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We experimentally demonstrate transverse load and strain sensing based on a fiber optic Fabry-Perot interferometer (FPI) with special air cavity, which was created by fusion splicing single mode fiber (SMF), hollow core fiber (HCF) and several electrical arc discharges. The cavity height of this structure is higher than the cladding diameter of SMF so that it can sense transverse load with high sensitivity. The transverse load sensitivity of this air cavity FPI sensor is 1.31 nm∕N and about 5 times more sensitive compared to the current fiber tip interferometer (0.2526 nm∕N). Meanwhile, this sensor also can measure strain and the strain sensitivity of 3.29 pm∕με is achieved. In addition, the low temperature sensitivity (1.08 pm/°C) of the sensor can reduce the temperature-induced measurement error. This novel air cavity FPI can be developed and used as high-sensitivity transverse load and strain sensor with temperature-insensitive.
© 2017 Optical Society of America
1. Introduction
Recently, varieties of fiber FPI sensors have been proposed and developed for several sensing applications such as temperature [1–3], refractive index [4–6], strain [7–9], magnetic field [10,11], and pressure [12], because of its advantages including compact, reliable, stable, and ease of fabrication. Some fabrication technologies for fiber optic FPI have been presented such as splicing technology [13–15], chemical etching [16], and a coating at the end of the fiber [17–19]. For sensing of physical quantities such as transverse load, strain, refractive index, the strong temperature dependence may increase temperature-induced cross-noise. In order to solve this problem, temperature compensation can be introduced, but it will enhance the complexity of the system. Thus, the investigation on temperature-insensitive fiber sensing has potential applications.
Measurement of transverse load is important for structural health monitoring. Fiber-tip FPI-like micro-cavity for measurement of transverse load has reported [20], and high transverse load sensitivity (~1.37 nm/N) and low temperature sensitivity (~2.1 pm/°C) were obtained in the two-wave mode. In the two-wave mode, this Fiber-tip micro-cavity needed to be immersed into a liquid with refractive index of 1.4 or outer surface of the tip was blackened by applying matt black aerosol paint, however it is obvious that this sensor is inconvenience for measurement of transverse load with temperature-insensitive.
In this work, a special air cavity FPI created by fusion splicing SMF, HCF and several electrical arc discharges is proposed for transverse load and strain measurements. This sensor acquires high transverse load sensitivity of 1.31 nm∕N and is approximate 5 times more sensitive than that fiber tip modal interferometer for the transverse load measurement (sensitivity of 0.2526 nm∕N in previous report [21]). In addition to transverse load measurement, sensitivity of strain measurement is 3.29 pm/µε. Finally, the sensor presented low temperature sensitivity (1.08 pm/°C) which is ~10 times lower than that of the popular Fiber Bragg Grating (FBG) (~10 pm/°C) [22]. This sensor can be used as a promising transverse load and strain sensor with desirable high response transverse load and strain.
2. Fabrication of high air cavity FPI
The fabrication process of this air cavity FPI is shown in Fig. 1. Firstly, a piece of HCF with inner diameter of 50 μm and external diameter of 125 μm, was spliced to SMF using an fiber electric-arc fusion splicer (Fujikura FSM-45PM), as shown in Fig. 1 (a). To ensure that air hole of HCF was not completely collapsed so that a silica-air reflective surface as shown M1 in Fig. 1 (b) was formed at the end face of the fiber, electrical arc deviating of 120 μm from splice point, lower arc power of −100 bit and shorter arc duration time of 400 ms were applied. Secondly, extremely strong arc power discharge of 70 bit, long arc duration time of 2000 ms, and arc deviating of 150-250 μm from splice point which affects shape of air-cavity, as shown arc in Fig. 1 (a), was used to guarantee that the air hole of HCF was completely collapsed and cut off. Hybrid structured FPI was created with three reflective surface, as shown in Fig. 1 (b). Finally, SMF was spliced to end face of air cavity as shown in Fig. 1 (c) so that this structure could achieve strain sensing and minimally reduce optical reflection at M3.
Such a air cavity could be used to develop a promising fiber-optic FPI for transverse load and strain measurements. As shown in Fig. 2, the reflection spectrum of this structure was observed by a broadband light source, 3 dB coupler, and an optical spectrum analyzer. The free spectra range (FSR) of 120 × 141 μm (length × height), 90 × 130 μm, 82 × 146 μm, are 10.1, 13.7 and 14.7 nm, respectively and the extinction ratio (ER) are 10, 10 and 17 dB, respectively. Moreover, the air bubble wall thicknesses (the separation between M2 and M3) of 120 × 141 μm, 90 × 130 μm, 82 × 146 μm are respectively 8, 42 and 55 μm and maximum glass diameter are 198, 189 and 190 μm for 120 × 141 μm, 90 × 130 μm, 82 × 146 μm, respectively.
Fig. 2 (a), (b) and (c) Microscope images of the created air bubble with 120 × 141 μm (length × height), 90 × 130 μm, 82 × 146 μm, respectively;(d)–(f) the corresponding reflection spectra of 120 × 141 μm (length × high), 90 × 130 μm, 82 × 146 μm, respectively.
After fusion splicing SMF and HCF, the arc power discharge of 70 bit, arc duration time of 2000 ms, arc deviating of 250 μm from splice point, and several electrical arc discharges of 10 bit which can make air bubble bigger by melting silica wall were used to create air bubble of 120 × 141 μm. The arc power discharge and arc duration time were the same as air bubble of 120 × 141 μm, then arc deviating of 150 μm and 200 μm from splice point were respectively used to fabricate air bubble of 90 × 130 μm and 82 × 146 μm. During fabrication process of air bubble, the reflection spectrum was constantly observed so that appropriate parameters about fusion splicer was easily obtained and the sensor can be easily created by suitable parameters.
The intensity of the interference fringe of air cavity FPI can be expressed as
where I1 and I2 are the intensities of light reflected at the two cavity interfaces, respectively, and ϕ is the phase shift between the two reflected lights. FSR of the interference fringes of the air cavity FPI can be given bywhere λ represents the wavelength, and n is the refractive index of the air inside the air cavity, and L is the cavity length of the air bubble.3. Sensing applications and discussions
To investigate the transverse load sensing of this air cavity FPI, the air cavity FPI was horizontally placed between two parallel glass slides in the transverse load measurement, as shown in Fig. 3. As the load increased from 0 N to 3.63 N, reflectance spectrum had a redshift, as shown in Fig. 4. This may be caused by the influence of loads making the length of air cavity lengthen. For example, the transverse load sensitivity of the air cavity FPI was enhanced from 0.75 ± 0.01 to 1.31 ± 0.01 nm/N while the cavity length was shortened from 120 to 82 μm and their cavity heights are both higher than the cladding diameter (125 μm) of SMF in order to guarantee that this sensor can measure transverse load. The standard error of sensitivity to transverse load is 0.01 nm/N which perhaps result from the last digit uncertainty of OSA, temperature-induced error, and measurement uncertainty of load.
Fig. 4 (a) Transverse load response of the air cavity FPIs; (b) Reflection spectrum evolution of air cavity FPI with 82 × 146 μm while the load increases from 0 to 3.63 N.
The air cavity of higher indicates more sensitivity to transverse load and air cavity of thinner shows more sensitivity to transverse load and strain. Thus, transverse load sensitivity can be higher when the height/length ratio is larger. In addition, air bubble wall thickness and maximum glass diameter have little effect on transverse load sensitivity from Fig. 4. The air cavity of 82 × 146 μm achieved the highest transverse load sensitivity for the reason that it had a maximum height/length ratio in three cavities. Finally, the air bubble shape can be controlled by arc deviating and bigger air bubble can be created by melting silica wall.
To research the axial strain response of the air cavity FPI, we clipped a short section of fiber on each side of the air-cavity which was uncoated. One side which was glued to a translation stage was moved while the other side was glued to a fixed stage, as shown in Fig. 5. The wavelengths of the traces dips were recorded as the characteristic wavelengths of the air cavity FPI with the applied strain ranged from 0 µɛ to 1100 µɛ, as shown in Fig. 6. For instance, the strain sensitivity of the air cavity FPI was enhanced from 1.97 ± 0.01 to 3.29 ± 0.02 pm/με while the cavity length was shortened from 120 to 82 μm. The standard error of sensitivity to strain may possibly originate from the last digit uncertainty of OSA, temperature-induced error, and measurement uncertainty of strain.
Fig. 6 (a) Axial strain response of the air cavity FPIs; (b) Reflection spectrum evolution of the air cavity FPI with 82 × 146 μm while the strain increases from 0 to 1100 με.
For the temperature characterization of the air cavity FPI, the air cavity FPI was placed in a tube furnace to raise its temperature from 50 °C to 500 °C with a step of 50 °C. The reflectance spectrum of the air cavity FPI had a redshift with low temperature sensitivity of 1.08 pm/°C, as shown in Fig. 7. This sensor is temperature-insensitive because of its low temperature sensitivity of 1.08 pm/°C, which is ~10 times lower than that of the popular FBG (~10 pm/°C) [14].
Fig. 7 (a) The temperature response of the air cavity FPIs; (b) Reflection spectrum evolution of air cavity FPI with 82 × 146 μm while the temperature increases from 50 °C to 500 °C.
The wavelength of dip is λ0 = 2nL/m for air cavity, and FBG wavelength is λFBG = 2nL1, where m is integer, and L1 is FBG period. The wavelength shift of air cavity and FBG to temperature are given by
where ε = 5.5 × 10−7 and κ = 1.0 × 10−5 are respectively the thermal expansion coefficient and the thermo-optic coefficient for pure silica [20]. The wavelength shift of air cavity to temperature is only affected by the thermal expansion coefficient, but FBG is affected by the thermal expansion coefficient and the thermo-optic. Thus, air cavity is about 10 times less sensitive to temperature than standard FBG for the reason that the thermo-optic coefficient is over 10 times larger than the thermal expansion coefficient. In case no temperature compensation is done, the temperature-induced load measurement error is about 0.00075 N∕°C, and the temperature-induced strain measurement error is about 0.3 με∕°C.According to results from transverse load sensing, we can know that the transverse load sensitivity is enhanced as height/length ratio raising, and less dependent on other parameters such as air bubble wall thickness and maximum glass diameter. In addition, the strain sensitivity is mainly affected by air cavity length and increased with cavity length shortening in accordance to the references [7–9].
4. Conclusion
In summary, we demonstrate a special air bubble cavity FPI that its cavity height is higher than the cladding diameter of SMF for transverse load and strain measurements. The transverse load and strain sensitivities of this sensor are 1.31 pm/N and 3.29 pm/με, respectively. Meanwhile, this sensor has a small temperature-induced measurement error because of its low temperature sensitivity of 1.08 pm/°C. The results show this structure can be suitable for transverse load and strain measurements with advantages of low cost, compact and resistance to harsh environment.
Funding
The National Natural Science Foundation of China (NSFC) under Grant Nos. 61078006 and 61275066; the National Key Technology Research and Development Program of the Ministry of Science and Technology of China under Grants No. 2012BAF14B11.
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