Multipoint temperature-independent vector bending sensing with coupled-core fibers

36784 IEEE SENSORS JOURNAL, VOL. 24, NO. 22, 15 NOVEMBER 2024 Multipoint Temperature-Independent Vector Bending Sensing With Coupled-Core Fibers Marco A. Contreras-Teran , Amaia Berganza , Florian Lindner , Jörg Bierlich, Katrin Wondraczek, and Joel Villatoro , Senior Member, IEEE Abstract—Fiber optic bending sensing has potential use in industrial and medical applications. Thus, so far, several configurations have been reported with that end, but the state-of-the-art sensors are either complex, temperature dependent, or cannot be multiplexed easily. To circumvent these important limitations, we have developed a sensing platform based on an asymmetric coupled-core optical fiber that is combined with conventional Bragg gratings. The asymmetric fiber was designed with three cores arranged in an equilateral triangle. The said fiber supports supermodes that suffer drastic changes when it is bent. Consequently, the reflection of a Bragg grating inscribed close to the asymmetric fiber changes drastically, but its wavelength position is not altered. We demonstrate experimentally that our sensing platform allows the development of highly sensitive bending sensors that have important practical assets. The latter include compactness and simple fabrication, capability of distinguishing the direction of bending and simultaneous detection of temperature and bending. Moreover, the sensors can be multiplexed easily and can be interrogated with commercially available fiber optic sensor read out units. Index Terms—Bending sensors, coupled-core fibers (CCFs), fiber Bragg grating (FBG), mode interferometers, multicore fibers, optical fiber sensors, supermodes. I. INTRODUCTION THE effects of bending on the light guided by an optical fiber attracted research interest since the early days of Received 30 August 2024; accepted 7 September 2024. Date of publication 18 September 2024; date of current version 14 November 2024. This work was supported in part by MCIN/AEI/10.13039/501100011033, “ERDF A Way of Making Europe,” under Grant PID2023-152763NBI00, Grant PDC2022-133885-I00, Grant PID2021-122505OB-C31, and Grant TED2021-129959B-C21; in part by European Union “Next Generation EU/PRTR;” in part by the Gobierno Vasco/Eusko Jaurlaritza under Project IT1452-22; and in part by German Federal Ministry of Education and Research (BMBF): “RUBIN-QUANTIFISENS-TP11: Specialty Fibers and Fiber-Based Components for Omnifunctional Fiber Sensor Systems” under Grant FKZ: 03RU1U071J. The associate editor coordinating the review of this article and approving it for publication was Prof. Arnaldo Gomes Leal-Junior. (Corresponding author: Joel Villatoro.) Marco A. Contreras-Teran is with the Department of Electronic Engineering, Engineering Division Campus Irapuato-Salamanca, Universidad de Guanajuato, Salamanca 36885, Mexico (e-mail: [email protected]). Amaia Berganza is with the Department of Applied Mathematics, University of the Basque Country UPV/EHU, 48013 Bilbao, Spain (e-mail: [email protected]). Florian Lindner, Jörg Bierlich, and Katrin Wondraczek are with the Leibniz Institute of Photonic Technology (Leibniz IPHT), 07745 Jena, Germany (e-mail: flor[email protected]; joerg.bierlich@ leibniz-ipht.de; [email protected]). Joel Villatoro is with the Department of Communications Engineering, University of the Basque Country UPV/EHU, 48013 Bilbao, Spain, and also with IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain (e-mail: [email protected]). Digital Object Identifier 10.1109/JSEN.2024.3459623 fiber optics technology. It was soon demonstrated that bending or curvature applied on an optical fiber changes its refractive index profile due to the different mechanical and photograph elastic properties of the core (or cores) and cladding [1]. Consequently, pure bending alters the profile or distribution of the modes guided by an optical fiber and thereby altering its transmission properties [2]. Thus, by analyzing such transmission properties, it is possible to know how much an optical fiber has been bent. In principle, any type of optical fiber or combinations of different fibers can be used to develop a bending sensor, see, for example, [3]. However, in many cases, it is also important to know the bending direction along with (or independent of) other parameters, such as temperature and strain. Ideally, a fiber optic bending sensor must be compact, cost effective, and simple to interrogate. To achieve all these desirable features, the optical fiber sensor community has proposed several configurations. For instance, the changes of mode beating or speckle patterns in multimode fibers can be exploited to develop bending sensor [4],[5],[6],[7]. However, these sensors are affected by temperature; thus, additional sensors or a mechanism to compensate the effect of temperature may be necessary in practical applications. In addition, the multiplexing of multimode fiber bend sensors is complex. Long-period gratings can be used to device compact bending sensors, see, for example, [8],[9],[10],[11]. The disadvantage © 2024 The Authors. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. For more information, see https://creativecommons.org/licenses/by-nc-nd/4.0/ CONTRERAS-TERAN et al.: MULTIPOINT TEMPERATURE-INDEPENDENT VECTOR BENDING SENSING WITH CCFs 36785 of these gratings is their concurrent sensitivity to bending, strain, and temperature. On the other hand, their broad and multidip transmission spectrum complicates their multiplexing. Eccentric Bragg gratings, i.e., gratings that are inscribed off-axis inside the fiber core, offer multiple possibilities for sensing bending and its direction as well as temperature [12], [13],[14],[15],[16],[17]. However, the inscription of eccentric Bragg gratings requires careful positioning of a microscopic optical fiber core with respect to a periodic UV or femtosecond laser pattern. Moreover, eccentric gratings can excite cladding or higher order modes, which may lead to a multipeak reflection or transmission spectrum [12],[14], [16]. In addition, these gratings have concurrent sensitivity to strain, bending, and temperature. This imposes the use of sophisticated data processing to decouple all these parameters. Optical fibers with elliptical cores [18] or with more than one core are a good alternative to device sensors that can distinguish the bending direction as well as the bending degree. The cores can be uncoupled, i.e., they are isolated from each other [9],[10],[19],[20] or can be coupled [21],[22],[23], [24]. The disadvantage of sensors based on fibers with isolated cores includes the use of expensive fan-in/out devices to interrogate each individual core and the necessity to orient the cores with respect to the bending direction. This complicates the practical application of such sensors. Bending sensors built with coupled-core optical fibers are based on supermode interference and are sensitive to temperature and strain [25]. The multiplexing of supermode interferometers, placed in series or in parallel, is not straightforward as their transmission or reflection spectra are multiplied or added up [26], [27],[28]. The limitations and drawbacks of the state-of-the-art fiber optic bending sensors represent opportunities to develop innovative platforms that allow the development of compact, direction-sensitive bending sensors, ideally that are independent of temperature and that can be multiplexed easily. Thus, in this work, we propose a simple structure that is built with a short segment of asymmetric coupled-core fiber (CCF) and conventional fiber Bragg gratings (FBGs). The length of the segment of the asymmetric fiber and the Bragg wavelength of the gratings are chosen to minimize the insertion losses and to simplify the multiplexing of the sensors. It is demonstrated experimentally that the sensing architecture here proposed allows the development of bending sensors that can distinguish the bending direction and amplitude; in addition, they can monitor bending and temperature simultaneously and can be multiplexed easily. Moreover, their interrogation can be carried out with well-established FBG interrogators. We believe that all the distinctive features of our devices may be appealing in a variety of practical applications. II. MATERIAL AND METHODS A CCF with an asymmetric core distribution was designed at the University of the Basque Country (Spain) and fabricated at the Leibniz Institute of Photonic Technology (Jena, Germany). The CCF was designed with three identical photosensitive cores embedded in a pure silica cladding; one core was in the geometrical center of the CCF and other two cores Fig. 1. (a) Cross section of the designed CCF. (b) Photograph of the front view of the preform; the three cores are marked in red. Two SEM images of the fabricated CCF are shown in (c) and (d). were off center. The three cores were arranged in an equilateral triangle [see Fig. 1(a)]. To fabricate such an optical fiber, the modified chemical vapor deposition (MCVD) method was used to prepare a Ge-doped core material. First, the preform with the photosensitive cores was fabricated and drawn to rods of a diameter of 1 mm. Second, a hexagonally stacked preform was arranged consisting of pure silica rods of different diameters surrounding the three photosensitive rods. All the cores were located at the target position by means of the socalled stack-and-draw method [29]. In Fig. 1(b), we show a photograph of the front view of the stacked preform. The three cores are marked in red for visualization purposes. Finally, the preform was inserted into a silica cladding F300 tube and consolidated prior to drawing to final fiber. The fabricated CCF was inspected with a scanning electron microscope (SEM). Fig. 1(c) and (d) shows the images of the bright shining backscattered electrons (BSE) of the Ge-doped cores in the darker SiO2-glass matrix. To visualize the shape of the cores, an etching of the fiber end face was done. The slightly hexagonal shape of the cores and the layer structure of the deposition are visible in Fig. 1(d). All the cores of the CCF were designed to have similar numerical aperture (NA) of a standard single-mode fiber (SMF); 0.13 at 1550 nm. The diameter of the fiber with the photosensitive core and the SiO2-cladding was 125 µm, while the diameter of the standard acrylate coating of the CCF was 250 µm. The average diameter of each core of the fabricated CCF was 8.80 µm (±0.025 µm) and the average centerto-center separation between neighbor cores was 13.21 µm (±0.06 µm). The CCF with the above core arrangement allows to devise highly sensitive sensors that can distinguished the direction of bending independently of the core’s orientation with respect to the bending as previous publications suggest [22],[23]. The asymmetric CCF supports three polarization-degenerate supermodes; however, if light is launched in the central core of the CCF, with a standard SMF, for example, only two supermodes can be excited in the CCF [22]. In Fig. 2, we show a micrograph of the fabricated CCF and the 2-D profiles 36786 IEEE SENSORS JOURNAL, VOL. 24, NO. 22, 15 NOVEMBER 2024 Fig. 2. (a) Micrograph of the cross section of the fabricated CCF. For visualization purposes, the fiber was illuminated laterally with white light. (b) and (c) Two-dimensional profiles of two supermodes excited in the CCF. In both cases, the wavelength was 1550 nm and the simulation area was 55 ×55 µm2. Fig. 3. (a) Sketch of the sensor architecture placed on a temperaturecontrolled plate. FBG is the fiber Bragg grating; Lis the length of the CCF segment. The bending coordinate system is x,y.(b) Diagrammatic representation of the sensors interrogation; S1–S4 mean sensors 1–4, respectively. of the excited supermodes in such fiber. The profiles of the supermodes were analyzed with the finite difference method using the commercial simulation software (Ansys Lumerical MODE) assuming an excitation wavelength of 1550 nm. It can be noted that for both supermodes, the central core of the CCF has a good portion of guided light. The proposed sensing architecture, bending setup, and sensor interrogation are shown schematically in Fig. 3(a) and (b). The structure of the sensor consists of a segment of length Lof the asymmetric CCF fusion spliced with a conventional SMF (SMF-28 from Corning). After the CCF, a commercial FBG was fusion spliced. In other words, the sensing architecture is an SMF–CCF–FBG structure. To cut the segments of CCF with high precision, we used an automatic large diameter fiber cleaver (model CT-105 from Fujikura). Before cleaving the fibers, the protecting polymer coatings were removed; then, the bare fiber segments were cleaned with alcohol. The CCF was fusion spliced with standard SMF with a specialty fiber splicer (model FSM-100P+from Fujikura). The default programs set in the machine were used but with the cladding alignment mode. In this manner, the central core of the CCF and the unique core of the SMF were aligned and in direct physical contact. This splicing process ensures that light is launched in the central core of the CCF. FBGs with different Bragg wavelengths, all with a length of 5 mm and reflectivity circa 40%, were purchased from B-Sens (Belgium). In some experiments, commercial fiber retroreflectors (model P5-SMF28ER-P01-1 from Thorlabs) were used. Such retroreflectors operate in the 1260–1625-nm wavelength range. To characterize and test our devices, we used a broadband super luminescent light-emitting diode (model SLD1005S from Thorlabs) driven by homemade electronics Fig. 4. Maximum of the interference pattern as a function of the length of the CCF observed in three sets of SMF +CCF +SMF structures. The inset graph shows the reflection spectra of the samples built with different lengths of CCF. In all cases, a fiber retroreflector was used to measure the spectra. and an optical spectrum analyzer (model AQ6360 from Yokogawa). We also used a four-channel FBG interrogator (Hyperion Si155 from LUNA Innovations). The wavelength range in which we characterized our devices was from 1480 to 1620 nm. III. RESULTS AND DISCUSSION The proposed sensor architecture was designed to operate in reflection mode; it is sketched in Fig. 3(a). It can be noted that such a sensor architecture is compact and simple to assemble; only two fusion splices are required. The insertion losses of the SMF–CCF–SMF structures (without FBG) were minimal, less than 0.3 dB due to the good match between the NA and dimensions of the CCF and those of the SMF. The role of the CCF length in the performance of the devices was investigated first as it is known that an SMF–CCF–SMF structure is a supermode interferometer [21],[25],[30]. Thus, we fabricated three sets of samples with different lengths of CCF ranging from 9 to 13 mm in steps of 0.5 mm. Each supermode interferometer was characterized individually in reflection mode with the configuration shown in Fig. 3(b); FBGs were not used in this characterization. Instead, we used the commercial fiber retroreflector mentioned above. Light from a broadband source was launched to the interferometers by means of a fiber coupler; the reflected light was analyzed with the spectrum analyzer described in previous paragraphs. The normalized interference patterns of some samples are shown in the inset graph of Fig. 4. The figure also displays the peak wavelength (maximum of the interference pattern) as a function of the length of CCF. It can be concluded from Fig. 4 that for short segments of CCF, the position of the maximum of the interference pattern depends linearly on the CCF length. The samples with longer segments of CCF (L>20 mm) were also fabricated (results not shown). In these cases, the interference patterns exhibited more than one maximum. The SMF–CCF–SMF structure is sensitive to bending but also to temperature and strain as all these parameters perturb CONTRERAS-TERAN et al.: MULTIPOINT TEMPERATURE-INDEPENDENT VECTOR BENDING SENSING WITH CCFs 36787 Fig. 5. Reflection spectra observed when an SMF +CCF +FBG structure (L=11.60 mm) was bent in the +x-direction (top graph) and −x-direction (bottom graph) according to the coordinate system of Fig. 1(d). In all cases, the temperature was 21.5 ◦C. The values in the graphs are the bending angles in degrees. The vertical dotted lines show the wavelength position of the grating. the interfering supermodes of the CCF [21],[30],[31]. Thus, a single interference pattern cannot provide information of all such parameters. In addition, if several SMF–CCF–SMF structures—built with different lengths of CCF—are placed in series or in parallel, the resulting spectra can be the multiplication or sum of the individual spectra [26],[27],[28]. Thus, it would be difficult (if not impossible) to distinguish the effect of bending and temperature on an interferometer when it is multiplexed with others. To solve the above issues, we fusion spliced an FBG with suitable Bragg wavelength (λB)after the segment of CCF. The fiber after the Bragg grating was scissors cut; thus, the final configuration had an SMF–CCF–FBG structure as it is shown schematically in Fig. 3(a). The length of CCF and λBcannot be arbitrary as at certain wavelengths, the reflection spectrum of a supermode interferometer is zero or minimal (see the inset graph of Fig. 4). Thus, the maximum of the interference pattern was made to coincide with λBof the Bragg grating. This requirement does not add complexity to the sensing architecture as the length of CCF and the Bragg wavelength of the grating can be controlled with accuracy. Therefore, we fabricated several SMF–CCF–FBG structures with these conditions, and their performance for sensing bending and temperature was investigated. An SMF–CCF–FBG structure (L=11.60 mm and λB= 1550 nm) was fabricated and placed in cantilever-like position on a temperature-controlled breadboard (model PTC1/M from Thorlabs), as shown schematically in Fig. 3(a). The SMF– CCF–FBG structure was immobilized in the SMF and the CCF–FBG segment was bent with a translation stage. The temperature stability of the breadboard was 0.1 ◦C in the 15 ◦C–45 ◦C range. The CCF–FBG segment was bent in a plane parallel to the breadboard in the +xand −xdirections according to the coordinate system shown in Fig. 3(a). The orientation of the cores of the CCF with respect to the direction of bending was arbitrary because it is not crucial as reported Fig. 6. (a) Reflection spectra of an SMF +CCF +FBG structure (L=11.60 mm) when it was bent in the +x-direction at different temperatures. (b) Corresponding reflection spectra of the same structure when it was bent in the −x-direction. In each temperature, the steps of the bending angles were as those described in Fig. 5. in a previous publication [23]. The results of our experiments at constant temperature (21.5 ◦C) are shown in Fig. 5. The reflection of the SMF–CCF–FBG structure exhibited a single peak whose wavelength position can be denoted as λB. Note from Fig. 5that the height of the reflection peak increases or decreases depending on the direction of bending but its wavelength position is not altered. This means that at constant temperature, only the reflectivity of the FBG changes in proportion to the bending applied to the SMF–CCF–FBG structure. The results shown in Fig. 5can be explained as follows. In a bent CCF with asymmetric core arrangement, the central core and the off-center cores experience different stress [22], [23],[32]. As a consequence, the profile and the propagation constants of the interfering supermodes change. These changes give rise to a shift of the interference pattern. Hence, the amount of light that reaches and reflects from the FBG after the CCF changes drastically. As our CCF and FBGs are sensitive to temperature, we carried out bending experiments at different temperatures. Our 36788 IEEE SENSORS JOURNAL, VOL. 24, NO. 22, 15 NOVEMBER 2024 Fig. 7. Calibration curves obtained from the spectra shown in Fig. 6. The graph shows the normalized reflection of the SMF +CCF +FBG structure as a function of the bending angle at different temperatures. The inset plot shows the wavelength position of the FBG as a function of temperature. results are summarized in Fig. 6. The measured range, steps, and directions of bending at each temperature were the same as that shown in Fig. 5. The graphs displayed in Fig. 6(a) and (b) show that the reflectivity was well as the position of λB changes. The changes of reflectivity of the SMF–CCF–FBG structure relative to the unbent one as a function of the bending angle are shown in Fig. 7. The bending sensitivities in the +x and −xwere found to be 0.1326/◦and 0.1382/◦, respectively. The figure also shows the position of λBas a function of temperature. The temperature sensitivity was found to be 8.6 pm/◦C. The results shown in Fig. 7suggest that by monitoring the evolution of a single reflection peak of an SMF–CCF–FBG structure, it is possible to know the bending degree, the direction of bending, as well as temperature. This is possible because the reflectivity of the structure is not affected by temperature. The results shown in Fig. 7also suggest that by monitoring the reflectivity of an SMF–CCF–FBG structure, it seems feasible to carry out temperature-independent bending sensing. Based on the features of the SMF–CCF–FBG structures discussed above, we investigated their multiplexing. To this end, we fabricated four devices with the following lengths of CCF: 10.8, 11.60, 12.5, and 16 mm. The maximum of the interference patterns of such samples (before splicing the FBGs) were located at 1570, 1550, 1535, and 1525 nm, respectively. Thus, four Bragg gratings with λBthat coincided with such maxima were used. The four samples were then placed in parallel and interrogated with the configuration shown in Fig. 3(b). We induced bending in two devices denoted as S1 (λB=1525 nm) and S2 (λB=1535 nm) in the +xand −x directions and the other two devices (denoted as S3 and S4) were not bent. The experiments were carried out at constant temperature. The results of our experiments are summarized in Figs. 8and 9.Fig. 8shows that the reflectivity of the devices S1 and S2 that were subjected to bending increased Fig. 8. Reflection spectra observed when four devices were set in parallel and interrogated, as shown in Fig. 3(b). Bending was applied on sensors S1 and S2 (λB=1524.68 nm and λB=1534.62 nm) and the others (S3 and S4) were not bent. The values shown in the frame are bending angles in degrees. Fig. 9. Reflection changes for different bending angles observed in four SMF +CCF +FBG structures set in parallel when bending in the +xand −x-directions, according to Fig. 1(d), was applied to two of them. The reflection changes are relative to unbent structures. The solid lines are linear fittings to the data. The crosses are error bars in all cases. The measurements were carried out at room temperature. or decreased in proportion to the bending degree, but the reflectivity of the other devices (S3 and S4) was unaltered. Note also that the wavelength positions of the reflection of the four samples did not change when two SMF–CCF–FBG structures were subjected to bending. The calibration curve obtained with the spectra shown in Fig. 8is displayed in Fig. 9. It can be noted that one device subjected to bending has no effect on the others. Thus, it can be concluded that multipoint bending sensing with SMF–CCF–FBG structures interrogated with commercially available instrumentation is straightforward. The advantage in this case is that a conventional FBG interrogator can be used. Minor modifications in the software of such instruments can be required to track simultaneously the wavelength position and the relative reflectivity of narrowband and well-defined reflection peaks. CONTRERAS-TERAN et al.: MULTIPOINT TEMPERATURE-INDEPENDENT VECTOR BENDING SENSING WITH CCFs 36789 IV. CONCLUSION In this work, we have reported on a simple sensing platform composed by a segment of a few millimeters of asymmetric coupled-core optical fiber fusion spliced to conventional SMF and an FBG. The sensing platform may allow the development of bending sensors that can have the following practical assets: capability of monitoring the bending degree along with temperature (or independent of it) as well as the direction of bending. Moreover, multipoint bending sensing is feasible; additionally, the interrogation of the sensors can be carried out with commercially available FBG read out units. The performance of the devices reported here is due to the core arrangement of the CCF as well as on the selection of its length and the Bragg wavelength of the gratings. The optical properties of a bent CCF make the reflectivity of a Bragg grating located after such fiber change in proportion of the bending angle and direction. 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Villatoro et al., “Accurate strain sensing based on super-mode interference in strongly coupled multi-core optical fibres,” Sci. Rep., vol. 7, no. 1, Jun. 2017, Art. no. 4451. [31] J. E. Antonio-Lopez, Z. S. Eznaveh, and P. LiKamWa, “Multicore fiber sensor for high-temperature applications up to 1000 ◦C,” Opt. Lett., vol. 39, no. 15, pp. 4309–4312, 2014. [32] J. A. Flores-Bravo, J. Madrigal, J. Zubia, S. Sales, and J. Villatoro, “Coupled-core fiber Bragg gratings for low-cost sensing,” Sci. Rep., vol. 12, no. 1, Jan. 2022, Art. no. 1280. Marco A. Contreras-Teran received the M.Eng. degree from the University of Guanajuato, Salamanca, Mexico, in 2021, where he is currently pursuing the Ph.D. degree with the Engineering Division campus Irapuato-Salamanca (DICIS). His research focuses on the development of interferometric sensors and optical fiber lasers for applications in industrial, biosensing, and real-life scenarios. 36790 IEEE SENSORS JOURNAL, VOL. 24, NO. 22, 15 NOVEMBER 2024 Amaia Berganza received the M.Sc. degree in telecommunications engineering and the Ph.D. degree from the University of the Basque Country UPV/EHU, Bilbao, Spain, in 2004 and 2014, respectively. From that year to mid-2007, she worked with Gamesa Eólica and at GFI Norte, Zamudio, Spain, participating in different engineering projects. From 2007 to 2011, she had a Ph.D. Fellowship at the Applied Photonics Group (APG), Department of Electronics and Telecommunications, ETSI of Bilbao. As of 2011, she began to teach at the Department of Applied Mathematics, School of Engineering of Bilbao, while continuing with her research work at the APG Group. She has participated in numerous research projects, workshops, and national and international conferences. She has published numerous research papers. Besides, she has regularly taken part in various scientific dissemination activities. Her current scientific interests include modeling, simulation, and characterization of optical fibers and their use in different applications. Florian Lindner received the Ph.D. degree from the Friedrich-Schiller University of Jena, Jena, Germany, in 2016, with a focus on the modified chemical vapor deposition (MCVD) technology. He has been an Engineer of Material Science with the Department of Fiber Optics, Leibniz Institute of Photonic Technology Jena (IPHT), Jena, Germany, since 2008. From 2004 to 2011, he studied material science at the Friedrich-Schiller University Jena. He is engaged in the development of materials and gas phase deposition methods for the preparation of specialty optical fibers. His current research focuses on the incorporation of RE and other elements in silica glass with different doping and preparation methods for the realization of special fibers design. Jörg Bierlich received the Ph.D. degree from the University of Technology “Bergakademie” Freiberg, Freiberg, Germany, in 2008, focused on superconducting ceramic composites. He worked as a Graduate Engineer at the Department of Magnetics, Leibniz-IPHT, Jena, Germany, from 2001 to 2008. Since 2008, he has been working at the Department of Fiber Optics, Leibniz-IPHT, in the field of Fiber Drawing Technologies and the Development of Special Optical Fibers. Since 2021, he has been engaged in the preparation and characterization of microstructured fibers based on high silica and other glass materials at the Competence Center for Specialty Optical Fibers, Leibniz-IPHT. Katrin Wondraczek received the Ph.D. degree from the Clausthal University of Technology, Clausthal-Zellerfeld, Germany, in 2005. She has been working at the Corning European Research Center (CEEC) in Fontainebleau/France and Areva NP GmbH, Erlangen/DE prior joining Leibniz IPHT, Jena, Germany, since 2012, and starting research on optical fiber fabrication. Now, she is the Head of the Work Group “Optical Fiber Materials and Structures” focusing on the synthesis of high-purity, high-performance materials for specialty optical fibers. Joel Villatoro (Senior Member, IEEE) received the M.Sc. and Ph.D. degrees in optics from the INAOE-Instituto Nacional de Astrofísica, Óptica y Electrónica, Puebla, Mexico, in 1995 and 1999, respectively. He is currently an Ikerbasque Research Professor with the Faculty of Engineering of Bilbao, University of the Basque Country UPV/EHU, Bilbao, Spain. He has nine international patents and more than 93 journal and 74 conference papers to his credit. He has been SPIE and an OPTICA fellow, since 2020. His research interests include the development of advanced plasmonic and interferometric sensors based on conventional and specialty optical fibers, applications of such sensors in real-life scenarios, and development of optical sensors based on nanomaterials for applications in the biomedical, environment, and energy sectors.