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IEEE SENSORS JOURNAL, VOL. 13, NO. 11, NOVEMBER 2013
Single-Mode-Multimode Fiber Structure Based Sensor for Simultaneous Measurement of Refractive Index and Temperature Hongchao Xue, Hongyun Meng, Wei Wang, Rui Xiong, Qiqi Yao, and Ben Huang
Abstract— A fiber sensor for simultaneous measurement of refractive index (RI) and temperature in solutions based on multimode interference is presented. The intensity and the wavelength of the interference minimum will vary with the RI and the temperature of the solution, respectively. The sensitivity of the RI and the temperature are 94.58 dB/RI and 0.0085 nm/°C, respectively. Its ease of fabrication and low-cost offer the attractive applications in chemical and biological sensing. Index Terms— Fiber sensor, refractive index, temperature, SM fiber structure, multimode interference, fresnel reflection.
I. I NTRODUCTION
O
PTICAL fiber sensors have been widely studied in recent years, they have many advantages, such as small size, high sensitivity, immunity to electromagnetic interference, and so on [1]–[11]. They have generated great interest for remote sensing and process controlling, such as temperature, strain, RI, displacement and other physical parameters. In recent years, multimode interference is widely applied in the field of sensor, such as using a single-mode-multimode-single-mode (SMS) fiber structure [1]–[7], a SMS fiber structure combining a fiber Bragg grating [8], [9], a 3° slanted multimode fiber Bragg grating [10], a multimode-coreless-multimode (MCM) fiber structure [11]. Most of the methods mentioned above are based on the MMI in the fiber. But, these approaches proposed are mostly single parameter measurement and the SMS fiber structure proposed are mainly transmission-type which is difficult to immerse the multimode fiber (MMF) into the measured material completely. In this paper, we propose and demonstrate a single-mode-multimode (SM) fiber structure based sensor that can measure RI and temperature simultaneously. The interference will be formed between the different modes in the multimode fiber. The wavelength of the interference minimum will vary the temperature of the Manuscript received March 13, 2013; revised May 14, 2013; accepted May 16, 2013. Date of publication May 21, 2013; date of current version September 25, 2013. This work was supported in part by the National Natural Science Foundation of China under Grant 61275059 and the Natural Science Foundation of Guangdong Province, China under Grant 10151063101000014. The associate editor coordinating the review of this paper and approving it for publication was Dr. Anna Grazia Mignani. The authors are with the Laboratory of Nanophotonic Functional Materials and Devices, School for Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou 510006, China (e-mail:
[email protected];
[email protected]; 43671964@ qq.com;
[email protected];
[email protected]; huangben1989@ qq.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2013.2264460
Fig. 1. (a) The experiment schematic of the sensing system. (b) The SM fiber structure served as the sensing head.
solution. And the reflected intensity will vary with the RI of the solution due to the Fresnel reflection at the interface between the MMF and the solution. Then, the simultaneous measurement of the RI and the temperature of the solution can be achieved. Theory analysis and experiment are introduced and discussed in detail as follows. II. P RINCIPLE OF O PERATION Fig.1 shows the schematic of the RI and temperature simultaneous measurement sensor, which includes an amplified spontaneous emission (ASE) optical source as a broadband source (BBS), a circulator, a SM fiber structure sensing head, and an optical spectrum analyzer (OSA). The SM fiber structure sensing head is fabricated by fusion splicing a singlemode fiber (SMF) and a MMF together and the free end of the MMF is cleaved to achieve a surface perpendicular to the fiber axis to form a reflection mirror. When the light of different modes interfere in the MMF and couple back to the SMF at the splice, considering the two lowest-order modes only, the light intensity received by the OSA can be given by n co − n x 2 2πn L (1) I = I1 + I2 + 2 I1 I2 cos λ n co + n x where I1 and I2 are the power distributed in the first- and second-order modes, respectively. L is the double-length of the MMF, λ is the light wavelength, n is the difference of the two lowest-order mode indices. n co and n x are is the RI of the
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XUE et al.: SINGLE-MODE-MULTIMODE FIBER STRUCTURE BASED SENSOR
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multimode fiber core (MMFC) and the solution, respectively. Within the bandwidth of the source, the dependence of n co and n x on the wavelength can be ignored. From Eq. (1), it can be found that the optical intensity will vary with the RI of the solution. When the input light which has a field distribution of E(r, 0) launches the MMF, the input field can be decomposed by the eigenmodes {LPnm }of the multimode fiber. Due to the circular symmetric of input field and an ideal alignment, only the LP0m modes can be excited [12], denote the field profile of LP0m as Fm (r ), we have [13] E(r, 0) =
M
cm Fm (r )
(2)
m=1
where cm is the excitation coefficient of each mode, it can be given by [13] ∞ E(r, 0)Fm (r )r dr (3) cm = 0∞ 0 Fm (r )Fm (r )r dr As the light propagates in the multimode fiber section, the field at the propagation distance z can be given by [12]–[15] E(r, z) =
M
cm Fm (r ) exp(iβm z)
(4)
m=1
where βm is the propagation constant of each eigenmode of multimode fiber. As the excited modes of light propagate in the MMF, MMI will occur due to the different propagation constant of different mode. The light at z = L z has the same lateral profile as the input which is called re-imaging, where L z is given by [13] 4n co d 2 (5) Lz = λ where d is the diameter of the MMFC. It can be found in Eq. (5) that the period of the re-imaging is related to the wavelength of the light, the RI of the MMFC, and the diameter of the MMFC, respectively. The coupling efficiency to the SMF can be maximum at z = m L z , where m is a natural number. Usually, the re-imaging can’t gain when the length of the fiber is shorter than the L z . Oppositely, the re-imaging can gain but the coupling efficiency drops as the length of the fiber increasing. In the experiment, 60 mm was chosen to gain a suitable coupling efficiency. When the incident light injects into the multimode fiber, Fresnel reflection will happen at the interface between the MMF and the solution, then the light returns to the MMF and couples to the SMF and be received by the OSA at last. The intensity of the light will be maximum at the re-imaging distance. So, after the light of a certain wavelength range go through the SM structure and be reflected back to the OSA, the intensity of some wavelength is strong, but some is weak even zero. The wavelength of the interference minimum is determined by [2] d 2m n co (6) L So, when the temperature changes T , d, L, n co will change accordingly, and it will lead to the shift of the wavelength of λmin =
Fig. 2. Spectral response of the sensing system. The wavelength of the interference minimum is not affected by the variation of the RI. In contrast, the light intensity of each RI changes as a function of the RI.
the interference minimum, it can be given by λ0 min + λmin =
(d + d)2 m (n co + n co ) (L + L)
(7)
where d = k1 T , L = k1 T and n co = k2 T . The k1 and k2 are the thermal expansion coefficient and thermo-optic coefficient, respectively. λ0min is the initial wavelength of the interference minimum. From Eq. (7), it can be found that the λmin is only determined by the temperature variation T . III. R ESULTS AND D ISCUSSIONS To validate the technique, experiments are performed with the setup shown in Fig.1. We connect one Corning SMF-28 to the end of a 60 mm long YOFC SI 105/125-22/250 MMF with a core diameter of 105 μm and cladding diameter of 125 μm by fusion splicing. The optical fiber loss of fusion splicing is 0 dB in the experiment. The bandwidth and the maximum output power of the BBS are 1525-1565 nm and 13 dBm, respectively. In order to investigate the reflective spectra response of the sensor system at different RI, we put the sensing head in air, pure water and the NaCl solution with the concentration from 2.5% to 25%, respectively. The RI of the NaCl-water solution was calculated with [16]. The reflection spectral response is shown in Fig.2. As expected, the intensity at the wavelength of the interference minimum varies with the RI of the solution, and the wavelengths of the interference minimum are invariable due to the same temperature. One can see that there are two dips where interference minimum happen in the pattern. The temperature can be measured by detecting the light intensity of a dip. We choose the left dip (λ = 1544.76 nm) and detected its light intensity variation with the OSA when the concentration of the solution changes. The light intensity of the interference minimum as a function of the RI of the solution is shown in Fig. 3. Over the range from 1.3148 to 1.3534, the variation is 3.65 dB. The fitting results show that the variation is linear and the RI sensitivity is 94.58 dB/RI. In order to investigate the temperature property of the system, the sensor was immersed in the same solution at different temperature. Fig. 4 shows the optical spectra when
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IEEE SENSORS JOURNAL, VOL. 13, NO. 11, NOVEMBER 2013
IV. C ONCLUSION A novel simple sensor based on a SM fiber structure has been demonstrated for simultaneous measurement of refractive index and temperature. In this paper, the theory of the sensing system has been analyzed and the feasibility of this sensor is validated in experiment. This technology can effectively eliminate the cross sensitivity and has a high sensitivity. This low-cost, easy to fabricate sensor may find applications in many fields, including chemical and biological sensing. R EFERENCES Fig. 3.
The light intensity received by the OSA as a function of the RI.
Fig. 4. The measured spectra at the temperature from 25 °C to 95 °C of constant concentration of 5%.
Fig. 5. Plot of shift in wavelength of the interference minimum as a function of the temperature, fitted with a linear function. All of data are measured at the NaCl solution of constant concentration of 5%.
the sensor was immersed in the 5% NaCl-water solution at the temperature of 25, 60 and 95 °C. As expected, the wavelength of the interference minimum has a red shift when the temperature increases. The wavelength of the interference minimum as a function of the temperature of the solution is shown in Fig. 5. Over the range from 25 °C to 95 °C, the wavelength shift is 0.6 nm. The fitting results show that the shift is linear and the temperature sensitivity is 0.0085 nm/°. It should be note that the intensity varies weakly with the temperature because the RI of the solution is affected by the temperature slightly.
[1] Q. Wu, Y. Semenova, P. Wang, and G. Farrell, “High sensitivity SMS fiber structure based refractometer-analysis and experiment,” Opt. Exp., vol. 19, no. 9, pp. 7937–7944, Apr. 2011. [2] S. Taue, Y. Matsumoto, H. Fukano, and K. Tsuruta, “Experiment analysis of optical fiber multimode interference structure and its application to refractive index measurement,” J. Appl. Phys., vol. 51, p. 04DG14, Apr. 2012. [3] C. Zhang, E. Li, P. Lv, and W. Wang, “A wavelength encoded optical fiber sensor based on multimode interference in a coreless silica fiber,” Proc. SPIE, vol. 7157, p. 71570H, Feb. 2009. [4] Y. Liu and L. Wei, “Low-cost high-sensitivity strain and temperature sensing using graded-index multimode fibers,” Appl. Opt., vol. 46, no. 13, pp. 2516–2519, May 2007. [5] Q. Wu, Y. Semenova, P. Wang, A. M. Hatta, and G. Farrell, “Experimental demonstration of a simple displacement sensor based on a bent single-mode-multimode-single-mode fiber structure,” Meas. Sci. Technol., vol. 22, p. 025203, Jan. 2011. [6] Q. Wu, A. M. Hatta, P. Wang, Y. Semenova, and G. Farrell, “Use of a bent single SMS fiber structure for simultaneous measurement of displacement and temperature sensing,” IEEE Photon. Technol. Lett., vol. 23, no. 2, pp. 130–132, Jan. 2011. [7] A. Kumar, R. K. Varshney, and S. A. C. P. Sharma, “Transmission characteristics of SMS fiber optic sensor structures,” Opt. Commun., vol. 219, pp. 215–219, Feb. 2003. [8] D. Zhou, L. Wei, W. Liu, Y. Liu, and J. W. Y. Lit, “Simultaneous measurement for strain and temperature using fiber Bragg grating and multimode fibers,” Appl. Opt., vol. 47, no. 10, pp. 1668–1672, Apr. 2008. [9] Q. Wu, Y. Semenova, B. Yan, Y. Ma, P. Wang, C. Yu, and G. Farrell, “Fiber refractometer based on a fiber Bragg grating and single-modemultimode-single-mode fiber structure,” Opt. Lett., vol. 36, no. 12, pp. 2197–2199, Jun. 2011. [10] C. Zhao, X. Yang, M. S. Demokan, and W. Jin, “Simultaneous temperature and refractive index measurements using a 3°slanted multimode fiber bragg grating,” J. Lightw. Technol., vol. 24, no. 2, pp. 879–883, Feb. 2006. [11] Y. Jung, S. Kim, D. Lee, and K. Oh, “Compact three segmented multimode fibre modal interferometer for high sensitivity refractiveindex measurement,” Meas. Sci. Technol., vol. 17, pp. 1129–1133, Apr. 2006. [12] W. S. Mohammed, A. Mehta, and E. G. Johnson, “Wavelength tunable fiber lens based on multimode interference,” J. Lightw. Technol., vol. 22, no. 2, pp. 469–477, Feb. 2004. [13] Q. Wang, G. Farrell, and W. Yan, “Investigation on single-modemultimode-single-mode fiber structure,” J. Lightw. Technol., vol. 26, no. 5, pp. 512–519, Mar. 2008. [14] W. S. Mohammed, P. W. E. Smith, and X. Gu, “All-fiber multimode interference bandpass filter,” Opt. Lett., vol. 31, no. 17, pp. 2547–2549, Sep. 2006. [15] L. B. Soldano and E. C. M. Pennings, “Optical multi-mode interference devices based on self-imaging: Principles and applications,” J. Lightw. Technol., vol. 13 no. 4, pp. 615–627, Apr. 1995. [16] H. Su and X. Huang, “Fresnel-reflection-based fiber sensor for on-line measurement of solute concentration in solutions,” Sens. Actuators B, Chem., vol. 126 no. 2, pp. 579–582, Apr. 2007.
XUE et al.: SINGLE-MODE-MULTIMODE FIBER STRUCTURE BASED SENSOR
Hongchao Xue received the B.Sc. degree from Henan Normal University, Henan, China, in 2010. He is currently a Postgraduate with the Laboratory of Nanophotonic Functional Materials and Devices, School for Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou, China. His current research interests include fiber-optic sensors and optical communications.
Hongyun Meng was born in 1973. He received the M.S. and Ph.D. degrees from the Institute of Modern Optics, Nankai University, Tianjin, China, in 2000 and 2003, respectively. He was with South China Normal University, Guangzhou, China, from 2003 to 2005, and from 2007 to 2008, he was with the Korea Advanced Institute of Science and Technology, Daejeon, Korea, as a Post-Doctoral. He has been an Associate Professor with South China Normal University since 2005. He is currently a Professor with South China Normal University. He is the author or co-author of more than 60 journal and conference papers. His current research interests include optical amplifiers, optical lasers, and fiber sensors.
Wei Wang received the B.Sc. degree from the Nanjing University of Posts and Telecommunications, Nanjing, China, in 2006. He is currently a Postgraduate with the Laboratory of Nanophotonic Functional Materials and Devices, School for Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou, China. His current research interests include fiber-optic sensors and optical communications.
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Rui Xiong received the B.Sc. degree from Hubei Engineering University, Hubei, China, in 2011. He is currently a Postgraduate with the Laboratory of Nanophotonic Functional Materials and Devices, School for Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou, China. His current research interests include fiber-optic sensors and optical communications.
Qiqi Yao received the B.Sc. degree from the Wuhan Institute of Techology, Wuhan, China, in 2009. He is currently a Postgraduate with the Laboratory of Nanophotonic Functional Materials and Devices, School for Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou, China. His current research interests include fiber-optic sensors and optical communications.
Ben Huang received the B.Sc. degree from Hubei Engineering University, Hubei, China, in 2012. He is currently a Postgraduate with the Laboratory of Nanophotonic Functional Materials and Devices, School for Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou, China. His current research interests include fiber-optic sensors and optical communications.
