Jean-Michel Desse

Digital three-color holographic interferometry for flow analysis

A digital three-color holographic interferometer was designed to analyze the variations in refractive index induced by a candle flame. Color holograms are generated and recorded with a three layer photodiode stack sensor allowing a simultaneous recording with a high spatial resolution. Phase maps are calculated using Fourier transform and spectral filtering is applied to eliminate parasitic diffraction orders. Then, the contribution along each color is obtained with the simultaneous three wavelength measurement. Results in the case of the candle flame are presented. Zero order fringe, meaning zero optical path difference, can be easily extracted from the experimental data, either by considering a modeled colored fringe pattern or the wrapped phases along the three wavelengths.

Three-color differential interferometry

It is shown that differential interferometry using a Wollaston prism and a three-color laser source is an optical technique that has all the advantages of differential interferometry in polarized white light and of classical monochromatic interferometry. The interference fringe pattern obtained is very large and colored and presents a central white fringe that enables easy identification of the zero order of the interferogram. The three-color source is obtained by filtering the unwanted lines of the ionized laser (mixed argon and krypton) and balancing the three red, green, and blue lines by a technique that involves placing birefringent plates between the polarizer and the analyzer, the thickness of which has been calculated to create a natural filter. The unsteady aerodynamic flow downstream of a diamond shape airfoil has been visualized with this technique, which shows that the power of the light source is sufficient to record the interferograms at a high rate.

Real-time color holographic interferometry

A new optical technique based on real-time color holographic interferometry has been developed for analyzing unsteady aerodynamic wakes in fluid mechanics or for measuring displacements and deformations in solid mechanics. The techniques feasibility is demonstrated here. It uses three coherent wavelengths produced simultaneously by a cw laser (mixed argon and krypton). Holograms are recorded on single-layer panchromatic silver halide (Slavich PFG 03C) plates. Results show the optical setup can be adjusted to obtain a uniform background color. The interference fringe pattern visualized is large and colored and exhibits a single central white fringe, which makes the zero order of the interferogram easy to identify. An application in a subsonic wind tunnel is presented, in which the unsteady wake past a cylinder is recorded at high rate.

Sensor influence in digital 3λ holographic interferometry

In digital holographic interferometry, the resolution of the reconstructed hologram depends on the pixel size and pixel number of the sensor used for recording. When different wavelengths are simultaneously used as a luminous source for the interferometer, the shape and the overlapping of three filters of a color sensor strongly influence the three reconstructed images. This problem can be directly visualized in 2D Fourier planes on red, green and blue channels. To better understand this problem and to avoid parasitic images generated at the reconstruction, three different sensors have been tested: a CCD sensor equipped with a Bayer filter, a Foveon sensor and a 3CCD sensor. The first one is a Bayer mosaic where one half of the pixels detect the green color and only one-quarter detect the red or blue color. As the missing data are interpolated among color detection positions, offsets and artifacts are generated. The second one is a specific sensor constituted with three stacked photodiode layers. Its technology is different from that of the classical color mosaic sensor because each pixel location detects the three colors simultaneously. So, the three colors are recorded simultaneously with identical spatial resolution, which corresponds to the spatial resolution of the sensor. However, the spectral curve of the sensor is large along each wavelength since the color segmentation is based on the penetration depth of the photons in silicon. Finally, with a 3CCD sensor, each image is recorded on three different sensors with the same resolution. In order to test the sensor influence, we have developed a specific optical bench which allows the near wake flow around a circular cylinder at Mach 0.45 to be characterized. Finally, best results have been obtained with the 3CDD sensor.

Stochastic digital holography for visualizing inside strongly refracting transparent objects

This paper presents a digital holographic method to visualize and measure refractive index variations, convection currents, or thermal gradients, occurring inside a transparent and refracting object. The proof of principle is provided through the visualization of refractive index variation inside a lighting bulb. Comparison with transmission and reflection holography is also provided. A very good agreement is obtained, thus validating the proposed approach.

Real-time color holographic interferometry devoted to 2D unsteady wake flows

A new optical technique based on real time holographic interferometry in true colors has been implemented around the transonic wind tunnel of the ONERA-Lille center to analyze 2D unsteady wake flows. Tests realized in color interferometry, real time and double exposure, use simultaneously three wavelengths of a continuous waves laser (argon and krypton mixed) and holograms are recorded on silver-halide single-layer panchromatic Slavich PFG03c plates. The very principle of real-time true color holographic interferometry uses three primary wavelengths (red, green and blue) to record, under no-flow conditions, the interference among the three measurement beams and the three reference beams simultaneously on a single reference hologram. After the holographic plate is developed, it is placed on the test setup again in the position it occupied during exposure and the hologram is illuminated again by the three reference beams and three measurement beams. A flat, uniform color can then be observed behind the hologram. So a horizontal, vertical, or even circular fringe pattern can be formed and the achromatic central fringe can be made out very clearly. The single color is used to determine the path difference zero on the interferograms. The flow studied was the unsteady flow downstream of a cylinder placed crosswise in the test section. A sequence of hundred interferograms was recorded on the flow around the cylinder at Mach 0.37. The vortex formation and dissipation phases can be seen very clearly, along with the fringe beat to either side of the cylinder.

State of the Art of Color Interferometry at ONERA

At ONERA-Lille center a lot of studies have been conducted to characterize complex flows using an optical method based on differential interferometry with Wollaston prism and white polarized light source. Several applications are presented in two-dimensional and axisymmetric flows and in a gaseous mixture where the two gases interface is submitted to acceleration. Then, real-time color holographic interferometry (RCHI) has been developed to obtain the refractive index itself in two dimensional wake flow. The last improvements concern the extension of this method for analyzing three dimensional flows. The authors present a specific setup defined in a single sight direction, the aim being to reproduce the same optical setup along several sight directions, each shifted by a given angle. This optical technique uses reflection holograms where the diffraction efficiency of plates is strongly influenced by the variations in the gelatin thickness produced during the holograms treatment. Problems are discussed and solutions are proposed to control the gelatin shrinkage for two different types of used holograms. The results obtained in a one sight direction make it possible to build in the future an optical setup allowing several simultaneous line-of-sight optical measurements.

Robust processing of phase dislocations based on combined unwrapping and inpainting approaches

This Letter proposes a robust processing of phase dislocations to recover continuous phase maps. The approach is based on combined unwrapping and inpainting methods. Phase dislocations are determined using an estimator based on the second order phase gradient. The algorithm is validated using a realistic simulation of phase dislocations, and the phase restoration exhibits only weak errors. A comparison with other inpainting algorithms is also provided, demonstrating the suitability of the approach. The approach is applied to experimental data from off-axis digital holographic interferometry. The phase dislocation from phase data from a wake flow at Mach 0.73 are identified and processed. Excellent phase restoration can be appreciated.

Digital Mach-Zehnder Holographic Interferometer Using Pulsed Laser for Analyzing Large Flow Fields

A digital Mach-Zehnder holographic interferometer is designed for analyzing large flow fields. The optical setup produces very narrowed interference fringes which are analyzed by 2D FFT. Results are shown on the unsteady wake flow around a circular cylinder at Mach 0.75.

Wake Flows Analysis by Digital Color Holographic Interferometry

Digital 3? holographic interferometry is shown for analyzing the variations in the refractive index induced by the wakeflow around a circular cylinder.