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A miniature, robust, and highly sensitive optical fiber temperature sensor based on cascaded polymer-microbubble cavities was fabricated by polymer-filling and subsequent heat-curing process. The expansion of polymer cavity results in the compression of microbubble cavity when the sensor is heated. We demodulated the interference spectrum by means of the fast-Fourier transform (FFT) and signal filtering. Since the thermal response of the polymer cavity is positive and that of the microbubble cavity is negative, a high sensitivity of the temperature sensor is achieved by a subtraction between the two reciprocal thermal responses. Experimental results show that the sensitivity of the temperature sensor is as high as 5.013 nm/°C in the measurement range between 20 °C and 55 °C. Meanwhile, such a sensor has potential for mass production, owing to the simple, nontoxic, and cost-effective process of fabrication.
© 2016 Optical Society of America
1. Introduction
All-fiber Fabry-Pérot interferometer sensors (FPIs) are still appealing and indispensable because they have advantages of compact size, high integrality, electromagnetic immunity, and simple configuration. Such sensors have been built and utilized in many fields to detect a variety of parameters, such as strain [1–3], pressure [4–6], refractive index [7,8], and temperature [9–12]. Among the all-fiber FPIs, a single air-cavity based fiber sensor corresponding to two-beam FPI is usually insensitive to temperature because of the negligible thermal optic coefficient (TOC) of air and the low thermal expansion coefficient (TEC) of silica (5.5 × 10−7 /°C) [1–8]. The change of the cavity length induced by temperature variation is small and hence the temperature sensitivity is low. Based on three-beam FPI, the fiber sensor composed of cascaded air-silica cavities, is more sensitive to temperature variation [9–12]. Owing to the higher TOC of the silica (1.1 × 10−5 /°C), the optical path difference (OPD) between the two light beams in the silica cavity changes with temperature more than that in the air cavity. Therefore, the sensitivity of the fiber temperature sensor based on cascaded air-silica cavities can be enhanced [10,12]. Nevertheless, the temperature sensitivity can only be increased to 10 pm/°C scale because of the restriction of the silica material [13].
In recent years, methods to fuse materials of high TOC and TEC with FPIs have been presented to enhance the temperature sensitivity [14–19]. By dipping a cleaved single mode fiber (SMF) into liquid polymer and UV-curing subsequently, a solid polymer cavity based on FPI was formed on the end of the SMF [14,15]. Compared with that of the sensor based on cascaded air-silica cavities, the temperature sensitivity of such a sensor based on solid polymer cavity was raised one order of magnitude higher to 190 pm/°C [14] and 249 pm/°C [15]. Also, an air cavity between the end of SMF and material was fabricated to improve the temperature sensitivity [16–19] because it was apt to deform to change the OPD [19]. For example, air cavities were formed by filling a section of hollow core fiber spliced with SMF with polymer [16] or liquid [17], splicing a section of mercury-filled silica tube with a cleaved SMF [18] and aligning two cleaved SMFs in an iron V-groove [19]. The sensitivities of the fiber temperature sensors based on such air cavities were increased from 1.7 nm/°C [16] to 260 nm/°C [19]. However, several drawbacks existed in the above-mentioned methods, such as complicated fabrication process [17], toxic material [18], and narrow measurement range [19].
In this paper, we fabricate a temperature sensor based on cascaded polymer-microbubble FPI cavities by filling a section of silica capillary tube (SCT) spliced with SMF with polymer and heat-curing subsequently. The fabricated temperature sensor is based on three-beam FPI and demodulated by means of FFT and signal filtering. The OPDs in the polymer cavity and microbubble cavity change simultaneously with temperature variation. Meanwhile, the direction of wavelength shift caused by the polymer cavity is opposite to that caused by the microbubble cavity, for the reason that the microbubble cavity is embedded in the polymer and it is deformed by the thermal expansion of the polymer. Therefore, the thermal response of the polymer cavity is positive and that of the microbubble cavity is negative. The high sensitivity of such a temperature sensor is achieved by a subtraction between the two reciprocal thermal responses. The fabricated temperature sensor exhibits a sensitivity of 5.013 nm/°C in the measurement range between 20 °C and 55 °C. Moreover, such a sensor has potential for mass production, owing to the simple, nontoxic, and cost-effective process of fabrication.
2. Sensor fabrication and experiment
The polymer, Hasun RTV 901 (HR901), was used to fabricate the temperature sensor. HR901 is a kind of silicone material and can be cured by heat. After being cured, the HR901 becomes a stable and solid polymer with refractive index of 1.492. The TEC and TOC of the solid-state HR901 are 2.3 × 10−4 /°C and −1.3 × 10−4 /°C, respectively.
The fabrication process of the temperature sensor is shown in Fig. 1. First, as shown in Fig. 1(a), a section of cleaved SCT with outer diameter of 125 µm and inner diameter of 50 µm was spliced with a cleaved SMF (Corning, SMF-28e) by using a special fusion splicing method. During the process of fusion splicing, the discharge current and discharge position were adjusted manually through using a fusion splicer to avoid the collapse of the SCT. Second, as shown in Fig. 1(b), the spliced SCT with a length of 160 µm was cleaved precisely under a microscope. Third, as shown in Fig. 1(c), the cleaved SCT was fixed on a slide and dripped with liquid HR901. The cleaved SCT was filled with liquid HR901 gradually as a result of capillary action and the whole process was monitored in real time under a microscope. During the polymer-filling process, a microbubble formed when the liquid polymer HR901 approached the endface of the SMF. As a result of the intermolecular cohesion force of liquid, the residual air was wrapped in the liquid to form a microbubble [20]. Finally, as shown in Fig. 1(d), the cleaved SCT with a microbubble cavity embedded in the liquid polymer HR901 was placed into an oven with a temperature of 120 °C promptly and kept for 15 minutes to cure the liquid HR901 completely. After being cured, an optical fiber temperature sensor based on cascaded polymer-microbubble cavities was fabricated. Owing to the simple fabrication process and inexpensive raw material, the cost of the temperature sensor is very low.
A microscope image of the fabricated temperature sensor is shown in Fig. 2(a) and the schematic diagram is shown in Fig. 2(b). From Fig. 2(a), we can see that a tiny polymer cavity (cavity I) exists between the fiber end and the microbubble. A microbubble-shaped air cavity (cavity II) is embedded in the solid HR901. With a microscope, the measured length of cavity I is 6.15 µm and that of cavity II is 46.83 µm. The interface between the end of SMF and polymer together with the two opposite walls of the microbubble act as three reflectors (reflector 1, 2, 3) and constitute a three-beam FPI. From Fig. 2(b), we can see that when a light beam I0 travels from the lead-in SMF to the cascaded polymer-microbubble cavities, it is reflected by the three reflectors respectively. Three reflected light beams (namely I1, I2 and I3) are coupled back to the SMF and interfere with each other. Although there exists another light beam that travels in the longer polymer cavity (cavity III) on the right side of the microbubble cavity, it is weak enough to be neglected due to the propagation loss and absorption of the solid polymer HR901. The intensity of the reflected light can be expressed as [21]:
where I1, I2 and I3 are the intensities of the light beams reflected by the three reflectors separately, np and nair are the refractive index of the polymer and air, L1 is the length of the cavity I, L2 is the length of the cavity II, and φ10, φ20 and φ30 are the initial phases. From Eq. (1), we can see that the reflection spectrum is a superposition of three terms corresponding to different cavities. Each term is a result of two-beam interference. The spectrum was measured by an optical spectrum analyzer (OSA) (YOKOGAWA, AQ6370B) and a wide spectrum light source (1200-1700 nm). In addition, a circulator was used to transmit the output light and couple the reflected light back. The interference spectrum of the fabricated temperature sensor is shown in Fig. 2(c). The spectrum contains broad fringes, among which there are many fine fringes. The broad fringes correspond to the polymer cavity and the fine fringes are generated by the microbubble cavity and the hybrid polymer-microbubble cavity.
Fig. 2 Temperature sensor based on cascaded polymer-microbubble cavities: (a) Microscope image; (b) Schematic diagram; (c) Interference spectrum.
According to the Eq. (1), the dip wavelength of the spectrum can be deduced by Eq. (2) [8]:
where λm is the dip wavelength of the mth interference fringe, m is integer. For the three cavities, the free spectra ranges (FSR) can be calculated as follows:, and, where λ1, λ2 represent two adjacent wavelengths of the interference dips. The corresponding spatial frequencies can be expressed as: , , , respectively [10]. The interference spectrum was transformed into the spatial frequency spectrum by FFT and the results are shown in Fig. 3. As shown in Fig. 3, three main peaks exist in the spatial frequency spectrum: peak 1 is from the polymer cavity (cavity I); peak 2 is from the microbubble cavity (cavity II); peak 3 is from the hybrid polymer-microbubble cavity; other weak peaks are from the multiple reflections in the cavities [10]. To extract the interference spectrums corresponding to different cavities, we demodulated the interference spectrum by means of signal filtering [12]. According to the frequency values of the peaks, the interference spectrum of the polymer cavity can be extracted by low-pass filtering, and the interference spectrum of the microbubble cavity can be extracted by band-pass filtering. The demodulated interference spectrum of the polymer cavity is shown in Fig. 4(a) and that of the microbubble cavity is shown in Fig. 4(b). From Figs. 4(a) and 4(b), the cavity length can be calculated by using the relationship: , where n represents the refractive index of FPI cavity, L represents the cavity length and λ1, λ2 represent two adjacent dip wavelengths of the extracted interference spectrum. The calculated length of the microbubble cavity is 47.71 µm and that of the polymer cavity I is 6.64 µm, which are almost consistent with the measured results.Fig. 4 Demodulation of the interference spectrum by signal filtering. (a) Demodulated interference spectrum of the polymer cavity (cavity I); (b) Demodulated interference spectrum of the microbubble cavity (cavity II).
To investigate the thermal response of the fabricated sensor, the sensor head was placed into an electrical oven and the temperature response of the reflection spectrum was monitored by the OSA. The electrical oven was heated from 20 °C to 55 °C with an increment of 5 °C. During the heating process, the wavelength shifts corresponding to the polymer cavity and the microbubble cavity are shown in Figs. 5(a) and 5(b) separately. The thermal response of the polymer cavity can be ascribed to the thermal expansion and the thermal-optic effect. By taking a derivative of the Eq. (2), the dip wavelengths shift with the temperature variation can be expressed as Eq. (3):
the first term in the parenthesis is negative as it is associated with the thermal optic effect, and the second term in the parenthesis is positive as it is resulted from the thermal expansion effect. If is negative, the wavelengths shift to the shorter-wavelength direction in the heating process. On the contrary, the wavelengths shift to the longer-wavelength direction when is positive. As we can see from Fig. 5(a), the dip wavelengths of the extracted spectrum caused by the polymer cavity shift to the longer-wavelength direction with temperature increasing, namely “red shift”, which means that the thermal expansion of the polymer plays a dominant role in the thermal response of the polymer cavity. From Fig. 5(c), we can see that the thermal response that results from the polymer cavity is 1.904 nm/°C. Additionally, the dip wavelengths shift linearly with the temperature increasing. Contrary to the polymer cavity, as can be seen from Fig. 5(b), the dip wavelengths of the extracted spectrum associated with the microbubble cavity shift to the shorter-wavelength direction during the heating process, namely “blue shift”. In [16], the air gap cavity length was reduced by the only one-sided polymer expansion when heated. As the microbubble cavity was embedded in the polymer, the microbubble cavity length was squeezed to shorten by the two-sided polymer expansion. The two-sided polymer expansion produced a double effect to change the microbubble cavity length and contributed to the enhanced thermal response. As shown in Fig. 5(d), the microbubble cavity exhibits a thermal response of −3.109 nm/°C, which is almost twice higher than that of −1.7 nm/°C reported in [16]. Also, Fig. 5(d) shows a good linear relationship between the wavelength shift and the measured temperature.Fig. 5 Thermal response of the interference spectrum: (a) cavity I and (b) cavity II. (c) Thermal response of dip wavelength at 1415.9 nm caused by cavity I with the temperature increasing. (d) Thermal response of dip wavelength at 1583.6 nm caused by cavity II with the temperature increasing.
It is worth noting that the thermal responses of the two cavities are reciprocal because the expansion of the polymer cavity results in the compression of the microbubble cavity. Since the thermal response of the polymer cavity I is positive and that of the microbubble cavity is negative, a high sensitivity of such a temperature sensor is achieved by a subtraction between the two reciprocal thermal responses. The obtained temperature sensitivity is 5. 013 nm/°C. The FFT has been proven to be an accurate method to analyze the interference spectrum as it can provide the information of the frequency [22], amplitude [23]. The signal filtering accompanied with the FFT were used to extract the interference spectra associated with different resonant cavities, which can get the parameter response monitored more easily [12,24].
3. Conclusion
In this paper, a fiber optic FPI sensor with cascaded polymer-microbubble cavities was fabricated and used to measure the temperature. The sensing head of the temperature sensor was formed by filling a section of SCT spliced with SMF with polymer and heat-curing subsequently. Owing to the novel structure of the sensing head, a high sensitivity of the fabricated temperature sensor was achieved by a subtraction between the two reciprocal thermal responses corresponding to the cascaded dual cavities. Such a sensor exhibits a temperature sensitivity of 5.013 nm/°C and good linearity in the measurement range between 20 °C and 55 °C. In addition, the fabricated sensor is characterized in a simple, nontoxic and cost-effective process of fabrication, which provides it potential for mass production.
Funding
National Science Foundation under Grant (11374077, 11304064 and 11504070); The Fundamental Research Funds for the Central Universities under Grant (No.HIT.NSRIF.2016083); Discipline Construction Guidance Foundation of Harbin Institute of Technology at Weihai under Grant (WH20150209).
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