1. Il-Bum Kwon Korea Research Institute of Standards and Science Technologies on Fiber Optic Distributed Sensors 2008 The First Asia-Pacific Student Summer School on Smart Structure Technology

2. 1 OEMagazine 2005 NOV/DEC

3. 2 CONTENTS I. Brief Introduction of Fiber Optic Sensors II.Fiber Optic BOTDA Sensor III.Conclusion

4. FOS System Electrical to Optical Conversion Optical to Electrical Conversion Signal Processing Optical Configuration Transfer Function Demodulation Source Detector External Field

5. FOS Qualities Fiber optic sensors will not replace electrical sensor technology. Electrical sensors are mature, and too simple to use. FOS is useful in niche applications. Advantages: nEmbeddable nLong Gage Lengths (If Needed) nChemically Inert nSerial Multiplexibility (WDM) nNo Ground Loops Advantages: nCompatibility With Telecom nVery Small Gage Lengths (If Needed) nNo Sparking nCan Have Very Long Stand- off Distances nEMI no EMP

6. Basic Optics  1 = A 1 (x,y)exp[i(  t -  L 1 )]  2 = A 2 (x,y)exp[i(  t -  L 2 )] Electric Field (  ) 11 22 Spatial Coordinate (z) Frequency (2  c/  Propagation Constant (2  n/  Intensity Time  I = (  1 +  2 ) (  1 +  2 ) * = A 1 2 + A 2 2 + 2A 1 A 2 cos(  L) }

7. Basic Optics I = A + B cos(  )  = Relative Phase nL = Optical Path Length  nL = Optical Path Length Difference (OPD)  = (2  / )(n 2 L 2 - n 1 L 1 )

8. Different Sensors n Mach-Zehnder Sensors n Michelson Sensors n Polarimetric Sensors n Dual-Mode Sensors n Intrinsic Fabry-Perot Sensors n Bragg Grating Sensors n Extrinsic Fabry-Perot Sensors n In-line Fiber Etalon Sensors n Long Period Gratings

9. Mach-Zehnder Sensors I = A+Bcos{2  (nL 1 - nL 2 )/ } n Fibers Sensitive Between Couplers n Lead-Sensitivity is an Issue n Long Gage Lengths With Small OPDs (nL 1 - nL 2 ) Photo- diode Photo- diode Laser2x2 Coupler Keep Isolated Strain, etc. L

10. Michelson Sensors I = A+Bcos{4  (nL 1 - nL 2 )/ } n Fibers Sensitive Between Couplers n Lead-Sensitivity is an Issue n Long Gage Lengths With Small OPDs 2(nL 1 - nL 2 ) Strain, etc. Photo- diode Laser Mirror 2x2 Coupler Keep Isolated L

11. Polarimetric Sensors n Sensor Operates via Coherent Interference Two Orthogonal Polarization Components. n Use HiBi Fiber y y Polarized Light In Phase Out of Phase x Cladding Core x Polarizer Core Elliptical Clad Stress- Induced HiBi Fiber SAP y x

12. Dual-Mode Sensors n Use Elliptical Core (Geometry-induced HiBi) Fiber n Select Source Wavelength to Support Two Propagation Modes n Sensor Operates Via Coherent Interference Between the Supported Modes Cladding Core In Phase I = A+Bcos{(  11 -  12 )z} E-Core HiBi Fiber Core y x In Phase

13. Intrinsic Fabry-Perot (IFP) Sensors Photo- diode Laser 2x2 Coupler L I = A+Bcos{4  nL/ } n Sensor Operates via Coherent Interference Between Reflected Waves n First Real Localized Sensor to be Designed n Optical Phase Sensitive to Strain State and Temperature Cladding Internal Mirror L Thin Film Mirror

14. Extrinsic Fabry-Perot (EFP) Sensors Photo- diode Laser 2x2 Coupler L I = A+Bcos{4  nL/ } n Sensor Operates via Coherent Interference Between Reflected Waves n Localized Sensor n Optical Phase Sensitive to Axial Strain n Small Thermal Sensitivity LgLg Hollow Core Fiber Air Gap Cladding

15. In-Line Fiber Etalon (ILFE) Sensors Photo- diode Laser 2x2 Coupler L I = A+Bcos{4  nL/ } n Sensor Operates via Coherent Interference Between Reflected Waves n Localized Sensor n Optical Phase Sensitive to Axial Strain n Small Thermal Sensitivity L Hollow Core Fiber Air Gap Cladding

16. Bragg Grating Sensors Core Cladding P  P P n Sensor Operates via Mixing of Forward and Backward Propagating Waves n Localized Sensor n High Degree of Multiplexibility n Bragg Wavelength Sensitive to Strain State and Temperature Detector Broad Band Source 2x2 Coupler L Bragg Wavelength

17. Sensor Comparison Sensor Type Mach-Zehnder Michelson Polarimetric Dual-Mode IFP EFP ILFE Bragg Grating LPG Localized? No Yes Thermal Sensitivity High Low High Strain Sensitivity Strain State Axial Strain Strain State Strain Sate Ease of Multiplexing Difficult Moderate Easy Difficult n EFP and ILFE Sensors are the most attractive FOS analog to Resistance Strain Gages. n The Bragg grating is the undisputed multiplexing champion. n LPGs are new and are the subject of intense interest.

18. 17 Contents I. Backgrounds and Objective II. BOTDA Research and Products III. BOTDA Construction IV. BOTDA Signal V. Temperature measurement VI. Conclusion Fiber Optic Brillouin Optical Time Domain Analysis Sensor

19. 18 Backgrounds * Real-time health monitoring  Assuring the structural safety  More efficient than the previous electronic sensors * Advantages of Fiber optic sensor  easy to be embedded in composites  very sensitive  give some distributed information from one line  electro- magnetic immunity * Fiber optic BOTDA sensor  give long-range distributed temperature information along an optical fiber

20. 19 Objectives - Measuring distance : > 40 km - Spacial resolution : 10 m - Temperature resolution : < 1 °C 1. Development of fiber optic BOTDA sensor 2. Feasibility study of continuous distributed measurement - Temperature - Strain

21. 20 BOTDA Research and Products Japan ANDO (NTT) - Type : spontaneous Brillouin backscattering - Measuring range : 60 km @ 100m res. - Distance resolution : 2 m (min.) UK Univ. of Southampton - Type : spontaneous Brillouin backscattering - Measuring range : 40 km - Distance resolution : 2 - 3 m UK Kent Univ. - Type : stimulated Brillouin backscattering - Measuring range : 51 km - Distance resolution : 100 m AQ8602 LASBI9800 Switzerland Smartech Co. - Type : stimulated Brillouin backscattering - Measuring range : > 10 km - Distance resolution : < 1 m - Source power : 20 mW - Amplifier : Gain (30 dB), Max. power (15 dBm)

22. 21 Principle of BOTDA Temperature Index change Pulsed pumping lightCW probe light PD Test Fiber Index change heating

23. 22 Brillouin frequency & temperature Brillouin frequency Temperature Coefficient of Temperature (1MHz/°C)

24. 23 Building with an optical fiber Fiber length = 468 m Fiber length = 255 m Fiber length = 542 m Total Fiber length = 1265 m

25. 24 Brillouin gain spectrum from building At nightAt noon

26. 25 Software of BOTDA (DAQ Part) Pulse/Probe Setup BOTDA Spectrum Disp. Bias Optimization Temp. Monitoring Mode Rs232 Setup

27. 26 Software of BOTDA for Temperature Monitoring Disp. Of Temp. Distribution Setup for Monitoring Direction Setup for Monitoring Period Max/Min/Avg Temp. Analysis During Measurement Max/Min/Avg Temp. Analysis

28. 27 Experiment for a Building Temperature distribution measurement on a building South/West/North wall Total length : 1400 m 2003. 3. 25. 15:00 (north) Min: 15.1  C Max: 20.4  C Aver: 15.1  C 2003. 3. 25. 15:00 (south) Min: 15.1  C Max: 27.4  C Aver: 19.3  C Min: 13.5  C Max: 23.6  C Aver: 16.8  C 2003. 3. 25. 15:00 (west) South wall North wall West wall

29. 28 Temperature distribution (a) 11:00, Avr. T.: 16.8 ℃ (b) 15:00, Avr. T.: 19.3 ℃ (c) 19:00, Avr. T.: 11.0 ℃ (d) 23:00, Avr. T.: 5.2 ℃ (e) 03:00, Avr. T.: 4.7 ℃ (f) 07:00, Avr. T.: 4.0 ℃ 2003. 3. 25. ~ 3. 26., South wall

30. 29 Temperature Distribution (a) 11:00, Avr. T.: 11.3 ℃ (b) 15:00, Avr. T.: 15.1 ℃ (c) 19:00, Avr. T: 10.2 ℃ (d) 23:00, Avr. T.: 4.9 ℃ (e) 03:00, Avr. T.: 4.7 ℃ (f) 07:00, Avr. T.: 3.9 ℃ 2003. 3. 25. ~ 3. 26, North wall

31. 30 L Load, P Optical Fiber Sensors w h L/2 Strain measurement of a beam

32. 31 x = 0 but  B  0 b. x Spatial Resolution  x B 0 x a. L Measurement Range Actual Measurement Part x =  x B / 2 Compensation of spatial error

33. 32 Measured Strain

34. 33 Strain at low resolution

35. 34 Strain at mid-resolution

36. 35 Strain at high resolution

37. 36 Tension Section Compression Section a. b.c.

38. 37 Strain along the sensing fiber

39. 38 Deflection

40. 39 Conclusion Basic fiber optic sensors were reviewed shortly. Fiber optic BOTDA sensor was developed to monitor Temperature or strain. Future tasks of FOS for structural health monitoring are the installation techniques of sensors, the effective sensor probes and the cost-effective sensor systems. Thanks for your attention. Have a good time for the summer school in South Korea