A Relative Humidity Sensor Based on a Polarimetric Fiber Laser and Beat Frequency Demodulation

We propose and demonstrate a relative humidity (RH) sensor based on a polarimetric fiber laser and beat frequency (BF) demodulation. The laser cavity of the proposed fiber laser sensor consists of a pair of fiber Bragg gratings (FBGs) and a high-birefringence erbium-doped fiber (Hi-Bi EDF) coated by a thin polyvinyl alcohol (PVA) film layer. This thin-film layer responds to humidity changes in the environment by swelling or contracting, inducing stress effects on the EDF. The stress effect, in turn, results in changes to the polarization mode beat frequency (PMBF) detected by the photodetector (PD) in the sensing system. The inherent high-birefringence property of the Hi-Bi EDF significantly enhances the sensitivity of the PMBF to humidity-induced stress. Humidity sensing experiments are conducted utilizing the proposed sensor. Experiment results show that the sensor has a high average humidity sensitivity of 13.9 MHz/%RH within the humidity range of 20.4%RH–36.0%RH. A stability test is also conducted, demonstrating that the proposed sensor achieves an accuracy of 0.65%RH.