Study Of High-sensitive Fiber Bragg Grating Displacement Sensor Based On Power Demodulation

Fiber Bragg grating(FBG)has the advantages of the fine reflection peak,Bragg wavelength sensitive to external physical quantity,small vol?me,corrosion resistance and the fast response speed.Thus,optical fiber sensor based on the FBG is widely studied.In this paper,a high sensitivity Fiber Bragg grating displacement sensor based on power demodulation for measuring cylinder displacement is proposed.Firstly.The classification,application,sensing principle and characteristics of fiber optic sensors,the various measuring principle of the displacement sensor and the important application of Fiber Bragg Grating in displacement sensor are introduced in this paper.The classification,basic structure and working principle of single-mode fiber,Fiber Bragg grating and fiber coupler are introduced.Secondly.This paper introduces the theory and theoretical simulation of the double Bragg grating displacement sensor based on power demodulation.This paper uses two identical fiber Bragg gratings(FBG1,FBG2).The reflection spectr?m of FBG1 is incident to the FBG2.Two peaks(left,right)will appear on both sides of original Bragg wavelength in the transmission spectr?m of FBG2.Theoretical simulation found that: In the process of keeping the Bragg wavelength ?B1 constant and the Bragg wavelength ?B2 continuously redshif,the power difference(left peak minus peak)of the FBG2 transmission spectr?m is approximately linear with the redshift ??B2 of FBG2.This conclusion provides an important theoretical basis for power demodulated fiber Bragg grating displacement sensor.Through numerical simulation:It can be concluded that the smaller the transmittance of the Fiber Bragg grating,the higher the sensitivity of the fiber Bragg grating displacement sensor.Thirdly.The experimental and experimental verification of the power demodulated Fiber Bragg grating displacement sensor are carried out.This paper uses two identical fiber Bragg gratings of the Bragg wavelength ?B1=?B2= 1520 nm.In the experiment,during the cylinders vertically pressing down the FBG2,the Bragg wavelength ?B2 occurs to red shift continuously.The left peak power continues to increase,but the right peak power continues to decrease in the FBG2 transmission spectrum double peaks.In the cylinder displacement in the range of 0?m ~ 60?m,there is a good linear relationship between the cylindrical displacement and the bimodal power difference(left peak minus right)in the FBG2 transmission spectr?m.The displacement measurement sensitivity reaches to(7.46±0.17)×10-2d Bm/?m.The results are the same as the theoretical simulation,and the theory can be verified.Fourthly.In this paper,the causes of the small measurement of the Bragg grating displacement sensor are analyzed and the measurement range is increased by the segmented measurement method.When the FBG2 transmission spectr?m bimodal structure can not be resolved,pulling axially the FBG1 makes the Bragg wavelength ?B1 redshift until the peak power of the left and right peak again equal in the bimodal structure.Then the displacement of the cylinder can be measured continuously.By recycling the above process,we achieved a piecewise measurement of the cylinder displacement at the Bragg wavelength of 1520 nm nearly,the measuring range up to 230?m and the measurement sensitivity of up to(7.46±0.17)×10-2d Bm/?m.Finally.In this paper,the waveforms and linewidths of FBG1 and FBG2 are analyzed.During tracting axially the FBG1 and compressing vertically the FBG2,the linewidth of FBG1 and FBG2 are almost unchanged,and the spectr?m does not appear split.The results show that the nonuniform deformation of FBG1 and FBG2 is not serious and will not affect the experimental results.