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Reflective-distributed SPR sensor based on twin-core fiber
Optics Communications 366 (2016) 107–111
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Reflective-distributed SPR sensor based on twin-core fiber Zhihai Liu a,b, Yong Wei a, Yu Zhang a,b,n, Zongda Zhu a, Enming Zhao a, Yaxun Zhang a, Jun Yang a, Chunyu Liu c, Libo Yuan a a
Key Lab of In-fiber Integrated Optics, Ministry Education of China, Harbin Engineering University, China Centre for Micro-Photonics, Swinburne University of Technology, P.O. Box 218, Hawthorn, VIC 3122, Australia c Key lab of Electronic Engineering, Heilongjiang University, Harbin, 150080, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 7 September 2015 Received in revised form 7 December 2015 Accepted 11 December 2015
We propose and demonstrate a reflective-distributed surface plasmon resonance (SPR) sensor based on the twin-core fiber. Firstly, we study the effects of the fiber dual tapered (DT) probe grinding angle on the SPR dynamic range. The results show that for larger grinding angles, the resonance wavelength increases, resulting in a higher testing sensitivity. By using this method, we can make the sensor operate in an optimal waveband. Secondly, on the basis of the results above, we grind the DT probe to be an asymmetric wedge shape to configure two grinding angles (6° and 17°) to realize the distributed sensing. Results show that, with the refractive index detecting a range of 1.333–1.385, we can get two separated sensing zones, 591–715 nm and 729–966 nm, the testing sensitivity are 2385 nm/RIU and 4558 nm/RIU respectively. By using this method, we can detect multiple analytes in the same sensing area simultaneously, besides that we can also effectively compensate for the errors caused by background index interference, and the refractive index change resulting from the non-specific binding, or physical absorption and others. It worth to say that by using this method, we can adjust and control the resonance wavelength by changing the fiber grinding angle, while keeping the testing sensitivity is not reduced. For practical applications, this reflective distributed fiber-based sensor is much suitable for biochemical sensing, it has small size and can enter a small testing spaces (μm scale). The diameter of the twin-core fiber is the only 125 μm, which helps to be integrated into a micro-fluidic chip. In this paper, we integrate the fiber probe into an infusion needle to simulate blood vessels on-line monitoring. & 2015 Elsevier B.V. All rights reserved.
Keywords: Surface plasmon resonance Reflective-distributed Dynamic range adjustable Incidence angle changing Fiber grinding and polishing
1. Introduction Surface plasmon resonance (SPR) sensors have been increasingly used to detect biochemical reactions [1–4]. Normally, the common fiber-based SPR sensors are based on the multimode fibers. Some typical examples contain transmission type multimode fiber SPR sensors [5,6], reflective type multimode fiber SPR sensor [7], side polishing type (D type) multimode fiber SPR sensor [8], and heterogeneous core structure multimode fiber SPR sensors [9,10]. Besides those, there are multimode fiber-based DT (dual tapered) probe sensors [11–14] and ST (single tapered) probe sensors [15,16]. For the practical applications, multi-channel distributed SPR sensor needs to be developed to compensate for the background refractive index interference and reference the temperature change. However, the multimode fiber-based SPR sensors are not suitable to realize the distributed sensing. By using the n Corresponding author at: Key Lab of In-fiber Integrated Optics, Ministry Education of China, Harbin Engineering University, China. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.optcom.2015.12.018 0030-4018/& 2015 Elsevier B.V. All rights reserved.
multimode fiber with DT or ST shape tip, due to the presence of too many beam-propagating modes in the fiber core, and the unconcentration of multiple modes beams light power distribution, the distributed sensing experimental results are unsatisfactory. It is hard to adjust the dynamic range, and the testing sensitivity will reduce. Besides that, the modes mix seriously in the multimode fiber, therefore the background noise is large, and SPR resonance dips are shallow. Therefore, the single mode fiber is demanded. Here we employ a twin-core single mode fiber to realize the distributed SPR sensing. The twin cores in the fiber are spatially symmetry, which is suitable for the sensing beam launching and receiving. The diameter of the twin cores is 3.8 μm, meeting the single mode propagating condition within the light source waveband. Thus, only a fundamental mode beam can propagate in the fiber core, the sensitivity of the sensor can be greatly improved. The twin-core fiber is ground to be a DT shape, which helps to realize the Kretschmann configurations for sensing. Consequently, in this paper, we propose and demonstrate a reflective-distributed SPR sensor based on a twin-core fiber. We firstly study the effects of the DT probe grinding angle on the SPR dynamic range and sensitivity; and then on the basis of the study
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Z. Liu et al. / Optics Communications 366 (2016) 107–111
results, we develop a reflective type distributed SPR sensor based on the twin-core fiber with a DT probe. We grind the two side surfaces of the DT probe tip to be two different angles, which produces two different dynamic ranges to realize the distributed SPR sensing. This method has two advantages, the dynamic range is adjustable continuously, and the testing sensitivity is not reduced. This distributed SPR sensor can detect multiple analytes simultaneously, and has the ability to eliminate background interference refractive index and temperature from the reference. As far as we know, there is no report about the reflective-distributed single fiber SPR sensor before. Compared with the traditional transmitted distributed fiber-based SPR sensor, the reflective distributed fiber-based SPR sensor has much small size, being convenient to working in a small space (μm scales). Besides that, combining with the micro-fluid chip technology, we can improve the sample utilization efficiency, reducing the testing costs, increasing the testing speed, and saving the testing time.
2. Effects of grinding angle on dynamic range 2.1. Fiber DT probe We employ a twin-core fiber whose cladding diameter is 125 μm, and the core diameter is 3.8 μm to test. Fig. 1(a) provides the sensor probe structure, and the grinding angle1 is α, grinding angle2 is β, and the resonance angle is θ. Fig. 1(b) provides the image of the ground fiber tip. After grinding, we plate the gold film on the inclined surface of the fiber tip with the thickness of 50 nm. The film coating method and the measurement of the film thickness can be obtained in Ref [17]. 2.2. Effect of grinding angle on dynamic range By using the single mode fiber grinding and polishing method, we can adjust the SPR incidence angle. Here we test the grinding angle α of 10°, 12°, 14° and 16°; while the grinding angle β of 80°, 78°, 76° and 74° respectively. When the fiber grinding angle increases, SPR resonance wavelength moves to the long wavelengths. Fig. 2(a), (c), (e) and (g) provide the simulated results. We plate the gold film on the twin-core fiber wedge surface and realize the Kretchmann prism structure by using the twin cores structure. Therefore, the theoretical calculation mode is similar with the Kretchmann prism structure. The SPR sensing unit contains twin-core fiber, gold film and, the measured media, therefore, according to the Fresnel formula; we can derive the reflectance of the sensing device: 2
p R = r fiber 12 =
p
p
r fiber1 + r12e 2ikz1d1 p
p
1 + r fiber1r12e 2ikz1d1
2
(1)
According to the Fresnel formula, we can obtain the reflection
coefficient of each interface:
rikp =
(ε˜i − n2fiber sin2θfiber )1/2 /ε˜i − (ε˜k − n2fiber sin2θfiber )1/2 /ε˜k (ε˜i − n2fiber sin2θfiber )1/2 /ε˜i + (ε˜k − n2fiber sin2θfiber )1/2 /ε˜k
k z1 = k1 cos θ1 =
ω (ε˜1 − n2fiber sin2θfiber )1/2 c
(2)
(3)
Where, the subscripts fiber, 1 and 2 stand for the twin-core fiber, gold film and the measured medium respectively; the subscripts i, k¼ fiber, 1, 2; kZ1 is the z component of wave vector in the gold film; θfiber is the incident angle from the fiber to the gold film; d1 is the gold film thickness. The simulated conditions are: fiber core refractive index is 1.468, the thickness of the gold film is 50 nm, the dielectric constant of the gold film is obtained from Ref. [18]. According to the Fig. 2(a), (c), (e) and (g), in the refractive index range of 1.333–1.385, when the grinding angle α are 10°, 12°, 14°, and 16°, (SPR incidence angle are 80°, 78°, 76°, and 74°), the SPR dynamic range redshifts along with the incidence angle increasing, and the testing sensitivity increases from 2911 nm/RIU to 7548 nm/RIU, the dynamic range changes from 622–774 nm to 691–1084 nm. Correspondingly, we test the twin-core fiber DT probe with the grinding angle α are 10°, 12°, 14°, and 16°. The experimental results are shown in Fig. 2(b), (d), (f) and (h). In the refractive index range of 1.333–1.385, the SPR dynamic range redshifts along with the incidence angle increasing, and the testing sensitivity increases from 2307 nm/RIU to 5096 nm/RIU, and the dynamic range changes from 614–734 nm to 686–951 nm. The reasons why the experimental results of the sensitivity are lower than the simulated results may be the un-perfect grinding surface and the inhomogeneous thickness of the gold film. Fig. 3 provides the simulated and experimental results of the resonance wavelength when the grinding angles α are 10°, 12°, 14°, 16°. According to the Fig. 3, we can adjust the sensor working range by changing the fiber grinding angle. When the fiber grinding angle increases, the resonance wavelength will redshift, the resonance range will expand, and the testing sensitivity will be improved.
3. Distributed SPR sensing 3.1. DT probe for distributed sensing We employ the same twin-core fiber to fabricate the distributed DT probe. Fig. 4(a) provides the sensor probe structure, and the grinding angle1 is α, grinding angle2 is β. Fig. 4 (b) provides the image of the ground fiber tip. Here we employ the grinding angle γ to control the light beam reflection. Similarly, after grinding, we plate the gold film on the inclined surface of the
Fig. 1. Schematic diagram (a) and image(b) of the twin-core fiber DT probe.
Z. Liu et al. / Optics Communications 366 (2016) 107–111
109
Fig. 2. The simulated results with the grinding angle of (a)10°, (c)12°, (e)14° and (g)16°; the experimental results with the grinding angle of (b)10°, (d)12°, (f)14° and (h)16°.
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Fig. 3. The simulated (a) and experimental (b) results of the testing sensitivity with the grinding angle of 10°, 12 °, 14 ° and 16 °.
fiber tip with the thickness of 50 nm. The experimental setup is shown in Fig. 5. A super continuum light source (SuperK compact, NKT Photonics™) whose spectrum range is 450–2400 nm, is launched into one core of the twin-core fiber by using a lens transform system (seen Fig. 5(b), where we place the collimating lens in front of two single mode fibers while placing the microscope objective lens (the magnification of 25) in front of the twin-core fiber. The light power is launching into one core by adjusting the distance and position of the lens in the transform system), and the reflected beam is received by an optical spectrum analyzer (AQ6370C, Yokokawa™). We employ a programmable micro injection pumper (LSP01-1A, LongerPump™) to inject the Glycerine-aqueous solution to the micro fluidic channel. The Glycerine-aqueous solution index is calibrated by the Abbe refractometer (GDA-2S, Gold™). 3.2. Distributed sensing experimental results Based on the simulated and experimental results above, we grind the α to be 6°, β to be 17° and γ to be 79°. The simulated and experimental results are shown in the Fig. 6. Although the amplitude of the transmitted attenuating spectrum is not consistent with the simulated results, here we investigate the position of the resonance dip, which is consistent with the simulated results. Fig. 7 provides the simulated and experimental results of the resonance wavelength when the grinding angle α is 6° and β is 17°. According to the Fig. 7, we can adjust the sensor working range by changing the fiber grinding angle. When the fiber grinding angle increases, the resonance wavelength will redshift, and the resonance range will expand, and the testing sensitivity will be improved. According to Figs. 3 and 6, the single mode fiber SPR sensor resonance wavelength can shift to the much more sensitive band (near-infrared) by adjusting the single mode fiber grinding angle. Therefore, there are many advantages, such as, improving the sensitivity of SPR sensor, moving the working range of SPR sensor to the long wavelength, which is much suitable for a normal fiber light source. However, we have to admit that, the longer is the working wavelength, the larger is the noise.
Fig. 5. (a) The experiment setup sketch diagram of the reflective distributed twocore fiber SPR sensor; (b) the sketch diagram of the lens transform system; (c) the images the microfluidic device, where the inset image shows the details of the microfluidic channel and the fiber probe.
4. Conclusion In this paper, we study the effects of the fiber DT probe grinding angle on the SPR dynamic range. The larger is the fiber grinding angle, the longer is the resonance wavelength, and the higher is the testing sensitivity. By using this method, we can make the sensor operate in an optimal waveband, which helps to shorten the light source working waveband. The experimental show that, in the refractive index detecting the range of 1.333– 1.385, when the DT probe grinding angle increases from 10° to 16°,
Fig. 4. Schematic diagram of the reflective distributed twin-core fiber DT probe.
Z. Liu et al. / Optics Communications 366 (2016) 107–111
111
Fig. 6. (a) Simulated and (b) experimental results of the SPR spectrum response to different solution refractive index with fiber grind angle of 6° and 17°.
on-line monitoring.
Acknowledgment This work is supported by the National Natural Science Foundation of China (Grants no. 11204047, 61227013, 61275087, 61205071 and 61377085), and partially supported by the following grants: the 111 Project (B13015), Research Fund for the Doctoral Program of Higher Education of China (Grants no. 20112304 110017), Postdoctororal Science Foundation Fund of China (Grants nos. 2014M550181 and 2014M551217), and Fundamental Research Funds for Harbin Engineering University of China.
Fig. 7. The simulated (a) and experimental (b) results of the testing sensitivity with the grinding angle of 6° and 17 °.
the dynamic range moves from 614–734 nm to 686–951 nm, average testing sensitivity increases from 2308 nm/RIU to 5096 nm/RIU. On the basis of the results above, we propose and demonstrate a twin-core fiber reflected distributed SPR sensors. By using this sensor, we can detect multiple analytes in the same sensing area simultaneously, besides that we can also effectively compensate the errors caused by background interference, and the refractive index change resulting from the non-specific binding, or physical absorption and others. It worth to say that by using this method, we can adjust and control the resonance wavelength by changing the fiber grinding angle, and the testing sensitivity will not reduce after cascading. For an actual application, this reflective distributed fiber-based sensor is much suitable for biochemical sensing, it has a small size and can enter small testing spaces. The diameter of the twin-core fiber is only 125 μm, which helps to be integrated into a micro-fluidic chip, in this paper we integrate the fiber probe into an infusion needle to simulate blood vessels for
References [1] K.I. Nomura, S.C.B. Gopinath, T. Lakshmipriya, N. Fukuda, X.M. Wang, M. Fujimaki, Nat. Commun. 4 (2013) 2855. [2] J.H. Ahn, T.Y. Seong, W.M. Kim, T.S. Lee, I. Kim, K.S. Lee, Opt. Express 20 (2012) 21729. [3] J. Banerjee, M. Bera, M. Ray, J. Appl. Phys. 117 (2015) 113102. [4] C.L. Wong, M. Olivo, Plasmonics 9 (2014) 809. [5] R.C. Jorgenson, S.S. Yee, Sens. Actuators B 12 (1993) 213. [6] Y. Liu, Q. Liu, S.M. Chen, F. Cheng, H.Q. Wang, W. Peng, Sci. Rep. 5 (2015) 12864. [7] R.C. Jorgenson, S.S. Yee, Sens. Actuators B 43 (1994) 44. [8] W.H. Tsai, Y.C. Tsao, H.Y. Lin, B.C. Sheu, Opt. Lett. 30 (2005) 2209. [9] M. Iga, A. Seki, K. Watanabe, Sens. Actuators B 101 (2004) 368. [10] K. Takagi, H. Sasaki, A. Seki, K. Watanabe, Sens. Actuators A 161 (2010) 1. [11] L.A. Obando, D.J. Gentleman, J.R. Holloway, K.S. Booksh, Sens. Actuators B 100 (2004) 439. [12] Y.C. Kim, W. Peng, S. Banerji, K.S. Booksh, Opt. Lett. 30 (2005) 2218. [13] Y.C. Kim, J.F. Masson, K.S. Booksh, Talanta 67 (2005) 908. [14] J.F. Masson, Y.C. Kim, L.A. Obando, W. Peng, K.S. Booksh, Appl. Spectrosc. 60 (2006) 1241. [15] B. Grunwald, G. Holst, Sens. Actuators A 113 (2004) 174. [16] Y.Q. Yuan, D. Hu, L. Hua, M. Li, Sens. Actuators B 188 (2013) 757. [17] Z.H. Liu, Y. Wei, Y. Zhang, Y.X. Zhang, E.M. Zhao, J. Yang, L.B. Yuan, Opt. Lett. 40 (2015) 2826. [18] P.B. Johnson, R.W. Christy, Phys. Rev. B 6 (1972) 4370.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Reflective-distributed SPR sensor based on twin-core fiber Zhihai Liu a,b, Yong Wei a, Yu Zhang a,b,n, Zongda Zhu a, Enming Zhao a, Yaxun Zhang a, Jun Yang a, Chunyu Liu c, Libo Yuan a a
Key Lab of In-fiber Integrated Optics, Ministry Education of China, Harbin Engineering University, China Centre for Micro-Photonics, Swinburne University of Technology, P.O. Box 218, Hawthorn, VIC 3122, Australia c Key lab of Electronic Engineering, Heilongjiang University, Harbin, 150080, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 7 September 2015 Received in revised form 7 December 2015 Accepted 11 December 2015
We propose and demonstrate a reflective-distributed surface plasmon resonance (SPR) sensor based on the twin-core fiber. Firstly, we study the effects of the fiber dual tapered (DT) probe grinding angle on the SPR dynamic range. The results show that for larger grinding angles, the resonance wavelength increases, resulting in a higher testing sensitivity. By using this method, we can make the sensor operate in an optimal waveband. Secondly, on the basis of the results above, we grind the DT probe to be an asymmetric wedge shape to configure two grinding angles (6° and 17°) to realize the distributed sensing. Results show that, with the refractive index detecting a range of 1.333–1.385, we can get two separated sensing zones, 591–715 nm and 729–966 nm, the testing sensitivity are 2385 nm/RIU and 4558 nm/RIU respectively. By using this method, we can detect multiple analytes in the same sensing area simultaneously, besides that we can also effectively compensate for the errors caused by background index interference, and the refractive index change resulting from the non-specific binding, or physical absorption and others. It worth to say that by using this method, we can adjust and control the resonance wavelength by changing the fiber grinding angle, while keeping the testing sensitivity is not reduced. For practical applications, this reflective distributed fiber-based sensor is much suitable for biochemical sensing, it has small size and can enter a small testing spaces (μm scale). The diameter of the twin-core fiber is the only 125 μm, which helps to be integrated into a micro-fluidic chip. In this paper, we integrate the fiber probe into an infusion needle to simulate blood vessels on-line monitoring. & 2015 Elsevier B.V. All rights reserved.
Keywords: Surface plasmon resonance Reflective-distributed Dynamic range adjustable Incidence angle changing Fiber grinding and polishing
1. Introduction Surface plasmon resonance (SPR) sensors have been increasingly used to detect biochemical reactions [1–4]. Normally, the common fiber-based SPR sensors are based on the multimode fibers. Some typical examples contain transmission type multimode fiber SPR sensors [5,6], reflective type multimode fiber SPR sensor [7], side polishing type (D type) multimode fiber SPR sensor [8], and heterogeneous core structure multimode fiber SPR sensors [9,10]. Besides those, there are multimode fiber-based DT (dual tapered) probe sensors [11–14] and ST (single tapered) probe sensors [15,16]. For the practical applications, multi-channel distributed SPR sensor needs to be developed to compensate for the background refractive index interference and reference the temperature change. However, the multimode fiber-based SPR sensors are not suitable to realize the distributed sensing. By using the n Corresponding author at: Key Lab of In-fiber Integrated Optics, Ministry Education of China, Harbin Engineering University, China. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.optcom.2015.12.018 0030-4018/& 2015 Elsevier B.V. All rights reserved.
multimode fiber with DT or ST shape tip, due to the presence of too many beam-propagating modes in the fiber core, and the unconcentration of multiple modes beams light power distribution, the distributed sensing experimental results are unsatisfactory. It is hard to adjust the dynamic range, and the testing sensitivity will reduce. Besides that, the modes mix seriously in the multimode fiber, therefore the background noise is large, and SPR resonance dips are shallow. Therefore, the single mode fiber is demanded. Here we employ a twin-core single mode fiber to realize the distributed SPR sensing. The twin cores in the fiber are spatially symmetry, which is suitable for the sensing beam launching and receiving. The diameter of the twin cores is 3.8 μm, meeting the single mode propagating condition within the light source waveband. Thus, only a fundamental mode beam can propagate in the fiber core, the sensitivity of the sensor can be greatly improved. The twin-core fiber is ground to be a DT shape, which helps to realize the Kretschmann configurations for sensing. Consequently, in this paper, we propose and demonstrate a reflective-distributed SPR sensor based on a twin-core fiber. We firstly study the effects of the DT probe grinding angle on the SPR dynamic range and sensitivity; and then on the basis of the study
108
Z. Liu et al. / Optics Communications 366 (2016) 107–111
results, we develop a reflective type distributed SPR sensor based on the twin-core fiber with a DT probe. We grind the two side surfaces of the DT probe tip to be two different angles, which produces two different dynamic ranges to realize the distributed SPR sensing. This method has two advantages, the dynamic range is adjustable continuously, and the testing sensitivity is not reduced. This distributed SPR sensor can detect multiple analytes simultaneously, and has the ability to eliminate background interference refractive index and temperature from the reference. As far as we know, there is no report about the reflective-distributed single fiber SPR sensor before. Compared with the traditional transmitted distributed fiber-based SPR sensor, the reflective distributed fiber-based SPR sensor has much small size, being convenient to working in a small space (μm scales). Besides that, combining with the micro-fluid chip technology, we can improve the sample utilization efficiency, reducing the testing costs, increasing the testing speed, and saving the testing time.
2. Effects of grinding angle on dynamic range 2.1. Fiber DT probe We employ a twin-core fiber whose cladding diameter is 125 μm, and the core diameter is 3.8 μm to test. Fig. 1(a) provides the sensor probe structure, and the grinding angle1 is α, grinding angle2 is β, and the resonance angle is θ. Fig. 1(b) provides the image of the ground fiber tip. After grinding, we plate the gold film on the inclined surface of the fiber tip with the thickness of 50 nm. The film coating method and the measurement of the film thickness can be obtained in Ref [17]. 2.2. Effect of grinding angle on dynamic range By using the single mode fiber grinding and polishing method, we can adjust the SPR incidence angle. Here we test the grinding angle α of 10°, 12°, 14° and 16°; while the grinding angle β of 80°, 78°, 76° and 74° respectively. When the fiber grinding angle increases, SPR resonance wavelength moves to the long wavelengths. Fig. 2(a), (c), (e) and (g) provide the simulated results. We plate the gold film on the twin-core fiber wedge surface and realize the Kretchmann prism structure by using the twin cores structure. Therefore, the theoretical calculation mode is similar with the Kretchmann prism structure. The SPR sensing unit contains twin-core fiber, gold film and, the measured media, therefore, according to the Fresnel formula; we can derive the reflectance of the sensing device: 2
p R = r fiber 12 =
p
p
r fiber1 + r12e 2ikz1d1 p
p
1 + r fiber1r12e 2ikz1d1
2
(1)
According to the Fresnel formula, we can obtain the reflection
coefficient of each interface:
rikp =
(ε˜i − n2fiber sin2θfiber )1/2 /ε˜i − (ε˜k − n2fiber sin2θfiber )1/2 /ε˜k (ε˜i − n2fiber sin2θfiber )1/2 /ε˜i + (ε˜k − n2fiber sin2θfiber )1/2 /ε˜k
k z1 = k1 cos θ1 =
ω (ε˜1 − n2fiber sin2θfiber )1/2 c
(2)
(3)
Where, the subscripts fiber, 1 and 2 stand for the twin-core fiber, gold film and the measured medium respectively; the subscripts i, k¼ fiber, 1, 2; kZ1 is the z component of wave vector in the gold film; θfiber is the incident angle from the fiber to the gold film; d1 is the gold film thickness. The simulated conditions are: fiber core refractive index is 1.468, the thickness of the gold film is 50 nm, the dielectric constant of the gold film is obtained from Ref. [18]. According to the Fig. 2(a), (c), (e) and (g), in the refractive index range of 1.333–1.385, when the grinding angle α are 10°, 12°, 14°, and 16°, (SPR incidence angle are 80°, 78°, 76°, and 74°), the SPR dynamic range redshifts along with the incidence angle increasing, and the testing sensitivity increases from 2911 nm/RIU to 7548 nm/RIU, the dynamic range changes from 622–774 nm to 691–1084 nm. Correspondingly, we test the twin-core fiber DT probe with the grinding angle α are 10°, 12°, 14°, and 16°. The experimental results are shown in Fig. 2(b), (d), (f) and (h). In the refractive index range of 1.333–1.385, the SPR dynamic range redshifts along with the incidence angle increasing, and the testing sensitivity increases from 2307 nm/RIU to 5096 nm/RIU, and the dynamic range changes from 614–734 nm to 686–951 nm. The reasons why the experimental results of the sensitivity are lower than the simulated results may be the un-perfect grinding surface and the inhomogeneous thickness of the gold film. Fig. 3 provides the simulated and experimental results of the resonance wavelength when the grinding angles α are 10°, 12°, 14°, 16°. According to the Fig. 3, we can adjust the sensor working range by changing the fiber grinding angle. When the fiber grinding angle increases, the resonance wavelength will redshift, the resonance range will expand, and the testing sensitivity will be improved.
3. Distributed SPR sensing 3.1. DT probe for distributed sensing We employ the same twin-core fiber to fabricate the distributed DT probe. Fig. 4(a) provides the sensor probe structure, and the grinding angle1 is α, grinding angle2 is β. Fig. 4 (b) provides the image of the ground fiber tip. Here we employ the grinding angle γ to control the light beam reflection. Similarly, after grinding, we plate the gold film on the inclined surface of the
Fig. 1. Schematic diagram (a) and image(b) of the twin-core fiber DT probe.
Z. Liu et al. / Optics Communications 366 (2016) 107–111
109
Fig. 2. The simulated results with the grinding angle of (a)10°, (c)12°, (e)14° and (g)16°; the experimental results with the grinding angle of (b)10°, (d)12°, (f)14° and (h)16°.
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Fig. 3. The simulated (a) and experimental (b) results of the testing sensitivity with the grinding angle of 10°, 12 °, 14 ° and 16 °.
fiber tip with the thickness of 50 nm. The experimental setup is shown in Fig. 5. A super continuum light source (SuperK compact, NKT Photonics™) whose spectrum range is 450–2400 nm, is launched into one core of the twin-core fiber by using a lens transform system (seen Fig. 5(b), where we place the collimating lens in front of two single mode fibers while placing the microscope objective lens (the magnification of 25) in front of the twin-core fiber. The light power is launching into one core by adjusting the distance and position of the lens in the transform system), and the reflected beam is received by an optical spectrum analyzer (AQ6370C, Yokokawa™). We employ a programmable micro injection pumper (LSP01-1A, LongerPump™) to inject the Glycerine-aqueous solution to the micro fluidic channel. The Glycerine-aqueous solution index is calibrated by the Abbe refractometer (GDA-2S, Gold™). 3.2. Distributed sensing experimental results Based on the simulated and experimental results above, we grind the α to be 6°, β to be 17° and γ to be 79°. The simulated and experimental results are shown in the Fig. 6. Although the amplitude of the transmitted attenuating spectrum is not consistent with the simulated results, here we investigate the position of the resonance dip, which is consistent with the simulated results. Fig. 7 provides the simulated and experimental results of the resonance wavelength when the grinding angle α is 6° and β is 17°. According to the Fig. 7, we can adjust the sensor working range by changing the fiber grinding angle. When the fiber grinding angle increases, the resonance wavelength will redshift, and the resonance range will expand, and the testing sensitivity will be improved. According to Figs. 3 and 6, the single mode fiber SPR sensor resonance wavelength can shift to the much more sensitive band (near-infrared) by adjusting the single mode fiber grinding angle. Therefore, there are many advantages, such as, improving the sensitivity of SPR sensor, moving the working range of SPR sensor to the long wavelength, which is much suitable for a normal fiber light source. However, we have to admit that, the longer is the working wavelength, the larger is the noise.
Fig. 5. (a) The experiment setup sketch diagram of the reflective distributed twocore fiber SPR sensor; (b) the sketch diagram of the lens transform system; (c) the images the microfluidic device, where the inset image shows the details of the microfluidic channel and the fiber probe.
4. Conclusion In this paper, we study the effects of the fiber DT probe grinding angle on the SPR dynamic range. The larger is the fiber grinding angle, the longer is the resonance wavelength, and the higher is the testing sensitivity. By using this method, we can make the sensor operate in an optimal waveband, which helps to shorten the light source working waveband. The experimental show that, in the refractive index detecting the range of 1.333– 1.385, when the DT probe grinding angle increases from 10° to 16°,
Fig. 4. Schematic diagram of the reflective distributed twin-core fiber DT probe.
Z. Liu et al. / Optics Communications 366 (2016) 107–111
111
Fig. 6. (a) Simulated and (b) experimental results of the SPR spectrum response to different solution refractive index with fiber grind angle of 6° and 17°.
on-line monitoring.
Acknowledgment This work is supported by the National Natural Science Foundation of China (Grants no. 11204047, 61227013, 61275087, 61205071 and 61377085), and partially supported by the following grants: the 111 Project (B13015), Research Fund for the Doctoral Program of Higher Education of China (Grants no. 20112304 110017), Postdoctororal Science Foundation Fund of China (Grants nos. 2014M550181 and 2014M551217), and Fundamental Research Funds for Harbin Engineering University of China.
Fig. 7. The simulated (a) and experimental (b) results of the testing sensitivity with the grinding angle of 6° and 17 °.
the dynamic range moves from 614–734 nm to 686–951 nm, average testing sensitivity increases from 2308 nm/RIU to 5096 nm/RIU. On the basis of the results above, we propose and demonstrate a twin-core fiber reflected distributed SPR sensors. By using this sensor, we can detect multiple analytes in the same sensing area simultaneously, besides that we can also effectively compensate the errors caused by background interference, and the refractive index change resulting from the non-specific binding, or physical absorption and others. It worth to say that by using this method, we can adjust and control the resonance wavelength by changing the fiber grinding angle, and the testing sensitivity will not reduce after cascading. For an actual application, this reflective distributed fiber-based sensor is much suitable for biochemical sensing, it has a small size and can enter small testing spaces. The diameter of the twin-core fiber is only 125 μm, which helps to be integrated into a micro-fluidic chip, in this paper we integrate the fiber probe into an infusion needle to simulate blood vessels for
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