Center for Smart Structures & Materials - Northwestern University
Northwestern University
Fiber Optic Ultrasound Sensors for Smart Structures Applications Pavel Fomitchov John Dorighi Carmen Hernandez Sridhar Krishnaswamy Jan Achenbach
Northwestern University
Center for Quality Engineering
Northwestern University
OBJECTIVE: To develop, characterize and apply fiber optic ultrasound sensors for smart structures Fiber Optic Ultrasound Sensors
Application
Development Characterization
Center for Quality Engineering
smart skins
Northwestern University
Why use fiber-optic ultrasound sensors? Piezoelectric Transducers
Fiber Optic Sensors
• High temperature measurements • Temperature Limitations • Small size and light weight • Often bulky • Not easily embedded in Composites • Compatible with composite materials • Immune to EMI • Susceptible to EMI • Expensive • Inexpensive • New unproved technology • Proven reliable technology • Lower sensitivity • Higher sensitivity
Center for Quality Engineering
Cent er for Quality Engineering & Failure Northwestern Prev ention, University North west ern Univer
APPROACH • Conventional approach uses fiber breakage as indicator of damage and this requires dense distribution of fiber sensors since sensor must be in the vicinity of the damage. • Our approach uses ultrasonic energy to interrogate the structure for damage. The fiber sensors are simply used to detect the ultrasonic energy after it has interrogated the structure. • Advantages of our approach include: (a) no requirement for proximity of sensor to the damage area, allowing for relatively sparse distribution of fiber sensors ,and (b) better sensitivity to detect small flaws dictated by the wavelength of the ultrasound rather than the spacing of the fiber sensors. sensors detect ut
generated ultrasound Center for Quality Engineering
flaw
sensors in the shadow do not detect ultrasound
Northwestern University
Transduction mechanism How does ultrasound affect light propagating inside a fiber? ultrasound
optical fiber
• Impinging ultrasound causes strain in fiber • Strain in fiber leads affects optical transmission in fiber (via dimensional changes, refractive index changes...)
Center for Quality Engineering
Northwestern University
SENSOR DEVELOPMENT • fabricate the fiber sensors • build sensor stabilization control system Publication: • "Stabilization of an Embedded Fiber-Optic Fabry-Perot Sensor for Ultrasound Detection," J. F. Dorighi, Sridhar Krishnaswamy and J.D. Achenbach IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 42, No. 5, pp. 820-824. •P. Fomitchov, Sridhar Krishnaswamy, J.D. Achenbach, (1999), “Intrinsic and Extrinsic Sagnac Ultrasound Sensor,” to appear Optical Engineering.
Center for Quality Engineering
Northwestern University
SENSOR DEVELOPMENT:
Intrinsic Fiber Optic Fabry-Perot Ultrasound Sensor reference beam
sensing region
sensing beam partial mirrors
Pr = 2 R(1 − cosφ) Pi φ=4πnL λ
n: λ: L: R:
dφ
I
refractive index of cavity wavelength of light cavity length reflectivity of mirr ors
most sensitive ∆φ
least sensitive
Note: Sensor must be actively stabilized at most sensitive (steep part) of the response curve to detect ultrasound. Center for Quality Engineering
Northwestern University
SENSOR DEVELOPMENT:
Active Homodyne Stabilization of Sensor external cavity diode laser
optical isolator integrator
differential amplifier
epoxy plate 2x2 optical coupler
photodiode
index matching gel
reference voltage highpass filter oscilloscope
Center for Quality Engineering
fiber optic Fabry-Perot
Northwestern University
SENSOR DEVELOPMENT:
Detection of Ultrasound without stabilization
amplitude (volts)
0.006 0.004
0.004
0.002
0.002
0
0
-0.002
-0.002
-0.004
-0.004
-0.006
-0.006 0
.5
1.0
1.5
2.0
time (microsec)
with stabilization
0.006
2.5
0
.5
1.0
1.5
2.5
time (microsec)
Note: Sensor detects small amplitude ultrasonic signal only when sensor is stabilized about its most sensitive response point. Center for Quality Engineering
2.0
Northwestern University
Sagnac Ultrasound Sensor ls 2 PD Laser
l
structure 1
intrinsic probe mirrorized fiber-tip
• Common path --> noise insensitive • Easier to fabricate than Fabry-Perot • P. Fomitchov, Sridhar Krishnaswamy and J. D. Achenbach, (1997), "Compact phase-shifted Sagnac Interferometer for Ultrasound Detection," in Optics and Laser Technology, vol. 29, No. 6, pp.333-338. • P. Fomitchov, Sridhar Krishnaswamy and J. D. Achenbach, (1997), ”Extrinsic and Intrinsic Sagnac Interferometer for Ultrasound Detection," to appear in Optical Engineering.
Center for Quality Engineering
Northwestern University
Sagnac Ultrasound Sensor Bulk Wave Detection
Intrinsic probe PZT
Photodetector signal, mV
6 4 2 0 -2 -4 -6
Structure
Center for Quality Engineering
6
8
10 12 Time, µs
14
16
Northwestern University
Bragg-grating Ultrasound Sensors Incident Ultrasound
broadband incident reflected
Transmitted
Pitch Λ
Reflected Intensity
response curve λb
lock-point
Wavelength • local sensor • easy to fabricate • easy to multiplex several sensors in one fiber segment Center for Quality Engineering
Northwestern University
Sensor signal, mV
Bragg-grating Ultrasound Sensors 4 3 2 1 0 -1 -2 -3 -4
Time-domain signal
0
5
10 15 20 25 30 35 40 Time, µs
• we have obtained time-domain signals from a Bragg-grating subjected to an ultrasonic toneburst of 2.25MHz. • frequency response of Bragg-grating sensor still an issue Center for Quality Engineering
Northwestern University
SENSOR CHARACTERIZATION •what is the sensitivity of the sensor? •what is its frequency response? Publications: • “Response of an Embedded Fiber-Optic Ultrasound Sensor,” J.F. Dorighi, Sridhar Krishnaswamy and J.D. Achenbach Journal of the Acoustical Society of America, vol. 101, No. 1, pp. 257-263. • "Sensitivity of an Embedded Fiber-Optic Ultrasonic Sensor," J.F. Dorighi, Sridhar Krishnaswamy and J.D. Achenbach in Review of Progress in Quantitative Nondestructive Evaluation, ed. D.O. Thompson and D.E. Chimenti, vol. 16, Plenum Press, New York.
Center for Quality Engineering
Northwestern University
SENSOR CHARACTERIZATION: Frequency Response
Fiber Optic Ultrasound Sensors: Frequency Response x1 x3
elastodynamic analysis: calculate strain in fiber core due to impinging ultrasonic wave
(r,θ) x2
x1 k δ x2
strain-optic relations: host λ calculate the induced phase shift in the µρ fiber sensor due to strain in the fiber caused by the impinging ultrasonic wave 2 2
2
Center for Quality Engineering
sensor λ1 µ1 ρ1
Northwestern University
SENSOR CHARACTERIZATION: Frequency Response
Normally incident longitudinal wave fiber in water
1.5
fiber in epoxy
2.0
normalized phaseshift
phase shift along x2 static solution static solution phase shift along x3
1.5
1.0 1.0
0.5 0.5 phase shift along x2
phase shift along x3 0.0
0.0 0
1
2
3
4
5
6
frequency (MHz)
Center for Quality Engineering
7
8
0
1
2
3
4
5
6
frequency (MHz)
7
8
Northwestern University
SENSOR CHARACTERIZATION:Frequency Response
Obliquely incident longitudinal wave fiber in epoxy normalized phase shift
1.5
2o
1.5
incidence
4o incidence phase shift along x3
1.0
1.0 phase shift along x3 phase shift along x2
0.5
0.5
phase shift along x2 0.0
0.0 2
3
4
5
6
frequency (MHz) Center for Quality Engineering
7
8
2
3
4
5
6
7
frequency (MHz)
8
Northwestern University
SENSOR CHARACTERIZATION:Sensitivity
Sensitivity Full fringe
ultrasonic signal
1.4
0.04 0.03
1.2
amplitude (volts)
amplitude (volts)
1.3 1.1 1.0 0.9 0.8 0.7
Vo = 1.025 V 0.5
1
1.5 2 2.5 time (sec)
∂Vr
0.01
= 45 mV
0 -0.01 -0.02
0.6 0
0.02
3
3.5
4
Vo: quadrature offset voltage δVr: ultrasonic signal voltage Γ: fringe visibility Center for Quality Engineering
-0.03
0
1.0
2.0
3.0
4.0
time (microsec)
Phase-shift induced by ultrasound
∂φ =
∂Vr ΓVo
Northwestern University
Measured Sensitivity vs Theoretical Limit Shot Noise Limited Phase Shift
∂φ SNL =
Ultrasonic Phase Shift
1 hυB Γ ηRPi
∂ φ = 0.14 rad Smallest Measureable Phase Shift
∂ φ SNL = 2.2 x 10-7 rad/√Hz h: Planck’s constant ν: light frequency B: measurement bandwith
Measured Phase Shift
∂ φ = 2.0 x 10-6 rad/√Hz
η: quantum efficiency of detector R: mirror reflectivity Pi: total incident power into sensor
Center for Quality Engineering
Northwestern University
SENSOR CHARACTERIZATION: Sensitivity
Sensitivity in terms of Ultrasonic Amplitude 0.04
pzt transducer
amplitude (volts)
0.03 0.02 0.01 0 -0.01 -0.02 -0.03
Fizeau Interferometer (sensitive only to displacement)
0
1.0
2.0
3.0
4.0
time (microsec)
Ultrasonic Displacement δ = 11.6 nm
embedded FOFP sensor Smallest Measureable Displacement (sensitive to stress and displacement)
δ = 0.8 nm
Center for Quality Engineering
Northwestern University
SENSOR APPLICATION • detection of flaws • acoustic emission detection • cure monitoring of composites Publications: • "Laser Ultrasound System with Embedded Fiber-Optic Interferometric Sensor", J.F. Dorighi, Sridhar Krishnaswamy and J.D. Achenbach, SPIE Proceedings, vol. 2622. • "A fiber optic ultrasonic system to monitor the cure of epoxy," J.F. Dorighi, Sridhar Krishnasamy, and J.D. Achenbach, ASNT's Research in Nondestructive Evaluation (to appear).
Center for Quality Engineering
Northwestern University
SENSOR APPLICATIONS:
Flaw Detection Reflectivity
Specimen with a hole
epoxy plate
hole fsi-fofp
0
5
10
15 20
25
30 35
40
normalized amplitude (Vr/Vi)
0.08 0.07 0.06 0.05 0.04 0.03 0 pzt generator
scan
Center for Quality Engineering
10
20
30
distance (mm)
40
50
Northwestern University
SENSOR APPLICATIONS:
Simulated Acoustic Emission Event (Pencil Lead-Break Test) 0.02
amplitude (volts)
0 -0.02 -0.04 -0.06 -0.08 0 Center for Quality Engineering
50 100 time (microsec)
150
Northwestern University
FUTURE WORK
How about fiberized laser-generation of ultrasound? Embedded Fiber Generation
Surface Generation
Nd:Yag pulse
embedded fiber generator
glass slide
zinc coating
acoustic couplant absorbing layer
embedded FOFP
0.0016
0.0025
0.0014
amplitude (volts)
0.002
amplitude (volts)
Nd:Yag pulse
0.0015 0.001
0.0005 0
-0.0005
0.0012 0.001 0.0008 0.0006 0.0004 0.0002 0
-0.001 0 1.0
3.0 5.0 time (microsec)
Center for Quality Engineering
7.0
9.0
0
5.0 10.0 time (microsec)
15.0
Fiber Optic Ultrasound Sensors for Smart Structures Applications Pavel Fomitchov John Dorighi Carmen Hernandez Sridhar Krishnaswamy Jan Achenbach
Northwestern University
Center for Quality Engineering
Northwestern University
OBJECTIVE: To develop, characterize and apply fiber optic ultrasound sensors for smart structures Fiber Optic Ultrasound Sensors
Application
Development Characterization
Center for Quality Engineering
smart skins
Northwestern University
Why use fiber-optic ultrasound sensors? Piezoelectric Transducers
Fiber Optic Sensors
• High temperature measurements • Temperature Limitations • Small size and light weight • Often bulky • Not easily embedded in Composites • Compatible with composite materials • Immune to EMI • Susceptible to EMI • Expensive • Inexpensive • New unproved technology • Proven reliable technology • Lower sensitivity • Higher sensitivity
Center for Quality Engineering
Cent er for Quality Engineering & Failure Northwestern Prev ention, University North west ern Univer
APPROACH • Conventional approach uses fiber breakage as indicator of damage and this requires dense distribution of fiber sensors since sensor must be in the vicinity of the damage. • Our approach uses ultrasonic energy to interrogate the structure for damage. The fiber sensors are simply used to detect the ultrasonic energy after it has interrogated the structure. • Advantages of our approach include: (a) no requirement for proximity of sensor to the damage area, allowing for relatively sparse distribution of fiber sensors ,and (b) better sensitivity to detect small flaws dictated by the wavelength of the ultrasound rather than the spacing of the fiber sensors. sensors detect ut
generated ultrasound Center for Quality Engineering
flaw
sensors in the shadow do not detect ultrasound
Northwestern University
Transduction mechanism How does ultrasound affect light propagating inside a fiber? ultrasound
optical fiber
• Impinging ultrasound causes strain in fiber • Strain in fiber leads affects optical transmission in fiber (via dimensional changes, refractive index changes...)
Center for Quality Engineering
Northwestern University
SENSOR DEVELOPMENT • fabricate the fiber sensors • build sensor stabilization control system Publication: • "Stabilization of an Embedded Fiber-Optic Fabry-Perot Sensor for Ultrasound Detection," J. F. Dorighi, Sridhar Krishnaswamy and J.D. Achenbach IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 42, No. 5, pp. 820-824. •P. Fomitchov, Sridhar Krishnaswamy, J.D. Achenbach, (1999), “Intrinsic and Extrinsic Sagnac Ultrasound Sensor,” to appear Optical Engineering.
Center for Quality Engineering
Northwestern University
SENSOR DEVELOPMENT:
Intrinsic Fiber Optic Fabry-Perot Ultrasound Sensor reference beam
sensing region
sensing beam partial mirrors
Pr = 2 R(1 − cosφ) Pi φ=4πnL λ
n: λ: L: R:
dφ
I
refractive index of cavity wavelength of light cavity length reflectivity of mirr ors
most sensitive ∆φ
least sensitive
Note: Sensor must be actively stabilized at most sensitive (steep part) of the response curve to detect ultrasound. Center for Quality Engineering
Northwestern University
SENSOR DEVELOPMENT:
Active Homodyne Stabilization of Sensor external cavity diode laser
optical isolator integrator
differential amplifier
epoxy plate 2x2 optical coupler
photodiode
index matching gel
reference voltage highpass filter oscilloscope
Center for Quality Engineering
fiber optic Fabry-Perot
Northwestern University
SENSOR DEVELOPMENT:
Detection of Ultrasound without stabilization
amplitude (volts)
0.006 0.004
0.004
0.002
0.002
0
0
-0.002
-0.002
-0.004
-0.004
-0.006
-0.006 0
.5
1.0
1.5
2.0
time (microsec)
with stabilization
0.006
2.5
0
.5
1.0
1.5
2.5
time (microsec)
Note: Sensor detects small amplitude ultrasonic signal only when sensor is stabilized about its most sensitive response point. Center for Quality Engineering
2.0
Northwestern University
Sagnac Ultrasound Sensor ls 2 PD Laser
l
structure 1
intrinsic probe mirrorized fiber-tip
• Common path --> noise insensitive • Easier to fabricate than Fabry-Perot • P. Fomitchov, Sridhar Krishnaswamy and J. D. Achenbach, (1997), "Compact phase-shifted Sagnac Interferometer for Ultrasound Detection," in Optics and Laser Technology, vol. 29, No. 6, pp.333-338. • P. Fomitchov, Sridhar Krishnaswamy and J. D. Achenbach, (1997), ”Extrinsic and Intrinsic Sagnac Interferometer for Ultrasound Detection," to appear in Optical Engineering.
Center for Quality Engineering
Northwestern University
Sagnac Ultrasound Sensor Bulk Wave Detection
Intrinsic probe PZT
Photodetector signal, mV
6 4 2 0 -2 -4 -6
Structure
Center for Quality Engineering
6
8
10 12 Time, µs
14
16
Northwestern University
Bragg-grating Ultrasound Sensors Incident Ultrasound
broadband incident reflected
Transmitted
Pitch Λ
Reflected Intensity
response curve λb
lock-point
Wavelength • local sensor • easy to fabricate • easy to multiplex several sensors in one fiber segment Center for Quality Engineering
Northwestern University
Sensor signal, mV
Bragg-grating Ultrasound Sensors 4 3 2 1 0 -1 -2 -3 -4
Time-domain signal
0
5
10 15 20 25 30 35 40 Time, µs
• we have obtained time-domain signals from a Bragg-grating subjected to an ultrasonic toneburst of 2.25MHz. • frequency response of Bragg-grating sensor still an issue Center for Quality Engineering
Northwestern University
SENSOR CHARACTERIZATION •what is the sensitivity of the sensor? •what is its frequency response? Publications: • “Response of an Embedded Fiber-Optic Ultrasound Sensor,” J.F. Dorighi, Sridhar Krishnaswamy and J.D. Achenbach Journal of the Acoustical Society of America, vol. 101, No. 1, pp. 257-263. • "Sensitivity of an Embedded Fiber-Optic Ultrasonic Sensor," J.F. Dorighi, Sridhar Krishnaswamy and J.D. Achenbach in Review of Progress in Quantitative Nondestructive Evaluation, ed. D.O. Thompson and D.E. Chimenti, vol. 16, Plenum Press, New York.
Center for Quality Engineering
Northwestern University
SENSOR CHARACTERIZATION: Frequency Response
Fiber Optic Ultrasound Sensors: Frequency Response x1 x3
elastodynamic analysis: calculate strain in fiber core due to impinging ultrasonic wave
(r,θ) x2
x1 k δ x2
strain-optic relations: host λ calculate the induced phase shift in the µρ fiber sensor due to strain in the fiber caused by the impinging ultrasonic wave 2 2
2
Center for Quality Engineering
sensor λ1 µ1 ρ1
Northwestern University
SENSOR CHARACTERIZATION: Frequency Response
Normally incident longitudinal wave fiber in water
1.5
fiber in epoxy
2.0
normalized phaseshift
phase shift along x2 static solution static solution phase shift along x3
1.5
1.0 1.0
0.5 0.5 phase shift along x2
phase shift along x3 0.0
0.0 0
1
2
3
4
5
6
frequency (MHz)
Center for Quality Engineering
7
8
0
1
2
3
4
5
6
frequency (MHz)
7
8
Northwestern University
SENSOR CHARACTERIZATION:Frequency Response
Obliquely incident longitudinal wave fiber in epoxy normalized phase shift
1.5
2o
1.5
incidence
4o incidence phase shift along x3
1.0
1.0 phase shift along x3 phase shift along x2
0.5
0.5
phase shift along x2 0.0
0.0 2
3
4
5
6
frequency (MHz) Center for Quality Engineering
7
8
2
3
4
5
6
7
frequency (MHz)
8
Northwestern University
SENSOR CHARACTERIZATION:Sensitivity
Sensitivity Full fringe
ultrasonic signal
1.4
0.04 0.03
1.2
amplitude (volts)
amplitude (volts)
1.3 1.1 1.0 0.9 0.8 0.7
Vo = 1.025 V 0.5
1
1.5 2 2.5 time (sec)
∂Vr
0.01
= 45 mV
0 -0.01 -0.02
0.6 0
0.02
3
3.5
4
Vo: quadrature offset voltage δVr: ultrasonic signal voltage Γ: fringe visibility Center for Quality Engineering
-0.03
0
1.0
2.0
3.0
4.0
time (microsec)
Phase-shift induced by ultrasound
∂φ =
∂Vr ΓVo
Northwestern University
Measured Sensitivity vs Theoretical Limit Shot Noise Limited Phase Shift
∂φ SNL =
Ultrasonic Phase Shift
1 hυB Γ ηRPi
∂ φ = 0.14 rad Smallest Measureable Phase Shift
∂ φ SNL = 2.2 x 10-7 rad/√Hz h: Planck’s constant ν: light frequency B: measurement bandwith
Measured Phase Shift
∂ φ = 2.0 x 10-6 rad/√Hz
η: quantum efficiency of detector R: mirror reflectivity Pi: total incident power into sensor
Center for Quality Engineering
Northwestern University
SENSOR CHARACTERIZATION: Sensitivity
Sensitivity in terms of Ultrasonic Amplitude 0.04
pzt transducer
amplitude (volts)
0.03 0.02 0.01 0 -0.01 -0.02 -0.03
Fizeau Interferometer (sensitive only to displacement)
0
1.0
2.0
3.0
4.0
time (microsec)
Ultrasonic Displacement δ = 11.6 nm
embedded FOFP sensor Smallest Measureable Displacement (sensitive to stress and displacement)
δ = 0.8 nm
Center for Quality Engineering
Northwestern University
SENSOR APPLICATION • detection of flaws • acoustic emission detection • cure monitoring of composites Publications: • "Laser Ultrasound System with Embedded Fiber-Optic Interferometric Sensor", J.F. Dorighi, Sridhar Krishnaswamy and J.D. Achenbach, SPIE Proceedings, vol. 2622. • "A fiber optic ultrasonic system to monitor the cure of epoxy," J.F. Dorighi, Sridhar Krishnasamy, and J.D. Achenbach, ASNT's Research in Nondestructive Evaluation (to appear).
Center for Quality Engineering
Northwestern University
SENSOR APPLICATIONS:
Flaw Detection Reflectivity
Specimen with a hole
epoxy plate
hole fsi-fofp
0
5
10
15 20
25
30 35
40
normalized amplitude (Vr/Vi)
0.08 0.07 0.06 0.05 0.04 0.03 0 pzt generator
scan
Center for Quality Engineering
10
20
30
distance (mm)
40
50
Northwestern University
SENSOR APPLICATIONS:
Simulated Acoustic Emission Event (Pencil Lead-Break Test) 0.02
amplitude (volts)
0 -0.02 -0.04 -0.06 -0.08 0 Center for Quality Engineering
50 100 time (microsec)
150
Northwestern University
FUTURE WORK
How about fiberized laser-generation of ultrasound? Embedded Fiber Generation
Surface Generation
Nd:Yag pulse
embedded fiber generator
glass slide
zinc coating
acoustic couplant absorbing layer
embedded FOFP
0.0016
0.0025
0.0014
amplitude (volts)
0.002
amplitude (volts)
Nd:Yag pulse
0.0015 0.001
0.0005 0
-0.0005
0.0012 0.001 0.0008 0.0006 0.0004 0.0002 0
-0.001 0 1.0
3.0 5.0 time (microsec)
Center for Quality Engineering
7.0
9.0
0
5.0 10.0 time (microsec)
15.0
