【正文】 1 光纖 Bragg光柵傳感 Fiber Bragg Gratings 國(guó)立中山大學(xué) 電機(jī)工程研究所 林武文教授 2 ? Ref: Andreas Othonos amp。 Kyriacos Kalli, Fiber Bragg Gratings, Artech House , Norwood, 3 Bragg Grating Sensors Introduction Sensing External Fields Wavelength Demodulation of Bragg Grating Point Sensors Simultaneous Measurement of Temperature and Strain Polarization Stability of Interrogation Schemes 4 Bragg Grating Sensors Multiplexing Techniques Sensors Based on Chirped Bragg Gratings Distinguishing Bragg Grating Strain Effects Bragg Grating Fiber Laser Sensors Bragg Gratings as Interferometer Sensors and Reflective Markers 5 Fiber Bragg Gratings Other Bragg Sensor Types Applications of Bragg Grating Sensors 6 Introduction ? FBG is suitable for measuring Temperature strain pressure 7 Introduction ? The principal advantage is that the measured information is wavelength encoded flexibility to work linearity (from ppb~%) low insertion loss amp。 narrow band wavelength reflection easily embedded into materials 8 Introduction ? The key detection issue for FBG is the determination of small measurandinduced wavelength shifts. ? Category of FBG sensors point sensor array (WDM/TDM/SDM) or distributed a reflective marker (OTDR) extremely long grating 9 Sensing External Fields ? In we learned (1)Strain response of the grating arises from the physical elongation of the sensor, leading to a fractional change in the grating pitch with a corresponding change in the fiber index because of the photo elastic effect. 10 Sensing External Fields (2) Thermal response inherent thermal expansion of the fiber material. the temperature dependence of the refractive index. 11 Sensing External Fields Eq()amp。() amp。 () 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 relies on broad band illumination of the device. ? The grating used in sensor applications have bandwidth of to . 23 Passive Broadband Interrogation ? Broadband or edge filters provided a wavelength dependent loss when the cutoff is lose to the signal wavelength.((a)) 24 Passive Broadband Interrogation 25 Passive Broadband Interrogation ? Comparing light Xmitted thru the filter with light passed along a reference path recovers the wavelength shift of the sensor.((a)) 26 Passive Broadband Interrogation ? This approach provides (1) Very limited sensitivity for broadband filter. or(2) Limited range for edge filter. (3) Rely on bulk optic ponents。 alignment stability is critical which reduces portability. 27 Passive Broadband Interrogation }{???? ????OBRS AII () This system offer several advantages low cost ease of use low resolution 28 Passive Broadband Interrogation 29 Ratiometric Detection with a WDM Fiber Coupler ? All fiber approaches such as fiber WDM fusedtapered couplers provides a monotonic change in the coupling ratio between the two output ports. ((b)) ? A