METHOD FOR DETERMINING THE CURVATURE AND / OR TORSION OF AN OPTICAL FIBER

Description

[0001] The invention relates to a method for determining the curvature and / or torsion of an optical waveguide of a fiber optic sensor.

[0002] Within single-mode and multi-mode optical fibers, light is guided in defined and mathematically describable light modes. The light modes describe the distribution of the electromagnetic field of the light (light field or mode field) within the optical fiber. Optical fibers can be, for example, cores in glass fibers, laser-processed structures in glasses, or light-guiding structures in polymers. The light mode of a single-mode waveguide is characterized by the fact that the light intensity, i.e. the intensity of the light field, is maximum in the center of the light field, i.e. particularly in the center of the fiber core, and decreases radially outwards on all sides, similar to a Gaussian curve. The same applies to a superposition of many possible light modes within a multi-mode waveguide.

[0003] When the optical fiber is bent, the position of the maximum light intensity within the optical fiber shifts in the opposite direction to the curvature. This process can also be described or simulated, i.e., calculated, mathematically.

[0004] A Bragg grating is a region of periodic refractive index modulation in the material of the optical waveguide. The period of the refractive index modulation is also referred to as the grating constant and is in the range of the wavelength of the light being guided in the optical waveguide. This light is at least partially reflected by the Bragg grating, with the intensity of the reflected light depending in particular on its wavelength, the angle of incidence on the Bragg grating, and the grating constant of the Bragg grating.

[0005] DE 10 2014 211 918 A1 discloses a generic fiber optic sensor in which one of the Bragg gratings is incorporated into a central core and further Bragg gratings are incorporated into fiber cores in the cladding. The other fiber cores extend axially only over short sections of the optical fiber and conduct light from the evanescence region of the light in the central core to the other Bragg gratings. Such an optical fiber is complex to manufacture. This sensor can be used to determine the torsion of the optical fiber. The measurement principle underlying the torsion determination depends on wavelength changes of the light in the optical fiber, which occur due to rotations or temperature changes in the grating structure of the optical fiber. Accordingly, the torsion determination cannot simultaneously detect temperature-induced expansion of the optical fiber.

[0006] US 2007 / 0230861 A1 discloses an optical fiber comprising a core and a cladding, as well as several different Bragg gratings arranged at different locations within the optical fiber. However, this prior art does not describe how this method can be used to determine the curvature and / or torsion of an optical waveguide of a fiber optic sensor.

[0007] DE 10 2005 030 753 A1 describes an optical strain gauge for multiaxial strain measurement, which comprises at least two linear optical waveguide sections, each with a Bragg grating. This prior art does not disclose the presence of at least two Bragg gratings in an optical waveguide. Furthermore, this reference does not describe the strain gauge being used to determine the curvature and / or torsion of an optical waveguide of a fiber optic sensor.

[0008] Further methods for determining the curvature and / or torsion of an optical fiber are described in US 2006 / 215976 A1, JP H11 218450 A, US 2010 / 166358 A1, and US 2007 / 297712 A1. WO 2014 / 026839 A2 describes a method for determining mechanical stresses in an optical fiber.

[0009] It is known to measure the curvature and / or torsion via a wavelength change of individual Bragg gratings. This results in a shift in the wavelength of several Bragg gratings located at different locations in the fiber core as soon as the fiber is bent. The magnitude of the wavelength change for a specific curvature depends linearly on the distance of the grating structure from the neutral axis of the fiber (typically the center of the fiber core). The values ​​for several structures in a

[0010] The expected wavelength change in a fiber core (e.g., with a diameter of 4 µm) is therefore more than an order of magnitude smaller than that of Bragg gratings in multi-core fibers or waveguides in the cladding (e.g., with a diameter of more than 60 µm). Wavelength analysis due to the tiny signal change is almost impossible, even with large curvatures.

[0011] An object of the present invention is therefore to provide a method for determining the curvature and / or torsion of an optical waveguide of a fiber optic sensor, which reduces the disadvantages of the prior art, in particular enabling a simplified, reliable and improved measurement of the curvature and / or torsion.

[0012] The object is achieved according to the invention by a method according to claim 1. Advantageous developments of the invention can be found in the subclaims.

[0013] With the method according to the invention, the light intensities, in particular those of three or four different Bragg gratings located at one location, are measured instead of the absolute wavelengths as in the prior art. In addition to significantly higher measurement sensitivity, the method according to the invention can also measure the wavelength as a sensory parameter in order to further determine the temperature and / or the strain of the fiber based on this wavelength measurement.

[0014] For all single-mode waveguides, the intensity distribution of the guided light can be uniquely determined. Upon curvature, this mode field shifts, as will be described in detail below. Since the different Bragg gratings are located at fixed positions in the fiber core in the inventive method, the interactions between the mode field and the Bragg gratings change. In particular, from three Bragg gratings positioned at one location, the local curvature can be determined independently of the wavelengths. By comparing the measured intensities, it is also possible to carry out the inventive method independently of loss sources along the fibers.

[0015] A fiber optic sensor used according to the invention has an optical waveguide which forms an axial direction aligned in a light propagation direction and a radial direction aligned perpendicular thereto. The optical waveguide has a core which runs centrally in the axial direction and extends at least substantially over the entire length of the optical waveguide for guiding light, and a cladding (sheathing) which surrounds the core in the radial direction. Extending substantially over the entire length is understood to mean that, for example, at one end of the optical waveguide, a termination region can be provided into which the core does not extend. There is at least one section of the optical waveguide running in the axial direction, into which at least two Bragg gratings are incorporated. The Bragg gratings are radially spaced, i.e.arranged at different distances and / or different directions to the axis of symmetry of the cross-section.

[0016] The at least two Bragg gratings are arranged in a common cross-sectional plane through the optical fiber, extending radially. In other words, the Bragg gratings overlap at least partially in the axial direction. This means that although they are spatially spaced radially, their lengths overlap when projected onto the central axial direction.

[0017] According to the invention, the Bragg gratings are incorporated in the core and / or at the boundary between the core and the cladding and / or in an inner edge region of the cladding within an evanescence region (mode field edge) of the light. The evanescence region is the region outside the core in the cladding into which the light guided through the core enters due to its wave properties.

[0018] In this way, an optical sensor is provided which enables a method for amplitude evaluation of periodic index modulations for curvature and 3D shape detection of the optical waveguide. The fiber optic sensor used according to the invention is based on the possibility of processing several, i.e. at least two, in particular three or four regions of periodic refractive index modulation, i.e. Bragg gratings, within the same sections of an optical waveguide, wherein at least some of the Bragg gratings have different grating constants. In the manufacture of the sensor used in the invention, a standard fiber optic cable with existing cladding can be used as the optical waveguide. Structures with periodic index modulation, i.e. Bragg gratings, are locally introduced into the edge region of the core of the waveguide in order to locally disrupt the mode field propagation of the light guided through the optical waveguide.This allows for very cost-effective sensor production. The Bragg gratings can be positioned within the fiber core, at its edge, or near the fiber core in the cladding, in the region of the evanescent field of the waveguide, for example, by irradiation with focused laser light. If multiple regions with Bragg gratings are provided along the optical waveguide, the corresponding structures—that is, multiple Bragg gratings overlapping in the axial direction—are incorporated at various positions along the optical waveguide, e.g., an optical fiber.

[0019] Advantageously, the inner edge region of the cladding extends radially into the cladding by less than ten percent of the cladding thickness. Sufficient light intensity from the core still enters this region to achieve a sufficiently strong reflection of the light from a Bragg grating positioned there, allowing the intensity of the reflection to be accurately measured after exiting the optical fiber.

[0020] If three or four Bragg gratings are arranged in the section running through the cross-sectional plane, the curvature of the optical waveguide, e.g., a fiber, can be determined even if intensity losses are present in the optical waveguide. From any geometric arrangement of at least three distributed grating structures, i.e., Bragg gratings, the direction and amplitude of the fiber curvature can be determined. Even two grating structures are sufficient, provided an additional total light intensity measurement is performed or no intensity losses occur in the fiber.

[0021] If one of the Bragg gratings is arranged centrally in the core and two of the Bragg gratings are arranged in the inner edge region of the cladding or if the three Bragg gratings are arranged in the inner edge region of the cladding, their positions can be determined particularly well in the event of a bend in the optical fiber by measuring the intensity of the light reflections occurring at it.

[0022] If four Bragg gratings are arranged in the section running through the cross-sectional plane, with the four Bragg gratings all being arranged in the inner edge region of the cladding or all on the edge of the core or all in an outer edge region of the core, a redundancy of the position determination results, so that it can be determined more precisely while compensating for measurement inaccuracies.

[0023] It is advantageous if the Bragg gratings are arranged symmetrically around the center of the core, so that the locations in the optical fiber whose position is determined are evenly distributed within the optical fiber.

[0024] If the Bragg gratings have a maximum diameter less than half the maximum diameter of the core, their position can be determined with pinpoint accuracy. Typically, especially in optical fibers with a circular cross-section, Bragg gratings have an elliptical basic shape. The semi-major axis of the basic shape runs in the axial direction of the optical fiber, perpendicular to the grating bars.

[0025] According to the invention, at least some of the Bragg gratings have different grating constants. This allows different wavelengths of light in the optical waveguide, ie, their reflections at the Bragg gratings, to be evaluated separately.

[0026] A fiber optic sensor used according to the invention can be manufactured as follows: Providing an optical waveguide which has an axial direction aligned in a direction of light propagation and a radial direction aligned perpendicular thereto, wherein the optical waveguide has a core which runs centrally in the axial direction and extends substantially over the entire length of the optical waveguide for guiding light and a cladding which surrounds the core in the radial direction, and introducing at least two Bragg gratings which run through a common cross-sectional plane lying in the radial direction through the optical waveguide into a section of the optical waveguide running in the axial direction by irradiation with laser light, in particular a femtosecond laser, by varying the focus position of the laser light, wherein the Bragg gratings are introduced into the core and / or on the boundary between the core and the cladding and / or into an inner edge region of the cladding within an evanescence region of the light.

[0027] The Bragg grating can be created, for example, by modifying the core material using laser radiation. The laser radiation can be pulsed, for example, with a pulse length of less than 10 ns, less than 1 ns, or in the case of a femtosecond laser, less than 100 fs. Furthermore, the Bragg grating can be created by doping the core material. In either case, the Bragg grating contains a plurality of spatially defined regions whose refractive index differs from the refractive index of the surrounding core material. Thus, at the respective interfaces, part of the light propagating in the core is reflected and part is transmitted. Several such modified regions at a predeterminable distance form a Bragg grating, which, depending on its grating constant, reflects a wavelength range of the incident light and transmits radiation of other wavelength ranges.When the temperature changes or when mechanical stress is applied, the Bragg grating is stretched or compressed, so that the grating constant changes and can be determined by spectroscopic analysis of the transmitted or reflected light. The Bragg grating used according to the invention can be manufactured with an elliptical cross-section by laser material processing. In this case, the laser radiation can be focused onto the core by at least one cylindrical lens. Since the material modification by the laser radiation is limited to the focal point of the laser beam, an elliptical focus also results in a modified spatial region with the shape of an ellipsoid of revolution. The femtosecond writing technique is particularly suitable for the precise positioning of the index structures forming the Bragg gratings. The index structures are introduced into the optical fiber using femtosecond laser pulses.

[0028] The inventive method for determining a curvature and / or torsion, which can be caused, for example, by a temperature fluctuation, of the optical fiber of an inventive fiber optic sensor comprises the following method steps: Providing reference data of intensities of reflected light components of light coupled into the optical fiber as a function of known reference deformations of the optical fiber. The intensity data can be created, for example, by measurement and / or calculation. Measuring at least one light intensity of reflected light components of light coupled into the optical fiber, wherein the optical fiber has a deformation to be determined, and determining the deformation by comparing the light intensity with the reference data.

[0029] The light intensities reflected back by the structures forming the Bragg gratings are evaluated. In contrast to prior art approaches, the measurement principle according to the invention is independent of wavelength changes resulting from strain or temperature changes in the grating structures. Thus, in addition to shape detection, strain or temperature detection can be performed directly using known evaluation methods for Bragg grating reflections. The evaluation of the polarization dependence of the Bragg gratings for determining the torsion of the optical fiber, e.g., according to DE 102014211918 A1, is also possible independent of curvature and shape detection.

[0030] If at least two reflected light intensities are measured, a three-dimensional deformation can be determined by comparing the light intensities with the reference data. When determining curvature and / or torsion, the light intensities are advantageously evaluated using a spectrometer and / or an AWG (Arrayed Waveguide Grading) filter element and / or an FBG (Fiber Bragg Grading) filter element.

[0031] To determine the curvature and / or torsion, the invention performs wavelength division multiplexing and / or time-resolved multiplexing of the light coupled into the optical fiber, resulting in measurement signals whose errors can be minimized using statistical methods when determining the curvature and / or torsion. Furthermore, when determining the curvature and / or torsion, the reflected light intensities can be evaluated using wavelength division multiplexing and / or time-resolved multiplexing.

[0032] Particular embodiments of the present invention are explained in more detail below with reference to the accompanying drawings. They show: Fig. 1 a longitudinal section through a fiber optic sensor in a schematic representation, Fig. 2a to f different arrangements of Bragg gratings in a cross-section of a fiber optic sensor in a schematic representation, and Fig. 3a and 3b a representation of the principle underlying the method according to the invention for determining the curvature and / or torsion of the optical waveguide of a fiber optic sensor.

[0033] In Figure 1 1 shows a schematic representation of a longitudinal section through an embodiment of a fiber optic sensor 1. The longitudinal section runs in the axial direction z of the optical fiber 3 of the sensor 1, which represents the direction of light propagation in the core 5 of the optical fiber 3, which is made, for example, from a glass fiber. The core 5 runs centrally in the axial direction and extends essentially over the entire length of the optical fiber 3. In the radial direction r, i.e. perpendicular to the axial direction of the optical fiber 3, the core 5 is surrounded by a jacket 6, e.g. made of plastic, for its protection. The axial direction and the radial direction are shown as rz coordinate axes.

[0034] Two Bragg gratings (BG1 and BG2) 8, 9 are incorporated in a section of the optical waveguide running in the axial direction. Several such sections with Bragg gratings can also be provided in the axial direction of the optical waveguide 3, spaced apart from one another. The spaced-apart Bragg gratings 8, 9 run through a common, radially arranged cross-sectional plane 11 through the optical waveguide 3. The cross-sectional plane 11 is shown in the figure as a vertical line because it runs perpendicular to the plane of the drawing. Because the Bragg gratings 8, 9 have the same diameter, they completely overlap in the axial direction. One of the Bragg gratings 8 is incorporated in the core 5 in an outer edge region. The other Bragg grating 9 is incorporated in an inner edge region of the cladding 6 within the evanescence region of the light that is guided in the core 5 when the sensor 1 is used.The bars of the Bragg gratings 8, 9 run perpendicular to the axial direction. The bars do not need to have a significant longitudinal extension. They can also be essentially point-shaped. They are then arranged one behind the other in a line parallel to the axial direction, as shown.

[0035] In contrast to known superimposed Bragg gratings referred to as "superimposed," the structures of periodic refractive index modulation introduced into the optical waveguide 3 of a fiber optic sensor 1 according to the invention as Bragg gratings 8, 9 have a difference in their relative position to the center of the fiber core 5 and have a significantly smaller cross-section (spot diameter), i.e., maximum diameter, than the diameter of the core 5. The structures can be located both in the fiber core 5 and, in particular, at the edge or in the adjacent surrounding material of the cladding material 6 in the region of the evanescent interaction of the light, i.e., in the evanescence region, in the optical waveguide 3. The figure shows an example of the position of the periodic index modulation, i.e., the Bragg gratings 8, 9, in edge regions of the fiber core 5 and in the cladding material 6.

[0036] In the Figures 2a to 2fvarious arrangements of Bragg gratings are shown in a schematic representation, each in a cross-sectional plane through a section of the optical waveguide 3 of a fiber optic sensor according to the invention. The cross-sectional planes coincide with the plane of the drawing, which is why they are only represented by the circular circumference of the optical waveguide 3. Examples of possible geometric arrangements of periodic index structures, i.e. Bragg gratings 20, in particular at the mode field edge for a good 3D shape determination of the optical waveguide 3, i.e. its deformation, are shown. The cross-sectional planes each lie in the radial direction of the optical waveguide 3, i.e. their surface normals each run in the direction of the axial direction of the optical waveguide 3. The Bragg gratings 20, represented symbolically as points, all run through the cross-sectional plane of the associated optical waveguide 3. i.e. they lie on the cross-sectional planes.The Bragg gratings 20 have a maximum diameter less than half the maximum diameter of the core 5.

[0037] In the Figures 2d to 2f In each section, three Bragg gratings 20 are arranged, running through the cross-sectional plane. Figures 2d and 2e The three Bragg gratings 20 are arranged in the inner edge region of the cladding 6, whereby these Figure 2e are distributed symmetrically around the center of core 5. In Figure 2f one of the Bragg gratings 20 is arranged centrally in the core 5 and two of the Bragg gratings 20 are arranged in the inner edge region of the cladding 6.

[0038] In the Figures 2a to 2c Four Bragg gratings 20 are arranged in the respective section, running through the cross-sectional plane. Figure 2a the four Bragg gratings 20 are all arranged in the inner edge region of the cladding 6. In Figure 2b all Bragg gratings 20 are arranged on the edge of the core 5, whereas in Figure 2call Bragg gratings 20 are arranged in an outer edge region of the core 5. The four Bragg gratings 20 are each arranged symmetrically around the center of the core 5.

[0039] In the Figures 3a and 3b The principle underlying the method according to the invention for determining a deformation, ie a curvature and / or torsion, of the optical waveguide 3 of a fiber optic sensor is shown. In Figure 3a an optical waveguide 3 of a fiber optic sensor with two Bragg gratings 8,9 is shown undeformed, ie running straight. In Figure 3bThis is shown curved. The intensity distribution I (mode field intensity distribution) of the light introduced to determine the deformation is shown in the form of its spatial mode field distribution in the optical waveguide 3. The intensity distribution corresponds to a Gaussian distribution 30, which is plotted in a tilted xy coordinate system. In the undeformed optical waveguide 3, the Gaussian distribution 30 is evenly distributed around the center of the core 5, whereas in a curved waveguide, the Gaussian distribution 30 is shifted in the radial direction, thus extending further into the cladding 6 of the optical waveguide.

[0040] The lower part of the figures shows the light intensities 35 (intensity signal) after interaction of a spectrally broadband light field with a Bragg grating designed as a periodic index modulation at the edge of the mode field, i.e. the intensity of a reflected spectrum in each case. This shows how the intensity signal 35 changes when the optical fiber 3 is bent as shown. For a structure with periodic index modulation, i.e. a Bragg grating, at a specific location relative to the center of the core of an optical fiber (fiber core center), the intensity (I) of the reflected light changes when the mode field is shifted when it interacts with the structure. This leads to a change in the light intensity reflected by the Bragg grating, which can be detected or is used to measure the deformation.It can be seen that the intensity of the reflected light 35 from the individual Bragg gratings 8, 9 has changed due to the curvature. In particular, the intensity corresponding to the upper Bragg grating (BG1) 8 in the figure has increased, while the intensity corresponding to the lower Bragg grating (BG2) 9 in the figure has decreased.

[0041] From multiple intensity signals 35 from the same position of the waveguide, the displacement of the mode field and, from this, the local curvature of the fiber can be calculated. Interpolation of many such measurement points allows a complete reconstruction of the three-dimensional shape of the fiber and thus of the optical waveguide in space. Similar effects also occur during torsion and stretching of the optical waveguide and are evaluated for their determination.

[0042] Based on the illustrated intensity shifts of the light reflected at the Bragg gratings, the inventive method for determining the curvature and / or torsion of the optical waveguide of a fiber optic sensor comprises the following method steps: providing reference data (calibration parameters) of intensities of reflected light components (amplitude signals) of light coupled into the optical waveguide, in particular as a function of known reference deformations of the optical waveguide, measuring at least one light intensity of reflected light components of light coupled into the optical waveguide, wherein the optical waveguide has a deformation to be determined, and determining the deformation, i.e. evaluating the light intensities of the reflected light components, by comparing the light intensity with the reference data. The reference data can be calculated, for example, by means of simulation or measured.The reflected light components, ie their light intensities, can be evaluated particularly well to determine the deformation if the Bragg gratings are arranged in a geometric manner according to the . Figures 2 are incorporated into the optical fiber.

[0043] A possible evaluation of amplitude signals of the fiber optic sensor is based on the comparison of the current amplitude ratios of all sensors involved in a measuring plane, ie Bragg gratings, with the calibration parameters that correspond to the amplitude ratios of the non-curved sensor.

[0044] The determination of the amplitude signals, i.e., their magnitude, can be achieved using all known evaluation methods for Bragg gratings, e.g., conventional spectrometer-based FBG evaluation units, arrayed waveguide grading (AWG) systems, tunable laser diodes, time-of-flight selective interrogators in the frequency or time domain, or even with additional Bragg gratings. It can be achieved either by measuring an absolute value or by fitting a mathematical function to the raw data signal.

[0045] Every possible ratio of Bragg gratings of a measurement plane, ie Bragg gratings on a cross-sectional plane of the optical fiber, can be assigned to exactly one shift of the corresponding mode field. With fiber optic sensors with four orthogonal gratings, as in the Figures 2d to f, curvatures of the optical waveguide in both spatial directions can be determined separately.

[0046] More generally, assuming the mathematical mode field description of a non-bent waveguide, a unique displacement can be determined for three gratings positioned in an arbitrary two-dimensional arrangement using geometric and numerical methods. This displacement corresponds to the actually measured relative intensities of the three gratings. Instead of a static model for the intensity distribution of the light in the fiber core, simulated (i.e., calculated) mode fields can be stored as reference data for different curvatures. These mode fields exhibit a certain distortion of the intensity distribution depending on the radius of the curvature. This, in combination with numerical solution methods, promises the greatest possible accuracy in converting measured signal amplitudes into the deformation to be determined, e.g., curvature.

[0047] In the following, the method according to the invention with 3 or 4 superimposed FBGs at different positions of the fiber core, aligned at right angles to each other, is described by way of example in order to determine a curvature: (In the following description, the Z-axis is the propagation direction of the light, the X- and Y-axes are therefore the top view of the cross-section of the fiber) 1. The intensity distribution of the mode field for the given fiber optic parameters (core diameter, refractive index, numerical aperture of the fiber) can be approximated by a normal distribution. This can also be done using any more precise mathematical model. 2. Due to the sensor manufacturing process, the relative positions of the Bragg gratings used to each other and to the fiber core center are known (typically at the edge of the fiber core, for example, 2.2 µm from the center). For the sake of simplicity, two Bragg gratings at opposite positions to the fiber core are considered below on the X-axis and the further Bragg grating(s) perpendicular to them directly on the Y-axis (coordinates: Bragg grating 1: (2.2µm, 0µm), Bragg grating 2: (-2.2µm, 0µm), Bragg grating 3: (0µm, 2.2µm) and Bragg grating 4: (0µm, -2.2µm)).The measured (and process-dependently differing) amplitudes of all gratings are normalized for a non-bent fiber to the intensity that exhibits the perfectly centered mode field at its spatial position. This normalization represents a calibration constant. 4. The intensity ratio between the first two Bragg gratings corresponds to a non-bent fiber (along the x-axis). If the fiber is bent, the intensity ratio between the two opposite Bragg gratings changes. 5. It can be determined how far the mode field must have shifted in the x-direction for the new amplitude ratio to occur.

[0048] For an evaluation of 4 Bragg gratings at each measuring point: 6. If there are also 2 Bragg gratings opposite each other on the Y-axis, this procedure can also be carried out directly for the Y-axis

[0049] For an evaluation of 3 Bragg gratings at each measurement point (advantageous due to the smaller number of Bragg gratings required): 7. The expected intensity at position X = 0 (i.e., on the Y-axis) is derived for the displaced mode field along the X-axis determined in point 5. This intensity is used as a virtual fourth grating amplitude.

[0050] 8. Now the displacement of the mode field can also be determined on the Y-axis between the third Bragg grating and the new virtual Bragg grating analogously to points 4 and 5 above.

[0051] This allows both the direction and the strength of the mode field shift to be determined.

[0052] Using a suitable conversion model (in the simplest case an antiproportional dependency), the direction of curvature and the radius of the bend of the glass fiber can be directly determined.

[0053] Several such curvature measurement planes along a fiber optic cable allow complete 3D shape reconstruction.

[0054] It is advantageous if the following is used in the process according to the invention: the inscription of at least 3 Bragg gratings at different relative positions to a single-mode waveguide, an evaluation system which can evaluate and differentiate intensities of at least 3 Bragg gratings of different wavelengths, an algorithm (as described above) for determining the displacement of the mode field from at least 3 relative intensities of the Bragg gratings, and possibly the determination of the normalization constant which, for a non-bent fiber, corrects the random and manufacturing-related amplitude ratio of the Bragg gratings to the intensities of the mode field at the respective spatial position of the Bragg gratings.

[0055] Of course, the invention is not limited to the illustrated embodiments. The above description is therefore not to be considered limiting, but rather illustrative. The following claims are to be understood as meaning that a stated feature is present in at least one embodiment of the invention. This does not exclude the presence of further features. Where the claims and the above description define "first" and "second" embodiments, this designation serves to distinguish between two similar embodiments without establishing a priority.

Claims

1. Method for determining a curvature and / or torsion of an optical waveguide (3) of a fiber-optic sensor (1) with an optical waveguide (3) having an axial direction aligned in a direction of light propagation and a radial direction aligned perpendicular thereto, wherein the optical waveguide (3) hasa core (5) forconducting light, which core runscentrally in the axial direction and extendsat least substantially overthe entire length of the optical waveguide (3), and a cladding (6) surrounding the core (5) in the radial direction, and wherein at least one section of the optical waveguide (3), which runsin the axial direction, isprovided with at least two Bragg gratings (8, 9, 20), which are introduced therein and run through a common cross-sectional plane (11) through the optical waveguide (3), which cross-sectional plane liesin the radial direction, wherein the Bragg gratings(8, 9, 20) are introduced into the core (5) and / oron the boundary between the core (5) and the cladding (6) and / or in an inneredge region of the cladding (6) within an evanescent region of the light, and wherein at least some of the Bragg gratings(8, 9, 20) have different grating constants, comprising the method stepsof: (a) providing reference data of intensities of reflected light portions of light coupled into the optical waveguide (3), (b) measuring at least one light intensity (35) of reflected light portions of light coupled into the optical waveguide (3), wherein the optical waveguide (3) hasa deformation to be determined, and (c) determining the deformation by comparing the light intensity (35) with the reference data, wherein wavelength multiplexing and / or time multiplexing of the light coupled into the optical waveguide iscarried out to determine the deformation.

2. Method according to claim 1, wherein the inneredge region of the cladding (6) extendsinto the cladding (6) in the radial direction by lessthan ten percent of the thickness of the cladding (6).

3. Method according to claim 1 or 2, wherein three Bragg gratings(8, 9, 20) running through the cross-sectional plane (11) are arranged in the section.

4. Method according to claim 3, wherein one of the Bragg gratings(8, 9, 20) isarranged centrally in the core (5) and two of the Bragg gratings (8, 9, 20) are arranged in the inneredge region of the cladding (6) or the three Bragg gratings(8, 9, 20) are arranged in the inner edge region of the cladding (6).

5. Method according to any one of claims 1 or 2, wherein four Bragg gratings (8, 9, 20) running through the cross-sectional plane (11) are arranged in the section, in particular wherein the fourBragg gratings (8, 9, 20) are all arranged in the inneredge region of the cladding (6) or all on the edge of the core (5) or all in an outer edge region of the core (5).

6. Method according to any one of claims 1 to 5, wherein the Bragg gratings (8, 9, 20) are arranged in such a way that they are symmetrically distributed around the center of the core (5).

7. Method according to any one of claims 1 to 6, wherein the Bragg gratings(8, 9, 20) have a maximum diameterof less than half the maximum diameter of the core (5).

8. Method according to any one of claims 1 to 7, wherein at least two light intensities (35) are measured, wherein a three-dimensional deformation isdetermined by comparing the light intensities(35) with the reference data.

9. Method according to any one of the claims 1 to 8, wherein in determining the deformation, an evaluation of the light intensities(35) iscarried out by meansof a spectrometer and / or an AWG filter element and / or an FBG filter element.

10. Method according to any one of claims 1 to 9, wherein reference data of intensities of reflected light portions of light coupled into the opticalwaveguides(3) isprovided on the basisof known reference deformationsof the optical waveguide.