A Low-Cost Relative Humidity Sensor Using Polymer Optical Fiber Based on Intensity Variation

In sensor systems, most sensors are single-point detection, and it is challenging to develop a multipoint sensor where multiple points can be sensed. This work reports a low-cost relative humidity (RH) sensor relying on intensity variation based on the side coupling technique using polymer optical fiber (POF), where three sensors are integrated into a single illuminating fiber. The sensor structure contains the twisting of two fibers three times and is coated with agarose gel, while a rise in humidity causes a change in refractive index (RI) between two twisted fibers. The experimental results reveal that the sensors provide a linear relationship between 20% and 80% RH with a sensitivity of 0.4636 \mu {W} /%, 0.4402 \mu {W} /%, and 0.4153 \mu {W} /%. Detection limits are 1.57%, 1.79%, and 2.02%, for sensor-1, sensor-2, and sensor-3, respectively, while the bend radius of the sensor probe is 10 mm. When RH is increased from 80%, it results in more sensitivity, and the sensor's sensitivity also rises when the bend radius reduces. A lower detection limit and increased sensitivity are obtained with a coating of agarose gel. In addition, the novel RH sensor is a cost-effective solution that offers the benefits of decent reversibility, rapid reaction time (about 1 s), and temperature dependency.