Simultaneous measurement of strain and temperature using liquid core optical fiber sensors

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Virginia Tech


A liquid core fiber sensor can be used to sense both strain and temperature simultaneously. This liquid core fiber sensor is comprised of a hollow core optical fiber filled with a liquid of a known index of refraction which is slightly higher than that of the silica tube which acts as the cladding. The refractive index fluid is chosen such that the variation of its refractive index with strain and temperature is well defined and linear in the desired range of operation. The core of the sensing fiber contains a fluid which has a thermo-optic coefficient much larger in magnitude (-4.0x10⁻⁴/°C) than that of the silica cladding. This causes the fiber to be more sensitive to temperature changes than all-silica fibers. Both transmitted optical signal intensity and time-of-flight depend strongly on applied strain and temperature. Furthermore, the relative difference between the core and cladding refractive indices changes as a function of both parameters due to the inherently different material types used in the fiber construction. This results in critical strain and temperature regimes within which the refractive index difference is very small, and sensitivity is optimized. Testing of prototype sensors demonstrates these characteristics. A 0.47 m long liquid core fiber containing a liquid with a room temperature refractive index of 1.492 was analyzed. Both time- and intensity-domain behaviors around the device's critical temperature (95°C) confirm theoretical expectations. Simultaneous strain and temperature measurements were performed between 95 °C and 105 °C. Methods for multiplexing liquid core fibers for increasing the range of temperatures that can be monitored were also investigated as well as using those liquid core fibers for cooling purposes.