Relative humidity sensor based on long period grating with agarose gel coating

A long period grating (LPG) with agarose gel coating has been developed for relative humidity (RH) monitoring for first time. Increase of RH will result in volume expansion and refractive index increment of the agarose gel cladding coating. Since the LPG is sensitive to applied strain and ambient refractive index, the spectral properties of agerose-gel-coated LPG changes accordingly as the ambient humidity levels ranges from 25% to 95% RH. Both the resonance wavelength and coupling strength turn out particular responses to RH for two different RH ranges (25%-65% RH and 65%-95% RH). The coupling strength decreases within a lower RH range while increases throughout a higher RH range. The resonance wavelength is sensitive to RH only for higher RH levels, and the highest sensitivity reaches 108.67 pm/% RH, sharing a same RH turning point with coupling strength response. From a practical perspective, the proposed RH sensor would find its potential applications in temperature-independent RH sensing and multi-parameter sensing based on wavelength/power hybrid interrogation and even static RH alarm for automatic monitoring of a fixed RH value owing to its non-monotonic RH dependence of the transmission power within the whole tested RH range.

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