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/με in the range of 0-500με and a reduced temperature sensitivity of 0.05pm/°C in the range of (20-600)°C were obtained from the lowfrequency fringe, while a strain sensitivity of 5.38 pm/με and a temperature sensitivity of 10.81 pm/°C were obtained from the high-frequency fringe.
Fiber bragg grating has become one of the widely used optical measurement technologies in the field of aviation structure monitoring because of its light weight, high sensitivity and immunity to electromagnetic interference. A strain transfer model is established for the surface attached fiber grating, the function between real strain and sensing strain is obtained. Basing on finite element simulation, strain field distribution of the sensing structure is analyzed and the distribution curve of strain transform rate is drawn. The strain calibration system of fiber grating is built to verify the accuracy of the model. The average error of this model is superior to 5.5% in the measurement range of 0-3000με. The study improves the measurement precision of surface bonded fiber grating sensor, which also provides academic bases and references for the aircraft structural health monitoring.