Optical fiber long-period grating humidity sensor with poly(ethylene oxide)/cobalt chloride coating

A long-period fiber grating (LPFG) humidity sensor is reported utilizing poly(ethylene oxide)/cobalt chloride (PEO/CoCl2) as a hybrid hygrosensitive cladding coating. A thin overlay of the material is deposited on the LPFG and with exposure to different ambient humidity levels, its spectral properties are modified. The material parameters associated with the sensing mechanism may include those of refractive index, absorption, and morphological alterations of the overlaid material. Relative humidity variations in the range from 50% to 95% have been detected with a resolution better than 0.2%. The response time constant of the fiber sensor is of the order of a few hundred milliseconds.