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 relates generally 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 multiple fiber cores arranged at radial distances from a 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. Furthermore, the present invention relates to an optical shape sensing method using an optical fiber sensor. [Background technology]

[0002] Optical Shape Sensing (OSS) is a technique where the three-dimensional shape of special optical fibers can be reconstructed from the reflection of light within the fiber. This technique allows real-time 3D visualization of the complete shape of devices such as medical devices, e.g. catheters and guidewires. The shape of the medical device can be superimposed on X-ray images or pre-operative CT scans. In this way, physicians can navigate the device during the procedure without the need for X-ray tracking.

[0003] In optical shape sensing, a fiber optic sensor, also called optical shape sensing fiber, is interrogated with light coupled into the fiber core of the fiber, and the distributed strain and temperature signals are obtained from the backscattering spectrum obtained with an interrogator unit incorporating an interferometer. A standard fiber optic sensor has three outer fiber cores (herein, the fiber cores located away from the central axis of the fiber are also denoted as outer fiber cores) that are spirally wound around a fourth core located at the radial center of the fiber. The response of the fiber cores to strain and temperature is measured as a 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, e.g., a perfectly straight shape. From the phase difference of the fiber cores, the strain and temperature difference can be deduced for each fiber core. The strain signal is the sum of the bending strain in two orthogonal directions, as well as the torsional and axial strains, the latter being the longitudinal strain of the fiber optic 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 technology is to be able to distinguish the effects of temperature from the effects of axial strain. To do so, at least one additional core with a different temperature sensitivity is required, as described, for example, in WO 2016 / 099976.

[0005] As mentioned above, the geometry of an optical sensing fiber is calculated from position-dependent strain signals measured for several, typically four, cores inside the fiber. For example, bending the fiber in a plane defined by the fiber core and the fiber center will produce strain in the fiber core if the core is not located at the center of 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 measured here relative to the straight, 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), the periodic nature of the Bragg grating causes the sensor to reflect light at one specific wavelength, called the resonant wavelength. If the fiber core is elongated (positively strained) relative to the reference measurement, 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 in resonant wavelength δλ (in either the positive or negative direction, depending on the location of the fiber core at the bend). δλ=λ0ζε=(λ0ζα / r)sin(g twist (z) + φ) (1) where λ0 is the resonant wavelength of the fiber core, or more precisely, the FBG in the unstrained state, and ζ is the strain-optic coefficient (≈0.8) that accounts for the strain-induced change in the refractive index, affecting the relationship between the Bragg period and the wavelength. The sine function describes the changing position of the outer core as it is twisted helically around the fiber center. g twist is the cumulative twist angle of the core, which is the sum of the twist 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 in the reference position. For reasons of clarity, only the strain due to bending is assumed in equation (1).

[0006] When the 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 pre-shaped and changes its configuration during 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 vasculature, for example, can be very tortuous. To be able to access these types of blood vessels, more flexible devices are used. The optical fiber sensor in such medical devices should be able to withstand small radii of curvature. However, there are limitations related to the minimum measurable bending radius of the optical fiber sensor.

[0007] For shape sensing, a spectrum is typically recorded for each fiber core by scanning the light source over a fixed wavelength range Δλ centered on the resonance wavelength of the FBG in the unstrained state. The smallest bend radius that still has a resonance inside the measured spectrum is given by r min =(2λ0ξa) / Δλ (2) It is.

[0008] With a scan range centered at λ = 1545 nm, ξ = 0.8, and a = 35 μm, the smallest measurable bend radius is 5.1 mm. If the fiber optic sensor is bent to reduce the curvature, no signal is measured for the fiber core that is in the plane of the bend.

[0009] From equation (2), it can be seen that the minimum measurable bending radius can be reduced by decreasing the fiber core distance a and / or by increasing the scanning wavelength range Δλ. Reducing the outer fiber core distance a has the disadvantage of reducing the sensitivity to bending strain and also torsional strain, since the sensitivity to torsional strain scales with a?. The required precision in terms of twist is high, and therefore reducing the outer fiber core distance from the central axis of the fiber is not preferred. Increasing the scanning range Δλ is disadvantageous for other reasons. It reduces the signal-to-noise ratio, since the resonance peaks fill the spectrum relatively less. Furthermore, the delay length between two successive nodes (data points as a function of position on the fiber) is reduced, giving an increase in data points for the same physical length of fiber.

[0010] WO2018 / 075911A1 proposes to provide an optical fiber sensor with three or more outer fiber cores, where the fiber cores are arranged at different radial distances from the central axis of the fiber. For small bending radii to be measured, it is necessary to switch to a fiber core at a smaller distance, which may result in a decrease in the accuracy of the shape sensing measurement. Such an optical fiber sensor design therefore suffers from a loss of accuracy.

[0011] US2007 / 0297712A1 discloses an optical fiber sensor for detecting curvature of a body, the sensor having a cladding with an outer periphery, a central core having a Bragg grating and located in the mid-plane of the cladding, and peripheral cores for transmitting and receiving light.

[0012] US 2016 / 0047976 A1 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 substantially the entire length of the optical waveguide.

[0013] US 2006 / 0024008 A1 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 consisting of a plurality of cores extending along the fiber axis, a common cladding containing the core group, and a coating covering the outer periphery of the common cladding.

[0015] US2017 / 0123146A1 discloses a multi-core optical fiber having cores randomly arranged in 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 positioned within a sensing location. Summary of the Invention [Problem to be solved by the invention]

[0017] It is an object of the present invention to provide an optical fiber sensor that enables shape measurements at small bend radii without decreasing the fiber core distance from the central axis and / or increasing the scanning wavelength range.

[0018] It is a further object of the present invention to provide an apparatus having an improved fiber optic sensor.

[0019] It is a further object of the present invention to provide an optical shape sensing system that allows for improved shape sensing measurements.

[0020] It is a further object of the present invention to provide an optical shape sensing method that allows for improved shape sensing measurements. [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 having a plurality of at least four fiber cores embedded therein spaced apart from a central longitudinal 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 a shape sensing measurement of the fiber sensor, the fiber cores of the first subset being distributed azimuthally about the central axis with respect to each other, and each fiber core of the second subset being non-equidistantly positioned azimuthally about the central axis with respect to two neighboring fiber cores of the first subset.

[0022] Optionally, the fiber cores of the second subset that are disposed between two neighboring fiber cores of the first subset have an angular position closer to one of the two neighboring fiber cores of the first subset than the other of the two neighboring fiber cores of the first subset, and the fiber cores of the first subset are disposed equidistantly about the central axis relative to each other, and the fiber cores of the second subset are disposed equidistantly about the central axis relative to each other.

[0023] The optical fiber sensor according to the invention allows to measure smaller bending radii than a standard fiber sensor with only three outer cores by providing redundancy in the fiber cores. The invention is based on the insight that it is not necessary to provide the outer fiber cores at different distances from the central axis in order to reduce the minimum bending radius measurable with the fiber sensor. In the optical fiber sensor according to the 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 invention, at least one, preferably each fiber core of the second subset is arranged non-equidistantly in the azimuth direction around the central axis with respect to two neighboring fiber cores of the first subset. This means that the outer fiber core of the second subset, which is arranged between two neighboring 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 neighboring outer fiber cores of the first subset. This results in a certain asymmetry between the first and second subsets of outer fiber cores with respect to their angular position around the central axis. As described in more detail herein, such an arrangement of the outer fiber cores is suitable for reducing the minimum measurable bend radius without increasing the scanning wavelength range and / or reducing the fiber core distance from the central axis.

[0024] Although an asymmetry may exist between the angular arrangement of the fiber cores of the first subset and the fiber cores of the second subset, the overall arrangement of all fiber cores may be symmetric. For example, one outer fiber core of the first subset and one neighboring outer fiber core of the second subset may be considered to form a pair of outer fiber cores, and the pairs present in the fiber may form a symmetric arrangement of fiber core pairs if the difference in the angular positions of the fiber cores in each pair is the same and the pairs have equal differences in angular positions about the central axis relative to each other. However, it is also possible to vary the difference in angular positions between the two fiber cores in a pair and / or to vary the difference in angular positions 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 a generally symmetrical arrangement may be an arrangement in which the first and second subsets each include three outer fiber cores, and the outer fiber cores of the first subset may be positioned at 0°, 120°, and 240°, and the outer fiber cores of the second subset may be positioned at 30°, 150°, and 270° around the central axis.

[0026] The angle between the angular position of one fiber core of the second subset in the azimuthal direction about the central axis and the angular position of one of the two neighboring fiber cores of the first subset 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 neighboring 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 helically wound around a central axis of the fiber sensor, and the central fiber core may be disposed on the central axis and extend along the same axis.

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

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

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

[0031] The optical property in this regard may be a resonant wavelength of the fiber core in an unstrained state. In one embodiment, a first resonant wavelength of the fiber cores of the first subset responsive to light introduced into the fiber cores in an unstrained state and a second resonant wavelength of at least one fiber core of the second subset in an unstrained state may differ from each other. This measurement is also suitable for reducing the minimum measurable bend radius of the optical fiber.

[0032] A further measure for reducing the minimum measurable bend radius of an optical fiber that can also be combined with any of the above embodiments is to decenter a first resonant wavelength of the first subset of fiber cores and / or a second resonant wavelength of at least one fiber core of the second subset with respect to a central wavelength of the scanning wavelength range of light used to interrogate the fiber cores. In this embodiment, the first and second resonant wavelengths may be equal to or different from each other.

[0033] According to a second aspect of the present invention, there is provided an optical shape sensing device comprising a fibre optic sensor according to the first aspect and embodiments thereof.

[0034] The optical shape sensing device may be a medical device, in particular a catheter or a guidewire.

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

[0036] The optical shape sensing system according to the invention has the same or similar advantages as those described with respect to the optical fiber sensor according to the invention, in particular, the scanning wavelength range may be the same as for a standard fiber sensor having only three outer cores, and yet smaller bend radii may be measured in this scanning wavelength range than for the standard fiber sensor.

[0037] In one embodiment, the optical interrogation unit may be configured to set the scanning wavelength range such that a central wavelength of the scanning wavelength range is off-centered with respect to a first resonant wavelength of the fiber cores of the first subset, the resonant wavelength being responsive to light introduced into the fiber cores in an unstrained state of the fiber cores, and / or the optical interrogation unit is configured to set the scanning wavelength range such that a central wavelength of the scanning wavelength range is off-centered with respect to a second resonant wavelength of at least one fiber core of the second subset, the second resonant wavelength being responsive to light introduced into the at least one fiber core in an unstrained state of the at least one fiber core. In these embodiments, the scanning wavelength range is asymmetric with respect to the resonant wavelength of the outer fiber core of the optical fiber, which is also suitable for measuring smaller bend radii of the optical fiber sensor than possible with conventional systems.

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

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

[0040] It should be understood that all 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, an optical shape sensing method, all of which enable measuring the smallest possible bend radius of the optical fiber sensor.

[0041] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. [Brief description of the drawings]

[0042] [Figure 1] FIG. 1 shows a block diagram illustrating an example of an optical shape sensing system. [Diagram 2] 1 shows a perspective view of an example of a standard optical fiber sensor. [Figure 3A] 1 shows a cross section of a standard optical fiber sensor. [Figure 3B] 3B shows an illustration of a simulation result for the fiber optic sensor of FIG. 3A, in which the difference between the resonant wavelength and the center wavelength of the scanned wavelength range is plotted as a function of position on the fiber optic sensor with a bend. [Figure 4A] 1 illustrates an embodiment of a fiber optic sensor having six outer cores. [Figure 4B] FIG. 4B shows a diagram of simulation results for the fiber optic sensor of FIG. 4A, in which the difference between the resonant wavelength and the central wavelength of the scanned wavelength range is plotted as a function of position on the fiber optic sensor. [Figure 5A] 1 illustrates an embodiment of a fiber optic sensor having six outer cores. [Figure 5B] 5B shows a diagram of simulation results for the fiber optic sensor of FIG. 5A in which the difference between the resonant wavelength and the center wavelength of the scanned wavelength range is plotted as a function of position on the fiber optic sensor. [Figure 6A] 4B illustrates an embodiment of an optical fiber sensor having six outer cores similar to FIG. 4A. [Figure 6B] 6B shows an illustration of simulation results for the fiber optic sensor of FIG. 6A, in which the difference between the resonant wavelength and the central wavelength of the scanned wavelength range is plotted as a function of position on the fiber optic sensor. [Figure 7A] 4B illustrates an embodiment of an optical fiber sensor having six outer cores similar to FIG. 4A. [Figure 7B] 7B shows a diagram of simulation results for the fiber-optic sensor of FIG. 7A in which the difference between the resonant wavelength and the central wavelength of the scanned wavelength range is plotted as a function of position on the fiber-optic sensor. [Figure 8A] 7B shows the optical fiber sensor of FIG. 7A. [Figure 8B] 8B shows an illustration of simulation results for the fiber optic sensor of FIG. 8A, in which the difference between the resonant wavelength and the central wavelength of the scanned wavelength range is plotted as a function of position on the fiber optic sensor. [Figure 9A] 13 shows a plot of simulation results for gain coefficients f=rmin / r0 and r0 / rx as a function of central spectral eccentricity of the scanned wavelength range for two sets of outer core resonant frequencies. [Figure 9B] 13 shows a plot of simulation results for gain coefficients f=rmin / r0 and r0 / rx as a function of central spectral eccentricity of the scanned wavelength range for two sets of outer core resonant frequencies. [Figure 10]13 shows a plot of simulation results for the minimum radius of curvature measurable with the six outer cores as a function of the minimum radius still measurable with the four cores for various optical fiber sensor designs. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] FIG. 1 shows a schematic of a portion of a fiber optic sensor system 10 configured as a multi-channel optical frequency domain reflectometry (OFDR)-based and distributed strain sensing system for sensing a fiber optic sensor 12. The fiber optic sensor 12 has an optical fiber with multiple fiber cores 14, 16, 18, 20 embedded therein, in this example having four cores with one central core 16 and three outer cores 14, 18, 20. The fiber optic sensor shown in FIG. 1 is a standard fiber sensor. It is noted that the present invention proposes a fiber optic sensor design with more than three outer cores. FIG. 2 shows a length of fiber cores 14, 16, 18, 20 with the outer cores 14, 18, 20 spiraled around the central core 16. The central core 16 is located on the central axis of the fiber optic sensor 12. The outer fiber cores 14, 18, 20 are angularly spaced from one another in the azimuth direction about the central longitudinal axis of the fiber optic sensor 12. The central longitudinal axis coincides with the central core 16. With a plurality of four cores in this example, the angular spacing between adjacent outer cores may be 120°.

[0044] 1, the optical shape sensing system 10 includes an interrogator unit 21. The interrogator unit 21 may include 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 interferometric network 24 having optical channels 24a, 24b, 24c, 24d according to the multiple 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 interferometric network 24 may have a corresponding number of optical channels.

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

[0046] In one embodiment of the optical fiber sensor 12, the fiber cores 14, 16, 18, 20 may comprise fiber Bragg gratings (FBGs) formed by periodic variations in the refractive index. For simplicity, an FBG with a single resonant wavelength is considered here. The FBG reflects light of a certain wavelength (the resonant wavelength) that depends on the grating period of the FBG and transmits all other wavelengths. Upon bending of the optical fiber sensor 12, the grating period is affected by the strain and measurements of the reflected wavelength for any position along the fiber allow the local strain to be determined. The optical fiber sensor 12' according to the embodiment of the invention described below may comprise such an FBG.

[0047] Optical interrogation of the fiber optic sensor 12 can, in principle, provide the information needed to reconstruct the three-dimensional shape of the entire fiber sensor in real time. Given a suitable reference frame, it is possible to know the exact orientation and position of the complete fiber sensor 12 in real time.

[0048] When a fiber optic sensor such as fiber optic sensor 12 is used within a medical device, such as a catheter or guidewire, the device changes its configuration during handling of the device. For example, if the device is a catheter for introduction into the human vasculature, which may be very tortuous, the device, and therefore fiber optic sensor 12, experiences bending along its length that may have a radius of curvature that may be very small. However, with optical shape sensing technology, limitations exist related to the minimum measurable bending radius of the fiber optic 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 λ=1545 nm of the fiber core in the unstrained state, ξ=0.8, and a=35 μm (see above for the definition of these parameters). If 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 that is in the bending plane.

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

[0051] FIG. 4A illustrates an embodiment of a fiber optic sensor 12′ according to the present invention, which has a number of outer fiber cores greater than three. In other words, the fiber optic sensor 12′ provides redundancy by adding an additional outer fiber core that provides redundancy in the shape sensing measurement of the fiber 12′ to enable measuring a smaller bend radius of the fiber 12′ compared to the three outer fiber cores of the standard sensor 12. In FIG. 4A, the outer fiber cores are labeled with reference numbers 1 through 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, e.g., fiber cores 1, 3, 5, and three outer fiber cores of a second subset of fiber cores, e.g., 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). Furthermore, the fiber cores 1 to 6 are arranged equidistantly around the central axis in the azimuth direction. Therefore, the angle between two neighboring fiber cores of the fiber cores 1 to 6 is 60°. The fiber cores 1, 3, 5 may have a first single resonant wavelength in the unstrained state of the fiber 12′, and the fiber cores 2, 4, 6 may have a second single resonant wavelength in the unstrained state of the fiber sensor 12′. In this example, the first and second resonant wavelengths are equal.

[0053] The fiber cores of the second subset of fiber cores may be helically wound around a 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', which are bending strain, torsional strain and axial strain in two orthogonal directions, 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, with the resonant wavelength λ 1 as described with respect to FIG. 3B. res and the central wavelength λ of the scanning wavelength range c The difference between is plotted as a function of position on the optical fiber sensor 12'. The grey shaded area gives the scanning wavelength range required to include at least three resonances of the outer cores 1-6. In FIG. 4B, curves 41-46 show the simulation results for the outer fiber cores 1-6 (results for the central core 0 are again omitted in FIG. 4B). As can be seen from the diagram in FIG. 4B, the black dotted lines 51 and 52 are no longer the maximum of the fiber core signal, which means that smaller bending radii can be measured using the same scanning wavelength range (±8.4 nm). In this case, a minimum bending radius r of 4.5 millimeters is required. min3B and 4B reveals redundancy in the fiber cores, e.g., by providing six outer fiber cores instead of three, the minimum measurable bend radius can be reduced without increasing the scanned wavelength range and without decreasing the distance of the outer cores from the central axis.

[0056] In comparison to a standard optical fiber sensor having three outer cores, such as the optical fiber sensor 12 of FIG. 3A, a gain factor f may be calculated to account for the beneficial effect of redundancy in the outer fiber cores, which is obtained by redundancy with the amount of cores n without increasing the scanning wavelength range. f=cos((π / (n-1)floor((n / 2)-2)),n≧4 (3) where n is the total number of fiber cores (including the central core and n-1 outer fiber cores). For n=4 (standard optical fiber sensor), f is 1. For 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 (60° angle between two neighboring outer cores), the minimum measurable bending radius can be reduced by a factor of 0.87, i.e., from 5.1 mm to 4.5 mm, for the same scanning wavelength range.

[0057] The gain factor f, and therefore the minimum measurable bend radius, can be further reduced by one or more of the following measures, which are described in connection with further embodiments.

[0058] In general, the optimization of the gain coefficient f can be done by varying the fiber core angles relative to each other, and / or by varying the core optical properties, and / or by introducing an asymmetry between the scanning wavelength range and the resonant wavelength of the fiber core in the unstrained state. These strategies are described below.

[0059] FIG. 5A shows an embodiment of an optical fiber sensor 12′ having six outer cores 1-6, the difference from the embodiment in FIG. 4A being that the fiber cores 1-6 in FIG. 5A are not distributed equidistantly in the azimuth direction around a central core 0 extending along the central axis of the fiber 12′. In the embodiment of FIG. 5A, the angles between some of the neighboring fiber cores, e.g., fiber cores 2 and 3, are smaller than the angles between other neighboring fiber cores, e.g., fiber cores 1 and 2. For example, the smaller angles between fiber cores 2 and 3, between 4 and 5, and between 6 and 1 may be 30°, while the larger angles between fiber cores 1 and 2, between 3 and 4, and between 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 adopted as well, as long as there is redundancy. 5A, a first subset of outer cores 1, 3, 5 are positioned at 0°, 120°, and 240°, and a second subset of three cores 2, 4, 6 with the same relative angle are positioned under an angle of θ=30° with respect to the first subset of fiber cores 1, 3, 5. 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: f=max{cos((|θ|-π / 3) / 2), -sin((|θ|-2π / 3) / 2)}, -π / 3<θ≦π / 3 (4)

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

[0061] Thus, at an angle θ=30°, a reduction in the minimum measurable bend radius to 3.6 mm can be achieved, which is smaller than 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 approach to optimizing the minimum measurable bend radius of the optical fiber sensor is to appropriately select the optical properties of the fiber cores in the first and second subsets. Such an optical property that may differ between the fiber cores may be the resonant wavelength λ0 of the fiber cores in the unstrained state. FIG. 6A shows an embodiment of an optical fiber sensor 12′ that is geometrically identical to the embodiment of FIG. 4A, having a first subset of fiber cores, e.g., fiber cores 1, 3, 5, and a second subset of fiber cores, e.g., fiber cores 2, 4, 6. The difference with the embodiment of FIG. 4A is the resonant wavelength λ0 of the first subset of outer cores in the unstrained state. 0A is the resonant wavelength λ of the fiber cores of the second subset of fiber cores in the unstrained state. 0B The resonant wavelengths of the fiber cores of the first subset are different from the central wavelength λ of the scanned wavelength range. C and the resonant wavelengths of the fiber cores of the second subset may be offset from the scan wavelength range center λ C As an example, for a first subset of fiber cores, λ 0A -λ C may be 4.3 nm, and λ for a second subset of the outer fiber cores. 0B = λ C Also, λ 0A,0B -λ C 6B shows the simulation results of the optical fiber sensor 12′ of FIG. 6A, showing the resonant wavelength λ in the strained state. res and the central wavelength λ of the scanning wavelength range C The difference between is plotted as a function of the position on the sensor of the outer fiber cores 1-6, as described with respect to FIG. 3B.

[0063] It is also conceivable to combine the embodiment of FIG. 6A with the embodiment of FIG. 5A, ie, to vary the angular positions of the outer fiber cores 1-6 non-equidistantly, as shown in FIG. 5A.

[0064] A further option in combination with redundancy of the outer optical fiber core to reduce the minimum measurable bend radius is to introduce an asymmetry between the resonant wavelength of the FBG in the strain-free fiber core and the central wavelength of the scanning wavelength range used to interrogate the fiber core. This means that λ ≠ λ even if λ is the same for all fiber cores. C For this purpose, the interrogation unit 21 of the optical shape sensing system 10 of FIG. 1 selects a central wavelength λ 1 such that the central wavelength is different from the resonant wavelengths of the fiber cores 1 to 6. C 7A and 8A show in each case a cross-section of an optical fiber sensor 12' with six outer fiber cores 1-6 and one central core 0. The geometrical 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 scanning wavelength range is set so that the resonant wavelength λ0 of the undistorted fiber cores 1-6 is completely at the end of the scanning range, i.e. λ0-λ0 in this example. C = 8.4 nm. In this case, the minimum measurable bend radius is as low as 2.6 mm for a scan range of 16.7 nm. However, this configuration means that the resonances are in the scanned wavelength range only in the limit of a perfectly straight fiber. Let us define the minimum radius of curvature for which all fiber cores 1 to 6 are still within the measured spectrum as r x If we define it as r x =∞. This can be an undesirable situation since the redundancy of the fiber core can no longer be used for other things, even in the case of lower curvatures. C Lowering r x8A and 8B show the smallest measurable radius of the four fiber cores (including the central core), r min andr x For example, we provide an example where a trade-off is made between λ0-λ for all outer fiber cores 1 to 6. C = 2.3 nm, r min = 3.5mm and r x =7.0mm.

[0065] In Table 1 below, the simulation results for the standard case in FIG. 3A and the embodiments in FIGS. 4A, 5A, 6A, 7A and 8A are summarized. Table 1 shows the gain coefficients f, r min (the smallest radius of curvature that can still be measured for the four fiber cores, including the central core) and r x (the smallest radius of curvature for which all fiber cores are still within the measured spectrum) is listed. [Table 1]

[0066] Table 1 also shows the relationship between λ0 and λ C , for the outer fiber cores of the first subset and for the outer fiber cores of the second subset, deviates from zero, where λ - λ for the outer fiber cores of the first subset C = 2.8 nm, and λ - λ for the second subset of outer fiber cores. C =-2.8 nm.

[0067] All the above mentioned strategies for optimizing the design of the optical fiber sensor 12′ and optimizing the interrogator unit 21 (FIG. 1) that provides the scanning range for the interrogation of the fiber core of the optical fiber sensor 12′ can be combined for the application to be performed.

[0068] For example, the resonant wavelength λ in the unstrained state of the fiber core may deviate from one fiber core to another due to some other design constraint. For example, if it is desired to distinguish temperature from axial strain, at least one fiber core must be used that has a different temperature sensitivity than the other cores. This can result in a deviation of λ for this fiber core. For the case of a seven-fiber-core shape-sensing fiber of design similar to that of FIG. 6A, several options are described below. To this end, Δ=λ 0,A -λ 0,B where A and B denote two subsets of the three outer cores separated by 60°. 0,A -λ C For r min andr x It is possible to calculate the results shown in Figures 9A and 9B and in Figure 10.

[0069] In Figure 10, 1 / r x is the r for multiple Δ in the range from Δ = 0.0 nm to Δ = ±12.0 nm. min The gain factor f = r min 9B shows the simulation results for the quantity r0 / r x The simulation results for λ 0,A -λ C and λ 0,B -λ C r0 is the minimum measurable bend radius for a standard fiber design with three outer cores as shown in FIG. 3A.

[0070] The two plots, 9A and 9B, are combined in Figure 10. Figure 10 shows the highest measurable curvature (1 / r x ) to the minimum radius r that can still be measured by the four fiber cores (including the central core) of the fiber sensor 12′ for various sensor designs represented by Δ. min As a function of r minranges from 2.6 nm to 5.9 mm, and r x ranges from 5.1 nm to infinity. From Figures 9A, 9B, and 10, r min andr x For example, for Δ=0 nm, all curves with Δ≠0 nm are local optimal designs that maximize the tradeoff between r min Δ represents the optimum value, since it produces a larger or at most equal value for Δ. In the case of Δ=0 nm, it is possible to derive an expression for the minimum measurable bending radius as a function of the offset in the scanning wavelength range. For this purpose, the relative scanning wavelength range offset O f But, 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 formula: 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 Figure 10, f = 0.68 and r0 / r x = 0.73, so that O f It is clear that there is an optimum at r = (√3-1) / (√3+1). For a scan range of 16.7 nm and r = 5.1 mm, this means that r min = 3.5mm and r x= 7.0 mm (this is exactly the example given in FIG. 8A). min For r x is directly much larger. This local optimum is 0,A -λ C = 2.3 nm. Similar considerations can be made for other shape-sensing fiber designs.

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

[0073] Referring again to FIG. 6A, there is shown an optical fiber sensor 12′ having a total of seven cores, where the three temperature-sensitive fiber cores have different resonant wavelengths λ0 in the unstrained state, and thus the number of quantities to be measured (five) is less than the number of fiber cores (seven), so that 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 r = 3.2 mm, only the three outer fiber cores are still in the spectrum. With these three outer fiber cores and the central fiber core, the bending and torsional strains in the two orthogonal directions can still be measured, as well as the sum of the effects of axial strain and temperature. Because axial strain and temperature can no longer be separated, the accuracy of the other signals is compromised, but for small distances, the remaining accuracy may be sufficient. There may be many applications where there is a low probability that tight bends are present in the optical fiber sensor 12', and when they do occur, the length of the tight bends may be short and may be at the end of the shape, for example in medical devices, further reducing the impact on the overall shape accuracy. As mentioned before, the separation of temperature and axial strain where 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 may be interpolated or extrapolated from measurements closer in time to the measurement at .

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

[0075] While the 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 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 "comprise" does not exclude other elements or steps, and the indefinite article "a" or "an" does 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 embedded therein a plurality of at least four fiber cores spaced apart from a central longitudinal axis; In an optical fiber sensor having Each of the plurality of fiber cores is a fiber core of one of a first subset of at least two fiber cores or a second subset of at least two fiber cores, the fiber cores of the first subset and the fiber cores of the second subset have one or more fiber Bragg gratings along a length of the respective fiber core, the fiber cores of the second subset are configured to provide redundancy in a shape sensing measurement of the fiber sensor, the fiber cores of the first subset are distributed azimuthally about the central axis relative to each other, and each of the fiber cores of the second subset the fiber cores are non-equidistantly positioned in an azimuth direction around the central axis relative to two neighboring fiber cores of the first subset, a fiber core of the second subset positioned between two neighboring fiber cores of the first subset has an angular position closer to one of the two neighboring fiber cores of the first subset than to the other of the two neighboring fiber cores of the first subset, the fiber cores of the first subset are equidistantly positioned in an azimuth direction around the central axis relative to each other, and the fiber cores of the second subset are equidistantly positioned in an azimuth direction around the central axis relative to each other. Fiber optic sensors.

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

3. 2. The optical fiber sensor of claim 1, wherein an angle between an angular position of one fiber core of the second subset and an angular position of one of two neighboring fiber cores of the first subset in an azimuth direction about the central axis is at least 10% smaller than half the angle between the angular positions of the two neighboring fiber cores of the first subset.

4. 3. The optical fiber sensor of claim 2, wherein an angle between an angular position of one fiber core of the second subset in an azimuth direction about the central axis and an angular position of one of two neighboring fiber cores of the first subset is in a range of 20° to 40°.

5. 3. The optical fiber sensor of claim 2, wherein an angle between an angular position of one fiber core of the second subset in an azimuth direction about the central axis and an angular position of one of two neighboring fiber cores of the first subset is about 30°.

6. The optical fiber sensor of claim 1 , wherein the optical properties of the second subset of fiber cores are different than the optical properties of the first subset of fiber cores.

7. 2. The optical fiber sensor of claim 1, wherein the first subset of fiber cores have a first resonant wavelength responsive to light introduced into the fiber cores under an unstrained condition, and the second subset of fiber cores have a second resonant wavelength responsive to light introduced into the fiber cores under an unstrained condition, the second resonant wavelength being different from the first resonant wavelength.

8. The optical fiber sensor of claim 1 , wherein the first subset of fiber cores and the second subset of fiber cores have equal distances from the central axis.

9. The optical fiber sensor of claim 1 , wherein the optical fiber further comprises a central fiber core disposed on the central axis of the optical fiber.

10. An optical shape sensing device comprising an optical fiber sensor according to any one of claims 1 to 9.

11. An optical fiber sensor according to any one of claims 1 to 9; an optical interrogation unit configured to interrogate the first and second subset of fiber cores of the optical fiber sensor with light in a scanning wavelength range and measure reflection spectra received from the first and second subset of fiber cores of the optical fiber sensor; an evaluation unit configured to reconstruct the geometry of the fiber sensor using the reflection spectrum; An optical shape sensing system comprising:

12. 12. The optical shape sensing system of claim 11, wherein the optical interrogation unit is configured to set the scanning wavelength range such that a center wavelength of the scanning wavelength range is off-centered with respect to a first resonant wavelength of the first subset of fiber cores, the resonant wavelength responsive to light introduced into the first subset of fiber cores in an unstrained state of the fiber cores.

13. 12. The optical shape sensing system of claim 11, wherein the optical interrogation unit is configured to set the scanning wavelength range such that a center wavelength of the scanning wavelength range is off-centered with respect to a second resonant wavelength of the second subset of fiber cores, the second resonant wavelength being responsive to light introduced into the second subset of fiber cores in an unstrained state of the fiber cores.

14. Providing an optical fiber sensor according to any one of claims 1 to 9; optically interrogating the first subset of fiber cores and the second subset of fiber cores; measuring a reflectance spectrum of light returning from the first subset of fiber cores and the second subset of fiber cores; reconstructing the geometry of the optical fiber sensor using the reflectance spectrum; The optical shape sensing method comprises:

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