FP type vector bending sensor

Abstract

An FP type vector bending sensor relates to the field of optical fiber sensing, and is characterized in that a spherical microcavity with asymmetric radial wall thickness is integrated at the waist part of a conical single-mode optical fiber. An incident light beam deviates from the center of sphere and enters the microcavity, a reflection point is formed on the inner wall of the cavity, and the reflection point moves in the bending direction. When the optical fiber is bent, the displacement of the reflection point modulates an intracavity optical path track and an optical path difference, so that an interference spectrum has red shift or blue shift response along with different bending directions. Through the asymmetric wall thickness design, the stress of the thin wall side is concentrated when the thin wall side is bent, and the structural strength of the thick wall side is maintained, so that the direction sensitivity is remarkably improved and the sensitivity in each direction tends to be consistent. The sensor is compact in structure, fully sealed, insensitive to environmental refractive index and temperature change, suitable for high-precision curvature and direction measurement in complex environments such as biological fluid and seawater, and high in sensitivity and strong in direction discrimination capability.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing, and in particular to an FP type vector bending sensor. Background Art

[0002] Accurately detecting the magnitude and direction of curvature is a core requirement for analyzing object deformation characteristics, with important applications in structural health monitoring, soft robotics, minimally invasive surgery, and aerospace. Fiber-optic curvature sensors, with their high sensitivity, immunity to electromagnetic interference, and miniaturization, have gradually become a research hotspot in the field of curvature sensing. Key technologies include fiber Bragg gratings (such as fiber Bragg gratings and long-period gratings) and fiber interferometers (such as Fabry-Perot interferometers and Mach-Zehnder interferometers). However, fiber-optic Bragg grating sensors generally suffer from high cross-sensitivity and complex fabrication processes. While Mach-Zehnder interferometers offer high sensitivity, their long interference arms result in large device size, and the thermo-optical effects propagating through doped optical fibers can easily introduce significant temperature interference. In contrast, air-cavity-based Fabry-Perot interferometers, with their compact design and low thermal sensitivity, have demonstrated potential in biomedical microscale measurements. However, such sensors with both high sensitivity and directional discrimination capabilities remain rare.

[0003] In existing technologies, directional-sensitivity strategies are primarily implemented through asymmetric cavity structures or optical path offset designs. For example, adjusting the thickness differences of cylindrical air cavities to induce a directional response to bending strain, however, such open structures always weaken the stress on one side, resulting in uneven sensitivity, susceptibility to environmental contamination, and unsuitability for liquid environments. Another approach utilizes cascaded interferometer structures and the Vernier effect to enhance sensitivity, but this comes at the expense of increased device complexity, increased size, and decreased thermal stability. Another approach employs an optical path offset from the neutral axis to directly respond to tensile / compressive strain. However, due to the lack of deformation enhancement due to cavity wall thickness differences, directional sensitivity remains limited to a relatively low level, and the introduction of temperature-sensitive materials such as UV-curable adhesives further weakens environmental robustness. In summary, the planar reflective surface created by the cylindrical air cavity only detects the bending vector by measuring the beam's presence in the elongated and shortened regions of geometric deformation, relying solely on changes in the physical cavity length. Consequently, existing directional curvature sensors struggle to achieve the following core performance characteristics: highly sensitive directional response consistency, fully sealed miniaturized packaging, and strong immunity to temperature and environmental refractive index perturbations. Therefore, it is urgent to explore a new design that can break through the sensitivity bottleneck while meeting the comprehensive needs of high-precision and directional curvature measurement under complex working conditions. Summary of the Invention

[0004] The present invention aims to address the limited sensitivity, insufficient structural stability, and poor environmental immunity of existing fiber optic directional curvature sensors. The present invention provides an FP-type vector bend sensor. By integrating a single-mode optical fiber tapered region into a fully sealed asymmetric spherical air cavity, the sensor achieves precise determination of bending direction through a combination of geometric asymmetry and a dynamic response mechanism for incident light. The offset incidence point on the spherical arc-shaped inner surface and the asymmetric geometric wall thickness enhance and homogenize directional sensitivity. The reflection point undergoes a monotonic change, with the optical path continuously increasing or decreasing with the bending direction. This characteristic enables bending monitoring and directional identification of the object under test through redshift or blueshift of the interference spectrum. The fully enclosed design eliminates the need for heterogeneous splicing processes, effectively avoiding the sensitivity attenuation and environmental interference issues associated with open cavities. Furthermore, the uniform light field distribution of the asymmetric spherical air cavity significantly improves measurement reliability. This sensor combines high and uniform directional sensitivity with an ultra-compact package size and extremely low temperature / refractive index cross-sensitivity. It is particularly suitable for high-precision directional deformation monitoring in complex dynamic scenarios, providing a stable and robust curvature sensing solution for harsh environments.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for preparing an FP type vector bending sensor comprises the following steps:

[0007] 1) arcing the end faces of the first single-mode optical fiber and the second single-mode optical fiber respectively by pre-discharging in a fusion splicer;

[0008] 2) immersing the end face of the first single-mode optical fiber treated in step 1) in a hydrofluoric acid solution for etching, and forming a concave end face structure by utilizing the difference in etching rates of the hydrofluoric acid solution on the core and cladding materials;

[0009] 3) placing the first single-mode optical fiber and the second single-mode optical fiber processed in step 2) in a fusion splicer, fusing the two end faces to form a cone region, and directionally expanding the concave structure into a completely closed asymmetric spherical air cavity.

[0010] In step 3), offset taper welding is used to expand the concave structure to generate an asymmetric spherical air cavity that deviates from the central axis of the cone area.

[0011] An FP type vector bending sensor is manufactured using the above-mentioned manufacturing method.

[0012] The asymmetric spherical air cavity is embedded in the cone region of a single-mode optical fiber. Because the cone region's center is asymmetric and its diameter is smaller than the cone waist's diameter, it is a completely enclosed cavity with a relatively thin radial wall thickness. The asymmetric spherical air cavity causes the incident light to deviate from the neutral plane of the asymmetric spherical air cavity and results in an asymmetric radial wall thickness at the cone waist, with one side's wall thickness being smaller than the other. This causes strain concentration on the thin-walled side when bending. The incident light beam deviates from the center of the sphere before entering the cavity and is reflected by the inner wall surface. When the optical fiber bends, the position of the reflection point shifts with the bending direction. This shift modulates the optical path trajectory and optical path difference within the cavity, causing the interference spectrum to redshift or blueshift with different bending directions, enabling detection of curvature and its direction. Specifically, under opposite bending directions, the wavelength of the interference spectrum peak undergoes redshift and blueshift, respectively, to achieve differentiated detection of positive and negative curvatures.

[0013] The deviation of the incident light beam from the center of the sphere and the asymmetric distribution of the radial wall thickness of the cavity jointly cause the position of the reflection point in the cavity to change in different bending directions. The displacement of the reflection point further modulates the optical path difference in the cavity, thereby significantly enhancing the sensitive response of the sensor to the bending direction.

[0014] The temperature sensitivity of the FP type vector bending sensor is:

[0015]

[0016] Where Δλ is the resonant wavelength shift, ΔT is the temperature change, α is the thermal expansion coefficient of the material, κ is the thermo-optic coefficient, and λ is the resonant wavelength.

[0017] The present invention's ability to discern bending direction stems from the asymmetric displacement of incident light within the asymmetric spherical air cavity during the bending process, causing the optical path to monotonically increase or decrease. This bending-direction-dependent displacement of the reflection point effectively modulates the optical path difference, causing the interference spectrum to redshift or blueshift, thereby inverting the bending direction. The present invention concentrates bending stress in the cone region and creates an asymmetric stress distribution by constructing radial wall thickness differences, wherein the thin-wall side enhances the local strain response and the thick-wall side maintains structural stability. Under appropriate cone region parameters, the optical path asymmetry caused by the incident light's deviation toward the thick-wall side is coupled with the geometric asymmetry of the radial wall thickness to optimize directional curvature sensitivity.

[0018] The sensor has a compact and fully enclosed structure and is insensitive to changes in ambient refractive index and temperature, making it suitable for high-precision curvature and direction measurement in complex environments including biological fluids or seawater.

[0019] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0020] The FP-type vector bending sensor described in the present invention embeds a central asymmetric spherical air cavity in a tapered single-mode optical fiber through chemical etching and offset taper welding processes. Direction-sensitive optical path difference modulation is achieved through the different cone waist wall thicknesses and offset incidence brought about by the asymmetric spherical air cavity. Compared with the existing technology, this solution avoids heterogeneous fiber splicing and open cavity structure through an all-fiber integrated molding process, overcoming the problems of poor environmental stability, high temperature cross-sensitivity and complex manufacturing process of traditional sensors. By detecting the redshift / blueshift characteristics of the spectrum, the bending direction can be accurately mapped. At the same time, the sealing structure of the asymmetric spherical air cavity and the quartz-air material properties make it naturally resistant to refractive index changes and temperature fluctuations in the liquid environment. The present invention does not require complex packaging or special optical fibers, and can achieve high-sensitivity directional curvature detection only through single-mode optical fiber and welding, providing an innovative solution for miniaturized and interference-resistant dynamic vector monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a structural schematic diagram of the FP type vector bending sensor of the present invention.

[0022] Figure 2 Schematic diagram of the optical path changes of different structures under bending deformation analyzed through finite element simulation.

[0023] Figure 3 The influence of offset X and wall thickness difference ΔT on the directional sensitivity of the structure is demonstrated through finite element simulation analysis.

[0024] Figure 4 is the combined effect of the offset distance of the incident point (X), the wall thickness difference (ΔT), and the thin wall thickness on the directional curvature sensitivity (Sc).

[0025] Figure 5 Schematic diagram of the preparation of the present invention.

[0026] Figure 6 This is a diagram of the curvature detection experimental device of the present invention.

[0027] Figure 7 This is a fitting diagram of the maximum wavelength drift and a sensitivity radar diagram of the FP-type vector bending sensor of the present invention in different bending directions.

[0028] Figure 8 This is a linear fitting diagram of the FP type vector bending sensor of the present invention as it changes with temperature.

[0029] Figure 9 is the comparison of beam propagation under center incidence and offset incidence under different bending directions, where Figure 9 (a) to (c) show the beam propagation results based on the beam propagation method (BPM) at the center incidence at 90°, 0°, and 270° bending directions, respectively; Figure 9(d) to (f) correspond to the results under offset incidence, where the lateral displacement X introduces asymmetry in the light field distribution and reflection path. DETAILED DESCRIPTION

[0030] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention clearer and more understandable, the present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0031] The structural principle of the present invention is as follows Figure 1 As shown in Figure 1, its core is an asymmetric spherical air cavity Fabry-Perot interferometer integrated into the cone region. The asymmetric spherical air cavity is geometrically offset from the center of the cone region, resulting in a difference in radial wall thickness (upper wall thickness T1, lower wall thickness T2). After incident light (I0) is input from the left end of the optical fiber at an offset distance X from the center of the asymmetric spherical air cavity, it is partially reflected sequentially by the front and rear reflection surfaces (M1, M2) of the asymmetric spherical air cavity, generating two beams of reflected light (I1, I2). Due to the low reflectivity of the quartz / air interface (≈4%), higher-order reflections are negligible, and only I1 and I2 form a dual-beam interference spectrum. When an optical fiber is bent, the incident light path is asymmetrical due to the offset of the cavity axis, resulting in a displacement of the reflection point on the inner wall of the asymmetric spherical air cavity. During positive bending (toward 90°), the light is deflected toward the thicker wall (T1), and the reflection point shifts backward, increasing the optical path difference (OPD) and causing a red shift in the spectrum. During negative bending (toward 270°), the light is deflected toward the thinner wall (T2), and the reflection point shifts forward, reducing the OPD and causing a blue shift in the spectrum. By designing the incident point offset and wall thickness differences, the present invention further amplifies the OPD change, achieving enhanced sensitivity in directional curvature sensing.

[0032] Figure 2 The finite element simulation analysis of the optical path changes of different structures under bending deformation is shown, and bending in the +90° direction is defined as positive. Figure 2 As shown in (a), for a completely symmetrical spherical air cavity, the optical path is always shortened when bending in the opposite direction. Figure 2 (b) depicts the optical path change of a structure with offset incident light and asymmetric wall thickness. Specifically, when the curvature increases from 0 to 6m -1 When the curvature decreases from 0 to -6m, the optical path becomes longer, corresponding to the red shift of the resonance wavelength. -1 When , the optical path is shortened, resulting in a blue shift of the resonance wavelength.

[0033] Figure 3 The directional sensitivity of different structures is analyzed by finite element simulation. By fitting the wavelength drift of four different structures with the curvature, we get Figure 3The results are shown in (a). Among them, the black curve corresponds to a completely symmetrical structure, the red curve corresponds to a structure with only offset incidence, the blue curve corresponds to a structure with only asymmetric wall thickness, and the green curve corresponds to a structure with both offset incidence and asymmetric wall thickness. Obviously, the structure combining offset incidence and asymmetric wall thickness shows the largest wavelength drift amplitude, and the drift caused by wall thickness asymmetry exceeds the drift caused by offset incidence alone. Therefore, combining offset incidence with wall thickness asymmetry can significantly improve the sensitivity of completely symmetrical structures to mechanical strain. In order to further explore the influence mechanism of these two factors on structural sensitivity, a single variable control method is used for in-depth analysis. As Figure 3 As shown in (b) and (c), the waist diameter is fixed at 60 μm. Obviously, the sensitivity and redshift range of the spherical air cavity are positively correlated with these two factors.

[0034] Figure 4 Figures (a) and (b) show how the incident point offset and wall thickness difference affect the sensor sensitivity in the +90° and +270° directions, respectively. When X>0, these two parameters have a constructive sensitizing effect and gradually tend to saturate as the offset increases (for example, ΔT=8μm, 4μm<X<6μm). On the contrary, when X<0, their effects cancel each other out, resulting in a destructive effect. It can also be observed that for small absolute values ​​of X, changing its sign results in sensitivity changes of equal magnitude but opposite direction, which is consistent with theoretical predictions. This phenomenon occurs because the deformation effect is minimal near the neutral plane of the asymmetric spherical air cavity. However, for larger offsets, the cavity deformation becomes significant, enhancing the sensitivity when irradiating thicker walls and reducing the sensitivity when irradiating thinner walls. Therefore, the sensor sensitivity can be optimized by simultaneously increasing the wall thickness difference and moving the incident point toward the thicker wall side (+90° direction). In addition, Figure 4 (c) shows the dependence of sensitivity on wall thickness. Taking the bending direction of +270° as an example, decreasing the wall thickness increases sensitivity, indicating that wall asymmetry is another key factor in improving sensitivity.

[0035] See also Figure 5 , the preparation process of the present invention comprises the following steps:

[0036] 1) Figure 5 (a) arcing the end faces of the first single-mode optical fiber SMF1 and the second single-mode optical fiber SMF2 by pre-discharging in a fusion splicer;

[0037] 2) Figure 5 (b) immersing the end face of the first single-mode optical fiber SMF1 treated in step 1) in a 40% hydrofluoric acid solution for etching, and forming a concave end face structure by utilizing the difference in etching rates of the hydrofluoric acid solution on the core and cladding materials;

[0038] 3) Figure 5 In (c), the first single-mode optical fiber SMF1 and the second single-mode optical fiber SMF2 processed in step 2) are placed in a fusion splicer and offset tapered fusion is performed to melt the two end faces of the first single-mode optical fiber and the second single-mode optical fiber to form a cone region, and the concave surface structure is directionally expanded into a completely closed asymmetric spherical air cavity, finally obtaining an FP-type vector bending sensor.

[0039] The curvature detection experimental device of the present invention is shown in FIG. Figure 6 As shown in the figure, which contains a micrograph of the prepared embodiment. It can be seen that the asymmetric spherical air cavity has a length of 49.7μm and a height of 44.8μm. The waist diameter of the single-mode optical fiber is 51.7μm. The difference in thickness between the upper and lower walls of the asymmetric spherical air cavity in the cone region is about 6.9μm. In the experiment, a 3dB ring coupler was used to guide the light signal of the amplified spontaneous emission light source (ASE, 1525-1610nm) to the sensor. The reflected light was then coupled into an optical spectrum analyzer (OSA) for monitoring and recording the interference spectrum. On the optical table, the sensor was mounted on a rotating fixture on a pair of six-dimensional micro-displacement platforms (FT2200B-L, FT2200B-R), with a separation distance of w (0.2m). By adjusting the feed displacement z, the sensor produces bending deformation.

[0040] Figure 7 The maximum wavelength drift fitting diagram and sensitivity polar coordinate diagram of the present invention in different bending directions are studied in the embodiment of 0~4.35m -1 And the curvature characteristics in the bending direction of 0~360° (step size is 45°). Figure 7 (a) shows the maximum wavelength displacement of the sensor in different bending directions. It can be seen that the maximum wavelength displacement is approximately sinusoidal with the bending direction. The sensitivity of the sensor in different bending directions is as follows: Figure 7 The polar coordinates of (b) show that the curvature response has a clear angle dependence, and a clear figure-8 pattern can be observed. Therefore, the angle-dependent bending sensitivity of the sensor enables it to monitor directional curvature.

[0041] Figure 8 This is a linear fitting diagram of the present invention that changes with temperature. In practical applications, temperature and strain are both key factors affecting sensor accuracy. The temperature sensitivity of the sensor can be determined by the following formula:

[0042]

[0043] Where L is the optical path, ΔL is the optical path difference, λ is the resonance wavelength, Δn is the change in the refractive index of air, Δλ is the resonance wavelength drift, ΔT is the temperature change, α is the thermal expansion coefficient of the material, and κ is the thermo-optical coefficient.

[0044] It can be seen that the temperature sensitivity of the sensor is mainly determined by the thermal expansion coefficient, thermo-optic coefficient and wavelength of the interference peak of the material. The main materials of the sensor are silicon dioxide and air, which have low thermal expansion coefficient and thermo-optic coefficient, making it not easy to deform under temperature changes. Therefore, the sensor of the present invention has good resistance to temperature interference. The sensor is firmly fixed on the constant temperature platform using high-temperature resistant tape, and the two ends of the sensor are connected to the light source and the spectrometer to continuously monitor its response to temperature changes. The temperature of the constant temperature platform is gradually increased from 40°C to 160°C in steps of 20°C. The temperature is linearly fitted by calculating the average wavelength drift of the resonance dip near 1583nm. The temperature sensitivity is 1.09pm / °C and the linear fitting coefficient is 0.993.

[0045] Figure 9 Figures (a), (b), and (c) show BPM simulations for axially incident light, where the observed optical path length L1 > L2 = L3. This indicates that the optical path length is already at its maximum when the structure is unbent, resulting in a blue shift in the resonance wavelength regardless of the bending direction. Figure 9 Figures (d), (e), and (f) correspond to the case of off-axis incident light. In the unbent case, the reflection point is located at a1, with an optical path length of L5. When bent to +90°, the second reflection point moves from a1 to b1, increasing the optical path length and causing a red shift in the resonant wavelength. When bent to +270°, the second reflection point moves from a1 to c1, decreasing the optical path length and causing a blue shift in the resonant wavelength. Therefore, in theory, the reflection point vector movement mechanism caused by off-axis incidence can be used to implement vector curvature sensing in the sensor of the present invention without the need for structural bending deformation.

[0046] Table 1 provides a comprehensive comparison of the present invention and other sensors, including sensing length, curvature sensitivity, measurement range, temperature cross-sensitivity, and whether each device is a fully sealed, all-fiber structure. The present invention's sensor stands out for its ultra-compact size, nearly uniform bidirectional sensitivity, wide operating range, negligible thermal crosstalk, and fully sealed, all-fiber design—all of which ensure high-precision performance even in harsh liquid environments.

[0047] Table 1

[0048]

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[0050] [2]C.Jiang,Y.Liu,andC.Mou,"Polarization-MaintainingFiberLong-PeriodGratingBasedVectorCurvatureSensor,"IEEEPhoton.Technol.Lett.33(7),358–361(2021).

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[0053] [5]J.Kong,A.Zhou,andL.Yuan,"Temperatureinsensitiveone-dimensionalbendingvectorsensorbasedoneccentric-corefiberandaircavityFabry-Perotinterferometer,"J.Opt.19(4),045705(2017).

[0054] [6] R. Oliveira, M. Cardoso, and A. M. Rocha, "Two-dimensional vector bending sensor based on Fabry-Pérotcavities in multicore fiber," Opt. Express, OE30 (2), 2230–2246 (2022).

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[0056] In this invention, the asymmetric spherical air cavity is completely sealed, with no direct contact with the external environment. Therefore, the sensor can operate stably in solutions with varying refractive indices. The directional sensitivity of the sensor stems from the asymmetric displacement of the reflection point within the spherical cavity caused by the asymmetry of the incident light and the asymmetry of the radial wall thickness of the tapered waist during bending. This displacement effectively modulates the optical path difference, resulting in different spectral shifts (redshifts or blueshifts) depending on the bending direction.

[0057] The asymmetry of incident light caused by the asymmetric spherical air cavity and the asymmetry of the radial wall thickness of the tapered waist both affect the sensor's directional curvature sensing performance. When only one of these two factors exists, amplifying the asymmetry can enhance the sensor's sensitivity. When both factors exist simultaneously, increasing the asymmetry of the wall thickness while also introducing incident light offset toward the thicker side can further enhance the sensor's sensitivity. However, the sensitivity improvement achieved by the combined effect of these two factors is not linear, but rather saturates as the asymmetry increases. Therefore, by precisely designing these two parameters, higher sensitivity can be achieved.

[0058] The asymmetric distribution of the asymmetric spherical air cavity is coupled with the wall thickness gradient of the cone region, causing the path of the incident light to deviate and the reflection point to shift under the action of bending stress. The differentiated response of the interference spectrum is achieved through directional modulation of the optical path difference: positive bending corresponds to a red-shifted spectrum, and reverse bending corresponds to a blue-shifted spectrum, thereby accurately characterizing the spatial direction of the curvature vector.

Claims

1. A method for preparing an FP type vector bending sensor, characterized in that: The following steps are involved: 1) arcing the end faces of the first single-mode optical fiber and the second single-mode optical fiber respectively by pre-discharging in a fusion splicer; 2) immersing the end face of the first single-mode optical fiber treated in step 1) in a hydrofluoric acid solution for etching, and forming a concave end face structure by utilizing the difference in etching rates of the hydrofluoric acid solution on the core and cladding materials; 3) placing the first single-mode optical fiber and the second single-mode optical fiber processed in step 2) in a fusion splicer, fusing the two end faces to form a cone region, and directionally expanding the concave structure into a completely closed asymmetric spherical air cavity.

2. The method for preparing an FP type vector bending sensor according to claim 1, wherein: In step 3), offset taper welding is used to expand the concave structure to generate an asymmetric spherical air cavity that deviates from the central axis of the cone area.

3. An FP type vector bending sensor, characterized in that: Prepared by the preparation method according to any one of claims 1 to 2.

4. The FP type vector bending sensor according to claim 3, characterized in that: The radial wall thickness of the asymmetric spherical air cavity is asymmetric, with one side having a smaller wall thickness than the other side, so that strain concentration occurs on the thin-walled side when bending. An incident light beam deviates from the center of the sphere and enters the air cavity, and is reflected on the inner wall surface of the cavity. When the optical fiber bends, the position of the reflection point shifts along the bending direction. This displacement modulates the optical path trajectory and optical path difference within the cavity, causing the interference spectrum to redshift or blueshift in response to different bending directions, thereby enabling the detection of curvature and its direction.

5. The FP type vector bending sensor according to claim 4, characterized in that: The deviation of the incident light beam from the center of the sphere and the asymmetric distribution of the radial wall thickness of the cavity jointly cause the position of the reflection point in the cavity to change in different bending directions. The displacement of the reflection point further modulates the optical path difference in the cavity, thereby significantly enhancing the sensitive response of the sensor to the bending direction.

6. The FP type vector bending sensor according to claim 4, characterized in that: In opposite bending directions, the wavelength of the interference spectrum peak appears red-shifted and blue-shifted respectively, so as to achieve differentiated detection of positive and negative curvatures.

7. The FP type vector bending sensor according to claim 3, characterized in that: The temperature sensitivity of the sensor is: Where Δλ is the resonant wavelength shift, ΔT is the temperature change, α is the thermal expansion coefficient of the material, κ is the thermo-optic coefficient, and λ is the resonant wavelength.

8. Use of the FP type vector bending sensor according to any one of claims 3 to 7, characterized in that: Used for curvature and orientation measurements in complex environments including biological fluids or seawater.

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