Agarose gel-coated LPG based on two sensing mechanisms for relative humidity measurement.

A relative humidity (RH) sensor based on long-period grating (LPG) with different responses is proposed by utilizing agarose gel as the sensitive cladding film. The spectral characteristic is discussed as the ambient humidity level ranges from 25% to 95% RH. Since increment of RH will result in volume expansion and refractive index increment of the agarose gel, the LPG is sensitive to applied strain and ambient refractive index; both the resonance wavelength and coupling intensity present particular responses to RH within two different RH ranges (25%-65% RH and 65%-96% RH).