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Strain Sensitivity in Fiber Optic Sensors

Strain Sensitivity in Fiber Optic Sensors. PHANEENDRA MEDIDA. Briefly …. Optical Fiber Sensor Strain Concepts Interferometers Fabry-Perot interferometer Bragg grating fiber optic sensor Strain Sensitivity Calculations. Optical Fiber Sensor. Definition of Optical Fiber Sensor.

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Strain Sensitivity in Fiber Optic Sensors

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  1. Strain Sensitivity in Fiber Optic Sensors PHANEENDRA MEDIDA

  2. Briefly… • Optical Fiber Sensor • Strain Concepts • Interferometers • Fabry-Perot interferometer • Bragg grating fiber optic sensor • Strain Sensitivity Calculations

  3. Optical Fiber Sensor • Definition of Optical Fiber Sensor Block Diagram of Optical Fiber Sensor System Optical fibers & Actuators Optical Rx Optical Tx Data Acquisition And health Assessment Control system

  4. Optical Fiber Sensor (contd) • Large bandwidth and fast response. • Immunity to, and no emission of EMI. • Chemical and environmental ruggedness. • Small size and weight. • Cost effective.

  5. Strain Concepts • Strain is the relative change in shape or size of the body due to applied force or pressure. Hookes’s law = the stiffness matrix = the thermal expansion coefficient = the stress = F/A = the temperature change

  6. Strain Concepts (contd) • Far-field strains = displacement = strain= =shear strain Total strain inside the sensor is, is the residual strain and is the applied strain Far field strain are those strains which are present in the absence of sensor.

  7. Strain Concepts (contd) Optical fiber sensor, representing strain directions X2 X1 X3 The far field strain components are given by

  8. Interferometers • A fiber optic, interferometric strain gauge is based on the change in the optical path length caused by straining the fiber. • These strains cause a phase delay. • Strain optic effect – modulation of fiber refractive index. • Mode dispersion effect-due to change in the diameter of the fiber.

  9. Semi reflective Fusion splice Mirrored End Gauge Length (L) Semi reflective Fusion splice He-Ne Laser 3dB Coupler Mirrored End Detector FABRY-PEROT INTERFEROMETER • Semi-reflective fiber splices. • films are sputtered on the fiber end faces

  10. R1 R2 Pi Pt Pr L

  11. FABRY-PEROT INTERFEROMETER (contd) • Interference occurs at the half silvered separating the sensing portion of the fiber. Phase (Degrees) Strain Sensing Mechanism • A light radiation gets reflected from the semi-reflective splice. • Second radiation gets reflected from the mirror and then travels back to the fiber. • Two radiations overlap to give interference pattern.

  12. FABRY-PEROT INTERFEROMETER (contd) • Due to applied pressure, the phase changes with respect to the intensity, due to the change in the length of the gauge length.

  13. FABRY-PEROT INTERFEROMETER (contd) Relation between optical, geometrical properties and output change in length due to applied strain when light reflects back there will be two phase shifts, fast and slow varying terms

  14. FABRY-PEROT INTERFEROMETER (contd) Butter and Hocker Model S is the phase strain sensitivity is the effective strain-optic coefficient, For pure silica core and boron doped cladding the values of strain optic coefficient are =0.113 and =0.252 with n=1.458 and v=0.17

  15. Intracore Bragg Grating Optical Fiber Induced Grating Laser Beams BRAGG GRATING FIBER OPTIC SENSOR • periodic modulation of the core index • There is a strong back reflection at the Bragg wavelength, • Monitoring the wavelength of narrowband spectrum will help in determining the strain.

  16. Fiber Core Index Grating Reflected signal Signal OUT L L 1.46 Z Z1 I2 Back reflected Bragg signal Reflected Signals for 3 values of strain Bragg signal transmitted, missing signal

  17. BRAGG GRATING FIBER OPTIC SENSOR (contd) Sensing principle • When a strain is applied the reflected wavelength shifts and the shift is proportional to the amount of strain applied.

  18. BRAGG GRATING FIBER OPTIC SENSOR (contd) When stress is applied to the sensors, Taylor expansion of the Bragg’s relation Butter-Hocker model

  19. BRAGG GRATING FIBER OPTIC SENSOR (contd) Wavelength-strain sensitivity of the Bragg grating sensor, = is the index-weighted strain-optic coefficient

  20. Calculations Relation between Phase-strain sensitivity and refractive index for FP interferometer Strain Sensor k, is the free-space propagation constant /m n, is the refractive index P is the effective strain-optic coefficient,

  21. S_I is given in

  22. Relation between wavelength-strain sensitivity and refractive index of Bragg sensor S_B is given in pm

  23. Conclusion • Sensing mechanisms show that the strain is directly related to the phase change for the interferometric type. • Advantage of Bragg sensor is the Bragg’s wavelength is a linear function of the measurand. • Increasing change in the refractive index, the sensitivity increases for a Fabry-Perot sensor. • Decreases for a Bragg grating sensor • Selecting a strain sensor for particular range of sensitivity.

  24. References: 1.“Fiber Optic Sensors”, Eric Udd, John Wiley and Sons, Inc. 2.“An Introduction to Fiber Optic Systems”, john P. Powers, Aksen Associates Incorporation Publishers. 3.Single-Mode Optical Fiber Measurement: Characterization and Sensing”, Giovanni Cancellieri, Artech House, Inc. 4.“Selected Papers on Fiber Optic Sensors”, Reinhardt Willsch, Ralf Th. Kersten, SPIE Milestone Series. 5.“Strain and Temperature Measurement with Fiber Optic Sensor”, Regis J.Van Steenkiste, George S.Springer. 6.“Fiber Optic Fabry-Perot strain Gauge”, Tomas Valis, Dayle Hogg, and Raymond M.Measures, IEEE Photonics Technology Letters, Vol. 2, No. 3, 227-228, March 1990.

  25. 7.“Fiber Bragg grating temperature sensor with controllable sensitivity”, Jaehoon Jung, Hui Nam, Byoungho Lee, Jae Oh Byun, and Nam Seong Kim, APPLIED OPTICS , Vol. 38, No. 13 ,1 May 1999. 8.“Fiber Grating Sensors”, Alan D. Kersey, Michael A. Davis, Heather J. Patrick, Michel LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, and E. Joseph Friebele, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 15, NO. 8, AUGUST 1997. 9.“Fiber optic sensors in concrete structures: a review”, C I Merzbacher, A D Kersey and E J Friebele, Smart Mater. Struct. 5 (1996) 196–208. 10.“Optical fiber Fabry-Perot sensors for smart structures”, C E Lee, J J Alcoz, Y Yeh, W N Gibler, R A Atkins and H F Taylor, Smart Mater. Struct. 1 (1992) 123-127.

  26. Questions • What is Far field Strain? • State Hookes’s Law. • What are smart structures? • What is the principle of interferometry? • What is Bragg relation?

  27. THANK YOU pm72@drexel.edu

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