DOPPLER FREQUENCY-SHIFT COMPENSATED
PHOTOREFRACTIVE INTERFEROMETER FOR ULTRASOUND
DETECTION ON OBJECTS IN MOTION
B. Campagne, A. Blouin, C. Neron and J.-P. Monchalin
Industrial Materials Institute, National Research Council of Canada,
75 de Mortagne Blvd., Boucherville, Quebec, J4B 6Y4, Canada
ABSTRACT. Two-wave mixing based interferometry has been demonstrated to be a powerful
technique for non-contact, broadband and speckle insensitive measurements of the small surface
displacements produced by ultrasonic waves propagating in an object. When the object is in rapid
motion along the line-of-sight of the probing laser or when the laser beam is rapidly scanned on a
wavy surface, the two-wave mixing photorefractive interferometer loses sensitivity to the point it
could become useless. To circumvent the Doppler frequency-shift produced by this relative motion,
we propose a dynamic compensation scheme. We report a particularly simple scheme to implement
this concept by monitoring the low-frequency output signal of a balanced two-wave mixing
demodulator whose output is proportional to the frequency difference between the pump and signal
beams, and feeding this signal back to the acousto-optic shifter. With this new concept, the two-wave
mixing interferometer can operate on objects in rapid motion while maintaining its sensitivity to low
frequency ultrasound.
INTRODUCTION
The optical detection of the ultrasound is performed by a laser coupled to an optical
interferometer. The laser light scattered or reflected by the surface of the part to be probed
is phase-modulated by the small ultrasonic displacement of this surface. The conversion of
the phase-modulated optical beam into an intensity-modulated electrical signal is
performed by an interferometer followed by an optical detector. Several solutions have
been developed to detect and demodulate efficiently the scattered light in the practical
cases of industrial interest where the surface is rough and when the scattered light has
speckles.
A first type of solution is based on time-delay interferometers that have been fieldwiden [1,2]. Particularly useful and practical are the various confocal Fabry-Perot schemes.
This type of demodulator has a rather short response time to a change of phase or
frequency of the collected scattered light: values of 100 ns or shorter are typical and
depend upon the cavity length and mirror reflectivity. Therefore such systems easily
tolerate strong vibrations and motions of the probed object. Such motions cause not only a
change of the speckle pattern but also, a Doppler shift of the optical frequency when the
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
2003 American Institute of Physics 0-7354-0117-9
273
motions have a velocity component along the line-of-sight of the laser. It should be noted
motions
a velocity
along the based
line-of-sight
of the
the laser
laser.frequency
It should be
notedbe
that, for have
proper
operationcomponent
of the Fabry-Perot
devices,
should
that,
fortoproper
operation
of the
Fabry-Perot
devices,
the adjustable
laser frequency
shouldorbean
locked
the cavity
length,
which
requiresbased
a laser
with an
frequency
locked
to the
cavity
length,
which electronics.
requires a laser with an adjustable frequency or an
adjustable
cavity
length
and locking
adjustable
cavitysolution
length and
lockinginelectronics.
A second
consists
the various two-beam adaptive phase demodulator
A second
solution
consists
in the
two-beam
phasedirectly
demodulator
schemes,
in which
the beam
scattered
by various
the surface
is mixedadaptive
with a beam
derived
schemes,
in
which
the
beam
scattered
by
the
surface
is
mixed
with
a
beam
directly
from the laser (the pump beam) in a photorefractive material. This material has derived
trapping
from for
thethe
laser
(the pump
beam)
photorefractive
material.
material
sites
charges
produced
by in
thea photoelectric
effect.
AfterThis
charge
motionhas
bytrapping
diffusion
sites
for(if
theancharges
theapplied)
photoelectric
effect. After
charge
motion byisdiffusion
or
drift
electricproduced
field hasby
been
and trapping,
a charge
distribution
set in the
or drift (ifand
an electric
field
been applied)
and trapping,
charge distribution
is set in the
material
results in
an has
electric
field distribution
(theaspace-charge
field distribution).
material
and results
in for
an electric
field distribution
(theapproach
space-charge
fieldwell
distribution).
Two
different
schemes
demodulators
based on this
are now
known: the
Two different
schemes
for demodulators
based
this photo-electromotive
approach are now well
known:
the
two-wave
mixing
(TWM)
[3,4,5] scheme
andon the
force
(p-EMF)
two-wave
mixing
(TWM)
[3,4,5]
scheme
and
the
photo-electromotive
force
(p-EMF)
schemes [6]. The setup of the TWM optical demodulator is shown in Figure 1. In the
schemes
[6]. The setup
the at
TWM
opticalofdemodulator
shown in of
Figure
1. In beam
the
TWM
configuration,
the of
beam
the output
the crystal isiscomposed
the signal
TWM
configuration,
the
beam
at
the
output
of
the
crystal
is
composed
of
the
signal
beam
transmitted through the crystal and of the beam from the pump beam diffracted by the
transmitted
through
the crystal
and of
the signal
beam beam
from the
diffracted
by(LO).
the
grating,
which
is wavefront
adapted
to the
andpump
acts asbeam
a local
oscillator
grating,
which
is wavefront
adapted
to the
signalis beam
as optical
a local oscillator
(LO).
The
signal
beam
at the output
of the
crystal
then and
sent acts
to an
detector or
to a
The signal
beam at the output of the crystal is then sent to an optical detector or to a
balanced
receiver.
balanced
The receiver.
TWM adaptive demodulator makes a much more compact device than the
The Fabry-Perot
TWM adaptive
demodulator
makesThe
a much
device frequency
than the
confocal
based
demodulators.
TWMmore
also compact
has a broader
confocal Fabry-Perot based demodulators. The TWM also has a broader frequency
response, extending to low ultrasonic frequencies and without drop-off gaps at high
response, extending to low ultrasonic frequencies and without drop-off gaps at high
frequencies and does not require any active stabilization. For these reasons the TWM
frequencies and does not require any active stabilization. For these reasons the TWM
appears much more interesting than the confocal Fabry-Perot based demodulators. To
appears much more interesting than the confocal Fabry-Perot based demodulators. To
operate in the presence of strong vibrations or on moving objects, efforts have been
operate in the presence of strong vibrations or on moving objects, efforts have been
directed to shortening the response time of these two-beam adaptive phase demodulators.
directed to shortening the response time of these two-beam adaptive phase demodulators.
A
either obtained
obtained by
by design
design (e.g.
(e.g.small
smallangle
anglebetween
betweenthe
theinterfering
interfering
A short
short response
response time
time is
is either
beams)
and/or
by
strong
pumping.
In
spite
of
these
efforts
it
is
difficult
to
get
a
response
beams) and/or by strong pumping. In spite of these efforts it is difficult to get a response
time
as
short
as
the
confocal
Fabry-Perot
while
maintaining
a
reasonable
sensitivity.
time as short as the confocal Fabry-Perot while maintaining a reasonable sensitivity.
Therefore
phase demodulator,
demodulator, although
although very
very attractive
attractive inin several
several
Therefore the
the TWM
TWM adaptive
adaptive phase
aspects,
has
serious
shortcomings
when
the
object
is
in
motion.
Particularly
troublesome
aspects, has serious shortcomings when the object is in motion. Particularly troublesome isis
the
the line-of-sight
line-of-sight of
of the
thelaser,
laser,causing
causingnot
notonly
onlyaachange
change
the case
case where
where the
the motion
motion is
is along
along the
of
Doppler change
change of
of the
the optical
optical frequency
frequencyof
of the
the scattered
scattered
of the
the speckle
speckle pattern
pattern but
but also
also aa Doppler
light.
photorefractive interferometer
interferometer loses
loses sensitivity
sensitivitytoto
light. In
In that
that case
case the
the two-wave
two-wave mixing
mixing photorefractive
the
point
it
could
become
useless.
the point it could become useless.
λ/2
K/2
signal
PBS λ/4
Inspected part
part
Inspected
CW laser
L.O.
ν0
pump
ν0
λ/2 λ/4
signal
signal
signal / Photorefractive
Photorefractive crystal
crystal
Balanced
Balancedreceiver
receiver
FIGURE1.1.Basic
Basicsetup
setupofofaaTWM
TWM optical
optical demodulator
demodulator for the measurement
FIGURE
measurement of
of ultrasonic
ultrasonic wave
wave on
on rough
rough
surface.
surface.
274
To circumvent the Doppler frequency-shift produced by this relative motion, we
To acircumvent
the Doppler frequency-shift
producedanbyacousto-optic
this relativeshifter
motion,
propose
dynamic compensation
scheme. By inserting
on we
the
propose
a dynamic
compensation
scheme.
By insertingfor
anthe
acousto-optic
shifter on
theso
pump
or signal
beampaths,
it is possible
to compensate
Doppler frequency
shift
pump
signaland
beampaths,
it is have
possible
to compensate
for the Doppler frequency shift so
that
theorpump
signal beams
nearly
the same frequency.
that the
signal beams
have nearly
frequency.
We pump
reportand
a particularly
simple
schemethetosame
implement
this concept by monitoring the
We reportoutput
a particularly
schemetwo-wave
to implement
thisdemodulator
concept by monitoring
theis
low-frequency
signal ofsimple
a balanced
mixing
whose output
low-frequency
output
signal
of
a
balanced
two-wave
mixing
demodulator
whose
output
is
proportional to the frequency difference between the pump and signal beams, and feeding
proportional
to the
differenceshifter.
between
thescheme
pump and
signal beams,
this
signal back
to frequency
the acousto-optic
This
compensates
for and
the feeding
Doppler
this signal shift
back produced
to the acousto-optic
compensatesofforthe
thelaser.
Doppler
frequency
by object shifter.
motion This
alongscheme
the line-of-sight
The
frequency ofshift
by affected
object motion
along the
line-of-sight
laser. The
sensitivity
the produced
device is also
by the object
motion
transverseofto the
the line-of-sight
sensitivity
the device
is also
affected
by theinobject
motionoftransverse
to the
line-of-sight
of
the laser.ofThis
transverse
velocity
results
a variation
the speckle
pattern
collected
of
the
laser.
This
transverse
velocity
results
in
a
variation
of
the
speckle
pattern
collected
back from the surface and cannot be compensated by the proposed scheme. However,
we
backshow
fromthat
the the
surface
and cannot
be compensated
by demodulator
the proposedsensitivity
scheme. However,
will
transverse
velocities
for which the
is reducedweby
show
that isthetwo
transverse
for larger
which than
the demodulator
is reduced byof
awill
factor
a two
orders ofvelocities
magnitude
the velocity sensitivity
along the line-of-sight
a
factor
a
two
is
two
orders
of
magnitude
larger
than
the
velocity
along
the
line-of-sight
of
the laser. With this new concept, the two-wave mixing interferometer is allowed to operate
theobjects
laser. With
this new
concept,
two-wave mixing
interferometer
is allowed ultrasound.
to operate
on
in rapid
motion
whilethe
maintaining
its sensitivity
to low-frequency
on objects
in scheme
rapid motion
while
maintaining
its adaptive
sensitivitytwo-beam
to low-frequency
ultrasound.
The
proposed
can also
be used
with other
interferometers.
The proposed scheme can also be used with other adaptive two-beam interferometers.
VARIATION OF THE SENSITIVITY VERSUS THE DOPPLER FREQUENCY
VARIATION OF THE SENSITIVITY VERSUS THE DOPPLER FREQUENCY
SHIFT
SHIFT
In the TWM adaptive phase demodulator, a difference between the optical frequencies
In the TWM adaptive phase demodulator, a difference between the optical frequencies
of the pump and signal beams produces motion of the interference pattern. Since the
of the pump and signal beams produces motion of the interference pattern. Since the
buildup time of the space charge is finite, the space charge cannot exactly follow the
buildup time of the space charge is finite, the space charge cannot exactly follow the
moving
field amplitude
amplitude isis then
then reduced
reduced and
and
moving interference
interference pattern.
pattern. The
The space-charge
space-charge field
eventually,
for
a
large
enough
frequency
difference
the
space-charge
field
is
washed
out.
eventually, for a large enough frequency difference the space-charge field is washed out.
This
amplitude decreases
decreasesthe
thesensitivity
sensitivityofofthe
thedevice
devicetoto
This reduction
reduction of
of the
the space-charge
space-charge field
field amplitude
the
phase
modulation
induced
by
the
small
surface
displacements.
As
an
example,
the
the phase modulation induced by the small surface displacements. As an example, the
sensitivity
of
the
TWM-based
phase
demodulator
operated
with
a
GaAs
crystal
in
the
sensitivity of the TWM-based phase demodulator operated with a GaAs crystal in the
diffusion
frequency difference
difference between
between the
the signal
signal and
and pump
pump
diffusion regime
regime versus
versus the
the optical
optical frequency
beams
is
shown
in
Figure
2.
beams is shown in Figure 2.
Normalized sensitivity (a. u.)
-21.3
-21.3
Velocity (mm/s)
(mm/s)
Velocity
-10.6
0.0
10.6
-10.6
0.0
10.6
21.3
21.3
1.0
0.8
0.6
0.4
0.2
0.0
-40.0 -20.0
0.0
20.0
40.0
-40.0
20.0
40.0
Frequency difference
difference Av
Frequency
∆ν (kHz)
(kHz)
FIGURE2.2.Sensitivity
Sensitivityofofthe
theTWM-based
TWM-based phase
phase demodulator operated
FIGURE
operated with
with aa GaAs
GaAs crystal
crystal ininthe
thediffusion
diffusion
regimeversus
versusthe
theoptical
opticalfrequency
frequency difference
difference between
between the
the signal
signal and
regime
and pump
pump beams.
beams. An
An electro-optic
electro-opticphase
phase
modulatorwas
washarmonically
harmonicallydriven
drivenatat22 MHz
MHz to
to simulate
simulate the
the surface
modulator
surface displacement.
displacement. The
The response
responsetime
timeof
ofthe
the
crystalwas
wasabout
about35
35µs.
us.
crystal
275
CW laser
500 mW
λ/2
A/2
ν0
λ/2 λ/4
Phase
Phase
modulator
modulator
pump
Inspected part
+ ∆ν
Acousto-optic
shifter
ν0 + ∆ν
+—— signal
signal
Balanced receiver
receiver
Balanced
λ/4
*
Photorefractive material
material
Photorefractive
FIGURE3.3. Setup
Setupused
usedfor
forthe
themeasurements
measurementsshown
shownininFigure
Figure2.2.
FIGURE
The setup
setup for
for these
these measurements
measurements isis shown
shown in
in Figure
Figure 3.
3. The
The laser
laser was
was aa Nd:YAG
Nd:YAG
The
laser
at
1.064
(im
wavelength,
the
signal
beam
was
phase-modulated
using
an
electro-optic
laser at 1.064 µm wavelength, the signal beam was phase-modulated using an electro-optic
phasemodulator
modulatordriven
drivenatat22MHz
MHz and
andthe
the optical
optical frequency
frequency difference
difference between
between the
the pump
pump
phase
and
signal
was
produced
by
an
acousto-optic
shifter
on
the
signal
beam.
In
the
diffusion
and signal was produced by an acousto-optic shifter on the signal beam. In the diffusion
regime,the
thesensitivity
sensitivityisisreduced
reducedby
byaafactor
factor of
of approximately
approximately 22 for
for ∆ν
Av τ1 ≈~ 1/2π,
1/27C, where
where ∆ν
Av
regime,
is
the
optical
frequency
difference
between
the
signal
and
pump
beams
and
T
is
the
grating
is the optical frequency difference between the signal and pump beams and τ is the grating
build-uptime.
time. InIn this
thisexperiment,
experiment, τTwas
was about
about 35
35 µs
us and
and the
the sensitivity
sensitivity isis reduced
reduced by
by aa
build-up
factor 22 for
for an
an optical
optical frequency
frequency difference
difference of
of about
about 33 kHz.
kHz. In
In aa real
real application,
application, aa
factor
detectionlaser
laserofoftypically
typicallyofof1 1kW
kWand
and50
50µs
(isduration
durationisisused
usedtoto compensate
compensatefor
for the
the lowlowdetection
level
light
collected
back
from
optically
rough
surfaces.
With
such
a
high
peak
power
level light collected back from optically rough surfaces. With such a high peak power
pulsed detection
detection laser
laser we
we have
have measured
measured aa build-up
build-up time
time of
of 11 µs,
jus, and
and the
the ultrasonic
ultrasonic
pulsed
sensitivity
is
reduced
by
a
factor
of
2
for
an
optical
frequency
difference
of
200
kHz
[7].
sensitivity is reduced by a factor of 2 for an optical frequency difference of 200 kHz [7].
A
Doppler
optical
frequency
difference
between
the
pump
and
signal
beam
A Doppler optical frequency difference between the pump and signal beam isis
obtained
whencollecting
collectingthe
thelaser
laserlight
lightscattered
scatteredoff
offan
anobject
object in
in motion
motion along
along the
the line-ofline-ofobtained when
sight ofof the
the laser
laser oror inin motion
motion along
along the
the collection
collection direction.
direction. Alternatively,
Alternatively, aa Doppler
Doppler
sight
frequency shift
shift ofofthe
thescattered
scatteredlight
light isis also
also obtained
obtained when
when the
the scanning
scanning laser
laser beam
beam does
does
frequency
not
impinge
normally
to
the
surface
of
the
object,
for
example
when
scanning
a
contoured
not impinge normally to the surface of the object, for example when scanning a contoured
object oror probing
probing the
the side
side ofof aa spinning
spinning wheel.
wheel. When
When the
the incident
incident laser
laser beam
beam and
and the
the
object
collection
direction
are
collinear,
the
frequency
shift
of
the
scattered
beam
is
simply
collection direction are collinear, the frequency shift of the scattered beam is simply
proportionaltotothe
thecomponent
componentalong
along the
the line-of-sight
line-of-sight of
of the
the velocity
velocity of
of laser
laser spot
spot on
on the
the
proportional
object.
object.
As an example, with a laser wavelength of 1.064 urn, the 3 kHz half-reduced
As an example, with a laser wavelength of 1.064 µm, the 3 kHz half-reduced
sensitivity of Figure 2 corresponds to an object velocity of 1.6 mm/s along the line-of-sight
sensitivity of Figure 2 corresponds to an object velocity of 1.6 mm/s along the line-of-sight
of the laser. When the TWM is coupled to a high peak power pulsed laser, the 200 kHz
of the laser. When the TWM is coupled to a high peak power pulsed laser, the 200 kHz
half-reduced sensitivity corresponds to an object velocity of 100 mm/s. Such frequency
half-reduced sensitivity corresponds to an object velocity of 100 mm/s. Such frequency
shift may be found when inspecting objects submitted to large amplitude vibrations at low
shift may be found when inspecting objects submitted to large amplitude vibrations at low
frequencies or materials like a paper web or a metal sheet in rapid motion on a production
frequencies or materials like a paper web or a metal sheet in rapid motion on a production
line. For example, the in-plane Young's modulus of a paper web is deduced from the online. For example, the in-plane Young‘s modulus of a paper web is deduced from the online measurement of ultrasonic in-plane surface displacement. This in-plane displacement
line measurement of ultrasonic in-plane surface displacement. This in-plane displacement
measurement requires that the laser impinges at oblique incidence on the paper web, which
measurement requires that the laser impinges at oblique incidence on the paper web, which
results in a frequency shifted light collected back from the paper web. It is difficult to have
results in a frequency shifted light collected back from the paper web. It is difficult to have
a TWM system, even by using semiconductor photo refractive crystal with strong pumping
a TWM system, even by using semiconductor photo refractive crystal with strong pumping
that could tolerate the corresponding very large frequency offset. Furthermore, even if the
that
couldis tolerate
the corresponding
very large
frequency
Furthermore,
if the
system
made with
a very fast response,
it strongly
cutsoffset.
the low
ultrasonic even
frequencies
system
is
made
with
a
very
fast
response,
it
strongly
cuts
the
low
ultrasonic
frequencies
(below 1 MHz) that are needed for this application. Another important example is the
(below 1 MHz) that are needed for this application. Another important example is the
276
inspection of polymer-matrix composite objects used in the aerospace industry. Laserinspection of polymer-matrix composite objects used in the aerospace industry. Laserultrasonics is noteworthy for the ease of inspecting complex geometries and the technology
ultrasonics is noteworthy for the ease of inspecting complex geometries and the technology
has actually been commercialized by using a confocal Fabry-Perot as demodulator. The
has actually been commercialized by using a confocal Fabry-Perot as demodulator. The
use of based-based systems would be of interest for their sensitivities to low-frequency
use of based-based systems would be of interest for their sensitivities to low-frequency
ultrasound and for inspecting thick objects, but it should be realized that this application
ultrasound and for inspecting thick objects, but it should be realized that this application
requiresfast
fastscanning
scanning (one
(onemeter
meterper
persecond
secondand
andmore).
more).InInthe
thecase
caseofofcontoured
contouredobjects,
objects,
requires
since
there
is
oblique
incidence
of
the
probing
laser
beam
(of
45°
and
more),
very
large
since there is oblique incidence of the probing laser beam (of 45° and more), very large
frequency
shifts
then
result,
making
the
TWM
adaptive
phase
demodulator
insensitive
frequency shifts then result, making the TWM adaptive phase demodulator insensitive
withoutaacompensation
compensationscheme.
scheme.
without
VARIATIONOF
OFTHE
THESENSITIVITY
SENSITIVITYVERSUS
VERSUSTHE
THESPECKLE
SPECKLEEFFECT
EFFECT
VARIATION
The motion
motion of
of the
the object
object transverse
transverse toto the
the line-of-sight
line-of-sight ofof the
the laser
laser causes
causes
The
modification
of
the
speckle
pattern
to
which
the
space-charge
field
should
also
adapt.
The
modification of the speckle pattern to which the space-charge field should also adapt. The
experimental
setup
used
to
study
independently
the
effects
of
speckle
pattern
and
the
experimental setup used to study independently the effects of speckle pattern and the
Doppler
frequency
shift
is
shown
in
Figure
4.
A
spinning
wheel
whose
radial
velocity
can
Doppler frequency shift is shown in Figure 4. A spinning wheel whose radial velocity can
be controlled
controlled isisnow
nowthe
theinspected
inspectedpart.
part.The
Thelaser
laserbeam
beamimpinges
impingesononthe
theside
sideofofthe
thewheel.
wheel.
be
First, to
to study
study only
only the
the contribution
contribution ofof the
the speckle
speckle pattern,
pattern, the
the laser
laser impinges
impinges
First,
perpendicularly to
to the
the surface.
surface. In
Inthat
thatcondition,
condition,there
thereisisno
noDoppler
Dopplerfrequency
frequencyshift
shiftsince
since
perpendicularly
there
is
no
velocity
component
in
the
line-of-sight
of
the
beam.
When
the
wheel
is
moved
there is no velocity component in the line-of-sight of the beam. When the wheel is moved
up or
or down,
down, the
the velocity
velocity component
componentVD
vD in
in the
the line-of-sight
line-of-sight of
of the
the beam
beam isis no
no longer
longer zero,
zero,
up
and the
the scattered
scattered laser
laser light
light isis Doppler
Dopplerfrequency
frequencyshifted.
shifted.
and
We have
have found
found that
that the
theeffect
effectofofthe
thetransverse
transversemotion
motionisisless
lesssevere
severethan
thanthe
theeffect
effectofof
We
the motion
motion along
along the
the line-of-sight
line-of-sightofofthe
thelaser.
laser.More
Moreprecisely,
precisely,the
thetransverse
transversevelocity
velocityfor
for
the
which the
the TWM
TWM sensitivity
sensitivity isis reduced
reducedby
byaafactor
factorofof22isistwo
twoorders
ordersofofmagnitude
magnitudelarger
larger
which
than the
the velocity
velocity along
along the
the line-of-sight
line-of-sight ofof the
the laser
laser resulting
resulting inin the
the same
samesensitivity
sensitivity
than
reduction.
These
results
are
shown
on
the
Figure
5.
With
an
adequate
configuration
and
reduction. These results are shown on the Figure 5. With an adequate configuration and
sufficient pumping
pumping level,
level, the
the space-charge
space-charge field
field adapts
adapts itself
itself sufficiently
sufficientlyrapidly
rapidlytoto the
the
sufficient
speckle pattern
pattern variations
variations so
so there
there isis no
no significant
significant loss
loss ofof sensitivity.
sensitivity. Therefore
Therefore the
the
speckle
problem is
is essentially
essentially to
to overcome
overcome the
the large
large frequency
frequencyoffset
offsetlinked
linkedtotothe
thefast
fastmotion
motionofof
problem
either the
the object
object or
or the
the inspection
inspectionbeam
beamwhile
whilemaintaining
maintainingthe
thesensitivity
sensitivitytotolow
lowfrequency
frequency
either
ultrasound.
ultrasound.
CW laser
500 mW
λ/2
pump
ν0 + ∆ν
^—— signal
signal
Balanced receiver
λ/4
v
vt
vD
ν0
λ/2 λ/4
Phase
modulator
Spinning wheel
Doppler frequency shift
∆ν = ±2
vD
λ0
Photorefractive
Photorefractive crystal
crystal
FIGURE 4.
4. Setup
Setup for
FIGURE
for measurement
measurement of
of sensitivity
sensitivity versus
versus the
the transverse
transverse and
and longitudinal
longitudinal velocities
velocities of
of the
the
object. V
V is
is the
the tangential
velocity of
object.
tangential velocity
of the
the wheel,
wheel, V
VDD is
is the
the longitudinal
longitudinal velocity
velocity associated
associated to
to the
the Doppler
Doppler
effect, and
and V
Vtt is
is the
the transverse
transverse velocity
velocity associated
associated to
effect,
to the
the speckle
speckle pattern
pattern motion.
motion.
277
Normalized sensitivity (a. u.)
1.1
1.1 n
1.0
1.0*•*%
0.9
0.9:
0
••
B
0.80
l1
Doppler
Speckle
Do ppler *
Speckl
0.7
-m——--———
0.7:
m
•
•
0.6
0.6
0.5
0.5:
•
•
0.4
0.4•
0.3
0.3:
•
:
0.2
0.2
•
0.10.1
nn.
0.0
0.01 0.1
0.1 11 10
10 100
100 1000
100010000
0.01
10000
Velocity (mm/s)
Velocity
"V
FIGURE5.5. The
The drop
drop ofof sensitivity
sensitivity due
due to
to the
the speckle
speckle effect
effect (•)
FIGURE
(■) (velocity
(velocity of
of the
the side
side of
of the
the wheel
wheel
perpendiculartotothe
theline-of-sight
line-of-sightofofthe
thelaser
laser V
Vt)t) occurs
occurs at
at aa value
value much
much larger
larger than
than the
the one
one associated
associated to
to the
the
perpendicular
Dopplereffect
effect(●)
(•) (velocity
(velocityalong
alongthe
theline-of-sight
line-of-sight of
ofthe
the laser
laser V
VDD).).
Doppler
COMPENSATEDTWM
TWMDEMODULATOR
DEMODULATOR SCHEME
SCHEME
COMPENSATED
Tocircumvent
circumventthe
theDoppler
Doppler frequency
frequency shift
shift produced
produced by
by the
the velocity
To
velocity of
of the
the inspected
inspected
object
or
of
the
laser
spot
on
the
object,
we
propose
a
dynamic
compensation
object or of the laser spot on the object, we propose a dynamic compensation scheme.
scheme. The
The
approach isis based
based on
on two
two steps:
steps: first
first the
the velocity
velocity or
or the
approach
the frequency
frequency shift
shift is
is evaluated,
evaluated,
second, the
the frequency
frequency ofof one
one of
of the
the interfering
interfering beams
beams (preferably
(preferably the
the pump
pump beam)
second,
beam) is
is
shifted
in
such
a
way
that
both
beams
are
essentially
at
the
same
frequency.
Frequency
shifted in such a way that both beams are essentially at the same frequency. Frequency
shifting is conveniently performed by cascading two acousto-optic frequency shifters. The
shifting is conveniently performed by cascading two acousto-optic frequency shifters. The
first element up-shifts the optical frequency by a constant value + v (for instance v= 70
first element up-shifts the optical frequency by a constant value + ν (for instance ν= 70
MHz), whereas the second element down-shifts it by -v-Av, where Av is proportional to
MHz), whereas the second element down-shifts it by -ν-∆ν, where ∆ν is proportional to
the measured velocity. This second element is driven by a voltage-controlled oscillator,
the measured velocity. This second element is driven by a voltage-controlled oscillator,
which takes its control signal from the velocity measuring system. When the velocity is
which takes its control signal from the velocity measuring system. When the velocity is
zero the given shift is also zero. Since acousto-optic shifters operate best with small and
zero the given shift is also zero. Since acousto-optic shifters operate best with small and
collimated beams, the frequency compensation scheme is preferably applied on the
collimated
beams,
thethan
frequency
compensation
schemebyisthe
preferably
applied
pumped beam
rather
on the signal
beam scattered
object. This
beamonhasthea
pumped
beam
rather
than
on
the
signal
beam
scattered
by
the
object.
This
beam larger
has a
much larger etendue (or throughput) (which means for a given size a much
much
larger
etendue
(or
throughput)
(which
means
for
a
given
size
a
much
larger
divergence). The velocity can be measured by a number of ways depending upon the
divergence).
cansimple
be measured
by a scheme,
number an
of error
ways signal
depending
upon the
application. The
In a velocity
particularly
compensated
proportional
to
application.
In
a
particularly
simple
compensated
scheme,
an
error
signal
proportional
to
the velocity is obtained from the low frequency output of the balanced receiver.
the velocity
is obtained
low frequency
output
of the balanced
receiver.in Figure 6,
The basic
setup from
of thethecompensated
TWM
demodulator
is sketched
The basic
setup of
the demodulator
compensated(photo
TWMrefractive
demodulator
sketched
in Figure
6,
including
an adaptive
phase
crystalisand
a balanced
receiver)
including
an the
adaptive
(photo
refractive
crystal
and a waves
balanced
receiver)
to measure
small phase
surfacedemodulator
displacements
produced
by the
ultrasonic
propagating
toinmeasure
theinsmall
surface
displacements
the ultrasonic
wavesofpropagating
the object
motion
and an
acousto-opticproduced
frequencybyshifter
for the tuning
the signal
inbeam
the object
motion andThe
an Doppler
acousto-optic
frequency
for the
of the
opticalinfrequency.
frequency
shift isshifter
produced
by tuning
changing
the signal
radial
beam
optical
frequency.
The
Doppler
frequency
shift
is
produced
by
changing
the
velocity of the wheel. The small ultrasonic displacements (typically of a few nm, radial
much
velocity
thethe
wheel.
Thewavelength)
small ultrasonic
displacements
of aphase
few nm,
much
smaller of
than
optical
are simulated
by the (typically
electro-optic
modulator
smaller
optical
simulated
by theis electro-optic
phase
modulator
driven than
at 2 the
MHz.
The wavelength)
output of theare
balanced
receiver
fed back to the
acousto-optic
driven
MHz. The output
of the
balancedoffset
receiver
is fedtheback
to the
shifteratto2 compensate
the optical
frequency
between
signal
and acousto-optic
pump beams
shifter
to compensate
theofoptical
frequency offset between the signal and pump beams
produced
by the motion
the object.
produced by the motion of the object.
278
Reference
Reference
0
+
-+
-
0 signal
Error
Error signal
Ultrasonic signal
Ultrasonic
Ultrasonicsignal
signal
CW laser
CW
laser
500
mW
500 mW
Locking
Locking
network
network
Phase
Phase
modulator
modulator
+ν
−ν −∆ν
+ν
−ν −∆ν
+
Spinning wheel
Spinning
wheel
Spinning
wheel
ω
v
AcoustoAcousto-optic
AcoustoAcousto-optic
shifter
shifter
0
0
Voltage
Voltage
Controled
Controled
Oscillator
Oscillator
ω
v
vD
vD
ω
ω
+ ∆ν ∝ ω
+ ∆ν ∝ ω
+
Balanced receiver Photorefractive crystal
Balanced
Balancedreceiver
receiver Photorefractive
Photorefractive crystal
crystal
FIGURE 6. Principle of the dynamic compensation. The change of the radial velocity ω of the wheel creates
FIGURE 6.6. Principle
of the
compensation.
The
the radial
velocity
co ω
ofof
thethe
wheel
creates
Principle
thedynamic
compensation.
Thechange
changeofof
radial
velocity
wheel
creates
a FIGURE
Doppler frequency
shiftof∆ν
ofdynamic
the signal
beam that unbalalances
thethe
low
frequency
output
of the
balanced
aaDoppler
frequency
shift
Av
ofofthe
signal
beam
that
unbalalances
the
low
frequency
output
ofofthethebalanced
Doppler
frequency
shift
∆ν
the
signal
beam
that
unbalalances
the
low
frequency
output
balanced
receiver. This unbalanced output or error signal is continuously compared with a reference level. A Voltage
receiver.
unbalanced
output
ororerror
signal
isiscontinuously
compared with
level. A AVoltage
receiver.This
This
unbalanced
outputto
signal
continuously
witha areference
reference
Voltage
Controlled
Oscillator
connected
aerror
locking
network
drives ancompared
acousto-optic
shifter
thatlevel.
changes
and
Controlled
Oscillator
connected
toto a alocking
network
drives
ananacousto-optic
shifter
that
changes
and
Controlled
Oscillator
connected
locking
network
drives
acousto-optic
shifter
that
changes
and
compensates the Doppler frequency shift ∆ν.
compensates
compensatesthe
theDoppler
Dopplerfrequency
frequencyshift
shiftAv.
∆ν.
For
aasignal
beam
Doppler frequency
shifted by
motion
of
the object,
object, aa positive
positive or
or
For
beam
frequency
bybythe
the
motion
ofofthe
Forsignal
asignal
signal
beamDoppler
Doppler
frequencyshifted
shifted
thethe
motion
the
object, This
a positive
or
negative
appears
at
the
low-frequency
output
of
balanced
receiver.
signal
negative
signal
appears
atatthe
low-frequency
output
ofofthe
balanced
receiver.
This
signal
negative
signal
appears
the
low-frequency
output
the
balanced
receiver.
This
signal
could
then
used as
an error
signal
to
the
acousto-optic shifter.
It should
be noted
noted
could
then be
be
signal
totodrive
drive
the
shifter.
It
should
could
beused
usedas
asan
anerror
error
signal
drive
theacousto-optic
acousto-optic
shifter.
shouldbe
noted
that
itit isthen
necessary
to
find
first
the
proper
frequency
offset
in
order
toIt create
create
aabestable
stable
that
is
necessary
to
find
first
the
proper
frequency
offset
in
order
to
that
it
is
necessary
to
find
first
the
proper
frequency
offset
in
order
to
create
a
stable
grating
inside
the
material.
Once
such
a grating
is
the
feedback loop
loop can
can be
grating
inside
the
material.
Once
such
grating
isisestablished
established
the
feedback
grating
inside
the
material.
Once
suchaahigh
grating
established
the
feedback
loop canbebe
closed
and
if
the
loop
gain
is
sufficiently
there
is
continuous
tracking
and
continuous
closed
ififthe
gain
closedand
and
theloop
loopmotion.
gainisissufficiently
sufficientlyhigh
highthere
thereisiscontinuous
continuoustracking
trackingand
andcontinuous
continuous
detection
of
ultrasonic
detection
of
ultrasonic
motion.
detection of ultrasonic motion.
Doppler
frequency (kHz)
Doppler
Dopplerfrequency
frequency(kHz)
(kHz)
0
1
1 2x10
1
22
33
2x10
2x10
2x10
-1
0
1
2 2x10
3
2X10"
2x10°
2X10
2xl0
2xl0
2x10
2x10
2x10
2x10
2x10
1.1
1.1 1.1
1.0 ——•_•-•
ol.O'••• «f • • • •
1.0
d 0.9
0.9With
0.9
With
%
With
&0.8
0.8Without
compensation
ut
0.8 ~Withc
compensation
%
Without
0.7
7
compensation
|
O- " msation
compc
0.7 compensation
•
*| 0.6
0.6- compensation
§ 0.5
n0.6
c
•
0.5:
0.4
"S 0.4
0.4
10.3
0.3^
& 0.2
n0.3
o'
•
0.2
3 0.1
0^
0.1
1000
0.1
11
10
100
1000
0.1
10
100
0.1 Velocity
1 vV (mm/s)
10
100
1000
Velocity
D (rnm/s)
D
Velocity vD (mm/s)
Normalized sensitivity (a. u.)
Normalized sensitivity (a. u.)
-1
FIGURE7.7. Sensitivity
Sensitivityofofthe
theTWM-based
TWM-based demodulator
demodulator to
to the
the small
small amplitude,
amplitude, high
high frequency
frequency ultrasonic
ultrasonic
FIGURE
FIGURE
7.ananSensitivity
of theaaTWM-based
demodulator
the small amplitude,
highWhen
frequency
ultrasonic
motion
with
object having
having
velocity component
component
in the
thetoline-of-sight
line-of-sight
of the laser.
motion
with
object
velocity
in
of
the dynamic
motion with isan
objectthe
having
aisvelocity
component
inspeckle
the line-of-sight
compensation
isactive,
active,
the
system
isjust
justsensitive
sensitive
thespeckle
effect(■).
(•). of the laser. When the dynamic
compensation
system
totothe
effect
compensation is active, the system is just sensitive to the speckle effect (■).
279
Figure 7 shows the results obtained with and without dynamic compensation for an
object velocity along the line-of-sight of the laser. As expected, when the Doppler
frequency shift produced by the object velocity along the line-of-sight of the laser is
compensated, the sensitivity of the TWM demodulator to the ultrasonic motion remains
constant.
CONCLUSION
We have experimentally shown that the sensitivity of the TWM demodulator is
reduced when the object is in rapid motion along the line-of-sight of the probing. We have
also shown that the sensitivity to the motion of the probed surface along the line-of-sight of
the laser is larger than the one for transverse motion of the object. We have proposed and
experimentally tested a dynamic compensation scheme to compensate for the Doppler shift
induced by motion of the probing object along the line-of-sight of the laser. This scheme
allows to keep the low frequency sensitivity of the TWM while probing an object in
motion. The proposed scheme can also be used with other adaptive two-beam
interferometers such as the p-emf based demodulator.
REFERENCES
1. Monchalin, J.-P., Optical Detection of Ultrasound, IEEE Trans. Sonics, Ultrasonics,
Freq. Control, UFFC-33, p. 485-499 (1986).
2. Monchalin, J.-P., Heon, R., Bouchard, P., Padioleau, C., Appl. Phys. Lett. 55, 1612
(1989).
3. Ing, R. K., J.-P. Monchalin, J.-P., Broadband Optical Detection of Ultrasound by TwoWave Mixing in a Photo refractive Crystal, Appl.Phys.Lett. Vol. 59, pp.3233-3235,
1991.
4. Blouin, A., Monchalin, J.-P., Detection of ultrasonic motion of a scattering surface by
two-wave mixing in a photorefractive GaAs crystal, Appl. Phys. Lett. 65, 932, (1994).
5. Delaye, P., Blouin, A., Drolet, D., de Montmorillon, L.-A., Roosen, G., Monchalin, J.P. Detection of ultrasonic motion of a scattering surface by photorefr active InP:Fe
under an applied field, Journal of the Optical Society of America, B 14, 1723, (1997).P
6. Petrov, M.P., Sokolov, LA., Stepanov, S.I., Trofimov, G.S. Non-steady-state photoelectromotive-force induced by dynamic gratings in partially compensated
photoconductors, J. Appl. Phys. 68, 2216, (1990).
7. Drolet, D., Blouin, A., Neron, C., Monchalin, J.-P., Specifications of an ultrasonic
receiver based on two-wave mixing in aphotorefractive GaAs impelmented in a laserultrasonic system, Review of Progress in Quantitative Nondestructive Evaluation, 15,
637, (1996).
280