Relative Humidity Sensing based on Passively Mode-Locked Fiber Laser with Polyimide-Coated Fiber

Commercial polyimide-coated single mode fiber (PI-SMF) with excellent water affinity property has shown great potentials in realizing robust and efficient humidity sensing. In this paper, relative humidity (RH) sensing utilizing a passively mode-locked fiber laser (MLFL) combined with a beat frequency demodulation system is proposed and experimentally demonstrated. A PI-SMF in the fiber ring cavity is used as a sensing element, of which the length varies with humidity by volume-variation induced strain, leading to the frequency shift of the MLFL beat frequency signals. In the range of 25-85% RH, the frequency of the 2000 MHz beat frequency signal responds linearly with RH, with a sensitivity of -2.24 kHz/RH%. Thanks to the high frequency resolution of the beat frequency demodulation system, the RH resolution can reach 0.022 RH%. In addition, the experimental results demonstrate that the sensing approach has good long-term stability with root-mean-square (RMS) deviation of ≤0.2% RH, and repeatability with a sensitivity deviation of <0.2% over 4 weeks. These advantages of high resolution and excellent long-term stability make it suitable for potential applications in areas such as food safety, electronic storage, and pharmaceutical manufacturing.