- Email: [email protected]
Toluene optical fibre sensor based on air microcavity in PDMS
Optical Fiber Technology 34 (2017) 70–73
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Regular Articles
Toluene optical fibre sensor based on air microcavity in PDMS Daniel Kacik ⇑, Ivan Martincek Department of Physics, University of Zilina, Univerzitna 1, 01026 Zilina, Slovakia
a r t i c l e
i n f o
Article history: Received 21 July 2016 Revised 10 December 2016 Accepted 22 January 2017
Keywords: Optical fibre sensor Fabry-Perot interferometer PDMS Organic volatile compound Toluene
a b s t r a c t We prepared and demonstrated a compact, simple-to-fabricate, air microcavity in polydimethylsiloxane (PDMS) placed at the end of a single-mode optical fibre. This microcavity creates a Fabry-Perot interferometer sensor able to measure concentrations of toluene vapour in air. Operation of the sensor is provided by diffusion of the toluene vapour to the PDMS, and the consequent extension of length d of the air microcavity in PDMS. The sensor response for the presence of vapours is fast and occurs within a few seconds. By using the prepared sensor toluene vapour concentration in air can be measured in the range from about 0.833 g.m3 to saturation, with better sensitivity than 0.15 nm/g.m3 up to maximal sensitivity 1.4 nm/g.m3 at around concentration 100 g.m3 in time 5 s. Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction Optical fibre sensors have unique properties such as small size, light weight, high sensitivity, biocompatibility, corrosion resistance, immunity to electromagnetic interference, continuous measurements and in-situ monitoring of chemical parameters in industrial processes [1]. The sensors can work in different ways. One of the most sensitive ways is sensing measurands by the interference of light. Configuration of the interferometer depends on different parameters. Usually the interferometer consists of a set of mirrors dividing the light into arms, and then decoupled back to compare the phase of travelled light in particular arms. One interesting aspect of mirror creation is offered by a Fabry-Perot interferometer (FPI). The mirrors that form the Fabry-Perot cavity in fibre optics can be reflective splices between two identical optical fibres [2], the section of photonic crystal fibre between singlemode fibres [3] or etched polymer fibre with Bragg grating [4]. FPIs can be built not only as all-fibre (intrinsic), but also as extrinsic sensors. In the case of extrinsic sensors the typical FPI sensor is built between a cleaved optical fibre end and an elastic diaphragm, such as a polymer diaphragm [5], silica diaphragm [6] or metal diaphragm [7]. Sensors based on FPI have a wide range of applications similar to other optical-fibre sensors based on interference, such as temperature measurement, refractive index, pressure, transverse load, etc. [8]. ⇑ Corresponding author. E-mail addresses: [email protected] (D. Kacik), [email protected]. sk (I. Martincek). http://dx.doi.org/10.1016/j.yofte.2017.01.006 1068-5200/Ó 2017 Elsevier Inc. All rights reserved.
Recently, fibre-tip sensors with microcavities have been proposed. For example, microcavities fabricated by using a fusion splicer and pressurizing gas chamber [9], or applying polymer at the end of a small segment of photonic bandgap fibre [10]. The sensors were proposed in order to detect pressure, but when a polymer with favourable physicochemical properties is used, a similar configuration can also be used for the detection of organic volatile compounds (for example, toluene). The sensing of volatile organic compounds is of importance in a range of applications, for example, monitoring air quality in both indoor and outdoor environments. Volatile organic compounds can originate from fuel and petroleum products, from paints and by combustion processes, natural sources and farming. Human exposure to these chemicals, even at low concentration, can be hazardous due to producing short- and long-term adverse health effects. Sensitivities of particular configurations of sensors for different organic chemicals are different. A sensor based on a surface relief D-shaped fibre Bragg grating had sensitivity to acetone: 1.65 104 pm/ppm from concentration app. 6000 ppm [11]. For zeolite, a thin film-coated fibre sensor based on FPI, the sensitivity was 4.28 103 nm/ppm when the concentration of isopropanol ranged from 350 ppm to 2450 ppm [12]. In Ref. [15] the proposed sensor is based on long period grating coated with calixarene to form the nanostructured coating which achieved sensitivity to toluene 231 ppmv (ppm in volume) with spectrometer resolution of 0.3 nm and recovery time of order of 15 s. In this article, we report a preparation of polydimethylsiloxane (PDMS) FPI located at the end of a single-mode optical fibre (SMF). A fibre tip is treated by applying thin paraffin wax film to the fibre
D. Kacik, I. Martincek / Optical Fiber Technology 34 (2017) 70–73
end. Then the fibre end is packed with PDMS. After curing, the fibre end is heated to diffuse wax to the PDMS. This reduces the adhesion and allows the creation of the microcavity between the fibre end face and the PDMS layer. Then the length of microcavity is fixed by covering it with a second layer of PDMS. Based on toluene influence we investigated the function of the FPI fibre. When toluene is applied to such a FPI, the length of the microcavity will change. So we determine the sensor response on toluene concentrations in air. The main advantages of the developed sensor are its compactness and its very short analytical time (response and desorption time), which was found to be 5 s.
2. Operational principle The PDMS microcavity placed at the end of the SMF forms an extrinsic optical fibre FPI. The interferometer consists of two mirrors with reflectivity R1 and R2, respectively. The mirrors are separated by distance d. In our case, the first mirror of the interferometer is formed by the surface S1 with reflectivity R1 at the interface SMF/air, and the second mirror is formed by the surface S2 with reflectivity R2 at the interface air/PDMS at the end of the microcavity. SMF is used as input/output fibre of the interferometer. It is shown schematically in Fig. 1. If the light with intensity I0 is launched to the SMF, then the surface S1 will reflect part I1 of the light, and the rest will be transmitted. A similar effect will take place over an area S2 that reflects the intensity I2. Light reflected from both interfaces will be coupled back into the SMF core and interfere with each other. In our case, as the reflectance of the surfaces S1 and S2 is low, multiple reflections can be neglected and the intensities I1 and I2 can be written as I1 = R1I0 and I2 = (1 R1)R2I0, respectively. So the intensity IR of the light reflected from the air microcavity can be described as
pffiffiffiffiffiffiffiffi 4pd ; IR ¼ I1 þ I2 þ 2 I1 I2 cos k
71
3. Fabrication of air optical microcavity in PDMS For the fabrication of the air microcavity in the PDMS placed at the end of the optical fibre PDMS Sylgard 184 (Dow Corning) was used, which was supplied as two-part liquid component kits. PDMS was prepared by mixing prepolymer (part A) and curing agent (part B) in the ratio 10:1. PDMS possess properties such a hydrophobility, hydrolytic stability, non-flammable, high chemical stability, optically clear and its refractive index is close to that of glass [14]. The fabrication technological process of the air microcavity can be described as follows: after removing the primary fibre coating on the cladding at the end of the single-mode optical fibre a thin paraffin wax film was applied. However, it wasn’t applied to the cleaved face of the SMF. The paraffin wax film had a thickness of approximately 20 lm, and covered the fibre for a length of about 370 lm. Then the first layer of PDMS in a teardrop shape was applied to the end of the fibre up to the distance where the wax was. Also, the PDMS was applied to the face end of the SMF (Fig. 2A). After curing the fibre end with PDMS, the coating was heated to 70 °C for 60 min. During heating the paraffin wax changed to a liquid state. During this time, liquid paraffin wax diffused into the PDMS. As a result, the adhesion between the PDMS and the cladding of SMF was reduced. Then, at room temperature the PDMS layer was moved and between the fibre end and PDMS the air optical microcavity was created (Fig. 2B). The length of microcavity d was controlled by measurement of interference pattern and set to 70 lm. In order to fix the length of the optical microcavity at the end of the fibre, the first PDMS layer was encapsulated by a second layer of PDMS (Fig. 2C). The second layer of PDMS covered
ð1Þ
where d is the length of air microcavity in PDMS and k is wavelength of used light in a vacuum. It is known that liquid and gaseous toluene is well infiltrated into the PDMS, causing increases in volume [13] also like other organic volatile compounds. In our case, diffusion of toluene vapour to the PDMS caused increases of the air microcavity length. Thus, it results in an optical phase shift of the reflected intensity I2. The optical phase shift in the PDMS Fabry-Perot interferometer at constant temperature and constant pressure depends on the time of influence, and on the toluene vapour concentration in the PDMS.
Fig. 1. Schematic of the extrinsic optical fibre Fabry-Perot interferometer consisting of single-mode optical fibre (SMF) as input/output fibre and two mirrors with surfaces S1 and S2 separated by distance d.
Fig. 2. Technological process of preparation of air optical microcavity in polydimethylsiloxane (PDMS) located at end of single-mode fibre (SMF).
72
D. Kacik, I. Martincek / Optical Fiber Technology 34 (2017) 70–73
Fig. 3. Photograph of prepared air optical microcavity in PDMS placed at end of optical fibre.
the fibre for a length of about 700 lm from the end of fibre. The described technological process of the preparation of an optical microcavity is shown schematically in Fig. 2. Volume and shape of PDMS layers prepared by the fabrication process change only slightly so the sensitivity of the sensor change slightly, too. A photograph of the prepared air optical microcavity in PDMS is shown in Fig. 3. 4. Application to sensing of toluene concentration The experiment set-up for investigation of influence of toluene concentration based on FPI is shown in Fig. 4, where a broadband light source (SLED Safibra OFLS-6), with central wavelength 1550 nm and 100 nm spectral range is used as an incident light, and an optical spectral analyser (Anritsu MS9710B), with a resolu-
tion of 0.07 nm is used to record the reflected spectra, combined with a 3 dB coupler. As the gas chamber we used glass container with narrow neck with inner volume 1 L in which we left to evaporate defined volume of liquid toluene. The container was slowly rotated for the purpose of uniform distribution of toluene vapour in chamber. The temperature and pressure in the chamber was controlled and kept constant. In the experiment, the concentration of toluene in the chamber changed from 0.833 g.m3 to 140.8 g. m3. We placed the optical fibre sensor head into the chamber and measured the spectral dependence of FPI in selected time. After that time we took the sensor out of the chamber. Before another measurement we waited until measured spectral dependence of FPI was equal to the original spectral dependence without influence of toluene. Then we realized the measurement for another concentration of toluene. Fig. 5 shows the reflection spectra for the three different concentrations of toluene vapour in the wavelength range 1500 nm– 1600 nm, under constant atmospheric pressure and laboratory temperature 25 °C. Time of applying toluene vapours on sensor head was only 5 s. It is unlikely, that refractive index of PDMS is markedly changing due to infiltration of toluene vapour into the PDMS material. For such short time the toluene vapour only interacts with surface layers of PDMS coating, which stretched. Ultimately, thanks to the elastic properties of PDMS it is also reflected in the change of Fabry-Perot resonator length inside the PDMS. The refractive index of the fibre is 1.45 and the refractive index of PDMS is 1.40 at wavelength 1550 nm. So the reflectivity of the fibre end R1 is about 4 per cent, and the reflectivity of the polymer diaphragm R2 is about 3 per cent. As can be seen in Fig. 5, the intensity of reflected light at the interface between the fibre end and air I1 is approximately the same as I2 at the interface between the air and the polymer diaphragm. Also the particular reflection spectra have an equidistant period (in all spectra the space period between maxims is the same).
Fig. 4. Experimental set-up for investigation of toluene concentration consisting of broadband light source (SLED), optical spectral analyser (OSA) and 3 dB coupler.
Fig. 5. Comparison of reflected spectra for zero concentration of toluene in air with reflected spectra for concentration 33 g.m3 and 66 g.m3. Wavelength of reference maxima shifts to higher wavelengths when toluene vapour is applied. The spectral dependencies are not corrected to spectral characteristic of light source.
D. Kacik, I. Martincek / Optical Fiber Technology 34 (2017) 70–73
73
5. Conclusion
Fig. 6. Dependence of wavelength shift Dk on toluene concentration in air.
We prepared and demonstrated a compact, simple-to-fabricate, air microcavity in PDMS, placed at the end of a single-mode optical fibre for toluene vapour in air sensing. In the sensor, the wavelengths and distances of maximums and minimums of the reflected interference spectra depend on the toluene vapour concentration at measured points of space. For the operation of the sensor broadband light source can be used and low-resolution optical spectral analyser. The sensor response for the presence of vapours is fast and occurs within a few seconds. Sensitivity depends strongly on the concentration of toluene vapours in air. By using the prepared sensor, the toluene vapour concentration in air can be measured in the range from about 0.833 g.m3 to saturation, with a sensitivity of better than 0.15 nm/g.m3 up to maximal sensitivity of 1.4 nm/g m3 at around a concentration of 100 g.m3 in time 5 s. Acknowledgments This work was supported by Slovak National Grant Agency No. VEGA 1/0491/14, 1/0278/15 and the Slovak Research and Development Agency under the contract No. APVV-0395-12, APVV-150441 and the R&D operational program Centre of excellence of power electronics systems and materials for their components I. No. OPVaV-2008/2.1/01-SORO, ITMS 26220120003 funded by European regional development fund (ERDF) and the project ITMS 2610120021, co-funded from EU sources and European Regional Development Fund. References
Fig. 7. Dependence of sensitivity on toluene vapour concentration.
In our previous paper we investigated as the reflected spectra were evolving in time [14], but we realized that this technique was not suitable for sensor application, owing to the length of time (app. 40 min). So we decided to measure the value of concentration of toluene in time 5 s. The recovery time after measurement was better than 30 s. For evaluation, we chose one maximum (wavelength of the maxima) for reference at the reflected spectra for zero concentration of toluene in air, and compared it with the corresponding wavelength maxima of reflected spectra for particular toluene concentration. In free spectral range of Fabry-Perot interferometer it is possible to determine toluene vapour concentration up to 50 g.m3. After that as can be seen in Fig. 5, a 2p jump occurs. The dependence of the wavelength shift on the toluene concentration is shown in Fig. 6. Also the root mean square (rms) is shown for particular concentrations. For all concentrations the rms was better than 2 per cent. For example, for maximal concentration the value Dk was 60 nm ± 0.7 nm. The dependence of the wavelength shift on the toluene concentration in air has a non-linear character. So it does not have the same sensitivity in the investigated interval of toluene vapour concentration. The dependence of the sensitivity on the concentration obtained by derivation of the response function (shown in Fig. 6) is shown in Fig. 7. As can be seen in Fig. 7 the sensitivity is better than 0.15 nm/g. m3 in all investigated interval of toluene vapour concentrations in air. The highest sensitivity, 1.4 nm/g.m3 of proposed optical fibre Fabry-Perot interferometer, is around concentration 100 g.m3.
[1] E. Udd, W.B. Spillman Jr., Fiber Optics Sensors, second ed., John Wiley, Hoboken, New Jersey, 2011. [2] C.E. Lee, R.A. Atkins, H.F. Taylor, Performance of a fiber-optic temperature sensor from 200 to 1050 °C, Opt. Lett. 13 (1988) 1038–1040. [3] Y.J. Rao, T. Zhu, X.C. Yang, D.W. Duan, In-line fiber-optic etalon formed by hollow-core photonic crystal fiber, Opt. Lett. 32 (2007) 2662–2664. [4] K. Bhowmik, G.-D. Peng, E. Ambikairajah, V. Lovric, W.R. Walsh, B.G. Prusty, G. Rajan, Intrinsic high-sensitivity sensors based on etched single-mode polymer optical fibers, IEEE Photon. Technol. Lett. 27 (6) (2015) 604–607. [5] H. Bae, M. Yu, Miniature Fabry-Perot pressure sensor created by using UVmolding process with an optical fiber based mold, Opt. Express 20 (2012) 14573–14583. [6] D. Donlagic, E. Cibula, All-fiber high-sensitivity pressure sensor with SiO2 diaphragm, Opt. Lett. 30 (2005) 2071–2073. [7] F. Xu, D. Ren, X. Shi, C. Li, W. Lu, L. Lu, B. Yu, High-sensitivity Fabry-Perot interferometric pressure sensor based on a nanothick silver diaphragm, Opt. Lett. 37 (2012) 133–135. [8] Y.J. Rao, Recent progress in fiber/optic extrinsic Fabry-Perot interferometric sensors, Opt. Fiber Technol. 12 (3) (2006) 227–237. [9] J. Ma, J. Ju, L. Jin, W. Jin, A compact fiber-tip micro-cavity sensor for highpressure measurement, IEEE Photon. Technol. Lett. 23 (21) (2011) 1561–1563. [10] X. Tan, X. Li, Y. Geng, Z. Yin, L. Wang, W. Wang, Y. Deng, Polymer microbubblebased Fabry-Perot fiber interferometer and sensing applications, IEEE Photon. Technol. Lett. 27 (19) (2015) 2035–2038. [11] T.L. Lowder, J.D. Gordon, S.M. Schultz, R.H. Selfridge, Volatile organic compound sensing using a surface-relief D-shaped fiber Bragg grating and a polydimethylsiloxane layer, Opt. Lett. 32 (17) (2007) 2523–2525. [12] X. Ning, Ch. Zhao, F. Shi, J. Kang, Zeolite thin film-coated fiber sensors based on Fabry-Perot interferometer for detection of chemical vapors, Photonic Sensors 5 (2) (2015) 137–141. [13] J.N. Lee, C. Park, G.M. Whitesides, Solvent compatibility of poly (dimethylsiloxane)-based microfluidic devices, Anal. Chem. 75 (2003) 6544– 6554. [14] D. Kacik, I. Martincek, N. Tarjanyi, M. Kuba, Polydimethylsiloxane coated optical fiber sensor for detection of organic volatile compounds, Presented at 11th International conference ELEKTRO 2016, 16–18 May 2016. [15] S.M. Topliss, S.W. James, F. Davis, S.P.J. Higson, R.P. Tatam, Optical fibre long period grating based selective vapour sensing of volatile organic compounds, Sens. Actuators, B 143 (2010) 629–634.
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Regular Articles
Toluene optical fibre sensor based on air microcavity in PDMS Daniel Kacik ⇑, Ivan Martincek Department of Physics, University of Zilina, Univerzitna 1, 01026 Zilina, Slovakia
a r t i c l e
i n f o
Article history: Received 21 July 2016 Revised 10 December 2016 Accepted 22 January 2017
Keywords: Optical fibre sensor Fabry-Perot interferometer PDMS Organic volatile compound Toluene
a b s t r a c t We prepared and demonstrated a compact, simple-to-fabricate, air microcavity in polydimethylsiloxane (PDMS) placed at the end of a single-mode optical fibre. This microcavity creates a Fabry-Perot interferometer sensor able to measure concentrations of toluene vapour in air. Operation of the sensor is provided by diffusion of the toluene vapour to the PDMS, and the consequent extension of length d of the air microcavity in PDMS. The sensor response for the presence of vapours is fast and occurs within a few seconds. By using the prepared sensor toluene vapour concentration in air can be measured in the range from about 0.833 g.m3 to saturation, with better sensitivity than 0.15 nm/g.m3 up to maximal sensitivity 1.4 nm/g.m3 at around concentration 100 g.m3 in time 5 s. Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction Optical fibre sensors have unique properties such as small size, light weight, high sensitivity, biocompatibility, corrosion resistance, immunity to electromagnetic interference, continuous measurements and in-situ monitoring of chemical parameters in industrial processes [1]. The sensors can work in different ways. One of the most sensitive ways is sensing measurands by the interference of light. Configuration of the interferometer depends on different parameters. Usually the interferometer consists of a set of mirrors dividing the light into arms, and then decoupled back to compare the phase of travelled light in particular arms. One interesting aspect of mirror creation is offered by a Fabry-Perot interferometer (FPI). The mirrors that form the Fabry-Perot cavity in fibre optics can be reflective splices between two identical optical fibres [2], the section of photonic crystal fibre between singlemode fibres [3] or etched polymer fibre with Bragg grating [4]. FPIs can be built not only as all-fibre (intrinsic), but also as extrinsic sensors. In the case of extrinsic sensors the typical FPI sensor is built between a cleaved optical fibre end and an elastic diaphragm, such as a polymer diaphragm [5], silica diaphragm [6] or metal diaphragm [7]. Sensors based on FPI have a wide range of applications similar to other optical-fibre sensors based on interference, such as temperature measurement, refractive index, pressure, transverse load, etc. [8]. ⇑ Corresponding author. E-mail addresses: [email protected] (D. Kacik), [email protected]. sk (I. Martincek). http://dx.doi.org/10.1016/j.yofte.2017.01.006 1068-5200/Ó 2017 Elsevier Inc. All rights reserved.
Recently, fibre-tip sensors with microcavities have been proposed. For example, microcavities fabricated by using a fusion splicer and pressurizing gas chamber [9], or applying polymer at the end of a small segment of photonic bandgap fibre [10]. The sensors were proposed in order to detect pressure, but when a polymer with favourable physicochemical properties is used, a similar configuration can also be used for the detection of organic volatile compounds (for example, toluene). The sensing of volatile organic compounds is of importance in a range of applications, for example, monitoring air quality in both indoor and outdoor environments. Volatile organic compounds can originate from fuel and petroleum products, from paints and by combustion processes, natural sources and farming. Human exposure to these chemicals, even at low concentration, can be hazardous due to producing short- and long-term adverse health effects. Sensitivities of particular configurations of sensors for different organic chemicals are different. A sensor based on a surface relief D-shaped fibre Bragg grating had sensitivity to acetone: 1.65 104 pm/ppm from concentration app. 6000 ppm [11]. For zeolite, a thin film-coated fibre sensor based on FPI, the sensitivity was 4.28 103 nm/ppm when the concentration of isopropanol ranged from 350 ppm to 2450 ppm [12]. In Ref. [15] the proposed sensor is based on long period grating coated with calixarene to form the nanostructured coating which achieved sensitivity to toluene 231 ppmv (ppm in volume) with spectrometer resolution of 0.3 nm and recovery time of order of 15 s. In this article, we report a preparation of polydimethylsiloxane (PDMS) FPI located at the end of a single-mode optical fibre (SMF). A fibre tip is treated by applying thin paraffin wax film to the fibre
D. Kacik, I. Martincek / Optical Fiber Technology 34 (2017) 70–73
end. Then the fibre end is packed with PDMS. After curing, the fibre end is heated to diffuse wax to the PDMS. This reduces the adhesion and allows the creation of the microcavity between the fibre end face and the PDMS layer. Then the length of microcavity is fixed by covering it with a second layer of PDMS. Based on toluene influence we investigated the function of the FPI fibre. When toluene is applied to such a FPI, the length of the microcavity will change. So we determine the sensor response on toluene concentrations in air. The main advantages of the developed sensor are its compactness and its very short analytical time (response and desorption time), which was found to be 5 s.
2. Operational principle The PDMS microcavity placed at the end of the SMF forms an extrinsic optical fibre FPI. The interferometer consists of two mirrors with reflectivity R1 and R2, respectively. The mirrors are separated by distance d. In our case, the first mirror of the interferometer is formed by the surface S1 with reflectivity R1 at the interface SMF/air, and the second mirror is formed by the surface S2 with reflectivity R2 at the interface air/PDMS at the end of the microcavity. SMF is used as input/output fibre of the interferometer. It is shown schematically in Fig. 1. If the light with intensity I0 is launched to the SMF, then the surface S1 will reflect part I1 of the light, and the rest will be transmitted. A similar effect will take place over an area S2 that reflects the intensity I2. Light reflected from both interfaces will be coupled back into the SMF core and interfere with each other. In our case, as the reflectance of the surfaces S1 and S2 is low, multiple reflections can be neglected and the intensities I1 and I2 can be written as I1 = R1I0 and I2 = (1 R1)R2I0, respectively. So the intensity IR of the light reflected from the air microcavity can be described as
pffiffiffiffiffiffiffiffi 4pd ; IR ¼ I1 þ I2 þ 2 I1 I2 cos k
71
3. Fabrication of air optical microcavity in PDMS For the fabrication of the air microcavity in the PDMS placed at the end of the optical fibre PDMS Sylgard 184 (Dow Corning) was used, which was supplied as two-part liquid component kits. PDMS was prepared by mixing prepolymer (part A) and curing agent (part B) in the ratio 10:1. PDMS possess properties such a hydrophobility, hydrolytic stability, non-flammable, high chemical stability, optically clear and its refractive index is close to that of glass [14]. The fabrication technological process of the air microcavity can be described as follows: after removing the primary fibre coating on the cladding at the end of the single-mode optical fibre a thin paraffin wax film was applied. However, it wasn’t applied to the cleaved face of the SMF. The paraffin wax film had a thickness of approximately 20 lm, and covered the fibre for a length of about 370 lm. Then the first layer of PDMS in a teardrop shape was applied to the end of the fibre up to the distance where the wax was. Also, the PDMS was applied to the face end of the SMF (Fig. 2A). After curing the fibre end with PDMS, the coating was heated to 70 °C for 60 min. During heating the paraffin wax changed to a liquid state. During this time, liquid paraffin wax diffused into the PDMS. As a result, the adhesion between the PDMS and the cladding of SMF was reduced. Then, at room temperature the PDMS layer was moved and between the fibre end and PDMS the air optical microcavity was created (Fig. 2B). The length of microcavity d was controlled by measurement of interference pattern and set to 70 lm. In order to fix the length of the optical microcavity at the end of the fibre, the first PDMS layer was encapsulated by a second layer of PDMS (Fig. 2C). The second layer of PDMS covered
ð1Þ
where d is the length of air microcavity in PDMS and k is wavelength of used light in a vacuum. It is known that liquid and gaseous toluene is well infiltrated into the PDMS, causing increases in volume [13] also like other organic volatile compounds. In our case, diffusion of toluene vapour to the PDMS caused increases of the air microcavity length. Thus, it results in an optical phase shift of the reflected intensity I2. The optical phase shift in the PDMS Fabry-Perot interferometer at constant temperature and constant pressure depends on the time of influence, and on the toluene vapour concentration in the PDMS.
Fig. 1. Schematic of the extrinsic optical fibre Fabry-Perot interferometer consisting of single-mode optical fibre (SMF) as input/output fibre and two mirrors with surfaces S1 and S2 separated by distance d.
Fig. 2. Technological process of preparation of air optical microcavity in polydimethylsiloxane (PDMS) located at end of single-mode fibre (SMF).
72
D. Kacik, I. Martincek / Optical Fiber Technology 34 (2017) 70–73
Fig. 3. Photograph of prepared air optical microcavity in PDMS placed at end of optical fibre.
the fibre for a length of about 700 lm from the end of fibre. The described technological process of the preparation of an optical microcavity is shown schematically in Fig. 2. Volume and shape of PDMS layers prepared by the fabrication process change only slightly so the sensitivity of the sensor change slightly, too. A photograph of the prepared air optical microcavity in PDMS is shown in Fig. 3. 4. Application to sensing of toluene concentration The experiment set-up for investigation of influence of toluene concentration based on FPI is shown in Fig. 4, where a broadband light source (SLED Safibra OFLS-6), with central wavelength 1550 nm and 100 nm spectral range is used as an incident light, and an optical spectral analyser (Anritsu MS9710B), with a resolu-
tion of 0.07 nm is used to record the reflected spectra, combined with a 3 dB coupler. As the gas chamber we used glass container with narrow neck with inner volume 1 L in which we left to evaporate defined volume of liquid toluene. The container was slowly rotated for the purpose of uniform distribution of toluene vapour in chamber. The temperature and pressure in the chamber was controlled and kept constant. In the experiment, the concentration of toluene in the chamber changed from 0.833 g.m3 to 140.8 g. m3. We placed the optical fibre sensor head into the chamber and measured the spectral dependence of FPI in selected time. After that time we took the sensor out of the chamber. Before another measurement we waited until measured spectral dependence of FPI was equal to the original spectral dependence without influence of toluene. Then we realized the measurement for another concentration of toluene. Fig. 5 shows the reflection spectra for the three different concentrations of toluene vapour in the wavelength range 1500 nm– 1600 nm, under constant atmospheric pressure and laboratory temperature 25 °C. Time of applying toluene vapours on sensor head was only 5 s. It is unlikely, that refractive index of PDMS is markedly changing due to infiltration of toluene vapour into the PDMS material. For such short time the toluene vapour only interacts with surface layers of PDMS coating, which stretched. Ultimately, thanks to the elastic properties of PDMS it is also reflected in the change of Fabry-Perot resonator length inside the PDMS. The refractive index of the fibre is 1.45 and the refractive index of PDMS is 1.40 at wavelength 1550 nm. So the reflectivity of the fibre end R1 is about 4 per cent, and the reflectivity of the polymer diaphragm R2 is about 3 per cent. As can be seen in Fig. 5, the intensity of reflected light at the interface between the fibre end and air I1 is approximately the same as I2 at the interface between the air and the polymer diaphragm. Also the particular reflection spectra have an equidistant period (in all spectra the space period between maxims is the same).
Fig. 4. Experimental set-up for investigation of toluene concentration consisting of broadband light source (SLED), optical spectral analyser (OSA) and 3 dB coupler.
Fig. 5. Comparison of reflected spectra for zero concentration of toluene in air with reflected spectra for concentration 33 g.m3 and 66 g.m3. Wavelength of reference maxima shifts to higher wavelengths when toluene vapour is applied. The spectral dependencies are not corrected to spectral characteristic of light source.
D. Kacik, I. Martincek / Optical Fiber Technology 34 (2017) 70–73
73
5. Conclusion
Fig. 6. Dependence of wavelength shift Dk on toluene concentration in air.
We prepared and demonstrated a compact, simple-to-fabricate, air microcavity in PDMS, placed at the end of a single-mode optical fibre for toluene vapour in air sensing. In the sensor, the wavelengths and distances of maximums and minimums of the reflected interference spectra depend on the toluene vapour concentration at measured points of space. For the operation of the sensor broadband light source can be used and low-resolution optical spectral analyser. The sensor response for the presence of vapours is fast and occurs within a few seconds. Sensitivity depends strongly on the concentration of toluene vapours in air. By using the prepared sensor, the toluene vapour concentration in air can be measured in the range from about 0.833 g.m3 to saturation, with a sensitivity of better than 0.15 nm/g.m3 up to maximal sensitivity of 1.4 nm/g m3 at around a concentration of 100 g.m3 in time 5 s. Acknowledgments This work was supported by Slovak National Grant Agency No. VEGA 1/0491/14, 1/0278/15 and the Slovak Research and Development Agency under the contract No. APVV-0395-12, APVV-150441 and the R&D operational program Centre of excellence of power electronics systems and materials for their components I. No. OPVaV-2008/2.1/01-SORO, ITMS 26220120003 funded by European regional development fund (ERDF) and the project ITMS 2610120021, co-funded from EU sources and European Regional Development Fund. References
Fig. 7. Dependence of sensitivity on toluene vapour concentration.
In our previous paper we investigated as the reflected spectra were evolving in time [14], but we realized that this technique was not suitable for sensor application, owing to the length of time (app. 40 min). So we decided to measure the value of concentration of toluene in time 5 s. The recovery time after measurement was better than 30 s. For evaluation, we chose one maximum (wavelength of the maxima) for reference at the reflected spectra for zero concentration of toluene in air, and compared it with the corresponding wavelength maxima of reflected spectra for particular toluene concentration. In free spectral range of Fabry-Perot interferometer it is possible to determine toluene vapour concentration up to 50 g.m3. After that as can be seen in Fig. 5, a 2p jump occurs. The dependence of the wavelength shift on the toluene concentration is shown in Fig. 6. Also the root mean square (rms) is shown for particular concentrations. For all concentrations the rms was better than 2 per cent. For example, for maximal concentration the value Dk was 60 nm ± 0.7 nm. The dependence of the wavelength shift on the toluene concentration in air has a non-linear character. So it does not have the same sensitivity in the investigated interval of toluene vapour concentration. The dependence of the sensitivity on the concentration obtained by derivation of the response function (shown in Fig. 6) is shown in Fig. 7. As can be seen in Fig. 7 the sensitivity is better than 0.15 nm/g. m3 in all investigated interval of toluene vapour concentrations in air. The highest sensitivity, 1.4 nm/g.m3 of proposed optical fibre Fabry-Perot interferometer, is around concentration 100 g.m3.
[1] E. Udd, W.B. Spillman Jr., Fiber Optics Sensors, second ed., John Wiley, Hoboken, New Jersey, 2011. [2] C.E. Lee, R.A. Atkins, H.F. Taylor, Performance of a fiber-optic temperature sensor from 200 to 1050 °C, Opt. Lett. 13 (1988) 1038–1040. [3] Y.J. Rao, T. Zhu, X.C. Yang, D.W. Duan, In-line fiber-optic etalon formed by hollow-core photonic crystal fiber, Opt. Lett. 32 (2007) 2662–2664. [4] K. Bhowmik, G.-D. Peng, E. Ambikairajah, V. Lovric, W.R. Walsh, B.G. Prusty, G. Rajan, Intrinsic high-sensitivity sensors based on etched single-mode polymer optical fibers, IEEE Photon. Technol. Lett. 27 (6) (2015) 604–607. [5] H. Bae, M. Yu, Miniature Fabry-Perot pressure sensor created by using UVmolding process with an optical fiber based mold, Opt. Express 20 (2012) 14573–14583. [6] D. Donlagic, E. Cibula, All-fiber high-sensitivity pressure sensor with SiO2 diaphragm, Opt. Lett. 30 (2005) 2071–2073. [7] F. Xu, D. Ren, X. Shi, C. Li, W. Lu, L. Lu, B. Yu, High-sensitivity Fabry-Perot interferometric pressure sensor based on a nanothick silver diaphragm, Opt. Lett. 37 (2012) 133–135. [8] Y.J. Rao, Recent progress in fiber/optic extrinsic Fabry-Perot interferometric sensors, Opt. Fiber Technol. 12 (3) (2006) 227–237. [9] J. Ma, J. Ju, L. Jin, W. Jin, A compact fiber-tip micro-cavity sensor for highpressure measurement, IEEE Photon. Technol. Lett. 23 (21) (2011) 1561–1563. [10] X. Tan, X. Li, Y. Geng, Z. Yin, L. Wang, W. Wang, Y. Deng, Polymer microbubblebased Fabry-Perot fiber interferometer and sensing applications, IEEE Photon. Technol. Lett. 27 (19) (2015) 2035–2038. [11] T.L. Lowder, J.D. Gordon, S.M. Schultz, R.H. Selfridge, Volatile organic compound sensing using a surface-relief D-shaped fiber Bragg grating and a polydimethylsiloxane layer, Opt. Lett. 32 (17) (2007) 2523–2525. [12] X. Ning, Ch. Zhao, F. Shi, J. Kang, Zeolite thin film-coated fiber sensors based on Fabry-Perot interferometer for detection of chemical vapors, Photonic Sensors 5 (2) (2015) 137–141. [13] J.N. Lee, C. Park, G.M. Whitesides, Solvent compatibility of poly (dimethylsiloxane)-based microfluidic devices, Anal. Chem. 75 (2003) 6544– 6554. [14] D. Kacik, I. Martincek, N. Tarjanyi, M. Kuba, Polydimethylsiloxane coated optical fiber sensor for detection of organic volatile compounds, Presented at 11th International conference ELEKTRO 2016, 16–18 May 2016. [15] S.M. Topliss, S.W. James, F. Davis, S.P.J. Higson, R.P. Tatam, Optical fibre long period grating based selective vapour sensing of volatile organic compounds, Sens. Actuators, B 143 (2010) 629–634.