Research On The Demodulation Technique Based On Free Spectral Range Matched

The development of fiber optic sensors based on in-fiber Bragg gratings(FBGs) has been ofinterest due to its numerous applications in strain,temperature etc. Because of the wavelength-encoded nature of the FBG sensors,the common approaches for detection are the wavelengthinterrogation techniques. This paper realizes the interrogation of FBG sensing signals, based on theFabry-Perot free spectral range matched technique. The work that this paper has done are asfollows.First,the review of the techniques and applications of fiber Bragg grating sensing is done andbriefly introduce the demodulation techniques and multiplexing techniques. Then the basic theory offiber Bragg grating is analyzed by using classic methods, including coupled-mode theory andtransfer matrix method. By using the OptiGrating simulation sofeware, a fiber Bragg grating withuser defined apodization function and linearly chirped period is simulated. By using GratingMod, aFBG strain sensor is simulated. This will help the optimization of parameters of fiber Bragg gratingwhile designing and fabricating FBG sensors.Next, the basic principles and main parameters of Fabry-Perot filter are analyzed and throughnumerical simulation, the impact of parameters on Fabry-Perot filter is found. Then, a systematicanalysis method has been developed to study the performance, includeing peak intensity, vallyintensity and fringe contrast of an interferometric sensor based on the Fabry-Peort interferometer.Through numerical simulation, the impact of parameters on interferometric sensors based on theFabry-Peort interferometer is found. In total, the theoretic analysis and numerical simulation will dohelp in the process of fabricating the interferometric sensors.At last, analyze the demodulation technique based on the tunale Fabry-Perot optical filter. Basedon the traditional tunable Fabry-Perot filter demodulation technique, a free-spectral-range-matchedinterrogation (FSRMI) technique which integrates a tunable FFP filter followed by a multichannelbandpass filter is proposed. An n-channel FSRMI allows simultaneous scanning and thusimprovement of detection speed by n times compared to a nonfree-spectral-range-matchedinterrogation scheme. This technique simultaneously scans the multiple transmission peaks andprovides solutions to a wavelength-modulated distributed sensors with high sampling speed andwavelength accuracy.