Bimetal coated optical fiber sensors based on surface plasmon resonance induced change in birefringence and intensity Tan Tai Nguyen,1 Eun-Cheol Lee,1,2 and Heongkyu Ju1,2,3,∗ 1 Department
of Bionano technology, Gachon University, South Korea of Nanophysics, Gachon University, South Korea 3 Neuroscience Institute, Gil Hospital, Incheon, South Korea
2 Department
∗ [email protected]
Abstract: We present a surface plasmon resonance (SPR) based multimode fiber sensor with non-golden bimetallic coating. Our detection scheme used, which is capable of measuring the combined effects of SPR-induced birefringence and intensity changes, supported the minimum resolvable refractive index (RI) of 5.8 × 10−6 RIU with the operating RI range of 0.05 to be experimentally obtained at a single wavelength (632.8 nm) without non-spectroscopic techniques. The asymmetric profile of the thickness of the bimetal coating on the fiber core together with the inherent range of incidence angle for multimode propagation also contributed to the wide operating range. The SPR fiber device with the detection scheme demonstrated will be likely to be developed as a real-time label-free and highly sensitive diagnostic device of a wide operating range for biomedical and biochemical applications in a portable format. © 2014 Optical Society of America OCIS codes: (240.6680) Surface plasmons; (280.4788) Optical sensing and sensors; (060.2370) Fiber optics sensors.
References and links 1. B. Liedberg, C. Nylander, and I. Lundstr¨om, “Surface plasmon resonance for gas detection and biosensing,” Sens. Actuators 4, 299–304 (1983). 2. P. B. Daniels, J. K. Deacon, M. J. Eddowes, and D. Pedley, “Surface plasmon resonance applied to immunosensing,” Sens. Actuators 15, 11–17 (1988). 3. R. C. Jorgenson and S. S. Yee, “A fiber optica chemical sensor based on surface plasmon resonance,” Sens. Actuators B 12, 213–220 (1993). 4. S. Miwa and T. Arakawa, “Selective gas detection by means of surface plasmon resonance sensor,” Thin Solid Films 281, 466–468 (1996). 5. J. Melendez, R. Carr, D. Bartholomew, H. Taneja, S. Yee, C. Jung, and C. Furlong, “Development of a surface plasmon resonance sensor for commercial applications,” Sens. Actuators B Chem. 39, 375–379 (1997). 6. H. P. Chiang, C. W. Chen, J. J. Wu, H. L. Li, T. Y. Lin, F. J. S´anchez, and P. T. Leung, “Effects of temperature on the surface plasmon resonance at a metal-semiconductor interface,” Thin Solid Films 515, 6953–6961 (2007). 7. J. Homola, “Optical fiber sensor based on surface plasmon excitation,” Sens. Actuators B Chem. 29, 401–405 (1995). 8. R. Slavik, J. Homola, and J. Ctyrok´y, “Miniaturization of fiber optic surface plasmon resonance sensor,” Sens. Actuators B Chem. 51, 311–315 (1998). 9. A. J. C. Tubb, F. P. Payne, R. B. Millington, and C. R. Lowe, “Sing-mode optical fibre surface plasma wave chemical sensor,” Sens. Actuators B 41, 71–79 (1997).
#203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5590
10. R. K. Vema, A. K. Sharma, and B. D. Gupta, “Surface plasmon resonance based tapered fiber optic sensor with different taper profiles,” Opt. Commun. 281, 1486–1491 (2008). 11. R. Slavik, J. Homola, and J. Ctyrok´y, “Single-mode optical fiber surface plasmon resonance sensor,” Sen. Actuators B Chem. 54, 74–79 (1999). 12. W. B. Lin, N. J. Renault, A. Gagnaire, and H. Gagnaire, “The effects of polarization of the incident light modelling and analysis of a SPR multimode optical fiber sensor,” Sens. Actuators A Phys. 84, 198–204 (2000). 13. E. Fontana, “A novel gold-coated multimode fiber sensor,” IEEE Trans. Microwave Theory Technol. 50, 82–87 (2002). 14. H.-Y. Lin, W.-H. Tsa, Y.-C Tsao, and B.-C, Sheu, “Side-polished multimode fiber biosensors based on surface plasmon resonance with halogen light,” Appl. Opt. 46, 800–806 (2007). 15. M. Piliarik, J. Homola, Z. Manikova, and J. Ctyrok`y, “Surface plasmon resonance based on a single mode polarization maintaining optical fiber,” Sens. Actuators B Chem. 90, 236–242 (2003). 16. 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, 1120–1124 (2007). 17. M. Mitsushio, K. Miyashita, and M. Higo, “Sensor properties and surface characterization of the metal-deposited SPR optical fiber sensors with Au, Ag, Cu and Al,” Sens. Actuators A 125, 296–303 (2006). 18. A. K. Sharmal and G. J. Mohr, “On the performance of surface plasmon resonance based fibre optic sensor with different bimetallic nanoparticle alloy combinations,” J. Phys. D Appl. Phys. 41, 055106 (2008). 19. B. D. Gupta and A. K. Sharma, “Sensitivity evaluation of a multi-layered surface plasmon resonance-based fiber optic sensor: a theoretical study,” Sens. Actuators B Chem. 107, 40–46 (2005). 20. A. A. Kruchinin and Y. G. Vlasov, “Surface plasmon resonance monitoring by means of polarization state measurement in reflected light as the basis of a DNA-probe biosensor,” Sens. Actuators B 30, 77–80 (1996). 21. R. C. Weast, ed., CRC Handbook of Chemistry and Physics, 68th ed. (CRC Press, 1987), p. D-232. 22. S. G. Nelson, K. S. Johnston, and S. S. Yee, “High sensitivity surface plasmon resonance sensor based on phase detection,” Sens. Actuators B Chem. 35, 187–191 (1996). 23. S. Y. Wu, H. P. Ho, W. C. Law, C. L. Lin, and S. K. Kong, “Highly sensitive differential phase-sensitive surface plasmon resonance biosensor based on the Mach-Zehnder configuration,” Opt. Lett. 29, 2378–2380 (2004). ˇ 24. J. Dost´alek, J. Ctyrok´ y, J. Homola, E. Brynda, M. Skalsk´y, P. Nekvindov´a, J. Spirkov´a, J. Skvor, and J. Schr¨ofel, “Surface plasmon resonance biosensor based on integrated optical waveguide,” Sens. Actuators B 76, 8–12 (2001). 25. M. Zourob, S. Mohr, B. J. T. Brown, P. R. Fielden, M. B. McDonnell, and N. J. Goddard, “Bacteria detection using disposable optical leaky waveguide sensors,” Biosens. Bioelectr. 21, 293–302 (2005). 26. J.-G. Huang, C.-L. Lee, H.-M. Lin, T.-L. Chuang, W.-S. Wang, R.-H. Juang, C.-H. Wang, and C. K. Lee, “A miniaturized germanium-doped silicon dioxide-based surface plasmon resonance waveguide sensor for immunoassay detection,” Biosens. Bioelectr. 22, 519–525 (2006).
1.
Introduction
Optical sensors that utilize sensitivity of surface plasmon resonance (SPR) with respect to change in a refractive index of a sensed medium, have been extensively studied for a variety of biological and chemical applications [1–6]. Surface plasmon waves can be excited at the interface between metal and dielectric (sensed medium) with an aid of a transparent high-index material which is usually placed on the opposite side of the dielectric across the metal to provide the excitation light whose phase velocity is tuned for the SPR phase matching condition as given by ω εm εd l , (1) kex = Re c εm + ε d l is the longitudinal component of the propagation vector of excitation light at the where kex metal-dielectric interface, and εm and εd are the relative permittivities of the metal, and the sensed medium, respectively. Optical fibers with appropriate metal coating have also been suggested as a SPR sensing device that can benefit from the device flexibility, the potential use for remote sensing, extension of SPR interaction length, and small volume of analytes, device miniaturization capabilities [3, 7, 8]. Unlike the SPR coupling via a prism, light coupling into the fiber removes the need of angular adjustment of incident light for SPR excitation. The thickness of a metal film deposited
#203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5591
on the exposed core of the fiber which can result from removal of the local part of the fiber cladding, should be chosen for the wavelength in use for SPR. Surface plasmon waves can be excited by the evanescent waves of the fiber guided modes with their penetration depth into the sensing region, determined by the exponentially decreasing electric field Ed (r) in the sensing region (dielectric medium comprising aqueous solution) as given by Ed (r) = Ed (0)e
−r
2 −k2 ksp d
,
(2)
where ksp is the surface plasmon wave propagation constant, kd is the propagation constant in the dielectric (sensing region) and r is the coordinate in the orthogonal direction to the sensing surface. Various types of optical fibers including single mode fibers [9–11], multimode fibers [3, 12–14], polarization maintaining fibers [15], and D-shaped optical fibers [16] have been demonstrated for SPR sensors either on a basis of an wavelength or an intensity interrogation. The sensing operating RI range that had been limited to about 0.01-0.02 RIU in the fiber optic sensors has further been extended to at least 0.06 by utilizing either a tapered single mode fiber that resulted in the various depth of deposited metal [9] or a side-polished multimode fiber [14], both of which were based on wavelength interrogation. However, a spectroscopy technique that would usually make the detection module bulky, puts the limit on sensor device portability and miniaturization without sacrifice of spectral resolution. This leads to requirement of a detection module operated at a single wavelength that can remove the need of a spectrograph to meet the sensor device compactness and an inexpensive integration. In the mean time, non-straightforward process for elimination of silica cladding of the optical fiber does not ensure the homogeneous exposure of the core surface, resulting in difficulties of efficient treatment of the sensing surface, together with fragility of a thin bare core of fibers, in particular, in cases of single mode fibers.
Fig. 1. (a) Metal vapor deposition on the exposed core surface of the clad-free optical fiber. (b) Expected cross-section of the metal coating profile on the fiber core after the bimetallic deposition for SPR.
The theoretical investigation on SPR fiber optical devices with alloy metal deposition has reported that the deposition layer of bimetallic nanoparticle alloy can be better than that of the single-metal nanoparticle layer in terms of figure-of-merits of optical sensors such as a sensitivity, signal-to-noise ratio and an operating range of the sensed refractive index [17, 18]. Thus, the present study examined a SPR sensor device comprising a multimode fiber with bimetallic (Ag-Al) coating, using a non-spectroscopic detection scheme that is capable of de#203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5592
tecting signals sensitive to changes in SPR-induced birefringence and intensity. The minimum detectable RI of 5.8 × 10−6 with the operating RI range of 0.05 has been achieved at a single wavelength (632.8nm). Discussion on the sensing performance is provided with the mathematical description of the birefringence sensitive detection used in our work. 2.
Experimental techniques with mathematical description
We used the non-golden bimetallic coating for SPR, i.e., Ag and Al with different composition ratios, expecting the high enough sensitivity to the RI change while avoiding too much attenuation of light due to SPR for widening operating range [18]. This bimetal combination where Al is in a direct contact with the sensed medium is also expected to prevent Ag-inherent demerits such as the chemical instability and the vulnerability to corrosion, and to simultaneously allow immobilization of bio-receptor without difficulties possibly encountered in the case of sensing surface Ag [17, 19]. We used a polymer-clad multimode fiber (numerical aperture of 0.37, JFTLH, Polymicro Technologies) of 200 μ m diameter core, whose cladding of 10cm length can be removed via a soldering machine with the subsequent cleansing by a mixture of acetone and ethylene. The cladding removal is followed by the subsequent bimetallic film coating for SPR-sensing surface of 10 cm length. This fiber that was bent into a ring shape of 6 cm diameter constitutes the key element of the sensing device, which allows for the main modes to propagate with a number of reflections off the core-metal boundary, generating the multi-reflection enhanced SPR-induced change in the device output light properties. A thermal evaporator was used to coat the Ag and Al layers on the exposed fiber core. As shown in Fig. 1(a), in the vacuum chamber of the thermal evaporator, we placed the fibers perpendicular to the metal vapor deposition direction. After the first deposition of each metal on one side of the fiber, we rotated the fiber by 180 degrees along its axis for the second deposition of the same metal on the other side, expecting the asymmetric cross-sectional profile of coating thickness as shown in Fig. 1(b). Despite the fact that incomplete controllability of the evaporation direction including experimental errors in coating would produce a certain form of non-circular symmetric profile of thickness which would be different from that shown in Fig. 1(b), this asymmetry of the coated profile permits various penetration depth into the sensing region [9]. This causes both the various ksp in Eq. (2) and various incidence angles for SPR to be excited, and consequently can contribute to the enlarged operating range of RI sensing while the high enough sensitivity level boosted by multi-reflections (estimated as 18) off the fiber core surface through the ring-shaped fiber.
Fig. 2. Schematic of the experimental setup for analyte solution sensing. λ /4 and λ /2 represent a quarter- and a half-wave plates, respectively, PBS denotes a polarizing beam splitter, and BD a balanced detector.
Figure 2 shows schematic of the experimental setup with the fiber optical sensor device. A #203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5593
ring-shaped flow cell made of polydimethylsiloxane(PDMS) was fabricated to install the metal coated fiber on the groove built inside the flow cell along its ring-shaped path, with an inlet and an outlet ports made for the analyte solution. The total volume of the analyte solution that could fill the flow cell along the 10 cm sensing length was estimated to be 730μ l. A light source of a He-Ne laser which offered a linearly polarized light at 632.8 nm was passed through a quarterwave plate (λ /4) to make a circular polarization. We used a circularly polarized incident light to ensure that half of its light had the so called SPR-supporting transverse magnetic (TM)polarization at the interface between the sensed medium and the outer metal (Al), taking into account asymmetric cross-sectional profile of the coated metals around the fiber core, shown in Fig. 1(b) The metal coated fiber whose sensing surface interacting with analyte solution was assumed to act itself as a SPR-induced birefringent device where SPR-induced polarizationdependent phase shift occurs as well as polarization-dependent intensity change. The SPRinduced phase changes are denoted by δ φo and δ φe along the ordinary axis (o-axis) and the extra-ordinary axis (e-axis) of the SPR birefringent device, respectively. This is due to the fact that SPR is sensitive to the light polarizations, leading to polarization-dependent properties of light propagating through the fiber (i.e., intensity and phase) [20]. The electric field of fiber H ) and vertical (E V ) output light which is decomposed into the components of horizonal (Eout out polarizations, can be described as follows: ⎤ ⎡ −iΓ/2 ⎡ H ⎤
⎡ ⎤ 0 e 1 Eout ⎦ = R(−ξ ) ⎣ ⎦ R(ξ ) ⎣ ⎦ , ⎣ (3) V ±i Eout 0 eiΓ/2 where R(ξ ) is the rotation matrix with the ξ the the angle between the o-axis of the SPR fiber device and a laboratory horizontal axis, Γ ≡ δ φe − δ φo is the phase retardation of light through the SPR fiber device, and the ± sign denotes the right-handed and left-handed circular polarizations of input light, respectively. The optical power before the incident light coupling into the fiber was 15 mW while that of light coupled out of the fiber was about 97 μ W (∼ −22 dB) in the case of deionized-water filled in the flow cell for the Ag (36nm)-Al (4nm) coated fiber. The attenuation of output power reduces as the Ag thickness portion decreases for a total bimetal thickness of 40 nm kept. The fiber output light that was then passed through a rotatable half-wave plate (λ /2) can be written as ⎡
EλH/2,out
⎤
⎡
− cos 2ψ
⎦ = i⎣ ⎣ EλV/2,out sin 2ψ
sin 2ψ cos 2ψ
⎤⎡
H Eout
⎤
⎦⎣ ⎦, V Eout
(4)
where ψ is the angle between the λ /2 optic axis and a laboratory horizontal axis, thereby determining the relative ratio of optical power between the o- and e-axes polarization components, for each output port of the PBS. The following polarizing beam splitter (PBS) then split light from the λ /2 into two polarization components, i.e., horizontal (H) and vertical (V) polarizations, each of which entered the corresponding diode of a two-port balanced detector (BD) which gave the subtracted signal between the two photodiode signals. Note that each port of the PBS produced the in-line interference between two polarization components, i.e., light polarized along the o- and e-axes of the SPR fiber device. For the righthanded circular polarization of incident light, the optical intensity at the horizonal and vertical
#203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5594
ports of the PBS output can be written as
⎤ ⎡ ⎡ ⎤ /2) 1 − sin Γ sin(2 ξ − 4 ψ ) (I ⎢ out ⎥ IH ⎢ ⎥ ⎥ ⎣ ⎦=⎢ ⎢
⎥ , ⎣ ⎦ IV (Iout /2) 1 + sin Γ sin(2ξ − 4ψ )
(5)
where Iout is the total optical intensity at the fiber output. The subtraction of Ip from Is in the BD removes both the common mode noise present in the light source and Γ-independent terms (the first terms on the RHS) of the matrix element equations as shown in Eq. (5), consequently, giving the signal proportional to the twice the magnitude of the second terms on the RHS of those equations, that is, I− ≡ IV − IH = Iout sin Γ sin(2ξ − 4ψ ). The subtraction signal is subject to the device birefringence change as well as to the output intensity change for a given ξ and ψ . The scanning of ψ by 1800 rotation of the λ /2 plate then enables both the maximum and minimum values of the subtraction signal to be obtained regardless of the orientation of the oand e-axes of the SPR fiber device. Then the sensing signal S, defined as the difference between the maximum and minimum of the subtraction signal, is proportional to the product of the output intensity and the sin Γ S
≡ max(I− ) − min(I− ) = 2Iout sin Γ.
(6)
Fig. 3. Sensing of glycerol concentration (volume to volume ratio) for the fiber device with no metal coating.
The fiber device with the flow cell can be used as a refractometer for the refractive index change in the sensing region since the refractive index change causes the SPR resonance condition to change, resulting in the change both in Iout and Γ. For refractometer experiments, we used the glycerol solution which was one of the laboratory-available standard solvents whose concentration in volume-to-volume ratio (C in %) can be easily calibrated into a refractive index, n with the linear relation obtained using sodium D line (589 nm), n = 1.3330 + 0.0012ρ C (ρ : glycerol density) found in [21]. 3.
Results and discussion
First, we checked the glycerol concentration dependence of optical power at the fiber output both in cases of no metal coated fiber and the metal coated fiber to see the SPR-induced attenuation of intensity. In no metal coating case, the increased concentration of glycerol increases #203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5595
Fig. 4. Sensing of glycerol concentration (volume to volume ratio) for different ratios of Ag and Al coating thicknesses.
optical power at the fiber output due to the fact that the leakage of propagating mode energies over the clad-free sensing length is reduced. On the contrary, the application of metal coating on the fiber core decreases output power as the glycerol concentration increases, manifesting the SPR-induced attenuation of output power. We also measured the signal S in the case of no metal coating, as shown in Fig. 3. The highly nonlinear noise-like fluctuating behavior is visible over the entire concentration range used, indicating that the birefringence response of no metal coated fiber to the glycerol concentration change may not be useful for the sensing application. Figures 4(a)–4(d) show the measured signal S as the glycerol concentration increases, for the different composition ratios of thicknesses of Ag and Al deposited on the fiber core. The signal S decreases with the concentration for all composition ratios of Ag-Al coating thicknesses whilst exhibiting the different sensitivity characteristics, especially, near zero concentration. The coating thickness ratios of tAg /tAl =7nm/30nm and tAg /tAl =30nm/10nm produce more nonlinearly fluctuating signals near zero concentration than those of the other ratios of tAg /tAl =20nm/5nm and tAg /tAl =36nm/4nm. It is also seen that the two different sensitivity (slope) regions appear around the concentrations of about 1% and 0.05%, as shown in Figs. 4(c) and 4(d), respectively. It was also found that the sensitivity of the signal S to the concen-
#203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5596
tration (C), δ S/δ C =32.4mV/% achieved near zero concentration with the thickness ratio of tAg /tAl =36nm/4nm turned out to be better than the sensitivity (15.2mV/%) with the thickness ratio of tAg /tAl =20nm/5nm. We chose the fiber device of this bimetallic coating (tAg /tAl =36nm/4nm) to probe smaller concentrations near zero, and achieved the minimum detectable concentration of 5 × 10−3 % to which the increase of concentration from zero caused the signal change to be comparable or slightly greater than a standard deviation of the signal near zero as shown in the inset of Fig. 4(d), enabling one to distinguish the signal of the lowest possible concentration from that of the zero concentration. The measured signal standard deviation near zero using the experimentally achieved δ C/δ S lead to the minimum resolvable refractive index (MRI) of 5.8 × 10−6 RIU as experimentally achieved with our current setup. Note that, in spite of the single wavelength employed, the operating RI range demonstrated with this fiber device shows the potential operating range of concentration from zero to more than 35% as shown in Fig. 4(d). This corresponds to the potential RI operating range of > 0.05RIU which can be estimated by the linear relation between the refractive index and the concentration mentioned above. This RIU range turns out to be much wider than those demonstrated in the recent works [8, 11, 15, 16, 22, 23] which have achieved the comparable MRIs. It is estimated that the wide operating range is, partly, due to the broad SPR angle conditions available by the asymmetric metal coating profile of the fiber core, which can serve to provide various SPR probe angles together with the wide range of incident angles for fiber multimode propagation. In addition, it is seen that the non-golden bimetallic coating that was chosen to avoid too much SPR-induced attenuation while securing high enough device sensitivity, supports the wide operating RI range. The detection scheme used to observe the combination effects of SPR-induced birefringence and intensity attenuation, as shown in Eq. (5), also benefits extending the operating RI range. This was verified by comparing the above results with those obtained by measuring merely optical power at the output of the fiber (tAg /tAl =36nm/4nm) as a function of glycerol concentrations, as shown in Fig. 5. The operating range of concentration (RI) is restricted, compared to those shown in Fig. 4(d), and the unwanted nonlinearly fluctuating behavior near zero concentration was observed.
Fig. 5. glycerol concentration (volume to volume ratio) sensing by measurement of optical power at the fiber output.
4.
Conclusions
In conclusion, we demonstrated Ag-Al coated multimode fiber optical sensors by utilizing SPRinduced changes in birefringence and intensity using a non-spectroscopic technique. The min#203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5597
imum resolvable RI of 5.8 × 10−6 was experimentally attained with the operating RI range of 0.05 using a single wavelength operation. The wide operating range can be attributed, in part, to our detection scheme which is capable of detecting the combined effects of SPR-attenuated intensity and SPR-induced birefringence change. In addition, the various SPR angle available from the bimetal surface as a result of asymmetric profile of the metal coating on the fiber core, and the wide range of incident angles for multimode propagation can contribute to the wide operating range. This system with the fiber device requires no spectrograph, potentially allowing a costeffective integration and miniaturization of the system, while maintaining a high enough sensitivity with a wide operating range at a single wavelength for label-free biochemical and biomedical sensing applications. The sensing region length dependence of the device performance can be examined for the further optimization. An integrated optical waveguide device supporting SPR birefringence can also be further investigated, as replacing the fiber in our system [24–26]. The sensing platform with the fiber device demonstrated will be likely to be developed for biomedical and biochemical quantitative diagnosis as a label-free real-time monitoring sensing applications in a compact format. Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2011-0009353, NRF-2013R1A1A2006532).
#203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5598
of Bionano technology, Gachon University, South Korea of Nanophysics, Gachon University, South Korea 3 Neuroscience Institute, Gil Hospital, Incheon, South Korea
2 Department
∗ [email protected]
Abstract: We present a surface plasmon resonance (SPR) based multimode fiber sensor with non-golden bimetallic coating. Our detection scheme used, which is capable of measuring the combined effects of SPR-induced birefringence and intensity changes, supported the minimum resolvable refractive index (RI) of 5.8 × 10−6 RIU with the operating RI range of 0.05 to be experimentally obtained at a single wavelength (632.8 nm) without non-spectroscopic techniques. The asymmetric profile of the thickness of the bimetal coating on the fiber core together with the inherent range of incidence angle for multimode propagation also contributed to the wide operating range. The SPR fiber device with the detection scheme demonstrated will be likely to be developed as a real-time label-free and highly sensitive diagnostic device of a wide operating range for biomedical and biochemical applications in a portable format. © 2014 Optical Society of America OCIS codes: (240.6680) Surface plasmons; (280.4788) Optical sensing and sensors; (060.2370) Fiber optics sensors.
References and links 1. B. Liedberg, C. Nylander, and I. Lundstr¨om, “Surface plasmon resonance for gas detection and biosensing,” Sens. Actuators 4, 299–304 (1983). 2. P. B. Daniels, J. K. Deacon, M. J. Eddowes, and D. Pedley, “Surface plasmon resonance applied to immunosensing,” Sens. Actuators 15, 11–17 (1988). 3. R. C. Jorgenson and S. S. Yee, “A fiber optica chemical sensor based on surface plasmon resonance,” Sens. Actuators B 12, 213–220 (1993). 4. S. Miwa and T. Arakawa, “Selective gas detection by means of surface plasmon resonance sensor,” Thin Solid Films 281, 466–468 (1996). 5. J. Melendez, R. Carr, D. Bartholomew, H. Taneja, S. Yee, C. Jung, and C. Furlong, “Development of a surface plasmon resonance sensor for commercial applications,” Sens. Actuators B Chem. 39, 375–379 (1997). 6. H. P. Chiang, C. W. Chen, J. J. Wu, H. L. Li, T. Y. Lin, F. J. S´anchez, and P. T. Leung, “Effects of temperature on the surface plasmon resonance at a metal-semiconductor interface,” Thin Solid Films 515, 6953–6961 (2007). 7. J. Homola, “Optical fiber sensor based on surface plasmon excitation,” Sens. Actuators B Chem. 29, 401–405 (1995). 8. R. Slavik, J. Homola, and J. Ctyrok´y, “Miniaturization of fiber optic surface plasmon resonance sensor,” Sens. Actuators B Chem. 51, 311–315 (1998). 9. A. J. C. Tubb, F. P. Payne, R. B. Millington, and C. R. Lowe, “Sing-mode optical fibre surface plasma wave chemical sensor,” Sens. Actuators B 41, 71–79 (1997).
#203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5590
10. R. K. Vema, A. K. Sharma, and B. D. Gupta, “Surface plasmon resonance based tapered fiber optic sensor with different taper profiles,” Opt. Commun. 281, 1486–1491 (2008). 11. R. Slavik, J. Homola, and J. Ctyrok´y, “Single-mode optical fiber surface plasmon resonance sensor,” Sen. Actuators B Chem. 54, 74–79 (1999). 12. W. B. Lin, N. J. Renault, A. Gagnaire, and H. Gagnaire, “The effects of polarization of the incident light modelling and analysis of a SPR multimode optical fiber sensor,” Sens. Actuators A Phys. 84, 198–204 (2000). 13. E. Fontana, “A novel gold-coated multimode fiber sensor,” IEEE Trans. Microwave Theory Technol. 50, 82–87 (2002). 14. H.-Y. Lin, W.-H. Tsa, Y.-C Tsao, and B.-C, Sheu, “Side-polished multimode fiber biosensors based on surface plasmon resonance with halogen light,” Appl. Opt. 46, 800–806 (2007). 15. M. Piliarik, J. Homola, Z. Manikova, and J. Ctyrok`y, “Surface plasmon resonance based on a single mode polarization maintaining optical fiber,” Sens. Actuators B Chem. 90, 236–242 (2003). 16. 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, 1120–1124 (2007). 17. M. Mitsushio, K. Miyashita, and M. Higo, “Sensor properties and surface characterization of the metal-deposited SPR optical fiber sensors with Au, Ag, Cu and Al,” Sens. Actuators A 125, 296–303 (2006). 18. A. K. Sharmal and G. J. Mohr, “On the performance of surface plasmon resonance based fibre optic sensor with different bimetallic nanoparticle alloy combinations,” J. Phys. D Appl. Phys. 41, 055106 (2008). 19. B. D. Gupta and A. K. Sharma, “Sensitivity evaluation of a multi-layered surface plasmon resonance-based fiber optic sensor: a theoretical study,” Sens. Actuators B Chem. 107, 40–46 (2005). 20. A. A. Kruchinin and Y. G. Vlasov, “Surface plasmon resonance monitoring by means of polarization state measurement in reflected light as the basis of a DNA-probe biosensor,” Sens. Actuators B 30, 77–80 (1996). 21. R. C. Weast, ed., CRC Handbook of Chemistry and Physics, 68th ed. (CRC Press, 1987), p. D-232. 22. S. G. Nelson, K. S. Johnston, and S. S. Yee, “High sensitivity surface plasmon resonance sensor based on phase detection,” Sens. Actuators B Chem. 35, 187–191 (1996). 23. S. Y. Wu, H. P. Ho, W. C. Law, C. L. Lin, and S. K. Kong, “Highly sensitive differential phase-sensitive surface plasmon resonance biosensor based on the Mach-Zehnder configuration,” Opt. Lett. 29, 2378–2380 (2004). ˇ 24. J. Dost´alek, J. Ctyrok´ y, J. Homola, E. Brynda, M. Skalsk´y, P. Nekvindov´a, J. Spirkov´a, J. Skvor, and J. Schr¨ofel, “Surface plasmon resonance biosensor based on integrated optical waveguide,” Sens. Actuators B 76, 8–12 (2001). 25. M. Zourob, S. Mohr, B. J. T. Brown, P. R. Fielden, M. B. McDonnell, and N. J. Goddard, “Bacteria detection using disposable optical leaky waveguide sensors,” Biosens. Bioelectr. 21, 293–302 (2005). 26. J.-G. Huang, C.-L. Lee, H.-M. Lin, T.-L. Chuang, W.-S. Wang, R.-H. Juang, C.-H. Wang, and C. K. Lee, “A miniaturized germanium-doped silicon dioxide-based surface plasmon resonance waveguide sensor for immunoassay detection,” Biosens. Bioelectr. 22, 519–525 (2006).
1.
Introduction
Optical sensors that utilize sensitivity of surface plasmon resonance (SPR) with respect to change in a refractive index of a sensed medium, have been extensively studied for a variety of biological and chemical applications [1–6]. Surface plasmon waves can be excited at the interface between metal and dielectric (sensed medium) with an aid of a transparent high-index material which is usually placed on the opposite side of the dielectric across the metal to provide the excitation light whose phase velocity is tuned for the SPR phase matching condition as given by ω εm εd l , (1) kex = Re c εm + ε d l is the longitudinal component of the propagation vector of excitation light at the where kex metal-dielectric interface, and εm and εd are the relative permittivities of the metal, and the sensed medium, respectively. Optical fibers with appropriate metal coating have also been suggested as a SPR sensing device that can benefit from the device flexibility, the potential use for remote sensing, extension of SPR interaction length, and small volume of analytes, device miniaturization capabilities [3, 7, 8]. Unlike the SPR coupling via a prism, light coupling into the fiber removes the need of angular adjustment of incident light for SPR excitation. The thickness of a metal film deposited
#203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5591
on the exposed core of the fiber which can result from removal of the local part of the fiber cladding, should be chosen for the wavelength in use for SPR. Surface plasmon waves can be excited by the evanescent waves of the fiber guided modes with their penetration depth into the sensing region, determined by the exponentially decreasing electric field Ed (r) in the sensing region (dielectric medium comprising aqueous solution) as given by Ed (r) = Ed (0)e
−r
2 −k2 ksp d
,
(2)
where ksp is the surface plasmon wave propagation constant, kd is the propagation constant in the dielectric (sensing region) and r is the coordinate in the orthogonal direction to the sensing surface. Various types of optical fibers including single mode fibers [9–11], multimode fibers [3, 12–14], polarization maintaining fibers [15], and D-shaped optical fibers [16] have been demonstrated for SPR sensors either on a basis of an wavelength or an intensity interrogation. The sensing operating RI range that had been limited to about 0.01-0.02 RIU in the fiber optic sensors has further been extended to at least 0.06 by utilizing either a tapered single mode fiber that resulted in the various depth of deposited metal [9] or a side-polished multimode fiber [14], both of which were based on wavelength interrogation. However, a spectroscopy technique that would usually make the detection module bulky, puts the limit on sensor device portability and miniaturization without sacrifice of spectral resolution. This leads to requirement of a detection module operated at a single wavelength that can remove the need of a spectrograph to meet the sensor device compactness and an inexpensive integration. In the mean time, non-straightforward process for elimination of silica cladding of the optical fiber does not ensure the homogeneous exposure of the core surface, resulting in difficulties of efficient treatment of the sensing surface, together with fragility of a thin bare core of fibers, in particular, in cases of single mode fibers.
Fig. 1. (a) Metal vapor deposition on the exposed core surface of the clad-free optical fiber. (b) Expected cross-section of the metal coating profile on the fiber core after the bimetallic deposition for SPR.
The theoretical investigation on SPR fiber optical devices with alloy metal deposition has reported that the deposition layer of bimetallic nanoparticle alloy can be better than that of the single-metal nanoparticle layer in terms of figure-of-merits of optical sensors such as a sensitivity, signal-to-noise ratio and an operating range of the sensed refractive index [17, 18]. Thus, the present study examined a SPR sensor device comprising a multimode fiber with bimetallic (Ag-Al) coating, using a non-spectroscopic detection scheme that is capable of de#203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5592
tecting signals sensitive to changes in SPR-induced birefringence and intensity. The minimum detectable RI of 5.8 × 10−6 with the operating RI range of 0.05 has been achieved at a single wavelength (632.8nm). Discussion on the sensing performance is provided with the mathematical description of the birefringence sensitive detection used in our work. 2.
Experimental techniques with mathematical description
We used the non-golden bimetallic coating for SPR, i.e., Ag and Al with different composition ratios, expecting the high enough sensitivity to the RI change while avoiding too much attenuation of light due to SPR for widening operating range [18]. This bimetal combination where Al is in a direct contact with the sensed medium is also expected to prevent Ag-inherent demerits such as the chemical instability and the vulnerability to corrosion, and to simultaneously allow immobilization of bio-receptor without difficulties possibly encountered in the case of sensing surface Ag [17, 19]. We used a polymer-clad multimode fiber (numerical aperture of 0.37, JFTLH, Polymicro Technologies) of 200 μ m diameter core, whose cladding of 10cm length can be removed via a soldering machine with the subsequent cleansing by a mixture of acetone and ethylene. The cladding removal is followed by the subsequent bimetallic film coating for SPR-sensing surface of 10 cm length. This fiber that was bent into a ring shape of 6 cm diameter constitutes the key element of the sensing device, which allows for the main modes to propagate with a number of reflections off the core-metal boundary, generating the multi-reflection enhanced SPR-induced change in the device output light properties. A thermal evaporator was used to coat the Ag and Al layers on the exposed fiber core. As shown in Fig. 1(a), in the vacuum chamber of the thermal evaporator, we placed the fibers perpendicular to the metal vapor deposition direction. After the first deposition of each metal on one side of the fiber, we rotated the fiber by 180 degrees along its axis for the second deposition of the same metal on the other side, expecting the asymmetric cross-sectional profile of coating thickness as shown in Fig. 1(b). Despite the fact that incomplete controllability of the evaporation direction including experimental errors in coating would produce a certain form of non-circular symmetric profile of thickness which would be different from that shown in Fig. 1(b), this asymmetry of the coated profile permits various penetration depth into the sensing region [9]. This causes both the various ksp in Eq. (2) and various incidence angles for SPR to be excited, and consequently can contribute to the enlarged operating range of RI sensing while the high enough sensitivity level boosted by multi-reflections (estimated as 18) off the fiber core surface through the ring-shaped fiber.
Fig. 2. Schematic of the experimental setup for analyte solution sensing. λ /4 and λ /2 represent a quarter- and a half-wave plates, respectively, PBS denotes a polarizing beam splitter, and BD a balanced detector.
Figure 2 shows schematic of the experimental setup with the fiber optical sensor device. A #203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5593
ring-shaped flow cell made of polydimethylsiloxane(PDMS) was fabricated to install the metal coated fiber on the groove built inside the flow cell along its ring-shaped path, with an inlet and an outlet ports made for the analyte solution. The total volume of the analyte solution that could fill the flow cell along the 10 cm sensing length was estimated to be 730μ l. A light source of a He-Ne laser which offered a linearly polarized light at 632.8 nm was passed through a quarterwave plate (λ /4) to make a circular polarization. We used a circularly polarized incident light to ensure that half of its light had the so called SPR-supporting transverse magnetic (TM)polarization at the interface between the sensed medium and the outer metal (Al), taking into account asymmetric cross-sectional profile of the coated metals around the fiber core, shown in Fig. 1(b) The metal coated fiber whose sensing surface interacting with analyte solution was assumed to act itself as a SPR-induced birefringent device where SPR-induced polarizationdependent phase shift occurs as well as polarization-dependent intensity change. The SPRinduced phase changes are denoted by δ φo and δ φe along the ordinary axis (o-axis) and the extra-ordinary axis (e-axis) of the SPR birefringent device, respectively. This is due to the fact that SPR is sensitive to the light polarizations, leading to polarization-dependent properties of light propagating through the fiber (i.e., intensity and phase) [20]. The electric field of fiber H ) and vertical (E V ) output light which is decomposed into the components of horizonal (Eout out polarizations, can be described as follows: ⎤ ⎡ −iΓ/2 ⎡ H ⎤
⎡ ⎤ 0 e 1 Eout ⎦ = R(−ξ ) ⎣ ⎦ R(ξ ) ⎣ ⎦ , ⎣ (3) V ±i Eout 0 eiΓ/2 where R(ξ ) is the rotation matrix with the ξ the the angle between the o-axis of the SPR fiber device and a laboratory horizontal axis, Γ ≡ δ φe − δ φo is the phase retardation of light through the SPR fiber device, and the ± sign denotes the right-handed and left-handed circular polarizations of input light, respectively. The optical power before the incident light coupling into the fiber was 15 mW while that of light coupled out of the fiber was about 97 μ W (∼ −22 dB) in the case of deionized-water filled in the flow cell for the Ag (36nm)-Al (4nm) coated fiber. The attenuation of output power reduces as the Ag thickness portion decreases for a total bimetal thickness of 40 nm kept. The fiber output light that was then passed through a rotatable half-wave plate (λ /2) can be written as ⎡
EλH/2,out
⎤
⎡
− cos 2ψ
⎦ = i⎣ ⎣ EλV/2,out sin 2ψ
sin 2ψ cos 2ψ
⎤⎡
H Eout
⎤
⎦⎣ ⎦, V Eout
(4)
where ψ is the angle between the λ /2 optic axis and a laboratory horizontal axis, thereby determining the relative ratio of optical power between the o- and e-axes polarization components, for each output port of the PBS. The following polarizing beam splitter (PBS) then split light from the λ /2 into two polarization components, i.e., horizontal (H) and vertical (V) polarizations, each of which entered the corresponding diode of a two-port balanced detector (BD) which gave the subtracted signal between the two photodiode signals. Note that each port of the PBS produced the in-line interference between two polarization components, i.e., light polarized along the o- and e-axes of the SPR fiber device. For the righthanded circular polarization of incident light, the optical intensity at the horizonal and vertical
#203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5594
ports of the PBS output can be written as
⎤ ⎡ ⎡ ⎤ /2) 1 − sin Γ sin(2 ξ − 4 ψ ) (I ⎢ out ⎥ IH ⎢ ⎥ ⎥ ⎣ ⎦=⎢ ⎢
⎥ , ⎣ ⎦ IV (Iout /2) 1 + sin Γ sin(2ξ − 4ψ )
(5)
where Iout is the total optical intensity at the fiber output. The subtraction of Ip from Is in the BD removes both the common mode noise present in the light source and Γ-independent terms (the first terms on the RHS) of the matrix element equations as shown in Eq. (5), consequently, giving the signal proportional to the twice the magnitude of the second terms on the RHS of those equations, that is, I− ≡ IV − IH = Iout sin Γ sin(2ξ − 4ψ ). The subtraction signal is subject to the device birefringence change as well as to the output intensity change for a given ξ and ψ . The scanning of ψ by 1800 rotation of the λ /2 plate then enables both the maximum and minimum values of the subtraction signal to be obtained regardless of the orientation of the oand e-axes of the SPR fiber device. Then the sensing signal S, defined as the difference between the maximum and minimum of the subtraction signal, is proportional to the product of the output intensity and the sin Γ S
≡ max(I− ) − min(I− ) = 2Iout sin Γ.
(6)
Fig. 3. Sensing of glycerol concentration (volume to volume ratio) for the fiber device with no metal coating.
The fiber device with the flow cell can be used as a refractometer for the refractive index change in the sensing region since the refractive index change causes the SPR resonance condition to change, resulting in the change both in Iout and Γ. For refractometer experiments, we used the glycerol solution which was one of the laboratory-available standard solvents whose concentration in volume-to-volume ratio (C in %) can be easily calibrated into a refractive index, n with the linear relation obtained using sodium D line (589 nm), n = 1.3330 + 0.0012ρ C (ρ : glycerol density) found in [21]. 3.
Results and discussion
First, we checked the glycerol concentration dependence of optical power at the fiber output both in cases of no metal coated fiber and the metal coated fiber to see the SPR-induced attenuation of intensity. In no metal coating case, the increased concentration of glycerol increases #203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5595
Fig. 4. Sensing of glycerol concentration (volume to volume ratio) for different ratios of Ag and Al coating thicknesses.
optical power at the fiber output due to the fact that the leakage of propagating mode energies over the clad-free sensing length is reduced. On the contrary, the application of metal coating on the fiber core decreases output power as the glycerol concentration increases, manifesting the SPR-induced attenuation of output power. We also measured the signal S in the case of no metal coating, as shown in Fig. 3. The highly nonlinear noise-like fluctuating behavior is visible over the entire concentration range used, indicating that the birefringence response of no metal coated fiber to the glycerol concentration change may not be useful for the sensing application. Figures 4(a)–4(d) show the measured signal S as the glycerol concentration increases, for the different composition ratios of thicknesses of Ag and Al deposited on the fiber core. The signal S decreases with the concentration for all composition ratios of Ag-Al coating thicknesses whilst exhibiting the different sensitivity characteristics, especially, near zero concentration. The coating thickness ratios of tAg /tAl =7nm/30nm and tAg /tAl =30nm/10nm produce more nonlinearly fluctuating signals near zero concentration than those of the other ratios of tAg /tAl =20nm/5nm and tAg /tAl =36nm/4nm. It is also seen that the two different sensitivity (slope) regions appear around the concentrations of about 1% and 0.05%, as shown in Figs. 4(c) and 4(d), respectively. It was also found that the sensitivity of the signal S to the concen-
#203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5596
tration (C), δ S/δ C =32.4mV/% achieved near zero concentration with the thickness ratio of tAg /tAl =36nm/4nm turned out to be better than the sensitivity (15.2mV/%) with the thickness ratio of tAg /tAl =20nm/5nm. We chose the fiber device of this bimetallic coating (tAg /tAl =36nm/4nm) to probe smaller concentrations near zero, and achieved the minimum detectable concentration of 5 × 10−3 % to which the increase of concentration from zero caused the signal change to be comparable or slightly greater than a standard deviation of the signal near zero as shown in the inset of Fig. 4(d), enabling one to distinguish the signal of the lowest possible concentration from that of the zero concentration. The measured signal standard deviation near zero using the experimentally achieved δ C/δ S lead to the minimum resolvable refractive index (MRI) of 5.8 × 10−6 RIU as experimentally achieved with our current setup. Note that, in spite of the single wavelength employed, the operating RI range demonstrated with this fiber device shows the potential operating range of concentration from zero to more than 35% as shown in Fig. 4(d). This corresponds to the potential RI operating range of > 0.05RIU which can be estimated by the linear relation between the refractive index and the concentration mentioned above. This RIU range turns out to be much wider than those demonstrated in the recent works [8, 11, 15, 16, 22, 23] which have achieved the comparable MRIs. It is estimated that the wide operating range is, partly, due to the broad SPR angle conditions available by the asymmetric metal coating profile of the fiber core, which can serve to provide various SPR probe angles together with the wide range of incident angles for fiber multimode propagation. In addition, it is seen that the non-golden bimetallic coating that was chosen to avoid too much SPR-induced attenuation while securing high enough device sensitivity, supports the wide operating RI range. The detection scheme used to observe the combination effects of SPR-induced birefringence and intensity attenuation, as shown in Eq. (5), also benefits extending the operating RI range. This was verified by comparing the above results with those obtained by measuring merely optical power at the output of the fiber (tAg /tAl =36nm/4nm) as a function of glycerol concentrations, as shown in Fig. 5. The operating range of concentration (RI) is restricted, compared to those shown in Fig. 4(d), and the unwanted nonlinearly fluctuating behavior near zero concentration was observed.
Fig. 5. glycerol concentration (volume to volume ratio) sensing by measurement of optical power at the fiber output.
4.
Conclusions
In conclusion, we demonstrated Ag-Al coated multimode fiber optical sensors by utilizing SPRinduced changes in birefringence and intensity using a non-spectroscopic technique. The min#203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5597
imum resolvable RI of 5.8 × 10−6 was experimentally attained with the operating RI range of 0.05 using a single wavelength operation. The wide operating range can be attributed, in part, to our detection scheme which is capable of detecting the combined effects of SPR-attenuated intensity and SPR-induced birefringence change. In addition, the various SPR angle available from the bimetal surface as a result of asymmetric profile of the metal coating on the fiber core, and the wide range of incident angles for multimode propagation can contribute to the wide operating range. This system with the fiber device requires no spectrograph, potentially allowing a costeffective integration and miniaturization of the system, while maintaining a high enough sensitivity with a wide operating range at a single wavelength for label-free biochemical and biomedical sensing applications. The sensing region length dependence of the device performance can be examined for the further optimization. An integrated optical waveguide device supporting SPR birefringence can also be further investigated, as replacing the fiber in our system [24–26]. The sensing platform with the fiber device demonstrated will be likely to be developed for biomedical and biochemical quantitative diagnosis as a label-free real-time monitoring sensing applications in a compact format. Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2011-0009353, NRF-2013R1A1A2006532).
#203158 - $15.00 USD (C) 2014 OSA
Received 16 Dec 2013; revised 30 Jan 2014; accepted 10 Feb 2014; published 4 Mar 2014 10 March 2014 | Vol. 22, No. 5 | DOI:10.1364/OE.22.005590 | OPTICS EXPRESS 5598
