Optical waveguide for sensor applications, use of optical waveguide and method of manufacturing an optical waveguide

Abstract

The present invention relates to an optical waveguide for sensor applications, use of the optical waveguide as optical sensor as well as a method of manufacturing the same, and in particular to an optical waveguide which can be bent and comprises fiber Bragg grating structures. The optical waveguide being bendable and includes at least two Bragg grating structures providing continuous and broad spectral responses so as to enable high spatial resolution being sensitive to small shape changes.

Description

[0001] FiSens GmbH HE 277 926

[0002] OPTICAL WAVEGUIDE FOR SENSOR APPLICATIONS, USE OF OPTICAL WAVEGUIDE AND

[0003] METHOD OF MANUFACTURING AN OPTICAL WAVEGUIDE

[0004] Field of the Invention

[0005] The present invention relates to an optical waveguide for sensor applications, use of the optical waveguide as optical sensor as well as a method of manufacturing the same, and in particular to an optical waveguide which can be bent and comprises fiber Bragg grating structures.

[0006] Background

[0007] Moravec's Paradox highlights a striking counterintuitive observation that tasks requiring high-level reasoning (like playing chess or solving equations) are easier for Artificial Intelligence (Al) to learn, while tasks humans find simple, such as walking, recognizing objects, or navigating through space, are remarkably difficult for machines.

[0008] Current advancements in machine learning, particularly in robotics, show a need to emulate the multi-sensory richness of human perception. The self-awareness in space and time requires sensory technologies that can detect and process high-resolution spatial and temporal information, mimic biological systems' integration of touch, proprioception, and vision as well as facilitate real-time motion feedback and selfcalibration for robotic links and limbs.

[0009] Hence, there is a need for a simple, compact and cost-effective sensor system that enables motion sensing in artificial extremities achieving high spatial resolution to detect micro-movements and deformations, integrating seamlessly into robotic systems to provide comprehensive motion feedback. Similarly, such a sensor system may also be applied to humans, animals, or mechanical devices the motion of which needs to be monitored.

[0010] Fiber-optic shape sensors offer a highly promising approach to motion sensing due to their ultra-thin and flexible design, which provides an elongated medium for spatially distributed sensing and fast transportation of information with high bandwidths. They are easy to integrate into a robotic kinematic chain, mechanical joints or levers, garments, protheses, etc. and their energy-efficient, passive nature makes them ideal in environments with electric drives, power electronics and electromagnetic interferences. However, conventional fiber-optic shape sensing solutions lack high spatial resolution, being constrained by the practical density of sensing points along the fiber. They lack absolute production repeatability and robustness against environmental influences such as temperature, vibration and humidity. They require precise calibration procedures to compensate for their limited number of individually reflected light signals. Additionally, conventional solutions for measuring and processing light signals are bulky, heavy, energy inefficient and expensive.

[0011] For example, in US 10,969,541 B2 (reference 1) the number of sensor points is inherently limited, as each fiber Bragg grating (FBG) requires a specific spectral channel to ensure that the reflections by the Bragg gratings can be evaluated separately without crosstalk.

[0012] Within the constraints of a practically available bandwidth when using individual FBGs in curvature sensors, Waltermann et al. estimate in their paper in the Journal Applied Optics "Muliple off-axis fiber Bragg gratings for 3D shape sensing" (Vol. 57, No. 28, 1 October 2018) that 10 to 15 curvature sensors within a single fiber are achievable (reference 2).

[0013] However, for an optical fiber of several centimeters of length to achieve a good spatial resolution to measure different changes in shape, e.g. curvatures, more than 100 FBGs would be needed which is not practically achievable with known techniques due to the limitations discussed above with respect to references 1 and 2.

[0014] That is, when increasing the number of FBGs within the available spectrum, the separate FBG peaks become too close to each other making any analysis unreliable. The proximity of the FBG peaks results in crosstalk effects, ultimately causing errors, as the intensity change of one FBG signal and its side lobes interferes with the adjacent FBG signals leading to inaccuracies.

[0015] Summary

[0016] Therefore, an aim is to provide a novel optical waveguide, use of the optical waveguide as optical sensor as well as a method of manufacturing the optical waveguide.

[0017] Specifically, it is desired to overcome the above limitations and provide an optical waveguide with high spatial resolution being sensitive to small shape changes over several centimeters so as to enable shape reconstruction with high accuracy. According to an embodiment, an optical waveguide which is bendable and comprises a core and a cladding which surrounds the core. The core is configured to guide light which is incident on the waveguide (incident light) in a light propagation direction along a light propagation path. A cross-section of the waveguide is substantially perpendicular to the light propagation direction and comprises a first core area of a first core segment and a second core area of a second core segment. The first and second core areas are on opposite sides of the center of the core. Similarly, the first and second core segments may be on opposite sides of the center of the core so that the core may be considered to be comprised of two core segments. The optical waveguide comprises at least two Bragg grating structures. One Bragg grating structure is placed within the first core segment or along its boundary with the cladding and another Bragg grating structure is placed within the second core segment or along its boundary with the cladding. In other words, a Bragg grating structure may be placed in the core or in close proximity to the boundary of the core to the cladding, wherein along the boundary comprises locations on the boundary and / or in the cladding close to the boundary. Each Bragg grating structure extends over a length of at least 10000 times, preferably at least 50000 times, the central wavelength of the incident light in the light propagation direction. For example, considering incident light in the visible, near infrared or short-wavelength infrared range and a length of 10 000 times the central wavelength in these ranges, this would lead to a length of at least approximately 0.5 to 1 centimeter. Each Bragg grating structure includes a chirped Bragg grating or multiple Bragg gratings along the light propagation direction. The multiple Bragg gratings may be continuously, i.e. one after the other, inscribed in the optical waveguide to form a continuous optical waveguide structure.

[0018] Accordingly, a wide spectral response of each of the Bragg grating structures can be achieved over a long optical waveguide.

[0019] According to an advantageous example, the center wavelengths of the respective multiple Bragg gratings are all different, wherein the different center wavelengths of the subsequent Bragg gratings preferably differ by a few picometers or by value between 10 and 1000 pm.

[0020] According to another advantageous example, the center wavelengths of the respective multiple Bragg gratings continuously increase or decrease in the light propagation direction, wherein the continuously increasing or decreasing center wavelengths of the subsequent Bragg gratings preferably differ by a few picometers or by value between 10 and 1000 pm.

[0021] According to another advantageous example, the at least two Bragg grating structures are configured to reflect incident light in different wavelength ranges so that wavelengths of the incident light reflected by the first Bragg grating structure can be mapped to spatial positions in the light propagation direction in the first core segment and wavelengths of the incident light reflected by the second Bragg grating structure can be mapped to spatial positions in the light propagation direction in the second core segment. The different wavelength ranges dependent on the spectral response of the Bragg grating structure, wherein the spectral response of each Bragg grating structure is a combination of multiple reflected wavelengths of the multiple Bragg gratings or chirped Bragg grating. From the combination, an individual response of a Bragg grating cannot be resolved. Hence, the spectral response of each Bragg grating structure is a continuous spectral response. For example, the Bragg grating structures start at a common cross-section and have increasing or decreasing center wavelengths of the multiple Bragg gratings so that the center wavelengths can be mapped to positions having an increasing distance from the position of the common cross-section in light propagation direction.

[0022] According to another advantageous example, the optical waveguide comprises three or four Bragg grating structures and the cross-section comprises three or four core areas of three or four core segments, respectively. In the case of four Bragg grating structures the optical waveguide further comprises a third core area of a third core segment and a fourth core area of a fourth core segment both being on opposite sides of the center of the core.

[0023] According to another advantageous example, the length of each Bragg grating structure is at least 1 centimeter and the width of the wavelength range of the spectral response of each of the Bragg grating structures is at least 2 nanometers and preferably more than 10 nanometers or more than 20 nanometers. The spectral response of the Bragg grating structure is a combination of multiple reflected wavelengths of the multiple Bragg gratings or chirped Bragg grating; the combination being such that an individual peak of a Bragg grating response (Bragg grating peak) cannot be resolved. Preferably, the length of each Bragg grating structure is at least 3 centimeters or 5 centimeters. For example, a 10 cm long optical waveguide structure with a spectral range of 10 nanometers leads to a spatial resolution of 100 pm spectral response for each 1 mm optical waveguide length. According to another advantageous example, the optical waveguide is a single mode optical waveguide, preferably a single mode optical fiber.

[0024] According to another advantageous example, the multiple Bragg gratings include more than 10, preferably more than 25 or more than 50, Bragg gratings and / or include chirped Bragg gratings. Preferably, the spectral response of the Bragg grating structure includes the same number of center wavelengths as number of Bragg gratings so that the spectral response is a combination of individual responses of the multiple Bragg gratings, which cannot be resolved.

[0025] According to another advantageous example, the different wavelength ranges, and particularly the spectral responses, of the Bragg grating structures are spectrally separated by 0.5 nm to 3 nm.

[0026] According to another advantageous example, the spectral response of each Bragg grating structure constitutes an uninterrupted spectrum of light reflected from the multiple Bragg gratings or the chirped Bragg grating.

[0027] According to an embodiment, the above optical waveguide is used as optical sensor, comprising coupling the incident light into the optical waveguide; receiving back-reflected light which are wavelengths of the incident light reflected by the Bragg grating structures; generating at least two spectra of at least two different wavelength ranges, wherein the width of a spectral range of a spectra corresponds to the length of the Bragg grating structure; and comparing the at least two spectra to derive the shape of the optical waveguide.

[0028] According to an embodiment, a method of manufacturing an optical waveguide for sensor applications comprises providing a bendable optical waveguide having a core and a cladding which surrounds the core; the core being configured to guide incident light in a light propagation direction along a light propagation path, wherein a cross-section is substantially perpendicular to the light propagation direction and comprises a first core area of a first core segment and a second core area of a second core segment both being on opposite sides of the center of the core; and inscribing at least two Bragg grating structures in the bendable optical waveguide by focusing a pulsed laser beam into the same, wherein one Bragg grating structure is placed within the first core segment or along its boundary with the cladding and another Bragg grating structure is placed within the second core segment or along its boundary with the cladding; wherein each Bragg grating structure extends over a length of at least 10000 times the central wavelength of the incident light in the light propagation direction; and wherein each Bragg grating structure is inscribed to include a chirped Bragg grating or multiple Bragg gratings along the light propagation direction.

[0029] Brief description of the drawings

[0030] Fig. 1A illustrates a spatial mode-field distribution in an optical single mode fiber.

[0031] Fig. IB illustrates a Gaussian intensity profile of the cross-sectional mode-field.

[0032] Figs. 2A and 2B show two FBGs in a fiber according to the prior art.

[0033] Figs. 3A, 3B and 3C illustrate three exemplary shapes of an elongated optical fiber.

[0034] Figs. 4A and 4B illustrate schematically four core segments in three dimensional space and four core areas.

[0035] Fig. 5 schematically illustrates two Bragg grating structures along the core of a fiber.

[0036] Figs. 6A and 6B illustrate a fiber core with continuous FBG structures in its proximity and their spectral responses.

[0037] Fig. 7 illustrates four spectral responses in a spectral dimension.

[0038] Fig. 8 illustrates a two-dimensional view on each side of a straight optical fiber core.

[0039] Fig. 9 illustrates changes in intensity of a curved optical fiber.

[0040] Fig. 10 illustrates a simulation of a bending of a fiber.

[0041] Fig. 11 illustrates an error when comparing a reconstructed mode-field path to a simulate path.

[0042] Fig. 12 illustrates spectral response changes in time when bending a fiber.

[0043] Detailed description of the preferred embodiments It is known and shown in Figs. 1A and IB that a single mode-field has a gaussian intensity distribution over the cross-section of an optical waveguide, such as a single mode optical fiber.

[0044] It is also known in the prior art, such as in references 1 and 2 above, to use two, see e.g. Fig. 2A, or three FBGs in one waveguide location (common cross-section), and by measuring their intensities, see e.g. Fig. 2B, and comparing them to a reference signal to derive a shift of the mode-field and thereby a curvature at this location.

[0045] This is because, the mode-field intensity within the fiber will be slightly shifted if the fiber is curved. Hence, the coupling between the mode-field and an FBG will be influenced and the back- reflected light of the FBG will change. In other words, the mode-field within the fiber is dislocated due to bending so that the position of the mode-field or the curvature of the fiber, respectively, can be determined by monitoring the intensities of the back- reflected light. As example, Fig. 2B shows two different intensities reflected from BG1 and BG2, referenced as 8 and 9 in Fig. 2A, which are due to a curvature of fiber 3 having core 5 and cladding 6 shown in Fig. 2A.

[0046] However, this approach requires to evaluate each FBG separately and drastically limits the achievable number of derivable curvature points due to the required spectral bandwidth for distinguishable FBG signals. In order to calculate the full shape of a fiber this approach requires an interpolation between the derived curvature points along the fiber, which leads to errors and reduces the accuracy of the shape sensing.

[0047] Neither this approach nor the embodiments of the invention below are limited to optical fibers but any bendable optical waveguide can be used and inscribed with Bragg gratings (BGs) by point-by-point femtosecond laser processing as described in reference 2.

[0048] Figs. 3A to 3C show three exemplary shapes of an elongated optical fiber. It is understood that the propagation path of the single mode field along the fiber is inherently linked to the shape of the waveguide it is guided through. Like water flowing through a water hose, the trajectory of the mode field will be displaced relative to the bending of its confining waveguide.

[0049] In a straight waveguide the single mode field will propagate in a straight propagation path 2 constituting an axial direction (here assumed to be the z-direction), with the maximum intensity distributed in the center of the core of the optical fiber in a straight line along the optical fiber core 1.

[0050] If the waveguide is bent the path 3 (or trajectory) of the mode field is displaced towards the curvature and to the edges of the core of the fiber. Accordingly, the path (trajectory) of the mode-field intensity profile is displaced in the same way.

[0051] The present invention overcomes the limitations of the prior art by seamlessly measuring the continuous path (trajectory) or flow of the mode field along the optical waveguide, such as the fiber in Figs. 3A to 3C.

[0052] This is achieved by measuring continuous spectral responses of at least two, preferably four areas along the core of the fiber. These spectral responses are created by inscribing continuous BG structures into core segments. For example, four spectral responses are created by inscribing four continuous BG structures into four core segments, indicated in cross section as core areas A, B, C and D in Fig. 4B. Fig. 4A schematically illustrates the four core segments which are three-dimensional structures, e.g. four cylinder segments if the fiber is approximated as cylinder, in three dimensional space extending in the light propagation direction corresponding to the z-direction in the figure.

[0053] Alternatively, fewer core segments and thus core areas can be used, e.g. a first and a second core segment. That is the case if instead of four quadrants in Fig. 4B two halves of the shown circular cross section are defined, e.g. combining core areas A and C to a first core area of a first core segment and combining core areas B and D to a second core segment.

[0054] The Bragg grating (BG) structure placed, e.g. inscribed by a laser, in one core segment or along its boundary with the cladding, i.e. in close proximity to the cladding, can be either a continuously chirped BG, or it can be multiple of BGs tightly spaced with center wavelengths differing only a few picometer, preferably lO-lOOOpm. That is, the center wavelengths are much too close for a typical FBG interrogation which would result in crosstalk.

[0055] This continuous BG structure responds with an uninterrupted spectrum (spectral response), instead of separated distinguishable BG peaks. Depending on the desired length of the optical fiber and required spatial resolution of shape sensing, the FBG structure can be comprised of hundreds or even thousands of FBGs. It is obvious that one continuously chirped BG is understood as one BG structure with virtually hundreds or thousands of BGs slightly shifted in wavelength. Conversely multiple individual BG with slightly shifted center wavelengths can be understood as one quasi-continuously chirped BG structure. For an exemplary 10 cm long fiber length, the width of the FBG spectral response can be at least 2-10nm and preferably more than lOnm or even more than 20nm. A spectral response of lOnm of a 10cm fiber results in a high spatial resolution of 100pm spectral response for each 1mm length of fiber.

[0056] Fig. 5 shows a fiber core 1 with continuous FBG structures 7.1 and 7.2 in the cladding along the boundary between core and cladding.

[0057] Figs. 6A and 6B show a three-dimensional view on each side of a straight optical fiber core, visualizing the spectral responses of all four core segments of core areas A-D along the full fiber length in z-direction.

[0058] By mapping the different core segments A, B, C and D to respective continuous spectral ranges of the spectral responses 8.1, 8.2, 8.3 and 8.4 one overcomes the limitation of separate and distinguishably FBG channels, i.e. the need in the prior art to separate the induvial peaks of each FBG. Within this spectral-spatial arrangement, adjacent points along the fiber curve are also adjacent within the spectrum and thereby facilitate an interrogation of the full spectral response, i.e. full trajectory of the mode-field and thereby the shape of the optical fiber. For example, if the center wavelengths of the respective multiple Bragg gratings along the light propagation direction are selected to continuously increase or decrease, the spectral response includes these increasing or decreasing center wavelengths and can be mapped to the positions from where the reflections originate in the fiber.

[0059] With this arrangement there is virtually no limit in spatial resolution, since any additional FBG peak will lead to a higher and more detailed description of the shape.

[0060] The four spectral responses 8.1 to 8.4 from the four BG structures in the fiber of Figs. 6A and 6B are illustrated in a spectral dimension in Fig. 7. Each local position along the continuous spectral responses of the core segments with inscribed FBG structures suffer from inaccuracies due to slight production tolerances. If one used only a few separated local curvature sensing points, i.e. only a few individual FBGs with separable responses, this would exaggerate the error propagation. However, according to the present invention the continuous structure intrinsically balances slight inconsistencies along the fiber length. This results from the seamless and overlapping peaks of multiple FBGs along the entire fiber length.

[0061] An advantage of the present invention is that it can utilize nearly all available pixels of a spectrometer image detector sensing the fiber shape, whereby each imaged spectrum pixel approximates one position along the fiber. This is contrary to the prior art, where only a few channels of the spectrum are used for shape measurement, and multiple empty spaces in-between are left blank on purpose to avoid errors in the FBG peak detection.

[0062] Fig. 8 shows a two-dimensional view on each side of a straight optical fiber core, with 9.1 and 9.2 showing the spectral responses of both core segments A and B or C and D. Note that in the figure the spectral responses are mapped to the z-direction of the fiber, i.e. different intensity values at different wavelengths can be attributed to different positions.

[0063] By bending the optical fiber and displacing the mode-field path (trajectory), the spectral responses will change accordingly. As depicted in Fig. 9, the spectral response A (C) increases in intensity over the core segment in the same direction as the curvature (see spectrum 11.1), whereas spectral response B (D) decreases in intensity over the core segment in the opposite direction of the curvature (see spectrum 11.2). The spectra 10.1. and 10.2 illustrate the situation without bending.

[0064] It is understood that one respective spectral response A (C) or B (D) would be sufficient to calculate the induced displacement of the mode-field path and the fiber shape in one dimension (X or Y). But due to global intensity losses and polarization effects, it is beneficial to derive the change in path by comparison of two opposite spectral responses A (C) and B (D), e.g. by generating a differential signal out of both spectral responses.

[0065] A global attenuation or drop in light power will result in intensity losses for both opposite spectral responses, but not affect the differential signal, which is only created by bending of the mode-field path. Such an opposite pair is particularly immune against polarization effects, since any birefringence and change of the global polarization state will induce the same magnitude of intensity change on both sides, but will not alter the differential signal. Figures 10 and 11 represent a simulation of a bending of a first fiber shape (e.g. straight fiber) into a random second fiber shape and visualizing the spectral responses of two opposite fiber core areas A (C) and B (D) as well as the displaced trajectory of the modefield along the fiber length. Accounting for noise in the intensity detection and assuming an exemplary 15cm long fiber, the simulation shows very low mean error in Fig. 11 resulting into high accuracy of determining the full shape and motion of the fiber.

[0066] One advantage of the measurement of continuous FBG spectra in response to the change of the fiber shape, is the ability to permanently capture any small changes in time, i.e. the motion of the fiber shape. By knowing the gaussian intensity profile and measuring relative changes in the FBG spectral responses (t -> t+1), one can estimate any relative motion of the fiber. A bending motion of a fiber over time is illustrated in Fig. 12 by illustrating four spectra corresponding to four different subsequent time points during which a fiber is being bend.

[0067] With the prior art it is not possible to measure the relative motion from one shape to the next without calibration to known shapes, since each curvature sensing point represents only a cutout of the full shape and motion. Contra rily, by utilizing a continuous spectral response no information of the fiber shape is lost, and the seamless relative movement of the full fiber length is captured.

[0068] Accordingly, the present invention enables a simple solution for tracking robotic movements and in particular simultaneous localization and mapping (SLAM) approaches, since the complete information of one shape to a relative next shape is captured and available for navigating in space and time.

[0069] The above described optical waveguides can be used as optical sensor, comprising coupling the incident light into the optical waveguide; receiving back-reflected light which are wavelengths of the incident light reflected by the Bragg grating structures; generating at least two spectra of at least two different wavelength ranges wherein the width of a spectral range corresponds to the length of the Bragg grating structure; and comparing the at least two spectra to derive the shape of the optical waveguide.

[0070] For manufacturing the above described optical waveguides a commercial regenerative Ti:Sa amplifier system can be used to create laser pulses at 800 nm with an energy of approximately 150 nJ and 100 fs duration. A suitable system for manufacturing is discussed in reference 2. Accordingly, a method of manufacturing the above optical waveguide for sensor applications comprises providing a bendable optical waveguide having a core and a cladding which surrounds the core; the core being configured to guide incident light in a light propagation direction along a light propagation path, wherein a cross-section is substantially perpendicular to the light propagation direction and comprises a first core area of a first core segment and a second core area of a second core segment both being on opposite sides of the center of the core; and inscribing at least two Bragg grating structures in the bendable optical waveguide by focusing a pulsed laser beam into the same, wherein one Bragg grating structure is placed within the first core segment or along its boundary with the cladding and another Bragg grating structure is placed within the second core segment or along its boundary with the cladding; wherein each Bragg grating structure extends over a length of at least 10000 times the central wavelength of the incident light in the light propagation direction; and wherein each Bragg grating structure is inscribed to include a chirped Bragg grating or multiple Bragg gratings along the light propagation direction.

Claims

CLAIMS1. Optical waveguide for sensor applications, the optical waveguide being bendable and comprising a core and a cladding which surrounds the core; the core being configured to guide incident light in a light propagation direction along a light propagation path, wherein a cross-section is substantially perpendicular to the light propagation direction and comprises a first core area of a first core segment and a second core area of a second core segment both being on opposite sides of the center of the core; at least two Bragg grating structures, wherein one Bragg grating structure is placed within the first core segment or along its boundary with the cladding and another Bragg grating structure is placed within the second core segment or along its boundary with the cladding; wherein each Bragg grating structure extends over a length of at least 10000 times the central wavelength of the incident light in the light propagation direction; and wherein each Bragg grating structure includes a chirped Bragg grating or multiple Bragg gratings along the light propagation direction.

2. The optical waveguide of claim 1, wherein the center wavelengths of the respective multiple Bragg gratings are all different, wherein the different center wavelengths of the subsequent Bragg gratings preferably differ by a few picometers or by value between 10 and 1000 pm.

3. The optical waveguide of claim 1 or 2, wherein the center wavelengths of the respective multiple Bragg gratings continuously increase or decrease in the light propagation direction, wherein the continuously increasing or decreasing center wavelengths of the subsequent Bragg gratings preferably differ by a few picometers or by value between 10 and 1000 pm.

4. The optical waveguide of claim 1, 2 or 3, wherein the at least two Bragg grating structures are configured to reflect incident light in different wavelength ranges so that wavelengths of the incident light reflected by the first Bragg grating structure can be mapped to spatial positions in the light propagation direction in the first core segmentand wavelengths of the incident light reflected by the second Bragg grating structure can be mapped to spatial positions in the light propagation direction in the second core segment.

5. The optical waveguide of one of claims 1 to 4, wherein the optical waveguide comprises three or four Bragg grating structures and the cross-section comprises three or four core areas of three or four core segments.

6. The optical waveguide of one of claims 1 to 5, wherein the length of each Bragg grating structure is at least 1 centimeter and the width of the wavelength range of the spectral response of each of the Bragg grating structures is at least 2 nanometers and preferably more than 10 nanometers or more than 20 nanometers.

7. The optical waveguide of one of claims 1 to 6, wherein the optical waveguide is a single mode waveguide, preferably a single mode optical fiber.

8. The optical waveguide of one of claims 1 to 7, wherein the multiple Bragg gratings include more than 10, preferably more than 25 or more than 50, Bragg gratings and / or include chirped Bragg gratings, and wherein in one example the multiple Bragg gratings are continuously inscribed in the optical waveguide to form a continuous optical waveguide structure.

9. The optical waveguide of one of claims 4 to 8, wherein the different wavelength ranges of the multiple Bragg grating structures are spectrally separated by 0.5 nm to 3 nm.

10. The optical waveguide of one of claims 1 to 9, wherein the spectral response of each Bragg grating structure constitutes an uninterrupted spectrum of light reflected from the multiple Bragg gratings or the chirped Bragg grating.

11. Use of the optical waveguide of one of claims 1 to 10 as optical sensor, comprising coupling the incident light into the optical waveguide; receiving back-reflected light which are wavelengths of the incident light reflected by the Bragg grating structures;15 generating at least two spectra of at least two different wavelength ranges wherein the width of a spectral range corresponds to the length of the Bragg grating structure; and comparing the at least two spectra to derive the shape of the optical waveguide.

12. Method of manufacturing an optical waveguide for sensor applications, comprising: providing a bendable optical waveguide having a core and a cladding which surrounds the core; the core being configured to guide incident light in a light propagation direction along a light propagation path, wherein a cross-section is substantially perpendicular to the light propagation direction and comprises a first core area of a first core segment and a second core area of a second core segment both being on opposite sides of the center of the core; and inscribing at least two Bragg grating structures in the bendable optical waveguide by focusing a pulsed laser beam into the same, wherein one Bragg grating structure is placed within the first core segment or along its boundary with the cladding and another Bragg grating structure is placed within the second core segment or along its boundary with the cladding; wherein each Bragg grating structure extends over a length of at least 10000 times the central wavelength of the incident light in the light propagation direction; and wherein each Bragg grating structure is inscribed to include a chirped Bragg grating or multiple Bragg gratings along the light propagation direction.

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