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Experimental demonstration of offset-induced sensitivity enhancement in SMS-based temperature and strain sensing

Experimental demonstration of offset-induced sensitivity enhancement in SMS-based temperature and... Applied Physics Express 16, 052003 (2023) LETTER https://doi.org/10.35848/1882-0786/acd046 Experimental demonstration of offset-induced sensitivity enhancement in SMS-based temperature and strain sensing 1* 2 3 1 1 1 1 Kun Wang , Yosuke Mizuno , Heeyoung Lee , Xingchen Dong , Wolfgang Kurz , Maximilian Fink , Martin Jakobi , and Alexander W. Koch Institute for Measurement Systems and Sensor Technology, TUM School of Computation, Information and Technology, Technical University of Munich, Theresienstraße 90, Munich D-80333, Germany Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan College of Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan E-mail: kun88.wang@tum.de Received February 20, 2023; revised March 31, 2023; accepted April 24, 2023; published online May 11, 2023 A simple, inexpensive, and high-sensitivity temperature and strain sensor based on a single-mode–multimode–single-mode (SMS) structure with core offset is developed and experimentally characterized. This sensor does not require specialty fibers and can be fabricated using a standard fiber fusion splicer. The dependencies of the temperature and strain sensitivities on the core-offset amplitudes at the input and output single-mode/ multimode fiber boundaries are investigated. The results indicate that the maximum temperature and strain sensitivities are two times and eight times higher than those of the standard SMS structure, respectively. The limit of the sensitivity enhancement by core offset is also revealed. 2023 The Japan Society of Applied Physics Supplementary material for this article is available online ptical fiber sensors based on multimode interference be fabricated using a standard fiber fusion splicer. We have been intensively developed and widely studied experimentally investigate the sensitivity dependencies on O over the last decades because of their advantages like the core-offset amplitudes at the input and output SMF/MMF compact size, uncomplicated fabrication, low cost, and easy boundaries. The results clearly show that the temperature and 1–6) compatibility with other photonic devices and structures. strain sensitivities become higher than those of the aligned The most broadly used simple configuration is a so-called SMS sensor, probably because the core offset promotes the single-mode–multimode–single-mode (SMS) structure, which excitation of higher-order modes in the MMF. However, the means that a multimode fiber (MMF), working as a sensing larger core offset also results in a more vulnerable structure. fiber, is sandwiched between two single-mode fibers (SMFs). The measured results also reveal the limit of the sensitivity Various sensing applications have been developed and re- enhancement by core offset, as the largest core offset does 7,8) 9,10) ported, including temperature sensing, strain sensing, not induce the highest sensitivity. 11,12) 13,14) refractive index (RI) sensing, curvature sensing, The operating principle of conventional SMS sensing is 15,16) 17) humidity sensing, and breath state monitoring. Due to based on the assumption that the MMF and two SMFs are the increased demand for sensing stability and high-sensitivity, perfectly aligned. The injected light is guided from the input several methods have been studied, such as SMS structure SMF into the MMF and propagates along with the MMF. At 18,19) 20,21) based on polished MMFs, tapered MMFs, polymer the first SMF/MMF boundary, the spot-size difference 22,23) 24) optical fibers, and specialty fibers such as microfibers between the fundamental modes in the SMF and MMF 25) and square-core fibers. excites the first few modes in the MMF, which propagate 29) However, these high-sensitivity methods usually suffer with different propagation constants. At the second SMF/ from sophisticated devices, cost, fabrication complexity, or MMF boundary, the net field coupled to the output SMF is the use of specialty fibers. Recently, another structure, i.e. determined by the relative phase differences among the many 26) core-offset structure, has attracted considerable attention. modes guided in the MMF. According to the detailed 30,31) A Mach–Zehnder interferometer based on the core-offset calculation, the sensing principle can be summarized as SMS structure comprising a few-mode fiber has led to a strain 2 2 iL () bb - PA=+ ∣ A e −1 27) out 0 1 sensitivity of 3.35 pm μò within the range of 0–1000 μò. iL () bb - 2 ++ Ae ...∣( , 1) The offset structure also has the advantage of high RI 2 sensitivity; for instance, by core-offset splicing of a triple cladding quartz specialty fiber between two SMFs, the RI where P is the power in the output SMF, A is defined as out i −1 sensitivity of as high as 543.75 nm RIU (refractive index the field amplitude of the ith mode at the first SMF/MMF 28) unit) has been achieved. However, these core-offset boundary, β is the propagation constant of the ith mode, and structures utilized specialty fibers to enhance the sensitivities L is the length of the MMF. and had drawbacks, such as fabrication difficulty, low However, this principle cannot be directly applied to the repeatability, and high costs. Besides, for most core-offset core-offset SMS sensing, where a larger number of higher- structures, the balance between sensitivity and mechanical order modes are excited. It seems reasonable to assume that strength is another issue that needs to be studied. A larger the excitation of higher-order modes may lead to higher core-offset value usually leads to higher sensing sensitivity sensitivities; to verify this assumption, we experimentally but, on the contrary, results in a vulnerable structure. characterize the core-offset SMS structure and investigate the In this work, we exploit a simple, inexpensive, and highly limit of the induced sensitivity enhancement. sensitive core-offset SMS sensor for temperature and strain The schematic diagram of the core-offset SMS structure is sensing. This sensor does not include specialty fibers and can shown in Fig. 1(a). In this experiment, one side of the SMF is 052003-1 2023 The Japan Society of Applied Physics © Appl. Phys. Express 16, 052003 (2023) K. Wang et al. core offset spliced. To find the optimal core-offset SMS core-offset SMS. In addition, the temperature sensitivity structure, different samples are fabricated utilizing standard exhibits a non-monotonic relationship with increasing core SMFs (9/125) and a commonly used 6 cm-long step-index offset, and the highest temperature sensitivity of −1 MMF (50/125) with an inexpensive fiber fusion splicer 13.92 pm °C is achieved at 20 μm core offset. The max- (INNO View 7). The splicing settings are kept constant imal temperature sensitivity is approximately two times the −1 during the fabrication process of all the samples to ensure the value of the aligned SMS sensor (5.83 pm °C , refer to Fig. consistency of the quality of the core offset. As the radius of S4 in Supplementary). the MMF core is 25 μm, samples with core-offset amplitudes Subsequently, the strain sensing is carried out at room of 10, 15, 20, 25, and 30 μm are fabricated (refer to Fig. S1 in temperature (∼20 °C), and the results for the input core-offset Supplementary). An aligned SMS sensor is also prepared in SMS structure are illustrated in Fig. 5. The measured the same condition as a reference. The sensing characteristics dependencies of the spectral dip on strain are presented in of the input core-offset and output core-offset SMS structures Figs. 5(a)–5(c) for the core offsets of 10, 20, and 30 μm, are investigated separately. respectively. The spectral dips shift to shorter wavelengths The experimental setup is illustrated in Fig. 2. The MMF with increasing strain. The corresponding dip wavelength −1 section is heated and clamped on two precision translation dependence on strain leads to coefficients of −0.8 pm μò , −1 −1 stages, which can apply varying axial tensile stress to the −1.19 pm μò , and −0.63 pm μò , as displayed in sample. The applied axial strain can be calculated by Figs. 5(d)–5(f) (The results of the output core-offset SMF are presented in Supplementary). Figure 6 summarizes the DL  =,2 () strain sensitivities with the input and output core-offset SMS structures. It indicates that the strain sensitivities (absolute where L is the initial length and ΔL is the additional change values) with the input core-offset SMS are also higher than in length of the MMF section when longitudinal strain is those with the output core-offset SMS. It also exhibits a non- applied. monotonic relationship with increasing core-offset amplitude, The temperature measurement without applied strain is and the maximal strain sensitivity (absolute value of −1 performed in the range of 30 °C to 70 °C in steps of 5 °C. −1.19 pm μò ) is obtained at the 20 μm core offset (where Examples of the temperature sensing results for the input the highest temperature sensitivity is obtained). Note that SMF with core-offset amplitudes of 10, 20, and 30 μm are 20 μm is the largest offset when the core of the SMF is shown in Fig. 3. The spectral dips exhibit redshifts in the spliced to the core of the MMF; with offsets of >20 μm, the wavelength domain when the temperature increases, as core of the SMF is (partially or totally) spliced to the cladding shown in Figs. 3(a)–3(c). The corresponding resulting of the MMF, which does not promote the excitation of the wavelength shifts of the spectral dips against the increasing higher-order modes in the MMF core. Compared to the strain −1 temperature are fitted and plotted in Figs. 3(d)–3(f). The sensitivity of the aligned SMS sensor (−0.15 pm μò , refer intervals between the curves are not uniform, and the vertical to Fig. S4 in Supplementary), the maximal strain sensitivity power change exhibits non-monotonic behavior against (absolute value) of the core-offset SMS sensor is enhanced by temperature. This can be attributed to non-uniform heating approximately eight times. across the entire sensing section, resulting in a slight A high-sensitivity, easy-to-fabricate, and low-cost SMS reduction in the linearity of the dip wavelength dependences. sensor based on core offset was developed and experimen- Specifically, the coefficients for the dip wavelength depen- tally investigated for temperature and strain sensing. This −1 dences are measured to be 7.63 pm °C (10 μm), sensor is implemented with a commonly used MMF and can −1 −1 13.92 pm °C (20 μm), and 8.48 pm °C (30 μm) (the be fabricated by a commercial splicer. The sensing char- results for the output core-offset SMF are presented in acteristics of the SMS sensors with core offsets of 10, 15, 20, Supplementary). The summary of the temperature sensing 25, and 30 μm at the input and output SMF/MMF boundaries with input and output core-offset SMS structures is illustrated are investigated, and the obtained maximal temperature and −1 −1 in Fig. 4, which shows that the temperature sensitivities with strain sensitivities are 13.92 pm °C and −1.19 pm με , the input core-offset SMS are higher than those with output which are ∼two times and ∼eight times the values of the (a) (b) Fig. 1. (a) Schematic diagram of the core-offset SMS structure with an offset at the input SMF/MMF boundary. (b) Image of the core offset spliced (offset = 20 μm). 052003-2 2023 The Japan Society of Applied Physics © Appl. Phys. Express 16, 052003 (2023) K. Wang et al. Fig. 2. Schematic diagram of the experimental setup for temperature and strain measurement; BLS, broadband light source (wavelength range of 100 nm and central wavelength of 1550 nm); SMF, single-mode fiber; MMF, multimode fiber; OSA, optical spectrum analyzer. (a) (b) (c) (e) (f) (d) Fig. 3. Temperature measurement results for the input core-offset SMS structure. Measured spectral dependencies on temperature with core offsets of (a) 10 μm, (b) 20 μm, and (c) 30 μm. Spectral dip shifts plotted as a function of temperature with core offsets of (d) 10 μm, (e) 20 μm, and (f) 30 μm. Fig. 4. Summary of the temperature measurement with the input and output core-offset SMS structure. 052003-3 2023 The Japan Society of Applied Physics © Appl. Phys. Express 16, 052003 (2023) K. Wang et al. (b) (a) (c) (d) (e) (f) Fig. 5. Strain measurement results for the input core-offset SMS structure. Measured spectral dependencies on strain with core offsets of (a) 10 μm, (b) 20 μm, and (c) 30 μm. Spectral dip shifts plotted as a function of strain with core offsets of (d) 10 μm, (e) 20 μm, and (f) 30 μm. Fig. 6. Summary of the strain measurement with the input and output core-offset SMS structures. aligned SMS sensor, respectively. The results also indicate structure, it may be beneficial to explore the potential of that the maximal sensitivities for both temperature and strain utilizing graded-index MMFs or step-index MMFs with are obtained with the 20 μm core offset, not the largest core larger core diameters in future work. Lastly, with its offset of 30 μm, which indicates the existence of a limit of the simplicity, cost efficiency, and robustness, we believe that offset-induced sensitivity enhancement. In addition, both our method will be a valuable tool for enhancing the temperature and strain sensitivities with the input core-offset sensitivities of SMS temperature and strain sensors in the SMS are higher than those with the output core-offset SMS. future. Note that pull tests of the core-offset SMS structures are Acknowledgments The authors would like to thank the China additionally performed, indicating that they can withstand at Scholarship Council (CSC) (Grant No. 201808340074) for supporting this work. least 1.5% strain regardless of the offset amplitudes. To Y.M. is indebted to the Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant No. 21H04555), and the research grants from the Murata further enhance the performance of the proposed sensor 052003-4 2023 The Japan Society of Applied Physics © Appl. Phys. Express 16, 052003 (2023) K. Wang et al. Science Foundation, the Telecommunications Advancement Foundation, the 15) X. Wang, K. Tian, L. Yuan, E. Lewis, G. Farrell, and P. Wang, J. Light. Takahashi Industrial and Economic Research Foundation, the Yazaki Memorial Technol. 36, 2730 (2018). Foundation for Science and Technology, and the Konica Minolta Science and 16) L. Xia, L. Li, W. Li, T. Kou, and D. Liu, Sensors Actuator A 190, 1 (2013). Technology Foundation. 17) X. Li et al., Meas. Sci. Technol. 28, 035105 (2017). ORCID iDs Kun Wang https://orcid.org/0000-0003-0410- 18) X. Wang, J. Zhang, K. Tian, S. Wang, L. Yuan, E. Lewis, G. Farrell, and 204X Yosuke Mizuno https://orcid.org/0000-0002-3362- P. Wang, Opt. Express 26, 26534 (2018). 4720 Heeyoung Lee https://orcid.org/0000-0003-3179-0386 19) J. Tang, J. Zhou, J. Guan, S. Long, J. Yu, H. Guan, H. Lu, Y. Luo, J. Zhang, and Z. Chen, IEEE J. Sel. Top. Quantum Electron. 23, 238 (2017). 20) P. Wang, G. Brambilla, M. Ding, Y. Semenova, Q. Wu, and G. Farrell, Opt. Lett. 36, 2233 (2011). 1) X. Wang and O. S. Wolfbeis, Anal. Chem. 92, 397 (2020). 21) K. Tian, M. Zhang, G. Farrell, R. Wang, E. Lewis, and P. Wang, Opt. Express 26, 33982 (2018). 2) J. Chen, A. Hangauer, R. Strzoda, M. Fleischer, and M.-C. Amann, Opt. Lett. 35, 3577 (2010). 22) Y. Mizuno, S. Hagiwara, T. Kawa, H. Lee, and K. Nakamura, Jpn. J. Appl. Phys. 57, 058002 (2018). 3) K. Wang, X. Dong, M. H. Köhler, P. Kienle, Q. Bian, M. Jakobi, and A. W. Koch, IEEE Sens. J. 21, 132 (2021). 23) K. Wang, Y. Mizuno, K. Kishizawa, Y. Toyoda, H. Lee, K. Ichige, W. Kurz, X. Dong, M. Jakobi, and A. W. Koch, Jpn. J. Appl. Phys. 61, 118001 4) Q. Wu et al., IEEE Sens. J. 21, 12734 (2021). 5) Y. Zhao, L. Cai, X.-G. Li, and F.-C. Meng, Sensors Actuators B 196, 518 (2022). 24) H. Luo, Q. Sun, Z. Xu, D. Liu, and L. Zhang, Opt. Lett. 39, 4049 (2014). 6) S. Silva, O. Frazão, J. Viegas, L. A. Ferreira, F. M. Araújo, F. X. Malcata, (2014). and J. L. Santos, Meas. Sci. Technol. 22, 085201 (2011). 25) K. Wang, Y. Mizuno, X. Dong, W. Kurz, M. Fink, M. Jakobi, and 7) Y. Liu and L. Wei, Appl. Opt. 46, 2516 (2007). A. W. Koch, Jpn. J. Appl. Phys. 61, 078002 (2022). 8) Q. Wu, A. M. Hatta, P. Wang, Y. Semenova, and G. Farrell, IEEE Photon. 26) H. Niu, S. Zhang, W. Chen, Y. Liu, X. Li, Y. Yan, S. Wang, T. Geng, Technol. Lett. 23, 130 (2011). W. Sun, and L. Yuan, IEEE Sens. J. 21, 22388 (2021). 9) J. Huang, X. Lan, H. Wang, L. Yuan, T. Wei, Z. Gao, and H. Xiao, Opt. 27) B. Wang, W. Zhang, Z. Bai, L. Zhang, T. Yan, L. Chen, and Q. Zhou, IEEE Lett. 37, 4308 (2012). Photon. Technol. Lett. 28, 71 (2016). 10) G. Numata, N. Hayashi, M. Tabaru, Y. Mizuno, and K. Nakamura, IEEE 28) X. Fu, F. Liu, Y. Zhang, J. Wen, D. Wang, H. Xie, G. Fu, and W. Bi, Opt. Fiber Technol. 46, 63 (2018). Photonics J. 6, 1 (2014). 11) L. Yang, L. Xue, D. Che, and J. Qian, Opt. Lett. 37, 587 (2012). 29) W. S. Mohammed, P. W. E. Smith, and X. Gu, Opt. Lett. 31, 2547 (2006). 12) P. Wang, S. Zhang, R. Wang, G. Farrell, M. Zhang, T. Geng, E. Lewis, and K. Tian, Opt. Express 27, 13754 (2019). 30) A. Kumar, R. K. Varshney, S. A. C, and P. Sharma, Opt. Commun. 219, 215 (2003). 13) X. Zhang, C. Liu, J. Liu, and J. Yang, IEEE Sens. J. 18, 8375 (2018). 14) Y. Gong, T. Zhao, Y.-J. Rao, and Y. Wu, IEEE Photon. Technol. Lett. 23, 31) S. M. Tripathi, A. Kumar, R. K. Varshney, Y. B. P. Kumar, E. Marin, and 679 (2011). J.-P. Meunier, J. Lightwave Technol. 27, 2348 (2009). 052003-5 2023 The Japan Society of Applied Physics http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Physics Express IOP Publishing

Experimental demonstration of offset-induced sensitivity enhancement in SMS-based temperature and strain sensing

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© 2023 The Author(s). Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd
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1882-0778
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1882-0786
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10.35848/1882-0786/acd046
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Abstract

Applied Physics Express 16, 052003 (2023) LETTER https://doi.org/10.35848/1882-0786/acd046 Experimental demonstration of offset-induced sensitivity enhancement in SMS-based temperature and strain sensing 1* 2 3 1 1 1 1 Kun Wang , Yosuke Mizuno , Heeyoung Lee , Xingchen Dong , Wolfgang Kurz , Maximilian Fink , Martin Jakobi , and Alexander W. Koch Institute for Measurement Systems and Sensor Technology, TUM School of Computation, Information and Technology, Technical University of Munich, Theresienstraße 90, Munich D-80333, Germany Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan College of Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan E-mail: kun88.wang@tum.de Received February 20, 2023; revised March 31, 2023; accepted April 24, 2023; published online May 11, 2023 A simple, inexpensive, and high-sensitivity temperature and strain sensor based on a single-mode–multimode–single-mode (SMS) structure with core offset is developed and experimentally characterized. This sensor does not require specialty fibers and can be fabricated using a standard fiber fusion splicer. The dependencies of the temperature and strain sensitivities on the core-offset amplitudes at the input and output single-mode/ multimode fiber boundaries are investigated. The results indicate that the maximum temperature and strain sensitivities are two times and eight times higher than those of the standard SMS structure, respectively. The limit of the sensitivity enhancement by core offset is also revealed. 2023 The Japan Society of Applied Physics Supplementary material for this article is available online ptical fiber sensors based on multimode interference be fabricated using a standard fiber fusion splicer. We have been intensively developed and widely studied experimentally investigate the sensitivity dependencies on O over the last decades because of their advantages like the core-offset amplitudes at the input and output SMF/MMF compact size, uncomplicated fabrication, low cost, and easy boundaries. The results clearly show that the temperature and 1–6) compatibility with other photonic devices and structures. strain sensitivities become higher than those of the aligned The most broadly used simple configuration is a so-called SMS sensor, probably because the core offset promotes the single-mode–multimode–single-mode (SMS) structure, which excitation of higher-order modes in the MMF. However, the means that a multimode fiber (MMF), working as a sensing larger core offset also results in a more vulnerable structure. fiber, is sandwiched between two single-mode fibers (SMFs). The measured results also reveal the limit of the sensitivity Various sensing applications have been developed and re- enhancement by core offset, as the largest core offset does 7,8) 9,10) ported, including temperature sensing, strain sensing, not induce the highest sensitivity. 11,12) 13,14) refractive index (RI) sensing, curvature sensing, The operating principle of conventional SMS sensing is 15,16) 17) humidity sensing, and breath state monitoring. Due to based on the assumption that the MMF and two SMFs are the increased demand for sensing stability and high-sensitivity, perfectly aligned. The injected light is guided from the input several methods have been studied, such as SMS structure SMF into the MMF and propagates along with the MMF. At 18,19) 20,21) based on polished MMFs, tapered MMFs, polymer the first SMF/MMF boundary, the spot-size difference 22,23) 24) optical fibers, and specialty fibers such as microfibers between the fundamental modes in the SMF and MMF 25) and square-core fibers. excites the first few modes in the MMF, which propagate 29) However, these high-sensitivity methods usually suffer with different propagation constants. At the second SMF/ from sophisticated devices, cost, fabrication complexity, or MMF boundary, the net field coupled to the output SMF is the use of specialty fibers. Recently, another structure, i.e. determined by the relative phase differences among the many 26) core-offset structure, has attracted considerable attention. modes guided in the MMF. According to the detailed 30,31) A Mach–Zehnder interferometer based on the core-offset calculation, the sensing principle can be summarized as SMS structure comprising a few-mode fiber has led to a strain 2 2 iL () bb - PA=+ ∣ A e −1 27) out 0 1 sensitivity of 3.35 pm μò within the range of 0–1000 μò. iL () bb - 2 ++ Ae ...∣( , 1) The offset structure also has the advantage of high RI 2 sensitivity; for instance, by core-offset splicing of a triple cladding quartz specialty fiber between two SMFs, the RI where P is the power in the output SMF, A is defined as out i −1 sensitivity of as high as 543.75 nm RIU (refractive index the field amplitude of the ith mode at the first SMF/MMF 28) unit) has been achieved. However, these core-offset boundary, β is the propagation constant of the ith mode, and structures utilized specialty fibers to enhance the sensitivities L is the length of the MMF. and had drawbacks, such as fabrication difficulty, low However, this principle cannot be directly applied to the repeatability, and high costs. Besides, for most core-offset core-offset SMS sensing, where a larger number of higher- structures, the balance between sensitivity and mechanical order modes are excited. It seems reasonable to assume that strength is another issue that needs to be studied. A larger the excitation of higher-order modes may lead to higher core-offset value usually leads to higher sensing sensitivity sensitivities; to verify this assumption, we experimentally but, on the contrary, results in a vulnerable structure. characterize the core-offset SMS structure and investigate the In this work, we exploit a simple, inexpensive, and highly limit of the induced sensitivity enhancement. sensitive core-offset SMS sensor for temperature and strain The schematic diagram of the core-offset SMS structure is sensing. This sensor does not include specialty fibers and can shown in Fig. 1(a). In this experiment, one side of the SMF is 052003-1 2023 The Japan Society of Applied Physics © Appl. Phys. Express 16, 052003 (2023) K. Wang et al. core offset spliced. To find the optimal core-offset SMS core-offset SMS. In addition, the temperature sensitivity structure, different samples are fabricated utilizing standard exhibits a non-monotonic relationship with increasing core SMFs (9/125) and a commonly used 6 cm-long step-index offset, and the highest temperature sensitivity of −1 MMF (50/125) with an inexpensive fiber fusion splicer 13.92 pm °C is achieved at 20 μm core offset. The max- (INNO View 7). The splicing settings are kept constant imal temperature sensitivity is approximately two times the −1 during the fabrication process of all the samples to ensure the value of the aligned SMS sensor (5.83 pm °C , refer to Fig. consistency of the quality of the core offset. As the radius of S4 in Supplementary). the MMF core is 25 μm, samples with core-offset amplitudes Subsequently, the strain sensing is carried out at room of 10, 15, 20, 25, and 30 μm are fabricated (refer to Fig. S1 in temperature (∼20 °C), and the results for the input core-offset Supplementary). An aligned SMS sensor is also prepared in SMS structure are illustrated in Fig. 5. The measured the same condition as a reference. The sensing characteristics dependencies of the spectral dip on strain are presented in of the input core-offset and output core-offset SMS structures Figs. 5(a)–5(c) for the core offsets of 10, 20, and 30 μm, are investigated separately. respectively. The spectral dips shift to shorter wavelengths The experimental setup is illustrated in Fig. 2. The MMF with increasing strain. The corresponding dip wavelength −1 section is heated and clamped on two precision translation dependence on strain leads to coefficients of −0.8 pm μò , −1 −1 stages, which can apply varying axial tensile stress to the −1.19 pm μò , and −0.63 pm μò , as displayed in sample. The applied axial strain can be calculated by Figs. 5(d)–5(f) (The results of the output core-offset SMF are presented in Supplementary). Figure 6 summarizes the DL  =,2 () strain sensitivities with the input and output core-offset SMS structures. It indicates that the strain sensitivities (absolute where L is the initial length and ΔL is the additional change values) with the input core-offset SMS are also higher than in length of the MMF section when longitudinal strain is those with the output core-offset SMS. It also exhibits a non- applied. monotonic relationship with increasing core-offset amplitude, The temperature measurement without applied strain is and the maximal strain sensitivity (absolute value of −1 performed in the range of 30 °C to 70 °C in steps of 5 °C. −1.19 pm μò ) is obtained at the 20 μm core offset (where Examples of the temperature sensing results for the input the highest temperature sensitivity is obtained). Note that SMF with core-offset amplitudes of 10, 20, and 30 μm are 20 μm is the largest offset when the core of the SMF is shown in Fig. 3. The spectral dips exhibit redshifts in the spliced to the core of the MMF; with offsets of >20 μm, the wavelength domain when the temperature increases, as core of the SMF is (partially or totally) spliced to the cladding shown in Figs. 3(a)–3(c). The corresponding resulting of the MMF, which does not promote the excitation of the wavelength shifts of the spectral dips against the increasing higher-order modes in the MMF core. Compared to the strain −1 temperature are fitted and plotted in Figs. 3(d)–3(f). The sensitivity of the aligned SMS sensor (−0.15 pm μò , refer intervals between the curves are not uniform, and the vertical to Fig. S4 in Supplementary), the maximal strain sensitivity power change exhibits non-monotonic behavior against (absolute value) of the core-offset SMS sensor is enhanced by temperature. This can be attributed to non-uniform heating approximately eight times. across the entire sensing section, resulting in a slight A high-sensitivity, easy-to-fabricate, and low-cost SMS reduction in the linearity of the dip wavelength dependences. sensor based on core offset was developed and experimen- Specifically, the coefficients for the dip wavelength depen- tally investigated for temperature and strain sensing. This −1 dences are measured to be 7.63 pm °C (10 μm), sensor is implemented with a commonly used MMF and can −1 −1 13.92 pm °C (20 μm), and 8.48 pm °C (30 μm) (the be fabricated by a commercial splicer. The sensing char- results for the output core-offset SMF are presented in acteristics of the SMS sensors with core offsets of 10, 15, 20, Supplementary). The summary of the temperature sensing 25, and 30 μm at the input and output SMF/MMF boundaries with input and output core-offset SMS structures is illustrated are investigated, and the obtained maximal temperature and −1 −1 in Fig. 4, which shows that the temperature sensitivities with strain sensitivities are 13.92 pm °C and −1.19 pm με , the input core-offset SMS are higher than those with output which are ∼two times and ∼eight times the values of the (a) (b) Fig. 1. (a) Schematic diagram of the core-offset SMS structure with an offset at the input SMF/MMF boundary. (b) Image of the core offset spliced (offset = 20 μm). 052003-2 2023 The Japan Society of Applied Physics © Appl. Phys. Express 16, 052003 (2023) K. Wang et al. Fig. 2. Schematic diagram of the experimental setup for temperature and strain measurement; BLS, broadband light source (wavelength range of 100 nm and central wavelength of 1550 nm); SMF, single-mode fiber; MMF, multimode fiber; OSA, optical spectrum analyzer. (a) (b) (c) (e) (f) (d) Fig. 3. Temperature measurement results for the input core-offset SMS structure. Measured spectral dependencies on temperature with core offsets of (a) 10 μm, (b) 20 μm, and (c) 30 μm. Spectral dip shifts plotted as a function of temperature with core offsets of (d) 10 μm, (e) 20 μm, and (f) 30 μm. Fig. 4. Summary of the temperature measurement with the input and output core-offset SMS structure. 052003-3 2023 The Japan Society of Applied Physics © Appl. Phys. Express 16, 052003 (2023) K. Wang et al. (b) (a) (c) (d) (e) (f) Fig. 5. Strain measurement results for the input core-offset SMS structure. Measured spectral dependencies on strain with core offsets of (a) 10 μm, (b) 20 μm, and (c) 30 μm. Spectral dip shifts plotted as a function of strain with core offsets of (d) 10 μm, (e) 20 μm, and (f) 30 μm. Fig. 6. Summary of the strain measurement with the input and output core-offset SMS structures. aligned SMS sensor, respectively. The results also indicate structure, it may be beneficial to explore the potential of that the maximal sensitivities for both temperature and strain utilizing graded-index MMFs or step-index MMFs with are obtained with the 20 μm core offset, not the largest core larger core diameters in future work. Lastly, with its offset of 30 μm, which indicates the existence of a limit of the simplicity, cost efficiency, and robustness, we believe that offset-induced sensitivity enhancement. In addition, both our method will be a valuable tool for enhancing the temperature and strain sensitivities with the input core-offset sensitivities of SMS temperature and strain sensors in the SMS are higher than those with the output core-offset SMS. future. 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Journal

Applied Physics ExpressIOP Publishing

Published: May 1, 2023

Keywords: multimode fiber; single-mode–multimode–single-mode fiber sensor; fiber sensor; core-offset

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