Humidity sensor based on optical fiber attached with hydrogel spheres

•Humidity sensor based on bare fiber core attached with hydrogel spheres is proposed.•Sensitivity of the proposed sensor is superior than hydrogel coating based ones.•Size and number of hydrogel spheres are two main factors that influence sensitivity.•Enhancement of sensitivity by increasing sphere number beyond three is minimal.

This paper reports a new type of intensity-based fiber optic humidity sensor formed by solidifying mini hydrogel spheres on bare fiber cores. Upon variation of relative humidity, the refractive index of hydrogel spheres changes and leads to the intensity change of the transmitted light through the fiber. The dependence of the transmission ratio on the refractive index, size, number and separated distance of hydrogel spheres is simulated and optimized by using commercial software program TracePro. Simulation results suggest great dependence of the transmission ratio on the refractive index change of hydrogel spheres compared to even-thickness coating, because the spherical geometry is much more effective to couple light out of the fiber core. For example, the transmission ratio of the fiber core attached with a single hydrogel sphere in diameter of 2 mm could be reduced to 9% by changing refractive index of hydrogel while 2 mm coating could only achieve 87% for the same fiber core. Larger spheres reduce transmission as expected for longer coupling length. Increasing sphere number also cut transmission but the magnitude become minimal for four and more spheres. Preliminary experimental studies were carried out and demonstrated the idea of the sensor.