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Simultaneous measurement of temperature and refractive index with F–P microcavity sensor based on graded-index few mode fiber
Optics Communications 455 (2019) 124577
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Simultaneous measurement of temperature and refractive index with F–P microcavity sensor based on graded-index few mode fiber Xinghu Fu ∗, Lianxu Liu, Shuming Huang, Guangwei Fu, Wa Jin, Weihong Bi School of Information Science and Engineering, The Key Laboratory for Special Fiber and Fiber Sensor of Hebei Province, Yanshan University, Qinhuangdao 066004, China
ARTICLE Keywords: F–P microcavity Few mode fiber Etch Temperature Refractive index
INFO
ABSTRACT In this paper, a Fabry–Perot (F–P) microcavity sensor based on graded-index few mode fiber is proposed and fabricated. One end of the graded-index few mode fiber is etched by hydrofluoric(HF) acid to form an air microcavity. The end with air microcavity is spliced with single-mode fiber, and another end is cut flat to make sensing head. Due to the generation of reflection surface, the F–P microcavity structure is formed between the surfaces of microcavity and the fiber tip. Simulation and experimental results show that the reflection spectrum will shift regularly with the change of external temperature, and the extinction ratio of that will also change regularly with the change of external solution concentration. The temperature and refractive index can be measured by detecting the wavelength shift and extinction ratio change in spectrum. When the temperature changes between 30◦ C∼90◦ C, the temperature sensitivity can be up to 10.52pm/◦ C. When the refractive index changes between 1.3331∼1.3568, the maximum refractive index sensitivity is -16.03dB/RIU. This fiber F–P microcavity sensor has different responses to the change of temperature and liquid refractive index, it can be used to measure both parameters simultaneously.
1. Introduction Fabry–Perot (F–P)interferometer sensor is expected to achieve an intelligent measurement, so it has been paid more and more attention in research of fiber sensor. Especially, the fiber F–P sensor have many advantages in small size, anti-electromagnetic interference and high sensitivity, it has been widely used in the high-precision measurement [1–3]. In F–P sensor, the change of optical path difference (OPD) is affected by cavity length, refractive index(RI) in cavity, and facet reflectivity. It will cause the wavelength shift or extinction ratio change in spectrum. Thereby, different types of F–P sensors are used to measure various parameters, such as temperature [4], RI [5,6], atmospheric pressure [7,8], strain and curvature [9–11]. At present, the fabrication of fiber F–P interferometer sensors based on endface micromachining mainly include arc discharge, chemical corrosion and laser processing method. In 2012, Pevec and Donlagic [12] proposed an F–P interferometer for simultaneous measurement of pressure and temperature, and these F–P cavities were prepared with different lengths. In 2013, Di Wu et al. [13] used two SMFs to form an intrinsic fiber–optic F–P interferometer. At the fusion point, the film is formed by the arc discharge and regard as a reflector, the reflected light intensity is increased. This sensor can realize the measurement of temperature and RI. ChengLing Lee et al. [14] proposed an ultra compact microcavity fiber F–P interferometer based on an ultra thin
film of gold (Au) embedded in a SMF endface, this sensor can measure the RI of surrounding and high temperature. In 2015, Di Wu et al. [15] used a large fusion power and a long fusion time to splice the SMF and multimode photonic crystal fiber(MPCF), the air-holes of MPCF were collapsed to form a thin film and regard as the reflection surface, this sensor can realize the measurement of temperature and RI simultaneously. Jiacheng Li et al. [16] etched the photonic crystal fiber with HF acid to remove the cladding of the core air-hole, and then spliced with the SMF to form an F–P microcavity sensor, which is insensitive to temperature and has an external RI sensitivity of 358.27 dB/RIU. In 2017, Pevec et al. [17] proposed the stacking three different F–P cavities to make a multi-cavity F–P sensor for measuring gas pressure, RI, temperature and thermal conductivity. In 2018, Yinggang Liu et al. [18] used excimer lasers to process microcavities on SMF, the F–P sensor is performed for temperature and pressure measurements with sensitivity of 10.8 pm/◦ C and 4.1587 nm/MPa, respectively. Based on the above research results, we can see that there are many studies on the dual-parameter measurement of F–P microcavity sensor. Especially, some methods are applied to increase the reflectivity at the fusion point and can be used to measure the temperature and RI simultaneously. In this paper, a temperature and RI sensor based on a graded-index few mode fiber(GI-FMF) F–P microcavity structure is proposed. The sensor consists of a GI-FMF, an air microcavity and a SMF. One end of GI-FMF is etched with HF acid and then spliced with SMF, and another
∗ Corresponding author. E-mail addresses: [email protected] (X. Fu), [email protected] (W. Bi).
https://doi.org/10.1016/j.optcom.2019.124577 Received 19 June 2019; Received in revised form 6 August 2019; Accepted 12 September 2019 Available online 16 September 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.
X. Fu, L. Liu, S. Huang et al.
Optics Communications 455 (2019) 124577
Fig. 3. Photomicrograph of F–P microcavity structure.
discharge fusion splice, and tip cutting. The schematic diagram of sensor fabrication process is shown in Fig. 2. In fabrication process, the main instruments are high-precision fiber cutting device and fiber fusion splicer. The fabrication steps are as follows. Firstly, a section of GI-FMF is used with no coating, and one end of that is cut flat. Secondly, the GI-FMF with one end cut flat is placed vertically in 40% HF acid for 12 min at room temperature. Thirdly, the etched end of GI-FMF is put into the distilled water for 3–5 min to remove the residual HF acid. Fourthly, the end of the GIFMF with the microcavity is manually spliced with SMF by the fiber fusion splicer, wherein the SMF is used for the input and output of optical signals. Finally, another end of the GI-FMF is cut flat, and a sensing head has been fabricated successfully. The photomicrograph of F–P microcavity structure is shown in Fig. 3. In Fig. 3, M1, M2 is the front and back reflection surface of the air microcavity, and 𝐿0 is the length of that. Similarly, M2, M3 is also the reflection surface of GI-FMF microcavity, and L is the length of that.
Fig. 1. Cross-section and RI profile of GI-FMF.
end is cut flat as sensing head. This sensor is an all-fiber structure and have many advantages, such as small in size, easy to manufacture, and simple in structure. It only needs to insert the sensing head into the environment to be tested, and can measure temperature and RI simultaneously. 2. Sensor fabrication and principle 2.1. Fiber parameters The cross-section and RI profile of GI-FMF are shown in Fig. 1. The GI-FMF core radius a is 9.93 μm, the inner cladding radius b is 15.49 μm, and the outer cladding radius c is 62.5 μm. It is fabricated by Yangtze Optical Fiber and Cable Joint Stock Limited Company, China. The SMF core radius is 4.5 μm and the cladding radius is 62.5 μm.
2.3. Sensing principle According to Fig. 3, the schematic diagram of microcavity structure is shown in Fig. 4. In Fig. 4, it consists of three reflection surfaces as M1, M2, M3. The sensor has three F–P microcavities, the first one is an etched groove microcavity F-P1 , which involves M1 and M2. The second microcavity F-P2 and third microcavity F-P3 of that involve M2 and M3, M1 and M3, respectively. Due to the difference in optical path difference
2.2. Sensor fabrication The fabrication of the sensor is mainly divided into the following five steps, including endface cut flat, HF acid etch, cleaning,
Fig. 2. Fabrication process of sensor: (a) endface cut flat (b) HF acid etch (c) cleaning (d) discharge fusion splice (e) tip cutting.
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Optics Communications 455 (2019) 124577
Fig. 4. Schematic diagram of F–P microcavity structure.
Fig. 5. The reflection spectrum of F–P microcavity structure with different L: (a) 200 μm (b) 400 μm (c) 1000 μm.
(OPD) between the two cavities of F-P2 and F-P3 is small, that is, the contribution of free spectral range (FSR) in two cavities to the spectral intensity is almost similar, so it is difficult to distinguish. Therefore, we can think that the high-frequency interference fringes are the result of the interaction of two F–P cavities. In the proposed F–P microcavity structure, the effective RI of the SMF core and the GI-FMF core are 1.4675 and 1.4682 respectively. There is a small difference between them, which is only 0.0007. If the SMF and GI-FMF are directly spliced together to form an SMF– FMF tip, the reflected light intensity at fusion point between SMF and GI-FMF will be extremely small. At the end of the GI-FMF, the reflected light intensity at the interface between GI-FMF and air is very strong. The F–P interference is formed by two reflected lights and not obvious. The interference intensity is small, which is difficult to use in sensing experiments. We consulted the literature similar to the structure of SMF–FMF tip [13–15] and found that the introduction of
a new reflection surface at fusion point not only improves the facet reflectivity, but also increases the reflected light intensity. Therefore, an air microcavity is introduced in the proposed sensor structure. In experiment, we have etched the end of GI-FMF by HF acid, and an air microcavity is formed at the fusion point with SMF, so that the sensor have three reflection surfaces and three beam interference occur. Due to the introduction of air microcavity, each reflection surface is located at the interface between the fiber core and air. In this way, it improves the facet reflectivity and increases the reflected light intensity, so that the interference effect is more obvious. For the air microcavity consisting of M1 and M2, the reflection spectrum is typical of two-beam interference. The reflectances of M1 and M2 is defined as 𝑅1 and 𝑅2 , respectively. The light intensity [19] of beam entering the SMF after two reflections can be expressed as [( )( ) √ ]2 𝑅2 𝐼 𝑟 = 𝑅1 + 1 − 𝑘 1 1 − 𝑅1 3
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Optics Communications 455 (2019) 124577
Fig. 6. The reflection spectrum of F–P microcavity structure with different 𝐿0 : (a) 10 μm (b) 20 μm (c) 30 μm.
Fig. 7. Simulation results of F–P sensor: (a) temperature (b) RI.
( )( )√ + 2𝑅1 1 − 𝑘1 1 − 𝑅1 𝑅2 cos(2𝜑1 )
(1)
Three-beam interferometer is composed of M1, M2 and M3. The reflectance of M3 is 𝑅3 , and the total light intensity after three reflections [19] can be expressed as [( )( ) √ ]2 𝑅2 𝐼𝑟𝑟 = 𝑅1 + 1 − 𝑘1 1 − 𝑅1 [( )( )( )( ) √ ]2 + 1 − 𝑘 1 1 − 𝑘 2 1 − 𝑅1 1 − 𝑅2 𝑅3 [ ( )] (2) ( )( )( )( )√ +2 1 − 𝑘1 1 − 𝑘2 1 − 𝑅1 1 − 𝑅2 𝑅1 𝑅3 cos 2 𝜑1 + 𝜑2 √ ( )( ) ( ) +2 1 − 𝑘1 1 − 𝑅1 𝑅1 𝑅2 cos 2𝜑1 ( )2 ( )( )2 ( )√ ( ) +2 1 − 𝑘1 1 − 𝑘 2 1 − 𝑅1 1 − 𝑅2 𝑅2 𝑅3 cos 2𝜑2
where 𝑘1 is the transmission loss of air microcavity. 𝜑1 = 2𝜋𝑛1 𝐿1 ∕𝜆, 𝜑1 is the phase shift caused by the transmission of light in air microcavity, 𝑛1 is the effective RI of air in microcavity, 𝜆 is the wavelength of
where, 𝑘2 is the transmission loss of GI-FMF microcavity. 𝜑2 = 2𝜋𝑛2 𝐿2 ∕ 𝜆, 𝜑2 is the phase shift caused by the transmission of light in GI-FMF microcavity, 𝑛2 is the effective RI of material in GI-FMF microcavity.
incident light. 4
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Optics Communications 455 (2019) 124577
Fig. 8. The measuring device of F–P microcavity sensor.
Fig. 9. Temperature experimental spectrum of different L: (a) 194.65 μm (b) 401.19 μm (c) 1000 μm.
Due to the thermal expansion and thermo-optic effect of material
coefficient of air microcavity is about −5.6 × 10−7 /◦ C at room temper-
in the interferometer, the phase shift will changes with temperature,
ature and standard atmospheric pressure. For the core material in GI-
which causes the interference wavelength to shift. The thermal expan-
FMF microcavity, the thermo-optic coefficient is about 10−5 ∼10−6 /◦ C.
sion coefficient of quartz is about 5.5 ×
10−7 /◦ C.
The thermo-optic
We can see that the FSR1 of air microcavity changes very little with 5
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Optics Communications 455 (2019) 124577
Fig. 10. Temperature experimental spectrum of different L in high-frequency band: (a) 194.65 μm (b) 401.19 μm (c) 1000 μm.
3. Experiment and analysis
temperature [20], which is negligible compared with the temperature change in the FSR2 of GI-FMF microcavity.
3.1. Temperature experiment
According to the Eq. (2), when 𝐿0 is 20 μm and L is 200 μm, 400 μm and 1000 μm, the reflection spectrum of F–P microcavity structure is
The measuring device of F–P microcavity sensor is shown in Fig. 8, which composed of broadband optical source (ASE, wavelength range of 1520∼1610 nm), optical spectrum analyzer (OSA, AQ6375), optical circulator and F–P microcavity sensor. The temperature control device(WHL-30B) have a adjustable range of 0∼100 ◦ C and a resolution of 0.1 ◦ C. In experiment, this sensor is placed in different temperature for detect its characteristic. The light of ASE broadband source enters into one end of the circulator through the SMF, and exits from another end into the sensor. The reflected signal is generated by F–P interferometer in sensor, then enter into the OSA through one end of circulator. In this experiment, the F–P microcavity sensor is placed in a temperature control device, the temperature is raised from 30 ◦ C to 90 ◦ C and record data every 10 ◦ C. Whenever the temperature reaches the predicted value, it is held for 5 min under the constant condition to obtain an accurate result. When L is 194.65 μm, 401.19 μm and 1000 μm, the experimental spectra in different temperature are shown in Fig. 9. Meanwhile, 𝐿0 is 17.94 μm. It can be seen from Fig. 9, as the length of L increases, the spectral density becomes large. The result is the same as simulation result in Fig. 5. Their spectral periods are slightly different, mainly due to the difference in 𝐿0 caused by microcavity fabrication. Moreover, the spectrum does not change in low-frequency band (spectral envelope), but in high-frequency band (spectral thin fringe) occurs red shift. GIFMF microcavity (F–P2 ) is the main factor causing the red shift of the
shown in Fig. 5, respectively. When L is 200 μm and 𝐿0 is 10 μm, 20 μm and 30 μm, the reflection spectrum of that is shown in Fig. 6, respectively. It can be seen from Fig. 5, when 𝐿0
is constant, the spectral
density increases gradually with the increase of L. In Fig. 6, when L is constant, the spectral envelope period decreases gradually with the increase of 𝐿0 . Therefore, the length of air microcavity 𝐿0 affects the size of spectral envelope and FSR1 , and the larger 𝐿0 is, the smaller the spectral envelope period is. The length of GI-FMF microcavity L affects the spectral density and FSR2 , the larger L is, the denser the spectrum is. Moreover, we analyze the temperature and RI characteristic of F–P sensor, 𝐿0 and L is defined as 20 μm and 200 μm, respectively. When the external temperature increase from 0 ◦ C to 100 ◦ C, the reflection spectrum is shown in Fig. 7(a). When the RI of solution is increased from 1.3 to 1.4, the simulation result is shown in Fig. 7(b). In Fig. 7, it can be seen that when the temperature rises, the spectrum shifts to the long wavelength band, which is a red shift phenomenon. When the RI increases, the contrast of reflection spectrum decreases significantly. Next, we conduct temperature and RI experiments to verify the theoretical analysis results. 6
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Optics Communications 455 (2019) 124577
Fig. 11. Temperature fitting results of different L: (a) 194.65 μm (b) 401.19 μm (c) 1000 μm.
spectrum in temperature rise, and air microcavity (F–P1 ) has almost no effect on the temperature. In entire reflection spectrum, the peaks or troughs near the wavelength to be measured change in the same trend. Therefore, the temperature or refractive index sensitivity can be analyzed at different wavelengths, and the results obtained are almost equal. In order to facilitate the analysis of the change in reflectance spectrum, we have modified the abscissa to the same wavelength range. The experimental spectra in high-frequency band of three sensors for detecting the temperature are shown in Fig. 10, and the corresponding fitting results between temperature and wavelength are shown in Fig. 11. It can be clearly seen that when the temperature increase from 30 ◦ C to 90 ◦ C, the spectrum occur red shift. The corresponding temperature sensitivity is 9.04 pm/◦ C, 10.52 pm/◦ C, and 10.11 pm/◦ C.
data are the same as simulation results. The reason is that three beam interference occurs in microcavity structure, when the concentration of external solution changes, the reflection efficiency changes when the third beam is reflected. The RI of solution is closer to the that of core, and reflection efficiency is lower. Therefore, when the RI of solution increase from 1.3331 to 1.3568, reflection efficiency of the third beam decreases, so that the energy and EXT of interference spectrum decrease. It can be seen from the relationship between RI and EXT, the RI sensitivity of three sensors are −12.99 dB/RIU, −10.49 dB/RIU and −16.03 dB/RIU, respectively. The linearity is good. 3.3. Sensor verification experiment When the sensor measures the external physical quantity, if the different outcome variables are produced by different physical quantities and do not affect each other, we can first detect these physical quantities separately to obtain their sensitivities, and then verification experiment are carried out. In this way, we can demonstrate that the sensor can measure two physical quantities simultaneously. There are many scholars who have done similar research. For example, Ben Xu et al. [4] proposed a hybrid fiber F–P interferometer, and achieved simultaneous measurement of temperature and RI by observing the wavelength shift and the change of light intensity respectively. X. L. Tan et al. [6] measured the temperature and RI sensitivity of the microhemisphere-based fiber–optic F–P sensor respectively. The verification experiments showed that the results of temperature and RI were no cross influence, so simultaneous measurement of temperature and RI were achieved. Yinggang Liu et al. [18] used the fiber FPI sensor to measure temperature and gas pressure respectively, and verified by experiments that the spectral changes caused
3.2. Refractive index experiment The experimental setup is shown in Fig. 8. At room temperature(25 the sensor is placed in different concentrations of NaCl solution to measure the RI sensing characteristics. The RI range is from 1.3331 to 1.3568, which corresponding to different concentrations of NaCl solution. The experimental spectra in high-frequency band of three sensors for detecting the RI are shown in Fig. 12, and corresponding fitting results between RI and Extinction Ratio (EXT, the difference in intensity between the peak and trough near the wavelength to be measured) are shown in Fig. 13. It can be seen from Figs. 12 and 13 that when the concentration of external solution changes, the contrast of spectrum changes. Further, the EXT decreases with the increase of external RI, the experimental ◦ C),
7
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Optics Communications 455 (2019) 124577
Fig. 12. RI experimental spectrum of different L in high-frequency band: (a) 194.65 μm (b) 401.19 μm (c) 1000 μm.
matrix of temperature and RI can be obtained as [ ] [ ][ ] 𝛥𝜆 𝐾1 0 𝛥𝑇 = 𝛥𝐼 0 𝐾2 𝛥𝑁
by the two physical quantities do not affect each other. So the simultaneous measurement of temperature and gas pressure was achieved by observing the changes of low-frequency and high-frequency waveforms. In our study, we know that the EXT of reflection spectrum is directly related to RI, and the wavelength shift of spectrum is directly related to temperature, The results do not affect each other. Therefore, the simultaneous measurement of temperature and RI can be achieved by detecting the EXT change and wavelength shift of reflection spectrum. In order to verify the sensing ability to measure both temperature and RI at the same time, the sensor is placed into solution to detected the EXT change and wavelength shift at different temperatures. The RI of solution is 1.3331. When the water bath temperature is increased from 30 ◦ C to 70 ◦ C, the EXT change and wavelength shift are shown in Fig. 14. It can be seen from Fig. 14 that when temperature increase, EXT hardly changes, and the wavelength appears red shift, which is consistent with experimental results and the sensitivity does not interfere with each other. When the external temperature and RI change, wavelength shift 𝛥𝜆 is approximately linear with the change of temperature 𝛥𝑇 . The same goes for the change in EXT 𝛥𝐼 and RI 𝛥𝑁. These linear relationship can be expressed as 𝛥𝜆 = 𝐾1 𝛥𝑇
(3)
𝛥𝐼 = 𝐾2 𝛥𝑁
(4)
[
The inverse matrix of Eq. (5) can be obtained ] [ ]−1 [ ] 𝛥𝑇 𝐾1 0 𝛥𝜆 = 𝛥𝑁 0 𝐾2 𝛥𝐼
(5)
(6)
Thereby, we can see that when wavelength shift 𝛥𝜆 and EXT change 𝛥𝐼 is obtained by OSA in experiment, then the data is substituted into Eq. (6), the simultaneous measurement of temperature and RI can be realized. For example, when the length of air microcavity is 17.94 μm and the length of remaining GI-FMF microcavity is 194.65 μm, the temperature and RI sensitivity is 9.04 pm/◦ C and −12.99 dB/RIU respectively, that is 𝐾1 = 9.04 and 𝐾2 = 12.99, the matrix equation of temperature and RI for the F–P microcavity sensor is obtained as [ ] [ ]−1 [ ] 𝛥𝑇 9.04 0 𝛥𝜆 = (7) 𝛥𝑁 0 −12.99 𝛥𝐼 4. Conclusion A sensor based on the GI-FMF to simultaneous measurement of temperature and RI is proposed. One end of GI-FMF is etched by HF acid and is spliced with SMF, and another end is cut flat to form a sensing head. Three sensing heads with GI-FMF microcavity length of 194.65 μm, 401.1 μm, and 1000 μm are selected to conduct simulation
where 𝐾1 and 𝐾2 are the temperature and RI sensitivity coefficients of the sensor, respectively. Combination with Eqs. (3) and (4), the change 8
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Optics Communications 455 (2019) 124577
Fig. 13. RI fitting results of different L: (a) 194.65 μm (b) 401.19 μm (c) 1000 μm.
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Fig. 14. EXT change and wavelength shift at different temperature.
and experimental measurement of temperature and RI. The experimental results show that the temperature sensitivity can be up to 10.52 pm/◦ C, and the maximum RI sensitivity is −16.03 dB/RIU. Due to the advantages of all-fiber, microprobe structure and high sensitivity, it can be used for temperature and RI measurement in extreme environments.
Acknowledgments
This work has been supported by National Natural Science Foundation of China (61575170, 61605168), the State Scholarship Fund of China (201708130199), the key basic research program of Hebei province (17961701D). 9
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Simultaneous measurement of temperature and refractive index with F–P microcavity sensor based on graded-index few mode fiber Xinghu Fu ∗, Lianxu Liu, Shuming Huang, Guangwei Fu, Wa Jin, Weihong Bi School of Information Science and Engineering, The Key Laboratory for Special Fiber and Fiber Sensor of Hebei Province, Yanshan University, Qinhuangdao 066004, China
ARTICLE Keywords: F–P microcavity Few mode fiber Etch Temperature Refractive index
INFO
ABSTRACT In this paper, a Fabry–Perot (F–P) microcavity sensor based on graded-index few mode fiber is proposed and fabricated. One end of the graded-index few mode fiber is etched by hydrofluoric(HF) acid to form an air microcavity. The end with air microcavity is spliced with single-mode fiber, and another end is cut flat to make sensing head. Due to the generation of reflection surface, the F–P microcavity structure is formed between the surfaces of microcavity and the fiber tip. Simulation and experimental results show that the reflection spectrum will shift regularly with the change of external temperature, and the extinction ratio of that will also change regularly with the change of external solution concentration. The temperature and refractive index can be measured by detecting the wavelength shift and extinction ratio change in spectrum. When the temperature changes between 30◦ C∼90◦ C, the temperature sensitivity can be up to 10.52pm/◦ C. When the refractive index changes between 1.3331∼1.3568, the maximum refractive index sensitivity is -16.03dB/RIU. This fiber F–P microcavity sensor has different responses to the change of temperature and liquid refractive index, it can be used to measure both parameters simultaneously.
1. Introduction Fabry–Perot (F–P)interferometer sensor is expected to achieve an intelligent measurement, so it has been paid more and more attention in research of fiber sensor. Especially, the fiber F–P sensor have many advantages in small size, anti-electromagnetic interference and high sensitivity, it has been widely used in the high-precision measurement [1–3]. In F–P sensor, the change of optical path difference (OPD) is affected by cavity length, refractive index(RI) in cavity, and facet reflectivity. It will cause the wavelength shift or extinction ratio change in spectrum. Thereby, different types of F–P sensors are used to measure various parameters, such as temperature [4], RI [5,6], atmospheric pressure [7,8], strain and curvature [9–11]. At present, the fabrication of fiber F–P interferometer sensors based on endface micromachining mainly include arc discharge, chemical corrosion and laser processing method. In 2012, Pevec and Donlagic [12] proposed an F–P interferometer for simultaneous measurement of pressure and temperature, and these F–P cavities were prepared with different lengths. In 2013, Di Wu et al. [13] used two SMFs to form an intrinsic fiber–optic F–P interferometer. At the fusion point, the film is formed by the arc discharge and regard as a reflector, the reflected light intensity is increased. This sensor can realize the measurement of temperature and RI. ChengLing Lee et al. [14] proposed an ultra compact microcavity fiber F–P interferometer based on an ultra thin
film of gold (Au) embedded in a SMF endface, this sensor can measure the RI of surrounding and high temperature. In 2015, Di Wu et al. [15] used a large fusion power and a long fusion time to splice the SMF and multimode photonic crystal fiber(MPCF), the air-holes of MPCF were collapsed to form a thin film and regard as the reflection surface, this sensor can realize the measurement of temperature and RI simultaneously. Jiacheng Li et al. [16] etched the photonic crystal fiber with HF acid to remove the cladding of the core air-hole, and then spliced with the SMF to form an F–P microcavity sensor, which is insensitive to temperature and has an external RI sensitivity of 358.27 dB/RIU. In 2017, Pevec et al. [17] proposed the stacking three different F–P cavities to make a multi-cavity F–P sensor for measuring gas pressure, RI, temperature and thermal conductivity. In 2018, Yinggang Liu et al. [18] used excimer lasers to process microcavities on SMF, the F–P sensor is performed for temperature and pressure measurements with sensitivity of 10.8 pm/◦ C and 4.1587 nm/MPa, respectively. Based on the above research results, we can see that there are many studies on the dual-parameter measurement of F–P microcavity sensor. Especially, some methods are applied to increase the reflectivity at the fusion point and can be used to measure the temperature and RI simultaneously. In this paper, a temperature and RI sensor based on a graded-index few mode fiber(GI-FMF) F–P microcavity structure is proposed. The sensor consists of a GI-FMF, an air microcavity and a SMF. One end of GI-FMF is etched with HF acid and then spliced with SMF, and another
∗ Corresponding author. E-mail addresses: [email protected] (X. Fu), [email protected] (W. Bi).
https://doi.org/10.1016/j.optcom.2019.124577 Received 19 June 2019; Received in revised form 6 August 2019; Accepted 12 September 2019 Available online 16 September 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.
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Optics Communications 455 (2019) 124577
Fig. 3. Photomicrograph of F–P microcavity structure.
discharge fusion splice, and tip cutting. The schematic diagram of sensor fabrication process is shown in Fig. 2. In fabrication process, the main instruments are high-precision fiber cutting device and fiber fusion splicer. The fabrication steps are as follows. Firstly, a section of GI-FMF is used with no coating, and one end of that is cut flat. Secondly, the GI-FMF with one end cut flat is placed vertically in 40% HF acid for 12 min at room temperature. Thirdly, the etched end of GI-FMF is put into the distilled water for 3–5 min to remove the residual HF acid. Fourthly, the end of the GIFMF with the microcavity is manually spliced with SMF by the fiber fusion splicer, wherein the SMF is used for the input and output of optical signals. Finally, another end of the GI-FMF is cut flat, and a sensing head has been fabricated successfully. The photomicrograph of F–P microcavity structure is shown in Fig. 3. In Fig. 3, M1, M2 is the front and back reflection surface of the air microcavity, and 𝐿0 is the length of that. Similarly, M2, M3 is also the reflection surface of GI-FMF microcavity, and L is the length of that.
Fig. 1. Cross-section and RI profile of GI-FMF.
end is cut flat as sensing head. This sensor is an all-fiber structure and have many advantages, such as small in size, easy to manufacture, and simple in structure. It only needs to insert the sensing head into the environment to be tested, and can measure temperature and RI simultaneously. 2. Sensor fabrication and principle 2.1. Fiber parameters The cross-section and RI profile of GI-FMF are shown in Fig. 1. The GI-FMF core radius a is 9.93 μm, the inner cladding radius b is 15.49 μm, and the outer cladding radius c is 62.5 μm. It is fabricated by Yangtze Optical Fiber and Cable Joint Stock Limited Company, China. The SMF core radius is 4.5 μm and the cladding radius is 62.5 μm.
2.3. Sensing principle According to Fig. 3, the schematic diagram of microcavity structure is shown in Fig. 4. In Fig. 4, it consists of three reflection surfaces as M1, M2, M3. The sensor has three F–P microcavities, the first one is an etched groove microcavity F-P1 , which involves M1 and M2. The second microcavity F-P2 and third microcavity F-P3 of that involve M2 and M3, M1 and M3, respectively. Due to the difference in optical path difference
2.2. Sensor fabrication The fabrication of the sensor is mainly divided into the following five steps, including endface cut flat, HF acid etch, cleaning,
Fig. 2. Fabrication process of sensor: (a) endface cut flat (b) HF acid etch (c) cleaning (d) discharge fusion splice (e) tip cutting.
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Optics Communications 455 (2019) 124577
Fig. 4. Schematic diagram of F–P microcavity structure.
Fig. 5. The reflection spectrum of F–P microcavity structure with different L: (a) 200 μm (b) 400 μm (c) 1000 μm.
(OPD) between the two cavities of F-P2 and F-P3 is small, that is, the contribution of free spectral range (FSR) in two cavities to the spectral intensity is almost similar, so it is difficult to distinguish. Therefore, we can think that the high-frequency interference fringes are the result of the interaction of two F–P cavities. In the proposed F–P microcavity structure, the effective RI of the SMF core and the GI-FMF core are 1.4675 and 1.4682 respectively. There is a small difference between them, which is only 0.0007. If the SMF and GI-FMF are directly spliced together to form an SMF– FMF tip, the reflected light intensity at fusion point between SMF and GI-FMF will be extremely small. At the end of the GI-FMF, the reflected light intensity at the interface between GI-FMF and air is very strong. The F–P interference is formed by two reflected lights and not obvious. The interference intensity is small, which is difficult to use in sensing experiments. We consulted the literature similar to the structure of SMF–FMF tip [13–15] and found that the introduction of
a new reflection surface at fusion point not only improves the facet reflectivity, but also increases the reflected light intensity. Therefore, an air microcavity is introduced in the proposed sensor structure. In experiment, we have etched the end of GI-FMF by HF acid, and an air microcavity is formed at the fusion point with SMF, so that the sensor have three reflection surfaces and three beam interference occur. Due to the introduction of air microcavity, each reflection surface is located at the interface between the fiber core and air. In this way, it improves the facet reflectivity and increases the reflected light intensity, so that the interference effect is more obvious. For the air microcavity consisting of M1 and M2, the reflection spectrum is typical of two-beam interference. The reflectances of M1 and M2 is defined as 𝑅1 and 𝑅2 , respectively. The light intensity [19] of beam entering the SMF after two reflections can be expressed as [( )( ) √ ]2 𝑅2 𝐼 𝑟 = 𝑅1 + 1 − 𝑘 1 1 − 𝑅1 3
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Optics Communications 455 (2019) 124577
Fig. 6. The reflection spectrum of F–P microcavity structure with different 𝐿0 : (a) 10 μm (b) 20 μm (c) 30 μm.
Fig. 7. Simulation results of F–P sensor: (a) temperature (b) RI.
( )( )√ + 2𝑅1 1 − 𝑘1 1 − 𝑅1 𝑅2 cos(2𝜑1 )
(1)
Three-beam interferometer is composed of M1, M2 and M3. The reflectance of M3 is 𝑅3 , and the total light intensity after three reflections [19] can be expressed as [( )( ) √ ]2 𝑅2 𝐼𝑟𝑟 = 𝑅1 + 1 − 𝑘1 1 − 𝑅1 [( )( )( )( ) √ ]2 + 1 − 𝑘 1 1 − 𝑘 2 1 − 𝑅1 1 − 𝑅2 𝑅3 [ ( )] (2) ( )( )( )( )√ +2 1 − 𝑘1 1 − 𝑘2 1 − 𝑅1 1 − 𝑅2 𝑅1 𝑅3 cos 2 𝜑1 + 𝜑2 √ ( )( ) ( ) +2 1 − 𝑘1 1 − 𝑅1 𝑅1 𝑅2 cos 2𝜑1 ( )2 ( )( )2 ( )√ ( ) +2 1 − 𝑘1 1 − 𝑘 2 1 − 𝑅1 1 − 𝑅2 𝑅2 𝑅3 cos 2𝜑2
where 𝑘1 is the transmission loss of air microcavity. 𝜑1 = 2𝜋𝑛1 𝐿1 ∕𝜆, 𝜑1 is the phase shift caused by the transmission of light in air microcavity, 𝑛1 is the effective RI of air in microcavity, 𝜆 is the wavelength of
where, 𝑘2 is the transmission loss of GI-FMF microcavity. 𝜑2 = 2𝜋𝑛2 𝐿2 ∕ 𝜆, 𝜑2 is the phase shift caused by the transmission of light in GI-FMF microcavity, 𝑛2 is the effective RI of material in GI-FMF microcavity.
incident light. 4
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Optics Communications 455 (2019) 124577
Fig. 8. The measuring device of F–P microcavity sensor.
Fig. 9. Temperature experimental spectrum of different L: (a) 194.65 μm (b) 401.19 μm (c) 1000 μm.
Due to the thermal expansion and thermo-optic effect of material
coefficient of air microcavity is about −5.6 × 10−7 /◦ C at room temper-
in the interferometer, the phase shift will changes with temperature,
ature and standard atmospheric pressure. For the core material in GI-
which causes the interference wavelength to shift. The thermal expan-
FMF microcavity, the thermo-optic coefficient is about 10−5 ∼10−6 /◦ C.
sion coefficient of quartz is about 5.5 ×
10−7 /◦ C.
The thermo-optic
We can see that the FSR1 of air microcavity changes very little with 5
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Optics Communications 455 (2019) 124577
Fig. 10. Temperature experimental spectrum of different L in high-frequency band: (a) 194.65 μm (b) 401.19 μm (c) 1000 μm.
3. Experiment and analysis
temperature [20], which is negligible compared with the temperature change in the FSR2 of GI-FMF microcavity.
3.1. Temperature experiment
According to the Eq. (2), when 𝐿0 is 20 μm and L is 200 μm, 400 μm and 1000 μm, the reflection spectrum of F–P microcavity structure is
The measuring device of F–P microcavity sensor is shown in Fig. 8, which composed of broadband optical source (ASE, wavelength range of 1520∼1610 nm), optical spectrum analyzer (OSA, AQ6375), optical circulator and F–P microcavity sensor. The temperature control device(WHL-30B) have a adjustable range of 0∼100 ◦ C and a resolution of 0.1 ◦ C. In experiment, this sensor is placed in different temperature for detect its characteristic. The light of ASE broadband source enters into one end of the circulator through the SMF, and exits from another end into the sensor. The reflected signal is generated by F–P interferometer in sensor, then enter into the OSA through one end of circulator. In this experiment, the F–P microcavity sensor is placed in a temperature control device, the temperature is raised from 30 ◦ C to 90 ◦ C and record data every 10 ◦ C. Whenever the temperature reaches the predicted value, it is held for 5 min under the constant condition to obtain an accurate result. When L is 194.65 μm, 401.19 μm and 1000 μm, the experimental spectra in different temperature are shown in Fig. 9. Meanwhile, 𝐿0 is 17.94 μm. It can be seen from Fig. 9, as the length of L increases, the spectral density becomes large. The result is the same as simulation result in Fig. 5. Their spectral periods are slightly different, mainly due to the difference in 𝐿0 caused by microcavity fabrication. Moreover, the spectrum does not change in low-frequency band (spectral envelope), but in high-frequency band (spectral thin fringe) occurs red shift. GIFMF microcavity (F–P2 ) is the main factor causing the red shift of the
shown in Fig. 5, respectively. When L is 200 μm and 𝐿0 is 10 μm, 20 μm and 30 μm, the reflection spectrum of that is shown in Fig. 6, respectively. It can be seen from Fig. 5, when 𝐿0
is constant, the spectral
density increases gradually with the increase of L. In Fig. 6, when L is constant, the spectral envelope period decreases gradually with the increase of 𝐿0 . Therefore, the length of air microcavity 𝐿0 affects the size of spectral envelope and FSR1 , and the larger 𝐿0 is, the smaller the spectral envelope period is. The length of GI-FMF microcavity L affects the spectral density and FSR2 , the larger L is, the denser the spectrum is. Moreover, we analyze the temperature and RI characteristic of F–P sensor, 𝐿0 and L is defined as 20 μm and 200 μm, respectively. When the external temperature increase from 0 ◦ C to 100 ◦ C, the reflection spectrum is shown in Fig. 7(a). When the RI of solution is increased from 1.3 to 1.4, the simulation result is shown in Fig. 7(b). In Fig. 7, it can be seen that when the temperature rises, the spectrum shifts to the long wavelength band, which is a red shift phenomenon. When the RI increases, the contrast of reflection spectrum decreases significantly. Next, we conduct temperature and RI experiments to verify the theoretical analysis results. 6
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Optics Communications 455 (2019) 124577
Fig. 11. Temperature fitting results of different L: (a) 194.65 μm (b) 401.19 μm (c) 1000 μm.
spectrum in temperature rise, and air microcavity (F–P1 ) has almost no effect on the temperature. In entire reflection spectrum, the peaks or troughs near the wavelength to be measured change in the same trend. Therefore, the temperature or refractive index sensitivity can be analyzed at different wavelengths, and the results obtained are almost equal. In order to facilitate the analysis of the change in reflectance spectrum, we have modified the abscissa to the same wavelength range. The experimental spectra in high-frequency band of three sensors for detecting the temperature are shown in Fig. 10, and the corresponding fitting results between temperature and wavelength are shown in Fig. 11. It can be clearly seen that when the temperature increase from 30 ◦ C to 90 ◦ C, the spectrum occur red shift. The corresponding temperature sensitivity is 9.04 pm/◦ C, 10.52 pm/◦ C, and 10.11 pm/◦ C.
data are the same as simulation results. The reason is that three beam interference occurs in microcavity structure, when the concentration of external solution changes, the reflection efficiency changes when the third beam is reflected. The RI of solution is closer to the that of core, and reflection efficiency is lower. Therefore, when the RI of solution increase from 1.3331 to 1.3568, reflection efficiency of the third beam decreases, so that the energy and EXT of interference spectrum decrease. It can be seen from the relationship between RI and EXT, the RI sensitivity of three sensors are −12.99 dB/RIU, −10.49 dB/RIU and −16.03 dB/RIU, respectively. The linearity is good. 3.3. Sensor verification experiment When the sensor measures the external physical quantity, if the different outcome variables are produced by different physical quantities and do not affect each other, we can first detect these physical quantities separately to obtain their sensitivities, and then verification experiment are carried out. In this way, we can demonstrate that the sensor can measure two physical quantities simultaneously. There are many scholars who have done similar research. For example, Ben Xu et al. [4] proposed a hybrid fiber F–P interferometer, and achieved simultaneous measurement of temperature and RI by observing the wavelength shift and the change of light intensity respectively. X. L. Tan et al. [6] measured the temperature and RI sensitivity of the microhemisphere-based fiber–optic F–P sensor respectively. The verification experiments showed that the results of temperature and RI were no cross influence, so simultaneous measurement of temperature and RI were achieved. Yinggang Liu et al. [18] used the fiber FPI sensor to measure temperature and gas pressure respectively, and verified by experiments that the spectral changes caused
3.2. Refractive index experiment The experimental setup is shown in Fig. 8. At room temperature(25 the sensor is placed in different concentrations of NaCl solution to measure the RI sensing characteristics. The RI range is from 1.3331 to 1.3568, which corresponding to different concentrations of NaCl solution. The experimental spectra in high-frequency band of three sensors for detecting the RI are shown in Fig. 12, and corresponding fitting results between RI and Extinction Ratio (EXT, the difference in intensity between the peak and trough near the wavelength to be measured) are shown in Fig. 13. It can be seen from Figs. 12 and 13 that when the concentration of external solution changes, the contrast of spectrum changes. Further, the EXT decreases with the increase of external RI, the experimental ◦ C),
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Optics Communications 455 (2019) 124577
Fig. 12. RI experimental spectrum of different L in high-frequency band: (a) 194.65 μm (b) 401.19 μm (c) 1000 μm.
matrix of temperature and RI can be obtained as [ ] [ ][ ] 𝛥𝜆 𝐾1 0 𝛥𝑇 = 𝛥𝐼 0 𝐾2 𝛥𝑁
by the two physical quantities do not affect each other. So the simultaneous measurement of temperature and gas pressure was achieved by observing the changes of low-frequency and high-frequency waveforms. In our study, we know that the EXT of reflection spectrum is directly related to RI, and the wavelength shift of spectrum is directly related to temperature, The results do not affect each other. Therefore, the simultaneous measurement of temperature and RI can be achieved by detecting the EXT change and wavelength shift of reflection spectrum. In order to verify the sensing ability to measure both temperature and RI at the same time, the sensor is placed into solution to detected the EXT change and wavelength shift at different temperatures. The RI of solution is 1.3331. When the water bath temperature is increased from 30 ◦ C to 70 ◦ C, the EXT change and wavelength shift are shown in Fig. 14. It can be seen from Fig. 14 that when temperature increase, EXT hardly changes, and the wavelength appears red shift, which is consistent with experimental results and the sensitivity does not interfere with each other. When the external temperature and RI change, wavelength shift 𝛥𝜆 is approximately linear with the change of temperature 𝛥𝑇 . The same goes for the change in EXT 𝛥𝐼 and RI 𝛥𝑁. These linear relationship can be expressed as 𝛥𝜆 = 𝐾1 𝛥𝑇
(3)
𝛥𝐼 = 𝐾2 𝛥𝑁
(4)
[
The inverse matrix of Eq. (5) can be obtained ] [ ]−1 [ ] 𝛥𝑇 𝐾1 0 𝛥𝜆 = 𝛥𝑁 0 𝐾2 𝛥𝐼
(5)
(6)
Thereby, we can see that when wavelength shift 𝛥𝜆 and EXT change 𝛥𝐼 is obtained by OSA in experiment, then the data is substituted into Eq. (6), the simultaneous measurement of temperature and RI can be realized. For example, when the length of air microcavity is 17.94 μm and the length of remaining GI-FMF microcavity is 194.65 μm, the temperature and RI sensitivity is 9.04 pm/◦ C and −12.99 dB/RIU respectively, that is 𝐾1 = 9.04 and 𝐾2 = 12.99, the matrix equation of temperature and RI for the F–P microcavity sensor is obtained as [ ] [ ]−1 [ ] 𝛥𝑇 9.04 0 𝛥𝜆 = (7) 𝛥𝑁 0 −12.99 𝛥𝐼 4. Conclusion A sensor based on the GI-FMF to simultaneous measurement of temperature and RI is proposed. One end of GI-FMF is etched by HF acid and is spliced with SMF, and another end is cut flat to form a sensing head. Three sensing heads with GI-FMF microcavity length of 194.65 μm, 401.1 μm, and 1000 μm are selected to conduct simulation
where 𝐾1 and 𝐾2 are the temperature and RI sensitivity coefficients of the sensor, respectively. Combination with Eqs. (3) and (4), the change 8
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Optics Communications 455 (2019) 124577
Fig. 13. RI fitting results of different L: (a) 194.65 μm (b) 401.19 μm (c) 1000 μm.
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Fig. 14. EXT change and wavelength shift at different temperature.
and experimental measurement of temperature and RI. The experimental results show that the temperature sensitivity can be up to 10.52 pm/◦ C, and the maximum RI sensitivity is −16.03 dB/RIU. Due to the advantages of all-fiber, microprobe structure and high sensitivity, it can be used for temperature and RI measurement in extreme environments.
Acknowledgments
This work has been supported by National Natural Science Foundation of China (61575170, 61605168), the State Scholarship Fund of China (201708130199), the key basic research program of Hebei province (17961701D). 9
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