Temperature and vector bending sensing with a supermode fiber Bragg grating

PAPER • OPEN ACCESS Temperature and vector bending sensing with a supermode fiber Bragg grating To cite this article: J Villatoro et al 2026 J. Phys. Photonics 8 015015 View the article online for updates and enhancements. You may also like Correlating the viscoelasticity of breast cancer cells with their malignancy Yasaman Nematbakhsh, Kuin Tian Pang and Chwee Teck Lim - Temperature and mixing effects on electrical resistivity of carbon fiber enhanced concrete Christiana Chang, Gangbing Song, Di Gao et al. - Switchable dual-wavelength Ho-doped fiber laser using thin core fiber filter operating at the extended 2 m wavelength region Harith Ahmad, Khalil Kamaruzzaman and Muhamad Zharif Samion - This content was downloaded from IP address 82.130.128.46 on 27/11/2025 at 11:27 J. Phys. Photonics 8(2026) 015015 https://doi.org/10.1088/2515-7647/ae2170 Journal of Physics: Photonics OPEN ACCESS RECEIVED 29 July 2025 REVISED 6 November 2025 ACCEPTED FOR PUBLICATION 19 November 2025 PUBLISHED 27 November 2025 Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. PAPER Temperature and vector bending sensing with a supermode fiber Bragg grating J Villatoro1,2,∗, M Alonso-Murias3, D Maldonado-Hurtado4, A Zornoza5, F Lindner6, J Bierlich6, S Sales4and K Wondraczek6 1Departamento de Ingeniería de Comunicaciones, University of the Basque Country UPV/EHU, 48013 Bilbao, Spain 2IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain 3Centro de Investigaciones en ´ Optica A. C., Loma del Bosque 115, Lomas del Campestre, León 37150 Guanajuato, Mexico 4Photonics Research Labs, iTEAM Research Institute, Universitat Politècnica de València, Camino de Vera s/n, 46022 Valencia, Spain 5Department of Applied Mathematics, University of the Basque Country UPV/EHU, 48013 Bilbao, Spain 6Leibniz Institute of Photonic Technology (Leibniz IPHT), 07745 Jena, Germany ∗Author to whom any correspondence should be addressed. E-mail: agustinjoel.vil[email protected]us Keywords: coupled-core fiber, multicore fibers, fiber Bragg gratings, optical fiber sensors, supermode interference, bending sensors, temperature sensors Abstract Due to their multiple advantages, single-mode fiber Bragg gratings (FBGs) are widely used in a myriad of practical sensing applications. However, their concurrent sensitivity to strain and temperature makes a reference sensor necessary in several situations. Here, we demonstrate that a single supermode FBG can be used to measure bending and temperature. A specially designed two coupled-core optical fiber (TCCF) that supports two supermodes was fabricated in which gratings were inscribed with femtosecond or ultraviolet lasers. The interrogation of the gratings was carried out with a conventional FBG sensor interrogator. It was found that the reflection spectrum of the supermode grating depended on its inscription in the TCCF. So, it was possible to fabricate samples in which the reflection spectra exhibited two narrow peaks very close to each other with well-defined Bragg wavelengths and reflectivities. Cross sensitivities and polarization effects on the devices were studied. It was found that two Bragg wavelengths and two reflectivities provided abundant data to measure bending of the TCCF, temperature, and to discriminate in which direction the TCCF was bent. Thus, we believe that the results reported here can pave the way for next-generation grating-based specialty optical fiber devices that are capable of multiparameter sensing. The results and approaches proposed here can also expand the use of Bragg grating technology. 1. Introduction Sensors based on single-mode fiber Bragg gratings (FBGs) have distinctive advantages which include reliability, absolute (wavelength encoded) measurements, immunity to electromagnetic interference, and multiplexing capability, among others [1–4]. Such sensors are commercially available and widely used in a myriad of practical applications. An important disadvantage of conventional single-mode FBG sensors is their concurrent sensitivity to temperature and strain. Thus, in several situations a reference sensor (an auxiliary Bragg grating) or a mechanism to compensate for temperature or the effect of environmental disturbances, may be necessary. However, additional sensors or temperature compensation mechanisms may be impractical in several applications where there are limitations in space, size, or weight. To circumvent the issues mentioned above, the optical fiber sensor community has long been striving to achieve multiparameter sensing with a single device. Ideally, such a device must be able to provide simultaneous information about a target parameter, independently or along with other environmental parameters, mainly temperature. To this end, different complex sensing architectures or combinations of © 2025 The Author(s). Published by IOP Publishing Ltd J. Phys. Photonics 8(2026) 015015 J Villatoro et al different platforms have been proposed, see for example [5–7]. The disadvantage of such multiparameter sensing platforms is their elaborate fabrication process, which may compromise sensor reproducibility and reliability. An alternative to achieve multiparameter sensing is by means of Bragg gratings inscribed with a tilt angle in single-mode fibers (SMFs) [8–10]. However, the interrogation of tilted gratings is complex and expensive as it entails polarization controllers and high spectral resolution over a broadband, at least 60 nm [11–13]. It is therefore complicated to interrogate more than one tilted grating if they are placed in series. Bragg gratings inscribed in birefringent (polarization maintaining) optical fibers can be used to simultaneously monitor strain and temperature as demonstrated in [14]. Another option to devising multiparameter sensors is to inscribe gratings in multimode fiber (MMF) structures. MMF Bragg gratings have been studied theoretically and experimentally for more than two decades, see for example [15, 16]. The reflection of MMF Bragg gratings features several broad reflection peaks that can be several nanometers apart. Such peaks can be tracked and correlated with more than one parameter [17,18]. Nevertheless, the difficulty in controlling the number and type of modes that can be excited in a MMF, along with the instability of the grating due to mode beating complicates the analysis of MMF Bragg gratings. Herein, we report on the inscription of Bragg gratings in an optical fiber comprising two identical photosensitive cores separated by approximately 14 µm. The two coupled core fiber (TCCF) was designed to allow optical coupling between the cores. It was observed that depending on the inscription of the grating, its reflection spectrum exhibited two narrow peaks whose wavelength position and reflectivity could be correlated with temperature and bending. The effect of polarization and cross sensitivities on the devices were studied. It was found that with a single supermode FBG, it was possible to monitor TCCF bends in terms of both bending direction and bending angle as well as temperature. The main advantage is that the interrogation of our multiparameter sensing device can be carried out with a commercial FBG read out unit. Thus, the sensors based on supermode FBGs proposed here can be used in several practical applications. 2. Results and discussion 2.1. Design and fabrication of the two coupled-core fiber The TCCF consists of two ultraviolet (UV) sensitive cores embedded in a silica matrix, where the offcenter-core is located at 14 µm from the central core. Each core of the TCCF had a numerical aperture like that of a conventional SMF, which is 0.14 at 1550 nm. Photosensitivity was obtained using GeO2in silica. The UV sensitive GeO2-doped silica cores were fabricated using the modified chemical vapor deposition method. In total, 26 GeO2-doped silica layers were deposited inside a F300 silica tube of outer diameter of 22 mm, yielding a consolidated GeO2doped core with a core-to-clad diameter ratio (CCDR) of 2.4; i.e. 5 mm core and 12 mm preform diameter. The obtained preform was then ground and polished to achieve a CCDR of 1.6. The ground preform was then elongated to 1 mm rods. Two rods were then stacked together with 1 mm-diameter silica rods as schematically depicted in figure 1(a) and inserted into a silica cladding tube (F300) to final fiber preform. By using the so-called stack-and-draw method [19], the doped cores were located at the target position in the final TCCF, figure 1(b). A sample of the fabricated TCCF was etched to observe the shape of the cores; such a sample was inspected with a scanning electron microscope (SEM). In figure 1(c), it can be observed that the cores are not perfectly circular. In the figure, the multi-layer structure of the GeO2-doped cores is visible. The cladding of the fiber was made of pure silica and had a diameter of 125 µm, this means, the same diameter and material of a conventional SMF. The TCCF shown in figure 1(b) supports only two supermodes. The profile of one of them is shown in figure 2(a). The effect on the supermode intensity profile when the TCCF was subjected to a curvature of 1 ×10−5m−1is shown in figure 2(b). The results shown in the figure were calculated at 1550 nm with commercial simulation software (MODE from Lumerical). From figure 2(b), it can be noticed that the intensity in the cores of the TCCF depends on the direction of curvature. 2.2. Inscription of Bragg gratings in the TCCF The inscription of Bragg gratings in the TCCF was carried out by means of two well-established techniques. The first technique involved a femtosecond (fs) laser equipped with a frequency-doubled Ybbased laser that emits at 515 nm and operates at a pulse rate of 1 kHz. This laser was focused through an oil immersion objective (Plan-Apochromat 63x/1.4 Oil M27) to create periodic variations in the 2 J. Phys. Photonics 8(2026) 015015 J Villatoro et al Figure 1. (a) Photograph of hexagonal stack of the preform with silica rods (white rods) and the two photosensitive rods (highlighted in red). (b) Microscopic image of a fabricated polymer-coated coupled-core fiber. (c) SEM image of cross section of etched TCCF end for visualizing the GeO2-doped cores. (d) Sketch of an SMF-TCCF +FBG structure that was subjected to bending angles (θ) in the +xand −xdirections. Lis the length of the segment that was bent. Figure 2. (a) Profile of a supermode supported by the TCCF. (b) Effect of curvature of 1 ×10−5m−1on the supermode profile. All calculations were done at 1550 nm with commercially available simulations software. refractive index within the core of the TCCF. This process was achieved using a point-by-point inscription technique as reported in [20,21]. The second technique involved a UV laser with a wavelength of 193 nm and a phase mask [22,23]. Before the inscription of the gratings, approximately 4 cm of TCCF were fusion spliced to a lead-in and lead-out segments of SMF. The splicing of the SMF and the TCCF was by means of the clad alignment method with a fusion splicer (model FSM-100P+from Fujikura). Thus, the central core of the TCCF and the unique core of the SMF were axially aligned. Under these conditions, the two supermodes of the TCCF were excited by the fundamental mode of the SMF. This excitation is quite efficient as the numerical apertures of both waveguides are the same. The polymer coating of the TCCF was totally removed with acetone. The Bragg gratings were inscribed approximately in the middle of the TCCF segment. Due to the flexibility of the inscription with fs lasers, gratings with different lengths were inscribed in the TCCF. In addition, single gratings were inscribed in the central or the off-center core. Figures 3(a) and (b) summarize our results. It can be noticed from these figures that the reflection spectrum of a short fs grating is broader than that of a long one. Note also that when the grating is inscribed only in the off-center core, two Bragg wavelengths appear. Another collection of gratings, all with the same length, were inscribed in the TCCF with UV laser and the phase mask technique. Figures 3(c) and (d) display the reflection spectra of six samples. We observed reflection patterns with two Bragg wavelengths, separated approximately 190 pm, in 60% of the fabricated samples and one Bragg wavelength in the rest of samples. This can be attributed to the exposure of the two photosensitive cores to the UV laser pattern. Thus, to ensure reflection spectra with two Bragg wavelengths it is important that the two photosensitive cores are equally exposed to the UV laser pattern. This can be achieved by orienting the TCCF before the gratings are inscribed. However, as the fabrication of Bragg gratings can be monitored in real time, samples that do not have two Bragg wavelengths can be discarded for dual parameter sensing applications for the reasons discussed in the following sections. 2.3. Bending and temperature sensing A simple set up was implemented to carry out the bending experiments. The TCCF with Bragg grating was cleaved with an angle to avoid Fresnel reflection. The SMF-TCCF+FBG structure was then placed in cantilever position, see figure 1(d). To secure the structure we used a high precision fiber rotator (HFR007, from Thorlabs). The distance from the point where the structure was immobilized to 3 J. Phys. Photonics 8(2026) 015015 J Villatoro et al Figure 3. Spectra of Bragg gratings inscribed in the TCCF with a femtosecond laser in the central core (a) and off-center core (b). The grating lengths are indicated. (c) and (d) Spectra of samples of 6 mm-long gratings inscribed in the TCCF with a UV laser and phase mask. the point where it was bent, Lin figure 1(d), was 15.30 ±0.01 mm. The bending angle of the SMFTCCF +FBG segment was varied by displacing it in steps of 100 ±1 micrometers in the +xand −x directions, according to the coordinate system shown in figure 1(d). The reflection spectrum at each bending angle was monitored with a commercial FBG interrogator (Hyperion Si155 from LUNA Innovations) connected to a laptop. An ad hoc LabVIEW program was implemented to monitor the Bragg wavelengths (λB) and their reflectivities. The FBG interrogator and laptop were configured in a local area network via TCP-IP protocol to communicate with each other. The reflection spectrum of the SMF-TCCF +FBG structure was displayed in linear scale. The Bragg wavelengths positions and their reflectivities were found by applying the Peak Detector VI from LabVIEW. Figures 4(a) and (b) display, respectively, the reflection spectra observed when the TCCF was bent at different angles in the +xand −xdirections. Note that the positions of λB1 and λB2 do not change when the TCCF is bent; see the vertical dotted lines in figures 4(a) and (b). However, the reflectivity of the two peaks changes in opposite manner. Figure 4(a) shows that R1(reflectivity of peak located at λB1) increases with the bending angle but R2(reflectivity of peak located at λB2) decreases. If the direction of bending changes, R1decreases and R2increases, see figure 4(b). This means that the TCCF bending angle and direction can be correlated with R1and R2, which are simple to monitor. To compensate for possible fluctuations of the light source, we correlated the bending angle with the ratio of R2/R1. The initial orientation of the TCCF cores with respect to the bending direction was arbitrary. This initial position was considered to be 0◦. From this position, the TCCF was then rotated 30◦, 60◦, and 90◦and was bent in +xand −xdirections with the procedure described above. In all cases, we collected the reflection spectra and measured R2and R1. The calibration curves that were obtained for different orientations of the TCCF cores with respect to the direction of the bending are displayed in figure 5. We observed that at larger angles, the behavior of the sensor was nonlinear. From figure 5the bending sensitivities were calculated. The maximum sensitivities in the +xand −x directions were found to be, respectively, 0.23 and −0.279 a.u. per degree. These sensitivities are ∼500% higher than those published in [24]. From the figure, it can be noted that independently of the TCCF core orientation with respect to the direction of bending, from the ratio R2/R1it is possible to quantify the bending degree and to know if the TCCF was bent to the +xor to the −xdirection. Note that in previous work [24], this was not possible. In addition, in [24], light fluctuations could not be compensated for as reflectivity changes of a single peak were monitored. Moreover, in our case, the positions of λB1 and λB2 can be used to monitor temperature with high accuracy. Thus, sensing of temperature and directional bending is feasible with a single supermode grating. To investigate the effect of temperature on the supermode Bragg grating, the SMF-TCCF+FBG structure was placed in a temperature calibrator (Fluke model 9103). Note that for real-world bending measurements, the TCCF must be coated, protected or packaged. So, the analyzed temperature range was from −20 ◦C to +90 ◦C, which covers a wide range of environments and the temperature operating range of polymeric coatings of optical fibers. The observed spectra and calibration curve are shown in 4 J. Phys. Photonics 8(2026) 015015 J Villatoro et al Figure 4. Experimental reflection spectra observed when the TCCF fiber was bent different angles in the +xdirection (a) and the −xdirection (b). In both graphs, λB1,λB2,R1, and R2denote, respectively, Bragg wavelengths and reflectivity of the peaks. Figure 5. Calibration curves observed when the cores of the TCCF were placed at different orientations with respect to the xdirection (the direction of bending). The initial orientation of the TCCF cores was denoted as 0◦. figure 6. It can be noted from the figure that the positions of the two Bragg wavelengths changed linearly with temperature. The temperature sensitivity, calculated with either of the two Bragg wavelengths, was found to be 9.9 pm ◦C−1. The ratio R2/R1as a function of temperature is also shown in the bottom inset graph of figure 6. It was observed that the value R2/R1changed approximately ±1.3% in the measured temperature range. This value is much lower than those observed when the TCCF was bent, which ranges from 50% to 75%. Thus, temperature-independent bending or dual parameter sensors can be developed with the structure reported here. In either case, the sensor interrogation can be carried out with widely available commercial instruments. 5 J. Phys. Photonics 8(2026) 015015 J Villatoro et al Figure 6. Bragg wavelengths as a function of temperature. The top inset graph shows the observed spectra at different temperatures, and the bottom inset graph shows the R2/R1ratio versus temperature. Figure 7. Reflection spectra of a supermode Bragg grating at 40 different states of polarization (SOP) of the input light. The inset graph shows the normalized R2/R1ratio observed at the 40 SOP. Guide modes, and supermodes in particular, are, in general, sensitive to polarization, so, the performance of a sensing device based supermodes may be affected by changes of polarization. Thus, the effect of polarization on the reflection spectra of the supermode gratings and on R2,R1,λB1 and λB2 was investigated. To do so, an electronic fiber polarization controller (model PC-15-1-1-2, from Phoenix Photonics) that operates from 1300 to 1640 nm was used in the experiments. The controller allowed us to launch any state of polarization (SOP) to the supermode Bragg grating. Our findings after launching 40 different SOP to the device are shown in figure 7. Note that although the two maxima and the minimum (dip) between them change, the R2/R1ratio varies less than 2%, see the inset graph of figure 7. The changes of λB1 and λB2 that were observed were less than 6 pm. Therefore, we can conclude that the supermode Bragg grating proposed here can operate with any SOP of the input light. The operation mechanism of supermode Bragg gratings has been discussed in different publications, see for example [24–27]. The condition of Bragg reflection indicates that specific wavelengths are reflected and that the intensity of the Bragg wavelengths depends on the coupling coefficient of the forward and backward propagating supermodes. In general, two Bragg wavelengths (λB1 and λB2) can be expected. However, our experimental results suggest that it depends on the inscription of the Bragg grating in the TCCF. Previous works have demonstrated that coupled-core fibers are highly sensitive to bending [24,28]. The latter induces stress to the fiber cores and alters their refractive indices [29]. Consequently, the field intensities of the supermodes are modified, thus leading to energy transfer between them. However, bending has no effect on the Bragg wavelengths. For these reasons, a single supermode Bragg grating can be used to monitor two parameters: bending and temperature with minimal or negligible cross sensitivity. 6 J. Phys. Photonics 8(2026) 015015 J Villatoro et al 3. Conclusions In this work, we have reported on the fabrication of an optical fiber comprising two identical photosensitive cores separated approximately 14 micrometers and the inscription of Bragg grating in one or in both cores. It was demonstrated that a single supermode FBG that has the potential to monitor bending angle and direction as well as temperature. The interaction of the gratings with two supermodes supported by the two coupled-core fiber makes the reflection spectrum have two well-defined peaks that are close to each other. It was observed that bending of the TCCF changed only the reflectivity of the two peaks and temperature altered only the Bragg wavelengths. The effect of polarization on the reflection spectra of the gratings was studied. Our results suggest that polarization has no significant effect on the performance of our sensor. Interestingly, the two Bragg wavelengths and the two values of reflectivity provide abundant data to measure temperature and bending degree of the TCCF as well as to discriminate in which direction the TCCF was bent. These features were observed at different orientations of the cores with respect to the direction of bending. Therefore, the position of the TCCF is not relevant. As the reflection of supermode Bragg gratings has two narrow peaks that are separated less than 200 pm, their multiplexing is straightforward. Any commercially available FBG read out unit can be used to interrogate the gratings. Therefore, the supermode FBGs reported here can be attractive in applications where it is important to monitor bending degree and temperature and to discriminate the direction of bending of the optical fiber with a single compact device. Data availability statement The data that support the findings of this study are available upon reasonable request from the authors. Acknowledgments This work was funded in part through Projects PDC2022-133885-I00; PID2023-152763NB-I00; INNOFIBER PID2023-152314OB-I00, and SMARTMOORING Cetp-2022-00382, PCI2023-145987-2 funded by the Spanish MCIU/AEI /10.13039/501100011033 and the European Union Next Generation EU/PRTR, IT11452-22 funded by the Basque Government and P2022-06 funded by the EU Horizon2020 Actphast4R GA 825051. It is also partially supported by the Cátedra Chip Fotónico UPV TSI-0691002023-0002, and PROMETEO 2021/015 Research Excellency Award funded by the Generalitat Valenciana. M. Alonso-Murias acknowledges the support received from SECIHTI through the National Postdoctoral Stays Program (Grant No. 787628). The authors acknowledge Shrabya Shukla for her assistance in some experiments. Author contributions J Villatoro 0000-0002-0842-9173 Conceptualization (equal), Formal analysis (equal), Investigation (equal), Supervision (equal), Writing – review & editing (equal) M Alonso-Murias 0000-0002-5161-9562 Data curation (equal), Formal analysis (equal), Software (equal) D Maldonado-Hurtado Investigation (equal), Methodology (equal) A Zornoza 0000-0003-2127-6789 Data curation (equal), Formal analysis (equal), Methodology (equal), Software (equal) F Lindner Investigation (equal), Methodology (equal), Resources (equal) J Bierlich Formal analysis (equal), Investigation (equal), Methodology (equal) S Sales Funding acquisition (equal), Investigation (equal), Resources (equal) K Wondraczek 0000-0002-6268-9136 Funding acquisition (equal), Project administration (equal), Resources (equal), Supervision (equal) 7 J. Phys. Photonics 8(2026) 015015 J Villatoro et al References [1] Hill K O and Meltz G 1997 Fiber Bragg grating technology fundamentals and overview J. 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