Abstract: In order to improve the sensitivity of liquid refractive index measurement and make the measurement process more convenient, U-shaped POF(Plastic Optical Fiber) probes for RI(Refractive Index) sensing with high sensitivity is prepared by wet etching. The principle of the multi-mode POF macro-bending RI sensor is analyzed. The light field transmitted in the POF is approximated by the ray method, and the expression of mode loss caused by macro-bending in the multi-mode POF is obtained. The cladding and part of the core are removed through a mixed solution of acetone and methanol. The surface morphology of the optical fiber after corrosion with different etchant concentrations is observed, and the best etchant concentration is 80% with acetone. The corroded optical fiber is produced to macro-bending structure by thermal setting method, and the sensor performance is optimized by changing the fiber core diameter and bending radius. The sensitivity of 618% / RIU is obtained in the large index range of 1. 30 to 1. 45 at wavelength of 655 nm. It is proved that the sensitivity of the sensor can be improved through the macro-bending processing, and the U-shaped probes make the
measurement more convenient, which can be widely used in refractive index sensing.

Key words: optoelectronics, fiber optic sensors, plastic optical fibers, refractive index sensor