The fiber geometry, fiber parameters and mode-guiding properties are crucial for realizing high-performance fiber-based sensors. In this work, we propose and demonstrate a few-mode fiber (FMF)–based surface plasmon resonance (SPR) biosensor. The FMF-SPR sensor was fabricated via side-polishing a few-mode fiber and coating a thin layer of gold film, on the basis of the optimization of fiber geometry, thickness of the gold film and mode selection, which were performed with the finite element method. The refractive index (RI) sensing performance of three such sensors with different residual fiber thicknesses were investigated. In the RI range from 1.333 to 1.404, the highest sensitivity up to 4903 nm/RIU and a figure of merit of 46.1 RIU−1 are achieved. For testing the bovine serum albumin (BSA) solution, an averaged BSA RI sensitivity of 6328 nm/RIU and an averaged BSA concentration sensitivity of 1.17 nm/(mg/ml) are realized. Benefiting from only a few modes supported in the FMF, a smaller line-width of the SPR spectrum is obtained, which further results in a higher figure of merit (FOM). Moreover, when combined with the superiority of the mode-multiplexing technology brought by the FMF, the FMF-SPR sensors may find applications in biochemical analysis with high performance and high throughputs.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The few mode fibers (FMFs) provide unique properties in mode-guiding and have attracted intensive attention in the field of optical communication based on mode division multiplexing. FMFs are being fabricated into various passive and active optical communication devices, like mode-division multiplexer , mode converter , generator of orbital angular momentum , mode-selective couplers  and mode selective fiber laser . However,in the domain of optical fiber sensing application , FMFs have been scarcely reported to be fabricated into fiber sensors, even though they show uniqueness in geometry and guiding properties when compared to both single-mode fiber (SMF) and multimode fiber (MMF).
Since the first fiber surface plasmon resonance (SPR) sensor was proposed by Jorgenson and Yee , SPR fiber sensors have been extensively investigated for the environmental monitoring, disease diagnostics, biological analysis, etc [8–11], due to their advantages such as high sensitivity, miniature size, easy fabrication, all-fiber structure, and low cost [12–17]. In order to realize the coupling between the core mode and surface plasmon (SP) mode at the metal-dielectric interface, fiber SPR sensors with various interrogation methods [18–23], fiber geometries [24,25], and plasmonic coatings [26–28] have been proposed. The common method is to remove the cladding entirely or partially , such as side-polished fiber (SPF) [30–32], tapered fiber , etched fiber , and D-shaped fiber . Fiber gratings including fiber tilted fiber gratings  and long-period fiber gratings  have also been developed to diffract the guided light from the core into the cladding for exciting the plasmon resonance. Compared to the other structures mentioned above, the SPF shows advantages of low manufacturing cost, easy fabrication and consisting of an ideally flat polished surface, which benefits it to be a promising candidate to construct SPR sensors.
The sensing performance of the fiber based SPR sensors, which are usually characterized by the sensitivity, full width at half maximum (FWHM), and figure of merit (FOM) , depends upon the fiber geometry, fiber parameters and metallic coatings. Different kinds of fibers have been used to construct SPR fiber sensors, including MMF [38,39], SMF [40,41], microstructured optical fiber , twin-core fiber , polarization-maintaining fiber , polymer fiber , coreless fiber , and two mode fiber (TMF) [46,47]. MMFs have relatively large core diameters providing high coupling efficiency with light source, which makes the polishing depth controlling is relatively simple. However, the large amount of propagating modes (up to a few hundreds) supported in the MMF will cause the satisfaction of the phase matching condition between the core modes and the SP modes at a number of wavelengths, and subsequently broaden the FWHM of the resonance dip. Reducing the number of modes that satisfy the phase matching condition is one way to narrow the FWHM, which can be realized by using the SMF. However, it suffers from the reduction of coupling efficiency between the light source and the SMF, and requires fine controlling in the polishing process. Therefore, a trade-off between the size of the core diameters and the number of the core modes exist in the design for a fiber based SPR sensor. Recently, few mode fiber, which possesses a compromising core diameter compared to the MMF and SMF, has been side-polished and fabricated into SPR sensor for detection of prostate specific antigen with a sensitivity of 2500 nm/RIU . More importantly, SPR sensor based on side-polished TMF operating in the near-infrared region was proposed , by coating a film of indium tin oxide and the sensitivity is numerically demonstrated to be higher than 10000 nm/RIU. However, the TMF SPR sensor has not been investigated experimentally, and the FWHM and FOM have not been analyzed as well.
Here we present, for the first time as the best of our knowledge, a comprehensive analysis of the gold film coated SPR sensor based on side-polished FMF (FMF-SPR), experimental fabrication and characterization of its sensing performance. In the simulations, the effect of gold (Au) film thickness and residual fiber thickness (RFT) on the sensing performance was analyzed and optimized design was obtained for the FMF based SPR sensor. Three FMF-SPR sensors with different RFTs were fabricated and characterized. The highest RI sensitivity at surrounding refractive index (SRI) of 1.404 is 4903 nm/RIU for the sensor with the RFT of 72.24 μm. Further biological analyte was tested with BSA solutions and the sensitivity was obtained as 1.17 nm/(mg/ml).
2. Simulation and optimization
In order to find the optimized parameters for the FMF-SPR sensor, numerical simulations were performed using the finite element method. The parameters of the FMF used in simulations are the same as those of the FMF used in our experiments. The structure diagram of the FMF-SPR sensor is shown in Fig. 1(a), where a section of FMF is polished to a certain depth and a layer of gold film is coated on the polished region (~10 mm in length). The diameters for the core and cladding are 19 and 125 μm, with corresponding refractive index are 1.449 and 1.444, respectively. The numerical aperture of the FMF is 0.12. The simulation parameters of the FMF are the same as those of the commercial TMF (Two-Mode Step-Index Fiber, OFS). The cross section of the FMF-SPR senor is shown in Fig. 1(b), where the symbols of dAu and dRFT denote the thickness of gold and the side-polished FMF, respectively.
The FMF supports the LP01 and LP11 modes. The LP11a mode of the FMF has higher sensitivity, which has been reported in our previous works . The electrical field amplitude showing the coupling between the core mode and SP mode at 545 nm is presented in Fig. 2. The arrows specify the orientation of electric field. The sharp peak in the electrical field amplitude near 10 μm confirms the SPR for the LP01 mode, as shown in Fig. 2(a). The inserts of Fig. 2(a) are the electric field distributions showing the coupling between the LP01 and the SP mode. Similarly, the Fig. 2(b) shows the coupling between the LP11a mode and the SP mode. The zoom-in of the coupling region shown in Figs. 2(a) and 2(b) indicates the LP11a mode has higher coupling efficiency than the LP01 mode. The LP11a mode has higher sensitivity than the LP01 mode, and the penetration depth of evanescent wave of the LP11a mode is larger .
The transmission spectra of the sensor are obtained using the following equation :
Figure 3(a) shows the dispersion curves of the LP11a mode (solid line) and SP mode (dashed line). At the wavelength around 545 nm, the phase matching condition is satisfied where the real part of of the core mode (black solid line) is approximately equal to the real part of of the SP mode (black dashed line).” The SPR is also verified by the imaginary part of of the core mode (red solid line). The inset is the electric field distributions of the SP mode. The loss spectra of the LP11a mode and SP mode are calculated with ne = 1.33, as shown in Fig. 3(b).
In order to find the SPR resonance wavelength, the effective indices of the transverse magnetic (TM) modes were calculated, because the resonance wavelengths correspond to the peak of the imaginary parts of the effective mode-indices. The sensing performance can be further analyzed by varying the surrounding refractive indices (SRI). Figure 4(a) shows the calculated transmission spectra for the LP11a mode with dAu = 40 nm and dRFT = 72 μm with the SRI changing from 1.33 to 1.39. It can be observed that the resonance wavelength shifts to the longer wavelength when the SRI increases. In addition, the resonance spectrum broadens as the SRI increases. Figure 4(b) shows the resonant wavelength increases with the surrounding refractive index. Since the FOM is defined as the ratio of the sensing sensitivity and the FWHM, there is a tradeoff between the sensitivity and FWHM of the SPR spectrum. Therefore, it is necessary to optimize the parameters of the thickness of the residual fiber and the gold film, for achieving the highest FOM for the FMF-SPR sensors.
Figure 5 shows the transmittance spectra for different Au film thickness and RFTs for the LP11a mode. When the SRI is changed from 1.33 to 1.34, the resonance wavelength shifts to longer wavelength and the depth of resonance dip becomes smaller. As shown in Figs. 5(a) and 5(b), the depth of resonance dip decreases when the Au film thickness increases from 30 to 70 nm for the LP11a mode. The transmittance spectra also change with the RFT. As shown in Figs. 5(c) and 5(d), the depth of resonance dip decreases with the RFT in the range from 69 to 73 μm. Note that the resonance wavelength changes approximately from 505 to 575 nm when the Au film thickness is changed from 30 to 70 nm, and the resonance wavelength changes form 549 to 538 nm when the RFT is increased from 69 to 73 μm.
In order to present a comprehensive analysis, Fig. 6 summarize all the dependence of the parameters, such as the thickness of Au film and RFT on the depth of resonance dip, FWHM, sensitivity and FOM of the FMF-SPR sensors, respectively. As shown in Fig. 6(a), the depth of resonance dip decreases with the Au film thickness. At fixed Au film thickness, the depth of resonance dip is reduced with larger RFT. Figure 6(b) depicts the change of FWHM with Au film thickness and RFT. The FWHM decreases with the Au film thickness for RFTs of 69, 70 and 71 μm. For RFTs of 72 and 73 μm, the FWHM increases with the Au film thickness. The FWHM also changes with the RFT, smaller FWHM can be obtained with larger RFT. Figure 6(c) shows the variation of sensitivity with the Au film thickness and RFT. The sensing performance is degraded with larger FWHM. The FOM is calculated by taking into account both the sensitivity and FWHM. The influence of Au film thickness and RFT on the FOM is studied. As shown in Fig. 6(d), the FOM changes with the Au film thickness and RFT. The FOM has a maximum value of 100 when the Au film thickness and RFT are 30 nm and 73 μm, respectively. The optimized sensor parameters are obtained by considering the depth of resonance dip and FOM. As can be seen in Fig. 6(d), RFTs of 72 and 73 μm have larger FOM. The RFT of 72 μm provides much better depth of resonance dip. The FOM is 86.9 RIU−1 when the film thickness is 40 nm for RFT = 72 μm, as shown in Fig. 6(d). Therefore, the optimized Au film thickness and RFT are 40 nm and 72 μm, respectively. The sensitivity is 2512.3 nm/RIU with the optimized sensor parameters. The coupling efficiencies between SP modes and core modes can be verified by the depth of resonance dip of the SPR transmission. As shown in Fig. 7(a), the LP21a mode has larger coupling efficiency than the LP01, LP11a, LP11b, and LP21b modes. In order to investigate the influence of other fiber modes to the FWHM of SPR spectrum. The spectrum of superposition of the LP01, LP11a, LP11b, LP21a, LP21b modes is calculated. Figure 7(b) plots the calculated transmission spectra of the LP11a mode and the superposition mode . The FWHM of the LP11a mode and superposed spectra are 40.1 nm and 46.2 nm, respectively.
3. Fabrication and characterization
Base on the simulation results, we fabricated and characterized the SPR fiber sensors. Firstly, the FMF was side-polished by using the wheel polishing technique. The polished length is totally ~10 mm, and the length of the flat region is ~6 mm. Three FMFs were side-polished with different RFTs, which were measured along the polished region in a step of 1 mm and shown in Figs. 8(a)–(c). The RFT is determined by the averaged values of the flat region in Figs. 8(b)–(d), which were obtained from the images under a microscope. The RFTs in the flat region were measured to be 69.89, 71.31, and 72.24 μm, respectively, and the corresponding FMF-SPR sensors are marked as #1, #2, and #3 for the simplicity.
Then the Au film was coated onto the polished FMFs using vacuum evaporating method. The chamber was vacuumed down to the pressure of Pa. The deposition rate of the gold film was 2 Å/s, with the deposition time of 200 s. The atomic force microscopy (AFM) was measured to characterize the surface roughness and thickness of the coated Au film on the side-polished fiber. The selected area of 3D AFM image is 5 × 5 μm2 in Fig. 9(a) shows the thickness of the Au film is 50 nm with the average roughness of 5.7 nm. The scanning electron microscopy (SEM) image of the cross section of the sensor is shown in Fig. 9(b). The total thickness of the side-polished fiber #3 together with an additional gold film on its top is measured to be 72 μm, which confirms the measurements shown in Fig. 8(d).
The sensing performance of the fabricated FMF-SPR sensors were characterized using the experimental setup as shown in Fig. 10. The light from the tungsten halogen (AvaLight-HAL-(S)-Mini, China) was coupled to the FMF. The fiber mode was coupled to the SP mode of the sensor. The transmitted light was measured by a spectrometer with the operating wavelengths from 300 to 1100 nm (AvaSpec-ULS2048XL, China). A computer was used to record the transmission spectrum from the spectrometer. In order to make sure the sensor is straight during sensing measurements, the two ends of sensor was fixed to a glass slide using UV glue. The analyte solutions with different refractive indices were obtained by mixing distilled water and ethylene glycol with different volume ratios. The refractive indices of the obtained analytes were measured by an Abbe refractometer (Edmund NT52-975, Edmund Optics Co., Ltd.) at room temperature of 25°C. Ten solutions with refractive indices from 1.333 to 1.404 were prepared for the RI measurement. Before measurement, the transmission spectrum in air was recorded as the reference. For RI measurement, the analyte solutions were flowed to the sensing area, and the normalized transmission spectrum was recorded. Then the sensor was cleaned with anhydrous ethanol to make sure the SPR spectra recover to the reference spectrum. The next measurement was carried out after the spectrum recovered.
Figures 11(a)–(c) show the measured transmission spectra of sensors with the RFTs of 69.89, 71.31, and 72.24 μm, respectively. The resonance dip shifts to longer wavelength when SRI increases from 1.333 to 1.404. The neff of the core mode and SP mode are increased with the SRI. However, the increments in the real parts of the effective index, Re(neff), of the core mode and SP mode are different, resulting in the resonance wavelength shifting towards longer wavelength. The wavelength shifts are 183.9, 175.5, and 203.2 nm for the three FMF-SPR sensors, respectively. Moreover, the FWHM of the SPR spectrum increases with the higher SRI, as shown in Figs. 11(a)–(c). It is believed that the broadening of FWHM is caused by neighboring resonance peaks which arise from the increased SRI. The FWHM can be further reduced with optimized sensor length, the SRI, and the roughness of side-polished surface. The polynomial fitting results of the resonant wavelength as a function of SRI are shown in Figs. 11(d)–(f). The resonance wavelength has a larger shift in the higher RI region. Maximum RI sensitivities are obtained when the RI is changed from 1.394 to 1.404, which are 4796, 3557, 4903 nm/RIU for the FMF-SPR sensor #1, #2 and #3, respectively. Since there is a trade-off between the sensitivity and FWHM of the SPR spectrum, the FOM is calculated to characterize the sensing performance. The maximum FOMs are 46.1, 26.9, 34.8 the FMF-SPR sensor #1, #2 and #3, respectively. The differences between the calculated results and experimental results might be caused by the roughness of side-polished surface, the RFT measurement error and the coupling between the light source and fiber sensor. Different SPR sensor configurations are summarized in Table 1. In comparison, the demonstrated structure has higher FOM than other SPR sensor configurations.
4. Bio-sensing application
In order to illustrate the potential applications of the proposed sensor, we applied this sensor for detecting the concentration of bovine serum albumin (BSA). The BSA solutions with different concentrations were prepared by dispersing BSA powder in phosphate buffer saline via ultrasonic treatment (Scientz-IID, Ningbo Xingzhi Biotechnology Co., Ltd, China) for 30 mimutes. The RI of the BSA solution was calibrated by an Abbe refractometer (Edmund NT52-975, Edmund Optics Co., Ltd, China) at a room temperature of 25 °C. The RI of the BSA solution increases with the BSA concentration. Then the BSA solutions were used for sensing experiments for the 72.24 μm sensor. The experimental setup for BSA measurement is shown in Fig. 12(a). The measured transmission spectra under various BSA concentrations are plotted in Fig. 12(b). As can be seen, the transmission spectrum shifts to longer wavelength when the BSA concentration increases. In addition, the resonant dip becomes deeper with larger BSA concentrations. Additionally, we measured the RIs of different BSA solutions with Abbe refractometer. Therefore, the dependence of the resonant wavelength on the RI can be yielded and are depicted in Fig. 12(c). The resonant wavelength increases with BSA concentration linearly. The linear fitting shows an average RI sensitivity of 6328 nm/RIU, corresponding to a BSA concentration sensitivity of 1.17 nm/(mg/ml). The results confirm the ability of the proposed sensor in measuring the concentration of bio-molecules.
The RI sensitivity of BSA solution is higher than the RI sensitivity of analyte solution shown in Fig. 11, this might be explained by the fact that SPR sensor shows higher sensitivity in surface-adsorbing events (called surface sensitivity) than that in bulk solutions (called bulk sensitivity) .
We have designed and demonstrated SPR sensors based on side-polished FMF. The FMF provides the SPR sensors with a relatively smaller FWHM for the SPR spectrum, a high sensitivity up to 4903 nm/RIU and thus an FOM of 46.1 RIU−1, when SRI is varied from 1.333 to 1.404. The averaged sensitivity of 6328 nm/RIU and the averaged concentration sensitivity of 1.17 nm/(mg/ml) in the testing of BSA solutions confirm the capability of the FMF-SPR sensors for the bio-sensing applications. Moreover, when the mode-multiplexing advantages brought by the few-mode fiber are fully integrated into the sensing system, the FMF-SPR sensors would find great potential applications in bio-chemical, environmental, and food safety assessing with high sensitivities and high throughputs.
National Natural Science Foundation of China (NSFC) (61575084, 61805108, 61475066, 61705087, 61705046); Natural Science Foundation of Guangdong Province (2015A030313320, 2014A030313377, 2016A030311019); Guangdong Science and Technology Department (2016B010111003, 2016A010101017, 2014B010120002, 2014B090905001, 201707010500, 201807010077, 201605030002, 201704030105); Joint Fund of Pre-research for Equipment, Ministry of Education of the People’s Republic of China (6141A02022124); Special Research Fund for Central Universities (21618404, 21617332).
1. A. Li, YA. Al Amin, X. Chen, and TW. Shieh, “Reception of mode and polarization multiplexed 107-Gb/s CO-OFDM signal over a two-mode fiber,” in Optical Fiber Communication Conference/National Fiber Optic Engineers Conference 2011 (Optical Society of America, 2011), paper PDPB8.
2. J. Dong and K. S. Chiang, “Temperature-insensitive mode converters with CO2-laser written long-period fiber gratings,” IEEE Photonics Technol. Lett. 27(9), 1006–1009 (2015). [CrossRef]
3. Y. Zhao, Y. Liu, C. Zhang, L. Zhang, G. Zheng, C. Mou, J. Wen, and T. Wang, “All-fiber mode converter based on long-period fiber gratings written in few-mode fiber,” Opt. Lett. 42(22), 4708–4711 (2017). [CrossRef] [PubMed]
5. J. Dong and K. S. Chiang, “Mode-locked fiber laser with transverse-mode selection based on a two-mode FBG,” IEEE Photonics Technol. Lett. 26(17), 1766–1769 (2014). [CrossRef]
6. L. Zhang, Y. Liu, Y. Zhao, and T. Wang, “High sensitivity twist sensor based on helical long-period grating written in two-mode fiber,” IEEE Photonics Technol. Lett. 28(15), 1629–1632 (2016). [CrossRef]
7. R. C. Jorgenson and S. S. Yee, “A fiber-optic chemical sensor based on surface plasmon resonance,” Sens. Actuators B Chem. 12(3), 213–220 (1993). [CrossRef]
9. J. Pollet, F. Delport, K. P. F. Janssen, K. Jans, G. Maes, H. Pfeiffer, M. Wevers, and J. Lammertyn, “Fiber optic SPR biosensing of DNA hybridization and DNA-protein interactions,” Biosens. Bioelectron. 25(4), 864–869 (2009). [CrossRef] [PubMed]
10. S. Herranz, M. Bocková, M. D. Marazuela, J. Homola, and M. C. Moreno-Bondi, “An SPR biosensor for the detection of microcystins in drinking water,” Anal. Bioanal. Chem. 398(6), 2625–2634 (2010). [CrossRef] [PubMed]
11. S. Zeng, D. Baillargeat, H. P. Ho, and K. T. Yong, “Nanomaterials enhanced surface plasmon resonance for biological and chemical sensing applications,” Chem. Soc. Rev. 43(10), 3426–3452 (2014). [CrossRef] [PubMed]
14. C. Caucheteur, V. Voisin, and J. Albert, “Near-infrared grating-assisted SPR optical fiber sensors: design rules for ultimate refractometric sensitivity,” Opt. Express 23(3), 2918–2932 (2015). [CrossRef] [PubMed]
15. M. Mitsushio, S. Higashi, and M. Higo, “Construction and evaluation of a gold-deposited optical fiber sensor system for measurements of refractive indices of alcohols,” Sens. Actuators A Phys. 111(2–3), 252–259 (2004). [CrossRef]
16. H. Wang, H. Zhang, J. Dong, S. Hu, W. Zhu, W. Qiu, H. Lu, J. Yu, H. Guan, S. Gao, Z. Li, W. Liu, M. He, J. Zhang, Z. Chen, and Y. Luo, “Sensitivity-enhanced surface plasmon resonance sensor utilizing a tungsten disulfide (WS 2) nanosheets overlayer,” Photon. Res. 6(6), 485–491 (2018). [CrossRef]
17. M. Yang, X. Xiong, R. He, Y. Luo, J. Tang, J. Dong, H. Lu, J. Yu, H. Guan, J. Zhang, Z. Chen, and M. Liu, “Halloysite nanotube-modified plasmonic interface for highly sensitive refractive index sensing,” ACS Appl. Mater. Interfaces 10(6), 5933–5940 (2018). [CrossRef] [PubMed]
18. J. Zhao, S. Cao, C. Liao, Y. Wang, G. Wang, X. Xu, C. Fu, G. Xu, J. Lian, and Y. Wang, “Surface plasmon resonance refractive sensor based on silver-coated side-polished fiber,” Sens. Actuators B Chem. 230, 206–211 (2016). [CrossRef]
19. S. Ju, S. Jeong, Y. Kim, S.-H. Lee, K. Linganna, C. J. Kim, and W.-T. Han, “Effect of heat treatment of optical fiber incorporated with Au nano-particles on surface plasmon resonance,” Opt. Mater. Express 5(6), 1440–1449 (2015). [CrossRef]
20. T. Schuster, R. Herschel, N. Neumann, and C. G. Schaffer, “Miniaturized long-period fiber grating assisted surface plasmon resonance sensor,” J. Lightwave Technol. 30(8), 1003–1008 (2012). [CrossRef]
21. M. H. Chiu, C. H. Shih, and M. H. Chi, “Optimum sensitivity of single-mode D-type optical fiber sensor in the intensity measurement,” Sens. Actuators B Chem. 123(2), 1120–1124 (2007). [CrossRef]
22. H. Moayyed, I. T. Leite, L. Coelho, J. L. Santos, and D. Viegas, “Analysis of a plasmonic based optical fiber optrode with phase interrogation,” Photonic Sens. 6(3), 221–233 (2016). [CrossRef]
23. H. Moayyed, I. T. Leite, L. Coelho, J. L. Santos, and D. Viegas, “Analysis of phase interrogated SPR fiber optic sensors with bimetallic layers,” IEEE Sens. J. 14(10), 3662–3668 (2014). [CrossRef]
24. Y. N. Kulchin, O. B. Vitrik, and A. V. Dyshlyuk, “Analysis of surface plasmon resonance in bent single-mode waveguides with metal-coated cladding by eigenmode expansion method,” Opt. Express 22(18), 22196–22201 (2014). [CrossRef] [PubMed]
25. Y. C. Kim, W. Peng, S. Banerji, and K. S. Booksh, “Tapered fiber optic surface plasmon resonance sensor for analyses of vapor and liquid phases,” Opt. Lett. 30(17), 2218–2220 (2005). [CrossRef] [PubMed]
26. S. K. Mishra and B. D. Gupta, “Surface plasmon resonance-based fiber-optic hydrogen gas sensor utilizing indium–tin oxide (ITO) thin films,” Plasmonics 7(4), 627–632 (2012). [CrossRef]
27. J. A. Kim, T. Hwang, S. R. Dugasani, R. Amin, A. Kulkarni, S. H. Park, and T. Kim, “Graphene based fiber optic surface plasmon resonance for bio-chemical sensor applications,” Sens. Actuators B Chem. 187, 426–433 (2013). [CrossRef]
28. J. Zhao, S. Cao, C. Liao, Y. Wang, G. Wang, X. Xu, C. Fu, G. Xu, J. Lian, and Y. Wang, “Surface plasmon resonance refractive sensor based on silver-coated side-polished fiber,” Sens. Actuators B Chem. 230, 206–211 (2016). [CrossRef]
29. 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 (Basel) 17, 12 (2016). [CrossRef] [PubMed]
30. Y. Luo, C. Chen, K. Xia, S. Peng, H. Guan, J. Tang, H. Lu, J. Yu, J. Zhang, Y. Xiao, and Z. Chen, “Tungsten disulfide (WS2) based all-fiber-optic humidity sensor,” Opt. Express 24(8), 8956–8966 (2016). [CrossRef] [PubMed]
31. H. Zhang, Y. Chen, X. Feng, X. Xiong, S. Hu, Z. Jiang, J. Dong, W. Zhu, W. Qiu, H. Guan, H. Lu, J. Yu, Y. Zhong, J. Zhang, M. He, Y. Luo, and Z. Chen, “Long-Range Surface Plasmon Resonance Sensor Based on Side-Polished Fiber for Biosensing Applications,” IEEE J. Sel. Top. Quantum Electron. 25(2), 1–9 (2019). [CrossRef]
32. Z. Jiang, J. Dong, S. Hu, Y. Zhang, Y. Chen, Y. Luo, W. Zhu, W. Qiu, H. Lu, H. Guan, Y. Zhong, J. Yu, J. Zhang, and Z. Chen, “High-sensitivity vector magnetic field sensor based on side-polished fiber plasmon and ferrofluid,” Opt. Lett. 43(19), 4743–4746 (2018). [CrossRef] [PubMed]
33. A. Díez, M. V. Andres, and J. L. Cruz, “In-line fiber-optic sensors based on the excitation of surface plasma modes in metal-coated tapered fibers,” Sens. Actuators B Chem. 73, 95–99 (2001). [CrossRef]
34. L. C. C. Coelho, J. M. M. M. de Almeida, H. Moayyed, J. L. Santos, and D. Viegas, “Multiplexing of surface plasmon resonance sensing devices on etched single-mode fiber,” J. Lightwave Technol. 33(2), 432–438 (2015). [CrossRef]
36. T. Allsop, R. Neal, S. Rehman, D. J. Webb, D. Mapps, and I. Bennion, “Generation of infrared surface plasmon resonances with high refractive index sensitivity utilizing tilted fiber Bragg gratings,” Appl. Opt. 46(22), 5456–5460 (2007). [CrossRef] [PubMed]
37. T. Schuster, R. Herschel, N. Neumann, and C. G. Schaffer, “Miniaturized long-period fiber grating assisted surface plasmon resonance sensor,” J. Lightwave Technol. 30(8), 1003–1008 (2012). [CrossRef]
38. P. Mao, Y. Luo, C. Chen, S. Peng, X. Feng, J. Tang, J. Fang, J. Zhang, H. Lu, J. Yu, and Z. Chen, “Design and optimization of surface plasmon resonance sensor based on multimode fiber,” Opt. Quantum Electron. 47(6), 1495–1502 (2015). [CrossRef]
39. Y. Zhang, P. Liang, Y. Wang, Y. Zhang, Z. Liu, Y. Wei, Z. Zhu, E. Zhao, J. Yang, and L. Yuan, “Cascaded distributed multichannel fiber SPR sensor based on gold film thickness adjustment approach,” Sens. Actuators A Phys. 267, 526–531 (2017). [CrossRef]
40. L. Coelho, J. de Almeida, J. L. Santos, R. A. S. Ferreira, P. S. André, and D. Viegas, “Sensing structure based on surface plasmon resonance in chemically etched single mode optical fibres,” Plasmonics 10(2), 319–327 (2015). [CrossRef]
43. M. Piliarik, J. Homola, Z. Manıková, and J. Čtyroký, “Surface plasmon resonance sensor based on a single-mode polarization-maintaining optical fiber,” Sens. Actuators B Chem. 90(1–3), 236–242 (2003). [CrossRef]
44. S. Cao, Y. Shao, Y. Wang, T. Wu, L. Zhang, Y. Huang, F. Zhang, C. Liao, J. He, and Y. Wang, “Highly sensitive surface plasmon resonance biosensor based on a low-index polymer optical fiber,” Opt. Express 26(4), 3988–3994 (2018). [CrossRef] [PubMed]
45. Z. W. Ding, T. T. Lang, Y. Wang, and C.-L. Zhao, “Surface plasmon resonance refractive index sensor based on tapered coreless optical fiber structure,” J. Lightwave Technol. 35(21), 4734–4739 (2017). [CrossRef]
46. H. S. Jang, K. N. Park, C. D. Kang, J. P. Kim, S. J. Sim, and K. S. Lee, “Optical fiber SPR biosensor with sandwich assay for the detection of prostate specific antigen,” Opt. Commun. 282(14), 2827–2830 (2009). [CrossRef]
47. Y. Wang, J. Dong, Y. Luo, J. Tang, H. Lu, J. Yu, H. Guan, J. Zhang, and Z. Chen, “Indium Tin Oxide Coated Two-Mode Fiber for Enhanced SPR Sensor in Near-Infrared Region,” IEEE Photonics J. 9(6), 1–9 (2017). [CrossRef]
48. Y. Al-Qazwini, P. T. Arasu, and A. S. M. Noor, “Numerical investigation of the performance of an SPR-based optical fiber sensor in an aqueous environment using finite-difference time domain,” in Proceedings of the 2nd International Conference on Photonics, (IEEE, 2011), pp. 1–4. [CrossRef]
49. J. N. Dash and R. Jha, “On the performance of graphene-based D-shaped photonic crystal fibre biosensor using surface plasmon resonance,” Plasmonics 10(5), 1123–1131 (2015).
50. S. Shukla, N. K. Sharma, and V. Sajal, “Sensitivity enhancement of a surface plasmon resonance based fiber optic sensor using ZnO thin film: a theoretical study,” Sens. Actuators, B 206(1), 463–470 (2015).
51. T. Huang, “Highly sensitive SPR sensor based on D-shaped photonic crystal fiber coated with indium tin oxide at near-infrared wavelength,” Plasmonics 12(3), 583–588 (2017).
53. K. Takagi, H. Sasaki, A. Seki, and K. Watanabe, “Surface plasmon resonances of a curved hetero-core optical fiber sensor,” Sens. Actuators B Chem. 161(1–2), 1–5 (2010).
54. M. Piliarik, J. Homola, Z. Maníková, and J. Čtyroký, “Surface plasmon resonance based on a polarization-maintaining optical fiber,” Sens. Actuators B Chem. 90(1–3), 236–242 (2003). [CrossRef]
55. P. Berini, “Bulk and surface sensitivities of surface plasmon waveguides,” New J. Phys. 10(10), 105010 (2008). [CrossRef]