An investigation of interference/intensity demodulated fiber-optic Fabry–Perot cavity sensor

Optical fiber sensors based on interference/intensity demodulated extrinsic fiber Fabry–Perot (F–P) cavity are described. An optical passive device, Coarse Wavelength Division Multiplexer (CWDM), is used to split the reflected lights from F–P cavity into two branches: one as signal channel with a narrowband spectrum and the other as reference channel with a broadband spectrum. Theoretical analysis shows that the optimum initial length of the F–P cavity should be set at the vicinity of the first phase reverse point of the broadband channel interference signal. Although the ratio of these two channel signals eliminates the influence of the light source output power and fiber loss fluctuation, an accurate temperature and driving current stabilizing system for light source is necessary for long-term high accuracy operation. In the experiment, the calibration curve was obtained by fitting experiment data to theoretical model with Levenberg–Marquardt nonlinear fitting algorithm. By controlling the length of two fusion points of the sensor head, the gauge range of 20–200, 100–300 °C for temperature sensors and 0–10, 0–20 MPa for pressure sensors were obtained. Under long-term measurements, an accuracy of ±0.1 °C over the full temperature range and a pressure accuracy of 0.2% over the full scale were obtained.