Fiber Bragg Grating-Based Quasi-Distributed Temperature Sensor for Down-Hole Monitoring

To overcome disadvantages of traditional electronic sensors, such as low reliability, large drift at high temperature, limited to a single point measurement and so on, a quasi-distributed temperature sensor system for down-hole monitoring based on the fiber Bragg grating (FBG) is designed in this paper. The working principle, package structure and signal demodulating method of this system are proposed in detail. Compared with other peak-detected algorithms, the direct Gaussian fitting algorithm was used to achieve a high precision and steady demodulation result of the reflection spectrum of the FBG. A triple layer metal tubing protection scheme was proposed to meet the critical measurement requirement in the harsh environment of the oil well. Experimental results show that the measurement accuracy is less than 0.26% with the full scale (F.S.) of 250 degrees C; the maximum of absolute-resolution is 0.15 degrees C, corresponding to a relative-resolution of 0.06% F.S. (250 degrees C); the repeatability is less than 0.13% F.S. (250 degrees C), and the long-term stability at 250 degrees C is less than 0.11% F.S.