光纤布拉格光栅.ppt

1、1,光纖布拉格光柵,Fiber Bragg Gratings 國立中山大學 電機工程研究所 林武文教授 2006.11.1,2,Ref: Andreas Othonos alignment stability is critical which reduces portability.,27,7.3.1.1 Passive Broadband Interrogation,(7.6),This system offer several advantages - low cost - ease of use - low resolution,28,7.3.1.1 Passive Broadband

2、 Interrogation,29,7.3.1.2 Ratiometric Detection with a WDM Fiber Coupler,All fiber approaches such as fiber WDM fused-tapered couplers provides a monotonic change in the coupling ratio between the two output ports. (ref.Fig.7.2(b) Advantages: (All-fiber type) - low power loss - low cost - 1% accurac

3、y,30,7.3.1.3 Interrogation via Scanning Optical Filter,The demodulated output is the convolution of the tunable filter spectrum with that of the grating. (ref.Fig.7.1(b).and the output is optimized when the spectrum of the tunable filter matches that of the grating.,31,7.3.1.3 Interrogation via Scan

4、ning 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.,32,7.3.1.3 Interrogation via Scanning Optical Filt

5、er, 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,(7.7),Where R: the grating reflectivity. I: the spectral brightness of the source : the gratings spectral width.,33,7.3.1.3 Interrogation via Scanning Optical Filt

6、er,(7.8),The scanning filter further limits the detection energy,Where : the filter bandwidth : the width of the scanned wavelength range.,34,7.3.1.3 Interrogation via Scanning Optical Filter,(1)Tunable Wavelength Fiber Fabry-Derot Filter (FPF) - used in optical fiber communication system to remove

7、ASE (amplified spontaneous emission) noise emanating from EDFA.,35,7.3.1.3 Interrogation via Scanning Optical Filter,(2)Tunable Fiber Bragg Grating Filters (3) Acoustic-Optic Tunable Filter (4) Other Tunable Filter Types,36,7.3.1.3 Interrogation via Scanning Optical Filter,(1)Tunable Wavelength Fibe

8、r Fabry-Derot Filter The characteristics of the filters - band pass resonance of Lorentzian line shapes - bandwidth (BW)0.3nm. Fig.7.3 shows how a funable filter is used. 2 operating schemes - Tracking (closed-loop) mode (for 1 FBG) - scanning mode (for 2 or-mol FBG),37,7.3.1.3 Interrogation via Sca

9、nning Optical Filter,38,7.3.1.3 Interrogation via Scanning Optical Filter,(1)Tunable Wavelength Fiber Fabry-Derot Filter Fig 7.4 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

10、a BW from DC360Hz.,39,7.3.1.3 Interrogation via Scanning Optical Filter,40,7.3.1.3 Interrogation via Scanning Optical Filter,(1)Tunable Wavelength Fiber Fabry-Derot Filter Fig.7.5 (inset) shows the output of one sensor subject to sine-wave modulation of periodicity of 2 minutes.,41,7.3.1.3 Interroga

11、tion via Scanning Optical Filter,42,7.3.1.3 Interrogation via Scanning Optical Filter,(2) Tunable FBG filters (TBGF) - by Jackson et al. (ref.Fig.7.6) - A reflectrometric FBG based tunable filter scheme. modified by Brady et al the parallel topology by using a series array of receiving grating. - a

12、reflect metric approach. - Advantage (1)more efficient. (2)reduce system components.,43,7.3.1.3 Interrogation via Scanning Optical Filter,44,7.3.1.3 Interrogation via Scanning Optical Filter,(2) Tunable FBG filters (TBGF) - by Davis et. al. (ref.Fig.7.7) - using the receiving gratings in an efficien

13、t Xmissive mode Minimize the effect of light loss. Improving sensor sensitivity.,45,7.3.1.3 Interrogation via Scanning Optical Filter,46,7.3.1.3 Interrogation via Scanning Optical Filter,(3) Acoustic-Optic Tunable Filters (AOTF) - is a solid-state optical filter - the wavelength of the diffracted li

14、ght is selected by RF. - large tuning range - fast access time, 5kHz - narrow spectral bandwidth.,47,7.3.1.3 Interrogation via Scanning Optical Filter,48,7.3.1.3 Interrogation via Scanning Optical Filter,- AOTF is attractive for wavelength multiplexing very large Bragg grating arrays with the provis

15、o that a suitable broadband source , or array sources is available. - Suitable for optical fiber.,49,7.3.1.3 Interrogation via Scanning Optical Filter,(7.9),For a given grating and AOTF bandwidth there is an ideal freq derivation for maximizing the tracking error signal given by,Where is the AOTF ba

16、ndwidth. Note (7.9) is independent of the filter Xmission ,grating reflectivity, and intensity noise.,50,7.3.1.3 Interrogation via Scanning Optical Filter,One of the notable advantages of the AOTF over all other filters is the possibility of driving the device at multiple RF signals to allow for tru

17、e parallel processing of multiple wavelength signals using a single filter and detector. Volanthen ef al. demo the monitoring of 2 FBG written at 1300 and 1550 nm using a ATOF. (ref. Fig.7.9),51,7.3.1.3 Interrogation via Scanning Optical Filter,圖7.9,52,7.3.1.4 Bragg Grating Interrogation Using Wavel

18、ength Tunable Source,Advantage - To improve S/N as the measurement defermines a max in grating reflected power and it dispenses with the need for optical filtering.,53,7.3.1.5 Recovery of Bragg Grating Wavelength Shift Using CCD Spectrometer,54,7.3.1.5 Recovery of Bragg Grating Wavelength Shift Usin

19、g CCD Spectrometer,(7.10),(7.11),Peak-Wavelength-Detection Algorithms. The ceutroid detection algorithm (CDA) use Eq(710) to get,Where are the intensity and center wavelength of the CCD pixel.,Where (ref. Fig.7.11(a) ),55,7.3.1.5 Recovery of Bragg Grating Wavelength Shift Using CCD Spectrometer,56,7

20、.3.1.6 Analysis of Bragg Grating Wavelength Shift Using Fourier Transform Spectroscopy,Fourier transform spectroscopy (FTS) - by Davis ,where,and system loss,is the imbalance length between fiber arms,is the reflected wavelength from FBG,is the interference fringe visibility,is a bias phase offset o

21、f the MZI resetting from slowly varying as the tamp rises , strain on the grating is progressively released.,85,7.4.2 Extrinsic Temperature Compensation,2. Packaging the FBG in a liquid crystalline polymer has resulted in an improvement in temp stability by a factor of 10. 3. A cantilever is used wi

22、th 2 FBG mounted on opposite surface, with one grating stretched while the other is compressed. Here ,the difference in the Bragg grating wavelengths is temp. independent because both Bragg gratings have the same temp sensitivity.,86,7.4.3 Intrinsic Temperature Compensation,7.4.3.1 Reference Grating

23、 7.4.3.2 Tapered Grating 7.4.3.3 Dual-Wavelength Superimposed Gratings 7.4.3.4 Multiple Bragg Grating Orders 7.4.3.5 Simultaneous Measurement of Temperature and Strain Using PANDA 7.4.3.6 Gratings in Dissimilar Diameter Fibers,87,7.4.3 Intrinsic Temperature Compensation,7.4.3.7 Hybrid Bragg Grating/

24、Long Period Grating 7.4.3.8 Superimposed Gratings and Polarization-Rocking Filters 7.4.3.9 Bragg Gratings and Brillouin Scattering 7.4.3.10 Combined Grating and In-Line/Extrinsic Fiber Etalon Sensor Methods 7.4.3.11 Bragg Grating Strain Rosettes 7.4.3.12 Dual-Core Fiber Bragg Gratings,88,7.4.3 Intri

25、nsic Temperature Compensation,(7.26),89,7.5 Polarization Stability of Interrogation Schemes,Ecke et al. have found that laterally compressing or bending a fiber lead produces dynamical changes due to polarization-mode conversion, resulting in severe noise and systematic errors. Fig.7.21 shows that f

26、or a simple broadband source and spectrometer-CCD arrangement, the passive Lyon depolarizer gives a dramatic improvement to the system stability, reducing wavelength errors to less then 1pm.,90,7.5 Polarization Stability of Interrogation Schemes,91,7.6 Multiplexing Techniques,7.6.1 Wavelength Divisi

27、on Multiplexing (WDM) 7.6.2 Time Division Multiplexing (TDM) 7.6.3 Spatial Division Multiplexing (SDM) 7.6.4 Combined SDM/WDM/TDM,92,7.6 Multiplexing Techniques,By sharing the source and processing electronics. - the cost per sensor is reduced. - reduces the overall system weight while enhancing dur

28、ability. For FOS - SDM (spatial Division Multiplexing ) - TDM - FDM - WDM - CDM (coherence domain multiplexing),93,7.6 Multiplexing Techniques,Limited to 10 FOS ,due to - speed - crosstalk - S/N - bandwidth,94,7.6.1 Wavelength Division Multiplexing (WDM),7.6.1.1 Parallel and Serial WDM Topologies 7.

29、6.1.2 WDM Schemes with Tunable Filters 7.6.1.3 Combined WDM and Interferometric Detection,95,7.6.1.1 Parallel and Serial WDM Topologies,- WDM encodes each BGS with a unique slice of the available source spectrum , which defines the sensors operating rouge and is also associated with a specific spati

30、al location along the optical fiber. Advantage- - The physical spacing between individual gratings may be as short as desired and the need for high-speed electrical signal processing, as is often required in TDM, is removed.,96,7.6.1.1 Parallel and Serial WDM Topologies,97,7.6.1.1 Parallel and Seria

31、l WDM Topologies,98,7.6.2 Time Division Multiplexing (TDM),7.6.2.1 Combined TDM and Interferometric Detection 7.6.2.2 TDM and WDM,99,7.6.2 Time Division Multiplexing (TDM),100,7.6.3 Spatial Division Multiplexing (SDM),7.6.3.1 SDM and WDM 7.6.3.2 SDM and TDM,101,7.6.3 Combined SDM/WDM/TDM,- The seria

32、l multiplexing schemes of WDM & TDM and combination make efficient use of the source power.,102,7.6.4 Combined SDM/WDM/TDM,103,7.7 Sensors Based on Chirped Bragg Gratings,7.7.1 Broadband Chirped Grating Sensor 7.7.2 Tapered Chirped Grating Sensor 7.7.3 Asymmetrically Chirped Grating Sensor 7.7.4 Int

33、ragrating Sensing,104,7.7.1 Broadband Chirped Grating Sensor,105,7.7.1,(7.27),106,7.7.2 Tapered Chirped Grating Sensor,107,108,7.7.3 Asymmetrically Chirped Grating Sensor,109,7.7.3 Asymmetrically Chirped Grating Sensor,110,7.7.4 Intragrating Sensing,7.7.4.1 Intensity Reflection Spectrum Analysis 7.7

34、.4.2 Intragrating Sensing Through Phase Measurement 7.7.4.3 Combined Reflection Spectrum and Phase Measurement 7.7.4.4 Low Coherence Reflectivity Measurement,111,7.7.4.1 Intensity Reflection Spectrum Analysis,(7.28),(7.29),112,7.7.4.1 Intensity Reflection Spectrum Analysis,113,7.7.4.2 Intragrating S

35、ensing Through Phase Measurement,(7.30),(7.31),114,7.7.4.2 Intragrating Sensing Through Phase Measurement,115,7.7.4.3 Combined Reflection Spectrum and Phase Measurement,116,7.7.4.3 Combined Reflection Spectrum and Phase Measurement,117,7.7.4.4 Low Coherence Reflectivity Measurement,118,7.7.4.4 Low C

36、oherence Reflectivity Measurement,119,7.8 Distinguishing Bragg Grating Strain Effects,(7.32),(7.33),The reflection distribution from a grating experiencing a general strain state is given by,With are given by,Where are the time averaged scalar magnitudes of the square of the electric fields in x and

37、 y direction.,120,7.8 Distinguishing Bragg Grating Strain Effects,(7.34a),(7.34b),For a grating experiencing the most general thermo- mechanical strain conditions, the change in the reflection wavelength is given by,Where the component of the light with its polarization Vector in x and y direction.,

38、121,7.8 Distinguishing Bragg Grating Strain Effects,122,7.9 Bragg Grating Fiber Laser Sensors,7.9.1 Single and Multipoint Bragg Grating Laser Sensors 7.9.2 Ultra-High Resolution Bragg Grating Laser Sensors Demodulation,123,7.9 Bragg Grating Fiber Laser Sensors,FBG may also be used as narrowband refl

39、ector for forming in-fiber laser cavities. The basic FBG laser sensor employs 2 Bragg grating of matched wavelength to create an in-fiber cavity or one grating combined with a broadband reflector with Er-doped fiber as the usual gain medium. ref.Fig.7.32,124,7.9.1 Single and Multipoint Bragg Grating

40、 Laser Sensors,125,7.9.1 Single and Multipoint Bragg Grating Laser Sensors,126,7.9.1 Single and Multipoint Bragg Grating Laser Sensors,(7.35),127,7.9.1 Single and Multipoint Bragg Grating Laser Sensors,128,7.9.2 Ultra-High Resolution Bragg Grating Laser Sensors Demodulation,(7.36),129,7.9.2 Ultra-Hi

41、gh Resolution Bragg Grating Laser Sensors Demodulation,130,7.9.2 Ultra-High Resolution Bragg Grating Laser Sensors Demodulation,131,7.10 Bragg Gratings as Interferometric Sensors and Reflective Markers,7.10.1 Reflectometric Sensing Arrays Using Bragg Reflectors 7.10.2 Nested Fiber Interferometers Us

42、ing Bragg Reflectors 7.10.3 Bragg Grating-Based Fabry-Perot Sensors 7.10.4 Collocated Fabry-Perot Cavities with Wavelength Addressable Cavity Lengths,132,7.10.1 Reflectometric Sensing Arrays Using Bragg Reflectors,133,7.10.1 Reflectometric Sensing Arrays Using Bragg Reflectors,134,7.10.2 Nested Fibe

43、r Interferometers Using Bragg Reflectors,(7.37),(7.38),Interrogation of the,The phase at the jth,135,7.10.2 Nested Fiber Interferometers Using Bragg Reflectors,136,7.10.3 Bragg Grating-Based Fabry-Perot Sensors,137,7.10.4 Collocated Fabry-Perot Cavities with Wavelength Addressable Cavity Lengths,138

44、,7.10.4 Collocated Fabry-Perot Cavities with Wavelength Addressable Cavity Lengths,139,7.11 Other Bragg Sensor Types,New specially modified or tailored gratings - - phase shift devices - multimode gratings - superstructure gratings,140,7.12 Applications of Bragg Grating Sensors,7.12.1 Introduction t

45、o Aerospace Applications 7.12.2 Bragg Grating Sensors in Marine Applications 7.12.3 Applications to Civil Engineering Structural Monitoring 7.12.4 Bragg Grating for Medical Applications,141,7.12 Applications of Bragg Grating Sensors,7.12.5 Bragg Sensors Within the Nuclear Power Industry 7.12.6 Appli

46、cations to Power Transmission Lines 7.12.7 Other Applications,142,7.12.1 Introduction to Aerospace Applications,- Monitoring Reinforced carbon Fiber Composite - Adaptive Structure - Application to Gas Turbine Engines,143,7.12.1 Introduction to Aerospace Applications,144,7.12.1 Introduction to Aerospace Applications,145,7.12.1 Introduction to Aerospace Applications,146,7.12.1 Introduction to Aerospace Applications,147,7.12.1 Introduction to Aerospace Applications,148,7.12.1.2 Space Vehicular Ultization,149,7.12.