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Simultaneous measurement of refractive index, strain and temperature using a tapered structure based on SMF
Optics Communications 410 (2018) 70–74
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Simultaneous measurement of refractive index, strain and temperature using a tapered structure based on SMF Na Zhang a,b , Wei Xu c , Shanhong You a, *, Cheungchuen Yu c , Changyuan Yu d , Bo Dong e,f , Kunpu Li b,g a
School of Electronic & Information Engineering, Soochow University, Suzhou 215006, China National University of Singapore (Suzhou) Research Institute, Suzhou 215123, China c Anlight Optoelectronic Technology Inc., Suzhou 215123, China d Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong e Institute for Infocomm Research (I2R), Singapore 138632, Singapore f Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore g School of Electronic Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China b
a r t i c l e
i n f o
Keywords: Mach–Zehnder interferometer (MZI) Mode interference Fiber sensor Single mode fiber (SMF)
a b s t r a c t A novel fiber-optic sensing structure based on miniaturized modal interferometer (MMI) for simultaneous refractive index (RI), strain and temperature measurement is proposed. It is mainly based on Mach–Zehnder interferometer (MZI) and formed by introducing a down taper between two adjacent up tapers in one single mode fiber (SMF). Experimental results demonstrate a RI sensitivity of 131.93 nm/RIU, a strain sensitivity of 0.0007 nm/μ𝜀 and a temperature sensitivity of 0.0878 nm/◦ C respectively. The sensor is merely made of SMF which is cheap and available, and the whole fabrication process contains only cleaving and splicing and can be well controlled by a commercial fiber splicer. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, fiber optic sensors are widely used to monitor biological and chemical parameters, such as refractive index (RI) [1–3], strain [4– 6], temperature [7–9], humidity [10], liquid concentration [11], etc. Among the parameters mentioned above, RI, strain and temperature have important utility value and have attracted much research interests. So far, some fiber-based schemes have been proposed for simultaneous RI, strain and temperature measurement. For instance, Alberto et al. [12] proposed a tilted fiber Bragg grating (TFBG) which can realize three-parameter measurements, while the fabrication of tilted TFBG is complex. Lee et al. [13] presented an etched-core FBG sensor to achieve simultaneous RI, strain and temperature measurement. However the etching process is needed and cannot be well controlled. Mau et al. [14] designed a fiber sensor based on a polarization maintaining FBG (PMFBG) and a concatenated long period fiber grating (LPG) fabricated in the photosensitive fibers. Although the sensor could successfully monitor the stress, temperature and RI simultaneously, the sensing structure is relatively complex and the cost is a little bit high. Tong et al. [15] gave out another approach for three parameters monitoring mentioned
above, where the sensor was constructed by a core-mismatch-based multimode-single mode-multimode (MSM) structure and a PMFBG. In very recently, Oliveira et al. [16] proposed a structure realized by cascading two FBGs and a no-core fiber. It should be noted that the two FBGs were inscribed in two different parts: one in a tapered region and the other in an un-tapered region. Nevertheless, the above mentioned three-parameter sensing techniques are almost based on FBG/LPG or with special fibers-assisted, which leads to a complex fabrication process and relatively high cost. To overcome these drawbacks, totally SMF-based, without FBG/LPG and special fibers involved, RI, strain and temperature fiber-optic MMI sensors are another welcome and promising approach and have been extensively studied owing to the merits of easy fabrication and costeffectiveness. Many efforts have been done to try to realize simultaneous RI, strain and temperature monitoring with only SMF based MMIs, such as using a pair of up tapers only formed by SMF for simultaneous measurement of strain and temperature [17], utilizing a double-pass inline MZI to simultaneously measure RI and temperature [18], employing two different types of MZI for simultaneous detection of temperature
* Corresponding author.
E-mail address: [email protected] (S. You). https://doi.org/10.1016/j.optcom.2017.09.096 Received 13 July 2017; Received in revised form 24 September 2017; Accepted 25 September 2017 0030-4018/© 2017 Elsevier B.V. All rights reserved.
N. Zhang et al.
Optics Communications 410 (2018) 70–74
Fig. 1. (a) The schematic of the UDU structure, (b) microscope image of the fusion up taper, (c) microscope image of the fusion down taper.
and strain [19], etc. However all the techniques mentioned above just achieved dual-parameter measurement and could not get rid of the remainder parameter’s interference. In this paper, to our best knowledge, it is for the first time that a novel SMF-based up taper-down taper-up taper (UDU) structure is presented and realize simultaneous RI, strain and temperature measurement with a RI sensitivity of 131.93 nm/RIU, a strain sensitivity of 0.7 pm∕μ𝜀 and a temperature sensitivity of 87.8 pm∕◦ C respectively. The main sensing mechanism is based on Mach–Zehnder modal interferometer and scattered evanescent waves. All of the fabrication process only contains cleaving and splicing by using a commercial fiber splicer (Fujikura FSM-80s) and fiber cleaver (CT-30). It has to be noted that it is only the build-in taper fusion modes are used in the fabrication process without manual fusion mode involved which may bring somewhat uncertainty. Fig. 2. The transmission spectrum of the UDU structure.
2. Principle To achieve three-parameter monitoring, a UDU structure is designed and is simply realized by introducing a down taper just locating in the middle of two adjacent up tapers. The whole fabrication process contains only splicing and cleaving and can be simply realized by Fujikura FSM80S and CT-30. The relevant schematic is shown in Fig. 1. The distance between the two up-tapers is ∼5 cm. The length of the down-tapered region is ∼250μm, and the two up-tapers almost have the same diameters ∼ 160μm. All the tapers, including up-taper and down-taper, are simply realized by the built-in taper mode through setting the parameter of ‘‘overlap’’ and the ‘‘taper length’’. It is only SMF that we utilize to set up the UDU structure. As is shown in Fig. 1, when the injected light meets at the first up taper, the high-order cladding modes are efficiently excited due to the mismatch of the mode field diameter (MFD) and the injected light is split into two parts, one part transmitting in the core and the other in the cladding. When these lights meet at the second up taper, they will recouple and interference with each other and then transmit back into the core of the lead out SMF. The introduced down taper is used to excite more evanescent waves to enhance the sensing performance of the surrounding environments. Fig. 2 shows the transmission spectrum of the UDU structure. As can be seen, several maximums and minimums which can be monitored to sense the aforementioned parameters. The Fast Fourier transform (FFT) is used to gain the spatial frequency spectrum from the transmission spectrum which is shown in Fig. 3, where we can also see that besides the core mode, more than one high-order cladding modes are effectively stimulated and can be used in the RI, strain and temperature monitoring. Assuming that only the core mode and one of the high-order cladding modes are taken into consideration, then it can be simply expressed as:
Fig. 3. The spatial frequency spectrum of the UDU structure.
2 2 2 𝐸𝑜𝑢𝑡 = 𝐸𝑐𝑜𝑟𝑒 + 𝐸𝑐𝑙𝑎𝑑𝑑𝑖𝑛𝑔 + 2𝐸𝑐𝑜𝑟𝑒 𝐸𝑐𝑙𝑎𝑑𝑑𝑖𝑛𝑔 cos (𝜑)
(1)
where 𝐸𝑜𝑢𝑡 represents the amplitude of the transmitted interference signal, 𝐸𝑐𝑜𝑟𝑒 and 𝐸𝑐𝑙𝑎𝑑𝑑𝑖𝑛𝑔 represent the amplitudes of the core and cladding modes respectively, and 𝜑 represents the phase difference between the core mode and the cladding mode, which can be further described as 𝜑 = 2𝜋𝛿𝑛𝑒𝑓 𝑓 𝐿∕𝜆 71
(2)
N. Zhang et al.
Optics Communications 410 (2018) 70–74
Fig. 4. The experimental setup.
Considering that the 𝜑 equals to (2𝑘 + 1) 𝜋, i.e., the output intensity drops to a trough at a certain 𝜆𝑑𝑖𝑝 , the Eq. (2) can be rewritten as follows: 2𝜋𝛿𝑛𝑒𝑓 𝑓 𝐿∕𝜆𝑑𝑖𝑝 = (2𝑘 + 1) 𝜋.
(3)
Calculated from Eq. (3), one could obtain that Fig. 5. The transmission spectra of the UDU structure under different RIs.
𝜆𝑑𝑖𝑝 = 2𝛿𝑛𝑒𝑓 𝑓 𝐿∕ (2𝑘 + 1) .
(4)
As for temperature measurement, because of thermal-optic effects and the thermal expansion, the effective RI changing and fiber length are all functions of temperature. The corresponding wavelength shift caused by temperature can be given by: ( ) 1 𝜕𝛿𝑛𝑒𝑓 𝑓 1 𝜕𝐿 𝛥𝜆 = + 𝛥𝑇 (5) 𝜆 𝛿𝑛𝑒𝑓 𝑓 𝜕𝑇 𝐿 𝜕𝑇 1 𝜕𝛿𝑛𝑒𝑓 𝑓 is the thermo-optical coefficient, which can 𝛿𝑛𝑒𝑓 𝑓 𝜕𝑇 𝜕𝐿 noted as 𝜉, and 𝐿1 𝜕𝑇 is the thermal expansion coefficient, which written as 𝛼𝑓 . Then the Eq. (5) can be rewritten as:
where
) 𝛥𝜆 ( = 𝜉 + 𝛼𝑓 𝛥𝑇 𝜆
be decan be
(6)
As for the measurement of strain, the effective RI changing and fiber length are all functions of strain owing to the elasto-optical effect. And the relative shift of wavelength can be expressed as: ( ) 1 𝜕𝛿𝑛𝑒𝑓 𝑓 𝜕𝜆 = +1 𝜀 (7) 𝜆 𝛿𝑛𝑒𝑓 𝑓 𝜀
Fig. 6. The relationship between the wavelength and the RI at different dips.
3.1. RI experiment
where 𝜀 is the micro-strain of the fiber, which can be presented as 𝜕𝛿𝑛𝑒𝑓 𝑓 is the elasto-optical coefficient, and can be 𝜀 = 𝛥𝐿∕𝐿. 𝛿𝑛1 𝜀 𝑒𝑓 𝑓 expressed as 𝑃𝜀 . So the Eq. (7) can be overridden as: ) 𝛥𝜆 ( = 𝑃𝜀 + 1 𝜀 𝜆
We prepare a range of NaCl solutions in order to generate different RIs environments to do RI-dependent experiments. The RI range is controlled from 1.3211 to 1.3527 by preparing solutions with different concentrations range from 3% to 21% [20]. A pair of clamps is used to hold the UDU structure to make sure that the sensor is straight hung when it is completely immersed into the NaCl solutions. The transmission spectra of the UDU structure under different RI are shown in Fig. 5. Dip A, dip B and dip C are chosen as the observation points. With the increasing of the RI, the transmission spectra have obvious blue shifts at dip A and dip B, but almost remain stable at dip C. The detailed relationship between the wavelength and the RI at different dips can be seen from Fig. 6. The RI sensitivities are −131.93 nm/RIU at dip A, −22.875 nm/RIU at dip B and 0 nm/RIU at dip C respectively. The UDU structure is based on two up-tapers and one down-taper, at least three higher order cladding modes are excited. Hence, the corresponding interference contains at least 3 phase difference, and the interference between lower cladding mode and the fundamental mode is not so sensitive to RI. And it leads to the difference of RI sensitivities between the three dips.
(8)
In regard to the measurement of surrounding RIs, only the change of effective RI is its function. The relevant shift of wavelength is: 𝛥𝛿𝑛𝑒𝑓 𝑓 𝛥𝜆 = 𝜆 𝛿𝑛𝑒𝑓 𝑓
(9)
As the proposed sensor is a three-parameter measurement structure, based on the Eqs. (6), (8) and (9), the total shift of wavelength can be given by: 𝛥𝛿𝑛𝑒𝑓 𝑓 ) ( ) 𝛥𝜆 ( = 𝜉 + 𝛼𝑓 𝛥𝑇 + 𝑃𝜀 + 1 𝜀 + 𝜆 𝛿𝑛𝑒𝑓 𝑓
(10)
3. Experimental results and discussion The experimental setup is shown in Fig. 4. A C-band broadband source (BBS) is used as the light source and an optical spectra analyzer (OSA) is utilized to monitor the transmission spectra. With the UDU structure, we conduct a series of experiments which include RI, strain and temperature experiments. The strain and temperature experiments are carried out on an optical table, while the temperature-dependent experiments are carried out in an incubator with temperature control system.
3.2. Strain experiment A micro displacement platform is used to provide different strain in the strain experiments. The strains range from 0 to 423.529 μ𝜀 with steps of 70.588 μ𝜀. The transmission spectra of the UDU structure under different strains are shown in Fig. 7. We choose dip A, dip B and dip C as the observation points. The transmission spectra show blue shifts 72
N. Zhang et al.
Optics Communications 410 (2018) 70–74
Fig. 7. The transmission spectra of the UDU structure under different strains.
Fig. 9. The transmission spectra of the UDU structure under different temperature.
Fig. 10. The relationship between the wavelength and the temperature at different dips. Fig. 8. The relationship between the wavelength and the strain at different dips.
dip C can be expressed as with the increase of the strains at dip B and C, while there is no shift at dip A. The relationship between the wavelength and the strains at different dips are shown in Fig. 8. Experimental results achieve strain sensitivities of 0 pm/μ𝜀 at dip A, −0.6 pm/μ𝜀 at dip B and −0.7 pm/μ𝜀 at dip C respectively. This UDU structure is mainly based on up-tapers, which is not so sensitive to strain, so this structure has a relatively low strain sensitivity.
0 0.0662⎤ ⎡ 𝛥𝑛 ⎤ ⎡𝛥𝜆𝐴 ⎤ ⎡−131.93 ⎢𝛥𝜆 ⎥ = ⎢−22.875 −0.0006 0.0906⎥ ⎢ 𝛥𝜀 ⎥ 𝐵 ⎥⎢ ⎥ ⎢ ⎥ ⎢ −0.0007 0.0878⎦ ⎣𝛥𝑇 ⎦ ⎣𝛥𝜆𝐶 ⎦ ⎣ 0 In the above measurement matrix, the unit of the wavelength is nanometer, the unit of the strain is micro strain, and the unit of the temperature is degree centigrade. As can be seen, a simultaneous RI, strain and temperature measurement is achieved. A system sensitivity table is constructed to compare with the proposed sensors [13,14,16] which can realize the simultaneous measurement of RI, strain and temperature, as is shown in Table 1. The values shown in Table 1 tell that the UDU structure has a higher RI sensitivity and a relative high temperature sensitivity compared with the sensors found in literature. Also, the fabrication process is much easier, the cost is much lower and the experimental set up is much simpler.
3.3. Temperature experiment The temperature experiments are carried out by employing a thermostat, where the temperature is well controlled ranging from 23.0 ◦ C to 54.0 ◦ C. Fig. 9 shows the transmission spectra of the UDU structure under different temperatures. We also choose dip A, dip B and dip C as the observation points. The transmission spectra have a red shift with the increasing of the temperature, as is shown in Fig. 9. In addition, Fig. 10 shows the relationship between the wavelength and the temperature at different dips. Temperature sensitivities of 0.0662 nm/◦ C at dip A, 0.0906 nm/◦ C at dip B and 0.0878 nm/◦ C at dip C are achieved. As mentioned above, at least three higher order modes are excited, and all the interference between the modes are sensitive to temperature. Owing to that only SMF is utilized in the structure, so the temperature sensitivities at different dips almost have the same amount.
4. Conclusion A novel UDU structure for simultaneous measurement of RI, strain and temperature is proposed and investigated theoretically and experimentally. It is based on MZI and is realized by introducing a down taper into a fiber between two adjacent up tapers. It is only formed by SMF with low cost and easy fabrication without manual fusion mode using. Acknowledgments
3.4. Simultaneous RI, strain and temperature measurement This work is supported by National Natural Science Foundation of China (Grant No. 61501313 and 61471253), The Hong Kong Polytechnic University (Grant No. 1-ZVHA) and Open Foundation of State
Based on the above experimental results, the RI 𝛥𝑛, strain 𝛥𝜀 and temperature 𝛥𝑇 measurement matrix in nm unit for dip A, dip B and 73
N. Zhang et al.
Optics Communications 410 (2018) 70–74 Table 1 Sensitivity comparison.
𝑁 (nm/RIU) 𝜀 (nm/με) 𝑇 (nm/◦ C)
Proposed UDU structure
Sensor1 [13]
Sensor2 [14]
Sensor3 [16]
131.93 0.0007 0.0906
92.037 0.0009 0.032
42.975 0.01992 0.1445
116.5 0.00577 0.03482
Key Laboratory of Optical Communication Technologies and Networks (Grant No. 2015OCTN-02).
[10] W. Xu, W. Huang, X. Huang, C. Yu, A simple fiber-optic humidity sensor based on extrinsic Fabry–Perot cavity constructed by cellulose acetate butyrate film, Opt. Fiber Tech. 19 (2013) 583–586. [11] Y. Zhao, Z. Deng, Q. Wang, Fiber optic SPR sensor for liquid concentration measurement, Sensors Actuators B 192 (2014) 229–233. [12] N.J. Alberto, C.A. Marques, J.L. Pinto, R.N. Nogueira, Three-parameter optical fiber sensor based on a tilted fiber Bragg grating, Appl. Opt. 49 (2010) 6085–6091. [13] S. Lee, S.S. Saini, M.Y. Jeong, Simultaneous measurement of refractive index, temperature, and strain using etched-core fiber bragg grating sensors, IEEE Photonics Technol. Lett. 22 (2010) 1431–1433. [14] J.C. Mau, P.P. Wu, M.Y. Fu, W.F. Liu, Simultaneous measurement of stress, temperature and refractive index using an PMFBG cascaded with an LPG, Session 4A1 Fiber Optics, Optical Sensors, Mar. 759. [15] Z. Tong, J. Wang, W. Zhang, Y. Cao, Simultaneous measurement of refractive index, temperature and strain based on core diameter mismatch and polarizationmaintaining FBG, Optoelectron. Lett. 9 (2013) 238–240. [16] R. Oliveira, J.H. Osório, S. Aristilde, L. Bilro, R.N. Nogueira, C.M. Cordeiro, Simultaneous measurement of strain, temperature and refractive index based on multimode interference, fiber tapering and fiber bragg gratings, Meas. Sci. Technol. 27 (2016) 075107. [17] B. Dong, Y. Peng, Y. Wang, C. Yu, Mode division multiplexing in a fiber modal interferometer for dual-parameters measurement, IEEE Photon. Technol. Lett. 28 (2016) 143–146. [18] Y. Lu, C. Shen, C. Zhong, D. Chen, X. Dong, J. Cai, Refractive index and temperature sensor based on double-pass M–Z interferometer with an FBG, IEEE Photon. Technol. Lett. 26 (2014) 1124–1127. [19] M. Najari, A.M. Javan, N. Amiri, Hybrid all-fiber sensor for simultaneous strain and temperature measurements based on Mach–Zehnder interferometer, Optik-Int. J. Light Electron Opt. 126 (2015) 2022–2025. [20] J.E. Saunders, C. Sanders, H. Chen, H.P. Loock, Refractive indices of common solvents and solutions at 1550 nm, Appl. Opt. 55 (2016) 947–953.
References [1] Z. Tian, S. Yam, H. Loock, Refractive index sensor based on an abrupt taper Michelson interferometer in a single-mode fiber, Opt. Lett. 33 (2008) 1105–1107. [2] Q. Wang, L. Kong, Y. Dang, F. Xia, Y. Zhang, Y. Zhao, J. Li, High sensitivity refractive index sensor based on splicing points tapered SMF-PCF-SMF structure Mach–Zehnder mode interferometer, Sensors Actuators B 225 (2016) 213–220. [3] X. Sun, D. Dai, L. Thylén, L. Wosinski, High-sensitivity liquid refractive-index sensor based on a Mach–Zehnder interferometer with a double-slot hybrid plasmonic waveguide, Opt. Express 23 (2015) 25688–25699. [4] D. Wu, T. Zhu, K. Chiang, M. Deng, All single-mode fiber Mach–Zehnder interferometer based on two peanut-shape structures, J. Lightwave Technol. 30 (2012) 805–810. [5] Y. Weng, E. Ip, Z. Pan, T. Wang, Single-end simultaneous temperature and strain sensing techniques based on brillouin optical time domain reflectometry in fewmode fibers, Opt. Express 23 (2015) 9024–9039. [6] A. Li, Y. Wang, J. Fang, M. Li, B.Y. Kim, W. Shieh, Few-mode fiber multi-parameter sensor with distributed temperature and strain discrimination, Opt. Lett. 40 (2015) 488–1491. [7] S. Silva, E.G. Pachon, M.A. Franco, J.G. Hayashi, F.X. Malcata, O. Frazão, C.M. Cordeiro, Ultrahigh-sensitivity temperature fiber sensor based on multimode interference, Appl. Opt. 51 (2012) 3236–3242. [8] G. Woyessa, A. Fasano, A. Stefani, C. Markos, K. Nielsen, H.K. Rasmussen, O. Bang, Single mode step-index polymer optical fiber for humidity insensitive high temperature fiber Bragg grating sensors, Opt. Express 24 (2016) 1253–1260. [9] Y. Bao, G. Chen, High-temperature measurement with brillouin optical time domain analysis of an annealed fused-silica single-mode fiber, Opt. Lett. 41 (2016) 3177– 3180.
74
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Simultaneous measurement of refractive index, strain and temperature using a tapered structure based on SMF Na Zhang a,b , Wei Xu c , Shanhong You a, *, Cheungchuen Yu c , Changyuan Yu d , Bo Dong e,f , Kunpu Li b,g a
School of Electronic & Information Engineering, Soochow University, Suzhou 215006, China National University of Singapore (Suzhou) Research Institute, Suzhou 215123, China c Anlight Optoelectronic Technology Inc., Suzhou 215123, China d Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong e Institute for Infocomm Research (I2R), Singapore 138632, Singapore f Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore g School of Electronic Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China b
a r t i c l e
i n f o
Keywords: Mach–Zehnder interferometer (MZI) Mode interference Fiber sensor Single mode fiber (SMF)
a b s t r a c t A novel fiber-optic sensing structure based on miniaturized modal interferometer (MMI) for simultaneous refractive index (RI), strain and temperature measurement is proposed. It is mainly based on Mach–Zehnder interferometer (MZI) and formed by introducing a down taper between two adjacent up tapers in one single mode fiber (SMF). Experimental results demonstrate a RI sensitivity of 131.93 nm/RIU, a strain sensitivity of 0.0007 nm/μ𝜀 and a temperature sensitivity of 0.0878 nm/◦ C respectively. The sensor is merely made of SMF which is cheap and available, and the whole fabrication process contains only cleaving and splicing and can be well controlled by a commercial fiber splicer. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, fiber optic sensors are widely used to monitor biological and chemical parameters, such as refractive index (RI) [1–3], strain [4– 6], temperature [7–9], humidity [10], liquid concentration [11], etc. Among the parameters mentioned above, RI, strain and temperature have important utility value and have attracted much research interests. So far, some fiber-based schemes have been proposed for simultaneous RI, strain and temperature measurement. For instance, Alberto et al. [12] proposed a tilted fiber Bragg grating (TFBG) which can realize three-parameter measurements, while the fabrication of tilted TFBG is complex. Lee et al. [13] presented an etched-core FBG sensor to achieve simultaneous RI, strain and temperature measurement. However the etching process is needed and cannot be well controlled. Mau et al. [14] designed a fiber sensor based on a polarization maintaining FBG (PMFBG) and a concatenated long period fiber grating (LPG) fabricated in the photosensitive fibers. Although the sensor could successfully monitor the stress, temperature and RI simultaneously, the sensing structure is relatively complex and the cost is a little bit high. Tong et al. [15] gave out another approach for three parameters monitoring mentioned
above, where the sensor was constructed by a core-mismatch-based multimode-single mode-multimode (MSM) structure and a PMFBG. In very recently, Oliveira et al. [16] proposed a structure realized by cascading two FBGs and a no-core fiber. It should be noted that the two FBGs were inscribed in two different parts: one in a tapered region and the other in an un-tapered region. Nevertheless, the above mentioned three-parameter sensing techniques are almost based on FBG/LPG or with special fibers-assisted, which leads to a complex fabrication process and relatively high cost. To overcome these drawbacks, totally SMF-based, without FBG/LPG and special fibers involved, RI, strain and temperature fiber-optic MMI sensors are another welcome and promising approach and have been extensively studied owing to the merits of easy fabrication and costeffectiveness. Many efforts have been done to try to realize simultaneous RI, strain and temperature monitoring with only SMF based MMIs, such as using a pair of up tapers only formed by SMF for simultaneous measurement of strain and temperature [17], utilizing a double-pass inline MZI to simultaneously measure RI and temperature [18], employing two different types of MZI for simultaneous detection of temperature
* Corresponding author.
E-mail address: [email protected] (S. You). https://doi.org/10.1016/j.optcom.2017.09.096 Received 13 July 2017; Received in revised form 24 September 2017; Accepted 25 September 2017 0030-4018/© 2017 Elsevier B.V. All rights reserved.
N. Zhang et al.
Optics Communications 410 (2018) 70–74
Fig. 1. (a) The schematic of the UDU structure, (b) microscope image of the fusion up taper, (c) microscope image of the fusion down taper.
and strain [19], etc. However all the techniques mentioned above just achieved dual-parameter measurement and could not get rid of the remainder parameter’s interference. In this paper, to our best knowledge, it is for the first time that a novel SMF-based up taper-down taper-up taper (UDU) structure is presented and realize simultaneous RI, strain and temperature measurement with a RI sensitivity of 131.93 nm/RIU, a strain sensitivity of 0.7 pm∕μ𝜀 and a temperature sensitivity of 87.8 pm∕◦ C respectively. The main sensing mechanism is based on Mach–Zehnder modal interferometer and scattered evanescent waves. All of the fabrication process only contains cleaving and splicing by using a commercial fiber splicer (Fujikura FSM-80s) and fiber cleaver (CT-30). It has to be noted that it is only the build-in taper fusion modes are used in the fabrication process without manual fusion mode involved which may bring somewhat uncertainty. Fig. 2. The transmission spectrum of the UDU structure.
2. Principle To achieve three-parameter monitoring, a UDU structure is designed and is simply realized by introducing a down taper just locating in the middle of two adjacent up tapers. The whole fabrication process contains only splicing and cleaving and can be simply realized by Fujikura FSM80S and CT-30. The relevant schematic is shown in Fig. 1. The distance between the two up-tapers is ∼5 cm. The length of the down-tapered region is ∼250μm, and the two up-tapers almost have the same diameters ∼ 160μm. All the tapers, including up-taper and down-taper, are simply realized by the built-in taper mode through setting the parameter of ‘‘overlap’’ and the ‘‘taper length’’. It is only SMF that we utilize to set up the UDU structure. As is shown in Fig. 1, when the injected light meets at the first up taper, the high-order cladding modes are efficiently excited due to the mismatch of the mode field diameter (MFD) and the injected light is split into two parts, one part transmitting in the core and the other in the cladding. When these lights meet at the second up taper, they will recouple and interference with each other and then transmit back into the core of the lead out SMF. The introduced down taper is used to excite more evanescent waves to enhance the sensing performance of the surrounding environments. Fig. 2 shows the transmission spectrum of the UDU structure. As can be seen, several maximums and minimums which can be monitored to sense the aforementioned parameters. The Fast Fourier transform (FFT) is used to gain the spatial frequency spectrum from the transmission spectrum which is shown in Fig. 3, where we can also see that besides the core mode, more than one high-order cladding modes are effectively stimulated and can be used in the RI, strain and temperature monitoring. Assuming that only the core mode and one of the high-order cladding modes are taken into consideration, then it can be simply expressed as:
Fig. 3. The spatial frequency spectrum of the UDU structure.
2 2 2 𝐸𝑜𝑢𝑡 = 𝐸𝑐𝑜𝑟𝑒 + 𝐸𝑐𝑙𝑎𝑑𝑑𝑖𝑛𝑔 + 2𝐸𝑐𝑜𝑟𝑒 𝐸𝑐𝑙𝑎𝑑𝑑𝑖𝑛𝑔 cos (𝜑)
(1)
where 𝐸𝑜𝑢𝑡 represents the amplitude of the transmitted interference signal, 𝐸𝑐𝑜𝑟𝑒 and 𝐸𝑐𝑙𝑎𝑑𝑑𝑖𝑛𝑔 represent the amplitudes of the core and cladding modes respectively, and 𝜑 represents the phase difference between the core mode and the cladding mode, which can be further described as 𝜑 = 2𝜋𝛿𝑛𝑒𝑓 𝑓 𝐿∕𝜆 71
(2)
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Optics Communications 410 (2018) 70–74
Fig. 4. The experimental setup.
Considering that the 𝜑 equals to (2𝑘 + 1) 𝜋, i.e., the output intensity drops to a trough at a certain 𝜆𝑑𝑖𝑝 , the Eq. (2) can be rewritten as follows: 2𝜋𝛿𝑛𝑒𝑓 𝑓 𝐿∕𝜆𝑑𝑖𝑝 = (2𝑘 + 1) 𝜋.
(3)
Calculated from Eq. (3), one could obtain that Fig. 5. The transmission spectra of the UDU structure under different RIs.
𝜆𝑑𝑖𝑝 = 2𝛿𝑛𝑒𝑓 𝑓 𝐿∕ (2𝑘 + 1) .
(4)
As for temperature measurement, because of thermal-optic effects and the thermal expansion, the effective RI changing and fiber length are all functions of temperature. The corresponding wavelength shift caused by temperature can be given by: ( ) 1 𝜕𝛿𝑛𝑒𝑓 𝑓 1 𝜕𝐿 𝛥𝜆 = + 𝛥𝑇 (5) 𝜆 𝛿𝑛𝑒𝑓 𝑓 𝜕𝑇 𝐿 𝜕𝑇 1 𝜕𝛿𝑛𝑒𝑓 𝑓 is the thermo-optical coefficient, which can 𝛿𝑛𝑒𝑓 𝑓 𝜕𝑇 𝜕𝐿 noted as 𝜉, and 𝐿1 𝜕𝑇 is the thermal expansion coefficient, which written as 𝛼𝑓 . Then the Eq. (5) can be rewritten as:
where
) 𝛥𝜆 ( = 𝜉 + 𝛼𝑓 𝛥𝑇 𝜆
be decan be
(6)
As for the measurement of strain, the effective RI changing and fiber length are all functions of strain owing to the elasto-optical effect. And the relative shift of wavelength can be expressed as: ( ) 1 𝜕𝛿𝑛𝑒𝑓 𝑓 𝜕𝜆 = +1 𝜀 (7) 𝜆 𝛿𝑛𝑒𝑓 𝑓 𝜀
Fig. 6. The relationship between the wavelength and the RI at different dips.
3.1. RI experiment
where 𝜀 is the micro-strain of the fiber, which can be presented as 𝜕𝛿𝑛𝑒𝑓 𝑓 is the elasto-optical coefficient, and can be 𝜀 = 𝛥𝐿∕𝐿. 𝛿𝑛1 𝜀 𝑒𝑓 𝑓 expressed as 𝑃𝜀 . So the Eq. (7) can be overridden as: ) 𝛥𝜆 ( = 𝑃𝜀 + 1 𝜀 𝜆
We prepare a range of NaCl solutions in order to generate different RIs environments to do RI-dependent experiments. The RI range is controlled from 1.3211 to 1.3527 by preparing solutions with different concentrations range from 3% to 21% [20]. A pair of clamps is used to hold the UDU structure to make sure that the sensor is straight hung when it is completely immersed into the NaCl solutions. The transmission spectra of the UDU structure under different RI are shown in Fig. 5. Dip A, dip B and dip C are chosen as the observation points. With the increasing of the RI, the transmission spectra have obvious blue shifts at dip A and dip B, but almost remain stable at dip C. The detailed relationship between the wavelength and the RI at different dips can be seen from Fig. 6. The RI sensitivities are −131.93 nm/RIU at dip A, −22.875 nm/RIU at dip B and 0 nm/RIU at dip C respectively. The UDU structure is based on two up-tapers and one down-taper, at least three higher order cladding modes are excited. Hence, the corresponding interference contains at least 3 phase difference, and the interference between lower cladding mode and the fundamental mode is not so sensitive to RI. And it leads to the difference of RI sensitivities between the three dips.
(8)
In regard to the measurement of surrounding RIs, only the change of effective RI is its function. The relevant shift of wavelength is: 𝛥𝛿𝑛𝑒𝑓 𝑓 𝛥𝜆 = 𝜆 𝛿𝑛𝑒𝑓 𝑓
(9)
As the proposed sensor is a three-parameter measurement structure, based on the Eqs. (6), (8) and (9), the total shift of wavelength can be given by: 𝛥𝛿𝑛𝑒𝑓 𝑓 ) ( ) 𝛥𝜆 ( = 𝜉 + 𝛼𝑓 𝛥𝑇 + 𝑃𝜀 + 1 𝜀 + 𝜆 𝛿𝑛𝑒𝑓 𝑓
(10)
3. Experimental results and discussion The experimental setup is shown in Fig. 4. A C-band broadband source (BBS) is used as the light source and an optical spectra analyzer (OSA) is utilized to monitor the transmission spectra. With the UDU structure, we conduct a series of experiments which include RI, strain and temperature experiments. The strain and temperature experiments are carried out on an optical table, while the temperature-dependent experiments are carried out in an incubator with temperature control system.
3.2. Strain experiment A micro displacement platform is used to provide different strain in the strain experiments. The strains range from 0 to 423.529 μ𝜀 with steps of 70.588 μ𝜀. The transmission spectra of the UDU structure under different strains are shown in Fig. 7. We choose dip A, dip B and dip C as the observation points. The transmission spectra show blue shifts 72
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Optics Communications 410 (2018) 70–74
Fig. 7. The transmission spectra of the UDU structure under different strains.
Fig. 9. The transmission spectra of the UDU structure under different temperature.
Fig. 10. The relationship between the wavelength and the temperature at different dips. Fig. 8. The relationship between the wavelength and the strain at different dips.
dip C can be expressed as with the increase of the strains at dip B and C, while there is no shift at dip A. The relationship between the wavelength and the strains at different dips are shown in Fig. 8. Experimental results achieve strain sensitivities of 0 pm/μ𝜀 at dip A, −0.6 pm/μ𝜀 at dip B and −0.7 pm/μ𝜀 at dip C respectively. This UDU structure is mainly based on up-tapers, which is not so sensitive to strain, so this structure has a relatively low strain sensitivity.
0 0.0662⎤ ⎡ 𝛥𝑛 ⎤ ⎡𝛥𝜆𝐴 ⎤ ⎡−131.93 ⎢𝛥𝜆 ⎥ = ⎢−22.875 −0.0006 0.0906⎥ ⎢ 𝛥𝜀 ⎥ 𝐵 ⎥⎢ ⎥ ⎢ ⎥ ⎢ −0.0007 0.0878⎦ ⎣𝛥𝑇 ⎦ ⎣𝛥𝜆𝐶 ⎦ ⎣ 0 In the above measurement matrix, the unit of the wavelength is nanometer, the unit of the strain is micro strain, and the unit of the temperature is degree centigrade. As can be seen, a simultaneous RI, strain and temperature measurement is achieved. A system sensitivity table is constructed to compare with the proposed sensors [13,14,16] which can realize the simultaneous measurement of RI, strain and temperature, as is shown in Table 1. The values shown in Table 1 tell that the UDU structure has a higher RI sensitivity and a relative high temperature sensitivity compared with the sensors found in literature. Also, the fabrication process is much easier, the cost is much lower and the experimental set up is much simpler.
3.3. Temperature experiment The temperature experiments are carried out by employing a thermostat, where the temperature is well controlled ranging from 23.0 ◦ C to 54.0 ◦ C. Fig. 9 shows the transmission spectra of the UDU structure under different temperatures. We also choose dip A, dip B and dip C as the observation points. The transmission spectra have a red shift with the increasing of the temperature, as is shown in Fig. 9. In addition, Fig. 10 shows the relationship between the wavelength and the temperature at different dips. Temperature sensitivities of 0.0662 nm/◦ C at dip A, 0.0906 nm/◦ C at dip B and 0.0878 nm/◦ C at dip C are achieved. As mentioned above, at least three higher order modes are excited, and all the interference between the modes are sensitive to temperature. Owing to that only SMF is utilized in the structure, so the temperature sensitivities at different dips almost have the same amount.
4. Conclusion A novel UDU structure for simultaneous measurement of RI, strain and temperature is proposed and investigated theoretically and experimentally. It is based on MZI and is realized by introducing a down taper into a fiber between two adjacent up tapers. It is only formed by SMF with low cost and easy fabrication without manual fusion mode using. Acknowledgments
3.4. Simultaneous RI, strain and temperature measurement This work is supported by National Natural Science Foundation of China (Grant No. 61501313 and 61471253), The Hong Kong Polytechnic University (Grant No. 1-ZVHA) and Open Foundation of State
Based on the above experimental results, the RI 𝛥𝑛, strain 𝛥𝜀 and temperature 𝛥𝑇 measurement matrix in nm unit for dip A, dip B and 73
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Optics Communications 410 (2018) 70–74 Table 1 Sensitivity comparison.
𝑁 (nm/RIU) 𝜀 (nm/με) 𝑇 (nm/◦ C)
Proposed UDU structure
Sensor1 [13]
Sensor2 [14]
Sensor3 [16]
131.93 0.0007 0.0906
92.037 0.0009 0.032
42.975 0.01992 0.1445
116.5 0.00577 0.03482
Key Laboratory of Optical Communication Technologies and Networks (Grant No. 2015OCTN-02).
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