Mokrane Malek

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.

Three-dimensional velocity near-wall measurements by digital in-line holography: calibration and results

Velocity measurements in the vicinity of an obstacle remain very complicated even when optical diagnostics based on displacement of micrometric tracers are considered. In the present paper, digital in-line holography with a divergent beam is proposed to measure the three-dimensional (3D) velocity vector fields in a turbulent boundary layer and, in particular, on the near wall region of a wind tunnel. The seeding droplets (1-5 μm) transported by a turbulent airflow are illuminated by a couple of laser pulses coming from a fiber coupled laser diode. These double exposure holograms are then recorded through a transparent glass reticle specially designed for this application with an accurate surface positioning combined with a particularly attractive in situ calibration method of the investigation volume (less than 10 mm(3)). The method used for processing holograms recorded in such a configuration is detailed. Our original calibration procedure and the assessment of its accuracy are presented. Our holographic probe has been tested in a wind tunnel for a large range of different velocities. Then 3D velocity vector fields extracted from more than 13000 holograms are analyzed. Statistical results show the capability of our approach to access in a turbulent boundary layer. In particular, it leads to relevant measurements for fluid mechanics such as velocity fluctuation and the shear stress in the very close vicinity of a wall.

Complex field recovering from in-line digital holography

This Letter presents an in-line digital holographic system that can provide full amplitude and phase reconstruction without any reference wave, with a single recorded hologram. This major capability is obtained by using a coherent mixing between several object waves generated by a pure spatial phase modulation. A scaling parameter permits us to reconstruct the phase without any unwrapping. The capability of the method to provide quantitative phase contrast measurement and numerical refocusing is demonstrated through experimental results.

Microtomography imaging of an isolated plant fiber: a digital holographic approach

This paper describes a method for optical projection tomography for the 3D in situ characterization of micrometric plant fibers. The proposed approach is based on digital holographic microscopy, the holographic capability being convenient to compensate for the runout of the fiber during rotations. The setup requires a telecentric alignment to prevent from the changes in the optical magnification, and calibration results show the very good experimental adjustment. Amplitude images are obtained from the set of recorded and digitally processed holograms. Refocusing of blurred images and correction of both runout and jitter are carried out to get appropriate amplitude images. The 3D data related to the plant fiber are computed from the set of images using a dedicated numerical processing. Experimental results exhibit the internal and external shapes of the plant fiber. These experimental results constitute the first attempt to obtain 3D data of flax fiber, about 12  μm×17  μm in apparent diameter, with a full-field optical tomography approach using light in the visible range.

Full-field and contact-less topography of nanometric thin films based on multiwavelength interferometry

This paper discusses a method to measure the thickness of thin layers deposited on a reflective substrate. A Michelson type interferometer with three wavelengths produces color interferences. A color sensor records the tint that is produced. The color interferences are approximated by a model based on the measurement of the laser intensities obtained with the reference mirror only. An iterative process leads to unambiguous algorithmic convergence and high accuracy thickness measurement. This method is simple, robust, compact, and single shot. The method does not need for angular scanning over the field of measurement (about 75mm2). The measurement on the surface yields a histogram of the thickness distribution and there is no requirement for any reference points (e.g. no need to make a groove or a walk on the layer). A thickness measurement performance of 50nm was demonstrated for homogenous polymer films deposited on silicon wafer. Set-up and digital image processing are discussed.

Digital Holographic Imaging Based on Shearing Interferometry

In-line and off-axis digital holography are now used in a wide range of domains such as fluid mechanics [1,2] and microscopy imaging [3-6]. In the basic in-line holographic configuration (no phase shifting, no sequential recording), the reference and object waves are parallel and the path lengths of both waves are equal. For this reason, the in-line holography is only able to provide an amplitude image of the object. Therefore, it can not be used to measure the phase contrast of any transparent object. For a multiple object recovering, such as particles [1], this method suffers from the presence of the intrinsic speckle noise generated by the twin images during the reconstruction. In the off-axis holography (single-shot recording), the reference wave is shaped to provide a spatial separation in the reconstruction plane or in the Fourier plane of the hologram. In this case, the virtual and real images are well spatially separated during the reconstruction. The use of an independent reference wave induces a sensibility to external perturbations such as vibrations, temperature changes, etc., and leads to an increase in the setup complexity. So as to simplify the setup and to get a real immunity to external perturbations, the in-line configuration is well adapted, but the faculty for the phase contrast recovering has to be invented. In order to overcome both the problems related to the reference wave in off-axis holography and the impossibility of measuring the phase contrast in in-line holography, we propose in this paper a new technique having new features in digital holography: i) simplify the recording set-up by eliminating the reference wave and ii) measure of the phase contrast of the object. This new approach in digital holography is based on the use of a spatial phase modulation so as to produce multiple replicas of the incoming diffracted wave at a given distance.

Topography of nanometric thin films with three-wavelength digital interferometry

Abstract. This paper discusses a method to measure the thickness of thin layers deposited on a reflective substrate. An interferometer with three wavelengths produces color interferences. A color sensor records the tint that is produced. The color interferences are approximated by a model based on the measurement of the laser intensities obtained with the reference mirror only. An iterative process leads to unambiguous algorithmic convergence and high accuracy thickness measurement. This method is simple, robust, compact, and single shot. The method does not need angular scanning over the field of measurement (about 75  mm2). The measurement on the surface yields a histogram of the thickness distribution and there is no requirement for any reference points (e.g., no need to make a groove or a walk on the layer). A thickness measurement performance of 50 nm was demonstrated for homogeneous polymer films deposited on a silicon wafer. The setup and digital image processing are discussed.