(Received 27 June 2011; revised 26 October 2011; accepted for publication 22 November 2011;published 21 December 2011)Purpose: Measured values of ultrasound attenuation in bone represent a combination of differentloss mechanisms. As a wave is transmitted from a ﬂuid into bone, reﬂections occur at the interface.In the bone, mode conversion occurs between longitudinal and shear modes and the mechanicalwave is scattered by its complex internal microstructure. Finally, part of the wave energy isabsorbed by the bone and converted into heat. Due to the complexity of the wave propagation andthe difﬁculty in performing measurements that are capable of separating the various loss mecha-nisms, there are currently no estimates of the absorption in bone. The aim of this paper is, thus, toquantify the attenuation, scattering, and thermal absorption in bone.Methods: An attenuating model of wave propagation in bone is established and used to develop athree-dimensional ﬁnite difference time domain numerical algorithm. Hydrophone and optical het-erodyne interferometer measurements of the acoustic ﬁeld as well as a x-ray microtomography ofthe bone sample are used to drive the simulations and to measure the attenuation. The acousticmeasurements are performed concurrently with an infrared camera that can measure the tempera-ture elevation during insonication. A link between the temperature and the absorption via a three-dimensional thermal simulation is then used to quantify the absorption coefﬁcients for longitudinaland shear waves in cortical bone.Results: We demonstrate that only a small part of the attenuation is due to absorption in bone andthat the majority of the attenuation is due to reﬂection, scattering, and mode conversion. In the ninesamples of a human used for the study, the absorption time constant for cortical bone was deter-mined to be 1.04 ls628%. This corresponds to a longitudinal absorption of 2.7 dB/cm and a shearabsorption of 5.4 dB/cm. The experimentally measured attenuation across the approximately 8 mmthick samples was 13.360.97 dB/cm.Conclusions: This ﬁrst measurement of ultrasound absorption in bone can be used to estimate theamount of heat deposition based on knowledge of the acoustic ﬁeld.

Attenuation, scattering, and absorption of ultrasound in the skull bone

Lamb waves are extensively involved in plate structure inspection because of their guided nature. However, their dispersive nature often limits their use in flaw detection. In this paper we show that the use of a time-reversal mirror (TRM) allows to automatically compensate for the dispersive nature of Lamb waves. Experiments showing the spatial and temporal behavior of time-reversed Lamb waves, demonstrate the ability of TRMs to self-focus and to recompress dispersive pulses. This is demonstrated in a set of experiments in which a broadband ultrasonic laser source is used to simulate a point Lamb wave source and an optical interferometer is used to map the time reversed elastic field. We also show that TRM may work in pulse echo mode and allows to detect and to focus along large 2-D plates on any flaws located in the inspected area.

Time-reversed Lamb waves

We report on a noncontact photoacoustic imaging (PAI) technique in which a low-coherence interferometer [(LCI), optical coherence tomography (OCT) hardware] is utilized as the acoustic detector. A synchronization approach is used to lock the LCI system at its highly sensitive region for photoacoustic detection. The technique is experimentally verified by the imaging of a scattering phantom embedded with hairs and the blood vessels within a mouse ear in vitro. The system’s axial and lateral resolutions are evaluated at 60 and 30 μm, respectively. The experimental results indicate that PAI in a noncontact detection mode is possible with high resolution and high bandwidth. The proposed approach lends itself to a natural integration of PAI with OCT, rather than a combination of two separate and independent systems.

Noncontact photoacoustic imaging achieved by using a low-coherence interferometer as the acoustic detector.

Television holography (TVH) can be defined as `the family of optical measurement techniques based on the electronic recording and processing of holograms'. Image-plane TVH was introduced in the early 1970s with the name `electronic speckle-pattern interferometry' (ESPI). Since then, TVH has undergone an impressive development and become one of the most promising optical techniques for non-destructive testing and industrial inspection. The aim of this review is to propose an original scheme for the systematic treatment of TVH and to review the existing techniques according to it. In this approach we split the measurement process into four highly independent stages (illumination and observation geometry, temporal treatment, secondary-correlogram generation and fringe-pattern analysis) and establish a common notation to formulate the corresponding techniques. Such a strategy allows the free combination of the techniques proposed for each stage as building blocks to obtain every particular variant of the whole TVH measurement process, whether it has already been reported or not, and also the incorporation of new techniques while retaining compatibility with the existing variants of the previous and following stages.

/pdf/a-systematic-approach-to-tv-holography-44h8p1g35o.pdf

A systematic approach to TV holography

Laser welding has been applied to various industries, in particular, automotive, aerospace and microelectronics. However, traditional off-line testing of the welds is costly and inefficient. Therefore, on-line inspection systems with low cost have being developed to increase productivity and maintain high welding quality. This paper presents the applications of acoustic, optical, visual, thermal and ultrasonic techniques and latest development of laser welding monitoring. The advantages and limitations of these techniques are also discussed.

https://iopscience.iop.org/article/10.1088/1742-6596/15/1/017/pdf

Review of techniques for on-line monitoring and inspection of laser welding

Heterodyne interferometers using two‐wave mixing in photorefractive cubic crystals for ultrasound detection on rough surfaces are demonstrated. The speckled scattered beam from a rough surface sample interferes with a planar coherent pump beam inside a photorefractive crystal. A third frequency‐shifted beam is used to read the grating. The diffracted readout beam and the transmitted signal beam are wavefront matched, resulting in an optimal heterodyne interference signal. The signal to noise ratio for the two commonly used crystallographic configurations with cubic crystals, G∥〈110〉∥ and G∥〈001〉, where G is the grating wave vector, are investigated. Very good sensitivity is demonstrated for the detection of small amplitude ultrasonic surface displacements.

Heterodyne interferometer with two‐wave mixing in photorefractive crystals for ultrasound detection on rough surfaces

A video camera records a series of interferometric images of a test object, while the object is subjected to a repeated and varying level of stress. At least one pair of interferometric images, taken during the same video frame, and corresponding to different levels of stress, are added together, in an analog manner, to form a composite image which is then stored in digital form. During the next video frame, the phase of the light used to generate the interferometric image is altered, and the process is repeated. The digitally-stored composite images obtained during two successive video frames are subtracted from each other to produce a second-order composite image which reveals information about the condition of the object. The second-order composite image includes fringes of high contrast, and is not adversely affected by most ambient noise.

Apparatus and method for nondestructive testing using additive-subtractive phase-modulated interferometry

In this paper, a video-based speckle interferometric method using a continuous reference updating technique is presented. Unlike conventional ESPI techniques, this methodsynchronizes the optical interferometric detection system with the acoustic stressing of the test object, and includes continuous renewal of the reference image. It is shown that the susceptibility of the method to environmental noise caused by vibration, temperature gradients, or thermal currents is substantially lower than that of conventional techniques. The application of this technique to the detection of defects in adhesively bonded structures is demonstrated.

Synchronized reference updating technique for electronic speckle interferometry

A holographic interferometer that uses two-wave mixing in a photorefractive (Bi12SiO20) crystal under an applied ac field is described. The interferometer uses a repetitive sequence of separate record and readout times to obtain quasi real-time holographic interferograms of vibrating objects. It is shown that a good signal-to-noise ratio of the interferometer is obtained by turning off the object illumination and the applied ac field during readout of the hologram. The good signal-to-noise ratio of the resulting holographic interferograms enables phase measurement, which allows for quantitative deformation analysis.

Dynamic holographic interferometry by photorefractive crystals for quantitative deformation measurements.

We demonstrate the linear transformation of small phase excursions of a speckled wavefront into modulation of the transmitted intensity using the polarization self-modulation effect in photoconductive electro-optic crystals. The technique is more advantageous than the widely used photorefractive two-wave mixing technique.

Bruno F. Pouet

Papers

Heterodyne interferometer with two‐wave mixing in photorefractive crystals for ultrasound detection on rough surfaces

Apparatus and method for nondestructive testing using additive-subtractive phase-modulated interferometry

Synchronized reference updating technique for electronic speckle interferometry

Dynamic holographic interferometry by photorefractive crystals for quantitative deformation measurements.

Adaptive interferometer using self-induced electro-optic modulation