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Abstract
A compact and robust fiber temperature sensor based on a hermetically-sealed liquid-filling Fabry–Perot (FP) cavity was fabricated by low-cost but efficient processes, including fusion splicing, liquid injection, and fused tapering. Owing to the high thermal optical coefficient (TOC) of the ethanol, the optical path difference (OPD) in the FP cavity varied strongly with temperature, which consequently induced a drastic wavelength shift of the reflection spectrum. Meanwhile, the low freezing point of the ethanol caused the fiber sensor to have the ability of detecting the sub–zero temperatures. As a result, a linear sensitivity as high as 429 pm/°C was achieved in the range between -5 °C and 30 °C. In addition, our fiber temperature sensor also exhibited rapid response time, good repeatability, and stability. The biocompatible structure, low fabrication cost, and high performance of such a temperature sensor can provide it potential for biological applications.
© 2017 Optical Society of America
1. Introduction
In the fields of food engineering, pharmaceutical production and biosciences, the continuous monitoring of the temperature is of great importance to assess the process of the biochemical reactions. For such applications, the temperature sensors that are highly sensitive and that possess the biocompatibility and long-term stability are very desirable. To meet such engineering requirements, many fiber optical biocompatible temperature sensors have been proposed and improved in the past few years. Several all–fiber configurations such as fiber Bragg gratings (FBGs) [1, 2], long period fiber gratings (LPFGs) [3, 4] and fiber interferometers such as Fabry–Perot interferometers (FPIs) [5, 6], Mach–Zenhder interferometers (MZIs) [7–9], have been reported. However, the temperature sensitivities were usually very low due to the relatively low thermal optical coefficient (TOC) and thermal expansion coefficient (TEC) of the fiber material [10].
Recently, methods of fusing the thermal sensitive materials with the fiber configurations have been presented to improve the performance of the biocompatible temperature sensor. The fusing methods were mainly based on introducing thermal–sensitive material on the surface of the sensing part of the fiber optical sensor [11, 12] or infiltration of the thermal sensitive liquid into the air holes of the photonic crystal fiber (PCF) [13–16]. The temperature sensitivities have been increased from 0.15 nm/°C [11] to 16.49 nm/°C [16]. However, there are some drawbacks existed in the above methods, such as complex fabrication processes [11–16], fragile structure [12], bulky and costly processing equipment [11, 13], and expensive raw materials [13–16]. In addition, such temperature sensors are difficult to get into a narrow space for point detection because the temperature sensitivity was usually obtained by monitoring the transmission spectrum.
To enhance the performance of the fiber biocompatible temperature sensor, the fiber configuration based on the FP cavity is a promising candidate to be fused with thermal sensitive materials, owing to its inherent miniature size, simple configuration, and convenient reflection mode of operation. Hernández–Romano et al. presented a fiber temperature sensor based on the FP cavity covered by the biocompatible and nontoxic PDMS [17]. However, the fabrication process was complex and such a temperature sensor suffered from the power fluctuation of the light source. Zhang et al. proposed a simple method of fabricating a biocompatible temperature sensor based on a solid polymer FP cavity by just dipping a cleaved single mode fiber (SMF) into liquid polymer PDMS and heat–curing subsequently [18]. However, the low contrast of the reflection spectrum caused by the spherical reflector increased the difficulty of monitoring the wavelength shift. Apart from the solid polymer FP cavity, a biocompatible liquid–air based FP cavity was fabricated by Llera et al. to increase the temperature sensitivity [19]. However, such a fiber temperature sensor showed some hysteresis in the process of temperature detection, which was adverse for the practical application. In addition, the process of depositing a thin membrane to encapsulate the sensitive liquid increased the difficulty of the fiber device fabrication and the deposited membrane may drop out during the long–term measurement process. Recently, Li et al. reported a temperature sensor of high performance based on a liquid polymer filled FP cavity [20]. However, the use of the femtosecond laser increased the fabrication cost and the fabrication process is time consuming.
In this paper, we reported a high performance biocompatible temperature sensor based on an ethanol–filled Fabry–Perot (FP) cavity. The fiber temperature sensor was fabricated by the low-cost but efficient processes, including fusion splicing, liquid injection and fused tapering. The ethanol-filled FP cavity was hermetically sealed by the collapsed silica wall. The introduction of the thermal-sensitive liquid breaks the sensitivity limitation imposed by traditional all-silica FPI temperature sensors. Our temperature sensor with a full liquid-filled FP cavity was based on the simplest two-beam FPI, which will make the demodulation of the interference spectrum much easier. Owing to the high TOC of ethanol, the OPD between the reflected lights varied strongly with temperature, which in turn modified the interference spectrum and induced a drastic wavelength shift. In addition, such a fiber temperature sensor possessed the ability of detecting the sub–zero temperatures owing to the low freezing point of the ethanol. Experimental results showed that the linear temperature sensitivity of such a fiber sensor was as high as 429 pm/°C in the range between –5 °C and 30 °C. Our fiber temperature sensor also exhibited rapid response time, good repeatability and stability.
2. Sensor fabrication and principle of operation
The fabrication processes of the biocompatible fiber temperature sensor are shown in Fig. 1. Firstly, as shown in Fig. 1(a), a section of cleaved silica capillary tube (SCT1) with outer diameter of 125 µm and inner diameter of 50 µm was fusion spliced with a cleaved SMF (Corning, SMF–28e). Secondly, as shown in Fig. 1(b), the spliced SCT1 was cleaved precisely by using a house-made fiber cleaving platform. Thirdly, as shown in Fig. 1(c), the cleaved SCT1 was fusion spliced with another segment of SCT2 to fabricate a micro–cavity by using a fusion splicer. In the process of fusion splicing, the discharge time and power was adjusted manually to avoid the collapse of the SCT2. The SCT2 with an inner diameter of 5 µm and outer diameter of 125 µm was connected to an ethanol–filled syringe in advance. Fourthly, as shown in Fig. 1(d), the ethanol was injected into the micro–cavity slowly through the feeding SCT2 and the whole process was monitored in real time under a microscope. The real-time filling state was displayed on the screen of the fusion splicer. When the micro–cavity was full of ethanol, it was then moved by a distance of (5~6 mm) from the splicing joint by operating the fiber fixture of the fusion splicer and the SCT2 was tapered to crack by the arc–discharge, as shown in Fig. 1(e). Meanwhile, the ethanol–filled micro–cavity was sealed by the collapsed silica wall. After being tapered, the fabrication of the optical fiber temperature sensor based on an ethanol–filled micro–cavity was finished, as shown in Fig. 1(f). As shown in Figs. 1(c)–1(e), the three main processes to fabricate the liquid–filled micro–cavity were performed only by a fusion splicer. There is no other equipment needed in the process of fabricating the liquid–filled FP cavity. In addition, compared with other methods of sealing the liquid–filled micro–cavity reported in [19, 20], our method is simple but efficient and the ethanol–filled micro–cavity can be sealed permanently and firmly, which can ensure the stability of the fabricated temperature sensor. Owing to the simple fabrication process, the inexpensive raw material and the reusable fabrication equipment, the cost of the temperature sensor is very low.
A schematic diagram of the fabricated temperature sensor is shown in Fig. 2(a). The microscope image of the processed end of the SCT2 is shown in Fig. 2(b). From Fig. 2(a), we can see that the liquid–filled micro–cavity is embedded between the SMF and the feeding silica capillary tube (SCT2). The end of the feeding SCT2 is blocked by its collapsed wall. The interface between the ethanol and the endface of the SMF acts as the reflector 1 and the interface between the ethanol and the endface of the SCT2 acts as the reflector 2. When the FP cavity was filled with the ethanol, the reflectivity (R) of the two reflectors decreased according to the expression:, where nf is the refractive index of the fiber and nm is the refractive index of the medium in the FP cavity. The reflectivity of the silica/ethanol interface (~0.13%) is much lower than that of the silica/air interface (3.5%). Due to low reflectivity of the silica/ethanol interface, high-order FP interference could be neglected. The two reflectors 1 and 2 constitute a two-beam Fabry-Perot interferometer. When a light beam I0 travels from the lead–in SMF to the ethanol–filled FP cavity, it is reflected by the two reflectors respectively. The two reflected light beams (namely I1 and I2) are coupled back to the SMF and interfere with each other. The intensity of the reflected light can be expressed as [21]:
where I1 and I2 are the intensities of the light beams reflected by the two reflectors separately, is the refractive index of the ethanol, L is the length of the FP cavity, φ0 is the initial phase (normally equals to zero), and λ is the optical wavelength. There exists a π-phase shift at reflector 2 when light is reflected from an optically denser medium. According to Eq. (1), the dip wavelength of the spectrum can be deduced by Eq. (2):where λm is the dip wavelength of the mth interference fringe, m is an integer. According to Eq. (2), the free spectral range (FSR), which is the distance between two adjacent wavelengths, can be expressed as:Equation (3) indicates that the FSR can be optimized by changing the length L of the micro–cavity at given wavelengths. The spectrum was measured by an optical spectrum analyzer (OSA) (YOKOGAWA, AQ6370B) with a resolution of 20 pm and a broadband light source (1200–1700 nm). In addition, a circulator was used to transmit the output light and couple the reflected light back. Figure 3 shows the microscope images of the liquid–filled FP cavities with different cavity lengths and the corresponding reflection spectra. From Fig. 3, we can see that when the air medium in the FP cavity was replaced by the ethanol, the insertion loss of the spectra became larger due to the reduction of the Fresnel reflectivity of the two reflectors. Meanwhile, the FSRs of the reflection spectra decreased with the increase of the FP cavity lengths, which is consistent with Eq. (3). The FSRs of the reflection spectra corresponding to the liquid–filled FP cavities with different lengths have been measured to be 33, 23, 18 µm. The corresponding lengths of the FP cavity can be calculated as 25, 35, 44 µm by using Eq. (3), which agree well with the measured results by a microscope.
Fig. 3 (a)–(c) Microscope images of the ethanol-filled FP cavities with different cavity lengths of 24, 36, 45 um measured by a microscope. (d)–(f) The corresponding reflection spectra before and after ethanol filling.
3. Experiment and discussion
The optical fiber sensor with the FP cavity length of 36 µm [Fig. 3(b)] was tested for monitoring the temperature variation. The sensor tip was placed in an ethanol–filled high–low temperature tank with a precision of 0.1 °C. In addition, the ethanol–filled tank was also used to simulate a harsh environment. The temperature of the ethanol–filled tank was cooled down from 30 °C to –5 °C with a step of –5 °C. At each step, the temperature was maintained for 5 minutes to make sure that the temperature in the tank was stable. After each testing, the temperature of the ethanol–filled tank returned from –5 °C to 30 °C. In order to test the repeatability of the temperature sensor, the cooling process was repeated for three times.
During the cooling process, the wavelength shifts of the reflection spectra are shown in Fig. 4(a). By taking a derivative of Eq. (2), the dip wavelengths shift with the temperature variation can be expressed as Eq. (4):
where σTOC is the TOC of the ethanol, αTEC is the TEC of the silica. Since the TEC of the silica (~5.5 × 10−6) is much less than the TOC of the ethanol (~3.7 × 10−4) [22], it can be neglected. Equation (4) can be simplified as Eq. (5):Fig. 4 (a) Evolution of the reflection spectra with the variation of the temperature. (b) Relationship between the temperature variation and the dip wavelength shift.
According to Eq. (5), we can see that the dip wavelength shift is proportional to the variation of temperature. The linear temperature response greatly improves the measurement accuracy and makes the measurement easier. Since the TOC of the ethanol is negative, the dip wavelengths will shift toward the longer wavelength direction when the temperature is cooled down. Figure 4(a) shows the evaluation of the interference spectrum when the fiber temperature sensor was cooled down from 30 °C to –5 °C. During the cooling process, the dip wavelength of the interference spectrum shifted to the longer wavelength direction, namely “red shift”, which is consistent with the results of Eq. (5). Figure 4(b) shows the measured dip wavelength as a function of the temperature. The error bars indicate the standard deviation of three times measurements. One can see that the deviation of measurement results is very small. The sensitivity of the temperature sensor is –0.429 nm/°C and the linearity of the fitting curve is better than 0.999. In addition, given that the spectrum resolution of the OSA is 0.02 nm, so the temperature resolution is 0.047 degree [23]. No obvious change of the interference spectra was observed during the three times testing processes, which indicated that such a temperature sensor showed a good repeatability. Confined by the temperature control device in our lab, the experiment of the sub-zero temperature measurement was just conducted at –5 °C. However, since the optical fiber has the advantages of low temperature resistance [24] and the ethanol has low freezing point, it is envisioned that our temperature sensor has potential for measuring the lower temperatures.
To further investigate the repeatability and the stability of the temperature sensor, the temperature cycle experiments were also conducted. Figure 5(a) shows the response curve of the temperature sensor when it was placed directly from the room temperature of 25 °C to a sub–zero temperature of –5 °C for three cycles. During the process of each testing cycle, the variation of the dip wavelength around 1527.1 nm was recorded once a second. From Fig. 5(a), we can see that the temperature sensor possesses a good repeatability even though it experiences such a sudden temperature change, which also indicates the robustness of our temperature sensor. The response times for the three cooling down processes are 17, 15, 18 seconds separately and that for the three recovering processes are 14, 12, 13 seconds. Compared with that reported in [19, 20], the response time of our temperature sensor is obviously faster. The stability of the temperature sensor is an important issue to consider for the practical application. We put the temperature sensor in a tank full of ice–water mixture, which was used to create a relatively stable temperature. We monitored the variation of a dip wavelength around 1537.7 nm every two minutes. Figure 5(b) shows the dip wavelength drift as a function of time. Shifts of less than 18 pm are obtained, indicating a high stability of the temperature sensor. Considering the resolution of the OSA and the temperature variation during the heat exchange process in the ice–water mixture, the fluctuation of the dip wavelength was within the measuring error, which indicated that the temperature sensor also possessed a good stability when it was used for the continuous monitoring of a certain temperature. The good repeatability and stability is due to the novel method of sealing the liquid-filled micro–cavity, which can avoid the leakage of the liquid radically and consequently ensure the high performance of the temperature sensor.
Fig. 5 (a) Dip wavelength response to the three cycles of alternating temperature between 25 °C and –5 °C. (b) Dip wavelength variation for 100 times temperature measurements of the ice–water mixture.
4. Conclusion
In this paper, an optical fiber sensor based on an ethanol-filled FP cavity was fabricated and used to measure the temperature. The sensing head of the temperature sensor was fabricated by filling a section of SCT with ethanol and fused tapering to seal the micro–cavity subsequently. Owing to the high TOC of the ethanol, a high sensitivity of the fabricated temperature sensor was achieved. Meanwhile, the low freezing point of the ethanol made the fiber sensor capable of detecting the sub–zero temperatures. Owing to the novel method of sealing the ethanol filled FP cavity, the temperature sensor exhibited an excellent stability and repeatability. A high temperature sensitivity of 0.429 nm/°C and good linearity were obtained in the measurement range between –5 °C and 30 °C. In addition, the fabricated sensor is characterized in the simple, nontoxic and cost-effective processes of fabrication, which provide it potential for biological applications.
Funding
National Science Foundation (11574063, 11504070, 11374077); The Science and Technology Development Plan of Weihai (2015DXGJUS002).
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