Applied Physics Express 7, 022501 (2014) http://dx.doi.org/10.7567/APEX.7.022501
Solution concentration and refractive index sensing based on polymer microfiber knot resonator Huaqing Yu1,2*, Liangbin Xiong1, Zhihong Chen1, Qianguang Li1, Xunong Yi1, Yu Ding3, Feng Wang3, Hao Lv1, and Yaoming Ding1* 1
School of Physics and Electronic Information Engineering, Hubei Engineering University, Xiaogan 432000, China Hubei Key Laboratory of Quality Control of Characteristic of Fruits and Vegetables, Hubei Engineering University, Xiaogan 432000, China 3 College of Chemistry and Materials Science, Hubei Engineering University, Xiaogan 432000, China E-mail:
[email protected];
[email protected] 2
Received November 12, 2013; accepted December 13, 2013; published online January 8, 2014 We report a simple and highly sensitive solution concentration and refractive index (RI) sensor based on a microfiber knot resonator (MKR). The sensor was assembled using a flexible poly(trimethylene terephthalate) microfiber of 1.3 µm diameter. The resonance peak shifts due to the variation of the concentration of the ambient medium surrounding the MKR were detected. In the mass concentration range of 0.6–1.2%, the concentration sensitivity and resolution of the sensor are 1.7 nm/% and 1.2 ' 10%3%, respectively. The highest measured sensitivity of the sensor is about 95.5 nm/RIU (RI unit) and the RI detection limit is about 2.1 ' 10%5 in the RI range from 1.39 to 1.41. © 2014 The Japan Society of Applied Physics
he online monitoring of the concentration of solutions, such as stock solutions, dilutions, and water-soluble industrial fluids, is of significant importance in many quality control industrial processes to check concentration stability and abnormal aging effects. More importantly, an online measurement technique should make it possible to detect the concentration without the need to handle the substance (e.g., dangerous solutions or solutions that are not easily accessible) under test. Microfibers/nanofibers leave a large fraction of the guided field outside the fiber as evanescent waves, making it highly sensitive to the variation of the ambient medium. Compared with electronic and chemical counterparts, fiber optic sensors for concentration and refractive index (RI) sensing have been the subject of intense investigation owing to their compact size, immunity to electromagnetic interference, and real-time sensing characteristics.1–5) Recently, optical resonators (i.e., knot, loop, and coil) based on microfibers/nanofibers have received intense attention in many sensing fields owing to their high Q-factor, small footprints, simple structure, and low loss.6–10) In particular, optical microfiber knot resonators (MKRs) have been widely used in RI, temperature, humidity, and current sensing owing to their high stability (i.e., high robustness to environmental perturbations), high sensitivity, and fast response.11–15) Therefore, several approaches, including the fiber-based Mach–Zehnder interferometer,2–5) microfiber loop,16) and integrated MKR,11) have been proposed to detect RI, but the sensitivity of these devices is relatively low [15–40 nm/RIU (RI unit) in the RI range of 1.3–1.38]. Poly(trimethylene terephthalate) (PTT) possesses strong flexibility and more than 90% elastic recovery, a relatively large refractive index (1.638), low dielectric losses at room temperature,17) and good transparency for visible to near-infrared light.18) These properties make PTT suitable for integrated photonic devices.19,20) In this work, we experimentally demonstrated MKR-based concentration and RI sensors using flexible PTT microfibers. The sensitivity of the sensor is approximately 46.5 nm/RIU for the surrounding RI in the range from 1.35 to 1.38; the RI detection limit is about 4.3 © 10¹5. The PTT microfibers used in this work were drawn from the PTT melt by a one-step process that has been reported elsewhere.18,21) As-fabricated fibers have lengths up to
T
Fig. 1. (a) Schematic diagram of experimental setup. (b) Optical microscopy image of an 85-µm-diameter MKR assembled with a 1.3-µmdiameter PTT fiber.
several millimeters with diameters from 200 to 800 nm, which is an optimal diameter for near-infrared light with respect to easy handling and efficient in/out coupling. It is worth noting that the lengths and diameters of the PTT fibers can be adjusted by changing the drawing speed (e.g., fibers with large diameter can be obtained by decreasing the drawing speed). In the experiment, an as-fabricated microfiber was tightened into an MKR of desired size using a micromanipulation process under an optical microscope. Then, the two free ends of the MKR were fixed by two stages placed on a MgF2-coated glass substrate (refractive index n = 1.50), which was supported by a micromanipulator with a high precision. The thickness of the MgF2 (n = 1.39) coating is 0.2–0.5 mm. For optical characterization, fiber tapers I and II illustrated in Fig. 1(a), which were drawn from a standard optical fiber, were used to launch and collect the light by evanescent coupling, respectively. Since the fiber taper I or II and the freestanding end of the MKR were placed in parallel and contacted each other tightly by van der Waals and electrostatic forces, the maximum coupling efficiency reached up to 90%.20,21) The amplitude transfer function of a self-coupled microfiber resonator can be obtained using7)
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Appl. Phys. Express 7, 022501 (2014)
T¼
expðL=2Þ expðjLÞ sinðKÞ ; 1 expðL=2Þ expðjLÞ sinðKÞ
H. Yu et al. (a)
ð1Þ
where ¡ is the intensity attenuation coefficient, ¢ is the propagation constant along the microfiber, L is the length of the ring resonator (equal to ³D; D is the diameter of the ring), and sin(K) is the intensity coupling ratio. K is the coupling parameter and it can be expressed as K = kl, where k is the coupling coefficient and l is the coupling length. According to Eq. (1), we can see that the resonance spectrum occurs when ¢l = 2n³ and K is close to Km = (2m + 1)³/2, where n and m are integers. The sensing mechanism of MKRs is based on the shift of resonance wavelength r because of a variation of ambient refractive index caused by varying the ambient medium concentration. As the RI of the ambient material varies, na surrounding MKRs changes the effective index neff of the guided modes in the microfiber and then results in a shift in the resonance wavelength of the transmission spectrum. Conversely, one can determine the concentration or RI of the surrounding medium by measuring the resonance wavelength shift of the output spectra. By monitoring the resonance wavelength shifts of the spectra, we can obtain the sensitivity of the MKR, which is defined as22) S¼
@r @r @neff r @neff ¼ ¼ : @na @neff @na neff @na
ð2Þ
It was found that S can be improved in thinner microfibers. This is because the evanescent wave field in the fibers becomes stronger and increasingly interacts with the ambient solution, which has an RI lower than those of the fibers. The detection limit for a change in refractive index in the MKR can be defined as22) na ¼ r
na 1 ¼ r ; S r
(b)
Fig. 2. Typical transmission spectra of an 85-µm-diameter MKR assembled with a 1.3-µm-diameter PTT fiber. (a) Transmission spectrum in the wavelength range of 1516–1521 nm for MKR immersed in NaCl solution with a mass concentration of 0.6%. (b) A close-up view of a single resonance peak shift at around 1517 nm for the MKR immersed in NaCl solution with a mass concentration of about 0.8%.
ð3Þ
where ¤r is the spectral resolution of the measurement system. To evaluate the performance of these MKRs as concentration sensors, firstly, we tested the output spectra of the MKRs. In the experiment, a tunable laser (TSL-210F) was used as the input light source and the power of the output light was recorded using an optical spectrum analyzer (MS 9710C, spectral resolution ³2 pm). All the experiments were carried out at room temperature and atmospheric pressure. Figure 1(b) shows an optical microscopy image of a typical MKR using a 1.3-µm-diameter PTT fiber with a knot diameter of about 85 µm (l ³ 36 µm), which showed a smooth outer surface morphology without obvious structural defects and was favorable for low-loss optical waveguiding. Figure 2(a) shows a typical transmission spectrum of an 85µm-diameter PTT MKR [the same as in Fig. 1(b)], which is immersed in a sample cell containing NaCl aqueous solution with a mass concentration of 0.6%. The scanning range of the wavelength is from 1516 to 1521 nm. Figure 2(b) is a closeup view of a single resonance peak at around 1517 nm for the MKR immersed in NaCl solution with a mass concentration of about 0.8%. From Fig. 2(b), it can be seen that the MKR has a Q-factor of about 11000 and a free spectral range of about 1.2 nm; the corresponding extinction is about 16 dB.
Fig. 3. Typical concentration-dependent resonance wavelength shift of MKR when the mass concentration changes from 0.6 to 1.2% with a step of approximately 0.05%. The red line is the numerically fitted curve of the experimental data.
To obtain the resonance peak shift versus the change in the concentration of the ambient medium, the MKR was immersed in eight NaCl aqueous solutions (mass concentration: 0.6–1.2%, step change ³0.05%). After each measurement, the sensor was cleaned with purified water, dried, and then prepared for use with NaCl solution of different concentrations. Figure 3 shows the resonance wavelength shift
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(a)
Fig. 5. Resonance wavelength shift of spectra as a function of refractive index of ambient solution. The corresponding glycerol mole ratios in the solutions are 0.48, 0.50, 0.51, 0.52, 0.53, 0.54, 0.55, and 0.56. The red line is the numerically fitted curve.
(b)
Fig. 4. Refractive-index-dependent resonance wavelength shift of MKR in glycerol solutions with glycerol mole ratios of 0.22, 0.24, 0.26, 0.29, 0.31, 0.33, 0.35, and 0.37. (a) Spectral shifts of a resonance peak caused by an index change of the ambient solution. (b) Linear relationship between resonance wavelength and RI of solution. The red line is the numerically fitted curve.
versus the RI of the ambient solution. The red line in Fig. 3 is the numerically fitted curve of the experimental data. The obtained fitting formula is y = 1.7x ¹ 1.0. According to the definition, the sensitivity is the slope of the numerically fitted curve. Thus, the sensitivity of the sensor is 1.7 nm/%, and the corresponding concentration resolution is 1.2 © 10¹3%. This is one order of magnitude higher than that of the pressure-induced long-period grating-based solution concentration sensor (sensitivity ³0.17 nm/%, concentration resolution ³5.88 © 10¹3%).23) Besides functioning as a concentration sensor, the proposed MKR can be used for an RI sensor as well. To estimate the sensing performance of MKR in detecting different types of ambient medium (e.g., organic liquids), firstly, the MKR was immersed in the sample cell containing pure water, and then the refractive index was modified by adding glycerol using a microliter syringe. As shown in Fig. 4(a), the MKR is operated at around 1.54 µm wavelength with a Q-factor and FSR of about 5200 and 1.1 nm, respectively. At this wavelength, the refractive indices of pure water and pure glycerol are 1.3180 and 1.4594, respectively.24) The refractive indices of the solutions can be estimated according to the mole ratio of each component,25) na ¼ ½k n2glycerol þ ð1 kÞn2water 1=2 ;
ð4Þ
where nwater is the RI of pure water, nglycerol is the RI of pure glycerol, and k is the mole ratio of glycerol in the solution. The refractive indices of the solutions with glycerol mole ratios of 0.22, 0.24, 0.26, 0.29, 0.31, 0.33, 0.35, and 0.37 are 1.3505, 1.3534, 1.3563, 1.3607, 1.3635, 1.3664, 1.3693, and 1.3721, respectively. Figure 4(b) shows the resonance wavelength shift as a function of the RI of the ambient solutions, where the red line is the numerical fitting curve. The obtained numerical fitting formula is y = 46.5x + 1477.5. Thus, the sensitivity of the sensor is 46.5 nm/RIU. This is higher than that of the reported optical loop resonator (17.8 nm/RIU),16) tapered fiber MZI (15–40 nm/RIU),2–5) and integrated MKR (30.5 nm/RIU).11) The RI detection limit of the sensor is 4.3 © 10¹5, which is better than those of the tapered fiber MZI5) (5.8 © 10¹4) and integrated MKR (2.3 © 10¹3).11) It should be noted that in this work, the temperature is kept almost constant (around 25 °C) during the measurement, and the temperature-induced drift is very small. To detect a higher RI of the ambient solution, glycerolwater solutions with mole ratios of 0.48, 0.50, 0.51, 0.52, 0.53, 0.54, 0.55, and 0.56 were used. With an increase in the concentration of the specimen, the refractive index contrast between the fiber and the surrounding medium decreases. As a result, the confinement of the guided light in the fiber decreases, resulting in a higher optical loss and a possible termination of the resonance. Thus, operation at a shorter wavelength is able to overcome this obstacle to obtaining good resonance with a relatively low refractive index contrast. Figure 5 shows the spectral response of the MKR sensor to ambient solutions with different RIs, where the red line is the numerically fitted curve. The measured resonance wavelength of the spectra is around 1.32 µm. At this wavelength, the refractive indices of pure water and pure glycerol are 1.3222 and 1.4605, respectively.24) The calculated refractive indices of the glycerol-water solutions with corresponding mole ratios are 1.3903, 1.3927, 1.3944, 1.3958, 1.3972, 1.3986, 1.400, and 1.4013, respectively. The obtained numerical fitting formula is y = 95.5x + 1192.1. The sensitivity and detection limit of the sensor are 95.5 nm/RIU and 2.1 © 10¹5, respectively. This is comparable to those of the copper-rod-supported microfiber loops.16) This confirmed the results of the analysis described above, which indicate
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that the sensitivity of the sensor will be higher if the MKR sensors operate at a shorter wavelength for low-index sensing. In summary, we have demonstrated concentration and RI sensors based on polymer MKR. The MKR is about 85 µm in diameter and assembled with a 1.3-µm-diameter PTT microfiber, which exhibits a linear response to both concentration and RI variation. The relatively high concentration sensitivity (1.7 nm/%) and resolution (1.2 © 10¹3%) for the sensor are achieved in the mass concentration range of 0.6–1.2%. On the other hand, the measured RI sensitivities and detection limits of the sensor in the RI ranges of 1.35–1.38 (lowconcentration solutions) and 1.39–1.41 (high-concentration solutions) are 46.5 nm/RIU, 4.3 © 10¹5 and 95.5 nm/RIU, 2.1 © 10¹5, respectively. We believe that the MKR can serve as concentration and RI sensors with high stability, high sensitivity, and large dynamic range in physics, biology, biochemistry, and environmental sensing fields. Acknowledgments This work was supported by the Natural Science Foundation of Hubei Province, China (Grant No. ZRZ0009930), the Major Program of Educational Commission of Hubei Province of China (Grant No. Z20102701), and the Hubei Key Laboratory of Quality Control of Characteristic of Fruits and Vegetables, Hubei Engineering University (Grant Nos. 2013K22 and SWZ009).
1) R. Falciai, A. G. Mignani, and A. Vannini, Sens. Actuators B 74, 74 (2001).
2) Z. Tian, S. S.-H. Yam, and H.-P. Loock, Opt. Lett. 33, 1105 (2008). 3) Z. Tian, S. S.-H. Yam, J. Barnes, W. Bock, P. Greig, J. M. Fraser, H.-P. Loock, and R. D. Oleschuk, IEEE Photonics Technol. Lett. 20, 626 (2008). 4) P. Lu, L. Men, K. Sooley, and Q. Chen, Appl. Phys. Lett. 94, 131110 (2009). 5) D. Wu, T. Zhu, M. Deng, D.-W. Duan, L.-L. Shi, J. Yao, and Y.-J. Rao, Appl. Opt. 50, 1548 (2011). 6) L. Shi, Y. Xu, W. Tan, and X. Chen, Sensors 7, 689 (2007). 7) M. Sumetsky, Y. Dulashko, J. M. Fini, A. Hale, and D. J. DiGiovanni, J. Lightwave Technol. 24, 242 (2006). 8) M. Sumetsky, R. S. Windeler, Y. Dulashko, and X. Fan, Opt. Express 15, 14376 (2007). 9) F. Xu and G. Brambilla, Appl. Phys. Lett. 92, 101126 (2008). 10) F. Xu, P. Horak, and G. Brambilla, Opt. Express 15, 7888 (2007). 11) K.-S. Lim, I. Aryanfar, W.-Y. Chong, Y.-K. Cheong, S. W. Harun, and H. Ahmad, Sensors 12, 11782 (2012). 12) Y. Wu, Y.-J. Rao, Y.-H. Chen, and Y. Gong, Opt. Express 17, 18142 (2009). 13) Y. Wu, T. Zhang, Y. Rao, and Y. Gong, Sens. Actuators B 155, 258 (2011). 14) P. Wang, F. Gu, L. Zhang, and L. Tong, Appl. Opt. 50, G7 (2011). 15) K. S. Lim, S. W. Harun, S. S. A. Damanhuri, A. A. Jasim, C. K. Tio, and H. Ahmad, Sens. Actuators A 167, 60 (2011). 16) X. Guo and L. Tong, Opt. Express 16, 14429 (2008). 17) A. Sanz, A. Nogales, T. A. Ezquerra, M. Soccio, A. Munari, and N. Lotti, Macromolecules 43, 671 (2010). 18) X. Xing, Y. Wang, and B. Li, Opt. Express 16, 10815 (2008). 19) H. Yu, D. Liao, M. B. Johnston, and B. Li, ACS Nano 5, 2020 (2011). 20) H. Yu, R. Zhang, and B. Li, Nanotechnology 22, 335202 (2011). 21) H. Yu and B. Li, Sci. Rep. 3, 1674 (2013). 22) C.-Y. Chao and L. J. Guo, J. Lightwave Technol. 24, 1395 (2006). 23) S.-H. Shi, X.-J. Zhou, Z.-Y. Zhang, L. Lan, C. Yin, and Y. Liu, Chin. Phys. Lett. 29, 094206 (2012). 24) P. R. Cooper, Appl. Opt. 22, 3070 (1983). 25) C. Gaffney and C.-K. Chau, Am. J. Phys. 69, 821 (2001).
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