Relative Humidity Sensor based on Tapered Plastic Optical Fibre with Full-and Spiral-Patterned Agarose Gel Coating

A relative humidity sensor based on plastic optical fibre (POF) is constructed by using chemical etching method. Agarose gel was chosen as the sensing layer with two different patterns which are fully coated pattern and spirally coated pattern. The agarose gel acts as a moisture absorbent due to its hydrophilic property. Full and spirally agarose coated fibre synthesis will be compared in terms of their sensitivity and limit of detection. The measured result shows that the spirally agarose-coated fibre has better sensitivity compared to the fully agarose-coated fibre. The increment in relative humidity percentage affected the refractive index of the cladding and contributed to higher leakage of transmitted light. As the relative humidity percentage (RH%) of magnesium chloride (MgCl2) inside the chamber varies from 50 RH% to 85 RH%, the output voltage of the spiral patterned fibre decreases linearly from 9.1 mV to 8.4 mV. It demonstrates the better linearity slope of over 80.95% and highest sensitivity of 0.0162 mV/% compared to the full patterned fibre. Hence, humidity sensor based on spirally agarose-coated POF demonstrates a high-resistant and reliable sensitivity towards the relative humidity of the ambient.