Liquid-filled photonic-crystal-fiber-based multimodal interferometer for simultaneous measurement of temperature and force Wei Lin,1 Binbin Song,1 Yinping Miao,2 Hao Zhang,1 Donglin Yan,1 Bo Liu,1,* and Yange Liu1 1
Key Laboratory of Optical Information Science and Technology, Ministry of Education, Institute of Modern Optics, Nankai University, Tianjin 300071, China
2
Tianjin Key Laboratory of Film Electronic & Communication Devices, School of Electronics Information Engineering, Tianjin University of Technology, Tianjin 300384, China *Corresponding author: [email protected] Received 26 November 2014; revised 5 January 2015; accepted 11 January 2015; posted 12 January 2015 (Doc. ID 228480); published 12 February 2015
In this paper, a multimodal interferometer based on the liquid-filled photonic crystal fiber (PCF) has been proposed and experimentally demonstrated for simultaneous measurement of temperature and force. Experimental results show that different spectral minima have distinctive sensitivities to the temperature and force. The proposed interferometer shows the temperature sensitivities of −9.214 nm∕°C, −24.757 nm∕°C, and −12.543∕°C and the force sensitivities of 0 nm∕N, 4.978 nm∕N, and 0 nm∕N, respectively, for the three selected spectral minima. The sensing matrices are thus established and simultaneous measurement of temperature and force has been experimentally demonstrated. The proposed liquid-filled PCF-based multimodal interferometer would find potential applications in multiple-parameter sensing owing to its high sensitivity, compactness, ease of fabrication, and low cost. © 2015 Optical Society of America OCIS codes: (060.5295) Photonic crystal fibers; (060.2370) Fiber optics sensors; (060.2310) Fiber optics. http://dx.doi.org/10.1364/AO.54.001309
1. Introduction
Optical fiber sensors have been widely investigated and extensively applied in the field of optical sensing technology, such as the measurement of temperature, strain, press, refractive index (RI), and so on, owing to their various advantages compared with traditional electronic sensors, including portability, high geometric adaptability, and anti-interference, etc., [1]. In practical applications, a single parameter sensor is hard to be achieved due to the cross-sensitivity effect in the presence of other environmental parameters. Several methods have been proposed to resolve this 1559-128X/15/061309-05$15.00/0 © 2015 Optical Society of America
issue, including parameter compensation and multiplexing sensing [2,3]. And the multiplexing sensing is the most effective method which could deal with the problem of cross sensitivity as well as simultaneous measurement of the multiple parameters. The principle of the multiplexing sensing is often based on the sensing head having different sensitivities to the physical parameters, which could be achieved by cascading different sensor heads with different sensitivities [4–9], or utilizing different modes in a single fiber structure [10–12]. The second solution would be widely used owing to the compactness of the sensor system. Regarding this method, many techniques have been proposed to excite the multiple modes, such as multimode-fiber-based interferometer [12,13], selective infiltration of high RI liquids [10], off-set 20 February 2015 / Vol. 54, No. 6 / APPLIED OPTICS
1309
splicing fusion [11,14], and fiber taper [15,16]. Among them, the multimode-fiber-based interferometer is the most effective way to excite the multiple modes by simply splicing the multimode fiber with single-mode fiber, which can achieve the simultaneous measurement due to the different sensitivities of the spectral minima to the external parameters. However, the sensitivity of this device is limited when using the uniform multimode fibers. For the past few decades, photonic crystal fibers (PCFs) have attracted increasing research interest, owing to their capability of functional material infiltration [17,18]. In the field of optical sensing technology, the device sensitivity can be greatly enhanced by infiltrating an appropriate liquid [19–21]. This makes it possible to overcome the aforementioned problem. Recently, we have theoretically analyzed and experimentally demonstrated the multimodal transmission property of the PCFs infiltrated with the RI matching liquid whose RI is close to the silica background [22]. The multimodal transmission property makes it possible to fabricate a multimode interferometer by splicing it with two standard single-mode fibers (SMFs) in between. In this work, we have investigated a multimodal interferometer utilizing the liquid-filled PCF. This interferometer has several advantages. First, it is highly sensitive to environmental temperature owing to the high temperature coefficient of the RI liquid, compared with other multimodal interferometers based on the traditional multimode fibers [23]. Second, compared with the multi-parameter sensor devices based on selectively infiltrated PCFs, it is easier to fabricate by simply filling the liquid with appropriate RI into the PCF and splicing it between two standard SMFs [24,25]. Third, its interference spectral minima have different sensitivities to the external parameters which could help to overcome the cross sensitivity in sensor applications. And finally, this interferometer has a small size and is highly compact. Due to these advantages, this interferometer would find potential applications in the measurement of multiple parameters. 2. Experimental Setup and Operation Principle
Photonics, Denmark) is infiltrated with the thermally sensitive RI matching liquid produced by Cargille Labs, USA (RI is 1.46 0.0002 at 25°C for 589.3 nm; temperature coefficient is −0.000398 RIU∕°C [RIU: refractive index unit]). The PCF has a length of 7 mm with 3 mm segment filled with the index liquid. A liquid-filled PCF-based multimodal interferometer is achieved by simply splicing this PCF between two sections of SMFs. The temperature-controlled chamber is employed to increase the environmental temperature from room temperature to 99.9°C with an adjustable accuracy of 0.1°C. The fiber holder is used to fix the fiber and the pulley is introduced to load the axial force with the same height as the fiber holder. In addition, a preset force (about 0.049N) is loaded to keep the fiber straight. It should be mentioned that the wavelength resolution of the OSA used in our experiment is 0.5 nm. As analyzed in our previous work in [17], multiple eigenmodes will be excited when the light from the SBS launches into this liquid-filled PCF. These modes will be coupled back into the fiber core of the SMFs when they propagate into the lead out fiber. Owing to the effective RI difference, multimodal interference will occur and the interference spectrum can be observed through the OSA, as shown in Fig. 2. The spectral minima in the spectrum can be expressed as [22]: λp
2nieff λp − njeff λp L 2Δni;j eff λp L ; 2p 1 2p 1
(1)
p is integer. nieff λp and njeff λp are the effective RI of the i- and j-order modes, which are the major modes involved in the multimodal interference. When the external parameters (such as temperature and force) change, nieff λp and njeff λp will exhibit different sensitivities to these parameters. The effective RI difference of the i- and j-order modes Δni;j eff λp will change accordingly. And thus, the wavelength will shift with the variation of these external parameters. Considering the influences of the axial force and temperature, the wavelength shift of the spectral minima can be expressed as
Figure 1 illustrates the schematic diagram of the experimental setup. It consists of a homemade supercontinuum broadband source (SBS), a liquid-filled PCF, and an optical spectrum analyzer (OSA: Yokogawa AQ6370C, operation wavelength ranges from 600 to 1700 nm). The PCF (LMA-10, provided by NKT
Fig. 1. Schematic diagram of the experimental setup. 1310
APPLIED OPTICS / Vol. 54, No. 6 / 20 February 2015
Fig. 2. Transmission spectra of the 7 mm unfilled PCF and the 7 mm PCF with 3 mm segment filled under a room temperature of 19.5°C. Inset shows the transmission output spectra together with the output spectrum of the SBS.
dλ
∂λ ∂λ dT dF; ∂T ∂F
(2)
where eff λ ∂Δn ∂λ ∂T ; eff ∂T Δneff − λ ∂Δn ∂λ
(3)
eff λ ∂Δn dλ ∂F : eff dF Δneff − λ ∂Δn ∂λ
(4)
Due to the high temperature sensitivity of the infiltrated liquid, Δneff would be highly dependent on the environmental temperature T, leading to the high temperature sensitivity of this structure. In addition, each mode in the liquid infiltrated PCF exhibits different photoelastic sensitivities caused by the axial force F. And therefore, Δneff changes with the variation of the applied axial force. As a result, the wavelength shifts accordingly [26]. For the proposed multimodal interferometer, the spectral minima would exhibit different sensitivities to external parameters, owing to different modes involved in the multimodal interference. Thus, simultaneous measurement of force and temperature can be achieved by measuring the wavelength shift of two selected spectral minima within the linear operation wavelength range, using the following sensor matrix: ΔT K T1 K F1 −1 Δλ1 ; (5) K T2 K F2 Δλ2 ΔF where ΔT and ΔF refer to the variation of the measured parameters; Δλ1 and Δλ2 are the wavelength shift of the two spectral minima; K T1 and K F2 are the coefficients of the temperature and force for minimum 1, and K T2 and K F2 represent those for minimum 2. 3. Experimental Results and Discussion
The temperature property of this liquid-filled PCFbased multimodal interferometer is studied with a constant preset axial force. The transmission spectrum of the interferometer changes gradually with the increment of environmental temperature from 19.5°C to 22.5°C, as shown in Fig. 3. It is obvious that
Fig. 3. Transmission spectra of the spectral interference under different temperatures.
Fig. 4. Spectral minimum wavelength shift as a function of temperature.
all of the spectral minima exhibit blueshift as the temperature increases. Figure 4 illustrates the minimum wavelength shift as a function of temperature. It can be seen that minimum A, B, and C shift by −38.1 nm, −71.9 nm, and −27.2 nm, respectively, and the corresponding sensitivities are −9.214 nm∕°C, −24.757 nm∕°C, and −12.543∕°C, respectively. For a temperature range of 19.5°C–22.5°C, the response shows a linear temperature-dependent behavior. When the temperature is out of this range, the spectral minima would rapidly vanish and new minimum would turn up. This phenomenon is caused by the fact that more higher-order eigenmodes are involved in the multimodal interference with the increment of the temperature within this range [22]. In order to investigate the force sensing characteristics, the external temperature is kept at 19.5°C using the temperature-controlled chamber, and the axial force is gradually increased by utilizing different weights. Figure 5 shows the transmission spectra of the interference under different applied forces. The transmission spectrum changes gradually with the increment of the axial force, as shown in Fig. 5. Three spectral minima are selected to analyze the force sensing property of the proposed interferometer. Figure 6 shows the minimum wavelength and transmission in response to the applied axial force. As shown in Fig. 6(a), with the increment of the applied axial force, the minimum wavelength shifts from
Fig. 5. Transmission spectra of the proposed interference under different axial forces, insets (a), (b), and (c) show the enlarged transmission spectra for minimum A, B, and C, respectively. 20 February 2015 / Vol. 54, No. 6 / APPLIED OPTICS
1311
of 19.5°C–22.5°C and force measurement range of 0– 0.882 N. Compared with the pretty high temperature sensitivities, the temperature measurement range is rather small. This problem can be overcome by infiltrating a liquid with lower RI, which ensures that fewer modes are involved in the multimodal interference, as analyzed in our previous work. Thus, it is possible to design a sensor for simultaneous measurement of temperature and force with high sensitivity as well as large measurement range. 4. Conclusion Fig. 6. Transmission minimum response to the applied axial force (a) wavelength response, (b) transmission response.
1218.2 to 1218.4 nm and 1458.7 to 1459 nm for minimum A and C, respectively. Their wavelength shifts are smaller than 0.5 nm, which is below the resolution of the experimentally employed OSA. Therefore, the minimum wavelengths could be considered unchanged. However, the wavelength of minimum B increases linearly from 1333.8 to 1338.4 nm with the increment of the axial force and has a sensitivity of 4.978 nm∕N. As to the transmission response, it is obvious that the minimum transmission increases with the increment of the applied axial force, as shown in Fig. 6(b). The transmission of minimum A increases from −27.877 to −23.699 dB and exhibits a nonlinear behavior which could be expressed as: y −33.4 0.49382 expx 3.61689∕1.50455. And for minimum B, the transmission increases from −44.668 to −26.176 dB and the experiment data could be fitted by: y −19.25934 − 25.3601 exp−x∕0.67969. However, the transmission of minimum C increases linearly from −41.482 to −30.701 dB with a sensitivity of 12.56 dB∕N. Different force as well as temperature sensitivities of the transmission minima make it possible to fabricate a multi-parameter sensing system. The temperature and force coefficients of the different spectral minima of the liquid-filled PCF-based multimodal interferometer are obtained by linear fitting of the measurement data, as shown in Figs. 4 and 6(a), respectively. Therefore, Eq. (5) can be written as by selecting minimum A and B or minimum B and C:
ΔT ΔF
−9.721 nm∕°C 0 −24.757 nm∕°C 4.978 nm∕N
−1
ΔλA ; ΔλB (6)
ΔT ΔF
−24.757 nm∕°C 4.978 nm∕N −12.543 nm∕°C 0
−1
ΔλB : ΔλC (7)
By using the above matrices, the variations of the temperature and the axial force can be measured simultaneously with high sensitivity. The proposed sensor possesses a temperature measurement range 1312
APPLIED OPTICS / Vol. 54, No. 6 / 20 February 2015
In this paper, a liquid-filled PCF-based multimodal interferometer has been proposed to achieve simultaneous measurement of temperature and force. A temperature sensitivity of −24.757 nm∕°C has been experimentally obtained for a temperature range of 19.5°C–22.5°C, and a force sensitivity of 4.978 nm∕N has been achieved for a force range of 0–0.882 N. And a sensor matrix is acquired to effectively resolve the cross sensitivity. The proposed liquid-filled PCFbased multimodal interferometer has such advantages as high sensitivity, high compactness, ease of fabrication, and low cost, and it would have potential applications in the measurement of multiple parameters. This work was jointly supported by the National Natural Science Foundation of China under grants 61377095, 61322510, 11274182, 11204212, and 11004110; the National Key Basic Research and Development Program of China under grant 2010CB327605; the 863 National High Technology Program of China under grant 2013AA014201; Science & Technology Support Project of Tianjin under grant 11ZCKFGX01800; Key Natural Science Foundation Project of Tianjin under grant 13JCZDJC26100; China Postdoctoral Science Foundation Funded Project under grant 2012M520024; and the Fundamental Research Funds for the Central Universities. References 1. K. T. V. Grattan and T. Sun, “Fiber optic sensor technology: an overview,” Sens. Actuators A 82, 40–61 (2000). 2. H. Zhao, F. Sun, Y. Yang, G. Cao, and K. Sun, “A novel temperature-compensated method for FBG-GMM current sensor,” Opt. Commun. 308, 64–69 (2013). 3. H. Luo, Q. Sun, Z. Xu, D. Liu, and L. Zhang, “Simultaneous measurement of refractive index and temperature using multimode microfiber-based dual Mach–Zehnder interferometer,” Opt. Lett. 39, 4049–4052 (2014). 4. H. J. Patrick, G. M. Williams, A. D. Kersey, J. R. Pedrazzani, and A. M. Vengsarkar, “Hybrid fiber Bragg grating/long period fiber grating sensor for strain/temperature discrimination,” IEEE Photon. Technol. Lett. 8, 1223–1225 (1996). 5. D. Zhou, L. Wei, W. K. Liu, and J. W. Y. Lit, “Simultaneous measurement of strain and temperature based on a fiber Bragg grating combined with a high-birefringence fiber loop mirror,” Opt. Commun. 281, 4640–4643 (2008). 6. D. J. J. Hu, J. L. Lim, M. Jiang, Y. Wang, F. Luan, P. P. Shum, H. Wei, and W. Tong, “Long period grating cascaded to photonic crystal fiber modal interferometer for simultaneous measurement of temperature and refractive index,” Opt. Lett. 37, 2283–3385 (2012).
7. J. Li, W. Zhang, S. Gao, P. Geng, X. Xue, Z. Bai, and H. Liang, “Long-period fiber grating cascaded to an S fiber taper for simultaneous measurement of temperature and refractive index,” IEEE Photon. Technol. Lett. 25, 888–891 (2013). 8. Q. Zhang, T. Zhu, J. Zhang, and K. S. Chiang, “Microfiber-based FBG sensor for simultaneous measurement of vibration and temperature,” IEEE Photon. Technol. Lett. 25, 1751–1753 (2013). 9. Q. Liu, Z. L. Ran, Y. J. Rao, S. C. Luo, H. Q. Yang, and Y. Huang, “Highly integrated FP/FBG sensor for simultaneous measurement of high temperature and strain,” IEEE Photon. Technol. Lett. 26, 1715–1717 (2014). 10. H. Liang, W. Zhang, P. Geng, Y. Liu, Z. Wang, J. Guo, S. Gao, and S. Yan, “Simultaneous measurement of temperature and force with high sensitivities based on filling different index liquids into photonic crystal fiber,” Opt. Lett. 38, 1071–1073 (2013). 11. J. Zhou, C. Liao, Y. Wang, G. Yin, X. Zhong, K. Yang, B. Sun, G. Wang, and Z. Li, “Simultaneous measurement of strain and temperature by employing fiber Mach–Zehnder interferometer,” Opt. Express 22, 1680–1686 (2014). 12. 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. 23, 130–132 (2011). 13. W. Zhou, Y. Zhou, X. Dong, L. Y. Shao, J. Cheng, and J. Albert, “Fiber-optic curvature sensor based on cladding-mode Bragg grating excited by fiber multimode interferometer,” IEEE Photon. J. 4, 1051–1057 (2012). 14. C. Gouveia, P. A. S. Jorge, J. M. Baptista, and O. Frazão, “Temperature-independent curvature sensor using FBG cladding modes based on a core misaligned splice,” IEEE Photon. Technol. Lett. 23, 804–806 (2011). 15. S. Yang, H. Sun, L. Yuan, X. Zhang, L. Zhou, and M. Hu, “Refractive index and temperature sensor based on claddingmode Bragg grating excited by abrupt taper interferometer,” Chin. Opt. Lett. 11, 120604 (2013). 16. T. Qi, S. Xiao, J. Shi, L. Yi, Z. Zhou, M. Bi, and W. Hu, “Cladding mode backward recoupling based displacement sensor
17. 18. 19.
20. 21.
22. 23.
24.
25.
26.
incorporating fiber up taper and Bragg grating,” IEEE Photon. J. 5, 7100608 (2013). B. T. Kuhlmey, B. J. Eggleton, and D. K. C. Wu, “Fluid-filled solid-core photonic bandgap fibers,” J. Lightwave Technol. 27, 1617–1630 (2009). C. Markos, I. Kubat, and O. Bang, “Hybrid polymer photonic crystal fiber with integrated chalcogenide glass nanofilms,” Sci. Rep. 4, 6057 (2014). W. Qian, C. Zhao, S. He, X. Dong, S. Zhang, Z. Zhang, S. Jin, J. Guo, and H. Wei, “High-sensitivity temperature sensor based on an alcohol-filled photonic crystal fiber loop mirror,” Opt. Lett. 36, 1548–1550 (2011). D. K. C. Wu, B. T. Kuhlmey, and B. J. Eggleton, “Ultrasensitive photonic crystal fiber refractive index sensor,” Opt. Lett. 34, 322–324 (2009). Y. Geng, X. Li, X. Tan, Y. Deng, and X. Hong, “Compact and ultrasensitive temperature sensor with a fully liquid-filled photonic crystal fiber Mach–Zehnder interferometer,” IEEE Sens. J. 14, 167–170 (2014). W. Lin, Y. Miao, B. Song, H. Zhang, B. Liu, Y. Liu, and D. Yan, “Multimodal transmission property in a liquid-filled photonic crystal fiber,” Opt. Commun. 336, 14–19 (2015). S. Silva, O. Frazão, J. Viegas, L. A. Ferreira, F. M. Araújo, F. X. Malcata, and J. L. Santos, “Temperature and strainindependent curvature sensor based on a single-mode/ multimode fiber optic structure,” Meas. Sci. Technol. 22, 085201 (2011). H. Liang, W. Zhang, H. Wang, P. Geng, S. Zhang, S. Gao, C. Yang, and J. Li, “Fiber in-line Mach–Zehnder interferometer based on near-elliptical core photonic crystal fiber for temperature and strain sensing,” Opt. Lett. 38, 4019–4022 (2013). Y. Peng, J. Hou, Y. Zhang, Z. Huang, R. Xiao, and Q. Lu, “Temperature sensing using the bandgap-like effect in a selectively liquid-filled photonic crystal fiber,” Opt. Lett. 38, 263–265 (2013). S. M. Tripathi, A. Kumar, R. K. Varshney, Y. B. P. Kumar, E. Marin, and J. P. Meunier, “Strain and temperature sensing characteristics of single-mode–multimode–single-mode structures,” J. Lightwave Technol. 27, 2348–2356 (2009).
20 February 2015 / Vol. 54, No. 6 / APPLIED OPTICS
1313
Key Laboratory of Optical Information Science and Technology, Ministry of Education, Institute of Modern Optics, Nankai University, Tianjin 300071, China
2
Tianjin Key Laboratory of Film Electronic & Communication Devices, School of Electronics Information Engineering, Tianjin University of Technology, Tianjin 300384, China *Corresponding author: [email protected] Received 26 November 2014; revised 5 January 2015; accepted 11 January 2015; posted 12 January 2015 (Doc. ID 228480); published 12 February 2015
In this paper, a multimodal interferometer based on the liquid-filled photonic crystal fiber (PCF) has been proposed and experimentally demonstrated for simultaneous measurement of temperature and force. Experimental results show that different spectral minima have distinctive sensitivities to the temperature and force. The proposed interferometer shows the temperature sensitivities of −9.214 nm∕°C, −24.757 nm∕°C, and −12.543∕°C and the force sensitivities of 0 nm∕N, 4.978 nm∕N, and 0 nm∕N, respectively, for the three selected spectral minima. The sensing matrices are thus established and simultaneous measurement of temperature and force has been experimentally demonstrated. The proposed liquid-filled PCF-based multimodal interferometer would find potential applications in multiple-parameter sensing owing to its high sensitivity, compactness, ease of fabrication, and low cost. © 2015 Optical Society of America OCIS codes: (060.5295) Photonic crystal fibers; (060.2370) Fiber optics sensors; (060.2310) Fiber optics. http://dx.doi.org/10.1364/AO.54.001309
1. Introduction
Optical fiber sensors have been widely investigated and extensively applied in the field of optical sensing technology, such as the measurement of temperature, strain, press, refractive index (RI), and so on, owing to their various advantages compared with traditional electronic sensors, including portability, high geometric adaptability, and anti-interference, etc., [1]. In practical applications, a single parameter sensor is hard to be achieved due to the cross-sensitivity effect in the presence of other environmental parameters. Several methods have been proposed to resolve this 1559-128X/15/061309-05$15.00/0 © 2015 Optical Society of America
issue, including parameter compensation and multiplexing sensing [2,3]. And the multiplexing sensing is the most effective method which could deal with the problem of cross sensitivity as well as simultaneous measurement of the multiple parameters. The principle of the multiplexing sensing is often based on the sensing head having different sensitivities to the physical parameters, which could be achieved by cascading different sensor heads with different sensitivities [4–9], or utilizing different modes in a single fiber structure [10–12]. The second solution would be widely used owing to the compactness of the sensor system. Regarding this method, many techniques have been proposed to excite the multiple modes, such as multimode-fiber-based interferometer [12,13], selective infiltration of high RI liquids [10], off-set 20 February 2015 / Vol. 54, No. 6 / APPLIED OPTICS
1309
splicing fusion [11,14], and fiber taper [15,16]. Among them, the multimode-fiber-based interferometer is the most effective way to excite the multiple modes by simply splicing the multimode fiber with single-mode fiber, which can achieve the simultaneous measurement due to the different sensitivities of the spectral minima to the external parameters. However, the sensitivity of this device is limited when using the uniform multimode fibers. For the past few decades, photonic crystal fibers (PCFs) have attracted increasing research interest, owing to their capability of functional material infiltration [17,18]. In the field of optical sensing technology, the device sensitivity can be greatly enhanced by infiltrating an appropriate liquid [19–21]. This makes it possible to overcome the aforementioned problem. Recently, we have theoretically analyzed and experimentally demonstrated the multimodal transmission property of the PCFs infiltrated with the RI matching liquid whose RI is close to the silica background [22]. The multimodal transmission property makes it possible to fabricate a multimode interferometer by splicing it with two standard single-mode fibers (SMFs) in between. In this work, we have investigated a multimodal interferometer utilizing the liquid-filled PCF. This interferometer has several advantages. First, it is highly sensitive to environmental temperature owing to the high temperature coefficient of the RI liquid, compared with other multimodal interferometers based on the traditional multimode fibers [23]. Second, compared with the multi-parameter sensor devices based on selectively infiltrated PCFs, it is easier to fabricate by simply filling the liquid with appropriate RI into the PCF and splicing it between two standard SMFs [24,25]. Third, its interference spectral minima have different sensitivities to the external parameters which could help to overcome the cross sensitivity in sensor applications. And finally, this interferometer has a small size and is highly compact. Due to these advantages, this interferometer would find potential applications in the measurement of multiple parameters. 2. Experimental Setup and Operation Principle
Photonics, Denmark) is infiltrated with the thermally sensitive RI matching liquid produced by Cargille Labs, USA (RI is 1.46 0.0002 at 25°C for 589.3 nm; temperature coefficient is −0.000398 RIU∕°C [RIU: refractive index unit]). The PCF has a length of 7 mm with 3 mm segment filled with the index liquid. A liquid-filled PCF-based multimodal interferometer is achieved by simply splicing this PCF between two sections of SMFs. The temperature-controlled chamber is employed to increase the environmental temperature from room temperature to 99.9°C with an adjustable accuracy of 0.1°C. The fiber holder is used to fix the fiber and the pulley is introduced to load the axial force with the same height as the fiber holder. In addition, a preset force (about 0.049N) is loaded to keep the fiber straight. It should be mentioned that the wavelength resolution of the OSA used in our experiment is 0.5 nm. As analyzed in our previous work in [17], multiple eigenmodes will be excited when the light from the SBS launches into this liquid-filled PCF. These modes will be coupled back into the fiber core of the SMFs when they propagate into the lead out fiber. Owing to the effective RI difference, multimodal interference will occur and the interference spectrum can be observed through the OSA, as shown in Fig. 2. The spectral minima in the spectrum can be expressed as [22]: λp
2nieff λp − njeff λp L 2Δni;j eff λp L ; 2p 1 2p 1
(1)
p is integer. nieff λp and njeff λp are the effective RI of the i- and j-order modes, which are the major modes involved in the multimodal interference. When the external parameters (such as temperature and force) change, nieff λp and njeff λp will exhibit different sensitivities to these parameters. The effective RI difference of the i- and j-order modes Δni;j eff λp will change accordingly. And thus, the wavelength will shift with the variation of these external parameters. Considering the influences of the axial force and temperature, the wavelength shift of the spectral minima can be expressed as
Figure 1 illustrates the schematic diagram of the experimental setup. It consists of a homemade supercontinuum broadband source (SBS), a liquid-filled PCF, and an optical spectrum analyzer (OSA: Yokogawa AQ6370C, operation wavelength ranges from 600 to 1700 nm). The PCF (LMA-10, provided by NKT
Fig. 1. Schematic diagram of the experimental setup. 1310
APPLIED OPTICS / Vol. 54, No. 6 / 20 February 2015
Fig. 2. Transmission spectra of the 7 mm unfilled PCF and the 7 mm PCF with 3 mm segment filled under a room temperature of 19.5°C. Inset shows the transmission output spectra together with the output spectrum of the SBS.
dλ
∂λ ∂λ dT dF; ∂T ∂F
(2)
where eff λ ∂Δn ∂λ ∂T ; eff ∂T Δneff − λ ∂Δn ∂λ
(3)
eff λ ∂Δn dλ ∂F : eff dF Δneff − λ ∂Δn ∂λ
(4)
Due to the high temperature sensitivity of the infiltrated liquid, Δneff would be highly dependent on the environmental temperature T, leading to the high temperature sensitivity of this structure. In addition, each mode in the liquid infiltrated PCF exhibits different photoelastic sensitivities caused by the axial force F. And therefore, Δneff changes with the variation of the applied axial force. As a result, the wavelength shifts accordingly [26]. For the proposed multimodal interferometer, the spectral minima would exhibit different sensitivities to external parameters, owing to different modes involved in the multimodal interference. Thus, simultaneous measurement of force and temperature can be achieved by measuring the wavelength shift of two selected spectral minima within the linear operation wavelength range, using the following sensor matrix: ΔT K T1 K F1 −1 Δλ1 ; (5) K T2 K F2 Δλ2 ΔF where ΔT and ΔF refer to the variation of the measured parameters; Δλ1 and Δλ2 are the wavelength shift of the two spectral minima; K T1 and K F2 are the coefficients of the temperature and force for minimum 1, and K T2 and K F2 represent those for minimum 2. 3. Experimental Results and Discussion
The temperature property of this liquid-filled PCFbased multimodal interferometer is studied with a constant preset axial force. The transmission spectrum of the interferometer changes gradually with the increment of environmental temperature from 19.5°C to 22.5°C, as shown in Fig. 3. It is obvious that
Fig. 3. Transmission spectra of the spectral interference under different temperatures.
Fig. 4. Spectral minimum wavelength shift as a function of temperature.
all of the spectral minima exhibit blueshift as the temperature increases. Figure 4 illustrates the minimum wavelength shift as a function of temperature. It can be seen that minimum A, B, and C shift by −38.1 nm, −71.9 nm, and −27.2 nm, respectively, and the corresponding sensitivities are −9.214 nm∕°C, −24.757 nm∕°C, and −12.543∕°C, respectively. For a temperature range of 19.5°C–22.5°C, the response shows a linear temperature-dependent behavior. When the temperature is out of this range, the spectral minima would rapidly vanish and new minimum would turn up. This phenomenon is caused by the fact that more higher-order eigenmodes are involved in the multimodal interference with the increment of the temperature within this range [22]. In order to investigate the force sensing characteristics, the external temperature is kept at 19.5°C using the temperature-controlled chamber, and the axial force is gradually increased by utilizing different weights. Figure 5 shows the transmission spectra of the interference under different applied forces. The transmission spectrum changes gradually with the increment of the axial force, as shown in Fig. 5. Three spectral minima are selected to analyze the force sensing property of the proposed interferometer. Figure 6 shows the minimum wavelength and transmission in response to the applied axial force. As shown in Fig. 6(a), with the increment of the applied axial force, the minimum wavelength shifts from
Fig. 5. Transmission spectra of the proposed interference under different axial forces, insets (a), (b), and (c) show the enlarged transmission spectra for minimum A, B, and C, respectively. 20 February 2015 / Vol. 54, No. 6 / APPLIED OPTICS
1311
of 19.5°C–22.5°C and force measurement range of 0– 0.882 N. Compared with the pretty high temperature sensitivities, the temperature measurement range is rather small. This problem can be overcome by infiltrating a liquid with lower RI, which ensures that fewer modes are involved in the multimodal interference, as analyzed in our previous work. Thus, it is possible to design a sensor for simultaneous measurement of temperature and force with high sensitivity as well as large measurement range. 4. Conclusion Fig. 6. Transmission minimum response to the applied axial force (a) wavelength response, (b) transmission response.
1218.2 to 1218.4 nm and 1458.7 to 1459 nm for minimum A and C, respectively. Their wavelength shifts are smaller than 0.5 nm, which is below the resolution of the experimentally employed OSA. Therefore, the minimum wavelengths could be considered unchanged. However, the wavelength of minimum B increases linearly from 1333.8 to 1338.4 nm with the increment of the axial force and has a sensitivity of 4.978 nm∕N. As to the transmission response, it is obvious that the minimum transmission increases with the increment of the applied axial force, as shown in Fig. 6(b). The transmission of minimum A increases from −27.877 to −23.699 dB and exhibits a nonlinear behavior which could be expressed as: y −33.4 0.49382 expx 3.61689∕1.50455. And for minimum B, the transmission increases from −44.668 to −26.176 dB and the experiment data could be fitted by: y −19.25934 − 25.3601 exp−x∕0.67969. However, the transmission of minimum C increases linearly from −41.482 to −30.701 dB with a sensitivity of 12.56 dB∕N. Different force as well as temperature sensitivities of the transmission minima make it possible to fabricate a multi-parameter sensing system. The temperature and force coefficients of the different spectral minima of the liquid-filled PCF-based multimodal interferometer are obtained by linear fitting of the measurement data, as shown in Figs. 4 and 6(a), respectively. Therefore, Eq. (5) can be written as by selecting minimum A and B or minimum B and C:
ΔT ΔF
−9.721 nm∕°C 0 −24.757 nm∕°C 4.978 nm∕N
−1
ΔλA ; ΔλB (6)
ΔT ΔF
−24.757 nm∕°C 4.978 nm∕N −12.543 nm∕°C 0
−1
ΔλB : ΔλC (7)
By using the above matrices, the variations of the temperature and the axial force can be measured simultaneously with high sensitivity. The proposed sensor possesses a temperature measurement range 1312
APPLIED OPTICS / Vol. 54, No. 6 / 20 February 2015
In this paper, a liquid-filled PCF-based multimodal interferometer has been proposed to achieve simultaneous measurement of temperature and force. A temperature sensitivity of −24.757 nm∕°C has been experimentally obtained for a temperature range of 19.5°C–22.5°C, and a force sensitivity of 4.978 nm∕N has been achieved for a force range of 0–0.882 N. And a sensor matrix is acquired to effectively resolve the cross sensitivity. The proposed liquid-filled PCFbased multimodal interferometer has such advantages as high sensitivity, high compactness, ease of fabrication, and low cost, and it would have potential applications in the measurement of multiple parameters. This work was jointly supported by the National Natural Science Foundation of China under grants 61377095, 61322510, 11274182, 11204212, and 11004110; the National Key Basic Research and Development Program of China under grant 2010CB327605; the 863 National High Technology Program of China under grant 2013AA014201; Science & Technology Support Project of Tianjin under grant 11ZCKFGX01800; Key Natural Science Foundation Project of Tianjin under grant 13JCZDJC26100; China Postdoctoral Science Foundation Funded Project under grant 2012M520024; and the Fundamental Research Funds for the Central Universities. References 1. K. T. V. Grattan and T. Sun, “Fiber optic sensor technology: an overview,” Sens. Actuators A 82, 40–61 (2000). 2. H. Zhao, F. Sun, Y. Yang, G. Cao, and K. Sun, “A novel temperature-compensated method for FBG-GMM current sensor,” Opt. Commun. 308, 64–69 (2013). 3. H. Luo, Q. Sun, Z. Xu, D. Liu, and L. Zhang, “Simultaneous measurement of refractive index and temperature using multimode microfiber-based dual Mach–Zehnder interferometer,” Opt. Lett. 39, 4049–4052 (2014). 4. H. J. Patrick, G. M. Williams, A. D. Kersey, J. R. Pedrazzani, and A. M. Vengsarkar, “Hybrid fiber Bragg grating/long period fiber grating sensor for strain/temperature discrimination,” IEEE Photon. Technol. Lett. 8, 1223–1225 (1996). 5. D. Zhou, L. Wei, W. K. Liu, and J. W. Y. Lit, “Simultaneous measurement of strain and temperature based on a fiber Bragg grating combined with a high-birefringence fiber loop mirror,” Opt. Commun. 281, 4640–4643 (2008). 6. D. J. J. Hu, J. L. Lim, M. Jiang, Y. Wang, F. Luan, P. P. Shum, H. Wei, and W. Tong, “Long period grating cascaded to photonic crystal fiber modal interferometer for simultaneous measurement of temperature and refractive index,” Opt. Lett. 37, 2283–3385 (2012).
7. J. Li, W. Zhang, S. Gao, P. Geng, X. Xue, Z. Bai, and H. Liang, “Long-period fiber grating cascaded to an S fiber taper for simultaneous measurement of temperature and refractive index,” IEEE Photon. Technol. Lett. 25, 888–891 (2013). 8. Q. Zhang, T. Zhu, J. Zhang, and K. S. Chiang, “Microfiber-based FBG sensor for simultaneous measurement of vibration and temperature,” IEEE Photon. Technol. Lett. 25, 1751–1753 (2013). 9. Q. Liu, Z. L. Ran, Y. J. Rao, S. C. Luo, H. Q. Yang, and Y. Huang, “Highly integrated FP/FBG sensor for simultaneous measurement of high temperature and strain,” IEEE Photon. Technol. Lett. 26, 1715–1717 (2014). 10. H. Liang, W. Zhang, P. Geng, Y. Liu, Z. Wang, J. Guo, S. Gao, and S. Yan, “Simultaneous measurement of temperature and force with high sensitivities based on filling different index liquids into photonic crystal fiber,” Opt. Lett. 38, 1071–1073 (2013). 11. J. Zhou, C. Liao, Y. Wang, G. Yin, X. Zhong, K. Yang, B. Sun, G. Wang, and Z. Li, “Simultaneous measurement of strain and temperature by employing fiber Mach–Zehnder interferometer,” Opt. Express 22, 1680–1686 (2014). 12. 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. 23, 130–132 (2011). 13. W. Zhou, Y. Zhou, X. Dong, L. Y. Shao, J. Cheng, and J. Albert, “Fiber-optic curvature sensor based on cladding-mode Bragg grating excited by fiber multimode interferometer,” IEEE Photon. J. 4, 1051–1057 (2012). 14. C. Gouveia, P. A. S. Jorge, J. M. Baptista, and O. Frazão, “Temperature-independent curvature sensor using FBG cladding modes based on a core misaligned splice,” IEEE Photon. Technol. Lett. 23, 804–806 (2011). 15. S. Yang, H. Sun, L. Yuan, X. Zhang, L. Zhou, and M. Hu, “Refractive index and temperature sensor based on claddingmode Bragg grating excited by abrupt taper interferometer,” Chin. Opt. Lett. 11, 120604 (2013). 16. T. Qi, S. Xiao, J. Shi, L. Yi, Z. Zhou, M. Bi, and W. Hu, “Cladding mode backward recoupling based displacement sensor
17. 18. 19.
20. 21.
22. 23.
24.
25.
26.
incorporating fiber up taper and Bragg grating,” IEEE Photon. J. 5, 7100608 (2013). B. T. Kuhlmey, B. J. Eggleton, and D. K. C. Wu, “Fluid-filled solid-core photonic bandgap fibers,” J. Lightwave Technol. 27, 1617–1630 (2009). C. Markos, I. Kubat, and O. Bang, “Hybrid polymer photonic crystal fiber with integrated chalcogenide glass nanofilms,” Sci. Rep. 4, 6057 (2014). W. Qian, C. Zhao, S. He, X. Dong, S. Zhang, Z. Zhang, S. Jin, J. Guo, and H. Wei, “High-sensitivity temperature sensor based on an alcohol-filled photonic crystal fiber loop mirror,” Opt. Lett. 36, 1548–1550 (2011). D. K. C. Wu, B. T. Kuhlmey, and B. J. Eggleton, “Ultrasensitive photonic crystal fiber refractive index sensor,” Opt. Lett. 34, 322–324 (2009). Y. Geng, X. Li, X. Tan, Y. Deng, and X. Hong, “Compact and ultrasensitive temperature sensor with a fully liquid-filled photonic crystal fiber Mach–Zehnder interferometer,” IEEE Sens. J. 14, 167–170 (2014). W. Lin, Y. Miao, B. Song, H. Zhang, B. Liu, Y. Liu, and D. Yan, “Multimodal transmission property in a liquid-filled photonic crystal fiber,” Opt. Commun. 336, 14–19 (2015). S. Silva, O. Frazão, J. Viegas, L. A. Ferreira, F. M. Araújo, F. X. Malcata, and J. L. Santos, “Temperature and strainindependent curvature sensor based on a single-mode/ multimode fiber optic structure,” Meas. Sci. Technol. 22, 085201 (2011). H. Liang, W. Zhang, H. Wang, P. Geng, S. Zhang, S. Gao, C. Yang, and J. Li, “Fiber in-line Mach–Zehnder interferometer based on near-elliptical core photonic crystal fiber for temperature and strain sensing,” Opt. Lett. 38, 4019–4022 (2013). Y. Peng, J. Hou, Y. Zhang, Z. Huang, R. Xiao, and Q. Lu, “Temperature sensing using the bandgap-like effect in a selectively liquid-filled photonic crystal fiber,” Opt. Lett. 38, 263–265 (2013). S. M. Tripathi, A. Kumar, R. K. Varshney, Y. B. P. Kumar, E. Marin, and J. P. Meunier, “Strain and temperature sensing characteristics of single-mode–multimode–single-mode structures,” J. Lightwave Technol. 27, 2348–2356 (2009).
20 February 2015 / Vol. 54, No. 6 / APPLIED OPTICS
1313
