| Optical fiber extrinsic Fabry-Perot interferometic(EFPI)sensor has abundant features,which can obtain the physical,chemical,and biological parameters based on their different sensing principles.The wavelength demodulation and intensity demodulation are widely applied for dynamic signal recovery of EFPI sensors.However,the wavelength demodulation does not have a large dynamic range and a good capacity for fast demodulation;Intensity demodulation is limited by its drifting operation point and imbalance of light path.Therefore,it is necessary to exploit a new interrogation method to track the dynamic signal of EFPI sensors.In this paper,based on the analysis of the dynamic signal demodulation technologies,a polarization-switched phase interrogator for EFPI sensors is presented with the analysis of dual-wavelength demodulation method.The major achievements and innovations of this article are listed as below:(1)Based on the contrastive analysis of Fabry-Perot sensor demodulation methods,a polarization-switched structure is proposed and demonstrated using the reflected light beams of polarization-maintaining fiber Bragg grating(PM-FBG)and the polarization modulation mechanism,which can switch the wavelength of two light signals in the time domain.The demodulation of interferometric signal is achieved in totally one light path in the polarization-switched structure.(2)In order to overcome the exact demand of phase orthogonality in the dual-wavelength orthogonal demodulation algorism,an ellipse fitting parameter evaluation method is introduced to solve the direct current components in the output signal of EFPI sensor.Combined with the differential cross multiplication algorism,the phase variation of EFPI sensors with different cavity lengths can be obtained,which reduces the strict fabrication precision requirement of EFPI cavity effectively.(3)The polarization-switched phase demodulation test system is achieved and the principles of polarization modulation and switch technology are studied.The fundamental experiment results show that two linearly polarized beams can be switched in the time domain and the system can track the vibration signal of EFPI sensor with the cavity length of 120?m,300?m,500?m.(4)Due to the stability of demodulation system is affected by the drifting operation point of polarization modulator,a feedback control method is achieved and it is added into the phase demodulation system with the maximum light intensity scanning based.The modified phase demodulation system can realize the stable output of two linearly polarized beams and the stability of the system is promoted greatly with a peak power spectrum density fluctuation level of ±0.5dB within the dynamic measurement range of 80-1250 Hz.(5)The noise feature of the demodulation system is measured first,and a quasi-monochromatic light interference optical model is established to analyze the polarization switching crosstalk and features of the sensing unit on the system distortion and noise level for EFPI phase demodulation.There is a good consistency between the simulation result and measurement result,and the simulation analysis provides a theoretic base for noise suppression in the next step.The polarization-switched system of EFPI sensors for tracking the dynamic signal stablely suppresses the light source fluctuation and external disturbance,and it has achieved demodulating two interference signals in totally one light path.The work of this paper enriches and enhances the interrogation system of F-P sensors,and it pushes forward the application development of EFPI sensors in the commercial and engineering fields. |
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