02whole.pdf - INTRAGRATING SENSING USING CHIRPED OPTICAL FIBRE BRAGG GRATINGS A thesis submitted by Anbhawa Nand MSc BSc PGDip(PH GCTT GCEd(USP for the

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Unformatted text preview: INTRAGRATING SENSING USING CHIRPED OPTICAL FIBRE BRAGG GRATINGS A thesis submitted by Anbhawa Nand MSc, BSc, PGDip(PH), GCTT, GCEd (USP) for the degree of DOCTOR OF PHILOSOPHY Optical Technology Research Laboratory School of Electrical Engineering Faculty of Health, Engineering and Science Victoria University Australia 2007 To my wife Sarita and daughter Ashneeta ii Declaration “I, Anbhawa Nand, declare that the PhD thesis entitled, CHIRPED FIBRE BRAGG INTRAGRATING POSITION AND STRAIN MEASUREMENT SENSOR is no more than 100,000 words in length, exclusive of tables, figures, appendices, references and footnotes. The thesis contains no material that has been submitted previously, in whole or in part, for the award of any other academic degree or diploma. Except where otherwise indicated, this thesis is my own work”. ………………. Anbhawa Nand dated the 3rd day of April, 2007 iii Acknowledgements I am indebted to many individuals who have provided assistance and support during the period of this research. Foremost, I would like to thank my supervisors, Associate Professor Stephen Collins and Professor Greg Baxter for providing the opportunity to undertake this research and their constant guidance, useful comments, encouragement and discussions in all aspects of this research. I am also grateful to Dr Dan Kitcher for his unconditional support, helpful discussions and assistance in modelling of intragrating sensors during the course of this research. A special thank to Dr Scott Wade for his useful comments and help in grating fabrication. I would also like to thank my fellow post-graduate students, technical and administrative staff of the School of Electrical Engineering for their assistance and friendship. I also acknowledge the Victoria University Postgraduate Scholarship scheme for providing the scholarship for this research, the Australian Research Council for providing the financial support for the purchase of relevant equipment for this project and the use of Swinburne-Monash-DSTO grating fabrication system, Monash University’s thermal camera and Finite Element Method (FEM) software. Finally, I would like to express my deep appreciation to my wife Sarita and daughter Ashneeta for their unconditional support, patience and encouragement throughout the duration of this thesis. iv Abstract This thesis describes a study of intragrating position (localised heat source) and strain measurement sensor systems. The design, development and performance of intensity (power) reflection spectrum based intragrating sensing systems employing chirped fibre Bragg gratings (CFBGs) are investigated. Intragrating sensing is the process of obtaining a continuous profile of a measurand over the grating length from either the amplitude or phase, or both the amplitude and phase of its measured reflection response. Techniques for intragrating sensing employing conventional fibre Bragg gratings have been reviewed and analysed in order to design an intragrating sensor which overcomes the problems associated with the existing sensor systems. The inability to determine the position of the disturbance, direction of the strain gradient and the broadening of the spectra together with the reduction in reflectance complicates the grating inverse reconstruction technique to recover the disturbance profile and is also computationally expensive. Thus the selection of CFBGs is desirable because each Bragg wavelength in the broadband reflection spectrum from a CFBG corresponds to particular local position along the grating. Thus a change in the reflection intensity spectrum, due to a disturbance, at a particular wavelength can be quantified to a local position in the grating and hence the spatial distribution of the measurand can be evaluated. The thesis consists of four major sections: fabrication of CFBGs, sensor calibration, intragrating position and strain measurements. Techniques for the production and controlling the spectral response of CFBGs suitable for intragrating sensing application were investigated using two fabrication techniques; the prism interferometer and the scanning phase mask system. The modelling and experimental results showed that fabrication constraints exist for designing a suitable sensor for intragrating measurements with the prism technique, and thus most of the gratings used for this research were fabricated with the scanning phase mask system. The analysis of results for strain/temperature characterisation, showed that CFBGs fabricated in hydrogenated standard telecommunications fibre have a temperature coefficient approximately 20% higher than standard FBGs in the 1550 nm region with v identical strain coefficients. Thus a simple technique for strain-independent temperature measurement is proposed and demonstrated using a sensor head with a combination of a standard FBG and a CFBG. This is an additional finding of the research. The intragrating position measurements within a CFBG sensor were investigated as a function of grating strength. Five linear CFBGs of chirp rate 20 nm/cm, approximate length of 15 mm with varying levels of reflectance (6-53%), fabricated with the scanning phase mask technique were employed for the determination of the centre position of a localised heat source within a CFBG. An iterative approach in conjunction with a fast Fourier transform (FFT) was implemented for solving the inverse problem of obtaining the nonuniform temperature distribution from the measured intensity reflection spectrum. The extracted temperature distribution was characterised by the three parameters of amplitude, width and the centre position. The precision of the inferred centre position of the localised heat source was investigated as a function of grating strength. It was demonstrated that the centre position root mean square (rms) error generally improves with grating strength up to approximately 30% peak reflectance and then decreases with further increase in grating strength for both the high and moderate temperature amplitudes which were investigated. A position rms error below 0.03 mm and a repeatability rms error of 0.005 mm were obtained for the centre position measurements with the CFBG intragrating sensor for grating strengths in the range 20-30%. The position rms error reported is the best to date for CFBG based intragrating sensing systems. The intragrating strain measurements were investigated using three embedded CFBGs sensors to determine nonuniform strain distribution near a stress concentration within a notched aluminium specimen subjected to an axial tensile force. The strain distribution was determined from the analysis of the intensity reflection spectrum through the use of an integration method which did not require an initial strain distribution hypothesis. The best case performance was obtained with the sensor having a reflectance of around 23%. The shape of the recovered strain distributions were in reasonable agreement with that predicted by the finite element method (FEM) modelling however some discrepancies were noted. These are most likely due to incomplete transfer of surface strain within the specimen through 3-layers (host/adhesive/fibre) to the fibre core. A repeatability rms error of less than 4 με in the extracted strain profiles was obtained. vi Table of Contents Declaration ........................................................................................................ iii Acknowledgements ........................................................................................................ iv Abstract ......................................................................................................... v Table of Contents ....................................................................................................... vii List of Figures ........................................................................................................ xi List of Tables ....................................................................................................... xv CHAPTER 1 INTRODUCTION ......................................................................... 1 1.1 1.2 1.3 1.4 1.5 1.6 Background........................................................................................................ 1 Fibre Bragg Gratings ......................................................................................... 2 Advantages of Fibre Bragg Gratings ................................................................. 3 Chirped Fibre Bragg Gratings ........................................................................... 4 Fibre Bragg Intragrating Sensing ...................................................................... 6 Scope of the Thesis and Research Objectives ................................................... 8 CHAPTER 2 2.1 2.2 Introduction ..................................................................................................... 11 Distributed Optical Fibre based Sensing ......................................................... 12 2.2.1 2.2.2 2.2.3 2.2.4 2.3 Optical Time Domain Reflectometry (OTDR) technique ..........................................12 Rayleigh scattering.....................................................................................................14 Raman Scattering .......................................................................................................14 Brillouin scattering.....................................................................................................16 Distributed Fibre Bragg Grating based Sensors .............................................. 17 2.3.1 2.3.2 2.3.3 2.4 BACKGROUND ON DISTRIBUTED AND INTRAGRATING SENSING..................................................... 11 Wavelength-Division Multiplexing (WDM) ..............................................................18 Wavelength-Division Multiplexing (WDM) Interrogation Techniques .....................20 Quasi-distributed sensing using WDM techniques ....................................................22 Theory and Application of Intragrating Sensing ............................................. 25 2.4.1 2.4.2 2.4.3 2.4.4 Intensity-Spectrum Based (Reflected Power Spectrum) Intragrating Sensing ...........27 Phase-Spectrum Based Intragrating Sensing..............................................................34 Combined Intensity and Phase Measurements ...........................................................36 Low Coherence Reflectometry...................................................................................39 2.5 Nonuniform Strain Sensing using Embedded FBGs ....................................... 40 2.6 Chapter Summary ............................................................................................ 42 vii CHAPTER 3 3.1 3.2 Introduction ..................................................................................................... 45 Photosensitivity in Optical Fibres ................................................................... 46 3.2.1 3.2.2 3.3 The Prism Interferometer Technique (Victoria University) .......................................59 The Phase-mask based Scanning FBG Fabrication Technique (Swinburne University) .............................................................................................65 Fibre Bragg Grating Simulation Techniques................................................... 67 3.6.1 3.6.2 3.6.3 3.6.4 3.7 Interferometric (Holographic) Technique ..................................................................55 Chirped Phase Mask Technique .................................................................................57 Chirped Grating Fabrication Techniques Investigated in this Project............. 59 3.5.1 3.5.2 3.6 Interferometric (Holographic) Fabrication Technique ...............................................49 Phase-Mask Technique ..............................................................................................51 Point-by-Point Technique...........................................................................................53 Chirped Fibre Bragg Grating Fabrication Techniques .................................... 54 3.4.1 3.4.2 3.5 Photosensitivity Models .............................................................................................46 Photosensitisation Techniques ...................................................................................48 Fibre Bragg Grating Fabrication Techniques .................................................. 49 3.3.1 3.3.2 3.3.3 3.4 FABRICATION AND MODELLING OF CHIRPED FIBRE BRAGG GRATING SPECTRA ................ 45 Coupled-Mode Theory for two Mode Coupling in Bragg Gratings ...........................67 Transfer Matrix Approximation Method (TMM).......................................................70 FFT Grating Design Algorithm Operation .................................................................71 Modelling CFBG spectra using the FFT algorithm....................................................74 Chapter Summary ............................................................................................ 76 CHAPTER 4 TEMPERATURE AND STRAIN CHARACTERISATION OF CHIRPED FIBRE BRAGG GRATINGS ........................... 78 4.1 Introduction ..................................................................................................... 78 4.2 Theoretical Background on Temperature and Strain Characteristics of a Fibre Bragg Grating.................................................................................. 79 4.3 Temperature Calibration of Chirped Fibre Bragg Gratings ............................ 81 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.3.7 4.3.8 4.4 Annealing of the gratings ...........................................................................................81 Normalisation of reflectance spectra ..........................................................................82 Temperature calibration technique .............................................................................83 CFBG spectra analysis ...............................................................................................84 Temperature calibration of standard FBGs (Prism technique) ...................................85 Temperature calibration of standard FBGs (Phase mask technique)..........................87 Temperature calibration of CFBGs (Phase mask technique)......................................89 Summary of temperature sensitivities of the gratings ................................................91 Strain Calibration of Chirped Fibre Bragg Gratings ....................................... 93 4.4.1 4.4.2 4.4.3 4.4.4 Strain calibration technique........................................................................................93 Strain calibration of standard FBGs (Prism technique)..............................................94 Strain calibration of standard FBGs (Phase mask technique).....................................96 Strain calibration of CFBGs (Phase mask technique) ................................................97 viii 4.5 Strain-Independent Temperature Measurement ............................................ 100 4.5.1 4.5.2 4.5.3 4.6 Introduction and background....................................................................................100 Theory ......................................................................................................................101 Analysis....................................................................................................................102 Chapter Summary .......................................................................................... 105 CHAPTER 5 POSITION MEASUREMENT OF A LOCALISED HEAT SOURCE USING INTENSITY REFLECTION SPECTRUM BASED INTRAGRATING SENSING............. 106 5.1 Chapter Overview.......................................................................................... 106 5.2 Introduction ................................................................................................... 107 5.3. Experimental Techniques .............................................................................. 108 5.3.1 5.3.2 5.3.3 Grating fabrication ...................................................................................................108 Experimental arrangement .......................................................................................109 Temperature measurements using the IR camera.....................................................110 5.4. Principle of Sensor Operation ....................................................................... 112 5.4.1 5.4.2 Effect of localised temperature on local Bragg wavelength and reflectance of chirped gratings....................................................................................................112 Modelling the temperature disturbance using FFT grating simulation technique ...114 5.5. Data Analysis Techniques ............................................................................. 117 5.5.1 5.5.2 5.6. Results 5.6.1 5.6.2 5.6.3 Determination of the temperature profiles - Iterative hypothesis-testing method ...117 Grating Reconstruction Inversion Problem – Justification of the FFT Grating Simulation Technique ..............................................................................................121 ..................................................................................................... 123 Extraction of temperature profile using the FFT Technique ....................................123 Position Measurements ............................................................................................125 Repeatability of Position Measurements ..................................................................127 5.7. Performance of Sensors ................................................................................. 129 5.7.1 5.7.2 5.7.3 Summary of Sensor Performance.............................................................................129 Position Measurement Performance Testing............................................................130 Repeatability of Position Measurement Performance Testing .................................131 5.8. Discussion ..................................................................................................... 132 5.9. Chapter Conclusion ....................................................................................... 133 ix CHAPTER 6 LOCALISED STRAIN MEASUREMENTS USING THE INTENSITY REFLECTION SPECTRUM BASED INTRAGRATING SENSING................................................... 135 6.1 Chapter Overview.......................................................................................... 135 6.2 Introduction ................................................................................................... 136 6.3. Experimental Techniques .............................................................................. 137 6.3.1 6.3.2 Grating fabrication ...................................................................................................137 Experimental system arrangement ...........................................................................138 6.4. Principle of Sensor Operation ....................................................................... 141 6.4.1 Effect of localised (non-uniform) strain on local Bragg wavelength and reflectance of CFBGs ........................................................................................141 6.5. Data Analysis Techniques ............................................................................. 143 6.5.1 6.5.2 6.6. Results 6.6.1 6.6.2 6.6.3 Determination of the non-uniform strain profiles using the ISB intragrating sensing employing the integration method...............................................................143 FEM modelling of strain distribution along the aluminium...
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