Effect of constructional parameters on the performance of a surface plasmon resonance sensor based on a multimode polymer optical fiber Katarzyna Gasior,* Tadeusz Martynkien, and Waclaw Urbanczyk Institute of Physics, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland *Corresponding author: [email protected] Received 11 August 2014; revised 28 October 2014; accepted 30 October 2014; posted 4 November 2014 (Doc. ID 220819); published 1 December 2014
We experimentally studied the influence of different constructional parameters on the performance of surface plasmon resonance (SPR) sensors based on a commercially available polymer step-index multimode fiber. For the first time, to the best of our knowledge, we experimentally investigated the influence of polishing depth on the characteristics of SPR sensors based on a straight multimode fiber. We also examined the impact of sensing length on the spectral position and strength of the SPR in side-polished straight fibers. To clarify literature contradictions concerning the effect of fiber bending on SPR, we experimentally investigated the performance of U-bent SPR sensing probes based on multimode fibers. We have shown that the SPR can be significantly amplified by bending the polymer fiber with stripped cladding. We also demonstrated that the side-polishing of U-bent sensing probes has little impact on their performance. © 2014 Optical Society of America OCIS codes: (240.6680) Surface plasmons; (060.2370) Fiber optics sensors; (130.5460) Polymer waveguides. http://dx.doi.org/10.1364/AO.53.008167
1. Introduction
Optical sensors based on the surface plasmon resonance (SPR) effect are very promising for applications in many fields, including chemistry and biology [1–3]. The sensing principle of SPR sensors relies upon changes in the wavelength of resonant coupling between the plasmon and incident light, caused by a small variation in the refractive index of an analyte covering a metal layer [4]. In 1993 Jorgenson and Yee showed that the traditional Kretschmann’s configuration based on prism coupling can also be used in optical fibers [5]. Such a concept allows for a simple sensor design without the necessity of controlling the incidence angle as well as miniaturization and remote 1559-128X/14/358167-08$15.00/0 © 2014 Optical Society of America
sensing [6]. Different designs of fiber-optic SPR sensors have been reported in the literature so far, including sensors based on single-mode [4,6–8] and multimode [9–11] fibers. In multimode SPR fiber sensors, due to many guided modes, the spectral width of the resonance is broader, which results in less precise measurements of the resonance wavelength [12]. However, SPR sensors based on multimode fibers have several advantages, such as simplicity of construction, lower cost, and easiness of effective coupling with broadband light sources, which makes the resonance detection more convenient [10]. So far, to expose the core for evaporation of a metal layer, SPR sensors based on multimode fibers have been prepared by stripping the cladding [13], fabricating the sensing tip [10], tapering [14,15], and side-polishing [9,16–21]. In silica multimode fibers, the sidepolishing process was conducted until half of the core 10 December 2014 / Vol. 53, No. 35 / APPLIED OPTICS
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was approached [9,16] or reached [17,18]. The polishing depth in the SPR sensors based on polymer multimode fibers reported in [19–21] was not precisely defined. However, it is believed in the literature that to obtain the effective coupling of light guided in multimode fiber to the plasmon, the fiber core must be polished to half of its diameter to allow for interaction of the fundamental mode with the metal layer [9,16,18]. Literature reports on the effect of bending on the SPR resonance in multimode fibers are somewhat contradictory. It was shown in [22] by numerical simulations that increasing the curvature of the SPR U-bent sensing probe based on multimode silica fiber broadens the resonance, increases the sensitivity, and shifts the resonance position toward longer wavelengths by hundreds of nanometers. These theoretical predictions were not confirmed by experimental observations presented in [23], in which the performance of the so-called hetero-core optical fiber SPR sensor was studied. This sensor was composed of three sections of silica fibers, respectively multimode, single-mode (actually operating in a multimode regime as no-core fiber), and multimode fiber spliced together, with the single-mode fiber covered by a gold layer. Investigations of the SPR resonance versus the bending radius of the metal-coated singlemode section showed amplification of the resonance depth, a decrease in the resonance width, and a small shift toward longer wavelengths (by single nanometers) [23]. Discrepancies between the numerical simulations [22] and the experimental observations [23] are most likely caused by the fact that the simulations were based on a very simple two-dimensional model. Experimental studies of a polished U-shaped probe made of polymer multimode fiber were also undertaken in [21], but as the curvature of the fiber was reduced to improve signal quality, no conclusions can be drawn from this work on the dependence of the SPR resonance upon bending. The effect of bending was also employed to enhance the coupling between light guided in multimode fiber and metal-coated nanospheres attached to the cladding [24] and to strengthen the SPR resonance in specially designed microstructured single-mode fiber [25]. Today, most of the SPR optical fiber sensors presented in the literature are based on silica optical fibers. However, polymer optical fibers (POFs) can be more advantageous in some applications due to their low cost, flexibility, and simplicity of SPR sensors fabrication. Therefore, in this work we have investigated the influence of different constructional parameters on the performance of the SPR sensors based on multimode polymer fibers. For the first time, to the best of our knowledge, we report the results of systematic studies on the influence of the polishing depth on the characteristics of the SPR sensor based on straight multimode fiber. We also examined the impact of the sensing length on the spectral position and strength of the SPR in side-polished multimode fibers. Finally, to clarify 8168
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literature contradictions concerning the effect of fiber bending on SPR, we experimentally investigated the performance of U-bent SPR sensing probes. In particular, we have shown that the SPR can be significantly amplified by bending the polymer multimode fiber with stripped cladding. We also demonstrated that side-polishing of U-bent sensing probes has little impact on their performance. 2. Investigated Sensors
The investigated sensors were made of commercially available step-index POF (DD-1000, supplied by Conrad Electronics SE) with a polymethylmetacrylate (PMMA) core with diameter of 980 μm, covered by a thin cladding (10 μm thick) made of a fluorinated polymer. The numerical aperture of this fiber is equal to 0.5. In order to expose the fiber core to deposition of the thin gold layer, we used the side-grinding/ polishing method or mechanical stripping of the fiber cladding. In the grinding/polishing process part of the cladding and the core was removed, thus allowing us to deposit a metal layer on a flat core surface. To better control the polishing process we used flat, 35 mm long polymer plates made of PMMA with a v-groove on top, in which the polymer fiber was fixed with epoxy glue. Next, using a grinding machine with silicon carbide grinding powder, we removed part of the fiber core up to specified depth h measured from the core–cladding interface [Fig. 1(a)]. In the next step, the plate together with the glued fiber was polished using cerium oxide to obtain a smooth surface. Finally, the polished surface was covered with a gold layer with a thickness of 40 nm deposited in a plasma sputtering machine (Q150 T ES, Quorum Technologies). The surface of the polished polymer fiber before and after deposition of the 40 nm gold layer was scanned by an atomic force microscope (AFM) (Fig. 2). The presented AFM scans show that the roughness of the metal layer is determined by the quality of the polishing process. The surface profiles of the polished PMMA fiber and the deposited gold layer measured along selected lines are very similar, with root-mean-squared
Fig. 1. (a) Schematic view of the side-polished fiber. (b), (c) Photographs of the sensing probes based on (b) straight and (c) slightly curved fiber.
Fig. 2. Atomic force microscope (AFM) images of the sidepolished fiber (a) before and (b) after deposition of 40 nm thick gold layer with roughness profiles measured along selected lines (different for both scans).
roughness equal to 6.2 and 5.9 nm, respectively, for the polymer and metal surface. In order to examine the influence of the polishing depth h on the SPR resonance characteristics, we fabricated three sensors with different h parameters equal to 90, 290, and 490 μm, respectively, [Fig. 1(b)]. In this case, the length of the sensing part covered with a metal layer was equal to 35 mm for all the sensors, as only the area of a polymer plate with a glued straight fiber was polished. To study the influence of the length on the SPR resonance, we also fabricated three other probes by gluing slightly bent fiber in a v-groove made in the base polymer plate (fiber bending radius was about 30 cm) [Fig. 1(c)]. In this case, by adjusting the maximum polishing depth h, we could control the length L of the sensing section consisting of a partially polished core covered with a gold layer. We fabricated and characterized three probes with different lengths of the sensing part, L 15, 26, and 35 mm, which correspond to the following maximum polishing depths: h 90, 290, and 490 μm, respectively. Finally, we fabricated and studied the U-bent SPR sensors of two types. The first U-bent probe was made by mechanical stripping of the fiber cladding over a distance of 5 mm. After stripping the cladding, half of the exposed fiber core was covered by the gold layer deposited from one side. Due to the different incidence angles of gold atoms on the fiber core during the sputtering process, the gold layer had the greatest thickness (40 nm) on the top of the core, which gradually decreased to 0 nm according to Lambert’s law. The fiber with a deposited gold layer was bent and fixed in a v-groove of half-circular shape made in a rectangular polymer plate [Fig. 3(a)]. To investigate the effect of fiber curvature on the performance of the U-probe, we used plates of different radii, R 10, 15, 20, 30, 40, and 50 mm, respectively. We also fabricated and investigated a series of Ubent sensing probes grinded and polished from the top of bending to remove part of the core. In this case, to simultaneously control the bending radius and the
Fig. 3. Schematic view and photograph of (a) U-bent sensing probe made of mechanically stripped multimode fiber and (b) fiber polished from the top of bending.
grinding/polishing depth h, the fiber was placed in a v-grove of half-circular shape made in a rectangular polymer plate covered with another flat plate from the top and glued with epoxy [Fig. 3(b)]. Finally, the glued plates together with the bent fiber in between were grinded and polished from the top of bending. In this way a flat surface of the core was exposed on the top of bending and finally covered with a 40 nm thick gold layer. We used plates with bending radii equal to R 10, 15, and 20 mm. 3. Experimental Results
To characterize the SPR sensors we used an experimental setup shown in Fig. 4, which is based on the wavelength interrogation method. A halogen lamp was used as a broadband light source, while the spectrum of transmitted light was measured at the fiber output by means of a spectrometer (Ocean Optics USB4000) in the range of 400–1100 nm. The SPR transmission spectra registered after application of the analyte were normalized with respect to the air spectrum used as a reference. As an analyte we used glycerol–water mixtures with different refractive indices measured by an Abbe refractometer at the wavelength of 589 nm. As we analyzed the normalized spectra, the resonance depth was determined with respect to the reference level for the normalized transmission, which is equal to 100%. The resonance width was determined at half of its minimum (FWHM) measured with respect to 100% transmission level. A. Side-Polished SPR Sensing Probes
For side-polished SPR sensors we first investigated the effect of polishing/grinding depth h on the resonance characteristics. For this purpose we used a set of three sensors made of straight fiber with different
Fig. 4. Schematic diagram of the experimental setup. 10 December 2014 / Vol. 53, No. 35 / APPLIED OPTICS
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polishing depths, h 90, 290, and 490 μm, measured with respect to the core cladding interface and the same length of the sensing section equal to L 35 mm. In Fig. 5 we present the resonance curves obtained for the three sensors for different values of the refractive index of the analyte na , which was changed in the range from 1.332 to 1.422. The transmission characteristics obtained for na 1.332 clearly show that the resonance depth depends weakly on the
Fig. 5. SPR transmission spectra normalized to the air spectrum registered for different refractive indices of the analyte for three sensing probes made of straight fiber with polishing depths of (a) h 90 μm, (b) 290 μm, and (c) 490 μm. 8170
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parameter h. The resonance occurs at around 600 nm, and its depth reaches 17% for all the sensors. With an increasing value of na, the resonance width broadened and its depth gradually decreased down to about 12% for na ≈ 1.41. We did observe the dependence of the full width at half minimum (FWHM) versus polishing depth h. The FWHM of the resonance decreased from 80 nm at h 90 μm to 70 nm at h 490 μm for na 1.332. This effect is most likely related to the lower number of modes guided in the more polished sensing section, which makes the resonance condition less disperse. The same tendency was also observed for greater values of na, although the resonance width increased with increasing na for all the sensors. It should be noted that the FWHM of the SPR observed in our experiment is reduced twofold compared to the results reported in [19] for the SPR sensors based on the same polymer fiber. According to the theoretical investigations presented in [26], a significant reduction of the FWHM is most likely related to improved smoothness of the metal layer in our sensors, which was achieved by using a polishing machine, in contrast to a hand polishing used for fabrication of the sensors reported in [19]. Finally, as shown in Fig. 6, we observed greater sensitivity for lower polishing depth h. For instance, for na 1.352, the sensitivity (S dλSPR ∕dna ) calculated by polynomial fitting of the experimental data presented in Fig. 6 is S 1711, 1590, and 1375 nm/ RIU, respectively, for h 90, 290, and 490 μm, while for na 1.390 it increases to 2978, 2421, and 2469 nm/RIU, correspondingly. The obtained sensitivities take slightly lower values compared to the results reported in [19], but in our case the dependence of the resonance wavelength upon the refractive index change is clearly nonlinear. The influence of the sensing length L on the resonance depth was investigated using the sensors made of slightly curved fiber shown in Fig. 1(c). The increase in the sensing length resulted in a resonance amplification from 3% for L 15 mm to 10% for L 26 mm, and up to 17% for L 35 mm,
Fig. 6. Resonance wavelength λSPR as a function of refractive index of the analyte measured for different values of polishing depth h for sensors based on straight fibers.
Fig. 7. SPR transmission spectra normalized to the air spectrum registered for the sensing probes made of slightly curved fiber with different sensing length L and polishing depth h, na 1.332.
measured for na 1.332 (Fig. 7); other characteristics of the SPR such as the FWHM and the sensitivity (Fig. 8) obtained for different combinations of sensing length L and polishing depth h for slightly curved fiber showed similar behavior to that for straight fiber with the same h parameter. These observations lead to the conclusion that the length of the sensing part is responsible only for the resonance depth and does not noticeably influence other resonance characteristics (FWHM and sensitivity), which depend mostly on the polishing depth. Our studies of the side-polished SPR sensors based on straight multimode fiber did not confirm the belief spread in the literature that the optimum polishing depth h is close to 50% of the fiber core diameter [9,16,18]. In contrast, we have demonstrated that a polishing depth equal to 10% of the fiber diameter (for the 35 mm long sensing part) is enough to obtain resonance of high quality, which cannot be improved by further increasing the polishing depth. Moreover, a lower value of the h parameter results in greater sensitivity of the SPR sensor based on multimode fiber at the expense of only a small increase in the
Fig. 8. Resonance wavelength λSPR as a function of refractive index of the analyte measured for different values of sensing length L and polishing depth h for the sensing probes made of slightly curved fiber.
Fig. 9. SPR transmission spectra registered for various bending radii of the fiber with mechanically stripped cladding (na 1.332).
FWHM, compared to sensors with an h parameter equal to 50% of the core diameter. B. U-Bent Sensing Probes
We also characterized the SPR sensors based on U-bent multimode fibers. The first probe was made of fiber with mechanically stripped cladding. The exposed fiber core was covered with a gold layer over the distance of L 5 mm, and the fiber was bent at different radii. The surface of the core coated with metal was about 7.7 mm2 . In Fig. 9, we show the resonance characteristics of this sensing probe measured for na 1.332 as a function of bending radius R. We clearly see in this figure a significant increase in the resonance depth ΔT versus fiber curvature 1∕R. It changes from ΔT 4.5% for straight fiber up to ΔT 20% for R 10 mm (Fig. 10). It is important to note that the resonance depth in the bent fiber with R 10 mm is greater than in the sensor based on a straight fiber discussed in the previous section with seven times longer sensing length (L 35 mm). For the investigated U-probe, we observed a shift of the resonance wavelength from 578 to 620 nm in response to a curvature change from 0 to 0.1 mm−1 ; see Fig. 11. This behavior is caused by a decrease
Fig. 10. Resonance depth ΔT as a function of the fiber curvature 1∕R for sensor with mechanically stripped cladding. 10 December 2014 / Vol. 53, No. 35 / APPLIED OPTICS
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Fig. 11. Resonance wavelength λSPR as a function of the fiber curvature 1∕R obtained for sensor with mechanically stripped cladding.
(toward the critical angle) in the angle of light incidence on the metal layer due to bending. The FWHM of the resonance characteristic does not decrease monotonically with 1∕R as was reported in [23] for a U-bent hetero-core fiber SPR sensor. In our case the FWHM changes randomly in the range of 73– 89 nm versus the bending curvature. This is probably related to the fact that the bending plane could not be perfectly preserved during the experiment and as a consequence the phase matching condition between plasmon and guided modes was influenced by different thicknesses of the gold layer, which is the result of sputtering on the cylindrical fiber core surface from one direction only. The same effect is in part responsible for the greater resonance width observed for na 1.332 in a U-bent SPR probe (73–89 nm) compared to the probe based on straight sidepolished fiber (70 nm at h 90 μm. It should be noted that our experimental results do not match the theoretical analysis concerning the influence of bending on SPR sensor performance in multimode fibers [22], which predicts significant broadening of the resonance and a very large shift of the resonance wavelength induced by bending. For instance, according to theoretical predictions a change in λSPR should be observed from about 650 nm for R 30 mm to 1300 nm for R 10 mm. However, our observations related to resonance amplification and bent-induced shift of λSPR are consistent with experimental results for the hetero-core fiber reported in [23]. The contradiction between the simulation and the experimental results is most likely a result of the too simplified two-dimensional model used in [22]. We also examined the performance of the U-bent probes polished from the top of bending, which are shown in Fig. 3(b). The polishing depths h were adjusted so that the area of the core coated with gold took the same values equal, respectively, to A 4.1, 5.3, 6.5, and 6.9 mm2 , ΔA 0.1 mm2 , for the sensors with different bending radii, R 10, 15, and 20 mm. Detailed parameters of the investigated 8172
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Fig. 12. Resonance depth ΔT as a function of the fiber curvature 1∕R for sensors based on stripped and polished fibers (A denotes the surface of the sensing area).
sensors, including sensing area A, polishing depth h, and length of sensing section L are gathered in Table 1. The resonance characteristics obtained for na 1.332 and different bending radii are shown in Fig. 12. The resonance depth increases with the area of the core coated with a gold layer. A positive impact of bending on the resonance depth was only observed in the sensors with greater sensing area. The resonance depths of the U-bent polished sensor with A 6.9 mm2 and the U-bent sensor made of stripped fiber with A 7.7 mm2 are very similar. This proves that a laborious and complicated polishing process practically does not improve the performance of the U-bent sensors compared to sensors based on U-bent stripped fibers. Finally, we measured the change in the resonance wavelength λSPR in response to the variation of the refractive index of the analyte na for U-bent sensing probes of both types. The results obtained for the probes with mechanically stripped cladding are shown in Fig. 13. For na 1.352, the calculated sensitivity S is equal to 1395, 1297, and 1654 nm/RIU for bending radii of 50, 30, and 10 mm, respectively. We obtained similar values for the U-bent probes polished from the top with the same bending radii and a sensing area of A 6.9 mm2. The conducted Table 1. Parameters of the Sensors Based on Bent Multimode Polymer Fiber Polished from the Top of Bending
R 10 mm
R 15 mm
R 20 mm
A mm2
h [μm]
L [mm]
4.1 5.3 6.5 6.9 4.1 5.3 6.5 6.9 4.1 5.3 6.5 6.9
330 460 650 800 265 360 470 510 225 300 390 420
5.3 6.3 7.4 8.2 5.8 6.7 7.7 8.0 6.1 7.1 8.1 8.4
This work was supported by Wrocław Research Center EIT + Ltd. in the frame of the NanoMat project ‘Application of Nanotechnology in Advanced Materials’, within the European Funds for Regional Development, POIG, Sub-action 1.1.2. References
Fig. 13. Resonance wavelength λSPR as a function of the refractive index of the analyte measured for different sensors based on bent fibers.
experiments prove that bending the fiber causes a significant increase in the resonance depth accompanied by a small increase in the sensitivity. Nevertheless, for na 1.352 the sensitivity of the U-bent probe with the smallest bending radius R 10 mm (S 1654 nm∕RIU) is still by a few percent (∼3%) lower than the sensitivity of the probe based on straight fiber with a polishing depth of h 90 μm (S 1711 nm∕RIU). 4. Summary
We systematically investigated the impact of different constructional parameters on the performance of SPR optical fiber sensors based on low-cost multimode POFs. The obtained results can be useful for optimization of the side-polished and U-bent sensing probes. Our findings concerning the side-polished SPR sensors did not confirm the belief spread in the literature that the optimum polishing depth h should be close to 50% of the fiber core diameter. In contrast, we have demonstrated that a polishing depth equal to 10% of the fiber diameter is enough to obtain resonance of high quality (17% deep for 35 mm long sensing probe), which cannot be improved by further polishing. Moreover, a lower value of the h parameter results in greater sensitivity of the SPR sensor based on multimode fiber at the expense of only a small increase in the FWHM compared to the sensors with h equal to 50% of the core diameter. We have also demonstrated that significant enhancement of the SPR can be achieved by bending the multimode fiber covered with a metal layer. The resonance in a U-bent probe made of stripped fiber with bending radius R 10 mm and length L 5 mm is stronger than in the sensors based on straight fiber with a sensing part seven times longer, L 35 mm. So strong amplification of the resonance can potentially be exploited for miniaturization of the SPR sensing probes based on multimode fibers. Finally, we have shown that laborious polishing of the U-bent probes from the top of bending does not improve their performance compared to probes made of U-bent stripped fiber.
1. J. Homola, S. S. Yee, and G. Gauglitz, “Surface plasmon resonance sensors: review,” Sens. Actuators B 54, 3–15 (1999). 2. J. Homola, “Present and future of surface plasmon resonance biosensors,” Anal. Bioanal. Chem. 377, 528–539 (2003). 3. I. Abdulhalim, M. Zourob, and A. Lakhtakia, “Surface plasmon resonance for biosensing: a mini-review,” Electromagnetics 28, 214–242 (2008). 4. J. Homola, “Optical fiber sensor based on surface plasmon excitation,” Sens. Actuators B 29, 401–405 (1995). 5. R. C. Jorgenson and S. S. Yee, “A fiber-optic chemical sensor based on surface plasmon resonance,” Sens. Actuators B 12, 213–220 (1993). 6. J. Homola and R. Slavik, “Fibre-optic sensor based on surface plasmon resonance,” Electron. Lett. 32, 480–481 (1996). 7. R. Slavik, J. Homola, and J. Ctyroky, “Single-mode optical fiber surface plasmon resonance sensor,” Sens. Actuators B 54, 74–79 (1999). 8. R. Slavik, J. Homola, and E. Brynda, “A miniature fiber optic surface plasmon resonance sensor for fast detection of staphylococcal enterotoxin B,” Biosens. Bioelectron. 17, 591–595 (2002). 9. H. Y. Lin, W. H. Tsai, Y. C. Tsao, and B. C. Sheu, “Side-polished multimode fiber biosensor based on surface plasmon resonance with halogen light,” Appl. Opt. 46, 800–806 (2007). 10. H. Suzuki, M. Sugimoto, Y. Matsui, and J. Kondoh, “Effects of gold film thickness on spectrum profile and sensitivity of a multimode-optical-fiber SPR sensor,” Sens. Actuators B 132, 26–33 (2008). 11. C. Perrotton, N. Javahiraly, M. Slaman, B. Dam, and P. Meyrueis, “Fiber optic surface plasmon resonance sensor based on wavelength modulation for hydrogen sensing,” Opt. Express 19, A1175–A1183 (2011). 12. R. Slavik, J. Homola, J. Ctyroky, and E. Brynda, “Novel spectral fiber optic sensor based on surface plasmon resonance,” Sens. Actuators B 74, 106–111 (2001). 13. A. Trouillet, C. Ronot-Trioli, C. Veillas, and H. Gagnaire, “Chemical sensing by surface plasmon resonance in a multimode optical fibre,” Pure Appl. Opt. 5, 227–237 (1996). 14. N. Cennamo, G. D’Agostino, M. Pesavento, and L. Zeni, “High selectivity and sensitivity sensor based on MIP and SPR in tapered plastic optical fibers for the detection of L-nicotine,” Sens. Actuators B 191, 529–536 (2014). 15. F. De-Jun, L. Guan-Xiu, L. Xi-Lu, J. Ming-Shun, and S. Qing-Mei, “Refractive index sensor based on plastic optical fiber with tapered structure,” Appl. Opt. 53, 2007–2011 (2014). 16. W. H. Tsai, Y. C. Tsao, H. Y. Lin, and B. C. Sheu, “Cross-point analysis for multimode fiber sensor based on surface plasmon resonance,” Opt. Lett. 30, 2209–2211 (2005). 17. Y. C. Lin, W. H. Tsai, Y. C. Tsao, and J. K. Tai, “An enhanced optical multimode fiber sensor based on surface plasmon resonance with cascaded structure,” IEEE Photon. Technol. Lett. 20, 1287–1289 (2008). 18. Y. C. Lin, “Characteristics of optical fiber refractive index sensor based on surface plasmon resonance,” Microw. Opt. Technol. Lett. 55, 574–576 (2013). 19. N. Cennamo, D. Massarotti, L. Conte, and L. Zeni, “Low cost sensors based on SPR in a plastic optical fiber for biosensor implementation,” Sensors 11, 11752–11760 (2011). 20. N. Cennamo, D. Massarotti, R. Galatus, L. Conte, and L. Zeni, “Performance comparison of two sensors based on surface plasmon resonance in a plastic optical fiber,” Sensors 13, 721–735 (2013). 21. V. Munoz-Berti, A. Lopez-Perez, B. Alen, J. L. Costa-Kramer, A. Garcia-Martin, M. Lomer, and J. Lopez-Higuera, “Low cost plastic optical fiber sensor based on surface plasmon resonance,” Proc. SPIE 7653, 765327 (2010). 10 December 2014 / Vol. 53, No. 35 / APPLIED OPTICS
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We experimentally studied the influence of different constructional parameters on the performance of surface plasmon resonance (SPR) sensors based on a commercially available polymer step-index multimode fiber. For the first time, to the best of our knowledge, we experimentally investigated the influence of polishing depth on the characteristics of SPR sensors based on a straight multimode fiber. We also examined the impact of sensing length on the spectral position and strength of the SPR in side-polished straight fibers. To clarify literature contradictions concerning the effect of fiber bending on SPR, we experimentally investigated the performance of U-bent SPR sensing probes based on multimode fibers. We have shown that the SPR can be significantly amplified by bending the polymer fiber with stripped cladding. We also demonstrated that the side-polishing of U-bent sensing probes has little impact on their performance. © 2014 Optical Society of America OCIS codes: (240.6680) Surface plasmons; (060.2370) Fiber optics sensors; (130.5460) Polymer waveguides. http://dx.doi.org/10.1364/AO.53.008167
1. Introduction
Optical sensors based on the surface plasmon resonance (SPR) effect are very promising for applications in many fields, including chemistry and biology [1–3]. The sensing principle of SPR sensors relies upon changes in the wavelength of resonant coupling between the plasmon and incident light, caused by a small variation in the refractive index of an analyte covering a metal layer [4]. In 1993 Jorgenson and Yee showed that the traditional Kretschmann’s configuration based on prism coupling can also be used in optical fibers [5]. Such a concept allows for a simple sensor design without the necessity of controlling the incidence angle as well as miniaturization and remote 1559-128X/14/358167-08$15.00/0 © 2014 Optical Society of America
sensing [6]. Different designs of fiber-optic SPR sensors have been reported in the literature so far, including sensors based on single-mode [4,6–8] and multimode [9–11] fibers. In multimode SPR fiber sensors, due to many guided modes, the spectral width of the resonance is broader, which results in less precise measurements of the resonance wavelength [12]. However, SPR sensors based on multimode fibers have several advantages, such as simplicity of construction, lower cost, and easiness of effective coupling with broadband light sources, which makes the resonance detection more convenient [10]. So far, to expose the core for evaporation of a metal layer, SPR sensors based on multimode fibers have been prepared by stripping the cladding [13], fabricating the sensing tip [10], tapering [14,15], and side-polishing [9,16–21]. In silica multimode fibers, the sidepolishing process was conducted until half of the core 10 December 2014 / Vol. 53, No. 35 / APPLIED OPTICS
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was approached [9,16] or reached [17,18]. The polishing depth in the SPR sensors based on polymer multimode fibers reported in [19–21] was not precisely defined. However, it is believed in the literature that to obtain the effective coupling of light guided in multimode fiber to the plasmon, the fiber core must be polished to half of its diameter to allow for interaction of the fundamental mode with the metal layer [9,16,18]. Literature reports on the effect of bending on the SPR resonance in multimode fibers are somewhat contradictory. It was shown in [22] by numerical simulations that increasing the curvature of the SPR U-bent sensing probe based on multimode silica fiber broadens the resonance, increases the sensitivity, and shifts the resonance position toward longer wavelengths by hundreds of nanometers. These theoretical predictions were not confirmed by experimental observations presented in [23], in which the performance of the so-called hetero-core optical fiber SPR sensor was studied. This sensor was composed of three sections of silica fibers, respectively multimode, single-mode (actually operating in a multimode regime as no-core fiber), and multimode fiber spliced together, with the single-mode fiber covered by a gold layer. Investigations of the SPR resonance versus the bending radius of the metal-coated singlemode section showed amplification of the resonance depth, a decrease in the resonance width, and a small shift toward longer wavelengths (by single nanometers) [23]. Discrepancies between the numerical simulations [22] and the experimental observations [23] are most likely caused by the fact that the simulations were based on a very simple two-dimensional model. Experimental studies of a polished U-shaped probe made of polymer multimode fiber were also undertaken in [21], but as the curvature of the fiber was reduced to improve signal quality, no conclusions can be drawn from this work on the dependence of the SPR resonance upon bending. The effect of bending was also employed to enhance the coupling between light guided in multimode fiber and metal-coated nanospheres attached to the cladding [24] and to strengthen the SPR resonance in specially designed microstructured single-mode fiber [25]. Today, most of the SPR optical fiber sensors presented in the literature are based on silica optical fibers. However, polymer optical fibers (POFs) can be more advantageous in some applications due to their low cost, flexibility, and simplicity of SPR sensors fabrication. Therefore, in this work we have investigated the influence of different constructional parameters on the performance of the SPR sensors based on multimode polymer fibers. For the first time, to the best of our knowledge, we report the results of systematic studies on the influence of the polishing depth on the characteristics of the SPR sensor based on straight multimode fiber. We also examined the impact of the sensing length on the spectral position and strength of the SPR in side-polished multimode fibers. Finally, to clarify 8168
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literature contradictions concerning the effect of fiber bending on SPR, we experimentally investigated the performance of U-bent SPR sensing probes. In particular, we have shown that the SPR can be significantly amplified by bending the polymer multimode fiber with stripped cladding. We also demonstrated that side-polishing of U-bent sensing probes has little impact on their performance. 2. Investigated Sensors
The investigated sensors were made of commercially available step-index POF (DD-1000, supplied by Conrad Electronics SE) with a polymethylmetacrylate (PMMA) core with diameter of 980 μm, covered by a thin cladding (10 μm thick) made of a fluorinated polymer. The numerical aperture of this fiber is equal to 0.5. In order to expose the fiber core to deposition of the thin gold layer, we used the side-grinding/ polishing method or mechanical stripping of the fiber cladding. In the grinding/polishing process part of the cladding and the core was removed, thus allowing us to deposit a metal layer on a flat core surface. To better control the polishing process we used flat, 35 mm long polymer plates made of PMMA with a v-groove on top, in which the polymer fiber was fixed with epoxy glue. Next, using a grinding machine with silicon carbide grinding powder, we removed part of the fiber core up to specified depth h measured from the core–cladding interface [Fig. 1(a)]. In the next step, the plate together with the glued fiber was polished using cerium oxide to obtain a smooth surface. Finally, the polished surface was covered with a gold layer with a thickness of 40 nm deposited in a plasma sputtering machine (Q150 T ES, Quorum Technologies). The surface of the polished polymer fiber before and after deposition of the 40 nm gold layer was scanned by an atomic force microscope (AFM) (Fig. 2). The presented AFM scans show that the roughness of the metal layer is determined by the quality of the polishing process. The surface profiles of the polished PMMA fiber and the deposited gold layer measured along selected lines are very similar, with root-mean-squared
Fig. 1. (a) Schematic view of the side-polished fiber. (b), (c) Photographs of the sensing probes based on (b) straight and (c) slightly curved fiber.
Fig. 2. Atomic force microscope (AFM) images of the sidepolished fiber (a) before and (b) after deposition of 40 nm thick gold layer with roughness profiles measured along selected lines (different for both scans).
roughness equal to 6.2 and 5.9 nm, respectively, for the polymer and metal surface. In order to examine the influence of the polishing depth h on the SPR resonance characteristics, we fabricated three sensors with different h parameters equal to 90, 290, and 490 μm, respectively, [Fig. 1(b)]. In this case, the length of the sensing part covered with a metal layer was equal to 35 mm for all the sensors, as only the area of a polymer plate with a glued straight fiber was polished. To study the influence of the length on the SPR resonance, we also fabricated three other probes by gluing slightly bent fiber in a v-groove made in the base polymer plate (fiber bending radius was about 30 cm) [Fig. 1(c)]. In this case, by adjusting the maximum polishing depth h, we could control the length L of the sensing section consisting of a partially polished core covered with a gold layer. We fabricated and characterized three probes with different lengths of the sensing part, L 15, 26, and 35 mm, which correspond to the following maximum polishing depths: h 90, 290, and 490 μm, respectively. Finally, we fabricated and studied the U-bent SPR sensors of two types. The first U-bent probe was made by mechanical stripping of the fiber cladding over a distance of 5 mm. After stripping the cladding, half of the exposed fiber core was covered by the gold layer deposited from one side. Due to the different incidence angles of gold atoms on the fiber core during the sputtering process, the gold layer had the greatest thickness (40 nm) on the top of the core, which gradually decreased to 0 nm according to Lambert’s law. The fiber with a deposited gold layer was bent and fixed in a v-groove of half-circular shape made in a rectangular polymer plate [Fig. 3(a)]. To investigate the effect of fiber curvature on the performance of the U-probe, we used plates of different radii, R 10, 15, 20, 30, 40, and 50 mm, respectively. We also fabricated and investigated a series of Ubent sensing probes grinded and polished from the top of bending to remove part of the core. In this case, to simultaneously control the bending radius and the
Fig. 3. Schematic view and photograph of (a) U-bent sensing probe made of mechanically stripped multimode fiber and (b) fiber polished from the top of bending.
grinding/polishing depth h, the fiber was placed in a v-grove of half-circular shape made in a rectangular polymer plate covered with another flat plate from the top and glued with epoxy [Fig. 3(b)]. Finally, the glued plates together with the bent fiber in between were grinded and polished from the top of bending. In this way a flat surface of the core was exposed on the top of bending and finally covered with a 40 nm thick gold layer. We used plates with bending radii equal to R 10, 15, and 20 mm. 3. Experimental Results
To characterize the SPR sensors we used an experimental setup shown in Fig. 4, which is based on the wavelength interrogation method. A halogen lamp was used as a broadband light source, while the spectrum of transmitted light was measured at the fiber output by means of a spectrometer (Ocean Optics USB4000) in the range of 400–1100 nm. The SPR transmission spectra registered after application of the analyte were normalized with respect to the air spectrum used as a reference. As an analyte we used glycerol–water mixtures with different refractive indices measured by an Abbe refractometer at the wavelength of 589 nm. As we analyzed the normalized spectra, the resonance depth was determined with respect to the reference level for the normalized transmission, which is equal to 100%. The resonance width was determined at half of its minimum (FWHM) measured with respect to 100% transmission level. A. Side-Polished SPR Sensing Probes
For side-polished SPR sensors we first investigated the effect of polishing/grinding depth h on the resonance characteristics. For this purpose we used a set of three sensors made of straight fiber with different
Fig. 4. Schematic diagram of the experimental setup. 10 December 2014 / Vol. 53, No. 35 / APPLIED OPTICS
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polishing depths, h 90, 290, and 490 μm, measured with respect to the core cladding interface and the same length of the sensing section equal to L 35 mm. In Fig. 5 we present the resonance curves obtained for the three sensors for different values of the refractive index of the analyte na , which was changed in the range from 1.332 to 1.422. The transmission characteristics obtained for na 1.332 clearly show that the resonance depth depends weakly on the
Fig. 5. SPR transmission spectra normalized to the air spectrum registered for different refractive indices of the analyte for three sensing probes made of straight fiber with polishing depths of (a) h 90 μm, (b) 290 μm, and (c) 490 μm. 8170
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parameter h. The resonance occurs at around 600 nm, and its depth reaches 17% for all the sensors. With an increasing value of na, the resonance width broadened and its depth gradually decreased down to about 12% for na ≈ 1.41. We did observe the dependence of the full width at half minimum (FWHM) versus polishing depth h. The FWHM of the resonance decreased from 80 nm at h 90 μm to 70 nm at h 490 μm for na 1.332. This effect is most likely related to the lower number of modes guided in the more polished sensing section, which makes the resonance condition less disperse. The same tendency was also observed for greater values of na, although the resonance width increased with increasing na for all the sensors. It should be noted that the FWHM of the SPR observed in our experiment is reduced twofold compared to the results reported in [19] for the SPR sensors based on the same polymer fiber. According to the theoretical investigations presented in [26], a significant reduction of the FWHM is most likely related to improved smoothness of the metal layer in our sensors, which was achieved by using a polishing machine, in contrast to a hand polishing used for fabrication of the sensors reported in [19]. Finally, as shown in Fig. 6, we observed greater sensitivity for lower polishing depth h. For instance, for na 1.352, the sensitivity (S dλSPR ∕dna ) calculated by polynomial fitting of the experimental data presented in Fig. 6 is S 1711, 1590, and 1375 nm/ RIU, respectively, for h 90, 290, and 490 μm, while for na 1.390 it increases to 2978, 2421, and 2469 nm/RIU, correspondingly. The obtained sensitivities take slightly lower values compared to the results reported in [19], but in our case the dependence of the resonance wavelength upon the refractive index change is clearly nonlinear. The influence of the sensing length L on the resonance depth was investigated using the sensors made of slightly curved fiber shown in Fig. 1(c). The increase in the sensing length resulted in a resonance amplification from 3% for L 15 mm to 10% for L 26 mm, and up to 17% for L 35 mm,
Fig. 6. Resonance wavelength λSPR as a function of refractive index of the analyte measured for different values of polishing depth h for sensors based on straight fibers.
Fig. 7. SPR transmission spectra normalized to the air spectrum registered for the sensing probes made of slightly curved fiber with different sensing length L and polishing depth h, na 1.332.
measured for na 1.332 (Fig. 7); other characteristics of the SPR such as the FWHM and the sensitivity (Fig. 8) obtained for different combinations of sensing length L and polishing depth h for slightly curved fiber showed similar behavior to that for straight fiber with the same h parameter. These observations lead to the conclusion that the length of the sensing part is responsible only for the resonance depth and does not noticeably influence other resonance characteristics (FWHM and sensitivity), which depend mostly on the polishing depth. Our studies of the side-polished SPR sensors based on straight multimode fiber did not confirm the belief spread in the literature that the optimum polishing depth h is close to 50% of the fiber core diameter [9,16,18]. In contrast, we have demonstrated that a polishing depth equal to 10% of the fiber diameter (for the 35 mm long sensing part) is enough to obtain resonance of high quality, which cannot be improved by further increasing the polishing depth. Moreover, a lower value of the h parameter results in greater sensitivity of the SPR sensor based on multimode fiber at the expense of only a small increase in the
Fig. 8. Resonance wavelength λSPR as a function of refractive index of the analyte measured for different values of sensing length L and polishing depth h for the sensing probes made of slightly curved fiber.
Fig. 9. SPR transmission spectra registered for various bending radii of the fiber with mechanically stripped cladding (na 1.332).
FWHM, compared to sensors with an h parameter equal to 50% of the core diameter. B. U-Bent Sensing Probes
We also characterized the SPR sensors based on U-bent multimode fibers. The first probe was made of fiber with mechanically stripped cladding. The exposed fiber core was covered with a gold layer over the distance of L 5 mm, and the fiber was bent at different radii. The surface of the core coated with metal was about 7.7 mm2 . In Fig. 9, we show the resonance characteristics of this sensing probe measured for na 1.332 as a function of bending radius R. We clearly see in this figure a significant increase in the resonance depth ΔT versus fiber curvature 1∕R. It changes from ΔT 4.5% for straight fiber up to ΔT 20% for R 10 mm (Fig. 10). It is important to note that the resonance depth in the bent fiber with R 10 mm is greater than in the sensor based on a straight fiber discussed in the previous section with seven times longer sensing length (L 35 mm). For the investigated U-probe, we observed a shift of the resonance wavelength from 578 to 620 nm in response to a curvature change from 0 to 0.1 mm−1 ; see Fig. 11. This behavior is caused by a decrease
Fig. 10. Resonance depth ΔT as a function of the fiber curvature 1∕R for sensor with mechanically stripped cladding. 10 December 2014 / Vol. 53, No. 35 / APPLIED OPTICS
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Fig. 11. Resonance wavelength λSPR as a function of the fiber curvature 1∕R obtained for sensor with mechanically stripped cladding.
(toward the critical angle) in the angle of light incidence on the metal layer due to bending. The FWHM of the resonance characteristic does not decrease monotonically with 1∕R as was reported in [23] for a U-bent hetero-core fiber SPR sensor. In our case the FWHM changes randomly in the range of 73– 89 nm versus the bending curvature. This is probably related to the fact that the bending plane could not be perfectly preserved during the experiment and as a consequence the phase matching condition between plasmon and guided modes was influenced by different thicknesses of the gold layer, which is the result of sputtering on the cylindrical fiber core surface from one direction only. The same effect is in part responsible for the greater resonance width observed for na 1.332 in a U-bent SPR probe (73–89 nm) compared to the probe based on straight sidepolished fiber (70 nm at h 90 μm. It should be noted that our experimental results do not match the theoretical analysis concerning the influence of bending on SPR sensor performance in multimode fibers [22], which predicts significant broadening of the resonance and a very large shift of the resonance wavelength induced by bending. For instance, according to theoretical predictions a change in λSPR should be observed from about 650 nm for R 30 mm to 1300 nm for R 10 mm. However, our observations related to resonance amplification and bent-induced shift of λSPR are consistent with experimental results for the hetero-core fiber reported in [23]. The contradiction between the simulation and the experimental results is most likely a result of the too simplified two-dimensional model used in [22]. We also examined the performance of the U-bent probes polished from the top of bending, which are shown in Fig. 3(b). The polishing depths h were adjusted so that the area of the core coated with gold took the same values equal, respectively, to A 4.1, 5.3, 6.5, and 6.9 mm2 , ΔA 0.1 mm2 , for the sensors with different bending radii, R 10, 15, and 20 mm. Detailed parameters of the investigated 8172
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Fig. 12. Resonance depth ΔT as a function of the fiber curvature 1∕R for sensors based on stripped and polished fibers (A denotes the surface of the sensing area).
sensors, including sensing area A, polishing depth h, and length of sensing section L are gathered in Table 1. The resonance characteristics obtained for na 1.332 and different bending radii are shown in Fig. 12. The resonance depth increases with the area of the core coated with a gold layer. A positive impact of bending on the resonance depth was only observed in the sensors with greater sensing area. The resonance depths of the U-bent polished sensor with A 6.9 mm2 and the U-bent sensor made of stripped fiber with A 7.7 mm2 are very similar. This proves that a laborious and complicated polishing process practically does not improve the performance of the U-bent sensors compared to sensors based on U-bent stripped fibers. Finally, we measured the change in the resonance wavelength λSPR in response to the variation of the refractive index of the analyte na for U-bent sensing probes of both types. The results obtained for the probes with mechanically stripped cladding are shown in Fig. 13. For na 1.352, the calculated sensitivity S is equal to 1395, 1297, and 1654 nm/RIU for bending radii of 50, 30, and 10 mm, respectively. We obtained similar values for the U-bent probes polished from the top with the same bending radii and a sensing area of A 6.9 mm2. The conducted Table 1. Parameters of the Sensors Based on Bent Multimode Polymer Fiber Polished from the Top of Bending
R 10 mm
R 15 mm
R 20 mm
A mm2
h [μm]
L [mm]
4.1 5.3 6.5 6.9 4.1 5.3 6.5 6.9 4.1 5.3 6.5 6.9
330 460 650 800 265 360 470 510 225 300 390 420
5.3 6.3 7.4 8.2 5.8 6.7 7.7 8.0 6.1 7.1 8.1 8.4
This work was supported by Wrocław Research Center EIT + Ltd. in the frame of the NanoMat project ‘Application of Nanotechnology in Advanced Materials’, within the European Funds for Regional Development, POIG, Sub-action 1.1.2. References
Fig. 13. Resonance wavelength λSPR as a function of the refractive index of the analyte measured for different sensors based on bent fibers.
experiments prove that bending the fiber causes a significant increase in the resonance depth accompanied by a small increase in the sensitivity. Nevertheless, for na 1.352 the sensitivity of the U-bent probe with the smallest bending radius R 10 mm (S 1654 nm∕RIU) is still by a few percent (∼3%) lower than the sensitivity of the probe based on straight fiber with a polishing depth of h 90 μm (S 1711 nm∕RIU). 4. Summary
We systematically investigated the impact of different constructional parameters on the performance of SPR optical fiber sensors based on low-cost multimode POFs. The obtained results can be useful for optimization of the side-polished and U-bent sensing probes. Our findings concerning the side-polished SPR sensors did not confirm the belief spread in the literature that the optimum polishing depth h should be close to 50% of the fiber core diameter. In contrast, we have demonstrated that a polishing depth equal to 10% of the fiber diameter is enough to obtain resonance of high quality (17% deep for 35 mm long sensing probe), which cannot be improved by further polishing. Moreover, a lower value of the h parameter results in greater sensitivity of the SPR sensor based on multimode fiber at the expense of only a small increase in the FWHM compared to the sensors with h equal to 50% of the core diameter. We have also demonstrated that significant enhancement of the SPR can be achieved by bending the multimode fiber covered with a metal layer. The resonance in a U-bent probe made of stripped fiber with bending radius R 10 mm and length L 5 mm is stronger than in the sensors based on straight fiber with a sensing part seven times longer, L 35 mm. So strong amplification of the resonance can potentially be exploited for miniaturization of the SPR sensing probes based on multimode fibers. Finally, we have shown that laborious polishing of the U-bent probes from the top of bending does not improve their performance compared to probes made of U-bent stripped fiber.
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