【正文】 elies 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。 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。1 光纖 Bragg光柵傳感 Fiber Bragg Gratings 國立中山大學 電機工程研究所 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。() amp。 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)) ? Advantages: (Allfiber type) low power loss low cost 1% accuracy 30 Interrogation via Scanning Optical Filter ? The demodulated output is the convolution of the tunable filter spectrum with that of the grating. ((b)).and the output is optimized when the spectrum of the tunable filter matches that of the grating. 31 Interrogation via Scanning Optical Filter ? Drawback of the use of narrowband SOF sampling a narrow slice of the optical spectrum at a given time. the measured resolution is strongly dependent on S/N of the return signal and the line widths of the tunable filter and FBG. 32 Interrogation via Scanning Optical Filter Ex Repeated scanning of a grating array at a freq f results in an energy reflected by each grating per sampling period that is equal to fRIE gR??? () Where R: the grating reflectivity. I: the spectral brightness of the source : the grating’s spectral width. g??RE33 Interrogation via Scanning Optical Filter sfRDEE????() ?The scanning filter further limits the detection energy DEWhere : the filter bandwidth : the width of the scanned wavelength range. fs???34 Interrogation via Scanning Optical Filter (1)Tunable Wavelength Fiber FabryDerot Filter (FPF) used in optical fiber munication system to remove ASE (amplified spontaneous emission) noise emanating from EDFA. 35 Interrogation via Scanning Optical Filter (2)Tunable Fiber Bragg Grating Filters (3) AcousticOptic Tunable Filter (4) Other Tunable Filter Types 36 Interrogation via Scanning Optical Filter (1)Tunable Wavelength Fiber FabryDerot Filter ? The characteristics of the filters band pass resonance of Lorentzian line shapes bandwidth (BW)~. ? shows how a funable filter is used. ? 2 operating schemes Tracking (closedloop) mode (for 1 FBG) scanning mode (for 2 ormol FBG) 37 Interrogation via Scanning Optical Filter 38 Interrogation via Scanning Optical Filter (1)Tunable Wavelength Fiber FabryDerot Filter ? Fig Compares the strain monitored with a scanning filter demodulated grating and resistive strain gauge (RSG) when both are subject to a strain level ,with resolutions of for a BW from DC~360Hz. ~ 2022??~1???39 Interrogation