of a fiber optic accelerometer 110. Furthermore, a sensor cable can be provided for connecting and disconnecting from the field distributor to the measuring device or evaluation unit 114.According to some embodiments, the distributor 501 is arranged at the leaf sheath or in the leaf root.

[0048] As in Figure 4 As shown, a transmission optical fiber 503, for example another optical fiber, can be routed from the distributor 501 to the evaluation unit 114. For example, the transmission optical fiber 503 can be guided along or through a spring or a spiral, or a corresponding mechanical element, so that the transmission optical fiber 503 is not damaged when the rotor blade 100 rotates about its blade axis 101, particularly when the rotor blade 100 is pitched. The mechanical guidance of the transmission optical fiber 503 along or through a spiral thus allows for torsion of the transmission optical fiber 503.

[0049] Figure 4Figure 1 shows a fiber optic accelerometer 110 in each of the rotor blades 100. Furthermore, it is possible to measure the acceleration at multiple positions along the blade axis 101 of a rotor blade 100. For this purpose, several fiber optic accelerometers 110 can be provided at the respective positions. It can also be advantageous to use fiber optic accelerometers 110 near the rotor blade tip of a wind turbine, i.e., in radially outward regions of the rotor 500. For this purpose, when retrofitting a fiber optic accelerometer 110, an optical fiber 112 must be routed to the blade tip.

[0050] Figure 5Figure 600 schematically depicts a measuring system 600 with a fiber optic accelerometer 110 according to the embodiments described herein. The measuring system 600 comprises one or more fiber optic accelerometers 110. The measuring system 600 has a source 602 for electromagnetic radiation, such as a primary light source. The source 602 serves to provide optical radiation with which at least one fiber optic accelerometer 110 can be irradiated. For this purpose, an optical transmission fiber or a light guide 603 is provided between the primary light source 602 and a first fiber coupler 604. The fiber coupler 604 couples the primary light into the optical fiber 112. Source 602 can be, for example, a broadband light source, a laser, an LED (light emitting diode), an SLD (superluminescent diode), an ASE (amplified spontaneous emission) light source, or an SOA (semiconductor optical amplifier).For the embodiments described here, several sources of the same or different types can also be used.

[0051] According to embodiments, the optical fiber 112 of the fiber optic accelerometer 110 is optically coupled to an optical resonator 302 of the fiber optic accelerometer 110, wherein the optical resonator 302 comprises, in particular, the fiber end face 131 of the optical fiber 112, the vibration membrane 103, and the cavity 107 formed between the fiber end face 131 and a reflective membrane surface. The light reflected by the at least one fiber optic accelerometer 110 is again guided via the fiber coupler 604, which directs the light via a further transmission fiber 605 into a beam splitter 606. The beam splitter 606 divides the reflected light for detection by means of a first detector 607 and a second detector 608. The signal detected on the second detector 608 is first filtered by an optical filter device 609.The filter device 609 can detect the position of an interference maximum or minimum emitted from the optical resonator 302, or a change in wavelength due to the optical resonator 302, and thus an acceleration.

[0052] In general, a measuring system, such as the one described below for measuring system 700 in Figure 6The system can be operated even without the beam splitter 606 or the detector 607. However, the detector 607 enables normalization of the measurement signal from the fiber optic accelerometer 110 with respect to other intensity fluctuations, such as variations in the intensity of the source 602, variations due to reflections at interfaces between individual optical fibers, variations due to reflections at interfaces between the optical fiber 112 and the fiber coupler 604, or other intensity fluctuations. This normalization improves the measurement accuracy and reduces the dependence of the measurement system on the length of the optical fiber 112 provided between the evaluation unit 114 and the fiber optic accelerometer 110.

[0053] Figure 6Figure 1 shows an embodiment of an evaluation unit 114, wherein a signal from a fiber optic accelerometer 110 is transmitted to the evaluation unit 114 via an optical fiber 112. Figure 6A light source 602 is also shown, which can optionally be provided in the evaluation unit. However, the light source 602 can also be provided independently or outside of the evaluation unit 114. The optical signal of the fiber optic accelerometer 110, in particular the optical interference signal, which may exhibit interference maxima and interference minima, is converted into an electrical signal by a detector, for example, a transducer 702. The evaluation unit 114 can include an opto-electronic transducer for converting the optical signal into an electrical signal. For example, a photodiode, a photomultiplier (PM), or another optoelectronic detector can be used as the transducer. The electrical signal can be filtered with an analog anti-aliasing filter 703.Following analog filtering with an analog anti-aliasing filter or low-pass filter 703, the signal is digitized using an analog-to-digital converter 704. Figure 6 It also shows a digital evaluation unit 706, which may include, for example, a CPU, a memory and other elements for digital data processing.

[0054] The optical filter device 609 or additional optical filter devices for filtering the interference spectrum or for detecting interference maxima and interference minima may include an optical filter selected from the group consisting of one or more edge filters, a thin-film filter, a fiber Bragg grating, an arrayed waveguide grating (AWG), an echelle grating, a grating array, a prism, and any combination thereof.

[0055] According to one embodiment, which can be combined with other embodiments described herein, the described evaluation unit 114 can resolve a wavelength shift of up to 2 pm. With respect to interference minima, this corresponds to a change in the cavity length 111 of 0.001 µm.

[0056] According to embodiments, a method 800 for manufacturing a fiber optic accelerometer for determining an acceleration along an axis is specified, as for example in Figure 7Schematically illustrated. In block 810, method 800 comprises providing a transparent substrate. For example, the substrate can be made of glass, such as quartz glass or borosilicate glass. In block 820, a vibration membrane is selectively laser-etched from the transparent substrate. The vibration membrane comprises a membrane frame, a deflection mass, and connecting bridges, and the connecting bridges link the membrane frame and the deflection mass at at least two different axial positions. The vibration membrane can be configured according to the embodiments described herein. The selective laser etching includes, for example, imprinting a structure of the vibration membrane into the substrate. The imprinting includes, for example, modifying, in particular chemically modifying by laser radiation, the areas of the substrate that surround the structure of the vibration membrane.Selective laser etching can further include the selective etching of the substrate regions modified by the laser radiation. In particular, the modified regions of the substrate can be etched at a higher etch rate using an etchant. By selectively etching these modified regions, the vibrational membrane can be etched out of the substrate.

[0057] According to embodiments, method 800 can further comprise arranging the vibration diaphragm relative to a fiber end face of an optical fiber. In particular, the diaphragm frame of the vibration diaphragm and the optical fiber can be connected to a sensor housing. In particular, the vibration diaphragm and the optical fiber can be arranged such that the fiber end face is oriented at least substantially perpendicular to the axis.

Claims

1. A fibre-optic accelerometer (110) for determining an acceleration along an axis, comprising an optical fibre (112) having a fibre end surface (131); and a vibrating membrane (103) having a membrane frame (121) and a deflection mass (123); wherein the vibrating membrane (103) is configured to reflect at least partially a primary radiation exiting from the fibre end surface (131); wherein the vibrating membrane (103) is equipped with connecting bridges (125); and wherein the connecting bridges (125) connect the membrane frame (121) and the deflection mass (123) at at least two axial positions different from one another, characterised in that the connecting bridges (125) between the membrane frame (121) and the deflection mass (123) are implemented as meandering arms.

2. The fibre-optic accelerometer according to claim 1, wherein the vibrating membrane comprises at least two, particularly at least three, connecting bridges at each of the at least two axial positions.

3. The fibre-optic accelerometer according to any one of the preceding claims, wherein the connecting bridges comprise first connecting bridges at a first axial position of the at least two axial positions, and second connecting bridges at a second axial position of the at least two axial positions; and wherein the first connecting bridges are circumferentially arranged offset to the second connecting bridges around the axis.

4. The fibre-optic accelerometer according to any one of the preceding claims, wherein an axial distance between the at least two axial positions is at least 0.1 mm, particularly at least 0.2 mm, and / or not more than 5 mm, particularly not more than 3 mm.

5. The fibre-optic accelerometer according to any one of the preceding claims, wherein the vibrating membrane is configured for a mechanical deflection of the deflection mass of at least 0.25 nm / G, particularly of at least 0.5 nm / G, and / or of not more than 40 nm / G, particularly of not more than 30 nm / G.

6. The fibre-optic accelerometer according to any one of the preceding claims, wherein the vibrating membrane is manufactured in one piece, and / or wherein the vibrating membrane is made of glass, particularly of quartz glass.

7. The fibre-optic accelerometer according to any one of the preceding claims, wherein the vibrating membrane is manufactured by selectively laser etching.

8. The fibre-optic accelerometer according to any one of the preceding claims, wherein the vibrating membrane, particularly the deflection mass, has a highly reflective coating.

9. The fibre-optic accelerometer according to any one of the preceding claims, comprising a cavity formed between the fibre end surface and the vibrating membrane; wherein the deflection mass comprises an axial membrane surface facing the cavity; and wherein the deflection mass behind the membrane surface comprises an impurity region for diffuse scattering light.

10. The fibre-optic accelerometer according to any one of the preceding claims, wherein the fibre-optic accelerometer is configured as a Fabry-Pérot interferometer; wherein the fibre end surface of the optical fibre forms a first mirror of the Fabry-Pérot interferometer; and wherein the vibrating membrane forms a second mirror of the Fabry-Pérot interferometer.

11. A fibre-optic accelerometer system comprising at least two fibre-optic accelerometers according to any one of the preceding claims, wherein the fibre-optic accelerometer system is configured to determine accelerations along at least two axes.

12. A wind turbine comprising at least one fibre-optic accelerometer according to any one of claims 1 to 10 or at least one fibre-optic accelerometer system according to claim 11.

13. The wind turbine according to claim 12, comprising a rotor blade; wherein the at least one fibre-optic accelerometer or the at least one fibre-optic accelerometer system is arranged within or on the rotor blade.

14. A method for manufacturing a fibre-optic accelerometer for determining an acceleration along an axis, particularly of a fibre-optic accelerometer according to any one of claims 1 to 10, comprising - providing a transparent substrate; and - selectively laser etching a vibrating membrane from the transparent substrate, wherein the vibrating membrane comprises a membrane frame and a deflection mass; wherein the vibrating membrane comprises connecting bridges; and wherein the connecting bridges connect the membrane frame and the deflection mass at at least two axial positions different from one another, characterised in that the connecting bridges between the membrane frame and the deflection mass are implemented as meandering arms.