High-speed Demodulation System Research Of Identical Weak Reflection FBG Based On FDML Swept Laser

Fiber Bragg grating(FBG)sensor is a kind of optical fiber sensor,which can measure the change of the external parameter through monitoring the center wavelength shift of the FBG reflection spectrum.Compared with the traditional electric sensor,FBG sensor has many excellent properties,such as strong anti-electromagnetic interference capability,lower transmission loss,erosion resistance,small volume and high detection sensitivity,etc.Accordingly it's widely applied in civil engineering,aviation,vessels,electricity,petroleum,the structural health monitoring of buildings and dynamic monitoring of complicated mechanical system.With the development of science and progress of the era,the existing technologies of optical fiber grating sensing systems have been incapable of social development needs,large capacity,low cost and fast demodulation are being the essential development direction from now on.As the craftsmanship of the optical fiber grating advances,the emergence of weak reflection FBG has led the optical fiber grating sensing network to a further leap.However,the existing technology of weak reflection FBG sensing network demodulation still has some problems like slow responsive speed.Thus this paper proposes the high-speed demodulation system of identical weak reflection FBG based on Fourier domain mode-locked(FDML)swept laser which provide a new method for the technology of FBG sensing network demodulation.Main research works are as follows:(1)Studied the related theories about the formation of swept laser,built FDML swept laser,analyzed and tested the influence of different impact factors on the output spectrum characteristic of the FDML swept laser,then acquired the high-speed swept spectrum with the swept frequency at 120.99kHz,the range of swept spectrum at1292.5nm~1306nm,the wavelength scanning rate at 3.267×10~6nm/s through frequency multiplication.(2)Constructed the high-speed demodulation system of the identical weak reflection FBG on the basis of high-speed swept laser and the FDML swept laser can output the high-speed swept light whose periodic wavelength changes continuously.The reflected signals of identical FBG at each position distinguish with others in time domain through optical delay effect,so that the location information of each FBG could be acquired.Meanwhile,the wavelength information of all the weak reflection FBG which were optically delayed could be obtained by only once scanning.(3)Proposed the demodulating method based the wavelength of designed laser's emission light existed a linear relationship with period count value.This method turning the measurement of FBG center wavelength in frequency domain into time domain.At the same time,took the optical comb filter as the reference path to demarcate segmentally for the system,thereby decrease the demodulating deviation that temperature drift of fiber Fabry-Perot tunable filter and the nonlinear factors of piezoelectric ceramic brought.The result of the experiment shows that the stability and demodulating linearity of identical weak reflection FBG demodulation system based on FDML swept laser behave well with the demodulating speed reaching120.99kHz and R~2 being above 0.999.(4)Proposed the calibration algorithm for optical delay deviation based on switching the wavelength scanning rate of the system,which realized the each FBG's real center wavelength demodulation of identical weak reflection FBG array.The result of the experiment proves that the demodulating deviation of each optical real center wavelength is less than±15pm and demodulating linearity is good.