Research On Cavity Length Demodulation Technology Of SiC Fabry-Perot Cavity Sensor

With the development of optical fiber sensing technology,Fabry-Perot(FP)cavity sensors have been widely used in aerospace,bridge and dam,medical research and other fields due to their superior characteristics.By demodulating the cavity length change information from FP cavity sensors,different physical quantities such as pressure and temperature can be measured.As a third-generation wide-bandgap semiconductor material,SiC can withstand high temperature and high pressure,and has gradually become one research direction of the FP cavity sensors in extreme environments.For the practical application of SiC FP cavity sensors,the key issue is to extract the cavity length of FP cavity sensors.Since the SiC light-transmitting band is in the range of visible light,while the demodulation systems for FP cavity sensors at this stage are all in the near-infrared range,the demodulation accuracy of such demodulation devices for the SiC FP cavity sensors is too low,and it is difficult to obtain accurate cavity length values.Therefore,this thesis mainly studies the cavity length demodulation technology in the range of visible light for SiC FP cavity sensors.The main contents are as follows:Firstly,the sensing principle of the FP cavity sensors was introduced in detail,and the relationship between the phase of the reflected spectrum signal and the cavity length was deduced.The specific demodulation scheme of the SiC FP cavity sensor and the cavity length demodulation algorithm were proposed.The specific scheme includes the optical path of the system and the hardware circuit.Secondly,according to the light response characteristics of SiC,a demodulation system was built.Because SiC has a strong transmittance in the visible light band,the visible light LED was used as the system source,the same wavelength micro-fiber spectrometer was selected to collect the reflected spectral signal of the FP cavity sensor,and the A/D module is used to complete the AD conversion of the spectrum signal,and the data processing was performed through the FPGA control circuit to calculate the cavity length value.Then,the collected reflection spectrum signal of the SiC FP cavity sensor was analyzed,because the obtained signal has a weak contrast,the existing spectral demodulation method has large calculation errors.Therefore,a signal square-cross-correlation demodulation algorithm was proposed.Through the simulation of the cavity length of 15-25 ?m FP cavity,results shown that the maximum demodulation error of the algorithm was 0.005?m,and the relative error was 0.022%.Finally,an experimental setup of the demodulation system was built,the feasibility of the hardware circuit system and software algorithm was verified by using a packaged SiC FP cavity sensor and the FP cavity length was successfully extracted.A pressure measurement experiment on the SiC FP cavity sensor,shown that the demodulation speed can reach 500Hz,and the output cavity length of FP sensor was linear related to the input pressure value.The demodulation system can working normally.A sensitivity of 0.19357nm/KPa,was realized from which,a pressure value of 500KPa can be distinguished.