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Refractive index and temperature sensor based on Mach-Zehnder interferometer with thin fibers
Optical Fiber Technology 54 (2020) 102101
Contents lists available at ScienceDirect
Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
Refractive index and temperature sensor based on Mach-Zehnder interferometer with thin fibers
T
Wei Liu, Xuqiang Wu , Gang Zhang, Shili Li, Cheng Zuo, Shasha Fang, Benli Yu ⁎
Key Laboratory of Opto-Electronic Information Acquisition and Manipulation of Ministry of Education, Anhui University, Jiulong Road 111#, Hefei 230601, China
ARTICLE INFO
ABSTRACT
Keywords: Mach-Zehnder interferometric sensor Thin fiber Refractive index Temperature
An all-fiber Mach-Zehnder interferometric sensor for refractive index (RI) and temperature measurement is proposed and experimentally demonstrated. The proposed sensor is fabricated with three segments of two type of thin fibers sandwiched between two standard single-mode fibers (SMFs). The variation of ambient RI and temperature causes the change of phase difference between the cladding modes and the core mode, which leads to the shift of interference spectrum. The two resonance dips shift in the wavelength spectrum is used to investigate the RI and temperature characteristics of the sensor. Experimental results show that the two dips have different responses of RI and temperature, which indicates that the sensor can realize simultaneous RI and temperature measurement. The maximum sensing sensitivities are −169.0879 nm/RIU and 0.0464 nm/°C, respectively. The proposed sensor exhibits potential applications in physical, biological and chemical sensing fields due to its high sensitivity, good linearity, simple fabrication and low cost.
1. Introduction Optical fiber sensors (OFSs) have been intensively studied in the measurement of various parameters due to their characteristics of high sensitivity, compact size, explosion-proof, anti-corrosion, anti-electromagnetic, remote sensing, etc. Among them, OFSs for dual-parameter RI and temperature measurement have become the research hotspot in recent years. To date, numerous OFSs utilizing fiber Bragg gratings (FBGs) [1–3] and long-period fiber gratings (LPFGs) [4–6] have been developed for simultaneous RI and temperature measurement. Y. Dong et al. proposed a D-shaped fiber structure combined with a FBG for RI and temperature measurement. The sensitivities of −31.79 nm/RIU and 28.7 pm/°C were achieved [1]. Q. Han et al. proposed a kind of LPFG for RI and temperature measurement, which is written on a SMF and double-clad fiber with CO2 laser point-by-point irradiation [6]. The grating-based (FBG, LPFG) sensors have high sensitivity and large measurement range, but require expensive fabrication equipment and stringent procedures [7]. Moreover, OFSs based on sensing techniques such as Mach-Zehnder [7–12], Fabry-Pérot [13], Michelson [14,15], and Sagnac [16] have been widely used in RI and temperature measurement due to their high sensitivity, flexible structure, and compact size. Z. Tong et al. proposed a fewmode fiber and spherical structure based dual-parameter sensor, which presented the sensitivities of −48.82 nm/RIU and 0.059 nm/°C [8]. Y.
⁎
Chen et al. designed a hybrid multimode interference structure based sensor, which presented the sensitivities of 113.6 nm/RIU and 9.2 pm/°C [11]. H. Lu et al. fabricated a Sagnac loop based sensor by splicing polarization-maintaining fiber and D-shaped fiber. The sensor achieved the sensitivities of −1.804 nm/°C and −131.49 nm/RIU [16]. However, these interferometric OFSs usually have the following drawbacks such as complicated structure, tough fabrication, low sensitivity, or high cost. In this paper, we proposed an easily fabricated and low cost sensor for simultaneous RI and temperature measurement. The sensor consists of two types of thin fibers, named as thin fiber (TF) and ultrathin fiber (UTF), respectively. 2. Sensor fabrication and sensing principle The schematic diagram of the proposed sensor is shown in Fig. 1. The sensor is fabricated by fusion splicing a section of TF between two UTFs after fusion splicing the UTFs on the end faces of the lead-in SMF and the lead-out SMF. Since the diameters of the thin fibers are too small, the suitable arc power and duration of arc fusion splicer (FSM45PM) are required to ensure that the thin fibers will not be destroyed by high power discharge. When the arc power and duration are set as 10 bit and 450 ms, respectively, the splice loss of the sensor fabricated by the discharge is small, and the interference spectrum is substantially stable. The illustrations in Fig. 1 show the fusion splicing points a and b
Corresponding author. E-mail address: [email protected] (X. Wu).
https://doi.org/10.1016/j.yofte.2019.102101 Received 30 August 2019; Received in revised form 18 October 2019; Accepted 24 November 2019 1068-5200/ © 2019 Elsevier Inc. All rights reserved.
Optical Fiber Technology 54 (2020) 102101
W. Liu, et al.
Fig.1. The schematic diagram of the sensor. The illustrations depict the splicing points a and b observed under the microscope.
observed under the microscope. The core/cladding diameters of the SMF, TF and UTF are 9 μm/125 μm, 6.5 μm/80 μm and 4.2 μm/50 μm, respectively. The length of UTF determines the mode coupling coefficient when the light in lead-in SMF couples into the TF, which will affect the interference spectrum [9]. A relatively high coupling coefficient for a particular cladding mode can be obtained by properly choosing the length of UTF, and the length should be as short as possible so that the phase differences between the guided modes are negligible [10]. In the experiment, 1 mm length of the two UTFs is chosen to ensure compact size, low intensity loss and good core-cladding coupling. The TF with a length of 14 mm is the interference part of the sensor, and its core and cladding have refractive indices of 1.4578 and 1.444. Combined with the transmission spectrum analysis of the sensor, the choice of TF length was discussed in detail in Section 3. The light from an amplified spontaneous emission (ASE) light source is transmitted in the lead-in SMF core as a fundamental mode and then transmitted to the splicing point a. Due to the mismatch of the core diameter between the lead-in SMF and the UTF 1, the high-order cladding modes of the input light will be excited in the UTF 1. The UTF 1 acts as an input coupler. As the light continues to propagate, the core mode and the cladding modes will propagate in the core and cladding of the TF, respectively. When the light travels to the splicing point b, the high-order cladding modes will interfere with the core fundamental mode because of the mismatch of the core diameter between the TF and the UTF 2. Then the light is re-coupled into the lead-out SMF. The UTF 2 acts as an output coupler. For convenience, only the core mode and one cladding mode are taken into consideration. The phase difference between the core mode and the cladding mode can be defined as [17]:
=
2
neff L
=
c 2 (neff
cl neff )L
wheren1andn2 represent the initial ambient RI and the increased ambient RI. Since the ERI of the cladding mode will increase with the cl cl increase of ambient RI, it can be known that neff , n2 > neff , n1 . So dip will decrease as the RI increases from n1 to n2 . That means the dip will shift towards the shorter wavelength region. Assuming that the change of ERI difference between the two modes is n , The RI sensitivity Kn can be expressed as:
=
2 neff L 2k + 1
=
2L c (neff 2k + 1
= =
dip, n2
cl neff , n2 ) cl neff , n1 )]
c (neff , n1
(4)
neff
ncl =
cl cl neff T
T (5)
where and are the ERI changes of the core mode and the cladding mode. c and cl correspond to the thermal-optic coefficients of the core and the cladding, respectively. T stands for the variation of ambient temperature. By combining Eq. (5) and Eq. (2), the change of dip can be written as: dip
= = = =
ncl
dip, T2
dip, T1
2L c [(neff , T2 2k + 1 2L c [ n 2k + 1 2L T [ nc 2k + 1 c eff
cl neff , T2 )
c (neff , T1
cl neff , T1 )]
ncl] cl cl neff ]
(6)
Where T1 and T2 represent the initial ambient temperature and the increased ambient temperature. According to Eq. (6), since the thermooptic coefficient of the fiber core is higher than that of the fiber cladding and the ERI of core mode is higher than the ERI of cladding mode, dip will increase as the temperature increases from T1 to T2 . That means the dip will move to the longer wavelength region. Likewise, the ambient temperature sensitivity KT can be expressed as:
(2)
KT =
dip
T
=
2L c ( neff 2k + 1 c
cl cl neff )
(7)
According to the above analysis, it can be known that as the RI and temperature increase, the dip appears blue-shift and red-shift, respectively.
dip, n1
2L c [(neff , n2 2k + 1 2L cl [ (neff , n2 2k + 1
2L = 2k + 1
c c neff
nc
As the ambient RI increasing, the ERI of the cladding mode will increase, while the ERI of the core mode almost remains unchanged. Thus the variation of dip with the change of ambient RI can be deduced from (2) as: dip=
=
nc =
(1)
cl neff )
n
Due to the thermal expansion effect and thermo-optic effect of the TF, the ERI, diameter and length of the TF will change with the variation of ambient temperature. Since the thermal-optic coefficient is much larger than the thermal expansion coefficient for silica, the dimension change induced by temperature variation can be neglected [18]. Therefore, the ERI changes of the core mode and the cladding mode with the variation of temperature can be described by the following formulas:
where is the wavelength of the propagating light andL is the length of c cl the TF. neff and neff denote effective refractive indices of the core mode and the cladding mode. neff is the effective refractive index (ERI) difference between the core mode and the cladding mode. When phase = (2k + 1) , (k = 1, 2, 3. ..) , the difference satisfies the expression respective resonance dip can be written as [17]: dip
dip
Kn =
cl neff , n1 )]
3. Experimental results and discussion (3)
The experimental setup is illustrated in Fig. 2. Two ends of the 2
Optical Fiber Technology 54 (2020) 102101
W. Liu, et al.
=
neff L 0
(8)
2
where 0 and neff represent the center wavelength and the differential modal group index, respectively. The neff corresponding to peak 0.081 nm−1 is calculated equal to 0.0139, which is close to the material RI difference between the TF core and cladding. Hence the inhomogeneous interference spectrum should be mainly caused by interferences between the core mode and the cladding modes with different orders. The wavelength separation between two interference minima, known as free spectral range (FSR), can be approximated as [8]:
FSR
0
2
neff L
(9)
The FSR is calculated equal to 12.44 nm, which is close to the stripe interval between the dip A and B in the interference spectrum. The neff in Eq. (9) is the material RI difference between the TF core and cladding in the air, while the stripe interval of 12.8 nm in interference spectrum is recorded by immersing the sensor completely in the glycerin-water solution. Therefore, there is a slight difference between the FSR and the fringe interval. In the experiment, the dip A and dip B with the wavelength of 1546.2 nm and 1559 nm marked in Fig. 3(a) are selected to analyze their RI and temperature characteristics. The RI measuring performance was firstly evaluated by immersing the proposed sensor into six concentrations of solutions with the RI ranging from 1.3388 to 1.3604 (measured by an Abbe refractometer, WAY-2S) at room temperature. we accomplished each change of RI solutions by adding glycerin to the glassware and stirring at a low speed. The experimental data was recorded after the RI of the solution reached the target value and the transmission spectrum was completely stabilized. The Fig. 4 presents the measured wavelength shift of dip A and B as the ambient RI increases. It can be seen that the two dips experience blue-shift and the shift of dip A and B is −2.8 nm and −3.8 nm. The fitted RI sensitivities of dip A and B are KA,n = 132.1763 nm/RIU and KB,n = -169.0879 nm/RIU, respectively. In order to evaluate the temperature measurement performance of the proposed sensor, the sensor immersed into the 12% glycerin-water solution (RI = 1.3473) was fixed in the incubator. In the experiment, the ambient temperature rises from 30 °C to 75 °C with a temperature interval of 5 °C. The plots of the dips wavelength shift in response to the temperature change are shown in Fig. 5. One can see that the two dips show red-shift along with the temperature increasing and the shift of dip A and B is 3.4 nm and 2.6 nm. The fitted temperature sensitivities of dip A and B are KA,T = 0.0744 nm/°C and KB,T = 0.0555 nm/°C,
Fig. 2. Schematic diagram of the experiment facility.
fabricated sensor are connected to the ASE light source with a bandwidth of 1530 nm-1565 nm and the optical spectrum analyzer (OSA, AQ6370C) with a resolution of 0.02 nm, respectively. The sensor fixed in glassware is placed in an incubator with the resolution of 0.1 °C. Fig. 3(a) shows the transmission spectra when the sensors with different TF lengths are completely immersed in the glycerin-water solution (RI = 1.3388) at the room temperature of 25 °C. The black curve represents the spectrum of ASE light source when the sensor is not connected. By comparing the black curve with other color curves, the insertion losses between the ASE light source and the sensor transmission can be intuitively observed, as shown in Fig. 3 (a). By comparing the interference spectra of the sensors with different TF lengths, it is found that the interference spectrum has the highest extinction ratio of more than 18 dB when the TF length is 14 mm. So we chose the sensor with the TF length of 14 mm for simultaneous RI and temperature measurement. It can be seen from the blue curve that there are three dips in the interference spectrum, which obviously indicate that more than one cladding mode is involved in the interference. In order to get more information about the interference between propagating modes, the spatial frequency distribution is obtained by taking fast Fourier transform (FFT), as shown in Fig. 3(b). It can be seen that the spatial frequency spectrum has one dominant peak at 0.081 nm−1 and several weak peaks. The spatial frequency can be expressed as [18]:
Fig. 3. (a) Measured transmission spectra of the sensors. (b) Spatial frequency spectrum of the sensor. 3
Optical Fiber Technology 54 (2020) 102101
W. Liu, et al.
Fig.6. The RI of 12% glycerin-water solution versus the temperature. Fig. 4. The dips shift in response to the RI change. The inset depicts the transmission spectra for different RI of glycerol-water solutions.
temperature measurement. When the ambient RI and temperature change simultaneously, the dips wavelength shift can be expressed as: (11)
= Kn· n + KT · T
According to Eq. (11), the relationship between the dips shift and the changes of RI and temperature can be described as:
( Tn ) = K1
KB, T KB, n
KA, T KA, n
A
(12)
B
where K = KA, n·KB, T KA, T · KB, n . By substituting the measured sensitivity coefficients into Eq. (12), the matrix formula can be turned into:
1 0.0197 (169.0879 ( Tn ) = 5.2418
)
A B
(13)
The calculated cross sensitivity of the RI on the temperature is about 0.1908 °C/RIU. When the wavelength measurement resolution of OSA is 0.02 nm, the resolutions of RI and temperature can reach 1.1828 × 10-4 RIU and 0.431 °C. It is clear that the ambient RI and temperature can be measured simultaneously by monitoring the resonance dips shift. Since the sensing area of the TF have a relatively small core-cladding diameter difference. Evanescent waves are more likely to leak and the cladding ERI is more sensitive to the ambient liquid. The temperature sensitivity of the sensor is mainly controlled by the thermal-optic coefficient of TF. Therefore, compared with the sensors with similar structures mentioned in the references [8–12], the proposed sensor improves the ambient RI sensitivity while maintaining the temperature sensitivity substantially unchanged. The performance comparison of these sensors is shown in Table 1.
Fig. 5. The dips shift in response to the temperature change. The inset depicts the transmission spectra under different temperature.
respectively. Since the RI of the glycerin-water solution is affected by the ambient temperature, the fitted temperature sensitivities above should be the sum of the direct contribution of the temperature and the indirect contribution of the induced change of the RI of the solution [9]. Supposing the temperature sensitivity of the sensor can be linearly approximated as [10]:
Km, T = Km, T + Km, n RRI , T , (m = A, B )
0.0464 132.1763
4. Conclusions
(10)
In summary, we have proposed and demonstrated an all-fiber MachZehnder interferometric sensor based on UTF-TF-UTF structure. By measuring the wavelength shift of dips formed by the cladding modes interfering with the core mode, the simultaneous RI and temperature measurement have been realized. The sensitivities of RI and temperature can reach up to −169.0879 nm/RIU and 0.0464 nm/°C, and the measured RI and temperature resolutions are 1.1828 × 10-4 RIU and 0.431 °C, respectively. The sensor shows the advantages of high sensitivity, good linearity, low cost and simple fabrication, which is suitable for physical, biological and chemical sensing applications.
where Km, T and Km, T are the measured sensitivity and pure sensitivity of the temperature, respectively. The RRI , T is the dependency of the RI on temperature. In the experiment of evaluating the temperature characteristic of the sensor, after recording the spectral curves at different temperatures, we also measured the RI of the 12% glycerin-water solution at the corresponding temperature using the Abbe refractometer. The Fig. 6 shows the RI of the glycerin-water solution versus the temperature in glassware. The linear fittings to the data give a fitted value of −2.12 × 10-4 °C−1 and a value of R2 larger than 0.9969, within the temperature range from 25 to 75 °C. The pure sensitivities of the temperature of dip A and B are 0.0464 nm/°C and 0.0197 nm/°C. Due to the dip A and dip B present different spectral responses of RI and temperature, the sensor can realize simultaneous RI and
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to 4
Optical Fiber Technology 54 (2020) 102101
W. Liu, et al.
Table 1 The performance comparison of the sensors with similar structures (the FMF, MMF, NCF, and TCF represent few-mode fiber, multi-mode fiber, no-core fiber, and thincore fiber, respectively).
Ref. [8] Ref. [9] Ref. [10] Ref. [11] Ref. [12] The proposed sensor
Structure of sensor
RI sensitivity (nm/RIU)
Temperature sensitivity (nm/°C)
SMF ball-SMF-FMF-SMF SMF-MMF-un-coated SMF-MMF-SMF SMF-MMF-un-coated SMF-peanut-shape structure-SMF SMF-NCF-SMF-MMF-SMF SMF ball-TCF-SMF ball SMF-UTF-TF-UTF-SMF
−48.82 −37.9322 −86.7434 113.66 −119.9 −169.0879
0.059 0.0522 0.0590 0.0092 0.067 0.0464
influence the work reported in this paper.
refractive index, IEEE Photonics Technol. Lett. 24 (2012) 1130–1132. [7] P. Xian, G. Feng, Y. Ju, et al., Single-mode all-fiber structured modal interference for temperature and refractive index sensing, Laser Phys. Lett. 14 (2017) 058101. [8] Z. Tong, X. Wang, Y. Wang, et al., Dual-parameter optical fiber sensor based on fewmode fiber and spherical structure, Opt. Commun. 405 (2017) 60–65. [9] R. Xiong, H. Meng, Q. Yao, et al., Simultaneous measurement of refractive index and temperature based on modal interference, IEEE Sens. J. 14 (2014) 2524–2528. [10] H. Wang, H. Meng, R. Xiong, et al., Simultaneous measurement of refractive index and temperature based on asymmetric structures modal interference, Opt. Commun. 364 (2016) 191–194. [11] Y. Chen, Y. Wang, R. Chen, et al., A hybrid multimode interference structure based refractive index and temperature fiber sensor, IEEE Sens. J. 16 (2) (2016) 331–335. [12] Z. Tong, X. Wang, W. Zhang, L. Xue, Research on dual-parameter optical fiber sensor based on thin-core fiber and spherical structure, Laser Phys. 28 (2018) 045102. [13] Y. Gong, T. Zhao, Y.J. Rao, Y. Wu, Y. Guo, A ray-transfer-matrix model for hybrid fiber Fabry-Perot sensor based on graded-index multimode fiber, Opt. Express. 18 (2010) 15844–15852. [14] H. Fu, N. Zhao, M. Shao, Y. Xu, et al., In-fiber quasi-Michelson interferometer based on waist-enlarged fiber taper for refractive index sensing, IEEE Sens. J. 15 (2015) 6869–6874. [15] Q. Rong, X. Qiao, Y. Du, et al., In-fiber quasi-Michelson interferometer with a corecladding-mode fiber end-face mirror, Appl. Opt. 52 (2013) 1441–1447. [16] H. Lu, Y. Yue, J. Du, L. Shao, et al., Temperature and liquid refractive index sensor using P-D fiber structure-based Sagnac loop, Opt. Express. 26 (2018) 18920–18927. [17] H. Li, H. Li, F. Meng, et al., All-fiber MZI sensor based on seven-core fiber and fiber ball symmetrical structure, Opt. Lasers Eng. 112 (2019) 1–6. [18] H. Zhang, Z. Wu, P.P. Shum, et al., Highly sensitive strain sensor based on helical structure combined with Mach-Zehnder interferometer in multicore fiber, Sci. Rep. 7 (2017) 46633.
Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant No. 51627804), National Key R&D Program of China (Grant No. 2016YFC0301902), and Provincial Key R&D Program of Anhui (Grant No. 1804a0802214). References [1] Y. Dong, S. Xiao, B. Wu, H. Xiao, S. Jian, Refractive index and temperature sensor based on D-shaped fiber combined with a fiber Bragg grating, IEEE Sens. J. 19 (2019) 1362–1367. [2] Y. Zhao, L. Cai, X.G. Li, F.C. Meng, Liquid concentration measurement based on SMS fiber sensor with temperature compensation using an FBG, Sens. Actuators B: Chem. 196 (2014) 518–524. [3] Y. Bai, B. Yin, C. Liu, et al., Simultaneous Measurement of Refractive Index and Temperature Based on NFN Structure, IEEE Photonics Technol. Lett. 26 (2014) 2193–2196. [4] G. Yin, Y. Wang, C. Liao, et al., Simultaneous refractive index and temperature measurement with LPFG and liquid-filled PCF, IEEE Photonics Technol. Lett. 27 (2015) 375–378. [5] J. Li, W. Zhang, S. Gao, et al., Long-Period Fiber Grating Cascaded to an S Fiber Taper for Simultaneous Measurement of Temperature and Refractive Index, IEEE Photonics Technol. Lett. 25 (2013) 888–891. [6] Q. Han, X. Lan, J. Huang, et al., Long-period grating inscribed on concatenated double-clad and single-clad fiber for simultaneous measurement of temperature and
5
Contents lists available at ScienceDirect
Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
Refractive index and temperature sensor based on Mach-Zehnder interferometer with thin fibers
T
Wei Liu, Xuqiang Wu , Gang Zhang, Shili Li, Cheng Zuo, Shasha Fang, Benli Yu ⁎
Key Laboratory of Opto-Electronic Information Acquisition and Manipulation of Ministry of Education, Anhui University, Jiulong Road 111#, Hefei 230601, China
ARTICLE INFO
ABSTRACT
Keywords: Mach-Zehnder interferometric sensor Thin fiber Refractive index Temperature
An all-fiber Mach-Zehnder interferometric sensor for refractive index (RI) and temperature measurement is proposed and experimentally demonstrated. The proposed sensor is fabricated with three segments of two type of thin fibers sandwiched between two standard single-mode fibers (SMFs). The variation of ambient RI and temperature causes the change of phase difference between the cladding modes and the core mode, which leads to the shift of interference spectrum. The two resonance dips shift in the wavelength spectrum is used to investigate the RI and temperature characteristics of the sensor. Experimental results show that the two dips have different responses of RI and temperature, which indicates that the sensor can realize simultaneous RI and temperature measurement. The maximum sensing sensitivities are −169.0879 nm/RIU and 0.0464 nm/°C, respectively. The proposed sensor exhibits potential applications in physical, biological and chemical sensing fields due to its high sensitivity, good linearity, simple fabrication and low cost.
1. Introduction Optical fiber sensors (OFSs) have been intensively studied in the measurement of various parameters due to their characteristics of high sensitivity, compact size, explosion-proof, anti-corrosion, anti-electromagnetic, remote sensing, etc. Among them, OFSs for dual-parameter RI and temperature measurement have become the research hotspot in recent years. To date, numerous OFSs utilizing fiber Bragg gratings (FBGs) [1–3] and long-period fiber gratings (LPFGs) [4–6] have been developed for simultaneous RI and temperature measurement. Y. Dong et al. proposed a D-shaped fiber structure combined with a FBG for RI and temperature measurement. The sensitivities of −31.79 nm/RIU and 28.7 pm/°C were achieved [1]. Q. Han et al. proposed a kind of LPFG for RI and temperature measurement, which is written on a SMF and double-clad fiber with CO2 laser point-by-point irradiation [6]. The grating-based (FBG, LPFG) sensors have high sensitivity and large measurement range, but require expensive fabrication equipment and stringent procedures [7]. Moreover, OFSs based on sensing techniques such as Mach-Zehnder [7–12], Fabry-Pérot [13], Michelson [14,15], and Sagnac [16] have been widely used in RI and temperature measurement due to their high sensitivity, flexible structure, and compact size. Z. Tong et al. proposed a fewmode fiber and spherical structure based dual-parameter sensor, which presented the sensitivities of −48.82 nm/RIU and 0.059 nm/°C [8]. Y.
⁎
Chen et al. designed a hybrid multimode interference structure based sensor, which presented the sensitivities of 113.6 nm/RIU and 9.2 pm/°C [11]. H. Lu et al. fabricated a Sagnac loop based sensor by splicing polarization-maintaining fiber and D-shaped fiber. The sensor achieved the sensitivities of −1.804 nm/°C and −131.49 nm/RIU [16]. However, these interferometric OFSs usually have the following drawbacks such as complicated structure, tough fabrication, low sensitivity, or high cost. In this paper, we proposed an easily fabricated and low cost sensor for simultaneous RI and temperature measurement. The sensor consists of two types of thin fibers, named as thin fiber (TF) and ultrathin fiber (UTF), respectively. 2. Sensor fabrication and sensing principle The schematic diagram of the proposed sensor is shown in Fig. 1. The sensor is fabricated by fusion splicing a section of TF between two UTFs after fusion splicing the UTFs on the end faces of the lead-in SMF and the lead-out SMF. Since the diameters of the thin fibers are too small, the suitable arc power and duration of arc fusion splicer (FSM45PM) are required to ensure that the thin fibers will not be destroyed by high power discharge. When the arc power and duration are set as 10 bit and 450 ms, respectively, the splice loss of the sensor fabricated by the discharge is small, and the interference spectrum is substantially stable. The illustrations in Fig. 1 show the fusion splicing points a and b
Corresponding author. E-mail address: [email protected] (X. Wu).
https://doi.org/10.1016/j.yofte.2019.102101 Received 30 August 2019; Received in revised form 18 October 2019; Accepted 24 November 2019 1068-5200/ © 2019 Elsevier Inc. All rights reserved.
Optical Fiber Technology 54 (2020) 102101
W. Liu, et al.
Fig.1. The schematic diagram of the sensor. The illustrations depict the splicing points a and b observed under the microscope.
observed under the microscope. The core/cladding diameters of the SMF, TF and UTF are 9 μm/125 μm, 6.5 μm/80 μm and 4.2 μm/50 μm, respectively. The length of UTF determines the mode coupling coefficient when the light in lead-in SMF couples into the TF, which will affect the interference spectrum [9]. A relatively high coupling coefficient for a particular cladding mode can be obtained by properly choosing the length of UTF, and the length should be as short as possible so that the phase differences between the guided modes are negligible [10]. In the experiment, 1 mm length of the two UTFs is chosen to ensure compact size, low intensity loss and good core-cladding coupling. The TF with a length of 14 mm is the interference part of the sensor, and its core and cladding have refractive indices of 1.4578 and 1.444. Combined with the transmission spectrum analysis of the sensor, the choice of TF length was discussed in detail in Section 3. The light from an amplified spontaneous emission (ASE) light source is transmitted in the lead-in SMF core as a fundamental mode and then transmitted to the splicing point a. Due to the mismatch of the core diameter between the lead-in SMF and the UTF 1, the high-order cladding modes of the input light will be excited in the UTF 1. The UTF 1 acts as an input coupler. As the light continues to propagate, the core mode and the cladding modes will propagate in the core and cladding of the TF, respectively. When the light travels to the splicing point b, the high-order cladding modes will interfere with the core fundamental mode because of the mismatch of the core diameter between the TF and the UTF 2. Then the light is re-coupled into the lead-out SMF. The UTF 2 acts as an output coupler. For convenience, only the core mode and one cladding mode are taken into consideration. The phase difference between the core mode and the cladding mode can be defined as [17]:
=
2
neff L
=
c 2 (neff
cl neff )L
wheren1andn2 represent the initial ambient RI and the increased ambient RI. Since the ERI of the cladding mode will increase with the cl cl increase of ambient RI, it can be known that neff , n2 > neff , n1 . So dip will decrease as the RI increases from n1 to n2 . That means the dip will shift towards the shorter wavelength region. Assuming that the change of ERI difference between the two modes is n , The RI sensitivity Kn can be expressed as:
=
2 neff L 2k + 1
=
2L c (neff 2k + 1
= =
dip, n2
cl neff , n2 ) cl neff , n1 )]
c (neff , n1
(4)
neff
ncl =
cl cl neff T
T (5)
where and are the ERI changes of the core mode and the cladding mode. c and cl correspond to the thermal-optic coefficients of the core and the cladding, respectively. T stands for the variation of ambient temperature. By combining Eq. (5) and Eq. (2), the change of dip can be written as: dip
= = = =
ncl
dip, T2
dip, T1
2L c [(neff , T2 2k + 1 2L c [ n 2k + 1 2L T [ nc 2k + 1 c eff
cl neff , T2 )
c (neff , T1
cl neff , T1 )]
ncl] cl cl neff ]
(6)
Where T1 and T2 represent the initial ambient temperature and the increased ambient temperature. According to Eq. (6), since the thermooptic coefficient of the fiber core is higher than that of the fiber cladding and the ERI of core mode is higher than the ERI of cladding mode, dip will increase as the temperature increases from T1 to T2 . That means the dip will move to the longer wavelength region. Likewise, the ambient temperature sensitivity KT can be expressed as:
(2)
KT =
dip
T
=
2L c ( neff 2k + 1 c
cl cl neff )
(7)
According to the above analysis, it can be known that as the RI and temperature increase, the dip appears blue-shift and red-shift, respectively.
dip, n1
2L c [(neff , n2 2k + 1 2L cl [ (neff , n2 2k + 1
2L = 2k + 1
c c neff
nc
As the ambient RI increasing, the ERI of the cladding mode will increase, while the ERI of the core mode almost remains unchanged. Thus the variation of dip with the change of ambient RI can be deduced from (2) as: dip=
=
nc =
(1)
cl neff )
n
Due to the thermal expansion effect and thermo-optic effect of the TF, the ERI, diameter and length of the TF will change with the variation of ambient temperature. Since the thermal-optic coefficient is much larger than the thermal expansion coefficient for silica, the dimension change induced by temperature variation can be neglected [18]. Therefore, the ERI changes of the core mode and the cladding mode with the variation of temperature can be described by the following formulas:
where is the wavelength of the propagating light andL is the length of c cl the TF. neff and neff denote effective refractive indices of the core mode and the cladding mode. neff is the effective refractive index (ERI) difference between the core mode and the cladding mode. When phase = (2k + 1) , (k = 1, 2, 3. ..) , the difference satisfies the expression respective resonance dip can be written as [17]: dip
dip
Kn =
cl neff , n1 )]
3. Experimental results and discussion (3)
The experimental setup is illustrated in Fig. 2. Two ends of the 2
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=
neff L 0
(8)
2
where 0 and neff represent the center wavelength and the differential modal group index, respectively. The neff corresponding to peak 0.081 nm−1 is calculated equal to 0.0139, which is close to the material RI difference between the TF core and cladding. Hence the inhomogeneous interference spectrum should be mainly caused by interferences between the core mode and the cladding modes with different orders. The wavelength separation between two interference minima, known as free spectral range (FSR), can be approximated as [8]:
FSR
0
2
neff L
(9)
The FSR is calculated equal to 12.44 nm, which is close to the stripe interval between the dip A and B in the interference spectrum. The neff in Eq. (9) is the material RI difference between the TF core and cladding in the air, while the stripe interval of 12.8 nm in interference spectrum is recorded by immersing the sensor completely in the glycerin-water solution. Therefore, there is a slight difference between the FSR and the fringe interval. In the experiment, the dip A and dip B with the wavelength of 1546.2 nm and 1559 nm marked in Fig. 3(a) are selected to analyze their RI and temperature characteristics. The RI measuring performance was firstly evaluated by immersing the proposed sensor into six concentrations of solutions with the RI ranging from 1.3388 to 1.3604 (measured by an Abbe refractometer, WAY-2S) at room temperature. we accomplished each change of RI solutions by adding glycerin to the glassware and stirring at a low speed. The experimental data was recorded after the RI of the solution reached the target value and the transmission spectrum was completely stabilized. The Fig. 4 presents the measured wavelength shift of dip A and B as the ambient RI increases. It can be seen that the two dips experience blue-shift and the shift of dip A and B is −2.8 nm and −3.8 nm. The fitted RI sensitivities of dip A and B are KA,n = 132.1763 nm/RIU and KB,n = -169.0879 nm/RIU, respectively. In order to evaluate the temperature measurement performance of the proposed sensor, the sensor immersed into the 12% glycerin-water solution (RI = 1.3473) was fixed in the incubator. In the experiment, the ambient temperature rises from 30 °C to 75 °C with a temperature interval of 5 °C. The plots of the dips wavelength shift in response to the temperature change are shown in Fig. 5. One can see that the two dips show red-shift along with the temperature increasing and the shift of dip A and B is 3.4 nm and 2.6 nm. The fitted temperature sensitivities of dip A and B are KA,T = 0.0744 nm/°C and KB,T = 0.0555 nm/°C,
Fig. 2. Schematic diagram of the experiment facility.
fabricated sensor are connected to the ASE light source with a bandwidth of 1530 nm-1565 nm and the optical spectrum analyzer (OSA, AQ6370C) with a resolution of 0.02 nm, respectively. The sensor fixed in glassware is placed in an incubator with the resolution of 0.1 °C. Fig. 3(a) shows the transmission spectra when the sensors with different TF lengths are completely immersed in the glycerin-water solution (RI = 1.3388) at the room temperature of 25 °C. The black curve represents the spectrum of ASE light source when the sensor is not connected. By comparing the black curve with other color curves, the insertion losses between the ASE light source and the sensor transmission can be intuitively observed, as shown in Fig. 3 (a). By comparing the interference spectra of the sensors with different TF lengths, it is found that the interference spectrum has the highest extinction ratio of more than 18 dB when the TF length is 14 mm. So we chose the sensor with the TF length of 14 mm for simultaneous RI and temperature measurement. It can be seen from the blue curve that there are three dips in the interference spectrum, which obviously indicate that more than one cladding mode is involved in the interference. In order to get more information about the interference between propagating modes, the spatial frequency distribution is obtained by taking fast Fourier transform (FFT), as shown in Fig. 3(b). It can be seen that the spatial frequency spectrum has one dominant peak at 0.081 nm−1 and several weak peaks. The spatial frequency can be expressed as [18]:
Fig. 3. (a) Measured transmission spectra of the sensors. (b) Spatial frequency spectrum of the sensor. 3
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Fig.6. The RI of 12% glycerin-water solution versus the temperature. Fig. 4. The dips shift in response to the RI change. The inset depicts the transmission spectra for different RI of glycerol-water solutions.
temperature measurement. When the ambient RI and temperature change simultaneously, the dips wavelength shift can be expressed as: (11)
= Kn· n + KT · T
According to Eq. (11), the relationship between the dips shift and the changes of RI and temperature can be described as:
( Tn ) = K1
KB, T KB, n
KA, T KA, n
A
(12)
B
where K = KA, n·KB, T KA, T · KB, n . By substituting the measured sensitivity coefficients into Eq. (12), the matrix formula can be turned into:
1 0.0197 (169.0879 ( Tn ) = 5.2418
)
A B
(13)
The calculated cross sensitivity of the RI on the temperature is about 0.1908 °C/RIU. When the wavelength measurement resolution of OSA is 0.02 nm, the resolutions of RI and temperature can reach 1.1828 × 10-4 RIU and 0.431 °C. It is clear that the ambient RI and temperature can be measured simultaneously by monitoring the resonance dips shift. Since the sensing area of the TF have a relatively small core-cladding diameter difference. Evanescent waves are more likely to leak and the cladding ERI is more sensitive to the ambient liquid. The temperature sensitivity of the sensor is mainly controlled by the thermal-optic coefficient of TF. Therefore, compared with the sensors with similar structures mentioned in the references [8–12], the proposed sensor improves the ambient RI sensitivity while maintaining the temperature sensitivity substantially unchanged. The performance comparison of these sensors is shown in Table 1.
Fig. 5. The dips shift in response to the temperature change. The inset depicts the transmission spectra under different temperature.
respectively. Since the RI of the glycerin-water solution is affected by the ambient temperature, the fitted temperature sensitivities above should be the sum of the direct contribution of the temperature and the indirect contribution of the induced change of the RI of the solution [9]. Supposing the temperature sensitivity of the sensor can be linearly approximated as [10]:
Km, T = Km, T + Km, n RRI , T , (m = A, B )
0.0464 132.1763
4. Conclusions
(10)
In summary, we have proposed and demonstrated an all-fiber MachZehnder interferometric sensor based on UTF-TF-UTF structure. By measuring the wavelength shift of dips formed by the cladding modes interfering with the core mode, the simultaneous RI and temperature measurement have been realized. The sensitivities of RI and temperature can reach up to −169.0879 nm/RIU and 0.0464 nm/°C, and the measured RI and temperature resolutions are 1.1828 × 10-4 RIU and 0.431 °C, respectively. The sensor shows the advantages of high sensitivity, good linearity, low cost and simple fabrication, which is suitable for physical, biological and chemical sensing applications.
where Km, T and Km, T are the measured sensitivity and pure sensitivity of the temperature, respectively. The RRI , T is the dependency of the RI on temperature. In the experiment of evaluating the temperature characteristic of the sensor, after recording the spectral curves at different temperatures, we also measured the RI of the 12% glycerin-water solution at the corresponding temperature using the Abbe refractometer. The Fig. 6 shows the RI of the glycerin-water solution versus the temperature in glassware. The linear fittings to the data give a fitted value of −2.12 × 10-4 °C−1 and a value of R2 larger than 0.9969, within the temperature range from 25 to 75 °C. The pure sensitivities of the temperature of dip A and B are 0.0464 nm/°C and 0.0197 nm/°C. Due to the dip A and dip B present different spectral responses of RI and temperature, the sensor can realize simultaneous RI and
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to 4
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Table 1 The performance comparison of the sensors with similar structures (the FMF, MMF, NCF, and TCF represent few-mode fiber, multi-mode fiber, no-core fiber, and thincore fiber, respectively).
Ref. [8] Ref. [9] Ref. [10] Ref. [11] Ref. [12] The proposed sensor
Structure of sensor
RI sensitivity (nm/RIU)
Temperature sensitivity (nm/°C)
SMF ball-SMF-FMF-SMF SMF-MMF-un-coated SMF-MMF-SMF SMF-MMF-un-coated SMF-peanut-shape structure-SMF SMF-NCF-SMF-MMF-SMF SMF ball-TCF-SMF ball SMF-UTF-TF-UTF-SMF
−48.82 −37.9322 −86.7434 113.66 −119.9 −169.0879
0.059 0.0522 0.0590 0.0092 0.067 0.0464
influence the work reported in this paper.
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