Research On Demodulation System Of Fiber Fabry-perot Sensor Based On Polarization Low Coherence

In the fields of gas turbine monitoring and petroleum exploration,pressure/temperature measurement in extreme working environments with high temperature and high corrosion is essential for monitoring the working state of these applications.Comparing to conventional electrical sensors,Fiber-optic Fabry-Perot sensors are more suitable to be applied in the above situations because of their multiplexing convenience,electromagnetic interference resistance,light-weight and adaptation to extreme conditions.However,despite various demodulation methods of fiber-optic Faber-Perot sensors,including intensity demodulation method,spectral demodulation method,and phase carrier method,have been reported in the past decades,these methods generally cannot meet the demands of range,speed,and dynamic/static parameters detection at the same time.To solve this problem,a low-coherence interferometry is employed to achieve the simultaneous large-scale measurement of dynamic and static parameters.This interferometer is constructed by a polarization interferometer,which avoids the additional processing complexity and signal-to-noise ratio degradation introduced by the use of Fizeau interferometer.In order to avert the demodulating error caused by the crystal dispersion in conventional method,an improved stripe center algorithm is proposed to correct the demodulation results of the fiber-optic Fabry-Perot sensor.To accelerate the signal process,this paper also builds a rapid acquisition,transmission,and processing system based on DSP and FPGA,and establishes a hardware platform for real-time demodulation.Finally,the linearity,stability,resolution,and repeatability of the demodulation system were tested.The principal works of this article are as follows:Firstly,theoretical models of Fizeau interferometer and polarization interferometer is analyzed and compared.A demodulation scheme which takes both interference fringe contrast and wedge processing complexity into account is designed.According to this scheme,the light source,optical fiber,birefringent crystal wedge,camera and other devices in the system are selected.Finally,a complete fiber-optic Faber-Perot sensor demodulation system is built.Secondly,based on the theoretical model of polarization interferometer,the influences of crystal dispersion on low-coherent interference fringes,such as the relative movement between the interference fringes and the envelope curve,as well as the dismatch between the zero phase point and the center of the envelope curve,is studied.To overcome these issues,this paper proposes an improved stripe center algorithm,in which the linear relationship between the envelope center of the interference fringe and the 2m? phase point are utilized to demodulate the low-coherence interference fringe signal.After introducing the dispersion into the demodulation system,the validity of the algorithm is verified.Thirdly,considering the huge volume of data and calculation in the proposed demodulation algorithm,this paper builds a rapid acquisition,transmission,and processing system based on DSP and FPGA.The system uses FPGA,Camera Link camera,and acquisition card to achieve high-speed acquisition of lowcoherence interference fringes;uses PCIe protocol to perform high-speed data transmission;uses TMSC6657 dual-core DSP to make high-speed data processing and the demodulated data output in analog form.Finally,resolution,the error,repeatability,stability and dynamic response of the proposed demodulation system is tested.According to the experimental results,The demodulation system is capable of resolving 4 nm displacement,with the maximum error accounting for 0.63% of the full scale,the 30 min stability,2min stability and repeatability are 0.55%,0.083%,and 0.14%,respectively.This method can resolve a displacement variation of 4nm when changing ratio is lower than 900 Hz.This paper solves the problem that fiber Fabry-Perot sensor demodulation systems cannot simultaneously take into account the measurable range,demodulation speed,simultaneous measurement of dynamic and static parameters,and is conducive to the marketization of fiber-optic Faber sensors.