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Optical fiber relative-humidity sensor with evaporated dielectric coatings on fiber end-face
Optical Fiber Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Optical fiber relative-humidity sensor with evaporated dielectric coatings on fiber end-face Weijing Xie a, Minghong Yang a,⇑, Yun Cheng a, Dongwen Li a, Yi Zhang b, Zhi Zhuang b a b
National Engineering Laboratory for Fiber Optic Sensing Technology, Wuhan University of Technology, Wuhan 430070, China Institute of Structural Mechanics, CAEP, 621900 MianYang, Sichuan, China
a r t i c l e
i n f o
Article history: Received 16 June 2013 Revised 18 February 2014 Available online xxxx Keywords: Optical fiber sensor Relative-humidity Electron beam evaporation Porous films
a b s t r a c t An optical fiber relative-humidity sensor (OFRHS) with evaporated dielectric coatings is proposed and demonstrated. The sensitive coatings, composed of multilayers of Ti3O5 and SiO2, form an extrinsic Fabry–Perot cavity on the distal end of the multimode fiber. As the effective refractive index of the porous coatings were correlated with the change of ambient Relative-humidity (RH), which will at last result in the shift of interference fringe. By monitoring the drift of reflected interference fringe under different RH levels, the information about RH of the environment under test can be extracted. Experimental results show that the average sensitivity is 0.43 nm/% RH when environmental RH changes from 1.8% RH to 74.7% RH. The proposed sensor was proved to be high repeatability, little hysteresis and especially highly sensitive to lower moisture measure. Ó 2014 Published by Elsevier Inc.
1. Introduction
2. Sensing principle
Applications in medical, agricultural, aerospace, food process and storage, structural health monitoring (SHM) and many other fields require the monitor and control of relative-humidity levels [1–6]. Due to the advantages such as miniature, high sensitivity, fast response, and immune to electromagnetic interference, optical fiber RH sensor attracted considerable attention in the past decades. Optical fiber RH sensors based on different configurations such as long period gratings (LPGs) [7], tilted fiber Bragg’s grating (TFBG) [8], U-bend [9], hetero-core optical fiber [10,11] have been reported. Meanwhile, different techniques have been demonstrated to enhance sensitivity. One of the most popular solutions is based on the deposition of sensing layers [12,13]. However, most of these methods rely on absorption measurement [5,14,15], which can be affected by the instability of optical source or photo-detector. In this paper, we proposed a novel RH sensor with porous thin films as sensing elements. The sensor, with coatings on the distal end of the multimode fiber by Electron Beam Physical Vapor Deposition (EB-PVD) process, is based on a nano Fabry–Perot configuration [16–18]. Especially, the transducer in this work exhibits very good sensitivity to lower relative-humidity measurement down to 1.8% RH, which is typically difficult for some other RH sensors [19,20].
Schematic diagram of the sensing probe is presented in Fig. 1. Three-layers of optical thin films were deposited on a multimode fiber (MMF: 62.5 lm/125 lm) tip. Instead of a single layer, the multilayer structure was used to optimize the reflected spectrum, enhancing the intensity and visibility of the reflected light as shown in Fig. 2. The final parallel layer structure of the coating forms a multiple beam interference Fabry–Perot cavity. The dielectric films realized by e-beam evaporation without ion-source assistance have columnar and porous structures. As the Ti3O5 and SiO2 porous coating absorbs (or releases) of water molecules from the surrounding, its refractive index will change [21–23]. Further, varying refractive index would change the phase of light propagating in the element, leading a resonant wavelength drift. The refractive index (neff) of the porous coating calculated by the Bruggeman effective medium approximation theory [24] can be described as follows:
⇑ Corresponding author. E-mail address: [email protected] (M. Yang).
ð1 f Þ
n2sio2 n2eff n2sio2 þ 2n2eff
þ ðf VÞ
n2air n2eff n2air þ 2n2eff
þV
n2H2 O n2eff n2H2 O þ 2n2eff
¼0
ð1Þ
where f is the porosity of the coating, V means the volume of liquid water in the pores as the influence of capillary condensation, nsio2 is the refractive index of SiO2, nair presents the refractive index of the air, nH2O is the refractive index of the liquid water. Since the optical path of SiO2 film is far greater than of Ti3O5 films, the phase change would be basically due to the presence
http://dx.doi.org/10.1016/j.yofte.2014.03.008 1068-5200/Ó 2014 Published by Elsevier Inc.
Please cite this article in press as: W. Xie et al., Optical fiber relative-humidity sensor with evaporated dielectric coatings on fiber end-face, Opt. Fiber Technol. (2014), http://dx.doi.org/10.1016/j.yofte.2014.03.008
2
W. Xie et al. / Optical Fiber Technology xxx (2014) xxx–xxx
Fig. 1. Schematic diagram of the Ti3O5/SiO2/Ti3O5 combination of films F-P RH sensor.
Fig. 3. Schematic diagram of EB-PVD.
Fig. 2. Theoretical reflections of the fiber with three layers, with a SiO2 layer and without any deposition calculated by software.
of SiO2 coating, and the characteristic wavelength of spectrum peak can be approximate to kk ¼ 2neff L=k, where neff and L represent the equivalent refractive index and thickness of the SiO2 film respectively, k is integer.
Fig. 4. Scanning electron microscope image of the sensing coating.
3. Experiments 3.1. Sensor fabrication with EB-PVD process With EB-PVD technology, it is possible to control structure and morphology of the deposited coating by optimization of deposition process parameters [25,26]. Fig. 3 shows the schematic diagram of EB-PVD. The air pressure of all depositions was 0.01 Pa, filling gas is oxygen (O2) with a total flow velocity of 100 sccm. With selected baking temperature 100 °C, the deposition rate for the first and third layer was 0.2 nm/s (Ti3O5,168.55 nm respectively) and the second layer 0.5 nm/s (SiO2 1621.34 nm). The dielectric coating realized by e-beam evaporation without ion-source assistance has porous structures as shown in Fig. 4. Referring to the previous experiences in fabricating porous dielectric films, the sample should be kept in drying closet (RH 30%, temperature 25 °C) for aging at least 50 days to attain a good stability. 3.2. Experimental set-up As shown in Fig. 5, the RH sensing characterization system consists of a miniature broadband light source (HL-2000 Tungsten Halogen Light Sources from Ocean Optics, wavelength range: 360– 2500 nm), optical signal analyzer OSA (S3000-VIS Micro Spectrometer made from Seeman Technology, wavelength range:
320–1050 nm, wavelength resolution: 0.3 nm), multimode optical fiber coupler (OC, a fused tapered optical fiber 50/50 coupler at 850 nm) and the F-P sensor probe. Light from the broad-band light source (TLS) is used to illuminate the sensor through a 3-dB fiber coupler. The reflected optical spectrum was detected by the spectrometer (OSA), which is connected on the other arm of the optical coupler. The RH sensor was enclosed in a sealed receptacle with a cylindrical foam-rubber cushion, which was suspended in the air above saturated salt solutions to measure different RH. Table 1 shows RH in the air of the sealed receptacle for each saturated salt solution at 20 °C, according to the international standard of salt solution saturated humidity value issued by the International Organization of Legal Metrology (OIML). The actual humidity value was measured by a Center 313 Hygrothermograph (rang: temperature 20 to 60 °C, humidity 0–100% RH, accuracy of measurement: temperature ±0.7 °C, humidity ±2.5% RH) in the laboratory environment (18–22 °C, 56.4–75% RH).
4. Experimental results and discussion Fig. 6 shows experimental results of the fiber with 3 layers, with a SiO2 film and without any deposition on the fiber end-face. One
Please cite this article in press as: W. Xie et al., Optical fiber relative-humidity sensor with evaporated dielectric coatings on fiber end-face, Opt. Fiber Technol. (2014), http://dx.doi.org/10.1016/j.yofte.2014.03.008
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W. Xie et al. / Optical Fiber Technology xxx (2014) xxx–xxx
Fig. 5. Experimental set-up of the moisture measuring system.
Table 1 Humidity values above the air of each saturated salt solution at 20°. Solutions
LiBr (%)
LiCl (%)
MgCl2 (%)
NaBr (%)
NaCl (%)
KCl (%)
Humidity Actual humidity value
6.6 ± 0.6 10.1
12 14.3
33.1 ± 0.2 35.4
59.1 ± 0.5 60.5
75.5 ± 0.2 74.7
85.1 ± 0.3 83.1
reason for the discrepancy between actual and theoretical spectra could be the fact that the theoretical result is obtained by assuming a non-porous and optically flat layer whose effective refractive index is larger than that of porous film. Another reason could be the interference of modes existing in the multimode fiber, which is caused by the fiber itself and shows no response to the RH change. Multimode fiber was used in this work due to the advantage of simple structure, ease of processing and low cost. Fig. 7 depicts the reflection spectra (time of exposure 510 ms, time sampling interval 5 ms, spectrum smooth degree 10) of a OFRHS sample at different RH levels. Since the balance of RH in the containers could be damaged each time opening the lid of the bottle to change the environment RH, 5 min was given for reaching equilibrium at each RH level before recording and the measuring. Test sequence was from lower to higher RH level with the solutions of 14.3%, 35.4%, 60.5% and 74.7% RH respectively. Table 2 shows the spectra shift at different RH levels. The spectrum undergoes a red shift with the increase of RH level, corresponding to an increase of effective refractive index of the sensing films. This
can be explained that when RH increases, the dielectric coating absorbs more water molecules from the environment, and the increase of water molecules (refractive index = 1.33) filling air pores (refractive index = 1) in the coating will lead to an increase of effective refractive index in the sensing films. The test was repeated for 4 times in order to investigate the repeatability and long-term stability of RH sensor. Wavelength shifts of OFRHS under different humidity levels in both ascending and descending phase are shown in Fig. 8. The experimental results are stable over repeated measurements with small standard errors at each RH level. The stability of the sensor was also examined when the chamber maintained at RH levels of 14.3%, 34.5% and 60.5% RH for 24 h respectively. The data, recorded every 3 min, with small fluctuations in Fig. 9, indicates good stability of the sensor. The small random errors displayed in Figs. 8 and 9 can be attributed to the instability of micro spectrometer and temperature change during recordings.
Fig. 6. The experimental reflected spectra of the fiber with three layers, with a SiO2 film and without any deposition on the fiber end-face.
Fig. 7. Reflection spectra of the porous films sensor in different environment of varying RH.
Please cite this article in press as: W. Xie et al., Optical fiber relative-humidity sensor with evaporated dielectric coatings on fiber end-face, Opt. Fiber Technol. (2014), http://dx.doi.org/10.1016/j.yofte.2014.03.008
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Table 2 The corresponding wavelength values of the fourth trough and its variations in different humidity environment. Solution
LiBr
LiCl
MgCl2
NaBr
NaCl
Total drift
Actual humidity value Wavelength (nm) Drift (nm)
10.1% RH 647.3 0
14.3% RH 649.8 2.5
35.4% RH 657.5 7.7
60.5% RH 668.5 11
74.7% RH 671.5 3
24.2
Fig. 8. Wavelength shift in different RH, 4 tests.
Fig. 9. Shift of peak over time, when the F-P sensor was held at 14.3%, 34.5% and 60.5% RH for 24 h.
Fig. 10. Time response of the sensor, cycling 14.3% and 60.5% RH.
The sensing response time was also evaluated. The spectrum movement was recorded with a rate of 1 frame per second, and the peak shift is plotted as shown in Fig. 10. The sensor was placed in 14.3% and 60.5% RH for about 2 min respectively and consistently. A few cycles was repeated to determine the repeatability, rise time, decay time and hysteresis of the sensor. Experimental results show that the sensor has a response/recover time of 5 s between 14.3% and 60.5% RH, which is faster than 10 s given by optical fiber RH sensors with silica films [5,14]. It could be attributed to the hydrophilic of TiO2 films. This unique character of TiO2 surface is ascribed to the microstructure composition of hydrophilic phases, produced by ultraviolet irradiation [27]. With the water molecules adsorbed on the TiO2 surface uniformly, the SiO2 film can store water more quickly and easily. In order to increase humidity sampling points, an Accurate Humidity Generator (Model SRH-1 made by SHINYEI, Japan) was used to generate and control the RH and temperature of environment. The humidity characterization is carried out at room temperature of about 25 °C as shown in Fig. 11, while the performance of the sensor at low humidity can be seen in Fig. 12. From the humidity characterization, the average RH sensitivity of proposed sensor is calculated to be 0.43 nm/% RH. It should be mentioned that the spectrum shift reaches 8.2 nm in RH level ranging from 1.8% to 14.3% RH (humidity sensing test lower than 1.8% RH is not available because of the limit of experimental device), which means RH sensitivity of 0.66 nm/% RH. An agarose optical fiber humidity sensor based on resonance in the infrared region showed a dynamical range of 45 nm between 20% and 80% RH [28], another humidity sensor based on an agarose coated PCF interferometer presented wavelength shift of 43 nm from 40% to 98% RH [29], the proposed sensor in this work shows promising prospect in the lower humidity measurement. Taken temperature change into account, the characteristic wavelength shows blue shift with the increase of environmental temperature as shown in Fig. 13. The sample probe tested in a temperature control box was sealed in a glass tube to cut off the moisture from the air. The blue shift caused by temperature can be explained by the Kelvin’s relation [21,23].
Fig. 11. Wavelength shift of the proposed sensor at varying RH.
Please cite this article in press as: W. Xie et al., Optical fiber relative-humidity sensor with evaporated dielectric coatings on fiber end-face, Opt. Fiber Technol. (2014), http://dx.doi.org/10.1016/j.yofte.2014.03.008
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5
5. Conclusion
Fig. 12. Calibration curve of the fiber-optic RH sensor in the range of 1.8–14.3% RH.
A multimode fiber humidity sensor with multilayer porous films manufactured by EB-PVD technique on fiber end-face is proposed and demonstrated. Wavelength shift was 30.5 nm when RH ranges from 1.8% to 74.7%. The sensitivity is 0.66 nm/% RH in RH range from 1.8 to 14.3% RH, which shows good response to lower moisture measurement. Furthermore, the sensor performance was tested 4 times and displayed little degradation. A cycling time of 5 s for a rise/fall time was determined for the proposed probe, which also displayed little hysteresis and good repeatability. The sensor also exhibited cross-sensitivity of 0.1 nm/°C to temperature change. Temperature compensation experiments were developed and under work. Further investigations will be concentrated on the correlation of different film structures with RH sensitivity. In general, the combination of optical fiber sensor with thin film optics is very promising for application in relative- humidity sensing.
Acknowledgments This work is finically supported by the Project of National Science Foundation of China, NSFC (Number: 62190311) and the Program for New Century Excellent Talents in University (Number: NECT-10-0664).
References
Fig. 13. Wavelength shift with increasing temperature.
r K ¼ 2M c cos h=RqT ln HR
ð2Þ
where rK is Kelvin radius, HR is the relative humidity, c is the surface tension, R is the universal gas constant, T is the temperature in Kelvin, q and M are the density and molecular weight of water respectively, h is the contact angle. The capillary condensation will take place in pores with a radius less than rK at a particular RH and temperature. With the temperature increasing, rK decreases, also the adhesion of vapor molecules in the coating reduces, which results in the decrease of effective refractive index. The thermo-optic coefficient of the layers caused by temperature is too small to be considered, compared to the significant effect of porous element. Cross-sensitivity to RH and temperature exists for the proposed F-P optical fiber RH sensor. Since the frequently-used temperature region locates between 20 and 40 °C, wavelength shift due to temperature change is only about 2 nm in this range. The temperature sensitivity of the sensor is 0.1 nm/°C. The effect of temperature cross-sensitivity can be compensated by possible solutions. For example, a reference probe in parallel to the sensing element is employed. The reference probe was manufactured with the same procedure as sensing probe, but packaged in a glass tube filled with hygroscopic silica gel (where the humidity can be stable in 18% RH). An optical switch is used to obtain the reflected spectra of both sensing and reference probes respectively. Therefore, the drifts of two probes can be extracted, so as to reduce the affect caused by variation of temperature.
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Please cite this article in press as: W. Xie et al., Optical fiber relative-humidity sensor with evaporated dielectric coatings on fiber end-face, Opt. Fiber Technol. (2014), http://dx.doi.org/10.1016/j.yofte.2014.03.008
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[20] Y. Wu, T.H. Zhang, Y.J. Rao, Y. Gong, Miniature interferometric humidity sensors based on silica/polymer microfiber knot resonators, Sens. Actuators B: Chem. 155 (2011) 258–263. [21] Zhiquan Li, Rang Zhang, Study on a gas sensor based on the optical character of porous silicon microcavities, Chin. J. Sens. Actuators 20 (2007) 54–57. [22] S.J. Regg, K.S.W. Sing, Adsorption M. Surface Area and Porosity, second ed., Academic Press, London, 1982. [23] J.T.W. Yeow, J.P.M. She, Capacitive humidity sensing using carbon nanotube enabled capillary condensation, in: IEEE SENSORS 2006, EXCO, Daegu, Korea, October 22–25, 2006. [24] E.V. Astrova, V.A. Tolmachev, Effective refractive index and composition of oxidized porous silicon films, J. Mater. Sci. Eng. 2002 (60) (2000) 142–148. [25] K. Robbie, M.J. Brett, Sculptured thin films and glancing angle deposition: growth mechanics and applications, J. Vacuum Sci. Technol. A: Vacuum Surf. Films 15 (1997) 1460–1465.
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Please cite this article in press as: W. Xie et al., Optical fiber relative-humidity sensor with evaporated dielectric coatings on fiber end-face, Opt. Fiber Technol. (2014), http://dx.doi.org/10.1016/j.yofte.2014.03.008
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Optical fiber relative-humidity sensor with evaporated dielectric coatings on fiber end-face Weijing Xie a, Minghong Yang a,⇑, Yun Cheng a, Dongwen Li a, Yi Zhang b, Zhi Zhuang b a b
National Engineering Laboratory for Fiber Optic Sensing Technology, Wuhan University of Technology, Wuhan 430070, China Institute of Structural Mechanics, CAEP, 621900 MianYang, Sichuan, China
a r t i c l e
i n f o
Article history: Received 16 June 2013 Revised 18 February 2014 Available online xxxx Keywords: Optical fiber sensor Relative-humidity Electron beam evaporation Porous films
a b s t r a c t An optical fiber relative-humidity sensor (OFRHS) with evaporated dielectric coatings is proposed and demonstrated. The sensitive coatings, composed of multilayers of Ti3O5 and SiO2, form an extrinsic Fabry–Perot cavity on the distal end of the multimode fiber. As the effective refractive index of the porous coatings were correlated with the change of ambient Relative-humidity (RH), which will at last result in the shift of interference fringe. By monitoring the drift of reflected interference fringe under different RH levels, the information about RH of the environment under test can be extracted. Experimental results show that the average sensitivity is 0.43 nm/% RH when environmental RH changes from 1.8% RH to 74.7% RH. The proposed sensor was proved to be high repeatability, little hysteresis and especially highly sensitive to lower moisture measure. Ó 2014 Published by Elsevier Inc.
1. Introduction
2. Sensing principle
Applications in medical, agricultural, aerospace, food process and storage, structural health monitoring (SHM) and many other fields require the monitor and control of relative-humidity levels [1–6]. Due to the advantages such as miniature, high sensitivity, fast response, and immune to electromagnetic interference, optical fiber RH sensor attracted considerable attention in the past decades. Optical fiber RH sensors based on different configurations such as long period gratings (LPGs) [7], tilted fiber Bragg’s grating (TFBG) [8], U-bend [9], hetero-core optical fiber [10,11] have been reported. Meanwhile, different techniques have been demonstrated to enhance sensitivity. One of the most popular solutions is based on the deposition of sensing layers [12,13]. However, most of these methods rely on absorption measurement [5,14,15], which can be affected by the instability of optical source or photo-detector. In this paper, we proposed a novel RH sensor with porous thin films as sensing elements. The sensor, with coatings on the distal end of the multimode fiber by Electron Beam Physical Vapor Deposition (EB-PVD) process, is based on a nano Fabry–Perot configuration [16–18]. Especially, the transducer in this work exhibits very good sensitivity to lower relative-humidity measurement down to 1.8% RH, which is typically difficult for some other RH sensors [19,20].
Schematic diagram of the sensing probe is presented in Fig. 1. Three-layers of optical thin films were deposited on a multimode fiber (MMF: 62.5 lm/125 lm) tip. Instead of a single layer, the multilayer structure was used to optimize the reflected spectrum, enhancing the intensity and visibility of the reflected light as shown in Fig. 2. The final parallel layer structure of the coating forms a multiple beam interference Fabry–Perot cavity. The dielectric films realized by e-beam evaporation without ion-source assistance have columnar and porous structures. As the Ti3O5 and SiO2 porous coating absorbs (or releases) of water molecules from the surrounding, its refractive index will change [21–23]. Further, varying refractive index would change the phase of light propagating in the element, leading a resonant wavelength drift. The refractive index (neff) of the porous coating calculated by the Bruggeman effective medium approximation theory [24] can be described as follows:
⇑ Corresponding author. E-mail address: [email protected] (M. Yang).
ð1 f Þ
n2sio2 n2eff n2sio2 þ 2n2eff
þ ðf VÞ
n2air n2eff n2air þ 2n2eff
þV
n2H2 O n2eff n2H2 O þ 2n2eff
¼0
ð1Þ
where f is the porosity of the coating, V means the volume of liquid water in the pores as the influence of capillary condensation, nsio2 is the refractive index of SiO2, nair presents the refractive index of the air, nH2O is the refractive index of the liquid water. Since the optical path of SiO2 film is far greater than of Ti3O5 films, the phase change would be basically due to the presence
http://dx.doi.org/10.1016/j.yofte.2014.03.008 1068-5200/Ó 2014 Published by Elsevier Inc.
Please cite this article in press as: W. Xie et al., Optical fiber relative-humidity sensor with evaporated dielectric coatings on fiber end-face, Opt. Fiber Technol. (2014), http://dx.doi.org/10.1016/j.yofte.2014.03.008
2
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Fig. 1. Schematic diagram of the Ti3O5/SiO2/Ti3O5 combination of films F-P RH sensor.
Fig. 3. Schematic diagram of EB-PVD.
Fig. 2. Theoretical reflections of the fiber with three layers, with a SiO2 layer and without any deposition calculated by software.
of SiO2 coating, and the characteristic wavelength of spectrum peak can be approximate to kk ¼ 2neff L=k, where neff and L represent the equivalent refractive index and thickness of the SiO2 film respectively, k is integer.
Fig. 4. Scanning electron microscope image of the sensing coating.
3. Experiments 3.1. Sensor fabrication with EB-PVD process With EB-PVD technology, it is possible to control structure and morphology of the deposited coating by optimization of deposition process parameters [25,26]. Fig. 3 shows the schematic diagram of EB-PVD. The air pressure of all depositions was 0.01 Pa, filling gas is oxygen (O2) with a total flow velocity of 100 sccm. With selected baking temperature 100 °C, the deposition rate for the first and third layer was 0.2 nm/s (Ti3O5,168.55 nm respectively) and the second layer 0.5 nm/s (SiO2 1621.34 nm). The dielectric coating realized by e-beam evaporation without ion-source assistance has porous structures as shown in Fig. 4. Referring to the previous experiences in fabricating porous dielectric films, the sample should be kept in drying closet (RH 30%, temperature 25 °C) for aging at least 50 days to attain a good stability. 3.2. Experimental set-up As shown in Fig. 5, the RH sensing characterization system consists of a miniature broadband light source (HL-2000 Tungsten Halogen Light Sources from Ocean Optics, wavelength range: 360– 2500 nm), optical signal analyzer OSA (S3000-VIS Micro Spectrometer made from Seeman Technology, wavelength range:
320–1050 nm, wavelength resolution: 0.3 nm), multimode optical fiber coupler (OC, a fused tapered optical fiber 50/50 coupler at 850 nm) and the F-P sensor probe. Light from the broad-band light source (TLS) is used to illuminate the sensor through a 3-dB fiber coupler. The reflected optical spectrum was detected by the spectrometer (OSA), which is connected on the other arm of the optical coupler. The RH sensor was enclosed in a sealed receptacle with a cylindrical foam-rubber cushion, which was suspended in the air above saturated salt solutions to measure different RH. Table 1 shows RH in the air of the sealed receptacle for each saturated salt solution at 20 °C, according to the international standard of salt solution saturated humidity value issued by the International Organization of Legal Metrology (OIML). The actual humidity value was measured by a Center 313 Hygrothermograph (rang: temperature 20 to 60 °C, humidity 0–100% RH, accuracy of measurement: temperature ±0.7 °C, humidity ±2.5% RH) in the laboratory environment (18–22 °C, 56.4–75% RH).
4. Experimental results and discussion Fig. 6 shows experimental results of the fiber with 3 layers, with a SiO2 film and without any deposition on the fiber end-face. One
Please cite this article in press as: W. Xie et al., Optical fiber relative-humidity sensor with evaporated dielectric coatings on fiber end-face, Opt. Fiber Technol. (2014), http://dx.doi.org/10.1016/j.yofte.2014.03.008
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Fig. 5. Experimental set-up of the moisture measuring system.
Table 1 Humidity values above the air of each saturated salt solution at 20°. Solutions
LiBr (%)
LiCl (%)
MgCl2 (%)
NaBr (%)
NaCl (%)
KCl (%)
Humidity Actual humidity value
6.6 ± 0.6 10.1
12 14.3
33.1 ± 0.2 35.4
59.1 ± 0.5 60.5
75.5 ± 0.2 74.7
85.1 ± 0.3 83.1
reason for the discrepancy between actual and theoretical spectra could be the fact that the theoretical result is obtained by assuming a non-porous and optically flat layer whose effective refractive index is larger than that of porous film. Another reason could be the interference of modes existing in the multimode fiber, which is caused by the fiber itself and shows no response to the RH change. Multimode fiber was used in this work due to the advantage of simple structure, ease of processing and low cost. Fig. 7 depicts the reflection spectra (time of exposure 510 ms, time sampling interval 5 ms, spectrum smooth degree 10) of a OFRHS sample at different RH levels. Since the balance of RH in the containers could be damaged each time opening the lid of the bottle to change the environment RH, 5 min was given for reaching equilibrium at each RH level before recording and the measuring. Test sequence was from lower to higher RH level with the solutions of 14.3%, 35.4%, 60.5% and 74.7% RH respectively. Table 2 shows the spectra shift at different RH levels. The spectrum undergoes a red shift with the increase of RH level, corresponding to an increase of effective refractive index of the sensing films. This
can be explained that when RH increases, the dielectric coating absorbs more water molecules from the environment, and the increase of water molecules (refractive index = 1.33) filling air pores (refractive index = 1) in the coating will lead to an increase of effective refractive index in the sensing films. The test was repeated for 4 times in order to investigate the repeatability and long-term stability of RH sensor. Wavelength shifts of OFRHS under different humidity levels in both ascending and descending phase are shown in Fig. 8. The experimental results are stable over repeated measurements with small standard errors at each RH level. The stability of the sensor was also examined when the chamber maintained at RH levels of 14.3%, 34.5% and 60.5% RH for 24 h respectively. The data, recorded every 3 min, with small fluctuations in Fig. 9, indicates good stability of the sensor. The small random errors displayed in Figs. 8 and 9 can be attributed to the instability of micro spectrometer and temperature change during recordings.
Fig. 6. The experimental reflected spectra of the fiber with three layers, with a SiO2 film and without any deposition on the fiber end-face.
Fig. 7. Reflection spectra of the porous films sensor in different environment of varying RH.
Please cite this article in press as: W. Xie et al., Optical fiber relative-humidity sensor with evaporated dielectric coatings on fiber end-face, Opt. Fiber Technol. (2014), http://dx.doi.org/10.1016/j.yofte.2014.03.008
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Table 2 The corresponding wavelength values of the fourth trough and its variations in different humidity environment. Solution
LiBr
LiCl
MgCl2
NaBr
NaCl
Total drift
Actual humidity value Wavelength (nm) Drift (nm)
10.1% RH 647.3 0
14.3% RH 649.8 2.5
35.4% RH 657.5 7.7
60.5% RH 668.5 11
74.7% RH 671.5 3
24.2
Fig. 8. Wavelength shift in different RH, 4 tests.
Fig. 9. Shift of peak over time, when the F-P sensor was held at 14.3%, 34.5% and 60.5% RH for 24 h.
Fig. 10. Time response of the sensor, cycling 14.3% and 60.5% RH.
The sensing response time was also evaluated. The spectrum movement was recorded with a rate of 1 frame per second, and the peak shift is plotted as shown in Fig. 10. The sensor was placed in 14.3% and 60.5% RH for about 2 min respectively and consistently. A few cycles was repeated to determine the repeatability, rise time, decay time and hysteresis of the sensor. Experimental results show that the sensor has a response/recover time of 5 s between 14.3% and 60.5% RH, which is faster than 10 s given by optical fiber RH sensors with silica films [5,14]. It could be attributed to the hydrophilic of TiO2 films. This unique character of TiO2 surface is ascribed to the microstructure composition of hydrophilic phases, produced by ultraviolet irradiation [27]. With the water molecules adsorbed on the TiO2 surface uniformly, the SiO2 film can store water more quickly and easily. In order to increase humidity sampling points, an Accurate Humidity Generator (Model SRH-1 made by SHINYEI, Japan) was used to generate and control the RH and temperature of environment. The humidity characterization is carried out at room temperature of about 25 °C as shown in Fig. 11, while the performance of the sensor at low humidity can be seen in Fig. 12. From the humidity characterization, the average RH sensitivity of proposed sensor is calculated to be 0.43 nm/% RH. It should be mentioned that the spectrum shift reaches 8.2 nm in RH level ranging from 1.8% to 14.3% RH (humidity sensing test lower than 1.8% RH is not available because of the limit of experimental device), which means RH sensitivity of 0.66 nm/% RH. An agarose optical fiber humidity sensor based on resonance in the infrared region showed a dynamical range of 45 nm between 20% and 80% RH [28], another humidity sensor based on an agarose coated PCF interferometer presented wavelength shift of 43 nm from 40% to 98% RH [29], the proposed sensor in this work shows promising prospect in the lower humidity measurement. Taken temperature change into account, the characteristic wavelength shows blue shift with the increase of environmental temperature as shown in Fig. 13. The sample probe tested in a temperature control box was sealed in a glass tube to cut off the moisture from the air. The blue shift caused by temperature can be explained by the Kelvin’s relation [21,23].
Fig. 11. Wavelength shift of the proposed sensor at varying RH.
Please cite this article in press as: W. Xie et al., Optical fiber relative-humidity sensor with evaporated dielectric coatings on fiber end-face, Opt. Fiber Technol. (2014), http://dx.doi.org/10.1016/j.yofte.2014.03.008
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5. Conclusion
Fig. 12. Calibration curve of the fiber-optic RH sensor in the range of 1.8–14.3% RH.
A multimode fiber humidity sensor with multilayer porous films manufactured by EB-PVD technique on fiber end-face is proposed and demonstrated. Wavelength shift was 30.5 nm when RH ranges from 1.8% to 74.7%. The sensitivity is 0.66 nm/% RH in RH range from 1.8 to 14.3% RH, which shows good response to lower moisture measurement. Furthermore, the sensor performance was tested 4 times and displayed little degradation. A cycling time of 5 s for a rise/fall time was determined for the proposed probe, which also displayed little hysteresis and good repeatability. The sensor also exhibited cross-sensitivity of 0.1 nm/°C to temperature change. Temperature compensation experiments were developed and under work. Further investigations will be concentrated on the correlation of different film structures with RH sensitivity. In general, the combination of optical fiber sensor with thin film optics is very promising for application in relative- humidity sensing.
Acknowledgments This work is finically supported by the Project of National Science Foundation of China, NSFC (Number: 62190311) and the Program for New Century Excellent Talents in University (Number: NECT-10-0664).
References
Fig. 13. Wavelength shift with increasing temperature.
r K ¼ 2M c cos h=RqT ln HR
ð2Þ
where rK is Kelvin radius, HR is the relative humidity, c is the surface tension, R is the universal gas constant, T is the temperature in Kelvin, q and M are the density and molecular weight of water respectively, h is the contact angle. The capillary condensation will take place in pores with a radius less than rK at a particular RH and temperature. With the temperature increasing, rK decreases, also the adhesion of vapor molecules in the coating reduces, which results in the decrease of effective refractive index. The thermo-optic coefficient of the layers caused by temperature is too small to be considered, compared to the significant effect of porous element. Cross-sensitivity to RH and temperature exists for the proposed F-P optical fiber RH sensor. Since the frequently-used temperature region locates between 20 and 40 °C, wavelength shift due to temperature change is only about 2 nm in this range. The temperature sensitivity of the sensor is 0.1 nm/°C. The effect of temperature cross-sensitivity can be compensated by possible solutions. For example, a reference probe in parallel to the sensing element is employed. The reference probe was manufactured with the same procedure as sensing probe, but packaged in a glass tube filled with hygroscopic silica gel (where the humidity can be stable in 18% RH). An optical switch is used to obtain the reflected spectra of both sensing and reference probes respectively. Therefore, the drifts of two probes can be extracted, so as to reduce the affect caused by variation of temperature.
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Please cite this article in press as: W. Xie et al., Optical fiber relative-humidity sensor with evaporated dielectric coatings on fiber end-face, Opt. Fiber Technol. (2014), http://dx.doi.org/10.1016/j.yofte.2014.03.008