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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). The coupling intensity decreases within a lower RH range while it increases throughout a higher RH range. The resonance wavelength is sensitive to the higher RH levels, and the highest sensitivity reaches 114.7 pm/% RH, and shares the same RH turning point with coupling intensity response. From a practical perspective, the proposed RH sensor would find its potential applications in high humidity level, temperature-independent RH sensing and multiparameter sensing based on wavelength/power hybrid demodulation and even static RH alarm for automatic monitoring of a particular RH value owing to the nonmonotonic RH dependence of the transmission power within the whole tested RH range.


Yinping Miao, Kaikiang Zhang, Yujie Yuam, Bo Liu, Hao Zhang, Yan Liu, Jianquan Yao. Agarose gel-coated LPG based on two sensing mechanisms for relative humidity measurement. Applied optics. 2013 Jan 1;52(1):90-5

PMID: 23292379

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