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GCLAD
Abstracts
Preprints available upon request
James N.
Caron, Gregory P. DiComo, and Sergei
Nikitin, "Generation of ultrasound in
materials using continuous-wave
lasers," Opt. Lett. 37,
830-832 (2012).
Generating and detecting ultrasound is
a standard method of nondestructive
evaluation of materials. Pulsed lasers
are used to generate ultrasound
remotely in situations that prohibit
the use of contact transducers. The
scanning rate is limited by the
repetition rates of the pulsed lasers,
ranging between 10 and 100 Hz for
lasers with sufficient pulse widths
and energies. Alternately, a
high-power continuous-wave laser can
be scanned across the surface,
creating an ultrasonic wavefront.
Since generation is continuous, the
scanning rate can be as much as 4
orders of magnitude higher than with
pulsed lasers. This paper introduces
the concept, comparing the theoretical
scanning speed with generation by
pulsed laser.
J.N. Caron, "Displacement and
Deflection Sensitivity of Gas-coupled
Laser Acoustic Detection," to be
published
in Laser Ultrasonics,
2008.
Ultrasound
radiated
from a surface can change the path of an
optical beam,
directed through the acoustic field and
parallel to the surface,
through acousto-optic interaction.
Sensing of the beam motion
with a position-sensitive detector
produces a simple but effective
non-contact ultrasound detector,
designated Gas-coupled Laser Acoustic
Detection, or GCLAD. Recent
research has shown that the received
signal is a combination of the
deflection and displacement of
beam. The technique proved capable
of detecting displacements of
the beam, created by a
transducer-generated airborne ultrasound
wave,
of less than a micrometer.
Deflections were recorded that
measured less than a microradian.
The presented work estimates
the sensitivity of GCLAD to an
ultrasonic surface displacement.
The results are compared to the
sensitivities of more standard
ultrasound detection methods.
J.N. Caron, "Displacement and
Deflection of an Optical Beam by
Airborne Ultrasound," published
in Review of
Progress in Quantitative
Nondestructive Evaluation, ed.
by D.O.
Thompson and D.E. Chimenti, AIP, Vol.
27A, 2008, p. 247.
Gas-Coupled
Laser
Acoustic Detection (GCLAD) enables
laser-based sensing of
ultrasound from a solid without contact
of the surface, and independent
of the optical
properties of the solid surface. A
probe laser beam, directed
parallel to the surface, intercepts the
ultrasound wave after
transmission to air. A split-cell
position-sensitive
photodetector senses changes in the
optical beam path created by the
disturbance. The interaction between the
probe beam and acoustic field
has typically been modeled as creating a
deflection in the beam. To
first order, sensitivity to deflection
improves by increasing the
distance from the interaction point to
the photodetector.
This paper describes this interaction as
a combination of displacement
and deflection of the optical beam.
Displacement occurs when the beam
is deflected twice by the acoustic field
such that the probe beam
is translated perpendicular to the
optical axis.
Experiments show that the sensitivity of
the displacement response is
comparable to the deflection
response. Sensing the displacement
can significantly decrease the system's
dependence of length. This
enables the miniaturization of the GCLAD
technique.
J.N. Caron, "Progress
towards
a portable laser-based ultrasound
sensor using gas-coupled laser
acoustic detection," Review of
Progress in Quantitative
Nondestructive Evaluation , Vol.
24, 2005.
Gas-Coupled
Laser
Acoustic
Detection (GCLAD) has proven to be a
viable alternative to
interferometric detection of ultrasound
for noncontact inspection of
materials. Unlike other laser-based
detection techniques, GCLAD
operates independently of the optical
properties of the sample surface.
Instead, the probe laser intercepts the
ultrasound wave after it has
been transmitted to air. The concept is
being researched as part of an
efficient, ultrasound sensor, with
hangar-to-hangar portability, for
interrogating flight-critical aircraft
structural supports. Areas of
active research include improving system
sensitivity and extending the
frequency response out to 10 MHz.
Research to this point has shown that
higher frequency waveforms can be
detected using this technique and
provide good sensitivity. Well-resolved
waveforms have been detected in
the test sample at 2.25 MHz. More
research is necessary to reach the
goal of detecting the signal from a 10
MHz signal. Improvements in the
electronic, optical and signal
processing methods are being considered.
J.N.
Caron,
"Multiple-beam
detection using
Gas-coupled Laser Acoustic Detection," Review of
Progress in
Quantitative Nondestructive Evaluation,
vol 20, 2000.
A
novel laser-based technique for the
detection of ultrasound radiated
from solid materials has been
developed. In this approach, a
probe beam is directed parallel to the
surface of a sample.
Ultrasonic waves in the solid are
detected when an acoustic wave is
radiated from the surface into the
ambient air, where the density
variations cause a beam
deflection. Because the laser
beam is not
reflected from the sample surface, the
technique is not dependent upon
the surface optical properties of the
material under
investigation. It is
particularly useful for testing
graphite/polymer composites and other
materials with poorly reflecting
surfaces. Gas-coupled laser
acoustic detection (GCLAD) has been
used to record well-resolved
through-transmission and
surface-acoustic
waveforms in various materials.
GCLAD has also been incorporated
into a C-scanning system where it has
been used to image subsurface
flaws in graphite/polymer composite
panels. Recent studies have
investigated the inspection of curved
surfaces. To this end, the
flanges and corner of an angled
graphite-reinforced composite panel
were scanned using this
technique. In addition, the
prospect of
using surface acoustic waves (SAWs)
for the interrogation of the skins
on multi-layer materials has also been
studied. Using GCLAD, Lamb
and Rayleigh waves have been detected
in composites, polymers, thin
metal films, and metal plates.
J.N.
Caron, Y. Yang, J.B. Mehl, and K.V.
Steiner, ``Gas coupled laser
acoustic detection for ultrasound
inspection of composite materials,
Vol. 58, No. 5, 2001, p. 667.
A
novel laser-based technique for the
detection of ultrasound radiated
from solid materials has been
developed. In this approach, a
probe beam is directed parallel to the
surface of a sample.
Ultrasonic waves in the solid are
detected when an acoustic wave is
radiated from the surface into the
ambient air, where the density
variations cause a beam
deflection. Because the laser
beam is not
reflected from the sample surface, the
technique is not dependent upon
the surface optical properties of the
material under
investigation. It is
particularly useful for testing
graphite/polymer composites and other
materials with poorly reflecting
surfaces. Gas-coupled laser
acoustic detection (GCLAD) has been
used to record well-resolved
through-transmission and
surface-acoustic
waveforms in various materials.
GCLAD has also been incorporated
into a C-scanning system where it has
been used to image subsurface
flaws in graphite/polymer composite
panels. Recent studies have
investigated the inspection of curved
surfaces. To this end, the
flanges and corner of an angled
graphite-reinforced composite panel
were scanned using this
technique. In addition, the
prospect of
using surface acoustic waves (SAWs)
for the interrogation of the skins
on multi-layer materials has also been
studied. Using GCLAD, Lamb
and Rayleigh waves have been detected
in composites, polymers, thin
metal films, and metal plates.
J.N Caron, Y.
Yang, J.B. Mehl and K.V. Steiner, "Gas-coupled
Laser Acoustic Detection at Ultrasonic
and Audible Frequencies,"
Review of Scientific Instruments,
vol 69(8), 1998, p. 2912.
Airborne acoustic waves have been
detected by a laser-beam deflection
technique in both the ultrasonic and
audio frequency ranges. For
ultrasonic applications, a probe beam
is directed parallel to the
surface of a sample. Ultrasonic waves
in the solid are detected when an
acoustic wave is radiated from the
surface into the ambient air, where
the density variations cause a beam
deflection. Gas-coupled laser
acoustic detection GCLAD! has
been used to record well-resolved
through-transmission and
surface-acoustic wave forms in various
materials. GCLAD has also been
incorporated into a C-scanning system
where it has been used to image
subsurface flaws in graphite/polymer
composite panels. Because the laser
beam is not reflected from the
sample surface, the technique is not
dependent upon the surface optical
properties of the material under
investigation. It is particularly
useful for testing graphite/polymer
composites and other materials with
rough surfaces. The beam-deflection
technique has been tested
quantitatively in the kHz frequency
range by passing a probe beam
through a cylindrical resonator. The
acoustic spectrum of the resonator
was measured from 4 to 13.5 kHz by
scanning the frequency of a source
and recording the acoustic field with
both a microphone and the
beam-deflection system. The acoustic
fields of the lower-frequency
modes are well known and enable both
qualitative and quantitative tests
of the beam-deflection technique.
Measurements on the lowest-frequency
plane-wave mode were used for absolute
calibration of the microphone.
The noise level of the beam-deflection
measurements at 4.3 kHz was
found to be 0.05 nrad~rms!,
corresponding to an acoustic pressure
of
0.005 Pa~rms.
J.N. Caron, Y.
Yang, J.B. Mehl and K.V. Steiner, "Thermoelastic
and Ablative Laser Generation of
Ultrasonic Waveforms in
Graphite/Polymer Composite Materials,"
submitted to the Journal
for
Applied Physics for
publication, but was never published,
May
1998.
A laser-based ultrasonic system was
used to study thermoelastic and
ablative ultrasonic laser generation
mechanisms in
graphite/polymer composite
structures. Ultrasonic waveforms
were generated in 16-layer
quasi-isotropic AS-4/PEEK composite
and
8-layer thick AS-4/PEKK [0/90]_{2S}
composite panels. Waveforms
generated onone side of the samples
were observed on the opposite side
using a confocal Fabry-Perot (CFP)
based detection system. The
waveforms, as functions of the
generation-laser power density, show
that there are two distinct generation
mechanisms. Below a
well-defined threshold power density,
the observed signals were
proportional to the power density and
are assumed to be generated
thermoelastically. Above the
threshold the observed waveforms are
a superposition of a thermoelastic
(TE) and ablatic (AB)
waveforms. The relative
amplitudes of the TE and AB components
were determined as a function of power
density by fitting a theoretical
model to the data. The ablation
threshold was independently
observed through photodetection of the
light radiated by the ablation
plume. Further experimentation
partially characterized the
directivity of the generation
mechanisms for the graphite/PEEK
composite panel.
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