Investigation Of Extrinsic Fabry-Perot Interferometer Based Optical Fiber Sensors For High Temperature And High Pressure Measurements

This work was supported by the National Scientific Foundation Committee of China under the project number 60028505 and the Science and Technology Foundation Committee of Liaoning Province, China, under the project number 20042164.As an important member of optical fiber sensors, extrinsic Fabry-Perot interferometer (EFPI) based optical fiber sensor has won widely attention for its small size, compact configuration, good reliability and flexibility. It can be widely used in most harsh environments, such as oil well downhole, electric power industry and chemical industry. In this dissertation, several key problems are studied systematically, which include the modulation mechanism of the EFPI optical fiber sensor, the design and fabrication technique of EFPI sensor probe, and the performance evaluation of the sensing system. The main research works are outlined as followings:1. Based on the theoretical analysis and experimental study on the self-calibrated interferometer/intensity based fiber EFPI sensor, an improved self-compensated demodulation algorithm has been proposed for the first time: by setting the initial F-P cavity length to a special point at which the interferometric signals from broadband and narrowband channels show reversed phase, more effective compensation and higher resolution of the sensing system are achieved. The key optical passive device applied in the sensing system is a CWDM component that is used to split the reflected lights from F-P sensor head into two branches: one as signal channel with a narrowband spectrum, the other as reference channel with a broadband spectrum. Theoretical analysis shows that the ratio of the two channel signals, R_a, can eliminates the influence of the light source output power and fiber loss fluctuation, improves signal-to-noise ratio and sensitivity of the system. The coefficients of expression R_a are obtained by fitting the experiments data to the theoretical model with Levenberg-Marquardt non-linear fitting algorithm, the fitting correlative coefficients are more than 0.9999.2. Design and fabrication technique of the extrinsic Fabry-Perot interferometer sensor probe are investigated in detail. By using CO₂ laser thermal fusion method, multi-gauge range F-P sensor heads for temperature, pressure and vibration measurements are fabricated. In order to minimize the temperature and pressure cross-sensitivity, a pressure insensitive temperature sensor with a fused silica pressure shielding structure and a temperature insensitive pressure sensor with optimized sensor head geometry parameters and material have been designed.3. Measurement performance of the EFPI fiber optic sensor system has been investigated in the experiment. Measurement scales of different high temperature and high pressure are obtained, such as: 20²00℃,100³00℃, 0³00℃, 20⁴50℃ and 0¹4MPa,0¹9MPa. 0³0MPa. For high temperature gauge range more than 200 °C, a resolution less than 0.06°C is achieved, a repeatability of 0.1 %FS and a stability less than ±0.1 %FS in several days are obtained. For high pressure measuring gauge more than 20MP&, a resolution less than 6.4KPa, accuracy relative to calibrator of ±0.3%FS, repeatability of 0.7%FS and stability less than ±0.5%FS are obtained. For the low pressure gauge range sensor, the experiment also shows that a resolution of 200Pa and stability of 0.5% in 4 hours in the full scale range of 0.2MPa are obtained. For testing the time response behavior of the sensor, vibration measurement is also conducted with the sensor. The test result shows that the response frequency of 1kHz can be reached.4. For the applications of high temperature and pressure measurements in oil well, a time domain multiplexed (TDM) temperature and pressure sensor system has been constructed. Measurement test in a high temperature and pressure simulation environment in the lab showed that an accuracy of temperature measurement of 0.25%FS under a pressure variation range up to 20MPa, and a pressure measurement accuracy of 1% under a temperature environment of 20³00°C are achieved separately. The downhole measuring system is implementing in Liaohe oil field for real-time temperature and pressure measurement recently.