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Simultaneous measurement of temperature and liquid level base on core-offset singlemode–multimode–singlemode interferometer
Optics Communications 321 (2014) 134–137
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Simultaneous measurement of temperature and liquid level base on core-offset singlemode–multimode–singlemode interferometer$ Wei Han n, Zhengrong Tong nn, Ye Cao School of Computer and Communication Engineering, Tianjin University of Technology, Tianjin 300384, China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 November 2013 Received in revised form 25 December 2013 Accepted 28 December 2013 Available online 21 January 2014
A fiber sensor based on core-offset singlemode–multimode–singlemode (SMS) interferometer for simultaneous measurement of temperature and liquid level is proposed. The sensor is fabricated by a SMS interferometer spliced with a single mode fiber (SMF) in core-offset way. Since the cladding mode of SMF is excited in the core-offset SMS, two different kinds of interference dips which are formed by the SMF and the SMS interferometer respectively are obtained. Using the difference sensitivities to the two parameters of the selected dips, temperature and liquid level can be measured simultaneously. The chosen interference dips are at 1529.632 nm and 1553.18 nm. Experiments indicate that the dip at 1529.626 nm is insensitive to liquid level, the temperature sensitivity is 0.064 nm/1C. The temperature sensitivity of the dip at 1553.18 nm is 0.082 nm/1C. The temperature measurement precision is 0.001 nm/1C. When the liquid is water (n¼1.33), the sensitivity of the liquid level is 0.140 nm/mm. When the liquid is sodium chloride solution (n¼1.38), the sensitivity of the liquid level is 0.290 nm/mm. The liquid level measurement precision is 0.001 nm/mm. The interferometer also can be applied in other sensing fields. & 2014 Elsevier B.V. All rights reserved.
Keywords: Fiber sensor SMS interferometer Core-offset Temperature and liquid level measurement
1. Introduction Due to the advantages of high sensitivity and low cost, interferometric fiber sensors have been hot topic since early years of optical fiber sensors [1–3]. Nowadays interferometric fiber sensors are widely used in strain [4–6], vibration [7], refraction index (RI) [8–10], curvature, temperature [11,12] and liquid level [13] measuring. Since temperature and liquid level are important parameters in chemical industry, biological, medical and military field, the measurement of temperature and liquid level has been a research hotspot these years. In the interferometer fiber sensors, SMS interferometer structure has been widely applied. The basic structure of the sensor is a multimode fiber (MMF) spliced between two SMFs to make the core mode coupling. By observing the shift of the interference dips, the sensor can be used to measure many parameters such as temperature [14], curvature [15] and strain. It has advantage of simple structure and the low cost, but the cross-sensitivity may reduce its measurement accuracy. In this paper, the traditional SMS sensor is improved. A SMS interferometer is spliced with SMF in core-offset way to create two
$ Manuscript received October 13, 2013; This work was supported in part by Natural Science Foundation of China (NO. 61107052), in part by Youth Foundation of Tianjin (13JCQNJC01800), in part by National Natural Science Foundation of Tianjin (No. 11JCGJC00100). n Corresponding author. Tel.: þ 86 15620956623. nn Corresponding author. E-mail addresses: [email protected] (W. Han), [email protected] (Z. Tong).
0030-4018/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.12.078
kinds of interference dips. One is formed by the core modes of MMF, and the other is formed by the core mode and low-order cladding mode of SMF. Since the two kinds of interference dips have different sensitivities to temperature and liquid level, the sensor can be used to measure the two parameters simultaneously and eliminate the cross-sensitivity by sensitive matrix.
2. Principle The schematic diagram of the sensor is illustrated in Fig. 1(a). The light from the broadband light source (BBS) is coupled into the sensor head through the SMF. There are two kinds of modes coupling in the sensor head. One is formed by the core mode and low-order cladding mode of SMF1, the other is formed by the core modes of MMF. The interference light is transmitted into the optical spectrometer analyzer (OSA). The structure of the sensor head is shown in Fig. 1(b). The sensor head is made up of the SMS interferometer spliced with SMF in core-offset way. The cross section of the core-offset part is illustrated in Fig. 1(c), a is the radius of the SMF0 s core, H is the size of the core-offset. When the light is transmitted into SMF1 from SMF, part of the light is coupled into the core of SMF1, the other part of the light is coupled into the cladding of SMF1. The core mode and the cladding mode are excited. They will interfere with each other in MMF and create interference dips. Meanwhile, the fundamental mode and high-order modes of the MMF0 s core will also create interference dips. As the two kinds of interference dips have
W. Han et al. / Optics Communications 321 (2014) 134–137
135
where δnef f is the effective refraction index difference of SMF1, asf is the thermal expansion coefficient of SMF1. δnef f can be obtained as
δnef f ¼
½ΔλΔnef f L1 ðL1 þ asf ΔTÞ þ λ asf ΔT 2
Δnef f Δλ½ðL1 þ asf ΔTÞΔnef f L1 þ ðL1 þ asf ΔTÞλ2
ð4Þ
The wavelength shift Δλ is described as
Δλ ¼ K t ΔT þ K l Δl
ð5Þ
where ΔT is the change of the temperature, Δl is the change of the liquid level, K t is the temperature coefficient of sensitivity, K l is the liquid level coefficient of sensitivity. When the light is transmitted into MMF, the core modes of MMF will interfere with each other. The wavelength interval is given by Refs. [16,17]
λcore core ¼
8nmco a2m ð2N þ 1Þ ðm nÞ½2ðm þ nÞ 1L2
ð6Þ
where m and n are the order of the core mode (m 4n), am is the radius of MMF, L2 is the length of MMF, nmco is the refraction index of MMF. N is an integer. When the liquid level changes, the interference dip which is formed by core modes of MMF will not shift since the core is isolated to the liquid by the cladding. When the temperature changes, am, nmco and L2 will change because of the thermo-optic effect and the thermal expansion effect. The wavelength shift of the dip formed by the MMF0 s core modes can be set as 8ðnmco þ ξm ΔTÞ½am þ amf ΔT2 ð2N þ 1Þ 8nmco a2m ð2N þ 1Þ ðm nÞ½2ðm þ nÞ 1L2 ðm nÞ½2ðmþ nÞ 1ðL2 þ amf ΔTÞ ðnmco þ ξm ΔTÞðam þ amf ΔTÞ nco am 8ð2N þ 1Þ ¼ ð7Þ L2 þ amf ΔT L2 ðm nÞ½2ðm þ nÞ 1
Δλcore core ¼
where ξm is the thermo-optic coefficient of MMF0 s core, amf is the thermal expansion coefficient of MMF0 s core. Because the two kinds of interference dips have different sensitivities, the sensor can be used to measure temperature and liquid level simultaneously. Fig. 1. (a) Sensor system structure, (b) sensor head structure, (c) the cross section of the core-offset part.
3. Experimental result and discussion different sensitivities to temperature and liquid level, which can be measured by the shift of the interference dips. As the result of splicing in core-offset way, both cladding mode and core mode are excited in SMF. The phase difference of the cladding mode and core mode is given by
ϕ¼
2πΔnef f L1
In the experiment, the core diameter of SMF1 and SMF (SMF28, manufactured by YOFC) are 8 μm, the length of SMF1 is 2.5 cm, the size of core-offset is 4 μm. The core diameter of MMF (SI60, manufactured by YOFC) is 60 μm and length is 2.5 cm. The transmitted spectrum of the core-offset SMS structure is shown in Fig. 2.
ð1Þ
λ
where λ is the wavelength in vacuum, L1 is the length of SMF1, Δnef f is the effective refraction index difference between the core and the cladding. The wavelength interval of the interference dips which are formed by the interference of SMF1’s core mode and cladding mode is described as
λcore cladding
λ2 Δnef f L1
ð2Þ
As described from the above formula, Δλcore cladding is related with L1 and Δnef f . When temperature changes, L1 and Δnef f will change correspondingly. And Δnef f will change while the liquid level changes. The wavelength shift of the dip formed by the core mode and cladding mode of SMF1 is described as
Δλcorecladding
λ2
ðΔnef f þ δnef f ÞðL1 þ asf ΔTÞ
λ2
Δnef f L1
ð3Þ Fig. 2. The transmitted spectrum of the core-offset SMS structure.
136
W. Han et al. / Optics Communications 321 (2014) 134–137
According to Eqs. (2) and (6), the dips which belong to different interferometers have different wavelength intervals. The dips at 1529.632 nm and at 1553.18 nm are chosen for the measurement. The dip at 1529.632 nm is marked as dip1, the other dip is marked as dip2. Fig. 3 shows the transmitted spectrum of the selected dips.
The sensor is placed on thermostat plate heated from 25 1C to 60 1C, the wavelength shift is recorded every 5 1C. As shown in Fig. 4, both dip1 and dip2 have a red shift with the increase of temperature. The total wavelength shift of dip1 is 2.256 nm, the
Fig. 3. The transmitted spectrum of the selected dips.
Fig. 6. The response characteristics curve of dip1 when liquid level is below 10 mm.
Fig. 4. Temperature response characteristics curve.
Fig. 7. The response characteristics curve of dip2 when liquid level is below 10 mm.
Fig. 5. Schematic diagram of the liquid level experiment.
W. Han et al. / Optics Communications 321 (2014) 134–137
Fig. 8. The response characteristics curve of dip1 when liquid level is above 10 mm.
sensitivity to temperature is 0.064 nm/1C; the total wavelength shift of dip2 is 2.8 nm, the sensitivity to temperature is 0.082 nm/ 1C. The measurement precision of temperature is 0.001 nm/1C. Fig. 4 illustrates that the fitness of temperature measurement for dip1 and dip2 are 0.9915 and 0.9927, respectively, that means the performance of the sensor in actual temperature measurement is accurate. The sensor is fixed vertically inside the container. SMF1 is under the MMF. The schematic diagram of the liquid level experiment is shown in Fig. 5. When the liquid submerges SMF1, zero level is recorded. The liquid level is measured while the liquid are water (n¼ 1.33) and sodium chloride solution (n¼ 1.38), respectively. The wavelength shift is recorded every 2 mm. When the liquid level changes from 0 mm to 10 mm (below the surface where SMF1 is spliced with MMF), and dip1 is completely insensitive to liquid level. The sensitivity to liquid level of dip2 is 0.140 nm/mm while the liquid is water, and when the liquid is sodium chloride solution, the sensitivity is 0.290 nm/mm. The response characteristics curves of dip1 and dip2 are shown in Figs. 6 and 7. The fitness (0.9988 and 0.9968) also show the accuracy of performance in liquid level measurement. When the liquid level is above 10 mm, MMF is submerged. Both dip1 and dip2 do not have a wavelength shift when the liquid level changes from 10 mm to 20 mm. The response characteristics curves of dip1 and dip2 are shown in Figs. 8 and 9. The measurement precision of liquid level is 0.001 nm/mm. The experiment result indicates that dip1 is the interference dip formed by MMF because it is insensitive to liquid level, dip2 is the interference dip formed by SMF1. The larger the RI of liquid is, the higher sensitivity the sensor will have. When the temperature and the liquid level change simultaneously, the wavelength shifts of dip1 and dip2 are given by
Δλi ¼ K ti ΔT þ K li Δl
ð8Þ
where ΔT is the change of the temperature, Δl is the change of the liquid level, K ti is the temperature coefficient of sensitivity, K li is the liquid level coefficient of sensitivity, i ¼1,2 is corresponding dip1 and dip2, respectively. The sensitivity matrix can be given as # " # " #" K t1 K l1 Δλ1 ΔT ð9Þ ¼ K t2 K l2 Δλ2 Δl
137
Fig. 9. The response characteristics curve of dip2 when liquid level is above 10 mm.
And the sensing matrix can be given as " #" # " # K l1 Δλ1 ΔT 1 K l2 ¼ K t1 Δλ2 D K t2 Δl
ð10Þ
where D ¼ K t1 K l2 K t2 K l1 . So the temperature and the liquid level can be measured simultaneously by observing the two dips.
4. Conclusions In this paper, a fiber sensor is proposed which is made up of a SMS interferometer spliced with SMF in core-offset way. The sensitivities to temperature of the two selected dips are 0.0815 nm/1C and 0.0636 nm/1C, respectively, and the sensitivities to the liquid level of dip2 are 0.14 nm/mm and 0.29 nm/mm respectively when the liquid is water and sodium chloride solution. Dip1 is insensitive to liquid level. Due to the different sensitivities to the two parameters of the selected dips, temperature and liquid level can be measured simultaneously. The sensor has the advantage of low cost, simple structure and high sensitivity. It also can be used in refraction index measurement.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
C. Hill Bruce, E. Barelli Anthony, Rev. Sci. Instrum. 49 (6) (1978) 722. S.K. Sheem, Appl. Phys. Lett. 35 (12) (1979) 914. I. Bush, Quantum Electron. 17 (12) (1981) 2454. Juan Kang, Xinyong Dong, Chunliu Zhao, Opt. Commun. 284 (8) (2011) 2145. Ben Xu Miao Sun, Xinyong Dong, Opt. Commun. 285 (18) (2012) 3721. Huaping Gong, Xiaorui Li, Yongxing Jin, Sens. Actuators, A 187 (2012) 95. P.u. Wei, Xuekang Shan, Xiaohan Sun, Opt. Fiber Technol. 19 (1) (2013) 47. Fufei Pang, Huanhuan Liu, Hairun Guo, IEEE Sens. J. 11 (10) (2011) 2395. Qiangzhou Rong, IEEE Sens. J. 12 (7) (2012) 2501. Q.i. Liu, Q.i.n. Wang, Chin. Opt. Lett. 10 (9) (2012) 090601. Yuanhong Yang, Chin. Opt. Lett. 2 (5) (2004) 259. Y.i. Xin, Xinyong Dong, Qingqiang Meng, Sens. Actuators, A 193 (2013) 182. Tuan Guo, Qida Zhao, Qingying Dou, IEEE Sens. J. 17 (11) (2005) 2400. Saurabh Mani Tripathi, Ravi K Barshney, Jean-Pierre Meunier, J. Lightwave Technol. 27 (13) (2009) 2438. [15] Yuan Gong, Tian Zhao, Yun-Jiang Rao, IEEE Photonics Technol. Lett. 23 (11) (2011) 679. [16] D. Marcuse, J. Lightwave Technol. 5 (1) (1987) 125. [17] W.S. Mohammed, A. Mehta, E.G. Johnson, J. Lightwave Technol. 22 (2) (2004) 469.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Simultaneous measurement of temperature and liquid level base on core-offset singlemode–multimode–singlemode interferometer$ Wei Han n, Zhengrong Tong nn, Ye Cao School of Computer and Communication Engineering, Tianjin University of Technology, Tianjin 300384, China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 November 2013 Received in revised form 25 December 2013 Accepted 28 December 2013 Available online 21 January 2014
A fiber sensor based on core-offset singlemode–multimode–singlemode (SMS) interferometer for simultaneous measurement of temperature and liquid level is proposed. The sensor is fabricated by a SMS interferometer spliced with a single mode fiber (SMF) in core-offset way. Since the cladding mode of SMF is excited in the core-offset SMS, two different kinds of interference dips which are formed by the SMF and the SMS interferometer respectively are obtained. Using the difference sensitivities to the two parameters of the selected dips, temperature and liquid level can be measured simultaneously. The chosen interference dips are at 1529.632 nm and 1553.18 nm. Experiments indicate that the dip at 1529.626 nm is insensitive to liquid level, the temperature sensitivity is 0.064 nm/1C. The temperature sensitivity of the dip at 1553.18 nm is 0.082 nm/1C. The temperature measurement precision is 0.001 nm/1C. When the liquid is water (n¼1.33), the sensitivity of the liquid level is 0.140 nm/mm. When the liquid is sodium chloride solution (n¼1.38), the sensitivity of the liquid level is 0.290 nm/mm. The liquid level measurement precision is 0.001 nm/mm. The interferometer also can be applied in other sensing fields. & 2014 Elsevier B.V. All rights reserved.
Keywords: Fiber sensor SMS interferometer Core-offset Temperature and liquid level measurement
1. Introduction Due to the advantages of high sensitivity and low cost, interferometric fiber sensors have been hot topic since early years of optical fiber sensors [1–3]. Nowadays interferometric fiber sensors are widely used in strain [4–6], vibration [7], refraction index (RI) [8–10], curvature, temperature [11,12] and liquid level [13] measuring. Since temperature and liquid level are important parameters in chemical industry, biological, medical and military field, the measurement of temperature and liquid level has been a research hotspot these years. In the interferometer fiber sensors, SMS interferometer structure has been widely applied. The basic structure of the sensor is a multimode fiber (MMF) spliced between two SMFs to make the core mode coupling. By observing the shift of the interference dips, the sensor can be used to measure many parameters such as temperature [14], curvature [15] and strain. It has advantage of simple structure and the low cost, but the cross-sensitivity may reduce its measurement accuracy. In this paper, the traditional SMS sensor is improved. A SMS interferometer is spliced with SMF in core-offset way to create two
$ Manuscript received October 13, 2013; This work was supported in part by Natural Science Foundation of China (NO. 61107052), in part by Youth Foundation of Tianjin (13JCQNJC01800), in part by National Natural Science Foundation of Tianjin (No. 11JCGJC00100). n Corresponding author. Tel.: þ 86 15620956623. nn Corresponding author. E-mail addresses: [email protected] (W. Han), [email protected] (Z. Tong).
0030-4018/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.12.078
kinds of interference dips. One is formed by the core modes of MMF, and the other is formed by the core mode and low-order cladding mode of SMF. Since the two kinds of interference dips have different sensitivities to temperature and liquid level, the sensor can be used to measure the two parameters simultaneously and eliminate the cross-sensitivity by sensitive matrix.
2. Principle The schematic diagram of the sensor is illustrated in Fig. 1(a). The light from the broadband light source (BBS) is coupled into the sensor head through the SMF. There are two kinds of modes coupling in the sensor head. One is formed by the core mode and low-order cladding mode of SMF1, the other is formed by the core modes of MMF. The interference light is transmitted into the optical spectrometer analyzer (OSA). The structure of the sensor head is shown in Fig. 1(b). The sensor head is made up of the SMS interferometer spliced with SMF in core-offset way. The cross section of the core-offset part is illustrated in Fig. 1(c), a is the radius of the SMF0 s core, H is the size of the core-offset. When the light is transmitted into SMF1 from SMF, part of the light is coupled into the core of SMF1, the other part of the light is coupled into the cladding of SMF1. The core mode and the cladding mode are excited. They will interfere with each other in MMF and create interference dips. Meanwhile, the fundamental mode and high-order modes of the MMF0 s core will also create interference dips. As the two kinds of interference dips have
W. Han et al. / Optics Communications 321 (2014) 134–137
135
where δnef f is the effective refraction index difference of SMF1, asf is the thermal expansion coefficient of SMF1. δnef f can be obtained as
δnef f ¼
½ΔλΔnef f L1 ðL1 þ asf ΔTÞ þ λ asf ΔT 2
Δnef f Δλ½ðL1 þ asf ΔTÞΔnef f L1 þ ðL1 þ asf ΔTÞλ2
ð4Þ
The wavelength shift Δλ is described as
Δλ ¼ K t ΔT þ K l Δl
ð5Þ
where ΔT is the change of the temperature, Δl is the change of the liquid level, K t is the temperature coefficient of sensitivity, K l is the liquid level coefficient of sensitivity. When the light is transmitted into MMF, the core modes of MMF will interfere with each other. The wavelength interval is given by Refs. [16,17]
λcore core ¼
8nmco a2m ð2N þ 1Þ ðm nÞ½2ðm þ nÞ 1L2
ð6Þ
where m and n are the order of the core mode (m 4n), am is the radius of MMF, L2 is the length of MMF, nmco is the refraction index of MMF. N is an integer. When the liquid level changes, the interference dip which is formed by core modes of MMF will not shift since the core is isolated to the liquid by the cladding. When the temperature changes, am, nmco and L2 will change because of the thermo-optic effect and the thermal expansion effect. The wavelength shift of the dip formed by the MMF0 s core modes can be set as 8ðnmco þ ξm ΔTÞ½am þ amf ΔT2 ð2N þ 1Þ 8nmco a2m ð2N þ 1Þ ðm nÞ½2ðm þ nÞ 1L2 ðm nÞ½2ðmþ nÞ 1ðL2 þ amf ΔTÞ ðnmco þ ξm ΔTÞðam þ amf ΔTÞ nco am 8ð2N þ 1Þ ¼ ð7Þ L2 þ amf ΔT L2 ðm nÞ½2ðm þ nÞ 1
Δλcore core ¼
where ξm is the thermo-optic coefficient of MMF0 s core, amf is the thermal expansion coefficient of MMF0 s core. Because the two kinds of interference dips have different sensitivities, the sensor can be used to measure temperature and liquid level simultaneously. Fig. 1. (a) Sensor system structure, (b) sensor head structure, (c) the cross section of the core-offset part.
3. Experimental result and discussion different sensitivities to temperature and liquid level, which can be measured by the shift of the interference dips. As the result of splicing in core-offset way, both cladding mode and core mode are excited in SMF. The phase difference of the cladding mode and core mode is given by
ϕ¼
2πΔnef f L1
In the experiment, the core diameter of SMF1 and SMF (SMF28, manufactured by YOFC) are 8 μm, the length of SMF1 is 2.5 cm, the size of core-offset is 4 μm. The core diameter of MMF (SI60, manufactured by YOFC) is 60 μm and length is 2.5 cm. The transmitted spectrum of the core-offset SMS structure is shown in Fig. 2.
ð1Þ
λ
where λ is the wavelength in vacuum, L1 is the length of SMF1, Δnef f is the effective refraction index difference between the core and the cladding. The wavelength interval of the interference dips which are formed by the interference of SMF1’s core mode and cladding mode is described as
λcore cladding
λ2 Δnef f L1
ð2Þ
As described from the above formula, Δλcore cladding is related with L1 and Δnef f . When temperature changes, L1 and Δnef f will change correspondingly. And Δnef f will change while the liquid level changes. The wavelength shift of the dip formed by the core mode and cladding mode of SMF1 is described as
Δλcorecladding
λ2
ðΔnef f þ δnef f ÞðL1 þ asf ΔTÞ
λ2
Δnef f L1
ð3Þ Fig. 2. The transmitted spectrum of the core-offset SMS structure.
136
W. Han et al. / Optics Communications 321 (2014) 134–137
According to Eqs. (2) and (6), the dips which belong to different interferometers have different wavelength intervals. The dips at 1529.632 nm and at 1553.18 nm are chosen for the measurement. The dip at 1529.632 nm is marked as dip1, the other dip is marked as dip2. Fig. 3 shows the transmitted spectrum of the selected dips.
The sensor is placed on thermostat plate heated from 25 1C to 60 1C, the wavelength shift is recorded every 5 1C. As shown in Fig. 4, both dip1 and dip2 have a red shift with the increase of temperature. The total wavelength shift of dip1 is 2.256 nm, the
Fig. 3. The transmitted spectrum of the selected dips.
Fig. 6. The response characteristics curve of dip1 when liquid level is below 10 mm.
Fig. 4. Temperature response characteristics curve.
Fig. 7. The response characteristics curve of dip2 when liquid level is below 10 mm.
Fig. 5. Schematic diagram of the liquid level experiment.
W. Han et al. / Optics Communications 321 (2014) 134–137
Fig. 8. The response characteristics curve of dip1 when liquid level is above 10 mm.
sensitivity to temperature is 0.064 nm/1C; the total wavelength shift of dip2 is 2.8 nm, the sensitivity to temperature is 0.082 nm/ 1C. The measurement precision of temperature is 0.001 nm/1C. Fig. 4 illustrates that the fitness of temperature measurement for dip1 and dip2 are 0.9915 and 0.9927, respectively, that means the performance of the sensor in actual temperature measurement is accurate. The sensor is fixed vertically inside the container. SMF1 is under the MMF. The schematic diagram of the liquid level experiment is shown in Fig. 5. When the liquid submerges SMF1, zero level is recorded. The liquid level is measured while the liquid are water (n¼ 1.33) and sodium chloride solution (n¼ 1.38), respectively. The wavelength shift is recorded every 2 mm. When the liquid level changes from 0 mm to 10 mm (below the surface where SMF1 is spliced with MMF), and dip1 is completely insensitive to liquid level. The sensitivity to liquid level of dip2 is 0.140 nm/mm while the liquid is water, and when the liquid is sodium chloride solution, the sensitivity is 0.290 nm/mm. The response characteristics curves of dip1 and dip2 are shown in Figs. 6 and 7. The fitness (0.9988 and 0.9968) also show the accuracy of performance in liquid level measurement. When the liquid level is above 10 mm, MMF is submerged. Both dip1 and dip2 do not have a wavelength shift when the liquid level changes from 10 mm to 20 mm. The response characteristics curves of dip1 and dip2 are shown in Figs. 8 and 9. The measurement precision of liquid level is 0.001 nm/mm. The experiment result indicates that dip1 is the interference dip formed by MMF because it is insensitive to liquid level, dip2 is the interference dip formed by SMF1. The larger the RI of liquid is, the higher sensitivity the sensor will have. When the temperature and the liquid level change simultaneously, the wavelength shifts of dip1 and dip2 are given by
Δλi ¼ K ti ΔT þ K li Δl
ð8Þ
where ΔT is the change of the temperature, Δl is the change of the liquid level, K ti is the temperature coefficient of sensitivity, K li is the liquid level coefficient of sensitivity, i ¼1,2 is corresponding dip1 and dip2, respectively. The sensitivity matrix can be given as # " # " #" K t1 K l1 Δλ1 ΔT ð9Þ ¼ K t2 K l2 Δλ2 Δl
137
Fig. 9. The response characteristics curve of dip2 when liquid level is above 10 mm.
And the sensing matrix can be given as " #" # " # K l1 Δλ1 ΔT 1 K l2 ¼ K t1 Δλ2 D K t2 Δl
ð10Þ
where D ¼ K t1 K l2 K t2 K l1 . So the temperature and the liquid level can be measured simultaneously by observing the two dips.
4. Conclusions In this paper, a fiber sensor is proposed which is made up of a SMS interferometer spliced with SMF in core-offset way. The sensitivities to temperature of the two selected dips are 0.0815 nm/1C and 0.0636 nm/1C, respectively, and the sensitivities to the liquid level of dip2 are 0.14 nm/mm and 0.29 nm/mm respectively when the liquid is water and sodium chloride solution. Dip1 is insensitive to liquid level. Due to the different sensitivities to the two parameters of the selected dips, temperature and liquid level can be measured simultaneously. The sensor has the advantage of low cost, simple structure and high sensitivity. It also can be used in refraction index measurement.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
C. Hill Bruce, E. Barelli Anthony, Rev. Sci. Instrum. 49 (6) (1978) 722. S.K. Sheem, Appl. Phys. Lett. 35 (12) (1979) 914. I. Bush, Quantum Electron. 17 (12) (1981) 2454. Juan Kang, Xinyong Dong, Chunliu Zhao, Opt. Commun. 284 (8) (2011) 2145. Ben Xu Miao Sun, Xinyong Dong, Opt. Commun. 285 (18) (2012) 3721. Huaping Gong, Xiaorui Li, Yongxing Jin, Sens. Actuators, A 187 (2012) 95. P.u. Wei, Xuekang Shan, Xiaohan Sun, Opt. Fiber Technol. 19 (1) (2013) 47. Fufei Pang, Huanhuan Liu, Hairun Guo, IEEE Sens. J. 11 (10) (2011) 2395. Qiangzhou Rong, IEEE Sens. J. 12 (7) (2012) 2501. Q.i. Liu, Q.i.n. Wang, Chin. Opt. Lett. 10 (9) (2012) 090601. Yuanhong Yang, Chin. Opt. Lett. 2 (5) (2004) 259. Y.i. Xin, Xinyong Dong, Qingqiang Meng, Sens. Actuators, A 193 (2013) 182. Tuan Guo, Qida Zhao, Qingying Dou, IEEE Sens. J. 17 (11) (2005) 2400. Saurabh Mani Tripathi, Ravi K Barshney, Jean-Pierre Meunier, J. Lightwave Technol. 27 (13) (2009) 2438. [15] Yuan Gong, Tian Zhao, Yun-Jiang Rao, IEEE Photonics Technol. Lett. 23 (11) (2011) 679. [16] D. Marcuse, J. Lightwave Technol. 5 (1) (1987) 125. [17] W.S. Mohammed, A. Mehta, E.G. Johnson, J. Lightwave Technol. 22 (2) (2004) 469.