Abstract
Liquid sealing is an effective method to convert a fiber-optic refractometer into a simple and highly sensitive temperature sensor. A refractometer based on the thin-core fiber modal interferometer is sealed in a capillary tube filled with Cargille oil. Due to the thermo-optic effect of the sealing liquid, the high refractive-index sensitivity refractometer is subsequently sensitive to the ambient temperature. It is found that the liquid-sealed sensor produces a highest sensitivity of −2.30 nm/°C, which is over 250 times higher than its intrinsic sensitivity before sealing and significantly higher than that of a grating-based fiber sensors. The sensing mechanisms, including the incidental temperature-induced strain effect, are analyzed in detail both theoretically and experimentally. The liquid sealing technique is easy and low cost, and makes the sensor robust and insensitive to the surrounding refractive index. It can be applied to other fiber-optic refractometers for temperature sensing.
Highlights
Fiber-optic temperature sensors have received significant attention for their unique advantages such as immunity to electromagnetic interference and fast response, as well as remote sensing capabilities, a wide variety of temperature sensing schemes have been proposed and reported to date, such as Sensors 2014, 14 fiber knot [1,2], high-birefringence fibers (HBFs) [3], fiber Bragg gratings [4,5], long period fiber gratings [6], fiber in-line interferometers [7,8,9], and so on
The sensing device was embedded into a slot of a copper block which was pasted on a thermoelectric cooler (TEC)
Experimental results have shown that the temperature sensitivity of the proposed sensor was significantly enhanced compared to it before sealing and it is much higher than those of fiber grating-based sensors and the most popular interferometric fiber-optic sensors
Summary
Fiber-optic temperature sensors have received significant attention for their unique advantages such as immunity to electromagnetic interference and fast response, as well as remote sensing capabilities, a wide variety of temperature sensing schemes have been proposed and reported to date, such as Sensors 2014, 14 fiber knot [1,2], high-birefringence fibers (HBFs) [3], fiber Bragg gratings [4,5], long period fiber gratings [6], fiber in-line interferometers [7,8,9], and so on. The fiber in-line interferometer sensors, such as multimode- [7], Fabry-Perot- [8], Michelson- [9] and Mach-Zehnder [13,14], are easy-to-fabricate and low-cost. Their temperature sensitivities are still low (~10–100 pm/°C) due to the small optical path difference (OPD) between the interference modes varying with temperatures. A refractometer based on a thin-core fiber modal interferometer (TCFMI) was coated with poly (allylamine hydrochloride and acrylic acid) for sensing pH [20]. The refractometer based on a TCFMI was sealed in a capillary tube filled with Cargille oil, which has a high thermo-optic coefficient. The liquid sealing technique is easy to apply to other fiber-optic refractometers for temperature sensing
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