Novel optical fiber humidity sensor based on a no-core fiber structure

A novel and simple optical fiber structure with a section of no-core fiber for measuring relative humidity (RH) is presented. Moisture-sensitive materials HEC/PVDF are used to form the hydrogel coating on the no-core fiber by dip impregnation method. Under different humidity conditions, the humidity induced refractive index changes on the outside coating of no-core fiber will lead to the variations of the optical output power. The different lengths of no-core fiber are tested at the two wavelengths of 1310 nm and 1550 nm, respectively. The results show that the structure with 2 cm no-core fiber has a higher RH sensitivity of 0.196 dB/%RH at 1310 nm and a better linearly response when the RH is lower than 75%. Furthermore, the optical output powers are measured with the rise and drop of RH, and the four times repetition are also recorded, which all demonstrate the good stability and repeatability of our proposed structure.