Fiber-optic pulse width modulation sensor for low concentration VOC gas

In this paper, we proposed a highly sensitive fiber-optic pulse width modulation (PWM) sensor for volatile organic compounds (VOCs). By incorporating Nile red and polyvinylpyrrolidone (PVP), a sensing membrane coating was fabricated on a side-polished single-mode fiber. This coating utilized Nile red's solvatochromic properties of changing optical characteristics upon VOC absorption. The sensing membrane coating produced strong signals that were fast and reversible when exposed to VOC gases such as benzene. An oscilloscope, and optical spectrum analyzer were used to analyze the performance of the sensor device. The PWM sensing system was compared against the resonance wavelength shift test for varying benzene gas concentrations. The proposed PWM sensing system was found to provide superior performance. The results indicate that the sensitivity of the benzene gas was 7.79 ns/ppb with highly reproducible response and recovery times of less than 50 s.