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GCLAD Abstracts
Preprints available upon request
J.N. Caron
and P Kunapareddy, "Atypical Applications for
Gas-coupled Laser Acoustic Detection,"
presented on June 25, 2013, at the 3rd
International Symposium on Laser
Ultrasonics and Advanced Sensing,
in Tokahoma, Japan.
Gas-coupled laser acoustic detection
(GCLAD) was primarily developed to
sense laser-generated ultrasound in
composite materials. In a typical
setup, a laser beam is directed
parallel to the material
surface. Radiated ultrasound
waves deflect or displace the probe
beam resulting from changes in the
air's index of refraction. A
position-sensitive photodetector
senses the beam movement, and produces
a signal proportional to the
ultrasound wave. In this paper,
we discuss three applications of GCLAD
that take advantage of the unique
detection characteristics.
Directivity patterns of ultrasound
amplitude in water demonstrate the use
of GCLAD as a directional
hydrophone. We also demonstrate
the sensing of waveforms from a
gelatin. The gelatin mimics
ultrasound propagation through skin
tissues. Lastly, we show how
GCLAD can be used as a line receiver
for continuous laser generation of
ultrasound. CLGU may enable
ultrasound scanning at rates that are
orders of magnitude faster than
current methods.
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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