An ultra-low loss balloon-type single stress-applying optical fiber refractive index sensor

In the field of refractive index (RI) sensing, macro-bending sensors have been widely studied for their advantages of low loss and easy fabrication. Here, an ultra-low loss balloon-type single stress-applying fiber (SSAF) sensor is demonstrated. The sensor is manufactured by splicing SSAF between two single-mode fibers (SMFs) in a balloon shape. The sensing of different RI solutions is realized based on the strong evanescent field characteristics generated by the SSAF with a stress-sensitive zone during bending. Compared with other macro-bending sensors, the RI sensitivity of the proposed sensor with a bend diameter of 0.95 cm has been doubled, reaching up to 387.09 nm/RIU in the range of 1.3308–1.3628, with a resolution of 0.0013 and a loss of 2.22 dB. The sensor has a compact structure, low processing difficulty, strong reproducibility, and low loss, which provides a new way for the development prospects of biomedical science, environmental monitoring, food safety, etc.