Two-Wavelength Dye-Doped Swellable Clad POF Humidity Sensor

Because humidity is an important indicator used in many fields, the development of technology for its measurement is of great significance. We investigated humidity sensors that use a swellable polymer as the cladding material of plastic optical fibers (POFs). A POF humidity sensor using hydroxyethyl cellulose (HEC) as the cladding material has a linear response at approximately 20% relative humidity (RH), but its sensitivity is low. The sensitivity can be improved by adding brilliant blue (BB) as a dye to cladding. However, this makes the detection range of the sensor too narrow for practical use. Therefore, we change the incident light from a single wavelength to two wavelengths and use the fact that the output waveform of the dye-doped sensor depends on the wavelength of the incident light. The results of experiments show that a humidity sensor with high sensitivity and an RH detection range of 10%–90% can be developed by combining the output waveforms of a two-wavelength sensor with a dye doping level of 0.8% in cladding.