Instrumentation for ice crystal characterization in laboratory using interferometric out-of-focus imaging.

Airborne characterization of ice crystals has important applications. The extreme difficulty of realizing in situ tests requires the development of a complete instrumentation in the laboratory. Such an installation should enable design, development, test, and calibration of instruments in conditions as close as possible to real ones. We present a set of numerical and experimental tools that have been developed to realize ice crystal sensors based on interferometric particle imaging. The set of tools covers the development of complementary simulators for crystal growth and interferometric particle imaging predictions, experimental generation of "programmable" ice crystals, and instrumentation of a freezing column where different techniques as in-focus imaging, out-of-focus imaging, and digital in-line holography can be combined simultaneously for test and calibration.

3D-shape recognition and size measurement of irregular rough particles using multi-views interferometric out-of-focus imaging.

We realize simplified-tomography experiments on irregular rough particles using interferometric out-of-focus imaging. Using two angles of view, we determine the global 3D-shape, the dimensions, and the 3D-orientation of irregular rough particles whose morphologies belong to families such as sticks, plates, and crosses.

Interferometric out-of-focus imaging of ice particles with overlapping images.

It is shown that the size and relative positions of two irregular rough particles can be analyzed using interferometric out-of-focus imaging despite the overlapping of their out-of-focus images. Simulations are confirmed by experiments done with ice particles generated in a freezing column.

Dual-wavelength digital holography for 3D particle image velocimetry: experimental validation.

A multi-exposure digital in-line hologram of a particle field is recorded by two successive pulses of different wavelengths. During the reconstruction step, each recording can be independently analyzed by selecting a given wavelength. This procedure enables avoiding the superimposition of particle images that may be close to each other.

Simultaneous 3D location and size measurement of bubbles and sand particles in a flow using interferometric particle imaging.

We present a system to characterize a triphasic flow in a 3D volume (air bubbles and solid irregular particles in water) using only one CCD sensor. A cylindrical interferometric out-of-focus imaging setup is used to determine simultaneously the 3D position and the size of bubbles and irregular sand particles in a flow. The 3D position of the particles is deduced from the ellipticity of their out-of-focus image. The size of bubbles is deduced from analysis of interference fringes. The characteristics of irregular sand particles are obtained from analysis of their speckle-like pattern. Experiments are confirmed by simulations.

Long time exposure digital in-line holography for 3-D particle trajectography.

One advantage of digital in-line holography is the ability for a user to know the 3-D location of a moving particle recorded at a given time. When the time exposure is much larger than the time required for grabbing the particle image at a given location, the diffraction pattern is spread along the trajectory of this particle. This can be seen as a convolution between the diffraction pattern and a blurring function resulting from the motion of the particle during the camera exposure. This article shows that the reconstruction of holograms recorded under such conditions exhibit traces that could be processed for extracting 3D trajectories.

Suppression of the Moiré effect in sub-picosecond digital in-line holography.

Digital in-line holography is widely used in flow studies where the 3D position, size and velocity of particles or fibers have to be known at a given time. When holograms are recorded by using a sub-picosecond laser the enlargement of the spectral distribution acts as a spatial low-pass filter over the intensity distribution of the diffraction pattern. This is not a disadvantage as regards to the spatial sampling. Indeed, the Moiré effect, due to the sub-sampling of the fringe pattern is naturally reduced. Experimental results are provided.

Application of the fractional Fourier transformation to digital holography recorded by an elliptical, astigmatic Gaussian beam.

The authors have studied the diffraction pattern produced by a particle field illuminated by an elliptic and astigmatic Gaussian beam. They demonstrate that the bidimensional fractional Fourier transformation is a mathematically suitable tool to analyse the diffraction pattern generated not only by a collimated plane wave [J. Opt. Soc. Am A 19, 1537 (2002)], but also by an elliptic and astigmatic Gaussian beam when two different fractional orders are considered. Simulations and experimental results are presented.

Digital in-line holography: influence of the shadow density on particle field extraction.

We have used a digital in-line holography system with numerical reconstruction for 3D particle field extraction. In this system the diffraction patterns (holograms) are directly recorded on a charge-coupled device (CCD) camera. The numerical reconstruction is based on the wavelet transformation method. A sample volume is reconstructed by computing the wavelet components for different scale parameters. These parameters are related to the axial distance between a particle and the CCD camera. The particle images are identified and localized by analyzing the maximum of the wavelet transform modulus and the equivalent diameter of the particle image. The general process for the 3D particle location and data processing method are presented. As in classical holography we found that the signal to noise ratio depends only on the shadow density. Nevertheless, we show that both the volume depth and the shadow density affect the percentage of extracted particles.

Particle field digital holographic reconstruction in arbitrary tilted planes.

Digital holography is applied to the reconstruction of small particles in a plane whose orientation is arbitrary as specified by the user. The diffraction pattern produced by the particles is directly recorded by a conventional CCD camera. The digital recorded image enables the recovery of particle-images in several parallel planes of the probe volume. Afterwards, an interrogation slice corresponding to a thin layer around a theoretical arbitrary tilted plane is fixed. The pixels whose 3D coordinates belong to this slice are selected and juxtaposed to rebuild the particle images. The feasibility is demonstrated on a fiber tilted with respect to the camera plane. A second example is given on an experimental particle field. These results let us predict future applications such as the characterization of particle fields in planes other than those parallel with the camera plane.

Characterization of chirped pulses with the fractional-order Fourier transformation.

We show that the fractional-order Fourier transformation (FRT) is a suitable method to describe chirped pulses submitted to group-velocity dispersion in a linear dispersive medium. Amplitudes exhibiting different chirp coefficients are easily separated with the FRT, although they are temporally superposed.