Fiber-optic Fabry-Perot pressure sensor for down-hole application

With advantages of low cross-sensitivity, high-resolution and reliable structure, the extrinsic Fabry-Perot interferometric (EFPI) based optical fiber sensor is the best candidate for down-hole pressure monitoring. This paper provides a general overview of key technologies regarding to EFPI pressure sensors for down-hole monitoring. The fiber Bragg grating (FBG) sensor is employed to provide temperature measurement as well as to eliminate the temperature-pressure cross-sensitivity. To realize high resolution and large dynamic range measurement, a vernier-like demodulation algorithm is presented, which can provide a resolution of 0.08 nm over a dynamic range of 3 mm. The carbon coating is employed for the EFPI sensor ((CEFPI)-E-2) to improve the long-term stability. The stability, hysteresis, repeatability, accuracy, and the vibration-resistance ability of the (CEFPI)-E-2 sensor are examined experimentally. An accuracy of 0.05% F. S. and a hysteresis of 0.01% F. S. in the dynamic range of 72 Mpa have been obtained with good long-term stability and vibration-resistance ability. The high reliable packaging technology, as well as the multiplexing technology with the distributed temperature sensor (DTS), are also introduced. Two year's field test results in high-temperature oil well show that the present (CEFPI)-E-2 sensor has good long-term stability and reliability.