Temperature insensitive salinity sensor with U-shaped structure based on few-mode fiber

A novel U-shaped few-mode fiber (FMF) salinity sensor is proposed, which is insensitive to temperature. It consists of a section of FMF fused between two sections of single-mode fiber (SMF). The flame heating method is used to form the FMF into a U-shaped optical fiber sensor. Because the bending makes the light energy of the core leak into the cladding, higher-order cladding modes are excited and the sensor becomes more sensitive to the external environment. In the experiments, the sensors with different bending diameters are fabricated and studied for salinity and temperature. The results demonstrate that the sensitivity reaches up to 1.847 nm/% in the salinity range of 0 % to 10 %. In the temperature range of 5 °C to 30 °C, it shows temperature insensitive characteristics. Compared with the traditional optical fiber sensor, the sensor can avoid the effect of temperature on measurement results, and can be easily fabricated. It has a good application prospect in the field of seawater temperature and salinity measurement.