Simultaneous humidity and temperature measurement sensor based on two cascaded long-period fiber gratings

An optical fiber sensor based on two cascaded long-period fiber gratings (LPFG) for simultaneous measurement of humidity and temperature is proposed. In the cascade structure, the resonant peak wavelengths of the two LPFGs (i.e. LPFG1 and LPFG2) are in different bands, in which the resonant peak wavelengths of LPFG1 are between 1325 nm and 1465 nm, and the resonant peak wavelengths of LPFG2 are between 1470 nm and 1575 nm. Then, the humidity sensitive material and thermal sensitive material are combined with LPFG1 and LPFG2 respectively to achieve high-sensitivity humidity and temperature measurement. In our sensor structure, the mixture of humidity sensitive material polyimide (PI) and graphene quantum dots (GQDs) was coated on the surface of LPFG1, and the humidity sensitivity of 78 pm/%RH was obtained in the range of 45%RH∼75%RH. Then, the surface of LPFG2 was encapsulated with dimethyl silicone oil (DSO) thermal sensitive material, and the temperature sensitivity of LPFG2 reached 445 pm/∘C. Finally, the sensitivity measurement matrix is constructed by using the temperature and humidity sensitivities obtained by two LPFG, which can realize the simultaneous measurement of humidity and temperature and eliminate the cross-sensitivity. The sensor has the advantages of simple structure, good stability, low cost, high sensitivity and double parameter measurement, and it has a certain practical application prospect.