Effect of coating thickness on the sensitivity of a humidity sensor based on an Agarose coated photonic crystal fiber interferometer

We report the effect of coating thickness on the sensitivity of a relative humidity (RH) sensor based on an Agarose coated photonic crystal fiber interferometer for the first time. An experimental method is demonstrated to select an optimum coating thickness to achieve the highest sensitivity for a given RH sensing range. It is shown that the Refractive Index (RI) of the coating experienced by the mode interacting with the coating depends on the thickness of the coating. It is observed that the spectral shift of the interferometer depends on both the bulk RI change and the thickness change of the Agarose coating with respect to an RH change. The RH sensitivity of the sensor has a significant dependence on the thickness of the coating and the sensor with highest sensitivity shows a linear response for RH change in the range of 40-90% RH with a humidity resolution of 0.07%RH and a fast response time of 75 ms for an RH change from 50% to 90%.