Systems, methods and assemblies for single-input shape sensing - Patents.com

Abstract

Multicore fiber assemblies, in which multiple single-mode cores are coupled together to form a single path, reduce the complexity of fiber optic sensor measurements, provide low back reflections, and are capable of measuring a variety of parameters such as fiber twist, temperature, axial strain, and fiber geometry.

Description

[Technical field]

[0001] [Reference to Related Applications] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 317,114, filed Mar. 07, 2012, entitled “SYSTEMS, METHODS AND ASSMEBLIES FOR SINGLE INPUT SHAPE SENSING” by Jie Li et al., which is incorporated herein by reference in its entirety.

[0002] [Technical field] The present invention relates generally to the field of optical fibers, and more particularly to optical fiber turnarounds for use in fiber-based sensors and similar devices. Described herein are systems, methods, and articles of manufacture for shape sensing using multicore fiber sensors having at least one single-mode sensing core, which can be interrogated through a single optical path that can access all sensing cores in a single measurement. [Background technology]

[0003] Optical fibers are utilized in sensing applications, including measurements of distributed strain and temperature in the optical fiber, as well as acoustic signals impinging on the fiber. In one sensing application, an optical signal is transmitted into the optical fiber, perturbations in the fiber core result in backscattering, and the backscattering can be analyzed to obtain the geometry of the fiber. In another sensing application, the distributed scattering signal provides a measurement of the acoustic waves acting to strain the fiber. In such distributed sensors, measurements can be made at any part of the array. Thus, continuous measurements with a certain spatial resolution are possible along the distributed sensor.

[0004] Distributed sensors are fiber optic sensing devices that utilize both single-ended and dual-ended optical fibers as sensors to accurately detect and measure temperature, acoustic, and strain at selected locations along the length of a borehole, or for other applications where temperature, acoustic, and strain detection over long distances is desired. Dual-ended interrogation (i.e., the ability to launch optical signals to both ends of a fiber optic path and detect the resulting optical signals after transmission) is often preferred for several reasons, including improved accuracy, as well as to help compensate for measurement errors resulting from exposure and time-dependent hydrogen diffusion in the sensor fiber, as well as errors associated with splicing and connectors.

[0005] It is known in shape sensing using fibers, such as multi-core fibers as sensors. For example, it is known that fiber optic shape sensing can use optical frequency domain reflectometry ("OFDR") of weak distributed reflection from twisted multi-core fibers. Thus, one aspect of conventional fiber optic shape sensing systems is the requirement of a multi-core optical fiber, where all cores are offset from the center by a fixed distance. A central core with zero offset from the fiber axis must then be included as a reference to enable accurate measurement of temperature, axial strain, and / or twist. Each core acts as a local strain sensor and thus may detect fiber bend and twist when referenced to the central core. Such strain sensing must be performed along multiple paths through the fiber to determine the local state of bend and twist along the length of the fiber. Once the local bend and twist are determined, the shape of the fiber may be calculated using differential geometry equations, such as, for example, the Frenet-Serret equations.

[0006] As a result, the back-reflected signal of the OFDR detector must be collected through multiple optical paths through the fiber. A given core in an optical fiber represents one such path. Typically, the signal leaving the distal end of the fiber is scattered or absorbed at the termination so as not to corrupt the reflected signal from the fiber. Such conventional measurements require multiple interrogator components or some form of switching to measure all of the cores. For example, as shown in fiber end face 110 and fiber side 120 of FIG. 1 (PRIOR ART), the input signal may be split into many signals that are directed to each core. The resulting back-scattered signals from each core must then be directed to a selection of detectors to obtain the signal from each core in the sensor. Such photonic networks can include many optical components including couplers, isolators, and polarizing beam splitters. Alternatively, a single light source and a single detector can be used along with a switch that sequentially directs the light to different cores, as shown in fiber end face 210 and fiber side 220 of FIG. 2 (PRIOR ART). As a result, the complexity of the interrogator increases and / or the acquisition speed decreases from that of a single OFDR measurement.Therefore, there is a need to accommodate multiple sensing cores without increasing the complexity and cost of the interrogator. Summary of the Invention

[0007] One aspect of the invention relates to a multicore fiber assembly including a waveguide that receives an input signal from an interrogator via a multicore fan-out attached to a proximal end of the multicore fiber assembly and propagates the input signal in a first direction along a first core of the multicore fiber assembly. The multicore fiber assembly also includes a first turnaround that redirects the input signal at a distal end of the multicore fiber assembly, the waveguide propagating the input signal in a second direction along a second core of the multicore fiber assembly, and a second turnaround that redirects the input signal at the multicore fan-out, the waveguide propagating the input signal in the first direction along a third core of the multicore fiber assembly. The first turnaround redirects the input signal, the waveguide propagating the input signal in the second direction toward the multicore fan-out along a fourth core of the multicore fiber assembly, and the waveguide propagates a dispersed back-reflected signal to the interrogator.

[0008] Another aspect of the invention relates to an optical system for transmitting a single signal in multiple directions in a multicore fiber sensor. The optical system includes a multicore fiber having an interrogator single core to a multicore fan-out and a plurality of single mode cores including a proximal end and a distal end, a GRIN lens fixed to the distal end of the multicore fiber and having a length of less than 5 mm, and a micro turnaround attached to the proximal end of the multicore fiber. The multicore fiber is configured to receive a signal from the interrogator via the multicore fan-out attached to the proximal end of the multicore fiber, propagate the signal in a first direction along at least one core of the multicore fiber, redirect the signal with the GRIN lens attached to the distal end of the multicore fiber sensor, and propagate a dispersed back-reflected signal to the interrogator.

[0009] Yet another aspect of the present invention relates to a method for transmitting a single signal in multiple directions in a multicore fiber sensor, the method including: a) receiving a signal at the multicore fiber sensor from an interrogator via a multicore fan-out attached to a proximal end of the multicore fiber sensor, b) propagating the signal in a first direction along a first core of the multicore fiber sensor, c) redirecting the signal at a GRIN lens attached to a distal end of the multicore fiber sensor, the GRIN having a length of less than 5 mm, d) propagating the signal in a second direction along a second core of the multicore fiber sensor, e) redirecting the signal with a single-core turnaround attached to the multicore fan-out, f) propagating the signal in a first direction along a third core of the multicore fiber sensor, g) redirecting the signal with the GRIN lens, h) propagating the signal in a second direction along a fourth core of the multicore fiber sensor towards the multicore fan-out, and i) propagating the distributed back-reflected signal to the interrogator. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 (Prior Art) shows a fiber end view and a fiber side view in which an input signal may be split into many signals that are directed to each core. [Diagram 2] FIG. 2 (Prior Art) shows a fiber end view and a fiber side view in which a single light source and a single detector can be used with a switch to direct light to different cores. [Diagram 3] FIG. 3 illustrates an embodiment of a fiber having four cores, where a first pair of cores are offset from the center of the fiber by a distance R1 and a second pair of cores are offset by R2, according to one embodiment of the invention. [Figure 4] FIG. 4 shows a fiber and a GRIN lens coated with a material to absorb stray reflections, according to one embodiment of the present invention. [Diagram 5]FIG. 5 shows a multicore fiber having perturbations along the length of the fiber, according to one embodiment of the present invention. [Figure 6] FIG. 6 shows a multicore fiber with a refractive index perturbation over only a portion of the waveguide, according to one embodiment of the present invention. [Figure 7A] FIG. 7A shows the output being sent through an optical isolator to prevent further back reflections, which can be fed back to the OFDR interrogator and used to reduce phase variations in the interferometric measurements in OFDR according to one embodiment of the present invention. [Figure 7B] FIG. 7B shows how signals can be transmitted bidirectionally through the four cores of a multicore fiber to enable interrogation schemes such as BOTDA. [Figure 8] FIG. 8 shows a switch that allows measurements in either direction, such that signals acquired bidirectionally may improve the accuracy and signal-to-noise ratio of the sensor, according to one embodiment of the present invention. [Figure 9] FIG. 9 shows a fiber having a fifth core very close to the center of the fiber, according to one embodiment of the present invention. [Figure 10] FIG. 10 illustrates the use of a GRIN fiber turnaround on the proximal side of the fiber, in accordance with one embodiment of the present invention. [Figure 11] FIG. 11 shows an additional core in a multicore fiber connected to another interrogator, according to an embodiment of the present invention. [Figure 12] FIG. 12 illustrates a multicore fiber including a multimode core, where such a core may be used in any number of alternative applications in accordance with an embodiment of the present invention. [Figure 13A] FIG. 13A shows various intermediate 7-core fibers (a), (b), and (c) according to one embodiment of the present invention. [Figure 13B] FIG. 13B shows an intermediate 7-core fiber (b) according to one embodiment of the present invention. [Figure 13C] FIG. 13C shows an intermediate 7-core fiber (c) according to one embodiment of the present invention. [Figure 14A] FIG. 14A shows another embodiment in which a bubble or other cavity is etched into the material near the central core at the end of the fiber. [Figure 14B] FIG. 14B shows another embodiment in which a bubble or other cavity is etched into the material near the central core at the end of the fiber. [Figure 15] FIG. 15 shows a schematic diagram of the experimental setup, where the fiber under test (FUT) is a twisted multi-core fiber inscribed with a continuous weak fiber Bragg grating according to one embodiment of the present invention. [Figure 16A] FIG. 16A shows the local variation of the Bragg wavelengths of the four outer cores. [Figure 16B] FIG. 16B shows the OFDR trace of the shape sensing system. [Figure 16C] FIG. 16C shows the local variation of the Bragg wavelengths of the four outer cores, an OFDR trace of the shape sensing system. [Figure 16D] FIG. 16D is a labeled end view image of an FUT with a core. [Figure 17A] FIG. 17A shows the reconstruction of the FUT under various curvatures with the GRIN turnaround device. [Figure 17B] FIG. 17B shows the reconstruction of the FUT under various curvatures without the GRIN turnaround device. [Figure 17C] FIG. 17C shows a comparison of curvature results of current and previous shape sensing systems in accordance with one embodiment of the present invention. [Figure 18] FIG. 18 shows the corresponding shape reconstructions of a fiber wrapped around two posts at different angles: −90°, −45°, 0°, 45°, and 90°, according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] As discussed in detail below, the present invention relates to systems, methods, and assemblies for single-input shape sensing. Exemplary embodiments described herein include multicore fiber sensors that have at least one single-mode sensing core and can be interrogated through a single optical path that accesses all sensing cores in a single measurement. Further embodiments include multicore fiber sensors with graded-index or "GRIN" lensed fiber turnarounds on one or both ends that allow for measurement of multiple cores in a single measurement. Thus, such multicore fiber shape sensors can be interrogated using a single optical path from an interrogator.

[0012] As mentioned above, there is a need to address multiple sensing cores without increasing the complexity and cost of the interrogator. One way to overcome this limitation involves turning the optical fiber back on itself at the distal end of the sensor. Such a turnaround recycles the input signal from the first core and sends it to the second core. This approach requires the use of a multicore-to-single core fan-out device followed by a single core splice from one single core output to another. In this way, a single measurement can be used to interrogate two cores in a single multicore sensor. Such multicore fiber turnarounds can pose reliability and size issues as the fan-out device is bulky and the fiber turnaround must be larger than a minimum bend radius, typically 5 mm, to ensure mechanical stability.

[0013] A more compact method of folding back light within a fiber involves the use of a GRIN fiber spliced ​​onto the end of the fiber. With such a "micro-turnaround", light entering one core can be turned back into another core using a single short GRIN assembly spliced ​​onto the end of the fiber, requiring very little extra space. Such a micro-turnaround was applied to a fiber with two multimode cores and was intended for use in distributed temperature sensing using Raman scattering. However, multimode cores are not suitable for fiber shape sensing because of the many mode paths within such cores. As a result, such fibers are not well suited for fiber shape sensing. Furthermore, fiber shape sensing typically requires measurements from three or more cores, including the central core. However, because the GRIN lens must be aligned to the axis of the multicore fiber, the GRIN lens strongly reflects light from the core on the central axis, thereby compromising the distributed weaker reflections from the other cores.

[0014] Exemplary embodiments described herein provide a multicore fiber and turnaround assembly that can couple to multiple single mode cores through a single path while keeping back reflections low and enabling measurements of fiber twist, temperature, and axial strain in addition to fiber shape. As described in more detail below, an exemplary method enables collection of signals from a multicore fiber sensor having at least one single mode core using a single optical path. This single path accesses multiple cores, preferably at least four cores. Additionally, the exemplary multicore fiber shape sensor can be interrogated with a single measurement and does not require a bulky fold back at the far end of the fiber.

[0015] FIG. 3 shows a fiber end view 310 and a fiber side view 320 of an embodiment in which a fiber with four cores is manufactured. Two of the cores are offset from the center of the fiber by a distance R1, and the second pair is offset by R2. The lines connecting the pairs may be oriented at 90 degrees from each other, or at some other angle. The offset radii R1 and R2 may be the same, or preferably different. If they are different, twist measurements are possible. It is also possible for the two cores to have different levels of dopants and different dimensions. Possible dopants include Ge, F, B, P, Ce, Sn. For example, they may have different levels of Ge and F doping, and the cores may have different radii. By this means, their properties such as effective area, effective refractive index and UV light sensitivity may be different. In FIG. 3, the two pairs of cores are oriented at 90 degrees to each other, have different core radii and doping levels, and have different offset radii. They may be the same. The fiber side view 320 in Figure 3 shows the cores not twisted about their axis, however, preferably the cores are stranded along the fiber to form four interlocking helices.

[0016] Although FIG. 3 shows a fiber with only two pairs of cores, it may have more than two pairs. Thus, the fiber may have six cores arranged in pairs centered on the center of the fiber and with the same offset for each pair. Because there is no core in the center of the fiber, there is no reflection into the GRIN lens used to fold the signal back at the distal end. FIG. 3 further shows a multicore to single core fanout device. Such a device may be fabricated by fusing a single core fiber and tapering it to a geometry that matches the multicore fiber, and then splicing the tapered structure to the multicore fiber.

[0017] According to one exemplary embodiment, the single core outputs can be spliced ​​together to direct the light exiting the multicore fiber back into the multicore fiber. A length of GRIN fiber is spliced ​​to the distal end of the fiber. Preferably, the center of the GRIN fiber refractive index profile is aligned with the midpoint of the pairs of cores 1 and 3, and 2 and 4. The GRIN fiber length is adjusted so that the light exiting one of the two pairs of cores is reflected back into the other member of the pair of cores. Typically, a reflective film is added to the GRIN fiber end face to maximize reflection. In this way, the back-reflected light scattered from both cores can be collected in a single measurement.

[0018] As shown in fiber end view 410 and fiber side view 420 in FIG. 4, the fiber and GRIN lens are preferably coated with a material that absorbs stray reflections. This material may be a polymer whose refractive index is greater than the fiber refractive index. It may also exhibit very high attenuation at the wavelengths used by the interrogator. FIG. 3 shows how light can traverse all the cores. Light leaves the interrogator (which may be an OFDR) and travels to the multi-core fan-out from a single core that sends the light to core 1 of the multi-core fiber. The GRIN fiber sends this signal back to core 3 at the distal end of the fiber. When the light returns to the proximal end where the fan-out is located, the light from core 3 enters the single-core fiber that corresponds to core 3. This fiber is spliced ​​onto the single-core fiber that was connected to core 2 in the multi-core fan-out. The light in core 2 then propagates to the GRIN fiber at the distal end where it is folded back again and sent to core 4. When the light reaches the fan-out and enters the single mode fiber corresponding to core 4, it can be terminated for low reflection with an index-matching gel. Such a low reflection termination is required for interrogation methods such as OFDR.

[0019] Figure 3 shows the input interrogator signal as a solid line. It also shows a portion of the distributed back-reflected signal as a dashed line. Although Figure 3 shows only one point of reflection back to the interrogator, it will be understood that distributed reflections return from all portions of the waveguide.

[0020] It is also possible to modify the cores of a multicore fiber by introducing refractive index changes along the length of the multicore fiber. Such changes may be used to increase the back-reflected signal. Such changes may be periodic, chirped, or non-periodic. Fiber end view 510 and fiber side view 520 in FIG. 5 show a multicore fiber with changes along the entire length of the fiber. Alternatively, fiber end view 610 and fiber side view 620 in FIG. 6 show a multicore fiber with refractive index changes over only a portion of the waveguide. The strength of the refractive index changes may be different in different waveguides. For example, if the Ge doping in core 4 in FIG. 5 is greater than that of core 1, the refractive index changes in this waveguide will be greater when they are introduced by actinic radiation. Furthermore, UV irradiation through a phase mask of a given period may form Bragg gratings in all cores. If a given core has more Ge, it will show a grating with a greater reflection, since the UV light sensitivity depends on the level of Ge doping. Such a larger reflection can counteract the attenuation that may occur as the input light propagates through the core.

[0021] It should be noted that the signal exiting Core 4 may also be used for other sensing tasks. As shown in the fiber end view 710 and fiber side view 720 in FIG. 7A, this output may be sent through an optical isolator to prevent further back reflections. This signal may be fed back to the OFDR interrogator and used to reduce phase variations in the interferometric measurements in OFDR. For example, phase perturbations in the multicore fiber may be observed in the transmitted signal and used to correct for phase variations in the reflected signal. In one embodiment, the transmitted signal may be used as a local oscillator in the OFDR measurements. The signal exiting Core 4 may also be sent to another interrogator, such as an optical spectrum analyzer, to monitor the spectral changes through the sensor fiber. It is also possible to send signals down the fiber in the opposite direction if both ends of the optical path are accessible. Thus, although FIG. 7 shows signals propagating in only one direction, it is also possible for the interrogator to send signals bidirectionally through the multicore fiber. These signals may be sent simultaneously. In one embodiment, the coincidence counter propagation signals may be interfered with within the interrogator to generate a sensed output.

[0022] In another embodiment, the interrogator may be a Brillouin Optical Time Domain Analysis (BOTDA) interrogator. Such an interrogator requires the propagation of signals in both directions of the fiber core to achieve improved spatial resolution using Brillouin scattering. Fiber end view 730 and fiber side view 740 in FIG. 7B show how a BOTDA interrogator may be used to send signals bidirectionally in all four cores of a sensing fiber. The arrows pointing in both directions indicate that the BOTDA signal is propagating bidirectionally in all four cores in fiber side view 740 in FIG. 7B. Other interrogators that require access to both ends of the fiber may also be used instead of BOTDA. Instead of BOTDA, the interrogation scheme may be Brillouin Optical Correlation Domain Analysis (BOCDA). BOCDA also requires signals from both directions of the fiber.

[0023] As shown in fiber end view 810 and fiber side view 820 in FIG. 8, an embodiment may include a switch to allow measurement in either direction, so that signals taken in both directions may improve the accuracy and signal-to-noise of the sensor. In another embodiment, the pump signal may be propagated within the core to amplify the signal. Amplification may come from mechanisms such as rare earth ion dopants such as Er or Yb, or by Raman or Brillouin gain. In another embodiment, both ends of the optical path may be combined in an interferometer. The beat signal between the two paths may be used to sense rotation and other changes in the fiber.

[0024] Fiber end view 910 and fiber side view 920 in Figure 9 show a fiber with a similar design to that of Figure 3; however, it includes a fifth core much closer to the center of the fiber. Light returning to the fan-out a second time through core 4 is sent back to nearly central core 5. This core allows for better isolation of bends, twists and axial strains since it is so close to the center of the fiber where only axial strain and temperature affect the fiber core. Unlike the other cores of the fiber, when light from this core enters a GRIN fiber, the back-reflected light is directed into the non-guiding cladding of the fiber. As a result, there is no back-reflection from this core and it is therefore optically terminated.

[0025] Fiber end view 1010 and fiber side view 1020 in Figure 10 show the use of a GRIN fiber turnaround on the proximal side of the fiber. Specifically, Figure 10 shows how an offset GRIN fiber core can route light from one core to another while allowing an input fiber to connect to one of the other offset cores. The fourth core does not reflect back into the fiber and is therefore terminated.

[0026] As shown in fiber end view 1110 and fiber side view 1120 in Figure 11, an additional core may be present in the multicore fiber, which may be connected to another interrogator. For example, this core may have Bragg gratings inscribed at various locations to measure local strain. Alternatively, this additional core may allow temperature measurements along the fiber.

[0027] In a further embodiment, the fiber end view 1210 and fiber side view 1220 of FIG. 12 show a multi-core fiber containing a multi-mode core, which is used for any number of alternative applications, such as, but not limited to, imaging, carrying telecommunication signals, delivering high power radiation, etc. A fan-out device couples an input single-core multi-mode fiber to this multi-mode core. In an embodiment, a mode control device, such as a spatial light modulator (SLM) or digital mirror device (DMD), for beam control or alternatively, is used to launch light into this multi-mode core. Calibration of the multi-mode core determines the exact launch conditions to generate a given image or spot on the distal end of the fiber. Importantly, the reflector at the distal end is dichroic. This means that the reflector is highly reflective to the sensing light in all cores where the signal is inverted. However, light propagating in the multi-mode waveguide has wavelengths that pass through the reflector and form a spot or other pattern on the other side of the GRIN fiber. The reflector may pass all or only a small portion of the imaging light. In one embodiment, the sensing light has a vacuum wavelength in the range of 1500 nm to 1650 nm. In another embodiment, the imaging light has a vacuum wavelength in the visible region of the optical spectrum. In another embodiment, the transmitted light has a wavelength of 1000 nm to 1150 nm. In another embodiment, the transmitted light carries an electrical communication signal. In a preferred embodiment, the other core of the multicore fiber is used to determine the fiber's geometry, and this geometry is used to determine the required launch into the multimode fiber such that a desired output is formed beyond the GRIN fiber at the distal end. It is also possible for the multimode waveguide to overlap one or more of the single mode waveguides.

[0028] If a central core is required and back reflections from the turnaround must be minimized, an additional intermediate fiber can be added between the sensing fiber and the GRIN turnaround to reduce back reflections from the GRIN turnaround. The outer core of the intermediate fiber has the same structure (location, mode field diameter ("MFD"), numerical aperture "NA", etc.) as the outer core of the sensing fiber. The central core of the intermediate fiber may be absent or may have high absorption (preferably with index matched to the central core of the multicore fiber), or may be an off-center graded index fiber as shown in the various configurations (1310, 1320, 1330) illustrated in FIG. 13A. In all cases, it is preferred that the center of the intermediate fiber has a similar or the same index of refraction as the central core of the multicore fiber. High absorption can be achieved with dopants such as cobalt. In this way, reflectivity due to index mismatch is reduced. High attenuation may be achieved in other ways. For example, a tilted grating may be inscribed in the central core that scatters all incident light into the cladding or free space modes. In FIG. 13B, the intermediate fiber 1370 is shown spliced ​​to the multicore sensor fiber 1360 and then to the GRIN turnaround fiber. The intermediate fiber 1370 is a 7-core fiber with a centrally placed intermediate core, which is a GI fiber, and the outer six cores are aligned to the cores of the multicore sensor fiber 1360. Only the central core 1350 and one outer core 1340 are shown. The length of the intermediate fiber 1370 is selected to minimize back reflections to the central core 1350 and all of the other cores 1340 of the multicore fiber. In FIG. 13B, this length is selected to be half the pitch in the offset graded index (GI) cores. In this way, the output light is focused away from the center of the multicore fiber. FIG. 13C shows another design of cores. The sensing multicore fiber 1380 has a guiding central core. The intermediate fiber 1390 has no central core to prevent back reflections.Equivalently, the intermediate fiber 1370 can have a highly lossy central core, preferably index matched to the sensor fiber central core, or can have an offset multimode core. Additionally, the radii R1 and R2 can be different or the same. Finally, there can be more or less than five cores in the sensing fiber. FIG. 14A shows a side view of the fiber assembly. It shows a single-core to multicore fan-out connected to a 5-core fiber spliced ​​to an intermediate fiber 1390 that does not have a central core, which is then spliced ​​to a GRIN fiber. Light reflection from the end of the central core at the splice back into the intermediate fiber is minimized. This light is shown escaping into the intermediate fiber 1390, where it spreads and is absorbed in the coating, with minimal power backscattering into the central core. The length of the intermediate fiber 1390 is selected to reduce this backreflection.

[0029] Fiber end view 1410 and fiber side view 1420 in FIG. 14A and fiber end view 1430 and fiber side view 1440 in FIG. 14B show another embodiment. In this embodiment, a bubble or other cavity is etched in the material near the central core of the end of the fiber. In one embodiment, such a bubble may be etched in a silica waveguide using an intense light pulse from a femtosecond laser. The intense pulse may be introduced from the side of the fiber, along the axis of the fiber, or at an angle to the fiber axis. Preferably, the bubble or etched region is offset from the center of the fiber but overlaps with the central core of the fiber. Preferably, the bubble or etched cavity does not overlap any of the other cores, as shown in fiber end view 1430 in FIG. 14B. If the cavity is formed prior to splicing to the GRIN fiber, the cavity must survive the splicing procedure that attaches the GRIN fiber to the end of the multicore fiber. The effect of the bubble or cavity is to reflect all light entering the bubble or cavity from the central core out of the fiber. This can be most effective when the angle of the bubble or cavity wall with respect to the fiber axis is greater than the critical angle for total internal reflection of the guided light in the central core. In this way, no light is reflected back to the central core.

[0030] The exemplary embodiments described herein allow for the reduction of undesired reflections at the interface between a multicore fiber and a GRIN lens fiber. Undesired reflections arise from discontinuities in the waveguide. This discontinuity can result in back reflections and can also result in scattering of light from the GRIN fiber. For example, if a multicore fiber has a single mode offset core, it should have an effective index that matches or is less than the level of the GRIN fiber index at the radius where the offset core is spliced ​​onto the GRIN fiber. In this way, back reflections at the interface are reduced and the GRIN fiber can guide the light exiting this single mode fiber into the GRIN without waveguide losses.

[0031] Note that the GRIN fiber and the sensor fiber have coatings that absorb stray light, preventing it from being reflected back into the OFDR. Such stray reflections cause spurious signals and must be reduced for the OFDR to achieve maximum accuracy.

[0032] The exemplary embodiment described herein takes a different approach to shape sensor interrogation that measures fiber shape with a single high-performance OFDR measurement. For example, a twisted multicore fiber can be combined with a GRIN lens turnaround at the distal side of the fiber and spliced ​​on the proximal multicore fan-out so that a single input signal from the OFDR can provide back-reflected sensor signals from the four cores. At the distal side of the fiber, the GRIN lens turnaround reflects light from one of the cores to a core that has the same radial offset and 180° from the first core. At the fan-out, the return light is spliced ​​to a core that is 60° from the first core, and then this light is reflected to the next core. This embodiment demonstrates that the back-reflected light from these four cores can be effectively used in shape sensing. Similar performance was found compared to that of a single-ended shape sensor that uses a 1×4 switch to obtain signals from each core. A single-ended shape sensor fiber may also be wrapped around two posts and reconstruct this wrapped shape as the posts are rotated relative to each other. It is expected that single-ended shape sensors may be used in applications that require a compact distal fiber end and where cost and / or performance requirements dictate the use of a single channel OFDR system.

[0033] Distributed fiber optic sensors have been shown to be valuable tools in many applications such as structural health monitoring, acoustic wave sensing, and in biomedical applications due to their accuracy, sensitivity, flexibility, and resistance to EM interference. One of the most demanding sensing applications is shape sensing or fiber shape reconstruction. Fiber optic shape sensing refers to many different applications ranging from monitoring large structures such as aircraft wings, wind turbines, and bridges where single-core fibers are coupled to the structure, to the deflection of small objects such as needles, catheters, and other surgical tools that use multi-core fibers. Typically, these sensors use wavelength division multiplexed (WDM) fiber Bragg grating arrays, which are interrogated by measuring the optical reflectance spectrum of individual gratings. However, WDM Bragg gratings only measure strain at discrete locations, and such sensors are typically limited to less than 100 sensing points and spatial resolutions better than 1 mm. Another method of fiber shape sensing uses Brillouin scattering to measure distributed fiber strain in multi-core and single-core fibers. Such schemes can be applied over long lengths, but have spatial resolutions on the order of centimeters. Additionally, both the proximal and distal ends of the fiber must be accessible for the sensor to be interrogated.

[0034] For more accurate shape reconstruction, measurements of fiber strain with spatial resolutions below 1 mm are required. To date, such high spatial resolution can only be achieved using swept-wavelength interferometry (SWI), alternatively known as optical frequency-domain reflectometry (OFDR). In OFDR, the spatial resolution is governed by the wavelength scanning range, and the maximum measurable fiber length is determined by the wavelength step size. The spatial resolution can be less than 100 μm, and fibers over several meters can be measured. Strain and shape sensing have been shown by using discrete Bragg gratings or Rayleigh scattering with OFDR. Sensing with Rayleigh scattering is distributed along the fiber and therefore provides high spatial resolution, but this backscattered signal is very weak and exhibits a randomly varying phase, which limits the speed and stability of the interrogator. Attempts to improve backscattering by using UV-enhanced fiber have been shown, but this fiber still exhibits uncontrolled phase. Significant improvements in both signal noise and phase information have been demonstrated using a near-continuous array of weak uniform periodic gratings written into a multicore fiber. Furthermore, the outer cores in these waveguides were spun during drawing to form a helix around the central core. Such quasi-continuous gratings in twisted multicore fibers greatly improved the accuracy and speed of fiber shape reconstruction using OFDR. These fibers enabled OFDR interrogation to be used in applications such as medical shape sensing and sensing of micron-sized deformations along the fiber.

[0035] Compared to other methods, OFDR requires a more complex interrogator, which uses a precisely tuned narrow linewidth source and phase-sensitive measurements. Applying OFDR to four separate channels for shape sensing adds further complexity. Typically, this requires an optical switch to measure different cores successively, which can degrade signal quality in dynamic environments. Alternatively, an input / output network can be used to distribute the input signal to the four cores and collect the back-reflected signal from each core. To obtain simultaneous measurements, such a scheme would require four separate detector modules. This additional complexity can result in increased costs and compromise accuracy, speed, and stability.

[0036] The exemplary embodiments described herein employ a different approach for shape sensor interrogation, requiring only a single high spatial resolution OFDR measurement, while still maintaining a small distal fiber end compatible for use in medical catheters and other miniature applications. For example, a 498 μm long graded index (GRIN) fiber lens can be spliced ​​to the distal end of a twisted multi-core fiber to reflect light 180 degrees from a given outer core to a second, opposite core. In this way, both the input light and the dispersed backscattered light used in the OFDR interrogator follow a back and forth path through the multi-core fiber. It is noted that the exemplary GRIN fiber lens can be less than 5 mm in length, preferably less than 1 mm or less than 500 μm.

[0037] In the proximal fan-out, the single-core fiber output of the GRIN lens reflected signal can be spliced ​​to another single-core fiber that connects to the third outer core. The light entering this core is then reflected back by the same GRIN lens to the fourth core. Thus, it was shown that the back-reflected light from these four cores can be effectively utilized for bending shape sensing with a spatial resolution of 80 μm. Furthermore, the performance of the exemplary single-measurement micro-turnaround shape sensor is compared to a 1 × 4 optical switch system that interrogates each core sequentially, resulting in a 2.2 m resolution. -1 52m from -1 Similar performance was found for a range of curvatures up to 100 nm. Furthermore, shape reconstruction may be performed with the exemplary shape sensor fiber wrapped around two struts rotated relative to each other. The reconstructed fiber shapes showed good agreement with the experimental results. It is expected that the exemplary micro-turnaround shape sensor will find use in applications requiring a compact distal fiber end and where cost and / or performance requirements necessitate the use of a single-channel OFDR system.

[0038] Sensor and GRIN Design FIG. 15 shows a schematic diagram of the experimental setup, where the fiber under test (FUT) is a twisted multicore fiber inscribed with a continuous weak fiber Bragg grating according to one embodiment of the present invention. FIG. 15A shows the experimental setup including an OFDR (optical frequency domain reflectometer), a TFB (tapered fiber bundle), and a FUT (fiber under test). FIG. 15B shows a diagram of a twisted multicore fiber with a continuous grating. FIG. 15B also shows a side image of a GRIN fiber spliced ​​to the FUT before a metal coating is applied. FIG. 15C shows an end-face image of the FUT. FIG. 15D shows a ray tracing diagram of the FUT and turnaround device. For clarity, only the first four cores are shown, and the twist of the FUT is not shown. Thus, alternative and / or additional embodiments may feature any number of cores, and may or may not feature a twist.

[0039] The fiber has one central core and six outer cores, and an end-face image is shown in Figure 15D. The outer cores are 35 μm from the central core and are twisted along the length of the fiber with a period of 2 cm (or 50 twists / meter). To inscribe long lengths of continuous FBGs and maintain the mechanical strength of the fiber, the fiber is coated with a UV transparent coating. The FBGs are inscribed with a 248 nm excimer laser through a phase mask technique, giving a Bragg wavelength of approximately 1541 nm.

[0040] While conventional applications of GRIN lenses and fibers may refocus a point source, an exemplary embodiment of the present invention exploits this effect to enable OFDR to measure backscattered signals from all outer cores of the fiber. To interrogate the outer cores of the FUT sequentially, the proximal end of the fiber is spliced ​​to a tapered fiber bundle (TFB) multicore-to-single-core fan-out with seven single-core fiber inputs and one multicore fiber output. The distal end is spliced ​​to a GRIN turnaround device. A side view of the FUT spliced ​​to the GRIN fiber is shown in FIG. 15B. The GRIN fiber has a length of 498 μm, a core diameter of α=115.3 μm, a cladding diameter of 127 μm, and the following core refractive index profile:

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[0041] OFDR and Data Processing Dispersive strain can be measured along the fiber sensor using OFDR. This technique is well known and has been used to measure discrete attenuation, strain and temperature in optical fibers. Briefly, the OFDR technique sends a narrow linewidth signal into the fiber under measurement. The weakly dispersed back reflections from the entire fiber length are interfered with a reference beam from the same laser. As the laser is scanned over a given frequency range, a spectral interferogram R(ω) is recorded and every location on the fiber is represented by a spectral oscillation frequency. The Fourier transform of this spectral interference yields the time domain quantity:

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[0042] 16A-D show the OFDR trace of the shape sensing system (FIG. 16A), the cropped and inverted amplitudes of the four outer cores (FIG. 16B), the local variation of the Bragg wavelengths of the four outer cores (FIG. 16C), and the shape of the fiber cross section of the FUT with the cores labeled (FIG. 16D). OFDR The (z) OFDR trace is shown in FIG. 16A. The y-axis units are dB / mm (20 log 10 R OFDR) units of power. The OFDR trace shows back reflections from six transits through the six outer cores of the fiber, alternating between forward and reverse directions of light propagation. The output of the OFDR is spliced ​​to the TFB fan-out fiber corresponding to core 1 of the FUT (splice 1). A small increase in the backscattered signal is observed at splice 1 due to differences in Rayleigh scattering between the output fiber of the OFDR (SMF-28) and the TFB fan-out fiber. The large spike in the OFDR trace after the input splice is caused by the TFB itself. The enhanced backscattered signal of the FUT from the continuous grating is clearly seen (core 1). The GRIN turnaround device spliced ​​to the distal end of the FUT appears as a spike, followed by a drop in signal due to attenuation and reflection losses. We note that there was another FUT-FUT splice near the GRIN lens splice. The signal is then spatially inverted with respect to the signal from core 1 and propagates towards the fan-out at the proximal end of the FUT (core 2). The output of the Core 2 fanout is spliced ​​into the Core 3 fiber of the fanout. The signals in Core 3 and Core 4 also propagate in a similar manner to Core 1 and Core 2, but with lower amplitudes due to losses in the TFB and multicore splices. Core 5 and Core 6 are also spliced ​​into the signal path, and their OFDR signals can be seen as well. However, they were not used in this set of experiments because the signals were too heavily degraded by losses in the system. Most of this attenuation is due to the TFB and two unoptimized multicore fiber splices, not the GRIN lens turnaround. Each GRIN reflection, including the splice to the multicore fiber, has a round-trip loss of about 2 dB. Other losses are estimated as follows: Each one-way path between the TFB multicore output-FUT splice has a loss in the range of 1.6-3 dB, the TFB itself has a round-trip loss of 3 dB, and the FUT-multicore splice near the GRIN lens has a splice loss in the range of 0.3-1.4 dB. Also, the grating reflectivity was about 2 to 3 dB lower in core 3 and core 4.

[0043] Precise shape reconstruction requires measurements of the local curvature and twist along the fiber, typically with a spatial resolution of less than 1 mm (80 μm in this work). These parameters are derived from the strain field across the fiber cross section. To measure the local strain, data is extracted from the first four cores of the OFDR traces in Figure 16A. The trace for core 1 is obtained from the measurement in Figure 16A by cutting before splice 1 and after the first turnaround at 8.5 m. The trace for core 2 is obtained by cutting the data before the turnaround and after splice 2, then flipping the trace. The traces for core 3 and core 4 are generated in a similar manner. Core 3 is cut between splice 2 and the second turnaround at 20 m. Core 4 is cut between the second turnaround and splice 3, then spatially flipped. The four traces are then translated such that they are all aligned at the location of the turnarounds, with splice 1 just after z=0. The various components of the system can be seen aligned in Figure 16B. The turnaround points are aligned at ~5.6m, the splices to the multicore fiber are aligned near 2.6m, and the tapered region of the TFB is aligned at ~2m. Note that the splices between the single-core fanout outputs do not occur exactly at the midpoint between the micro-turnaround points. This is because the single-core pigtails in the fanout are not exactly the same length. Thus, unlike the turnaround and the multicore splices to the fanout, the reflections from these points are not in the same location from trace to trace. These reflections, in combination with other reflection spikes, can produce spurious peaks in the traces. Such reflection artifacts can also result from stray reflections at the GRIN micro-turnarounds.

[0044] The OFDR process converts the phase data of these traces into local Bragg wavelengths using equation (2), as described above. Figure 2(c) shows the difference in local Bragg wavelengths, Δλ, for a straight fiber and a fiber with constant curvature. Bragg(z) shows that when the fiber is bent, one side of the fiber is under tension and the other side is under compression. As the outer cores rotate along the fiber, each outer core alternates between tension and compression over one twist cycle of about 2 cm, so that Δλ Bragg (z) gives a sinusoidal profile. Note that Δλ of core 1 and core 2 Bragg (z) are out of phase with each other because the cores are 180° apart, and this is also true for cores 3 and 4. Furthermore, the vibrations in core 1 (and core 2) are 60 degrees out of phase with core 3 (and core 4) because these cores are rotated 60 degrees with respect to each other. Knowing the local change in Bragg wavelength, the strain in the outer core can then be compared to:

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[15] , our fiber plus turnaround device does not provide a signal from the central core. The central core serves as a reference since it experiences no strain when the fiber is bent. In our analysis, we measure λ in a calibration step where we measure the fiber while holding it perfectly straight with no applied axial strain. Bragg A reference value of was obtained.

[0045] According to an exemplary embodiment of the present invention, fiber shape reconstruction can be performed by integrating the following French-Cerrette equation:

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[0046] To calculate k(s) and τ(s), we use a vector pointing to the helical core,

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[0001] for the first fiber oriented in the z direction.

[0047] As shown in Figure 16A-D, the output of the OFDR is spliced ​​to the TFB fan-out fiber (splice 1) corresponding to core 1 of the FUT. A slight increase in the backscattered signal is observed at splice 1 due to the difference in Rayleigh scattering between the output fiber of the OFDR (SMF-28) and the TFB fan-out fiber. The spike in the OFDR trace after splicing is due to the tapered region of the TFB. The multi-core side of the fan-out is spliced ​​to the FUT. The enhanced backscattered signal of the FUT from the continuous grating is clearly seen (core 1). The GRIN fiber turnaround device spliced ​​to the distal end of the FUT appears as a spike, followed by a drop in the signal due to GRIN fiber attenuation and reflection losses. The signal is then inverted with respect to the signal from core 1 and propagates towards the fan-out at the proximal end of the FUT (core 2). The output of the core 2 fan-out is spliced ​​to the core 3 fiber of the fan-out. Signals in core 3 and core 4 also propagate similarly to core 1 and core 2, but with lower amplitudes due to various losses. Core 5 and core 6 were spliced ​​into the setup and can be seen in the OFDR traces, but were not used in this set of experiments because the losses in the system degraded the signal too much. According to one embodiment, each GRIN reflection can cause a loss of about 1.6 dB, and each path between the TFB and the FUT splice can cause a loss in the range of 0.8 to 2.2 dB. According to further embodiments, the acceptable amount of loss caused by the exemplary GRIN fiber and the exemplary splice can be less than 5.0 dB, more specifically, less than 3.0 dB. Furthermore, the loss of the exemplary multicore fiber assembly featuring the use of multiple turnarounds can be less than 20 dB. For example, as shown in FIG. 16C, the overall attenuation from core 1 (~82 dB) to core 4 (~100 dB) is less than 20 dB.

[0048] 17A-C show the reconstruction of the FUT under various curvatures with (FIG. 17A) and without (FIG. 17B) the GRIN turnaround device, as well as a comparison of the curvature results for the current and previous shape sensing systems (FIG. 17C). FIG. 17A shows the current shape sensing system tested for several curvatures. The FUT was measured at 52 m -1 From 2.2m -1 The fibers have been wound onto spools of various diameters with minimal twist to test their performance over a range of diameters. The present shape sensing system is also compared to one in which the fiber does not have a GRIN turnaround device and each core is interrogated individually through the use of an optical switch. As shown in Figure 17C, the curvature sensing capabilities of the present system are comparable to past systems as they are accurate and in good agreement with each other and with theoretical values.

[0049] Figures 18A-E show photographs 1810-1850 and corresponding shape reconstruction plots 1815-1855 of a fiber wrapped around two posts at different angles: -90° (Figure 18A), -45° (Figure 18B), 0° (Figure 18C), 45° (Figure 18D), and 90° (Figure 18E). Thus, these figures show the capabilities of the current system in 3D shape sensing. In one embodiment, the fiber can exhibit a non-zero curvature since curvature is required to measure torsion. Note that this is a limitation of the current fiber for twist sensitivity, not a limitation of the single-ended shape sensor system. It is possible to design a fiber with higher twist sensitivity and still have a turnaround at the distal end. The fiber was wrapped helically around two optical posts such that the fiber has a curvature throughout its length. These two posts were then bent at five different angles (-90°, -45°, 0°, 45°, and 90°) and the shape reconstruction plots 1815-1855 are shown in Figures 18A-E. A cylinder can be added to the shape reconstruction to show that the reconstructed shapes have indeed wrapped around a straight cylinder. The shape reconstruction gives an arbitrary frame, so the bottom post may be used as a reference. The shape reconstruction in each case was rotated so that the axis around which the bottom spiral rotates is always in the same orientation, and the azimuthal orientation of the top post is always the same. Qualitatively, the reconstructions match the photographs of the fiber well. All reconstructions wrap nicely around both posts, and the curvature is not significantly more or less than the half inch post. Also, each reconstruction shows six fiber rotations around the post, the same as shown in the photographs. For more quantitative results, the top posts were virtually created and fitted against the reconstructed shape to give an estimate of the angle between the posts, which provides angles of -85°, -44°, 0°, 52°, and 91°. Note that precise wrapping of the posts was not required and the fiber was not coupled with the posts during the entire experiment.

[0050] In conclusion, the exemplary embodiments described herein present a novel single-channel interrogation technique of multicore fibers for use in curvature and shape sensing. By using a GRIN micro-turnaround device spliced ​​to the distal end of the multicore fiber, multiple cores in a single fiber can be interrogated simultaneously without the need for an optical switch. Curvature measurements are shown to be accurate and comparable to conventional multi-channel methods. Furthermore, the capabilities of this novel system are demonstrated with several shape sensing examples. These results show that it is possible to reduce the complexity and increase the acquisition speed of conventional shape sensor devices relevant to the medical, aerospace, structural monitoring industries, etc.

[0051] The present disclosure has been described with reference to its exemplary embodiments. All exemplary embodiments and conditional descriptions disclosed in the present disclosure are intended to help those skilled in the art to which the present disclosure pertains to understand the principles and concepts of the present disclosure. Therefore, those skilled in the art to which the present invention pertains will understand that the present invention can be modified and implemented without departing from the spirit and scope of the present invention. Although a number of embodiments having various features are described herein, combinations of such various features in other combinations not discussed herein are contemplated to be within the scope of the embodiments of the present disclosure.

Claims

1. 1. A multicore fiber assembly for transmitting an input signal from an interrogator in multiple directions, comprising: a multicore waveguide for receiving the input signal from the interrogator; a multi-core fan-out coupled to a proximal end of the multi-core waveguide and positioned to propagate the input signal in a first direction along a first core of the multi-core waveguide; a first turnaround coupled to a distal end of the multicore waveguide for redirecting the input signal at the distal end of the first core of the multicore waveguide, the input signal being redirected in a second direction along a second core of the multicore waveguide; a second turnaround coupled to the multi-core fan-out for redirecting the input signal at the multi-core fan-out, the input signal being redirected in the first direction along a third core of the multi-core waveguide; the input signal from the third core of the multicore waveguide is redirected by the first turnaround in the second direction along a fourth core of the multicore waveguide towards the multicore fan-out, the multicore waveguide propagating a distributed back reflected signal to the interrogator.

2. 2. The multicore fiber assembly of claim 1, wherein an attenuation loss of the input signal from the first turnaround to the second turnaround is less than 5 dB.

3. 2. The multicore fiber assembly of claim 1, wherein an attenuation loss of the input signal from the first core to the fourth core is less than 20 dB.

4. 2. The multicore fiber assembly of claim 1, wherein the input signal is a single signal received by the interrogator.

5. 10. The multicore fiber assembly of claim 1, wherein the input signal from the interrogator is bidirectionally coupled to the fiber assembly, and the interrogator collects signals from one or both directions.

6. 2. The multicore fiber assembly of claim 1, wherein the interrogator is a Brillouin Optical Time Domain Analysis (BOTDA) interrogator.

7. 2. The multicore fiber assembly of claim 1, wherein the interrogator is an optical frequency domain reflectometry (OFDR) interrogator.

8. 2. The multicore fiber assembly of claim 1, wherein at least one of the first and second turnarounds is a GRIN lens having a length of less than 5 mm.

9. 9. The multicore fiber assembly of claim 8, wherein the GRIN lens further comprises a reflector coupled to the distal end of the GRIN lens, the reflector being dichroic.

10. The multicore fiber assembly of claim 1 , wherein the second turnaround is a single-core fiber coupled to the multicore fan-out.

11. 2. The multicore fiber assembly of claim 1, wherein at least two cores are offset from a center of the multicore fiber by a radius R1 and at least two further cores are offset from the center of the multicore fiber by a radius R2, where R1 ≠ R2.

12. 1. An optical system for transmitting signals in multiple directions in a multicore fiber sensor, comprising: An interrogator; a multicore fiber having a proximal end, a distal end, and a plurality of single mode cores; a single-core-to-multicore fan-out coupled to a proximal end of the multicore fiber; a GRIN lens coupled to a distal end of the multicore fiber and having a length of less than 5 mm; a micro-turnaround attached to the multi-core fanout; The multicore fiber comprises: a) receiving the signal from the interrogator via the multicore fan-out coupled to the proximal end of the multicore fiber; b) propagating the signal in a first direction along at least one core of the multicore fiber; c) redirecting the signal at the GRIN lens coupled to the distal end of the multicore fiber in a second direction; d) an optical system for conveying the distributed back-reflected signals to said interrogator;

13. 13. The optical system of claim 12, wherein an attenuation loss in the signal from the first direction to the second direction is less than 5 dB.

14. 13. The optical system of claim 12, wherein the attenuation loss of the signal traveling through the micro-turnaround is less than 20 dB.

15. 13. The optical system of claim 12, wherein said signal is a single signal received by said interrogator.

16. 13. The optical system of claim 12, wherein the signal from the interrogator is coupled into the multicore fiber in both directions, the interrogator collecting the signal from one or both directions.

17. 13. The optical system of claim 12, wherein the interrogator is a Brillouin Optical Time Domain Analysis (BOTDA) interrogator.

18. 13. The optical system of claim 12, wherein the interrogator is an optical frequency domain reflectometry (OFDR) interrogator.

19. 13. The optical system of claim 12, wherein at least two cores are offset from a center of the multicore fiber by a radius R1 and at least two further cores are offset from the center of the multicore fiber by a radius R2, where R1 ≠ R2.

20. The optical system of claim 12 , wherein at least two cores are twisted around a central axis of the multicore fiber.

21. 13. The optical system of claim 12, wherein at least one core includes an index change along its length for modifying said multiple cores.

22. The optical system of claim 12 , wherein the multicore fiber includes at least one multimode core.

23. 23. The optical system of claim 22, wherein the GRIN lens further comprises a reflector coupled to the distal end of the GRIN lens, the reflector being dichroic.

24. 13. The optical system of claim 12, further comprising an inner central core of the multicore fiber and an intermediate fiber between the multicore fiber and the GRIN lens, wherein a non-centered outer core of the intermediate fiber has the same structure as the non-centered outer core of the multicore fiber.

25. The optical system of claim 12 , wherein at least the multicore fiber and the GRIN lens are coated with a stray reflection absorbing material.

26. 13. The optical system of claim 12, further comprising an optical isolator between the multicore fiber and the interrogator, the optical isolator receiving a propagation-dispersed back-reflected signal from the multicore fiber before being fed back to the interrogator.

27. 1. A method for transmitting a single signal in multiple directions in a multicore fiber sensor, comprising: a) receiving, at the multicore fiber sensor, a signal from an interrogator via a multicore fan-out coupled to a proximal end of the multicore fiber sensor; b) propagating the signal in a first direction along a first core of the multicore fiber sensor; c) redirecting the signal at a GRIN lens coupled to a distal end of the multicore fiber sensor, the GRIN having a length of less than 5 mm; d) propagating the signal in a second direction along a second core of the multicore fiber sensor; e) redirecting the signal at a single core turnaround coupled to the multi-core fanout; f) propagating the signal in the first direction along a third core of the multicore fiber sensor; g) redirecting the signal with the GRIN lens; h) propagating the signal in the second direction along a fourth core of the multicore fiber sensor towards the multicore fan-out; i) transmitting the distributed back-reflected signals to said interrogator.

28. 30. The method of claim 27, further comprising terminating the signal in a single mode fiber of the multicore fan-out.

29. 28. The method of claim 27, wherein the signal from the interrogator is coupled bidirectionally into the multicore fiber, and the interrogator collects the signal from one or both directions.

30. transmitting said signal to an optical spectrum analyzer; The method of claim 27 further comprising monitoring a spectral change through the multicore fiber sensor.

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