Research On Demodulation Theory And Algorithm Of Optical Fiber Pressure Sensing Based On Polarized Low-coherence Interferometry For Aviation Application

Optical fiber Fabry-Perot sensor has advantages of being small size, lightweight,high measurement precision and immunity to electromagnetic interferencecharacteristic. The techniques combining optical fiber Fabry-Perot sensor anddemodulation method of low-coherence interferometry are very suitable for aviationapplication of pressure measurement with high precision. In this paper, we proposed ahigh-speed and high-precision demodulation method based on the speed requirementof array barometric pressure for aviation application. We also studied on the influenceof location-dependent dispersion on the interference pattern and the phase information.Then two birefringence dispersion compensation demodulation methods withhigh-precision were proposed in the spatial domain and spatial frequency domainrespectively and corresponding experiments were carried out to verify theireffectiveness.The major work of the thesis as follows:1、On the basis of the speed requirements of array barometric pressure foraviation application, we designed the multi-channel demodulation system with opticalfiber Fabry-Perot sensors array. Comparing to the traditional spatial-frequencydomain analysis algorithm with empirical parameter, the computation speed of theproposed method was improved by more than10times and kept the similardemodulation accuracy. Thus the proposed method could implement the requirementsof the real-time demodulation for the multi-channel optical fiber Fabry-Perot sensorsand the computing ability limitation of embedded system.2、 We investigated the influence of location-dependent dispersion oninterferogram and constructed the theory model about the location-dependentdispersion of demodulation system in spatial domain. Then a dispersion compensationdemodulation method base on the interference pattern was proposed. This methoddemodulated the low-coherence interference signals by use of the characteristics oflocation-dependent dispersion and the envelope information. The measurementprecision was improved by17times comparing with the traditional envelopedetection method. This method had the advantages of simple principle and highprecision, but had large computational load.3、By investigating the influence of location-dependent dispersion on phase information and constructing the theory model about the location-dependentdispersion of demodulation system in spatial-frequency domain, a dispersioncompensation demodulation method against the spatial-frequency nonlinearity wasproposed. This method effectively avoided the jump errors of the demodulation results,which were caused by the location-dependent dispersion, by the traditionalspatial-frequency domain analysis algorithm. The demodulation precision of theproposed method was enhanced by13times comparing with the traditional envelopedetection method. Besides, this method was applicable to any spectrum shape ofbroadband source and had only half calculation of the dispersion compensationdemodulation method base on the interference pattern.4、We experimentally studied the effect of the signal-to-noise ratio (SNR) andtemperature on the performance of the demodulation system. The experiment resultsshowed that the changing tendency of the demodulation precision had the remarkablechange when the SNR value was17.5dB. When the SNR value greater than theinflexion point, the demodulation precision kept a high level. But when the SNRvalue fell below the inflexion, the demodulation precision reduced sharply. Besides,the resolution of the demodulation system enhanced when the SNR value increased.The measured cavity length became greater with elevating environment temperature.This offset of the cavity length was related to the temperature and the initial cavitylength. The temperature response coefficient of our experimental demodulationsystem was in the range0.461~1.919nm/℃.