Optical fiber sensor for shape sensing, optical shape sensing device, system and method

Abstract

To provide an optical fiber sensor which can measure a shape with a small curved radius, without reducing a fiber core distance from a center axis and / or increasing a scan wavelength range.SOLUTION: An optical fiber sensor for shape sensing comprises: an optical fiber in which at least four fiber cores are embedded, the fiber cores being arranged at an interval from a longitudinal center axis of the optical fiber; a plurality of fiber cores which includes a first sub set of at least two fiber cores, and a second sub set of at least two fiber cores, where the fiber cores of the second sub set are configured to provide redundancy in shape sensing measurement of the fiber sensor. The fiber cores of the first sub set are distributed in an azimuth direction around a center axis mutually, and respective fiber cores of the second sub set are arranged at a non-equal distance in the azimuth direction around the center axis, with respect to the two adjacent fiber cores of the first sub set.SELECTED DRAWING: Figure 5A

Description

Technical Field

[0001] The present invention generally relates to the field of optical shape sensing. In particular, the present invention relates to an optical fiber sensor for use in optical shape sensing having a plurality of fiber cores disposed at a radial distance from the central axis of the fiber. The present invention relates to an optical shape sensing device having such an optical fiber sensor and an optical shape sensing system having such an optical fiber sensor. Further, the present invention relates to an optical shape sensing method using the optical fiber sensor.

Background Art

[0002] Optical shape sensing (OSS) is a technique in which the three-dimensional shape of a special optical fiber can be reconstructed from the reflection of light in the fiber. This technique enables, for example, real-time 3D visualization of the complete shape of medical devices such as catheters and guidewires. The shape of the medical device can be overlaid on an X-ray image or a preoperative CT scan. In this way, a physician can navigate the device during the procedure without the need for X-ray tracking.

[0003] In optical shape sensing, an optical fiber sensor, also referred to as an optical shape sensing fiber, is interrogated with light coupled into the fiber core of the fiber, and the dispersion strain and temperature signals are obtained from the backscattered spectrum obtained by an interrogator unit incorporating an interferometer. A standard optical fiber sensor has three outer fiber cores (in this specification, fiber cores disposed spaced apart from the central axis of the fiber are also shown as outer fiber cores) spirally wound around the periphery of a fourth core disposed at the radial center of the fiber. The response of the fiber core to strain and temperature is measured as the phase difference of the optical signal from the interferometer as a function of the delay position along the fiber sensor. The phase difference is obtained with respect to a reference measurement where the fiber sensor has a well-defined shape, for example, a perfectly straight shape. From the phase difference of the fiber core, the strain and temperature differences can be estimated for each fiber core. The strain signal is the sum of the bending strains in two orthogonal directions, as well as torsional strain and axial strain, the latter being the strain in the longitudinal direction of the optical fiber sensor. From these four position-dependent quantities, the shape of the fiber sensor can be reconstructed. For high-precision shape sensing, accurate fiber sensor characteristics are required in the shape reconstruction model. These characteristics can be determined for each individual optical fiber in a calibration process.

[0004] A further extension of the shape sensing technique is to enable the distinction of the influence of temperature from that of axial strain. To do so, for example, at least one additional core having a different temperature sensitivity is required, as described in WO 2016 / 099976 pamphlet.

[0005] As described above, the shape of the optical sensing fiber is calculated from position-dependent strain signals measured for some, typically four, cores inside the fiber. For example, bending the fiber in a plane defined by the fiber core and the fiber center causes strain in the fiber core if the core is not centered within the fiber. In this case, the strain ε is the quotient of the distance a of the core from the central axis of the fiber and the radius r of the bend of the core. The bending strain is here measured relative to the straight and unstrained state of the fiber. The magnitude of the strain can be inferred from the amount of spectral shift of the reflected light. If the fiber core contains a fiber Bragg grating (FBG), due to the periodic nature of the Bragg grating, the sensor reflects light at one specific wavelength, called the resonant wavelength. If the fiber core is elongated (positively strained) relative to a reference measurement value, the periodicity of the FBG increases, resulting in an increase in the resonant wavelength. On the other hand, in the case of compressive (negative) strain, the periodicity of the FBG decreases, resulting in a decrease in the resonant wavelength. The smaller the radius of curvature of the bend, the larger the shift δλ of the resonant wavelength (in either the positive or negative direction, depending on the position of the fiber core in the bend). δλ=λ0ζε=(λ0ζα / r)sin(g twist (z)+φ) (1) Here, λ0 is the resonant wavelength of the fiber core, or more precisely, of the FBG in the unstrained state, and ζ is the strain optical coefficient (≒0.8) that takes into account the strain-induced change in the refractive index and affects the relationship between the Bragg period and the wavelength. The sine function describes the changing position of the outer core when it is helically twisted around the fiber center. g twist is the cumulative twist angle of the core, which is the sum of the twist that is inherently present in the spun fiber and the externally applied twist. φ is the offset angle related to the orientation of the bending plane and the angle of the fiber core at the reference position. For reasons of clarity, in equation (1), only the strain due to bending is assumed.

[0006] When an optical fiber sensor is inserted, for example, into the lumen of a medical device, the optical fiber sensor experiences a changing radius of curvature. The medical device may be preformed and its form may change during the handling of the device. The minimum radius of curvature encountered by the optical fiber sensor depends on the design of the device, the optical fiber itself, and the environment in which it is used. For example, the human vascular system can be very tortuous, for example. More flexible devices are used so that these types of blood vessels can be accessed. The optical fiber sensor within such a medical device should be able to withstand a small radius of curvature. However, there are limits related to the minimum measurable bending radius of the optical fiber sensor.

[0007] In shape sensing, typically, the spectrum is recorded for each fiber core by scanning a light source over a fixed wavelength range Δλ centered on the resonance wavelength of the strain-free FBG. The minimum bending radius that still has a resonance inside the measured spectrum is r min =(2λ0ξa) / Δλ (2) is.

[0008] In a scan range centered on λ0 = 1545 nm, ξ = 0.8, and a = 35 μm, the minimum measurable bending radius is 5.1 mm. When the optical fiber sensor is bent to reduce the curvature, the signal is not measured for the fiber core in the bending plane.

[0009] From equation (2), it can be seen that the minimum measurable bending radius can be decreased by decreasing the fiber core distance a and / or by increasing the scan wavelength range Δλ. Decreasing the outer fiber core distance a increases the sensitivity to torsional strain by a 2Since it scales, it has the drawback of also reducing the sensitivity to bending strain and the sensitivity to torsional strain. The required accuracy regarding torsion is high, and thus, reducing the outer fiber core distance from the central axis of the fiber is not preferable. Increasing the scan range Δλ is disadvantageous for other reasons. This reduces the signal-to-noise ratio since the resonance peaks are relatively few and fill the spectrum. Furthermore, the delay length between two consecutive nodes (data points as a function of the position on the fiber) is reduced, giving an increase in the data points for the same physical length of the fiber.

[0010] WO2018 / 075911A1 proposes to provide an optical fiber sensor having three or more outer fiber cores, where the fiber cores are arranged at a plurality of different radial distances from the central axis of the fiber. To measure a small bending radius, it is necessary to switch to a fiber core at a smaller distance, which can result in a reduction in the accuracy of the shape sensing measurement. The design of such an optical fiber sensor thus suffers a loss of accuracy.

[0011] US2007 / 0297712A1 discloses an optical fiber sensor for detecting the curvature of a body, the sensor having a cladding with an outer periphery. The central core has a Bragg grating and is located at the neutral plane of the cladding. The peripheral core transmits and receives light.

[0012] US2016 / 0047976A1 discloses an optical fiber sensor having an optical waveguide having at least one first core and a cladding surrounding the first core, the first core extending over substantially the entire length of the optical waveguide.

[0013] US2006 / 0024008A1 discloses a composite waveguide having a central core and at least one side core helically wound around the central core in optical proximity thereto.

[0014] US2016 / 0238783A1 discloses an optical fiber having a core group composed of a plurality of cores extending along a fiber axis, a common cladding including the core group, and a coating covering an outer periphery of the common cladding.

[0015] US2017 / 0123146A1 discloses a multi-core optical fiber having cores randomly arranged within a cladding matrix.

[0016] WO2018 / 009342A1 discloses a fiber including M primary cores and N redundant cores, where M is an integer greater than 2 and N is an integer greater than 1. An interferometer circuit detects interference pattern data associated with the M primary cores and the N redundant cores when the optical fiber is disposed within a sensing position.

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0017] An object of the present invention is to provide an optical fiber sensor that enables shape measurement with a small bending radius without reducing the fiber core distance from a central axis and / or without increasing a scan wavelength range.

[0018] A further object of the present invention is to provide an apparatus having an improved optical fiber sensor.

[0019] A further object of the present invention is to provide an optical shape sensing system that enables improved shape sensing measurement.

[0020] A further object of the present invention is to provide an optical shape sensing method that enables improved shape sensing measurement.

MEANS FOR SOLVING THE PROBLEM

[0021] According to a first aspect of the present invention, there is provided an optical fiber sensor for shape sensing, the optical fiber sensor having an optical fiber with a plurality of at least four fiber cores embedded therein and spaced apart from a longitudinal central axis of the optical fiber, the plurality of fiber cores including a first subset of at least two fiber cores and a second subset of at least two fiber cores, the fiber cores of the second subset being configured to provide redundancy in shape sensing measurements of the fiber sensor, the fiber cores of the first subset being distributed in an azimuthal direction around the central axis relative to each other, and each fiber core of the second subset being arranged non-equidistantly in the azimuthal direction around the central axis with respect to two adjacent fiber cores of the first subset.

[0022] Optionally, a fiber core of the second subset arranged between two adjacent fiber cores of the first subset has an angular position closer to one of the two adjacent fiber cores of the first subset than to the other of the two adjacent fiber cores of the first subset, the fiber cores of the first subset being arranged equidistantly around the central axis relative to each other, and the fiber cores of the second subset being arranged in an equidistant direction around the central axis relative to each other.

[0023] The optical fiber sensor according to the present invention enables measurement of a smaller bending radius than a standard fiber sensor having only three outer cores by providing redundancy in a plurality of fiber cores. The present invention is based on the insight that it is not necessary to provide outer fiber cores at different distances from the central axis in order to reduce the minimum bending radius measurable with a fiber sensor. In the optical fiber sensor according to the present invention, the outer fiber cores of the first subset and at least one fiber core of the second subset may be arranged at the same radial distance from the central axis of the fiber, which is preferred but not essential. In the fiber sensor according to the present invention, at least one, preferably each, fiber core of the second subset is arranged at an unequal distance in the azimuthal direction around the central axis with respect to two adjacent fiber cores of the first subset. This means that an outer fiber core of the second subset arranged between two adjacent outer fiber cores of the first subset has an angular position closer to one of the adjacent outer fiber cores of the first subset than to the other of the two adjacent outer fiber cores of the first subset. This results in a specific asymmetry between the outer fiber cores of the first subset and the fiber cores of the second subset with respect to the angular position around the central axis. As will be described in more detail herein, such an arrangement of the outer fiber cores is suitable for reducing the minimum measurable bending radius without increasing the scan wavelength range and / or without reducing the fiber core distance from the central axis.

[0024] Asymmetry may exist between the angular arrangements of the fiber cores of the first subset and the fiber cores of the second subset, but the overall arrangement of all the fiber cores may be symmetric. For example, one outer fiber core of the first subset and one adjacent outer fiber core of the second subset can be considered to form a pair of outer fiber cores, and pairs existing within the fiber, where the difference in the angular positions of the fiber cores in each pair is the same and the pairs have equal differences in angular position around the central axis relative to each other, may form a symmetric arrangement of fiber core pairs. However, it is also possible to vary the difference in angular position between the two fiber cores in a pair and / or vary the difference in angular position between the pairs such that there is no symmetry in the overall arrangement of the fiber cores of the first and second subsets.

[0025] An example of an overall symmetric arrangement may be an arrangement where the first and second subsets each include three outer fiber cores, the outer fiber cores of the first subset may be arranged at 0°, 120°, and 240°, and the outer fiber cores of the second subset may be arranged at 30°, 150°, and 270° around the central axis.

[0026] The angle between the angular position of one fiber core of the second subset and the angular position of one of the two adjacent fiber cores of the first subset in the azimuthal direction around the central axis may be at least 10%, or at least 20%, or at least 40% less than half the angle between the angular positions of the two adjacent fiber cores of the first subset. In an embodiment of a fiber sensor having three outer cores in the first subset and three outer cores in the second subset, the angle may be in the range of 20° to 40°, for example, about 30°.

[0027] The fiber cores of the first and second subsets of fiber cores may be spirally wound around the central axis of the fiber sensor. The central fiber core may be arranged on the central axis and extend along the same axis.

[0028] The fiber cores of the first subset and the fiber cores of the second subset may each have one or more fiber Bragg gratings along the length of the respective fiber core.

[0029] The second subset of fiber cores may include three or more fiber cores.

[0030] In another embodiment that can be combined with any one of the foregoing embodiments, the optical properties of the fiber cores of the second subset are different from the optical properties of the fiber cores of the first subset.

[0031] The optical property may, in this regard, be the resonance wavelength of the fiber core in a non-distorted state. In one embodiment, the first resonance wavelength of the fiber cores of the first subset in response to light introduced into the fiber core in a non-distorted state and the second resonance wavelength of at least one fiber core of the second subset in a non-distorted state may be different from each other. This measurement is also suitable for reducing the minimum measurable bending radius of the optical fiber.

[0032] Another measure for reducing the minimum measurable bending radius of the optical fiber that can be combined with any of the above embodiments is to offset the first resonance wavelength of the fiber cores of the first subset and / or the second resonance wavelength of at least one fiber core of the second subset with respect to the central wavelength of the scan wavelength range of the light used to interrogate the fiber core. In this embodiment, the first and second resonance wavelengths may be equal to each other or different from each other.

[0033] According to a second aspect of the present invention, there is provided an optical shape sensing device having an optical fiber sensor according to the first aspect and its embodiments.

[0034] The optical shape sensing device may be a medical device, particularly a catheter or a guide wire.

[0035] According to a third aspect of the present invention, there is provided an optical shape sensing system having an optical fiber sensor according to the first aspect, an optical interrogation unit configured to interrogate at least one fiber core of a first subset of fiber cores and a second subset of fiber cores of the optical fiber sensor using light in a scan wavelength range, and to measure a reflection spectrum received from at least one fiber core of the first subset of fiber cores and the second subset of fiber cores of the optical fiber sensor, and an evaluation unit configured to reconstruct the shape of the fiber sensor using the reflection spectrum.

[0036] The optical shape sensing system according to the present invention has the same or similar advantages as those described with respect to the optical fiber sensor according to the present invention. In particular, the scan wavelength range may be the same as in the case of a standard fiber sensor having only three outer cores, and nevertheless, a smaller bending radius than in the case of a standard fiber sensor may be measured in this scan wavelength range.

[0037] In one embodiment, the optical interrogation unit may be configured to set the scan wavelength range such that the center wavelength of the scan wavelength range is offset with respect to the first resonance wavelength of the fiber cores of the first subset, the resonance wavelength responding to light introduced into the fiber cores in a state without strain of the fiber cores, and / or the optical interrogation unit may be configured to set the scan wavelength range such that the center wavelength of the scan wavelength range is offset with respect to the second resonance wavelength of at least one fiber core of the second subset, the second resonance wavelength responding to light introduced into at least one fiber core in a state without strain of at least one fiber core. In these embodiments, the scan wavelength range is asymmetric with respect to the resonance wavelength of the outer fiber cores of the optical fiber, which is also suitable for measuring a smaller bending radius of the optical fiber sensor than what is possible with conventional systems.

[0038] According to a further aspect, there is provided an optical shape sensing method, the method comprising providing an optical fiber sensor according to the first aspect; optically interrogating at least one fiber core of the first subset of fiber cores and at least one fiber core of the second subset of fiber cores; measuring the reflection spectrum of the light returning from at least one fiber core of the first subset of fiber cores and the fiber cores of the second subset of fiber cores; reconstructing the shape of the optical fiber sensor based on the reflection spectrum; and having.

[0039] The optical shape sensing method according to the present invention has the same or similar advantages as those described above.

[0040] It should be understood that all of the above-described embodiments can be combined with each other to provide an optical fiber sensor, an optical shape sensing device, an optical shape sensing system, and an optical shape sensing method, all of which make it possible to measure the bending radius of an optical fiber sensor as small as possible.

[0041] These and other aspects of the invention will become apparent from and be elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0042]

Figure 1

Figure 2

Figure 3A

Figure 3B

Figure 4A

Figure 4B

Figure 5A

Figure 5B

Figure 6A

Figure 6B

Figure 7A

Figure 7B

Figure 8A

Figure 8B

Figure 9A

Figure 9B

Figure 10

DETAILED DESCRIPTION OF THE INVENTION

[0043] FIG. 1 schematically shows a part of an optical fiber sensor system 10 configured as a multi-channel optical frequency domain reflectometry (OFDR)-based and distributed strain sensing system for sensing an optical fiber sensor 12. The optical fiber sensor 12 has an optical fiber with a plurality of fiber cores 14, 16, 18, 20 embedded therein. In this example, it has four cores including one central core 16 and three outer cores 14, 18, 20. The optical fiber sensor shown in FIG. 1 is a standard fiber sensor. Here, it should be noted that the present invention proposes an optical fiber sensor design having more than three outer cores. FIG. 2 shows a section of the lengths of the fiber cores 14, 16, 18, 20 having outer cores 14, 18, 20 spiraled around the central core 16. The central core 16 is disposed on the central axis of the optical fiber sensor 12. The outer fiber cores 14, 18, 20 are angularly spaced from each other in the azimuthal direction around the longitudinal central axis of the optical fiber sensor 12. The longitudinal central axis coincides with the central core 16. According to the plurality of four cores in this example, the angular interval between adjacent outer cores may be 120°.

[0044] Referring again to FIG. 1, the optical shape sensing system 10 has an interrogator unit 21. The interrogator unit 21 may have an adjustable light source 22 that can be swept over a range of optical frequencies, also referred to as a scan wavelength range. The light emitted by the light source 22 is coupled into an optical interference network 24 having optical channels 24a, 24b, 24c, 24d according to the plurality of fiber cores 14, 16, 18, 20 of the optical fiber sensor 12. If the optical fiber sensor 12 has more than four cores, the optical interference network 24 may have a corresponding number of optical channels.

[0045] When the adjustable light source 22 is swept over a range of optical frequencies, each channel 24a, 24b, 24c, 24d, and thus each fiber core 14, 16, 18, 20 of the optical fiber sensor 12, is optically interrogated simultaneously and independently. The interference signals based on the reflection spectra returning from each of the fiber cores 14, 16, 18, 20 are sent to the processing unit or data acquisition unit 26 via their respective photodetectors 25. Then, the distributed strain measurements from the cores 14, 16, 18, 20 using the multi-channel OFDR system can be exported to the evaluation unit 27 for further processing, particularly for three-dimensional shape reconstruction of the optical fiber sensor 12 and for visual display of the reconstructed three-dimensional optical fiber sensor 12.

[0046] In one embodiment of the optical fiber sensor 12, the fiber cores 14, 16, 18, 20 may have fiber Bragg gratings (FBGs) formed by periodic variations in the refractive index. For simplicity, an FBG having a single resonant wavelength is considered here. The FBG reflects light of a specific wavelength (resonant wavelength) that depends on the grating period of the FBG and transmits all other wavelengths. Due to the bending of the optical fiber sensor 12, the grating period is affected by strain, and measurement of the reflected wavelength at any position along the fiber makes it possible to determine the local strain. The optical fiber sensor 12' according to the embodiment of the present invention described below may have such an FBG.

[0047] The optical interrogation of the optical fiber sensor 12 provides, in principle, the information necessary to reconstruct the three-dimensional shape of the entire fiber sensor in real time. Given an appropriate reference frame, it is possible to know the exact orientation and position of the complete optical fiber sensor 12 in real time.

[0048] When an optical fiber sensor, such as the optical fiber sensor 12, is used within a medical device, such as a catheter or a guide wire, for example, the device changes its form during handling of the device. For example, if the device is a catheter for introduction into a human vascular system that may be very tortuous, the device, and thus the optical fiber sensor 12, may experience bending along a length with a radius of curvature that may be very small. However, in optical shape sensing technology, there are limits related to the minimum measurable bending radius of the optical fiber sensor.

[0049] Referring to Equation (2) above, the minimum measurable bending radius of the standard optical fiber sensor 12 is 5.1 mm for a scan range centered on the resonant wavelength λ0 = 1545 nm of the fiber core in the strain-free state, ξ = 0.8, and a = 35 μm (see above for the definition of these parameters). When the standard optical fiber sensor 12 is bent to a curvature with a lower curvature, i.e., a bending radius less than 5.1 mm, no signal is measured for the fiber core in the bending plane.

[0050] Figure 3A shows a cross-section of the standard optical fiber sensor 12, with the three outer cores labeled 1, 2, 3 and the central core labeled 0. Figure 3B shows the simulation results (see Equation (1)) for the optical fiber sensor 12 having a bend with a radius of 5.1 mm. In Figure 3B, λ res , i.e., the resonant wavelength for each of the bent fiber cores 1, 2, 3, and λ c, that is, the differences from the central wavelength of the scan wavelength range are plotted as a function of the position on the optical fiber sensor along the length for three outer cores 1, 2, and 3 that are separated by 120° in the azimuthal direction around the central axis (central core 0). The spectrum of the central core 0 is not depicted in FIG. 3B because there is no shift in the resonance wavelength due to the bending strain of the central core 0. The twist rate of the outer fiber cores 1, 2, and 3 is 50 turns per meter, and one index corresponds to 48.2 μm. The gray shaded area gives the scan wavelength range required to cover the shift in the resonance wavelength of the outer core due to bending. In FIG. 3B, curve 41 shows the simulation result for core 1, curve 42 shows the simulation result for core 2, and curve 43 shows the simulation result for core 3. The minimum scan range required to always cover a bending radius of 5.1 mm (i.e., for all orientations of the optical fiber sensor with respect to bending) is shown by dotted lines 51, 52 in FIG. 3B at λ res -λ c = ±8.4 nm.

[0051] FIG. 4A shows an embodiment of the optical fiber sensor 12' according to the present invention, which has a number of outer fiber cores greater than 3. In other words, the optical fiber sensor 12' provides redundancy in the shape sensing measurement of the fiber 12' in order to be able to measure a smaller bending radius of the fiber 12' compared to the three outer fiber cores of the standard sensor 12, by adding additional outer fiber cores that provide redundancy. In FIG. 4A, the outer fiber cores are labeled with reference numerals 1 to 6. The fiber sensor 12' also includes a central core 0, where 0 also indicates the central axis of the fiber sensor 12'. Thus, there are three outer fiber cores of a first subset of fiber cores, for example, fiber cores 1, 3, 5, and three outer fiber cores of a second subset of fiber cores, for example, fiber cores 2, 4, 6 (note that the assignment of fibers to the first and second subsets is not important).

[0052] As shown in Fig. 4A, the fiber cores 1, 3, 5 of the first subset and the fiber cores 2, 4, 6 of the second subset have the same radial distance from the central axis (central core 0). Further, the fiber cores 1 to 6 are arranged equidistantly around the central axis in the azimuthal direction. Therefore, the angle between two adjacent fiber cores among the fiber cores 1 to 6 is 60°. The fiber cores 1, 3, 5 may have a first single resonance wavelength in a strain-free state of the fiber 12', and the fiber cores 2, 4, 6 may have a second single resonance wavelength in a strain-free state of the fiber sensor 12'. In this example, the first and second resonance wavelengths are equal.

[0053] The fiber cores of the second subset of fiber cores may be wound spirally around the central axis of the sensor 12'.

[0054] In order to be able to distinguish the four position-dependent quantities required for shape reconstruction using the optical fiber sensor 12', these quantities are the bending strain, torsional strain, and axial strain in two orthogonal directions, and at least three signals of the central core 0 and the outer cores 1 to 6 should be known.

[0055] Fig. 4B shows the simulation results of the optical fiber sensor 12' of Fig. 4A. As described with respect to Fig. 3B, the difference between the resonance wavelength λ res and the central wavelength λ c of the scan wavelength range is plotted as a function of the position on the optical fiber sensor 12'. The gray shaded area gives the scan wavelength range required to include at least three resonances of the outer cores 1 to 6. In Fig. 4B, the curves 41 to 46 show the simulation results for the outer fiber cores 1 to 6 (the results for the central core 0 are again omitted in Fig. 4B). As can be seen from the figure of Fig. 4B, the black dotted lines 51 and 52 are no longer the maximum values of the fiber core signals, which means that a smaller bending radius can be measured using the same scan wavelength range (±8.4 nm). In this case, the minimum bending radius r of 4.5 millimeters mincan be measured. Comparing the simulation results of FIGS. 3B and 4B, for example, by providing six outer fiber cores instead of three, the redundancy in the fiber core is revealed, and the minimum measurable bending radius can be reduced without increasing the scan wavelength range and without decreasing the distance of the outer core from the central axis.

[0056] Compared with a standard optical fiber sensor having three outer cores, such as the optical fiber sensor 12 of FIG. 3A, in order to have a measure for the beneficial effect of redundancy in the outer fiber core, the gain coefficient f may be calculated, which is obtained by the redundancy due to the amount n of the core without increasing the scan wavelength range. f = cos((π / (n - 1)floor((n / 2) - 2)), n≧4 (3) Here, n is the total number of fiber cores (including the central core and n - 1 outer fiber cores). When n = 4 (standard optical fiber sensor), f is 1. When n = 7 (six outer cores and one central core), f is approximately 0.87. This means that for a symmetric arrangement of six outer cores (an angle of 60° between two adjacent outer cores), the minimum measurable bending radius can be reduced to 0.87 times, i.e., from 5.1 mm to 4.5 mm, in the same scan wavelength range.

[0057] The gain coefficient f, and thus the minimum measurable bending radius, can be further reduced by one or more of the following measures described in connection with further embodiments.

[0058] In general, the optimization of the gain coefficient f can be performed by changing the fiber core angles relative to each other, and / or by changing the core optical properties, and / or by introducing an asymmetry between the scan wavelength range and the resonant wavelength of the fiber core in the strain - free state. These measures are described below.

[0059] Figure 5A shows an embodiment of an optical fiber sensor 12' having six outer cores 1 to 6. The difference from the embodiment in FIG. 4A is that the fiber cores 1 to 6 in FIG. 5A are not equally distributed in the azimuthal direction around the central core 0 extending along the central axis of the fiber 12'. In the embodiment of FIG. 5A, the angle between some of the adjacent fiber cores, for example, between fiber cores 2 and 3, is smaller than the angle between other adjacent fiber cores, for example, between fiber cores 1 and 2. For example, the smaller angles between fiber cores 2 and 3, 4 and 5, and 6 and 1 may be 30°, while the larger angles between fiber cores 1 and 2, 3 and 4, and 5 and 6 may be 90°. It should be noted that the number of six outer fiber cores as shown in FIG. 5A is exemplary, and any other number of outer fiber cores can be similarly adopted as long as there is redundancy. In the embodiment of FIG. 5A, the first subset of outer cores 1, 3, 5 is arranged at 0°, 120°, and 240°, and the second subset of three cores 2, 4, 6 having the same relative angle is arranged at an angle of θ = 30° with respect to the fiber cores 1, 3, 5 of the first subset. For a 7-core optical fiber sensor (six outer fiber cores and one central core) with any θ, the gain coefficient f is given by the following equation. f = max{cos((|θ| - π / 3) / 2), -sin((|θ| - 2π / 3) / 2)}, -π / 3 < θ ≦ π / 3 (4)

[0060] The minimum gain coefficient f is obtained for θ = 30° (f = 0.71) in the 7-core fiber sensor 12'. FIG. 5B shows a figure similar to FIG. 4B of the simulation results for the six outer cores 1 to 6 in FIG. 5A. The gray shaded area again gives the scan wavelength range required to include the resonances of at least three outer cores, and the dotted lines 51, 52 show the minimum scan wavelength range required to cover the resonances of all fiber cores for a bending radius of 5.1 mm.

[0061] Thus, at an angle θ = 30°, a reduction to a minimum measurable bending radius of 3.6 mm can be achieved, which is smaller than in the more symmetric case of the embodiment in FIG. 4A where θ = 60° for the same fixed scan wavelength range of ±8.4 nm.

[0062] A further strategy for optimizing the minimum measurable bending radius of the optical fiber sensor is to appropriately select the optical properties of the fiber cores within the first and second subsets. Such optical properties that can differ between fiber cores may be the resonant wavelength λ0 of the fiber cores in a strain - free state. FIG. 6A shows an embodiment of an optical fiber sensor 12' that is geometrically identical to the embodiment of FIG. 4A, which has a first subset of fiber cores, for example, fiber cores 1, 3, 5, and a second subset of fiber cores, for example, fiber cores 2, 4, 6. The difference from the embodiment of FIG. 4A is that the resonant wavelength λ 0A of the first subset of the outer cores in a strain - free state is different from the resonant wavelength λ 0B of the fiber cores of the second subset of fiber cores in a strain - free state. The resonant wavelength of the fiber cores of the first subset may be offset from the central wavelength λ C of the scan wavelength range, and the resonant wavelength of the fiber cores of the second subset is maintained at the scan wavelength range center λ C . As an example, for the first subset of fiber cores, λ 0A - λ C may be 4.3 nm, and for the second subset of the outer fiber cores, λ 0B = λ C . Also, λ 0A,0B - λ C may deviate from zero for the first subset of three outer fiber cores and the second subset of three outer fiber cores. FIG. 6B shows the simulation results of the optical fiber sensor 12' of FIG. 6A, where the difference between the resonant wavelength λ res in a strained state and the central wavelength λ C of the scan wavelength range is plotted as a function of the position on the sensor of the outer fiber cores 1 to 6, as described with respect to FIG. 3B.

[0063] Also, it is conceivable to combine the embodiment of FIG. 6A with the embodiment of FIG. 5A, that is, to change the angular positions of the outer fiber cores 1 to 6 non-equidistantly as shown in FIG. 5A.

[0064] To reduce the minimum measurable bending radius, a further option in combination with the redundancy of the outer optical fiber core is, for example, to introduce an asymmetry between the resonant wavelength of the FBG of the strain-free fiber core and the central wavelength of the scan wavelength range used to interrogate the fiber core. This means that even when λ0 is the same for all fiber cores, λ0≠λ C This means that for this purpose, the interrogation unit 21 of the optical shape sensing system 10 of FIG. 1 is configured to set the central wavelength λ C so that it is different from the resonant wavelengths of the fiber cores 1 to 6. FIGS. 7A and 8A show cross-sections of an optical fiber sensor 12' having six outer fiber cores 1 to 6 and one central core 0 in each case. The geometric design of the optical sensing fiber 12' in FIGS. 7A and 8A is the same with respect to each other. In the embodiment of FIG. 7A, the scan wavelength range is set such that the resonant wavelengths λ0 of the strain-free fiber cores 1 to 6 are completely at the ends of the scan range, that is, in this example, λ0 - λ C = 8.4 nm. In this case, the minimum measurable bending radius is as low as 2.6 mm for a scan range of 16.7 nm. However, this configuration means that the resonance is in the scan wavelength range only in the case of a completely straight fiber. If the minimum radius of curvature r x is defined within the spectrum where all fiber cores 1 to 6 have still been measured, r x = ∞. This can be an undesirable situation because the redundancy of the fiber core can no longer be used for others even in the case of lower curvatures. Lowering the offset λ0 - λ C results in r xalso decreases. FIGS. 8A and 8B give an example of a trade-off between r min and r x For example, for all outer fiber cores 1 to 6, λ0 - λ C = 2.3 nm, r min = 3.5 mm and r x = 7.0 mm.

[0065] In Table 1 below, the simulation results of the standard case in FIG. 3A and the embodiments in FIGS. 4A, 5A, 6A, 7A and 8A are summarized. Table 1 lists the gain coefficient f, r min (the minimum radius of curvature still measurable with four fiber cores including the central core) and r x (the minimum radius of curvature at which all fiber cores are still within the measurement spectrum).

Table 1

[0066] Table 1 also includes, as described above, the embodiment in the fifth row of Table 1 where λ0 - λ C deviates from zero for the outer fiber cores of the first subset and for the outer fiber cores of the second subset, where for the outer fiber cores of the first subset λ0 - λ C = 2.8 nm and for the outer fiber cores of the second subset λ0 - λ C = -2.8 nm.

[0067] The above measures to optimize the design of the optical fiber sensor 12' and to optimize the interrogator unit 21 (FIG. 1) that provides the interrogation scan range of the fiber cores of the optical fiber sensor 12' can all be combined for the application being executed.

[0068] For example, the resonance wavelength λ0 in the state without distortion of the fiber core may deviate for each fiber core due to some other design constraints. For example, when it is desirable to distinguish temperature from axial strain, at least one fiber core having a temperature sensitivity different from that of other cores must be used. This can cause a deviation of λ0 for this fiber core. For the case of a 7-fiber core shape sensing fiber with a design similar to that in FIG. 6A, several options are described below. For this purpose, Δ = λ 0,A - λ 0,B is defined, where A and B represent two subsets of three outer cores separated by 60°. Here, for a specific Δ and λ 0,A - λ C it is possible to calculate r min and r x . The results are given in FIGS. 9A and 9B and FIG. 10.

[0069] In FIG. 10, 1 / r x is plotted as a function of r min for a plurality of Δ in the range from Δ = 0.0 nm to Δ = ±12.0 nm. FIG. 9A shows the simulation results for the gain coefficient f = r min / r0, and FIG. 9B shows the simulation results for the quantity r0 / r x , shown as functions of λ 0,A - λ C and λ 0,B - λ C respectively. r0 is the minimum measurable bending radius for a standard fiber design having three outer cores as shown in FIG. 3A.

[0070] The two plots of FIGS. 9A and 9B are combined in FIG. 10. FIG. 10 shows the highest curvature (1 / r x ) measurable with seven fiber cores as a function of the minimum radius r min still measurable with four fiber cores (including the central core) of the fiber sensor 12' for various sensor designs represented by Δ. r mintakes a range from 2.6 nm to 5.9 mm, and r x takes a range from 5.1 nm to infinity. From FIGS. 9A, 9B, and 10, it can be seen that there exists a "local" optimal design that maximally utilizes the trade-off between r min and r x . For example, when Δ = 0 nm, all curves with Δ≠0 nm yield larger or at most equal values for r min , representing the optimal value. When Δ = 0 nm, it is possible to derive an expression for the minimum measurable bending radius as a function of the offset in the scan wavelength range. For this purpose, the relative scan wavelength range offset O f starts at O f = |2(λ 0,A - λ C ) / Δλ| = |2(λ 0,B - λ C ) / Δλ|, where Δλ represents the full scan range, which is 16.7 nm in this example. The gain factor f = r min / r0 is given by the following equations. f = (1 / 2)·√3 / (1 + O f ), 0 ≦ O f ≦ (√3 - 1) / (√3 + 1) f = (1 / 2)·1 / (1 - O f ), (√3 - 1) / (√3 + 1) ≦ O f ≦ 1 / 3 f = 1 / (1 + O f ), 1 / 3 ≦ O f ≦ 1 r0 / r x = 1 - O f , 0 ≦ O f ≦ 1 (5)

[0071] From Equation (5) and FIG. 10, it is clear that an optimal value exists at O x = (√3 - 1) / (√3 + 1) such that f = 0.68 and r0 / r f = 0.73. For a 16.7 nm scan range and r0 = 5.1 mm, this corresponds to r min = 3.5 mm and r xconstitutes a design with r = 7.0 mm (this is exactly the example given in FIG. 8A). For a slightly smaller r min r x becomes directly much larger. This local optimum is achieved for λ 0,A -λ C = 2.3 nm. Similar considerations can be made for other shape-sensing fiber designs.

[0072] The above-described aspects are all effective in cases of redundancy, i.e., when the number of fiber cores in the fiber sensor is greater than the number of amounts required to accurately sense the shape of the optical fiber sensor 12'. However, strictly speaking, even when there is no overall redundancy, it can be advantageous to use the same aspects. It may be acceptable to lose information about less important amounts in order to create "redundancy" in time or space for the essential amounts required for shape sensing. For example, for some measurements, or at some specific positions, e.g., having short bends with smaller radii of curvature, only some signals of the fiber cores can be used so that smaller radii of curvature can still be probed. This may slightly compromise the accuracy, or it can be compensated by interpolation or extrapolation of the signals (in time or space). This will be explained in more detail below.

[0073] Referring again to FIG. 6A, an optical fiber sensor 12' having a total of 7 cores is shown. The three temperature-sensing fiber cores have different resonant wavelengths λ0 in the strain-free state. Thus, since the number of quantities to be measured (5) is less than the number of fiber cores (7), axial strain, temperature, bending strain in two orthogonal directions, and torsional strain can be measured. The minimum radius of curvature at which the aforementioned quantities can be measured is 10.6 mm (r x ). Below this radius and r minAbove =3.2 mm, only the three outer fiber cores are still within the spectrum. With these three outer fiber cores and the central fiber core, the bending strain and torsional strain in two orthogonal directions can still be measured, similar to the sum of the effects of axial strain and temperature. Since the axial strain and temperature can no longer be separated, the accuracy of other signals is impaired, but for small distances, the remaining accuracy may be sufficient. There may be many applications where the probability of severe bending existing within the optical fiber sensor 12' is low. When they occur, the length of the severe bending is short, for example, it may be the end of the shape in a medical device, further reducing the impact on the overall shape accuracy. As described above, the separation of temperature and axial strain where the seven fiber cores are still available may be interpolated or extrapolated to compensate for the loss of accuracy. Or, in other situations, r min <r < r x The separation of temperature and axial strain for measurements at r > r x can be interpolated or extrapolated from measurements that are temporally close to the measurements at r > r

[0074] The above-described embodiments suitable for reducing the minimum measurable bending radius using one or more embodiments of the optical fiber sensor 12' described above can be used in an optical shape sensing method. In this method, an optical fiber sensor (12') is provided. The fiber cores of the first subset of fiber cores (1, 3, 5) and the fiber cores of the second subset of fiber cores (2, 4, 6) are interrogated with light. The reflection spectra of the light returning from the fiber cores of the first subset of fiber cores (1, 3, 5) and at least one fiber core (2, 4, 6) of the second subset of fiber cores are measured, and the shape of the optical fiber sensor (12') based on the reflection spectra is reconstructed. This method can be executed in the system 10 of FIG. 1. As described above, the system 10 has a corresponding number of optical channels 24a to 24d greater than 4. The above-described fiber sensor 12' may be constituted by a medical device such as a catheter or a guide wire.

[0075] Although the present invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, and the present invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

[0076] In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0077] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. An optical fiber having a plurality of at least four fiber cores embedded therein, which are arranged at a distance from the longitudinal center axis, In an optical fiber sensor having, Each of the plurality of fiber cores is a fiber core of one subset of at least two fiber cores of a first subset or at least two fiber cores of a second subset, and the fiber cores of the first subset and the fiber cores of the second subset have one or more fiber Bragg gratings along the length of each fiber core, and the fiber cores of the second subset are configured to provide redundancy in the shape sensing measurement of the fiber sensor, and the fiber cores of the first subset are distributed in the azimuthal direction around the central axis with respect to each other, and each fiber core of the second subset is arranged non-equidistantly in the azimuthal direction around the central axis with respect to two adjacent fiber cores of the first subset, and the fiber core of the second subset arranged between two adjacent fiber cores of the first subset has an angular position closer to the other of the two adjacent fiber cores of the first subset than to one of the two adjacent fiber cores of the first subset, and the optical characteristics of the fiber cores of the second subset are different from the optical characteristics of the fiber cores of the first subset. Optical fiber sensor.

2. The optical fiber sensor according to claim 1, wherein the first subset of fiber cores includes three fiber cores arranged at a radial distance from the longitudinal center axis, and the second subset of fiber cores includes three fiber cores arranged at a radial distance from the longitudinal center axis.

3. The optical fiber sensor according to claim 1, wherein the angle between the angular position of one fiber core of the second subset and the angular position of one of the two adjacent fiber cores of the first subset in the azimuthal direction around the central axis is at least 10% smaller than half of the angle between the angular positions of the two adjacent fiber cores of the first subset.

4. The angle between the angular position of one fiber core of the second subset in the azimuthal direction around the central axis and the angular position of one of two neighboring fiber cores of the first subset is within a range of 20° to 40°. The optical fiber sensor according to claim 2.

5. The angle between the angular position of one fiber core of the second subset in the azimuthal direction around the central axis and the angular position of one of two neighboring fiber cores of the first subset is about 30°. The optical fiber sensor according to claim 2.

6. The optical characteristics of the fiber cores of the first subset are a first resonance wavelength at which the fiber cores of the first subset respond to light introduced into the fiber cores in a non-distorted state, and the optical characteristics of the fiber cores of the second subset are a second resonance wavelength at which the fiber cores of the second subset respond to light introduced into the fiber cores in a non-distorted state. The second resonance wavelength is different from the first resonance wavelength. The optical fiber sensor according to claim 1.

7. The fiber cores of the first subset and the fiber cores of the second subset have equal distances from the central axis. The optical fiber sensor according to claim 1.

8. The optical fiber further has a central fiber core disposed on the central axis of the optical fiber. The optical fiber sensor according to claim 1.

9. An optical shape sensing device having the optical fiber sensor according to any one of claims 1 to 8.

10. The optical fiber sensor according to any one of claims 1 to 8, an optical interrogation unit configured to interrogate the fiber cores of the first subset and the fiber cores of the second subset of the optical fiber sensor with light in a scanning wavelength range and measure the reflection spectra received from the fiber cores of the first subset and the fiber cores of the second subset of the optical fiber sensor, an evaluation unit configured to reconstruct the shape of the fiber sensor using the reflection spectra, An optical shape sensing system having the above components.

11. The optical interrogation unit is configured to set the scan wavelength range such that a center wavelength of the scan wavelength range is offset from a first resonance wavelength of a fiber core of the first subset, and the resonance wavelength responds to light introduced into the fiber core of the first subset in a state where the fiber core is not distorted. The optical shape sensing system according to claim 10.

12. The optical interrogation unit is configured to set the scan wavelength range such that a center wavelength of the scan wavelength range is offset from a second resonance wavelength of a fiber core of the second subset, and the second resonance wavelength responds to light introduced into the fiber core of the second subset in a state where the fiber core is not distorted. The optical shape sensing system according to claim 10.

13. Providing an optical fiber sensor according to any one of claims 1 to 8; Interrogating the fiber core of the first subset and the fiber core of the second subset with light; Measuring a reflection spectrum of light returning from the fiber core of the first subset and the fiber core of the second subset; Reconstructing a shape of the optical fiber sensor using the reflection spectrum; An optical shape sensing method comprising:

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