Research On Demodulation Technology Of Optical Fiber Double Cavity Sensor

Pressure measurements in high-temperature environments are in high demand across various fields,such as aerospace,marine and offshore,energy exploration,and petrochemical industries.However,measuring pressure in such extreme conditions can be challenging due to potential deviations or inoperability caused by the high temperatures.To address this issue,the fiber optic dual Fabry-Perot cavity sensor has been developed to withstand high temperatures and electromagnetic interference while simultaneously measuring both temperature and pressure.In this paper,we investigate the demodulation technology of the dual Fabry-Perot cavity sensor to achieve high-speed and high-accuracy demodulation of the sensor’s cavity length and thus enable temperature and pressure measurements.Furthermore,we explore the temperature drift compensation method of the dual Fabry-Perot high-temperature pressure sensor to resolve any pressure measurement deviations resulting from temperature coupling effects.Overall,the main research contents of this paper include the investigation of the demodulation technology of the dual Fabry-Perot cavity sensor,temperature drift compensation method,and pressure measurement in high-temperature environments.By analyzing the interference principle of the dual Fabry-Perot cavity structure,a mathematical model of the reflection spectrum is established and spectral simulation and analysis are performed.The results show that the spectral of the dual Fabry-Perot cavity is more complex than that of the single Fabry-Perot cavity.The large envelope is modulated by the short cavity and long Fabry-Perot cavity,while the small envelope is modulated by the long cavity and long Fabry-Perot cavity.At the same time,the temperature and pressure measurement principle of the dual Fabry-Perot cavity are analyzed,which provides theoretical support for the subsequent design of a suitable dual Fabry-Perot cavity demodulation algorithm for this article.Design a dual Fabry-Perot cavity sensor demodulation algorithm.First,a spectral correction algorithm is proposed to reduce the influence of the light source on the demodulation results.Then,the idea of decoupling the double Fabry-Perot cavity signal into two single-cavity signals is proposed to reduce the difficulty of demodulation.The Hilbert variation method,local peak detection method and Fourier variation method are analyzed and simulated respectively,and it is clear that the Fourier variation method is the best decoupling method.The decoupled singlecavity signal is demodulated,and the joint algorithm of peak-chasing and inter-correlation is proposed for fast demodulation,and the feasibility of the algorithm is verified by simulating the air cavity and the base cavity in the range of 10 μm cavity length change.The results show that the demodulation consistency errors(standard deviation)of the air cavity and the substrate cavity are 0.48 nm and 0.53 nm,respectively.A demodulation system for dual Fabry-Perot cavity sensor is designed from four aspects:optical circuit and device selection,system circuit design and hardware,driver module design,and software writing and function testing.Finally,we test the entire demodulation system to verify its capability to display the cavity length of the dual Fabry-Perot cavity sensor.The temperature and pressure sensitive characteristics of the dual Fabry-Perot cavity sensor are analyzed and established an air cavity temperature compensation model.By conducting hightemperature static pressure calibration experiments,we obtained specific temperature compensation model values in an experimental environment.To verify the validity of the compensation model,we performed high-temperature static pressure compensation experiments,which resulted in a maximum temperature measurement error of ±1.2%FS in the range of 1000℃and ±1.1%FS in the range of 1MPa pressure measurement after compensation.Experimental results of sinusoidal pressure loading show that the demodulation system can demodulate at 20 k Hz.The consistency errors(standard deviation)of the air cavity and the substrate cavity were0.83 nm and 0.86 nm,respectively.