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Abstract
A novel surface plasmon resonance (SPR) temperature sensor based on a silver-coated multi-hole optical fiber (SMHOF) is presented. The central and surrounding air holes of the SMHOF are filled with two kinds of thermosensitive liquid with high and low refractive index (RI), respectively. Two separated resonance dips, which are related to the high and low RI filled liquid respectively, are observed at different wavelength in the transmission spectrum. Advantageously, the two dips move towards opposite direction with the temperature variation. The interval between the two SPR dips is measured under different environmental temperature and exhibits a good linearity. The proposed sensor with different detection range is fabricated by changing the RIs of the filled thermosensitive liquids. The temperature sensitivity of 7.72 nm/°C and -7.81 nm/°C is obtained in the range of 20-60 °C and -20-20 °C, respectively. Owing to the high temperature sensitivity and tunable detection range, the proposed sensor is expected to find potential applications in biomedicine, health care and environmental monitoring.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Temperature measurement plays an important role in the field of biomedicine, chemical reaction and industrial production. Thus, temperature sensors with high sensitivity and wide detection range are highly desirable. Compared with conventional electrical sensors, optical fiber temperature sensors have the advantages of compact size, corrosion resistance, anti-electromagnetic interference, and the ability of remote on-line monitoring. Up to now, many types of optical fiber temperature sensors based on various principles and structures have been proposed and extensively studied, such as fiber Bragg gratings [1,2], Fabry-Perot interferometers [3,4], Mach-Zehnder interferometers [5,6], and Sagnac interferometers [7,8]. However, the sensitivities of the above sensors are relatively low, which limits their practical application.
Surface plasmon resonance (SPR) is a physical phenomenon excited at the interface between metal and dielectric medium, when the phase-matching condition is satisfied for the incident light wave and the surface plasmon wave (SPW) [9]. Due to its high sensitivity to the refractive index (RI) variations in the vicinity of the metal surface, optical fiber SPR sensors have been widely applied in biochemical sensing [10,11], food safety [12,13] and disease diagnosis [14,15]. By integrating with thermosensitive materials such as polydimethylsiloxane (PDMS) and ethanol, the optical fiber SPR sensor can act as a temperature sensor through monitoring the RI change induced by the temperature variation [16,17]. In recent years, SPR temperature sensors using various fiber configurations have been investigated numerically [18–20] or experimentally [21–23]. Usually, a multi-mode fiber (MMF) is etched or side-polished to remove a portion of cladding and coated with a thin metal layer to construct a SPR sensor, then it is immersed in the thermosensitive medium for temperature measurement [16,24]. However, the fabrication process is complex and the sensor probe is fragile. To overcome these issues, a section of single-mode fiber (SMF) is spliced with two segments of MMF and coated with gold/PDMS film for temperature sensing [25]. Cladding modes in the SMF are excited so that the evanescent field can reach the fiber surface and interact with the surrounding temperature-sensitive medium. Several kinds of optical fibers have also been applied in this scheme, including no-core fiber [26], photonic crystal fiber [27] and depressed double cladding fiber (DDCF) [28]. Recently, SPR temperature sensor based on silver-coated hollow fiber structure was demonstrated [29]. The thermosensitive liquid with RI higher than that of the fused silica was filled in the hollow core. The light transmitted in the liquid core underwent total internal reflection and SPW was excited at the interface between the silver layer and the supporting tube. Additionally, some special optical fiber structures such as helical-core fiber [30] and single-mode twin-core fiber [31] were employed to improve the sensitivity and figure of merit of the SPR temperature sensors. For the sensors mentioned above, the external temperature is measured by monitoring the wavelength shift of a single SPR dip. Thus, the sensitivity and detection range of the sensor is limited by the thermal optical coefficient (TOC) and RI of the adopted thermosensitive material. Supposing that two separated SPR dips moving towards opposite directions with temperature variation are excited simultaneously in a single fiber structure, the sensitivity of the temperature sensor will be significantly improved and the detection range will be expanded. To the best of our knowledge, there has been no research about optical fiber temperature sensor utilizing dual SPR dips with opposite moving direction.
In this paper, a high-performance temperature sensor based on a silver-coated multi-hole optical fiber (SMHOF) is proposed and demonstrated experimentally. The central and surrounding air holes of the SMHOF are filled with two kinds of thermosensitive liquid with high and low RI, respectively. Correspondingly, the light transmitted in the central liquid core and fused silica part of the SMHOF excites two individual SPR dips at different wavelength in the transmission spectrum. Advantageously, these two SPR dips move towards opposite direction with the temperature variation. The interval between the two SPR dips at different temperature is measured and exhibits good linearity. The proposed sensor with different detection range is also fabricated by changing the RI of the filled liquid. Taking advantage of the larger variation of the interval than that of the single SPR dip, the fabricated SMHOF sensor achieves the high sensitivity of 7.72 nm/°C and -7.81 nm/°C in the range of 20-60 °C and -20-20 °C respectively, which is much higher than that of most optical fiber SPR sensors for temperature sensing. Moreover, the detection range of the proposed sensor can be easily tuned by varying the RI of the thermosensitive liquid filled in the air holes of the SMHOF. Due to the high temperature sensitivity and wide detection range, the proposed sensor is expected to find potential applications in the field of biomedicine, health care and environmental monitoring. The idea of utilizing two oppositely shifting SPR dips to improve the sensitivity can be adopted in the design of different kinds of optical fiber SPR sensors as well and contribute to the research of optical fiber sensors.
2. Sensor design and fabrication
The structure of the proposed optical fiber temperature sensor is depicted in Fig. 1(a), which consists of a lead-in MMF, a SMHOF filled with two kinds of thermosensitive liquid, and a lead-out MMF. Two glass tubes filled with the same liquid as the central hole of the SMHOF are adopted to connect the MMFs and the SMHOF, which are sealed with epoxy glue. The cross section of the SMHOF using fused silica (SiO2) as the background material is shown in Fig. 1(b). The diameter of the SMHOF is 1.4 mm. A central air hole is surrounded by a ring of six air holes arranged in a hexagonal lattice with the hole diameter d = 275 µm and the hole pitch Λ=440 µm. A thin silver layer with the thickness of dozens of nanometers is coated on the inner surfaces of each air hole to support the excitation of the SPR phenomenon. Two kinds of thermosensitive liquid with RI higher and lower than that of the fused silica are filled in the central and surrounding air holes of the SMHOF, respectively. SPW is excited at the Ag/SiO2 interface by the light transmitted in the central liquid core. Meanwhile, the SPW is also excited at the interface between the silver layer and the low RI liquid filled in the surrounding holes by the light transmitted in the fused silica. If the light in the central liquid core and surrounding fused silica are all collected by the output MMF, both of the excited SPR phenomena will demonstrate themselves as an individual SPR dip in the transmission spectrum of the sensor. By properly adjusting the RIs of the two kinds of thermosensitive liquid, the two SPR dips can be located at different wavelengths and separate from each other in the transmission spectrum. Owing to the negative value of the TOC, the SPR dip corresponding to the high and low RI liquid undergoes a redshift and a blueshift with increasing temperature, respectively. In other words, the two SPR dips shift in the opposite directions when the temperature changes. Thus, by measuring the interval between the two oppositely shifting SPR dips instead of the shift of a single SPR dip, the sensitivity of the proposed sensor is significantly improved, which is almost the sum of the respective sensitivity of the each SPR dip.
Fig. 1. (a) Schematic diagram of the proposed SMHOF temperature sensor. (b) The cross section of the SMHOF filled with thermosensitive liquids.
A liquid-phase deposition method was employed to coat the silver layer in the multi-hole optical fiber as shown in Fig. 2 [32]. During the coating process, the fluid path was switched to either syringe pump or vacuum pump by a three-way valve. First, the SnCl2 solution was forced to flow through the air holes of the multi-hole optical fiber by the vacuum pump to activate the glass surface. Then a thin silver layer was coated employing the silver mirror reaction. The ammoniacal silver solution and glucose solution were used as the plating and reducing solution, respectively. The solutions were mixed and forced to flow through the multi-hole optical fiber by the syringe pump. The reduced silver particles adhere on the inner wall of the air holes and gradually form a dense silver layer. Finally, the SMHOF was carefully washed with deionized water and ethanol in succession by the vacuum pump and then dried with nitrogen gas. The flow rate of solutions, deposition time and temperature must be carefully controlled to obtain a uniform and smooth silver layer which is also thin enough for the SPR sensing. In our experiments, the flow rate of the syringe pump was set as 5 ml/min. The deposition time and temperature were controlled around 40 s and 20 °C, respectively.
The fabrication process of the proposed SMHOF temperature sensor is shown in Fig. 3. First, the SMHOF was fabricated by coating a thin silver layer with the thickness of 50 nm on the inner surfaces of all air holes of the multi-hole optical fiber. In the photo of the cross-section, it can be observed that the silver-coated air holes are brighter than the uncoated holes because of the relatively low transmission loss. Second, a 5-cm-long piece of fiber was cut off from the fabricated SMHOF and filled with low RI thermosensitive liquid through a syringe. Then, the six surrounding holes were sealed at both ends by transparent epoxy glue under the microscope. The liquid remaining in the central hole was cleaned by flushing with nitrogen gas. After that, both ends of the SMHOF were connected with a 1.5-cm-long glass tube with an inner diameter of 1.65 mm and an outer diameter of 1.95 mm by the epoxy glue. Then the whole structure, including the central hole of the SMHOF and the glass tubes at both ends, is filled with the high RI thermosensitive liquid. Finally, two segments of MMF with a core/cladding diameter of 400/440 µm were inserted into the glass tubes and sealed with epoxy glue. In this step, the resonance depth of the two SPR dips corresponding to the high and low RI liquids can be tuned by adjusting the positions of the lead-in and lead-out MMFs. Then, two SMA905 adaptors were installed on the ends of the MMFs to connect the fabricated sensor with the light source and the spectrometer. The photo of the completed sensor is also shown in Fig. 3.
3. Experimental setup
The TOC is an important characteristic of the thermosensitive material for temperature sensing. Higher absolute value of the TOC means greater change of the RI with certain temperature variation, which leads to better sensor performance. In the experiments, the thermosensitive liquids filled in the SMHOF sensor, both low and high RI liquids, are the mixed solutions of the butyl acetate and the polymethylphenyl silicone oil (PMPS) with different volume ratios. The RI of the mixed solution at 20 °C, which is denoted by n0, ranges from 1.3931 to 1.5796. Figure 4 shows the TOC of the mixed solution as a function of n0. The RI of thermosensitive liquids was measured at different temperatures by an Abbey refractometer as shown in the insets of Fig. 4, where the slope of the linear fitting curve represents the TOC of the mixed solution. The fitting curve of the measured data points shows that the TOC varies from -5.5161 × 10−4 to -4.4315 × 10−4 RIU/°C and changes linearly with the RI of the mixed solution as
The proposed SMHOF sensor realizes temperature sensing by measuring the RI change of the thermosensitive liquids, which is caused by the variation of the environmental temperature. Thus, the initial RI at 20 °C of the liquids filled in the air holes of the SMHOF should be appropriately selected according to the temperature detection range. The RIs and the TOCs of the thermosensitive liquids adopted to fabricate the SMHOF temperature sensors are listed in Table 1. For the sensor with the detection range of 20-60 °C, the thermosensitive liquids with RIs of 1.5076 and 1.3970 were filled into the central and surrounding air holes of the SMHOF respectively, while the liquids with RIs of 1.5232 and 1.4131 were selected for the sensor with the detection range of -20-20 °C.
Fig. 4. The relationship between the TOC and the RI of the mixed solution. The insets show the RI of butyl acetate and PMPS as a function of temperature.
Table 1. The RI and the TOC of the thermosensitive liquids used in fabrication.
The experimental system for measuring the transmission spectra of the SMHOF sensor under different temperatures is illustrated in Fig. 5. For the fabricated sensor with the detection range of 20-60 °C, the sensor was immersed in the water bath with temperature adjustment range from room temperature to 100 °C as shown in the upper panel of Fig. 5. For the fabricated sensor with the detection range of -20-20 °C, the sensor was placed in a homemade cooler using dry ice with temperature adjustment range from room temperature to -78 °C as shown in the lower panel of Fig. 5. Meanwhile, a thermometer was placed near the fiber probe to monitor the real-time environmental temperature. The broadband light emitted from a halogen lamp was launched into the SMHOF temperature sensor via the lead-in MMF. Then the spectrum of the light transmitted through the sensor was detected by a spectrometer (PG2000-pro, Ideaoptics) via the lead-out MMF.
Fig. 5. Schematic diagram of the experimental setup. The top-right and bottom-right inset shows the photograph of the SMHOF temperature sensor with detection range of 20-60 °C and -20-20 °C, respectively.
4. Results and discussion
According to the working principle of SPR sensors, we first carried out an experiment to demonstrate the RI sensing ability of the SMHOF. While SPR phenomenon is excited, part of the energy of the incident light transfers to the SPW, leading to an apparent decrease of reflectivity. It would exhibit a minimum in the transmission spectrum at a particular wavelength known as the resonance wavelength (RW), which changes along with the RI of the sensed medium. Butyl acetate and PMPS were mixed with different volume ratios to obtain a series of solutions with RI ranging from 1.3931 to 1.5796. Then the transmission spectrum of the SMHOF filled with each solution was measured as shown in Figs. 6(a) and 6(b). As depicted in the insets, the solution was filled in the specified holes of the SMHOF according to its RI. For the seven solutions with RI lower than that of the fused silica shown in Fig. 6(a), it was filled in the surrounding holes. In contrast, the seven solutions with RI higher than that of the fused silica shown in Fig. 6(b) was filled in the central hole. On both occasions, the rest holes were empty. A spectrum of the SMHOF without liquid filling was adopted as the background reference, which contains the light transmitted in the silver-coated central hole and the light transmitted in the silica part of the SMHOF. The RIs of the filled liquids are labeled in the figures, which were measured at the wavelength of 589 nm by the Abbe refractometer just before each measurement. As shown in Fig. 6(a), the RW of the SPR dip corresponding to the low RI liquid shifts towards a longer wavelength when the RI of the filled liquid increases. The width of the resonance dip increases gradually with the increasing RI, while the depth of the resonance dip is substantially unchanged. For the SPR dip corresponding to the high RI liquid, the RW shifts towards a shorter wavelength and the width of the resonance dip decreases when the RI of the filled liquid increases as shown in Fig. 6(b). That is, the smaller the RI difference between the medium on both sides of the silver film, the longer the RW and the wider the resonance dip. The sensitivity of the SPR sensor can be calculated from the shift of the RW (Δλres) due to the RI change of the sensed medium (Δn0), which is defined as
To estimate the sensing performance of the SMHOF SPR sensor, the exponential fitting curves with R-square of 0.9996 and 0.9989 for the RW versus RI are shown in Fig. 6(c) and 6(d), respectively. The proposed sensor is demonstrated to display a high wavelength sensitivity of 4132-13454 nm/RIU and 6450-14955 nm/RIU in the RI range of 1.3931-1.4258 and 1.4909-1.5214, respectively. The RI sensitivity of the SMHOF SPR sensor is higher than that of most solid-core fiber and hollow-core fiber SPR sensors [33,34], which is beneficial for temperature sensing.
Fig. 6. Normalized experimental transmission spectra of the SMHOF SPR sensor filled with (a) low RI and (b) high RI liquid. The exponential fitting curves of the RW as a function of RI in the range of (c) 1.3931-1.4258 and (d) 1.4909-1.5214.
For the proposed SMHOF temperature sensor, as shown in Fig. 1, both the central and surrounding holes of the SMHOF were filled with thermosensitive liquids with RI higher and lower than that of the fused silica, respectively. Therefore, the two SPR dips corresponding to the high and low RI liquids will both appear in the transmission spectrum of the sensor, which are denoted by Dip-H and Dip-L, respectively. Due to the negative value of the TOC, the RIs of the thermosensitive liquids decrease when the environmental temperature increases. As a result, according to Figs. 6(a) and 6(b), Dip-L and Dip-H would undergo a blueshift and redshift with the increasing temperature, respectively. By measuring the interval between the two resonance dips with opposite moving direction, high-performance temperature sensing can be realized. The detection range of the SMHOF temperature sensor depends on the operation wavelength range of the spectrometer and the RWs of Dip-L and Dip-H. At the lower and upper limits of the temperature detection range, either the two resonance dips are at the closest distance that can be distinguished or one of them is going to move out of the operation wavelength range. Therefore, taking the TOC curve shown in Fig. 4 and the RW-RI curves shown in Fig. 6 as reference, the exact detection range of the SMHOF temperature sensor can be tuned as required by appropriately selecting the thermosensitive liquids filled in the air holes. For convenience, two SMHOF temperature sensors named TS20 + and TS20- were fabricated in the experiments, both adopting 20 °C as one limit of the detection range. However, TS20 + takes 20 °C as the lower limit while TS20- takes it as the upper limit.
TS20 + was fabricated by filling thermosensitive liquids with RI of 1.5076 and 1.3970 in the central and surrounding holes of the SMHOF, respectively. The transmission spectra of TS20 + at room temperature (20 °C) are shown in Fig. 7(a). The individual SPR spectra corresponding to the high and low RI liquids were also shown for comparison. The RWs of the individual spectra are 838 nm and 742 nm, while the RWs of the Dip-H and Dip-L observed in the spectrum of TS20 + are 819 nm and 740 nm, respectively. The difference in the RW can be ascribed to the slight overlap of the two resonance dips. Figure 7(b) shows the normalized transmission spectra of TS20 + measured at different temperatures from 20 °C to 60 °C. In all spectra, Dip-L is on the left side of Dip-H. At 20 °C, the lower limit of the detection range, Dip-L and Dip-H are closest to each other. When the environmental temperature increases, the two dips move apart from each other since Dip-L undergoes a blueshift and dip-H undergoes a redshift. At 60 °C, Dip-H is close to the edge of the operation wavelength range 1100 nm. Thus, 60 °C is set as the upper limit of TS20 + approximately. The exponential fitting curves for the RW versus temperature are shown in Fig. 7(c), with the R-square of 0.9993 and 0.9982 for Dip-L and Dip-H, respectively. Figure 7(d) shows the interval between the two SPR dips as a function of temperature, which was obtained by subtracting the RW of Dip-L from that of Dip-H. It should be noted that the nonlinearity of the two SPR dips cancels each other out, leading to a good linear relationship (R-square = 0.9985) between the interval and the environmental temperature. Owing to the SPR dips with opposite moving direction, TS20 + exhibits a high temperature sensitivity of 7.72 nm/°C in the range of 20-60 °C.
Fig. 7. Normalized transmission spectrum of TS20 + at 20 °C and individual SPR spectra corresponding to high and low RI liquids. (b) Normalized transmission spectra of TS20 + under different temperatures in the range of 20-60 °C. (c) Exponential fitting curves of the RW as a function of temperature. (d) Linear fitting of the interval between the two SPR dips as a function of temperature.
For TS20-, thermosensitive liquids with RI of 1.5232 and 1.4131 were filled in the central and surrounding holes of the SMHOF, respectively. The transmission spectra of TS20- at 20 °C and the individual SPR spectra corresponding to the high and low RI liquids are shown in Fig. 8(a). Contrary to the case shown in Fig. 7(a), the SPR dip corresponding to the low RI liquid at 833 nm is on the right of that corresponding to the high RI liquid at 741 nm. Similarly, in the spectrum of TS20-, Dip-L at 836 nm is on the right side of Dip-H at 736 nm, with an interval of 100 nm. Figure 8(b) shows the normalized transmission spectra measured at different temperatures in the range of -20-20 °C. In all spectra, contrary to TS20+, Dip-L is on the right side of Dip-H. Similar to TS20+, for TS20-, Dip-L and Dip-H are closest to each other at 20 °C. However, 20 °C is the upper instead of lower limit of the detection range for TS20-. When the environmental temperature decreases, the two dips move apart from each other since Dip-L undergoes a redshift and Dip-H undergoes a blueshift. At -20 °C, Dip-L is close to the edge of the operation wavelength range 1100 nm. Thus, -20 °C is set as the lower limit of TS20- approximately. The exponential fitting curves of the RWs as a function of temperature are shown in Fig. 8(c), with the R-square of 0.9988 and 0.9982 for Dip-H and Dip-L, respectively. The linear fitting between the interval and the environmental temperature with the R-square of 0.9922 is shown in Fig. 8(d), demonstrating a sensitivity of -7.81 nm/°C in the range of -20-20 °C. The resolution of a SPR temperature sensor can be calculated by the equation R = abs(σ/ST), where ST is the temperature sensitivity and σ is the standard deviation of the interval between Dip-H and Dip-L. The calculated σ is 0.20 nm and 0.22 nm for TS20 + and TS20- respectively, which is statistically obtained by repeating the measurement for 10 times at the same condition. Thus, with the temperature sensitivity of 7.72 nm/°C and 7.81 nm/°C, the resolutions of TS20 + and TS20- are estimated to be 0.026 °C and 0.028 °C, respectively.
Fig. 8. Normalized transmission spectrum of TS20- at 20 °C and individual SPR spectra corresponding to high and low RI liquids. (b) Normalized transmission spectra of TS20- under different temperatures in the range of -20-20 °C. (c) Exponential fitting curves of the RW as a function of temperature. (d) Linear fitting of the interval between the two SPR dips as a function of temperature.
A comparison of the temperature sensitivity and detection range between the proposed SMHOF temperature sensor and several fiber-based SPR temperature sensors reported in recent years is shown in Table 2. The sensitivity of the proposed sensor reaches 7.72 nm/°C and -7.81 nm/°C in the temperature range of 20-60 °C and -20-20 °C respectively, which is higher than that of most optical fiber sensors. Moreover, the detection range of the SMHOF temperature sensor can be easily tuned by varying the RI of the thermosensitive liquid filled in the air holes.
Table 2. Comparison of fiber-based SPR sensors for temperature sensing.
5. Conclusion
In conclusion, a novel optical fiber SPR temperature sensor based on the SMHOF filled with two kinds of thermosensitive liquids is proposed. In the transmission spectrum of the proposed sensor, there are two SPR dips shifting towards opposite direction as temperature changes. Compared with the shift of single SPR dip, the larger variation of the interval between the two SPR dips to the temperature variations will definitely enhance the sensitivity. The proposed sensors with different temperature detection ranges are fabricated. The performance of the fabricated sensors is investigated by measuring the transmission spectra under different temperatures. A good linear relationship between the interval of the two SPR dips and the environmental temperature is demonstrated. The fabricated SMHOF temperature sensors exhibit the high sensitivity of 7.72 nm/°C and -7.81 nm/°C in the range of 20-60 °C and -20-20 °C, respectively. By changing the RI of the thermosensitive liquids filled in the air holes, we can obtain sensors with different detection ranges to meet different applications. The results of this work could contribute to the research of optical fiber sensors for temperature sensing. The idea of utilizing two oppositely shifting SPR dips to improve the sensitivity can be adopted in the design of different kinds of optical fiber SPR sensors as well.
Funding
National Natural Science Foundation of China (61975034).
Acknowledgment
The authors thank the Zhejiang Lab’s International Talent Fund for Young Professionals.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. R. Yang, H. Bao, S. Zhang, K. Ni, Y. Zheng, and X. Dong, “Simultaneous Measurement of Tilt Angle and Temperature With Pendulum-Based Fiber Bragg Grating Sensor,” IEEE Sens. J. 15(11), 6381–6384 (2015). [CrossRef]
2. Y. Du, Q. Han, H. Hu, M. Sang, X. Zhao, X. Song, H. Wang, and T. Liu, “High-sensitivity refractive index and temperature sensor based on cascading FBGs and droplet-like fiber interferometer,” Sens. Actuators, A 299, 111631 (2019). [CrossRef]
3. C. E. Domínguez-Flores, D. Monzón-Hernández, J. I. Moreno-Basulto, O. Rodríguez-Quiroz, V. P. Minkovich, D. López-Cortés, and I. Hernández-Romano, “Real-time temperature sensor based on in-fiber Fabry–Perot interferometer embedded in a resin,” J. Lightwave Technol. 37(4), 1084–1090 (2019). [CrossRef]
4. Q. Sheng, N. Uddin, B. Zhou, X. Wang, and M. Han, “Fabrication of silicon-tipped fiber-optic temperature sensors using aerogel-assisted glass soldering with precise laser heating,” Opt. Lett. 47(11), 2718–2821 (2022). [CrossRef]
5. Y. Li, L. Wang, Y. Chen, D. Yi, F. Teng, X. Hong, X. Li, Y. Geng, Y. Shi, and D. Luo, “High-performance fiber sensor via Mach-Zehnder interferometer based on immersing exposed-core microstructure fiber in oriented liquid crystals,” Opt. Express 28(3), 3576–3586 (2020). [CrossRef]
6. X. Wang, D. Chen, H. Li, G. Feng, and J. Yang, “In-Line Mach–Zehnder Interferometric Sensor Based on a Seven-Core Optical Fiber,” IEEE Sens. J. 17(1), 100–104 (2017). [CrossRef]
7. L. Y. Shao, Y. Luo, Z. Zhang, X. Zou, B. Luo, W. Pan, and L. Yan, “Sensitivity-enhanced temperature sensor with cascaded fiber optic Sagnac interferometers based on Vernier-effect,” Opt. Commun. 336, 73–76 (2015). [CrossRef]
8. Q. Liu, S. G. Li, and H. Chen, “Enhanced sensitivity of temperature sensor by a PCF with a defect core based on Sagnac interferometer,” Sens. Actuators, B 254, 636–641 (2018). [CrossRef]
9. J. Homola, S. S. Yee, and G. Gauglitz, “Surface plasmon resonance sensors: review,” Sens. Actuators, B 54(1-2), 3–15 (1999). [CrossRef]
10. S. Qian, M. Lin, W. Ji, H. Yuan, Y. Zhang, Z. Jing, J. Zhao, J.-F. Masson, and W. Peng, “Boronic acid functionalized Au nanoparticles for selective microRNA signal amplification in fiber-optic surface plasmon resonance sensing system,” ACS Sens. 3(5), 929–935 (2018). [CrossRef]
11. Y. Zhao, R. J. Tong, F. Xia, and Y. Peng, “Current status of optical fiber biosensor based on surface plasmon resonance,” Biosens. Bioelectron. 142, 111505 (2019). [CrossRef]
12. Vikas, M. K. Yadav, P. Kumar, and R. K. Verma, “Detection of adulteration in pure honey utilizing Ag-graphene oxide coated fiber optic SPR probes,” Food Chem. 332, 127346 (2020). [CrossRef]
13. M. Pesavento, L. Zeni, L. D. Maria, G. Alberti, and N. Cennamo, “SPR-Optical Fiber-Molecularly Imprinted Polymer Sensor for the Detection of Furfural in Wine,” Biosensors 11(3), 72 (2021). [CrossRef]
14. V. S. Chaudhary, D. Kumar, and S. Kumar, “Gold-Immobilized Photonic Crystal Fiber-Based SPR Biosensor for Detection of Malaria Disease in Human Body,” IEEE Sens. J. 21(16), 17800–17807 (2021). [CrossRef]
15. R. Kant and B. D. Gupta, “Fiber-Optic SPR Based Acetylcholine Biosensor Using Enzyme Functionalized Ta2O5 Nanoflakes for Alzheimer’s Disease Diagnosis,” J. Lightwave Technol. 36(18), 4018–4024 (2018). [CrossRef]
16. Y. Zhao, Z. Q. Deng, and H. F. Hu, “Fiber-optic SPR sensor for temperature measurement,” IEEE Trans. Instrum. Meas. 64(11), 3099–3104 (2015). [CrossRef]
17. L. X. Kong, M. J. Chi, C. Ren, L. F. Ni, Z. Li, and Y. S. Zhang, “Micro-Lab on Tip: High-Performance Dual-Channel Surface Plasmon Resonance Sensor Integrated on Fiber-Optic End Facet,” Sens. Actuators, B 351, 130978 (2022). [CrossRef]
18. S. Weng, L. Pei, J. Wang, T. Ning, and J. Li, “High sensitivity D-shaped hole fiber temperature sensor based on surface plasmon resonance with liquid filling,” Photonics Res. 5(2), 103–107 (2017). [CrossRef]
19. Y. Liu, S. Li, H. Chen, J. Li, W. Zhang, and M. Wang, “Surface Plasmon Resonance Induced High Sensitivity Temperature and Refractive Index Sensor Based on Evanescent Field Enhanced Photonic Crystal Fiber,” J. Lightwave Technol. 38(4), 919–928 (2020). [CrossRef]
20. X. Yang, L. Zhu, Y. Lu, and J. Yao, “Ultrasharp LSPR Temperature Sensor Based on Grapefruit Fiber Filled With a Silver Nanoshell and Liquid,” J. Lightwave Technol. 38(7), 2015–2021 (2020). [CrossRef]
21. S. Liu, S. Cao, Z. Zhang, Y. Wang, C. Liao, and Y. Wang, “Temperature Sensor Based on Side-Polished Fiber SPR Device Coated with Polymer,” Sensors 19(15), C1 (2019). [CrossRef]
22. Y. X. Tang, X. Zhang, X. S. Zhu, and Y. W. Shi, “Dielectric layer thickness insensitive EVA/Ag-coated hollow fiber temperature sensor based on long-range surface plasmon resonance,” Opt. Express 29(1), 368–376 (2021). [CrossRef]
23. L. Liu, Z. Liu, Y. Zhang, and S. Liu, “V-shaped micro-structure optical fiber surface plasmon resonance sensor for the simultaneous measurement of the refractive index and temperature,” Opt. Lett. 44(20), 5093–5096 (2019). [CrossRef]
24. J. Luo, G. S. Liu, W. Zhou, S. Hu, L. Chen, Y. Chen, Y. Luo, and Z. Chen, “A graphene–PDMS hybrid overcoating enhanced fiber plasmonic temperature sensor with high sensitivity and fast response,” J. Mater. Chem. C 8(37), 12893–12901 (2020). [CrossRef]
25. J. S. Velázquez-González, D. Monzón-Hernández, D. Moreno-Hernández, F. Martínez-Piñón, and I. Hernández-Romano, “Simultaneous measurement of refractive index and temperature using a SPR-based fiber optic sensor,” Sens. Actuators, B 242, 912–920 (2017). [CrossRef]
26. B. Li, X. Yan, X. Zhang, F. Wang, S. Li, T. Suzuki, Y. Ohishi, and T. Cheng, “No-core optical fiber sensor based on surface plasmon resonance for glucose solution concentration and temperature measurement,” Opt. Express 29(9), 12930–12940 (2021). [CrossRef]
27. Y. Wang, Q. Huang, W. Zhu, M. Yang, and E. Lewis, “Novel optical fiber SPR temperature sensor based on MMF-PCF-MMF structure and gold-PDMS film,” Opt. Express 26(2), 1910–1917 (2018). [CrossRef]
28. Z. Yang, J. Xia, S. Li, R. Qi, G. Zuo, and W. Li, “Ultrawide temperature range operation of SPR sensor utilizing a depressed double cladding fiber coated with Au-Polydimethylsiloxane,” Opt. Express 28(1), 258–269 (2020). [CrossRef]
29. M. Lu, X. Zhang, Y. Liang, L. Li, J. F. Masson, and W. Peng, “Liquid crystal filled surface plasmon resonance thermometer,” Opt. Express 24(10), 10904–10911 (2016). [CrossRef]
30. X. Wang, H. Deng, and L. Yuan, “Ultra-high sensitivity SPR temperature sensor based on a helical-core fiber,” Opt. Express 29(14), 22417–22426 (2021). [CrossRef]
31. Z. Zhu, L. Liu, Z. Liu, Y. Zhang, and Y. Zhang, “Surface-plasmon-resonance-based optical-fiber temperature sensor with high sensitivity and high figure of merit,” Opt. Lett. 42(15), 2948–2951 (2017). [CrossRef]
32. X. Zhang, X. S. Zhu, and Y. W. Shi, “Fiber optic surface plasmon resonance sensor based on a silver-coated large-core suspended-core fiber,” Opt. Lett. 44(18), 4550–4553 (2019). [CrossRef]
33. E. Klantsataya, P. Jia, H. Ebendorff-Heidepriem, T. M. Monro, and A. François, “Plasmonic Fiber Optic Refractometric Sensors: From Conventional Architectures to Recent Design Trends,” Sensors 17(18), C1 (2017). [CrossRef]
34. B. H. Liu, Y. X. Jiang, X. S. Zhu, X. L. Tang, and Y. W. Shi, “Hollow fiber surface plasmon resonance sensor for the detection of liquid with high refractive index,” Opt. Express 21(26), 32349–32357 (2013). [CrossRef]
