In this paper, a novel TiO2 nanoparticle thin film coated optical fiber Fabry-Perot (F-P) sensor had been developed for refractive index (RI) sensing by monitoring the shifts of the fringe contrast in the reflectance spectra. Using in situ liquid phase deposition approach, the TiO2 nanoparticle thin film could be formed on the fiber surface in a controlled fashion. The optical properties of as-prepared F-P sensors were investigated both theoretically and experimentally. The results indicated that the RI sensitivity of F-P sensors could be effectively improved after the deposition of nanoparticle thin-films. It was about 69.38 dB/RIU, which was 2.6 times higher than that of uncoated one. The linear RI measurement range was also extended from 1.333~1.457 to 1.333~1.8423. More importantly, its optical properties exhibited the unique temperature-independent performance. Therefore, owing to these special optical properties, the TiO2 nanoparticle thin film coated F-P sensors have great potentials in medical diagnostics, food quality testing, environmental monitoring, biohazard detection and homeland security, even at elevated temperature.
© 2013 OSA
Owing to low cost, easy production, miniature size, immunity to electromagnetism, low power consumption, non-corrosiveness and remote operation, the optical fiber based refractive index (RI) sensors, for instance of the whispering gallery mode (WGM) microresonator, Mach–Zehnder interferometer (MZI), fiber Bragg gratings (FBGs) and long-period fiber gratings (LPFGs), etc, have attracted increasing research interests in the diverse fields of medical diagnostics, food quality testing, environmental monitoring, biohazard detection and homeland security [1–15]. However, there are several disadvantages of these sensors that strongly suppress their further applications. For example, the WGM microresonators have ultrahigh quality factor and extremely high resolution to external RI changes [1–4], but the difficulty of surface modification and low reliability cannot be overcomed easily. In the case of MZI [5, 6], its fragile structure is always a big problem. In addition, prior to the RI measurement, the FBGs [7, 8] and LPFGs [9–12] have to suffer the dangerous etching process in order to achieve a high sensing sensitivity. Most importantly, the temperature cross-sensitivity of above-mentioned sensors is hard to eliminate so that most of them are not suitable to work under elevated temperature. Therefore, it is still a challenge to develop a new fiber based RI sensor, which can offer the desirable advantages, such as high sensitivity, large measurement range and temperature-immunity, etc [13–20].
To this reason, a novel optical fiber Fabry-Perot (F-P) sensor had been developed recently [16, 17]. As shown in Fig. 1(a) , F-P sensor contains an in-fiber air cavity near a cleaved end of a single-mode fiber (SMF). In contrary to other optical fiber sensors, its optical properties are insensitive to the temperature. There are two ways to measure the external RI changes using F-P sensors: the first one is to inject medium into the tiny F-P cavities and then evaluate the RI value by monitoring the stripe shift of the reflective spectra [13, 16]. Although the sensing sensitivity is very high using this method, the operation is too complicated because numerous steps of injection, suction, and cleaning should be required. The second one is to immerse the F-P sensor directly into the measured medium and then deduce the RI value through the fringe contrast of the reflectance spectra [14, 17]. It is a simple and convenient way. However, its sensing sensitivity is not high enough and its measurement range, which is defined as the regime of linear relationship between RI and the fringe contrast, is too narrow. Therefore, to enhance the sensing sensitivity and broaden the measurement ranges are the prerequisites for their further applications.
It had been demonstrated that the sensing sensitivity of fiber based optical sensor can be enhanced by coating a higher RI (HRI) thin film on the surface [21–27]. Numerous methods, for instance of Langmuir-Blodgett (LB) technique, Electrostatic Self-Assembly (ESA) approach and Sol-Gel method had been developed to produce the HRI overlays. Among them, the LB technique is readily adapted to facilitate uniform deposition of organic molecular thin films onto fiber surface at room temperature, giving the control of the film thickness at nanometer scale. For example, the side-polished optical fiber devices coated LB films have been used as wavelength filters  and chemical sensors . However, the strict operating conditions and the lack of suitable building materials strongly limit their applications. ESA technique, which is based on a continuous assembly between positively and negatively charged materials, offers an easy and inexpensive process for multilayer formation. E. Simoes  had deposited polyelectrolyte films (PEM) on the LPFG surface using this technology and proved that the cladding mode could be guided by overlays if the films were thick enough. Previously, we had also developed the novel PEM coated LPFGs for optical sensing of sucrose [24, 25]. It had been demonstrated that the RI and film thickness of PEM overlays played the important role on the sensing sensitivity of LPFGs [24, 25]. However, mainly due to time consuming for achieving optimizing thickness, these PEM coated LPFGs cannot meet the requirement of rapid fabrication of the RI sensors [23, 24]. Sol-Gel method can be used to produce a HRI thin film , however, it is very hard to precisely control the film structures.
Recently, a novel in situ liquid phase deposition (LPD) approach had been utilized to produce the nanoparticle thin film (for example of TiO2, V2O5, SnO2 and SiO2.) . This method not only holds great potential for reduced production costs and environmental impact, but also has a high level of control over composition, microstructure and growth rates of the resulting films. Thus, it is very interesting to investigate the influence of the nanoparticle thin films on optical properties and sensing sensitivity of F-P sensors. To this end, in this work, we produce a TiO2 nanoparticle thin film coated F-P sensor for RI sensing using LPD approach. The theoretical simulation and experimental results indicated that the RI sensitivity of the F-P sensor could be effectively improved after the deposition of nanoparticle thin-film. And the linear RI measurement range could be broadened by 40%. More importantly, their optical properties exhibited the unique temperature-independent performance. As far as known, such novel F-P sensors had not been reported yet.
2. Theory analysis and simulation
The schematic diagram of the thin film-coated F-P sensor is shown in Fig. 1(a), which contains an in-fiber air cavity near a cleaved end of a SMF and the deposited thin-film adhering to the fiber tip. For RI measurement, the sensor tip is dipped into the liquid medium. The reflection principle of F-R sensors is illustrated in Fig. 1(b). There are four reflection surfaces in the sensor head. The lengths of air cavity, fiber cavity and film cavity are L1, L2, and L3, respectively. While the cavity lengths of L1 and L2, are the constants. The RI of the fiber (n0) and RI of the film (nf) are also constants and in general, nf is larger than n0. The power reflection coefficients at surfaces 1, 2, 3 and 4 are R1, R2, R3, and R4, respectively. Here, R1 and R2 are both equal to (n0-1)2/(n0 + 1)2 = 0.034<<1; R3 is (n-nf)2/ (n0 + nf)2, (n is the RI of medium); and R4 is (n-nf)2/(n + nf)2,which is also much smaller than unity in practical situations. It should be noted that only the condition of nf > n is considered in following analysis.
The total reflected field from the sensor is given approximately by the sum of the first-order reflected fields from the four surfaces [13–15]. The total contribution from the high-order reflections is less than 0.1% because of the low reflection coefficients and therefore can be neglected. As shown in Fig. 2 , the total reflected electric field, Er can be thus given by
From Eq. (1), the normalized reflection spectrum REP(λ) is obtained as follows:
Equation (2) describes the interference pattern of the reflected light from the sensor head for nf >n. According to Eq. (2), the REP(λ) is independent of the power of the input light. In addition, since both the thermal expansion coefficient and the thermo-optic coefficient of the fiber are very small, the REP(λ) is insensitive to the temperature. Here, it should be noted that only the reflection coefficient R4 depends on n.
In order to simulate the interference spectrum, the parameters are set as L1 = 30μm, L2 = 1020 μm, L3 = 1 μm, n0 = 1.457, R1 = R2 = 0.034, R3 = 0.01386, R4 = 0.0883, nf = 1.8458, and n = 1 (air). The simulated reflective spectrum was depicted in Fig. 3(a) . The low frequency modulation caused by the SMF cap can be observed in the spectral range from 1510 nm to 1590 nm. Each reflective spectrum consists of a large number of the fringes, which is due to the air cavity and film cavity, as shown in Fig. 3(b). Normally, the fringe is identified by locating the absolute minimum dip (λ2) and the adjacent peak (λ1). For nf >n, the corresponding fringe contrast is given by P = 10log10 [REP(λ1)/REP(λ2)]. As shown in Fig. 3(b), the fringe contrasts are not uniform across the reflective spectrum. Among them, one can find the maximum fringe contrast, which can be used to evaluate the sensing sensitivity of F-P sensors.
3 Results and discussion
In this work, an F-P sensor with a circular hole of a depth of ~26 μm and a diameter of ~54 μm had been produced by using a 157 nm laser micro-maching system. Firstly, a circular micro-hole had been created at the fiber tip using the 157 nm laser. Then the micro-machined fiber was spliced to another cleaved fiber in order to obtain the air cavity. The length of the air cavity was 28 μm and the distance from the air cavity to the fiber end was 1018 μm. A reflective optical analyzer instrument (MOI SM125) was utilized to measure the reflective spectrum of the F-P sensor.
As discussed before, the TiO2 nanoparticle thin-films should be deposited on the surface of sensor tip in order to improve the sensing sensitivity. Using in situ LPD approach , the oxide thin films are obtained by the slowly hydrolyzation process. As shown in Fig. 4(a) , a baker containing 0.04 M titanium tetrafluoride (TiF4) aqueous solution was put in the water bath. The hydrolyzation temperature was set at 50 °C and the film-growth speed was controlled at ~1.5 nm/min.
The time-resolved reflection spectra of F-P sensor were recorded by MOI SM125 and used to monitor the film growth. As shown in Fig. 4(b), the reflective spectra were gradually changed with the increasing of reaction time (or film thickness). After 160 mins (from 15:30 to 18:10), the fringe contrast decreased from 34 dB to 0.41 dB. Then it increased back to 25.240 dB after 160 mins (from 18:10 to 20:50). The relationship between the fringe contrast and the reaction time (or film thickness) was shown in Fig. 4(c). It could be observed that the fringe contrast displays an up-and–down curve with the increase of TiO2 film thickness. Obviously, the maximum fringe contrast can be reached at the film thickness of ~240 nm.
After optimization, the coated F-P sensors with film thickness of 240 nm and fringe contrast of 34.89 dB had been used to measure sucrose solution (10% ~60%), chloroform (n = 1.4467), benzene (n = 1.5012) and CS2 (n = 1.6276) solution, respectively. The relationship between fringe contrast and external RI were shown in Fig. 5 . With the increase of n, the fringe contrast of coated F-P sensor linearly decreased. The slope, which is so called the fringe contrast sensitivity, is about 69.38 dB/RIU. It is 2.56 times more than that of the uncoated sensor (27 dB/RIU). On the other hand, the inflection point (IP) is 1.8423, according to the extension of red line, as shown in Fig. 5. It is almost equal to the nTiO2. Compared to the inflection point of 1.457 for the uncoated sensor, the RI measurement range is enlarged by ~40%. Therefore, after coating by TiO2 nanoparticle thin film, both the RI sensitivity and measurement range of the sensor are highly improved.
In addition, as shown in Fig. 6 , the fringe contrasts kept constant over the temperature range from 10 to 90 °C. This unique temperature independent feature is one of the main advantages of F-P sensors.
In conclusion, a novel TiO2 nanoparticle thin film had been deposited on surface of Fabry-Perot (F-P) sensor by in situ liquid phase deposition approach. With the help of TiO2 thin film, a RI sensing sensitivity of 69.38 dB/RIU was effectively achieved, which was about 2.6 times higher than that of uncoated one. The linear RI measurement range could be extended from 1.333~1.457 to 1.333~1.8423. More importantly, their optical properties exhibited the unique temperature-independent performance. Therefore, owing to these special optical properties, the as-formed coated F-P sensors have great potentials in medical diagnostics, food quality testing, environmental monitoring, biohazard detection and homeland security, even at elevated temperature.
The authors gratefully acknowledge financial support for this work from the National Natural Science Foundation of China (Grant No. 91123029, 61077066 and 61074163), the 863 project of China (Grant No. 2012AA063302) and the Natural Science Foundation of Shandong Province, China (Grant No. ZR2011FQ025, No. ZR2012CM029).
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