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Abstract
A compact fiber-optic Fabry-Perot interferometric (FFPI) volatile organic compounds (VOCs) sensor is fabricated based on a single mode fiber (SMF) and a polymethyl methacrylate (PMMA) film. The VOCs induce the swelling effect and refractive index changes of PMMA film. The swelling of PMMA film affects the cavity length of Fabry-Perot interferometer resulting in shift of resonant dip, while the change in refractive index influences reflection resulting in change of extinction ratio (ER). The sensitivities of 2.7 pm/ppm for ethanol and 2.17 pm/ppm for acetone are achieved. Moreover, this sensor is insensitive to the inorganic gases. In addition, it has the advantages of ease of fabrication and high sensitivity to VOCs.
© 2017 Optical Society of America
1. Introduction
Volatile organic compounds (VOCs) are a very common and vitally important substance. They are widely applied in various fields such as chemical industry, medical and health, food processing and industrial safety production. There is a great demand for the measurement of VOCs with high sensitivity and accuracy, such as ethanol and acetone. In recent years, many one-dimensional (1D) nanostructural materials, such as nanotubes [1, 2], nanowires [3, 4], nanorods [5, 6] and nanoparticles [7, 8] have been synthesized and used to fabricate ethanol and acetone gas sensors. However, these approaches have some limitations, the fabrication of nanostructural materials needs various complicated physical and chemical process, expensive apparatus and rigorous condition. It is also difficult to use the electrical sensors to detect the flammable and explosive gases such as ethanol and acetone. Therefore, it is of great importance to develop a sensor to detect these gases with high sensitivity, intrinsic safety and low cost.
Fiber-optic sensors can offer many advantages, such as light-weight, wholly passive, resistance to electromagnetic interference, and have already been applied for numerous sensing applications. The related fiber-optic sensors include fiber Bragg grating (FBG) [9–11], microfiber [12–15], photonic crystal fiber [16–18], Mach-Zehnder interferometer [19, 20] and Fabry-Perot interferometer [21–23]. Recently, increasing attention has been paid to combine gas sensitive materials with fiber-optic microstructures for various gas sensing, such as those based on metal oxide [24], grahene [25–28], catalysts [29] and polymer [30,31].
In this paper, we proposed a fiber-optic Fabry-Perot interferometric sensor based on a PMMA film with a thickness about 300 nm which shows a good sensitivity and selectivity for VOCs sensing. The Fabry-Perot cavity is formed by a fiber end and a PMMA film which coated at the tip of a glass tube. The PMMA exhibits a swelling effect and a refractive index change [32–34] when adsorbing the VOC molecules. The swelling of PMMA film affects the cavity length of Fabry-Perot interferometer resulting in shift of resonant dip, while the change in refractive index influences reflection and thus it’s ER. By monitoring the variation of resonant dip and ER, we can detect the concentration of the VOCs. In this experiment, the inorganic gases of ammonia (NH3), carbonic oxide (CO) and hydrogen (H2) are also used for sensing to demonstrate the selectivity of this VOCs sensor.
2. Sensing principle and fabrication of FFPI sensor
The schematic diagram of the FFPI sensor is shown in Fig. 1(a). The Fabry-Perot cavity consists of two reflective surfaces, they are, the end surface of SMF and PMMA film. In this structure, the reflection coefficient of SMF end surface is calculated to be approximately 3.6% based on the Fresnel reflection equation R = [(n1-n2)/(n1 + n2)]2 where n1≈1.47 is the refractive index of SMF core and n2≈1 is the refractive index of air. According to the two-beam interference, the maximum ER can be obtained when the intensity of the two beams are equal. Due to there is a divergence angle when the light is emitting from the end surface of SMF, the effective reflection coefficient of the PMMA film can be controlled by adjusting the distance between the fiber end and the PMMA film. Here, an optimized distance about 114 μm is gained to obtain the maximum ER. The reflective interference spectrum from the Fabry-Perot interferometer is shown in Fig. 1(b). The ER is ~33.6 dB, which is among the highest of FFPIs [35, 36].
Fig. 1 (A) The schematic diagram of the FFPI sensor. (B) The reflective interference spectrum of the FFPI sensor.
The reflective spectrum of the sensor can be expressed as
Here, IFiber (λ) is the reflective intensity of the fiber end and IFilm (λ) is the reflective intensity of the PMMA film, both of which are only slightly dependent on the incident wavelength. OPD = 2l is the optical path difference, and l is the length of cavity. The minimum intensity of the interference occurs at the point λmin when OPD = mλ + λ/2, where m is integer. When the VOCs molecules are absorbed into PMMA film, the PMMA film will expand to increase the OPD resulting in the resonant dip λm shifts to a longer wavelength based on the Eq. (1). Meanwhile, the ER of the interference fringe can be written asInitially, IFiber (λ) and IFilm (λ) are modulated almost equal to acquire the largest ER. When the VOC molecules are absorbed into PMMA film, the refractive index of PMMA film will increase. And the IFilm (λ) will increase resulting in the ER becomes smaller based on the Eq. (2). By measuring the shifts of resonant dip and the changes of ER, the external VOC concentrations can be determined.Figure 2 shows the fabrication process of the FFPI sensor. A mixed solution was prepared by putting the PMMA powder into ethyl lactate with a mass ratio of 4:100 and stirred by a magnetic stirrer for 48 hours in order to make PMMA dissolved into ethyl lactate completely. Then, the mixed solution was transferred onto a Cu foil and spin-coated by spin coater with a rotation speed of 5000 r/min for 30 seconds. The coated Cu foil was put onto a heating stage with the temperature of 150 °C to evaporate the ethyl lactate. Subsequently, the underlying Cu foil was etched by immersing it into a ferric chloride (FeCl3) solution. The PMMA film was washed in deionized water several times and then transferred onto the end surface of glass tube. Finally, a cleaved single mode fiber (SMF) was inserted into the glass tube and fixed by a UV glue to form a Fabry-Perot cavity.
3. Gas sensing experiments
In our experiment, the gas sensing responses of the sensor were tested in a constant-temperature environment. The experimental setup is shown in Fig. 3. A wavelength-swept laser with a wide tunable range was used as the light source (81960A, Agilent, USA). The reflected light from the sensor was collected and analyzed by a power detected-based optical spectrum analyzer (OSA, N744A, Agilent, USA). The FFPI sensor was placed in a gas chamber. The outlet of the gas chamber was connected with the external environment. So the pressure inside the gas chamber was equal to the atmospheric pressure. Firstly, the dry N2 gas was injected into chamber to exhaust the air and water vapor inside. Ethanol and acetone gases were used as examples of VOCs in the sensing experiments. The gas concentrations were obtained by evaporating the corresponding liquid in a container of a fixed volume.
The concentrations of ethanol and acetone were controlled between 0 and 1800 ppm with a step of 300 ppm. The reflection spectrum of the sensor shifts to the longer wavelength by 0.82 nm, 1.6 nm, 2.42 nm, 3.21 nm, 3.92 nm and 4.72 nm with the ethanol gas concentrations of 300, 600, 900, 1200, 1500, 1800 ppm respectively as shown in Fig. 4(a). Wavelength shifts of 0.68 nm, 1.31 nm, 1.95 nm, 2.53 nm, 3.06 nm and 3.74 nm were obtained with acetone gas concentrations of 300, 600, 900, 1200, 1500, 1800 ppm respectively as shown in Fig. 4(b). According to the interference theory of Fabry-Perot, there is a formula of λ = 2nL/(m + 1/2). Here, m is an integer, λ is the wavelength of resonant dip, n is the refractive index of cavity, and L is the length of cavity. The resonant dip λ shifts to longer wavelength as the gas concentration increases, indicating the swelling of PMMA film increases the length of interference cavity. Meanwhile, a significant ER change of the interference fringes can also be observed. For ethanol, the ER decreased by 3.1 dB, 7.34 dB, 10.22 dB, 12.71 dB, 14.28 dB and 14.95 dB, respectively. For acetone, the decreases of ER were 3.32 dB, 7.31 dB, 9.15 dB, 11.8 dB, 13.95 dB and 15.36 dB, respectively. The decrease of ER and the increase of maximum intensity λmax indicated the refractive index of PMMA film increased, resulting in the increase of reflected light intensity from the PMMA film.
Fig. 4 The experimental results of FFPI sensor for (A) ethanol, (B) acetone, (C) NH3, (D) CO and (E)H2 gas sensing.
To investigate the selectivity of the sensor, the inorganic gases of NH3, CO and H2 were detected at the same concentrations, as shown in Figs. 4(c)-4(e). The interference spectrum is enlarged and shown as a function of the gas concentration increases. The reflection spectrum of the sensor shifts to longer wavelength are 0.25 nm, 0.1 nm and 0.009 nm for NH3, CO and H2 with the concentration of 1800 ppm are shown in Figs. 4(c)-4(e), respectively. These experimental results demonstrate the low sensitivity to these gases, indicating that this sensor has excellent selectivity to acetone and ethanol. This selective response of PMMA film to organic molecules can be explained by following mechanism [37]. The PMMA reacts with most organics. They all have the same atomic bond structure of –CH2C–. The interaction of PMMA and VOC molecules was physical reaction instead of chemical reaction. When the VOC molecules diffused into the relatively large molecular gap of PMMA, the PMMA molecules were separated from each other resulting in the swelling effect. Since NH3, CO and H2 gases are not VOCs, they were difficult to diffuse into PMMA to induce the swelling effect.
The linear fitting relationship between resonant dip shift and gas concentration is shown in Fig. 5(a). The curve fitting relationship between ER change and gas concentration is shown in Fig. 5(b). It is indicated that resonant dip shift of sensor has a good linear response for ethanol and acetone gas sensing. Because of the refractive index change tended to be saturated when more and more gas molecules were absorbed into PMMA film. So the ER change tended to be saturated, the ER change is not linear when the gas concentration is up to 900 ppm. The swelling effect has a bigger dynamic range than the refractive index change of PMMA film indicates that this FFPI sensor could work over a wide range by measuring the resonant dip shift. The ER change has a sensitivity of 0.011 dB/ppm and 0.01 dB/ppm for ethanol and acetone sensing, respectively. Also, the resonant dip shift has a sensitivity of 2.7 pm/ppm and 2.17 pm/ppm and the minimum concentration level of this sensor is ∼0.74 ppm and 0.93 ppm for ethanol and acetone sensing, mainly limited by the resolution of 2 pm of the optical spectrum analyzer. Figure 5(c) presents the temporal recoverability of the FFPI sensor by cyclically exposing the sensor in ethanol and acetone gas environment. It is demonstrated that this sensor has good repeatability.
Fig. 5 (A) Dip shift and (B) ER change of the FFPI sensor as a function of the gas concentration increase. (C) The recoverability of the FFPI sensor.
4. Conclusion
In conclusion, a compact fiber-optic Fabry-Perot interferometric sensor based on PMMA film is proposed and experimentally demonstrated. This sensor shows the characteristics of highly sensitive and selective detection for VOCs concentration at room temperature. Moreover, the sensor shows good repeatability for VOCs sensing. The advantages of high sensitivity and selectivity for VOCs, easy to be fabricated and low cost make this sensor a big promising method for VOCs sensing applications. In our further work, we can combine fiber-optic Fabry-Perot interferometric with different high sensitive film such as graphene based functional materials and metal doped two-dimensional thin films, which can greatly promote its applications for different gas sensing.
Funding
National Natural Science Foundation of China (NSFC) (61290312, 61475032, and 61575039); Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT1218); 111 Project (B14039).
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