High-Sensitivity Transverse-Load and Axial-Strain Sensor Based on Air-Bubble Fabry–Pérot Cavity and Fiber Sagnac Loop Cascaded

We have designed a highly sensitive optical fiber transverse load and axial strain sensor based on the cascaded Fabry–Perot (F–P) cavity and fiber Sagnac loop. F–P cavity is composed of an air-bubble formed by a Section of quartz capillary discharge. Fiber Sagnac loop is made by winding a Section of panda fiber (PF) through a 3 dB coupler. When the free spectral ranges (FSRs) of two interferometers are similar, they are cascaded to form the optical Vernier effect, which amplifies the sensitivity of the sensor. The experimental results show that the sensitivities of transverse load and axial strain of the sensor are up to 19.1 nm/N and 28.9 pm/$\mu \varepsilon $, respectively. The sensitivities of a single F–P cavity for measuring the transverse load and the axial strain are 1.5 nm/N and 2.4 pm/$\mu \varepsilon $, respectively. It can be seen that by using the optical Vernier effect, the sensitivity of the transverse load and axial strain of the sensor is improved by 12.7 and 12 times, respectively. The measurement matrix is constructed by using the sensitivities of a single F–P cavity and the cascaded structure to realize the simultaneous measurement of the transverse load and axial strain and eliminate the cross-sensitivity between them. The designed sensor has a certain attraction in small load and small strain monitoring.