Spectral Demodulation Of Optical Fiber Pressure Sensors

Compared with traditional pressure sensors,fiber-optic Fabry-Perot(FP)pressure sensors show advantages of small size,low loss,high sensitivity,high temperature resistance,and easy to achieve multi-point measurement,etc.Especially,by the using of materials like SiC,sapphire and so on,Fiber-optic FP pressure sensors can be used in high temperature,vibration and corrosive environment.Fiber-optic FP pressure sensors are very suitable for harsh environment such as gas turbines and aerospace engines,they have been the mainstream research direction in the field of pressure sensors under extreme environment.For the application of fiber-optic FP pressure sensors,it's very important to realize accurate acquisition of their FP cavity lengths,and the demodulation technology is the key issue.This paper focused on the spectral demodulation technology of fiber-optic FP pressure sensors,proposed a two-parameter elliptical fitting algorithm for cavity length calculation,designed and built a corresponding demodulation system.The major work is as follows:1.Based on interference principle of single-cavity and compound FP sensors,the formula for calculating the cavity length was derived when the phase of reflection spectrum changes.The reflection spectrum characteristics of single-cavity and compound FP sensors were studied from the aspects of fineness and cavity length in detail.Principle for four typical demodulation methods of FP sensors was introduced,and the factors influencing demodulation accuracy were analyzed.2.Designed and established a demodulation system for fiber-optic FP pressure sensors,which includes optical path design,circuit design and software design.ASE was selected as the the light source of the system.The reflection spectrum of the FP pressure sensor is collected by a compact spectrometer module.The digital signal is transmitted to the FPGA circuit by A/D analog-to-digital conversion circuit.The cavity length is calculated by the algorithm integrated in the FPGA.3.Based on the analysis of the characteristics of reflection spectrum,shortcomings of typical demodulation methods and the characteristics of the self-built demodulation system,a two-parameter elliptical fitting algorithm was proposed.The research was carried out from the aspects of low frequency signal extraction of air cavity,Lissajous diagram drawing,ellipse equation and cavity length establishment.The similarities and differences between direct extraction and mean filter extraction of low frequency signal of air cavity were analyzed.The characteristics of Lissajous,the comparison of the complexity of establishing six-parameter and two-parameter elliptic equations.This two-parameter elliptic fitting algorithm improves the demodulation accuracy and simplifies the calculation process.The demodulation results of the composite FP sensor with air cavity varying from 76?m to 80?m were simulated.The results show that the maximum error of the two-parameter elliptical fitting algorithm is 0.05?m,and the curve fitting degree is 0.998.4.Tested the software and circuit parts of the demodulation system and verified the pressure experiment of the FP pressure sensor Firstly,the spectrometer driver,A/D data acquisition program and circuit demodulation system were tested.Furthermore,the pressure measurement experiment of FP sensor was carried out after the demodulation circuit can output signals of reflection spectra in a steady way.Experiments show that each module of the demodulation system works normally and can output reflection spectra with negligible noises.The demodulation speed can reach to 20 KHz,a pressure change of 3KPa can be distinguished under the condition that the sensitivity of the sensor is 0.19nm/KPa.The demodulation system has advantages of high precision and high speed.