【正文】 iple of Operation 82 Principle of Operation ? Fig (b) shows the low corresponding to and planes. ? When the angle between the lines at the intersection point=0, Eq.() A fundamental te is that the ratio of the strain responses of 2 gratings be different from the ratio of their temp responses. 1? 2?1 2 1 2//TTK K K K????83 Principle of Operation 1 1 1 12 2 2 2//)//TTKKTKK??? ? ? ??? ? ? ??????? ??????? ??() () ?Ideally , the error in temp and train, are amp。 random environmental perturbations acting on the product nd. 69 QuasiStrain Monitoring tLtLt gBM Z IBBM Z I ?????????? s in2s in2)(2?????????tB ?? sin? )(t??(716) ?For a dynamical straininduced modulation in the reflected from the FBG ,the change in the phase shift is With ,is the OPD (optical path difference) of the MZI ndL M ZI ???? is the dynamic strain subjected to grating g?is the normalized straintowavelengthshift responsivety of the Bragg grating with 1 (7 .1 7)BgBx??????If replaced by ()amp。 Kersey ref. A Michelson interferometer 57 Analysis of Bragg Grating Wavelength Shift Using Fourier Transform Spectroscopy 58 Analysis of Bragg Grating Wavelength Shift Using Fourier Transform Spectroscopy )(co s)()( 110 ???? III ??))(e x p ()()( 111 ??? ?IA ?])([)( 01 IIHA ?? ??() () () ?If the effective coherence length of the reflected light exceed the OPD ?The grating center wavelength can be found from the Periodicity of the interferogram, which can be measured by finding an analytic sigual H: Hilbert transform ? ??? i ii ii IAA ))((e x p)()()( ?????Where is the superposition of signals from the Individual gratings. )(?iA59 Analysis of Bragg Grating Wavelength Shift Using Fourier Transform Spectroscopy 60 ModeLocked Fiber Laser ? ref. A grating sensor has been implemented with broadband ultra short pulses generated with a passively modelocked fiber laser. 61 ModeLocked Fiber Laser 62 ModeLocked Fiber Laser 63 Other Lasers ? Yun and coworkers demo a line width, wavelengthswept EDF laser scanned over a 28 nm range to demodulate a BGS array to demodulate a BGS array with a strain of resolution of at 250Hz. rms??)/42( 21Hzn ?64 Dynamic Strain Sensing QuasiStrain Monitoring Dynamic Strain Sensing Simultaneous Interrogation of Bragg Gratings and Interferometric Sensors 65 Dynamic Strain Sensing ? Optical interferometer (MZ or Michelson) can be used to convert the wavelength shift from grating into a phase change. 66 QuasiStrain Monitoring ? Fig shows the principle of Grating sensor system by using fiber interferometer wavelength discriminator for dynamic measurement. 67 QuasiStrain Monitoring 68 QuasiStrain Monitoring )])(c o s (1[)( ???? ??? BB kAI() ?The unbalanced interferometer serves to filter the light with a raised cosine fun。 () works for measuring acceleration ultrasonic waves forces 12 Sensing External Fields Pressure Sensitivity Dynamic Magic Field Sensitivity 13 Pressure Sensitivity PPnnPnnBp ?????????????? )11()(??EPvLL )(???() () )2)(21(2 11122ppvEPnnn ????() ?A pressure change of wavelength shif p??:p???For SMF Where E is Young’s modulus 14 Pressure Sensitivity )2)(21(21)21(111122ppvEnpnnEvP????????????PppvEnE vBp ???????? )]2)(21(2)21([ 11122?? () () () ?Given ,the normalized pitch pressure is given by /LL???For the refractivepressure coefficients are given by Therefore ???/15 Dynamic Magic Field Sensitivity (DMFS) ? Faraday effect A longitudinal magic field applied to the grating changes the refractive index for the two cp (circular polarization ,CP) 2 Bragg conditions: ?22BnBn????? ? ?? ? ?Where “+” refer to right cp. “ ” refer to left cp. 16 Dynamic Magic Field Sensitivity (DMFS) ??2VHnn ????() FBG’s have been used for DMFS thru the Faraday effect. 17 Wavelength Demodulation of Bragg Grating Point Sensors QuasiStatic Strain Monitoring Dynamic Strain Sensing Simultaneous Interrogation of Bragg Gratings and Interferometric Sensors 18 Wavelength Demodulation of Bragg Grating Point Sensors ? For FBG point sensors 1 pm resolution is required to resolve By OSA+ Tunable laser. ~ 0 .1 1C o r ??o19 Wavelength Demodulation of Bragg Grating Point Sensors 20 QuasiStatic Strain Monitoring Passive Broadband Interrogation Ratio metric Detection with a WDM Fiber Coupler Interrogation via Scanning Optical Filter Bragg Grating Interrogation Using Wavelength Tunable Source 21 QuasiStatic Strain Monitoring Recovery of Bragg Grating Wavelength Shift Using CCD Spectrometer Analysis of Bragg Grating Wavelength Shift Using Fourier Transform Spectroscopy ModeLocked Fiber Laser Other Lasers 22 QuasiStatic Strain Monitoring ? A lot of schemes used for recovery the wavelength shift information is required for smart structure application. ? The most fundamental means for interrogating a FBG r