A fiber-optic temperature sensor by using a multi-cladding special fiber is presented. It works on the basis of leaky mode resonance from fiber core to outer cladding. With the thin-thickness inner cladding, the cladding mode is strongly excited and the resonant spectrum is very sensitive to the refractive index variation of coating material. By coating the special fiber with temperature-sensitive silicone, the temperature response was investigated experimentally from -20°C to 80°C. The results show high temperature sensitivity (240pm/°C at 20°C) and good repeatability.
© 2008 Optical Society of America
Fiber-optic temperature sensor is very attractive in many fields for its well-known advantages, such as fast response, high sensitivity, anti-EM interference, corrosion resistance, remote sensing capability and multiplexing ability . A considerable number of fiber temperature sensors with different structures and principles have been developed. Most of them were exploited by using special micro-structure in single mode fiber such as fiber Bragg gratings (FBGs) , long period gratings (LPGs) [3, 4], fiber Fabry-Perot interferometer (F-P) [5, 6], tapered fiber , D-shape fiber  etc. These micro-structures are post-fabricated by using various techniques such as UV-laser writing, laser machining, mechanical polishing, fusion tapering, chemical etching etc. Generally these techniques require complicated processes. In recent years, many fiber optic sensors have been proposed based on the special optical fiber, such as photonic crystal fiber, multi-core fiber, hollow fiber, polarization-maintaining fiber, multimode fiber, which are very convenient to be integrated into conventional single mode fiber sensing system. The special structure in the fiber is formed during the fabrication. Thus, it doesn’t need extra post processing for fabricating fiber sensing head. Enbang Li  demonstrated a multimode fiber interference temperature sensor. Jian Ju  studied the temperature sensitivity of two-mode photonic crystal fiber. D. S. Moon  researched a temperature sensor based on the Sagnac interferometer made of polarization-maintaining fiber.
In this paper, a novel fiber temperature sensor is proposed and demonstrated based on a multi-cladding special fiber whose core and inner cladding are made of pure silica and fluorine-doped silica deposited with the conventional modified chemical vapor deposition (MCVD) technique. Attributed to the thin thickness of the inner cladding, the core mode will leak to the outer cladding and propagates as cladding modes. And a resonant spectrum is presented, which is very sensitive to the refractive index change of the fiber coating. By coating the special fiber with the temperature-sensitive silicone, a fiber-optic temperature sensor with high sensitivity is demonstrated.
As shown in Fig. 1, the proposed special fiber comprises four-layer cylindrical structure: core, inner cladding, outer cladding and coating. The core and the outer cladding are with the same refractive index which is higher than that of the inner cladding and the coating. Since there is no difference in refractive index between the core and the outer cladding, the effective index of the core mode is absolutely lower than the refractive index of the outer cladding. Thus the core mode is a leaky one, and the optical power propagating in the fiber core will leak out into the outer cladding . The outer cladding, which is sandwiched between the inner cladding and the coating, can also support guiding mode. Based on the coupled mode theory , the optical power exchanging between the fiber core and the outer cladding can be treated equivalently as modes coupling to each other through the evanescent wave, namely, the core (rod waveguide) mode and the cladding (ring waveguide) mode. According to the coupled mode theory, the two coupled modes must satisfy the phase-matching condition:
where βcore=k 0 neff-co and βclad=k 0 neff-cl are propagation constants for the core mode and the cladding mode respectively. k 0 is the wave vector in vacuum, neff-co and neff-cl are the effective indexes of the core mode and the cladding mode respectively. According to the transcendental equations given by F. D. Nunes , we calculated the normalized propagation constant B=(neff 2-n 2 2)/(n 2 1-n 2 2) for the core mode and various the cladding modes of the special fiber with a=4.639µm, b=13µm, c=63µm, n1=n3=1.456, n2=1.453875, n4=1.4317. Their dispersion curves are depicted in Fig. 2(a). We can see that the curves for the core mode HE11 and the cladding mode HE15 cross over at λ=1.5µm, namely the resonant wavelength.
With this multi-cladding fiber structure, the optical power exchanges between the core and the outer cladding. The optical power propagating in the core can be calculated as follows :
where κ is the coupling coefficient which is calculated from the fields overlap integral of the core mode and the cladding mode . L is the length of the special fiber and Δβ=|βcore-βclad| is the difference in propagation constants between the core mode and the cladding mode. The transmission is normalized with initial power launched into the fiber core. By using Eq. (2), the resonant spectrum is shown in Fig. 3. Here the length of the special fiber is equal to Lo which satisfies the condition of κ·Lo=π/2. For this length, the optical power at the resonant wavelength is transferred from the core to the outer cladding completely.
The light power transferring from the core to the outer cladding can also be explained by the supermodes method which is to consider the four-layer structure as an entire waveguide . With a conventional single mode fiber output as exciting field, all circular symmetric modes of the entire fiber can be excited. For the adopted fiber parameters previously, there exist only six order guiding supermodes. Their field distributions are plotted in Fig. 2(b). Here the field distributions are normalized to carry a unit power. It can be seen that HE15 and HE16 contain major light in the core region, whereas other lower order modes contain very little in core region. Hence the HE15 and HE16 modes are excited preferentially because their fields distribution within the core are similar to the exciting field. The dispersion curves of these two excited supermodes are plotted in insert drawing of Fig. 2(b). To comparing with the coupled mode method, the dispersion curves of the core mode and the ring waveguide mode HE15 are also plotted together. The HE15 and HE16 modes have different propagation constant and beat to each other as propagating along the special fiber. Thus the light power at the phase-matching wavelength exchanges between the core and the outer cladding.
Although the supermodes method is more accurate than the coupled mode method for the analysis of special fiber, the latter method is clearer and more intuitive for explaining the sensor operation principle. So the coupled mode theory is applied to explain sensing principle. When temperature is varied, the refractive index of the coating changes according to the thermo-optic effect. As a result, the propagation constant of the outer cladding mode varies, and the resonant wavelength shifts accordingly. Figure 4 shows the relationship between the resonant wavelength and the refractive index of the coating. With the refractive index of the coating increasing, the resonant wavelength shifts toward a longer wavelength. To explain the impact of the coating straightforward, Fig. 5 depicts the variation of the phase-matching crosspoint. For the outer cladding, a lower refractive index of the coating will decrease the effective refractive index and shift the dispersion curve to the left side. For the core, the core and the inner cladding are made of pure silica and F-doped silica respectively, which have very low thermo-optical coefficient . Thus the effective index of the core mode can be treated as an unchangeable curve approximately. In this way, the temperature variation can be measured by the shifts of phase-matching wavelength.
3. Fabrication of the special fiber temperature sensor
The cladding-mode resonant special fiber was fabricated based on the MCVD technique. A high-purity fused silica tube (diameter, 20mm; wall thickness, 2mm) was as substrate. Firstly, the fluorine-doped layer was deposited with O2, Freon and SiCl4 as the starting reactants. Then the pure silica layer was deposited with O2 and SiCl4. The deposited temperature was 1500°C. After the deposition, the supporting tube was collapsed around 2000°C to a preform. At last, the preform was placed onto drawing tower and drawn into a fiber at 1800 °C. The refractive index profile around the core and inner cladding region of the special fiber was tested by the optical fiber analyzer (EXFO NR9200). As shown in Fig. 6, the special fiber core is about 9.3µm in diameter. And the refractive index difference between the core and the inner cladding is about 0.2%. Thus the fiber core as a rod waveguide only supports the fundamental mode. The outer cladding whose diameter is 125µm is a multi-modes ring waveguide. Through the thin-thickness inner cladding, the core mode and the cladding modes are coupled with each other based on the evanescent waves.
To apply the special fiber to temperature sensing, a segment of the special fiber was spliced into conventional single mode (SM) fiber forming the structure of SM fiber-special fiber-SM fiber by using fusion splicer (FITEL S177), as shown in Fig. 7. Then silicone gel with negative thermo-optic coefficient was dip-coated around the sensor fiber and dried in room temperature for 24h. Additionally, for aging purpose, the silicone-coated special fiber was thermally annealed at 100°C for 10 hours. One SM fiber inputs light wave into the special fiber at one end, and the other SM fiber collects output light wave from the special fiber core at the other end. In this way, the resonant spectrum of the special fiber and its temperature sensing characteristics can be observed and studied.
4. Experiments and discussions
Figure 7 shows the experimental setup for investigating temperature sensing characteristics. An 1550nm-wavelength superluminescent diode (SLED, SL3200-C42) and an optical spectrum analyzer (OSA, ANDO AQ-6315A) were used to record resonant spectrum. Temperature-controlled chamber (ESL-04KA) was utilized to adjust temperature. The length of the spliced special fiber was about 14mm. To avoid the impact of fiber bending or strain on temperature measuring results, the sensing fiber was hung vertically with a load.
The dependence of the resonant spectrum on temperature was investigated experimentally within the range from -20°C to +80°C. As shown in Fig. 8, with temperature increasing, the center wavelength of the spectrum shifts to shorter wavelength and the total wavelength shift is up to 21.17nm. Due to the negative thermo-optic coefficient of the silicone coating material, the refractive index of the coating decreases when temperature increases. According to the analysis in section 2, the resonant wavelength satisfying the phase-matching condition shifts toward shorter wavelength. Figure 9 shows experimental curve of the dependence of the resonant wavelength on temperature, which is consistent with the theory result in Fig. 4.
To investigate repeatability of temperature response, the resonant spectra were also tested with temperature decreasing from 80°C to -20°C. Compared with the response in temperature increasing process, the sensor demonstrates a good repeatability, as shown in Fig. 9.
From the experimental results shown in Fig. 8, we can also see that the optical power transmission at the resonant wavelength varies. Within the temperature range from -20°C to +80°C, the transmission at resonant wavelength becomes larger monotonically from -27dB to -22.5dB, as shown in Fig. 10. As far as a length-known special fiber is concerned, as indicated by Eq. (2), the transmission at resonant wavelength depends on the coupling coefficient κ which is calculated by the field overlap integral of the coupled modes . According to the study by A. Cusano , a decreasing refractive index of coating can shift the transverse field profile of the cladding modes to the inner cladding so that the evanescent wave is enhanced in the fiber core region. In this case, the fields overlap integral between the cladding and core mode becomes larger and the coupling coefficient also increases accordingly. As a result, the variation of the transmission at resonant wavelength can be observed in our experiment, shown in Fig. 10. From the monotone relationship, we can also conclude that the length of the spliced special fiber makes itself work at over coupling region, namely κ·L>π/2. The monotone response is quite important to sensing signal process in the practice application.
A novel fiber-optic temperature sensor based on SM fiber-special fiber-SM fiber structure is presented in this paper. The sensor utilizes the leaky mode resonance between the core and the outer cladding through the inner cladding. Theoretical studies were carried out and the shift of resonant wavelength versus refractive index of coating was calculated numerically. To achieve temperature sensor, the special fiber was coated with the temperature-sensitive silicone. The experimental result shows high temperature sensitivity (240pm/°C at 20°C). By testing the resonant wavelength shift from -20°C to 80°C, the total shift up to 21.17nm is achieved, which is much larger than that of the conventional FBGs sensor. And after the anneal processing, the sensor presented a good repeatability. With the attractive performance, the cladding-mode special fiber temperature sensor can be found wide applications.
This work was funded by the Natural Science Foundation of China (60677031 and 60577043), the Specialized Research Fund for the Doctoral Program of Higher Education of China (20060280001), Chenguang Program (2007CG54), Science and Technology Commission of Shanghai Municipality (STCSM) (07dz22024 and 075307017) and nano-technology special project of Shanghai (0652nm046), Shanghai Municipal Education Commission (06AZ032).
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