A Fabry-Perot interferometer for simultaneous measuring temperature and strain based on sapphire-derived fiber-air

A sapphire-derived fiber (SDF)-air based composite Fabry-Perot interferometer (FPI) for simultaneous measurement of strain and temperature has been proposed and demonstrated. The FPI was fabricated by cascading a lead-in single mode fiber and a short section of SDF with a small air cavity left in between. The structure was inserted into capillaries and fixed by an arc fusion. The air cavity was used for strain sensing. Its strain sensitivity was improved by increasing the ratio of capillary based active length to air based interference cavity length, while its temperature sensitivity was compensated by the SDF with a well-designed length and alumina content. The SDF cavity served as a thermometer. The reflection spectrum was demodulated by tracing the wavelength shifts of the low- and high-frequency fringes, which were caused by the air-cavity and the combination of the air and SDF based cavities, respectively. A dual-parameter sensitivity matrix was introduced to realize the simultaneous measurement of strain and temperature. In the experiments, an enhanced strain sensitivity of 55.52 pm/mu epsilon in the range of 0-500 mu epsilon and a reduced temperature sensitivity of 0.05pm/degrees C in the range of (20-600)degrees C were obtained from the low-frequency fringe, while a strain sensitivity of 5.38 pm/mu epsilon and a temperature sensitivity of 10.81 pm/degrees C were obtained from the high-frequency fringe.