Denis Mounier

Time-averaged digital holography

We demonstrate that it is possible to study the modal structures of a vibrating object with digitally recorded holograms by use of the time-averaging principle. We investigate the numerical reconstruction from a theoretical point of view, and we show that the numerically reconstructed object from a digital hologram is modulated by the zeroth-order Bessel function. Results of experiments in time-averaged digital holography are presented.

Some opportunities for vibration analysis with time averaging in digital Fresnel holography

Features offered by the combination of time averaging and digital Fresnel holography are investigated. In particular, we introduce the concept of the zero-crossing phase of Bessel fringes, which allows a highly contrasted determination of the dark fringes in the hologram. We discuss some particularities of the digital reconstruction and show how time-averaged digital holography can be used to study vibration drifts. Experiment results are presented in the case of a loudspeaker under a sinusoidal excitation; digital and analogical holography are also compared.

Twin-sensitivity measurement by spatial multiplexing of digitally recorded holograms

We present an all-numeric multiplexing/demultiplexing technique for digitally recorded holograms that allow the simultaneous determination of the in-plane and the out-of-plane components of the displacement vector of an object submitted to some loading. The twin-sensitivity measurement is obtained from two different illumination directions that give two sensitivity vectors. The spatial multiplexing is achieved by an incoherent mixing of two duplets of coherent waves that produce holograms carried with orthogonal polarized reference waves. The spatial demultiplexing uses the autocorrelation function of the multiplexed holograms as a position estimator. The estimator then enables the determination of the pixel-to-pixel correspondence between the holograms for the double component determination. Results of the experiment are presented.

High-resolution digital two-color holographic metrology.

An algorithm designed for digital color holography is presented. It is based on a filter bank allowing full object reconstruction with a pixel pitch being invariant along each wavelength. Its application to high-resolution simultaneous two-wavelength metrology is described.

Method of digital holographic recording and reconstruction using a stacked color image sensor

We discuss a method to record and reconstruct color holograms by using a stack of photodiode sensors associated to a one-way reference beam. The reconstruction algorithm follows a convolution strategy in which a transverse magnification leads to the full reconstruction of the object in the reconstructed horizon. The transverse magnification of the object depends on the curvature of the reference wave. Analysis of the spatial resolution indicates that it is linked to the transversal magnification but that no extra information is gained or lost in the process. Experimental results confirm the validity of the proposed approach for two-color digital holography. The error due to spectral mixing is investigated and found to be quite irrelevant compared to the range of the phase measurement.

Spatial bandwidth extended reconstruction for digital color Fresnel holograms

This paper presents a reconstruction algorithm based on the convolution formula of diffraction which uses the Fresnel impulse response of free space propagation. The bandwidth of the reconstructing convolution kernel is extended to the one of the object in order to allow the direct reconstruction of objects with size quite larger than the recording area. The spatial bandwidth extension is made possible by the use of a numerical spherical wave as a virtual reconstructing wave, thus modifying the virtual reconstruction distance and increasing the kernel bandwidth. Experimental results confirm the suitability of the proposed method in the case of the simultaneous recording of two-color digital holograms by using a spatial color multiplexing scheme.

Picosecond acoustics in p-doped piezoelectric semiconductors

We demonstrate by experiment and theoretical analysis that the presence of built-in electric fields near the (111) and (1¯1¯1¯) surfaces of p-doped GaAs causes efficient generation of acoustic waves due to the laser-induced inverse piezoelectric effect. At the same time, the generation efficiency from the electron-hole-phonon deformation potential is shown to be reduced. The polarity of the acoustic pulse is inverted when changing the laser irradiated surface from (111) to (1¯1¯1¯). The results have ramifications for optically controlled piezoelectric ultrasound transducers.

Jones matrix formalism for the theory of picosecond shear acoustic pulse detection

A theoretical analysis of the transient optical reflectivity of a sample by a normalized Jones matrix is presented. The off-diagonal components of the normalized matrix are identified with the complex rotation of the polarization ellipse. Transient optical polarimetry is a relevant technique to detect shear acoustic strain pulses propagating normally to the surface of an optically isotropic sample. Moreover, polarimetry has a selective sensitivity to shear waves, as this technique cannot detect longitudinal waves that propagate normally to the sample surface.

Laser ultrasonics detection of an embedded crack in a composite spherical particle

Laser ultrasonics was applied to the manufacturing control of the integrity (no failure) of coated spherical particles designed for High Temperature Reactors (HTR). This control is of major importance, since the coating of the nuclear fuel kernel is designed to prevent from the diffusion of fission products outside the particle during reactor operation. The SiC layer composing the coating is particularly important, since this layer must be an impenetrable barrier for fission products. The integrity of the SiC shell (no crack within the shell) can be assessed by the ultrasonic vibration spectrum of the HTR particle, which is significantly changed, compared to the reference spectrum of a defect-free particle. Spheroidal vibration modes of defect-free dummy particles with a zirconium dioxide (ZrO(2)) core were observed in the 2-5MHz range. A theoretical analysis is presented to account for the observed vibration spectra of defect-free or cracked HTR particles.

On the possibility of ultrashort shear acoustic pulse excitation due to the laser-induced electrostrictive effect

A simple theory is developed and numerical estimates presented to demonstrate the possibility of exciting ultrashort acoustic pulses in optically transparent materials by laser-induced electrostrictive surface mechanical stresses. Both longitudinal and shear coherent acoustic pulses can be excited. The conditions for which the polarization of ultrashort shear pulses are controlled by the polarization of an incident laser pulse are established. The profiles and durations of longitudinal and shear acoustic strain pulses are shown to coincide with laser pulse intensity envelope.