Photoacoustic tomography (PAT) and ultrasonography (US) of biological tissues usually rely on ultrasonic transducers
for the detection of ultrasound. For an optimum sensitivity, transducers require a physical contact with the tissue using a
coupling fluid (water or gel). Such a contact is a major drawback in important potential applications such as surgical
procedures on human beings and small animal imaging in research laboratories. On the other hand, laser ultrasonics (LU)
is a well established optical technique for the non-contact generation and detection of ultrasound in industrial materials.
In this paper, the remote optical detection scheme used in industrial LU is adapted to allow the detection of ultrasound in
biological tissues while remaining below laser exposure safety limits. Both non-contact PAT (NCPAT) and non-contact
US (NCUS) are considered experimentally using a high-power single-frequency detection laser emitting suitably shaped
pulses and a confocal Fabry-Perot interferometer in differential configuration. It is shown that an acceptable sensitivity is
obtained while remaining below the maximum permissible exposure (MPE) of biological tissues. Results were obtained
ex vivo on chicken breast specimens with embedded inclusions simulating blood vessels optical properties. Sub-mm
inclusions are readily detected at depths approaching 1 cm. The method is expected to be applicable to living tissues.
Different algorithms for performing Fourier transforms with unequally sampled data in wavenumber space for
Fourier-domain optical coherence tomography are considered. The efficiency of these algorithms is evaluated
from point-spread functions obtained with a swept-source optical coherence tomography system and from computational
time. Images of a 4-layer phantom processed with these different algorithms are compared. We show that convolving the data with an optimized Kaiser-Bessel window allowing a small oversampling factor before computing the fast Fourier transform provides the optimal trade-off between image quality and computational time.
Optical coherence tomography was used to collect cross-sectional images of glass powder beds consisting of microspheres with diameters ranging from 8 to 175 µm. Images were formed by a collection of individual interferogram envelopes that give the backscattered light amplitude as a function of the optical path in the glass powder bed. The diameter distribution, for microspheres located near the surface of the beds, is obtained by appropriate peak distance measurements on threshold-selected envelopes after having performed the surface profilometry. The measured distributions are in good agreement with those obtained by laser diffraction. When considering the whole powder volume, the evaluation of the mean light penetration depth inside the powder beds proves to be a useful approach to evaluate the mean particle diameter, although no information is obtained on the actual particle size distribution in this case. Two simplified models are introduced to understand the linear relationship observed between the penetration depth and the mean particle size.
We developed optical tissue phantoms with a novel combination of matrix and scatterers. These phantoms have a well
known scattering microstructure of monodisperse silica microspheres, embedded in elastic silicone. We characterize
their mechanical properties and, some of their optical properties. We also validate the control over the density of
scatterers achieved with our proposed fabrication technique. The properties obtained are a practical combination of
deformability, durability and simplicity of the microstructure. These are illustrated by results on speckle statistics in
optical coherence tomography.
Much of the current activity in optical coherence tomography aims at increasing the image resolution. Nowadays,
two kinds of OCT techniques are available. The first approach is the Time-Domain OCT (TD-OCT) which
usually relies on a moving part into the reference arm to probe the sample in depth. The second approach is
the Fourier-Domain OCT (FD-OCT) in which the signal is acquired as a function of the wavelength and the
depth profile of the sample is obtained by Fourier transform. Theoretically, in both techniques, the resolution is
limited by the central wavelength of the source and by its full width at half maximum. Nevertheless, it is shown
in this paper that this resolution may be improved by using deconvolution technique based on Wiener filtering
and Autoregressive Spectrum Extrapolation (ASE). In our experiment, thanks to deconvolution an improvement
of a factor up to 4 is obtained in TD-OCT and about 2 in FD-OCT. As an illustration, the approach is applied
to TD and FD-OCT measurements of the profile of a carbon-epoxy composite to evaluate the performance in
determining the thickness of the upper layer within a resolution better than that provided by the conventional
processing of the OCT envelope.
A laser-ultrasonic technique is described to non-destructively determine residual stresses in metals such as those produced by shot peening. The method is based on monitoring the small ultrasonic velocity change of the laser-generated surface skimming longitudinal wave (LSSLW) propagating just below the surface. The main advantage of using LSSLW is that the effect of surface roughness induced by shot peening is greatly reduced compared to using surface acoustic waves (SAW). To improve resolution in the measurement of small velocity changes, a cross-correlation technique is used with a reference signal taken on the same but unstressed material in similar conditions. Also, the low-frequency SAW can be used to correct the LSSLW results when affected by minute changes in the path length during the measurements. The validity of the approach is demonstrated by measuring quantitatively the near surface stress in a four-point bending experiment with different levels of surface roughness. Then, scanning results on properly and improperly laser shock peened samples are reported. In particular, the LSSLW velocity variations for the properly peened samples clearly show an increase in the laser-peened area well indicative of a compressive stress.
This paper discusses the design and fabrication of ultra lightweight laser scanning mirrors from two types of metal-matrix composites for the next generation Space Vision System (SVS). The materials selected for this study were SiC particulate reinforced aluminum composite and beryllium-aluminum (AlBeMet) composite. Three mirror designs were made and compared in terms of mass, rotating inertia and first mode natural frequency. Mirror surface layer selection and processing were discussed. Problems encountered during the mirror fabrication and the ways to solve it were presented.
Corrosion has been recognized as a serious problem in the maintenance of aging aircraft. The Industrial Materials Institute (IMI) has explored the use of laser-ultrasonics for the detection of hidden corrosion in metallic lap joint structures. For inspection with painted surfaces, IMI has shown that a resonance spectroscopy approach using a simple two-layer model can be used to determine the thickness of the paint layer and of the top metal skin. Validation of the model has been made using a test sample with a broad range of paint thickness. Once combined with a numerical inversion method, the model is used to produce a thickness map of the top metal skin from measured resonance frequencies. Results from standard samples with flat-bottom holes showed that the laser-ultrasonic technique could detect metal loss below 1%. The reliability of the method was also demonstrated on accelerated corrosion samples. Comparison to X-ray images showed that the laser-ultrasonic method presented a thickness map that had the same accuracy as the X-ray system without the need for dismantling the sample. These results indicated that laser-ultrasonics could be a useful tool not only to inspect aircraft during routine maintenance but also to provide valuable data in the study of corrosion inception and growth in lap joint structures.
In this paper we explore laser induced breakdown spectroscopy (LIBS) at relatively low energies in the range 10 -
350 tJ. We present measurements ofthe threshold laser energy needed for LIBS and the scaling of plasma size and crater
size with energy. The effects of the laser pulse length and gating of the detector on the LIB spectra are studied and we also
assess the use ofmicrojoule LIBS for the identification ofAl alloys.
In North America, most of the water mains pipes have been fabricated with either cast or ductile iron. It is well known that unprotected iron water mains are prone to corrosion resulting sometimes in serious metal loss and even major water leaks. At present time, water utilities have thus to replace parts of their water distribution pipes ont he basis of their average age, the number of breaks per kilometer per year, the hydraulic efficiency and water quality. In this paper, an ultrasonic technique which will enable the in-situ measurement of the pipe wall thickness is presented as an alternative to these inaccurate criteria. Such a technique can be used to map the defects along a water main and to evaluate their severity even if a mortar lining has been used to avoid the formation of tubercles. This technique uses water as the ultrasonic couplant and will not notably disturb the flow. The novelty of the approach relies on the use of a specialized model for ultrasonic propagation in multilayers coupled with a variable gain amplifier which increases significantly the dynamic range of the ultrasonic measurement systems. This system has been successfully used to produce images of corroded or graphitized areas in cast iron pipes of 6 inches nd 8 inches in diameter. The ultrasonic images agree well with the corresponding optical images. Thickness measurements have been also performed on water mains specimens around the defects found. The results indicate that the remaining iron thickness in the corroded regions can be estimated, in most cases, after proper signal processing.