Simultaneous Measurement of Humidity and Temperature Based on a Long-Period Fiber Grating Inscribed in Fiber Loop Mirror

A fiber loop mirror sensor with a long period grating (LPG) inscribed in the polarization maintaining fiber (PMF) is proposed and experimentally demonstrated for the measurement of humidity and temperature simultaneously. The LPG is inscribed in the PMF by a CO₂ laser and then connected with a 3-dB coupler to form a fiber loop mirror. The proposed sensor is created through coating a thin layer of polyvinyl alcohol onto the surface of PM-LPG, which can change its refractive index with different humidity levels. The resonance wavelength and its intensity varied with temperature and humidity, so humidity and temperature can be measured simultaneously. The humidity and temperature sensitivity of 0.1723 nm/%RH and 0.2174 nm/°C with the measurement errors of 2.6%RH and 0.2 °C were obtained.