Sensor fiber for temperature, strain and / or torsion detection in the form of a multi-core optical waveguide with a fiber Bragg grating structure

Description

[0001] The invention relates to a sensor fiber for temperature, strain and / or torsion detection in the form of a multi-core optical fiber with a fiber Bragg grating structure according to the preamble of claim 1.

[0002] Such sensor fibers are used to monitor materials and processes. By inscribing Fiber Bragg Gratings (FBGs) into the fiber core, sensitivity to length and / or temperature changes is achieved. By combining several such FBGs, material deflections and stresses can also be determined. To reduce errors caused by environmental conditions affecting the individual FBGs, several of these gratings are combined into a single fiber, which thus contains several such FBG fiber cores.

[0003] Several embodiments of such multi-core fibers are known from the prior art. For example, US Pat. No. 5,563,967 teaches a method for manufacturing a sensor fiber based on a design with two FBG fiber cores. Such a sensor fiber makes it possible to simultaneously determine temperature and length changes.

[0004] WO 2007 / 143369 A1 discloses a multi-core optical fiber sensor. According to the teaching contained therein, a pressure sensor in the form of a multi-core optical fiber is described. The structure disclosed therein is intended to determine external pressure by measuring fiber bending. To determine the bending of the fiber, both fiber cores are read. According to the teaching disclosed therein, only the relative behavior of both fiber cores must be determinable, from which the bending is determined. The position of the fiber cores, however, is not important.

[0005] WO 03 / 005080 A1 discloses a method in connection with optical fibers. The subject of the document is a manufacturing method for producing a fiber having capillaries. According to the teachings disclosed therein, these capillaries are filled with a material, preferably metal. The fiber is designed such that the optical properties of the fiber core can be modified by applying an electric field or selective heating.

[0006] US Pat. No. 7,317,849 B1 discloses an optical fiber sensor and a method for manufacturing such a sensor. The arrangement of the FBG fiber cores provided therein is intended to be used to detect bends within defined planes. According to the teaching disclosed therein, the assignment and orientation of the fiber cores is achieved by externally shaping the sensor fiber. This allows the fiber to be placed on a corresponding surface in a defined manner.

[0007] US 2012 / 0114292 A1 discloses multi-core optical fiber ribbons. In the fiber ribbons shown therein, at least two cores are combined parallel to each other in a plane within a surrounding cladding. Each of the cores can be configured as a so-called multimode core. The fiber shown therein is not a sensor fiber and does not contain any FBG fiber cores. The subject of the document is a practical implementation of a specific fiber density in a fiber ribbon.

[0008] US 2011 / 0052129 A1 discloses a multi-core optical fiber. The subject of the document is the reduction of so-called crosstalk between cores combined in a common fiber body. The document discloses a multi-core fiber for light transmission. The disclosed fiber cores are not FBG fiber cores.

[0009] However, the sensor fibers of this type known from the state of the art have the disadvantage that they do not readily allow for simultaneous measurement of length, bending, temperature, and torsion. To acquire the most comprehensive measurement data possible, many FBG fiber cores are required, and the analysis of the raw data generated by the sensor fibers is becoming increasingly complex.

[0010] The task is therefore to provide a sensor fiber of the type mentioned with the simplest possible structure, with which as many measurement parameters as possible can be measured simultaneously and with which it is particularly possible to make torsion effects measurable in addition to temperature, length and bending changes.

[0011] The object is achieved with a sensor fiber for temperature, strain, and / or torsion detection in the form of a multi-core optical waveguide with a fiber Bragg grating structure having the features of claim 1. This comprises at least two light-conducting FBG fiber cores provided with the fiber Bragg grating structure and a common cladding surrounding the FBG fiber cores.

[0012] According to the invention, the sensor fiber is characterized by a reference point present within the cladding in the form of one or more differentiation and orientation means arranged within the sensor fiber for generating a marking that uniquely identifies each individual FBG fiber core, wherein the individual FBG fiber cores combined in the sensor fiber are distinguishable, so that the respective measurement signals from each individual FBG fiber core can be detected by a measuring apparatus and can be uniquely assigned to each individual fiber.

[0013] According to the invention, the discrimination and orientation means address the individual FBG fiber cores, so that in a measuring apparatus downstream of the sensor fiber, information is always available about which FBG fiber core within the sensor fiber delivers which signal and which FBG fiber core thus contributes in which way to the measurement signal.

[0014] According to the invention, the one or more differentiation and orientation means arranged within the sensor fiber are designed in the form of at least one clearly detectable marking zone arranged at selected positions within the cross-section of the sensor fiber in the cladding, wherein the at least one marking zone is a rod inducing mechanical stresses, wherein the mechanical stresses cause a clearly detectable change in the light-guiding properties in a nearby and thereby marked FBG fiber core.

[0015] The idea underlying the sensor fiber according to the invention is thus to design the individual FBG fiber cores combined in the sensor fiber so that they can be distinguished, so that the respective measurement signals from each individual FBG fiber core can be recorded very easily by a measuring device and, above all, can be clearly assigned to each individual fiber. These differentiation and orientation means make it possible, in particular, not only to perform strain, length, and bending measurements, but also to determine the temperature and, above all, the torsion present in the fiber. The latter is possible because the differentiation and orientation means address the individual FBG fiber cores, so that in a measuring device downstream of the sensor fiber, information is always available about which FBG fiber core within the sensor fiber is delivering which signal and which FBG fiber core therefore contributes in what way to the measurement signal.

[0016] In a first embodiment not according to the invention, the distinguishing and orientation means is a non-cylindrically symmetrical arrangement of the individual FBG fiber cores within the cladding, wherein the FBG fiber cores are provided in a number ensuring the unambiguous marking.

[0017] The individual FBG fiber cores are distributed across the cross-section of the sensor fiber in such a way that their asymmetric arrangement allows for a clear differentiation of the FBG fiber cores.

[0018] In a further embodiment, the distinguishing and orientation means is designed in the form of clearly detectable marking zones arranged in the cladding at selected positions within the cross-section of the sensor fiber. In such a case, the sensor fiber provides the downstream measuring device with at least one reference point or reference signal that indicates the arrangement of the FBG fiber cores and thus their distinguishability.

[0019] The marking zones can be designed in different ways.

[0020] In a first embodiment, the marking zone is a capillary embedded in the cladding. This capillary is characterized by its "hole" structure in a way that is easily detected optically, thus establishing a reference point within the cross-section of the sensor fiber, by which the individual FBG fiber cores can be individually distinguished.

[0021] In an embodiment not according to the invention, the capillary is filled with an analyte and / or a marker. This allows additional measurement signals to be derived from the capillary and / or significantly increases the detectability of the reference point established by the capillary.

[0022] According to the invention, the marking zone is designed as a rod inducing mechanical stresses, wherein the mechanical stresses cause a clearly detectable change in the optical fiber properties of a nearby and thus marked FBG fiber core.

[0023] In this embodiment, the differentiation of the individual FBG fiber cores is thus achieved by the fact that at least one of the FBG fiber cores is optically clearly different from the other FBG fiber cores and can thus be used as a reference from which the individual other FBG fiber cores can be addressed by the measuring apparatus.

[0024] In a further embodiment not according to the invention, the marking zone is designed as a defect in the form of a rod made of the material of the cladding, which defect disrupts a given arrangement symmetry of the FBG fiber cores and cannot be used for light propagation.

[0025] The differentiation of the individual FBG fiber cores is achieved here by the downstream measuring equipment being set up for a specific arrangement symmetry of the FBG fiber cores within the sensor fiber, but due to the rod consisting of the cladding, it detects precisely this arrangement symmetry as being disturbed. This disturbance then forms a reference point from which the individual FBG fiber cores can be addressed and thus distinguished.

[0026] In a further embodiment not according to the invention, the distinguishing and orientation means is formed by an asymmetrically ablated material formed on the respective FBG fiber core. The FBG fiber core is thereby modified with regard to its optical properties and thus forms a clearly detectable reference point for addressing the remaining FBG fiber cores in the sensor fiber.

[0027] Additionally, each individual FBG fiber core can be surrounded by a layer with a reduced refractive index that prevents optical waveguide transmission between the individual FBG fiber cores. This supports the unambiguous allocation of the measurement signals within the individual FBG fiber cores and improves the quality of the respective measurement signal.

[0028] The sensor fiber according to the invention will be explained in more detail below using exemplary embodiments. Figuren 1 bis 6 The same reference symbols are used for identical or equivalent parts.

[0029] It shows: Fig. 1 shows a cross section of a first embodiment of the sensor fiber not according to the invention, Fig. 2 shows a cross section of a first embodiment of the sensor fiber not according to the invention with marking zones arranged within the cladding, Fig. 3 shows a cross section of a first embodiment of the sensor fiber not according to the invention with marking zones in the form of a capillary with and without markers and / or analytes, Fig. 4 shows a cross section of an embodiment of the sensor fiber not according to the invention with a rod for inducing mechanical stresses, Fig. 5 shows a cross section of an embodiment of the sensor fiber not according to the invention with a symmetry-disturbing defect, Fig. 6 shows a cross section of an embodiment of the sensor fiber not according to the invention with a modified FBG fiber core, Fig. 7 shows a cross section of an embodiment of the sensor fiber not according to the invention with an inhomogeneity arranged in the outer contour of the fiber.

[0030] All embodiments described below with multiple FBG fiber cores are created by combining doped and / or undoped glass rods. These are either single-mode or multimode preforms. Furthermore, doped / undoped glass rods can also be inserted to serve as placeholders or to induce mechanical stresses. These individual cores are arranged in a tube in such a way that the individual cores can be distinguished even after fusing and drawing the fiber. Some exemplary embodiments are described in more detail below.

[0031] The cross-sectional size of the marking zones is not crucial. They should be at least large enough to be reliably detected optically. In particular, it is sufficient that the marking zone can be clearly identified as a "defect" or "inhomogeneity."

[0032] Fig. 1 shows a cross-section of a first embodiment of the sensor fiber 1 not according to the invention. In the present example, the sensor fiber consists of three individual FBG fiber cores 2, each having a fiber Bragg grating (FBG). The three FBG fiber cores are surrounded by a cladding 3, whereby the sensor fiber formed thereby has a circular cross-sectional shape in the present example. Other cross-sectional shapes of the sensor fiber, in particular elliptical shapes, are also possible.

[0033] At the Fig. 1 In the embodiment shown, three FBG fiber cores are arranged in the cladding, which do not adopt a cylindrically symmetrical distribution. The means of differentiation and orientation for the entire sensor fiber, and thus the identification of each individual sensor core, is ensured by the asymmetrical distribution of the sensor cores. For example, the fiber core located in the 12 o'clock direction forms a first core, the fiber core located in the 3 o'clock direction forms the second core, and the fiber core located at 6 o'clock forms the third core, whereby it is specified that the asymmetry of the sensor fiber lies in the 9 o'clock direction. This asymmetry can be easily detected. The orientation of the sensor fiber in an external reference system, in particular its twist angle, can be arbitrary.Once the asymmetry has been detected, the individual FBG fiber cores are addressable, so their position relative to the external reference system can be easily corrected using appropriate transformations within the measurement device. The corresponding assignment can be performed within the measurement device. However, it is important to ensure the correct direction of radiation through the sensor fiber.

[0034] The Fig. 1 The asymmetrical arrangement of the FBG fiber cores shown can be very easily determined during the production of the sensor fiber preform. The preform FBG cores are arranged accordingly and cladding them in the usual way. The arrangement of the FBG cores is maintained during the subsequent drawing process and marks the asymmetry present in the finished sensor fiber.

[0035] Fig. 2 shows a non-inventive embodiment with four FBG fiber cores 2 in a cylindrically symmetrical arrangement. In the present example, a marking zone 4 is provided within the cladding 3, which is enclosed between two FBG fiber cores 2 and thus designates a marking location in the cladding. The marking zones are areas within the cladding that differ in their properties from the surrounding cladding. In the embodiment in Fig. 4 This is a marking zone with a triangular cross-section. It is relatively small compared to the FBG fiber cores, but sufficiently large to be reliably detected.

[0036] The marking zones can, in particular, be designed to be light-conducting, but without actually being used as light-conducting cores or sensor cores. Their function essentially consists only in providing and defining an orientation signal and thus a reference point for the entire sensor fiber.

[0037] One way to install one or more marking zones in Cladding 3 is in Fig. 3 shown. In the present case, which is not according to the invention, two capillaries 5 are embedded in the cladding, with one of the capillaries being filled with an analyte and / or a marker 6. The marker can, for example, be a fluorescent substance whose fluorescent light is registered by a downstream measuring device. In cross-section, the capillaries appear as "holes" that are either filled or unfilled. A sensor fiber designed as in this example ensures, on the one hand, the distinguishability of the individual FBG fiber cores 2, for example by designating the FBG fiber core arranged between the capillaries as the first core and the FBG fiber cores following it clockwise as the second, third, and fourth FBG fiber cores, and are thus detectable.Furthermore, both capillaries are designed to be distinguishable, so that the direction of radiation through the sensor fiber can be clearly determined, while the unique addressing of the individual FBG fiber cores is maintained at all times. Specifically, the orientation of the sensor fiber and its individual FBG fiber cores is determined in such a way that a downstream measuring device registers the light signals arriving from the 5 capillaries accordingly and then addresses the individual FBG fiber cores in an internal calibration process.

[0038] An inventive possibility for marking and addressing the FBG fiber cores is shown Fig. 4 In the present example, a rod 7 is provided within the cladding 3. This rod is arranged near one of the FBG fiber cores 2 and induces mechanical stresses in this FBG fiber core. This changes the optical behavior of the corresponding FBG fiber core. This thus forms a reference point that can be clearly detected by a measuring device, from which the remaining FBG fiber cores can be distinguished and addressed.

[0039] Fig. 5 shows a further possibility for marking and addressing the FBG fiber cores, not according to the invention. In the example presented here, a cylindrically symmetrical arrangement of eight FBG fiber cores 3 within the common cladding 3 is provided. However, the eight-fold symmetry is disturbed by the fact that one of the FBG fiber cores has been replaced by a rod 8. This rod 8 has, in particular, the refractive index of the cladding and can also be made of the same material as the cladding. Of course, the rod can also differ from the cladding in terms of material and refractive index. In any case, the rod breaks the symmetry of the arrangement of the FBG fiber cores and thus sets a marking reference point.

[0040] Fig. 6 shows another exemplary embodiment of the sensor fiber 1, not according to the invention, with four FBG fiber cores 2 within the cladding 3. In the present embodiment, one of the FBG fiber cores is provided with a modification 9, which thus establishes a reference point for the arrangement of the FBG fiber cores. Furthermore, all FBG fiber cores are surrounded by a layer 10 with a reduced refractive index.

[0041] Modification 9 can be achieved in particular by modifying the FBG fiber core in question by asymmetrically removing material.

[0042] Fig. 7shows a non-inventive embodiment of the sensor fiber 1 with four FBG fiber cores 2 within the cladding 3. In this exemplary embodiment, the marking zone is formed as an inhomogeneity 11 within the outer contour of the sensor fiber. In the present case, the inhomogeneity is a depression in the form of a notch. The notch within the sensor fiber can be created by milling a sufficiently deep and wide initial notch over the entire length of the preform intended for production of the sensor fiber at a defined location or by introducing it using other corresponding machining methods. During the subsequent drawing process, the initial notch rounds and flattens somewhat, but remains sufficiently clear and can be used as a detectable marking.

[0043] During preform and fiber manufacturing, the individual cores are fused to the surrounding tube. In some cases, the fiber Bragg gratings, as well as the modifications to be made to the individual FBG fiber cores, can also be engraved directly into the sensor fiber during the fiber drawing process.

[0044] The sensor fiber was explained in more detail using examples.

[0045] The invention is defined by the appended claims. List of reference symbols

[0046] 1Sensor fiber 2FBG fiber core 3Cladding 4Marking zone 5Capillary 6Marker / analyte 7Stress-inducing rod 8Void rod 9Fiber core modification 10Refractive index-reduced layer 11Inhomogeneity in outer contour

Claims

1. Sensor fiber (1) for temperature, expansion and torsion detection in the form of a multi-core optical fiber with a fiber Bragg grating structure, comprising at least two light-conducting FBG fiber cores (2) provided with the fiber Bragg grating structure and a common cladding (3) surrounding the FBG fiber cores, characterized by a reference point present within the cladding in the form of one or more distinguishing and orientation means arranged within the sensor fiber for producing a marking which uniquely identifies each individual FBG fiber core, wherein the individual FBG fiber cores combined in the sensor fiber are distinguishable, so that the respective measurement signals from each individual FBG fiber core can be detected by a measuring apparatus and can be uniquely assigned to each individual fiber, wherein the distinguishing and orientation means address the individual FBG fiber cores, so that a measuring apparatus downstream of the sensor fiber always has information about which FBG fiber core within the sensor fiber supplies which signal and which FBG fiber core thus contributes to the measuring signal in which way, wherein the one or more distinguishing and orientation means arranged within the sensor fiber is designed in the form of at least one clearly detectable marking zone (4) arranged at selected positions within the cross-section of the sensor fiber (1) in the cladding (3), wherein the at least one marking zone is a rod (7) inducing mechanical tensions, wherein the mechanical tensions cause a clearly detectable change in the light guiding properties of an FBG fiber core located in the vicinity and marked thereby.

2. Sensor fiber according to claim 1, characterized in that each individual FBG fiber core (2) is surrounded by a refractive index-reduced layer (10) which prevents light wave conduction occurring between the individual FBG fiber cores.