Bernard Tilkens

Almost-common path interferometer using the separation of polarization states for digital phase-shifting shearography

An original experimental setup for shearography with metrological applications is presented. The simplicity and the efficiency of the setup are provided by a shearing device, a prism that separates the TE and TM polarization modes with a coating and a thin glass plate placed on its face. The use of this shearing device enables an in-line and almost-common path configuration for the shearing interferometer, a path that leads to high stability and a low sensitivity to external disturbances. Moreover, the sensitivity of the interferometer can be easily adjusted for different applications by varying the shearing amount with glass plates of different thicknesses or by moving the shearing device between two lenses along the optical axis. The temporal phase-shifting method is applied through the use of a liquid crystal variable retarder.

High transmission efficiency for surface plasmon resonance by use of a dielectric grating

The efficiency of the transmission of surface plasmon waves by use of a dielectric diffraction grating is discussed. The Kretschmann device allows us to obtain a surface plasmon resonance that consists of an absorption peak in the reflection spectrum. When surface plasmon resonance occurs, the TM-polarization mode of the incident electromagnetic wave is neither transmitted nor reflected. The procedure to transform an 4bsorption peak into a transmission peak is described. Transmittivity of 68% is obtained for a simple structure that consists of a thin-film layer of Ag coated on a volume diffraction grating and embedded between two dielectric media. The results presented herein were obtained by numerical simulations that were carried out by use of an algorithm based on the rigorous coupled-wave theory.

Uncertainties in the values obtained by surface plasmon resonance

Surface plasmon resonance (SPR) is a suitable technique to optically characterize thin layers. It has often been stated that SPR cannot determine simultaneously both the thickness and the dielectric constant (possibly complex) of the layer. We demonstrate that this idea arises from an error caused by the method used in the simulation of the reflectivity curve. So we do not have to design complex systems to solve this problem as did previous authors. In this paper, we report on a simulation based on the Fresnel equations to calculate the reflectivity curve from the critical angle (?c ) to cover a wider range of ? values. With a matrix formalism, we can pick the layers we want very easily. With our process, results are excellent for all kind of noise distributions even at high noise levels. A metallic layer was first characterized; then an ultrathin (12 A) dielectric layer was added. We also calculated the standard deviations for all cases to prove that the SPR technique is a very sensitive probe. In all studies, the results showed very good agreement between the true values of the parameters and the simulated ones, as well as small standard deviations.

Interferometric fringes projection system for 3D profilometry and relief investigation

We present a new 3D full-frame profilometer based on structured laser light projection method. This device takes advantage of the polarization states splitting technique for producing and shifting multiple sinusoidal Youngs interference patterns that are projected on the inspected surface. The principle of the technique is presented and we discuss the advantages of monochromatic light projection method as a mean to overcome ambient lighting for in-situ measurement. Some results that we obtained on objects from the Egyptian Department of the British Museum are presented to demonstrate that 3D laser profilometry is a worthwhile technique for epigraphic investigations where naked-eye inspections fail.

Highlighting properties of filters for their application in temporal phase shifting interferometry

The goal of this work is to develop a simple and systematic method to highlight the properties of filters for their application in temporal phase shifting interferometry. In this study, the effects of elementary filters (mean, gaussian and median masks) are analyzed. In order to compare those filters, correlation fringes were numerically synthesized and a Gaussian noise has been added. The advantages and the failures of each studied filtering mask have been enhanced thanks to the comparison of different profiles and fidelity functions. Finally, this study is applied to the filtering of a shearogram recorded in our laboratory.

Quantification of defect size in shearing direction by shearography and wavelet transform

Shearography is a recognized interferometric technique in non-destructive testing to detect defects. Defects are detectable in wrapped phase maps because they are characterized in their neighborhood by singular fringes. They are detectable in unwrapped phase maps, because they induce unexpected phase values. By analyzing the length of unexpected phase values area in shearing direction, and by taking into consideration shearing amount, defect size can be locally estimated. To examine this length, we propose to locally determine borders of unexpected phase values region by analyzing wavelet transform of unwrapped phase map profiles. The borders of defect area are found by examining the convergence at fine scales of lines of wavelet modulus maxima. To have a physical interpretation of this convergence, second derivate of a Gaussian is employed as mother wavelet: estimated borders of defect region are some maximal curvature points of unwrapped phase map profile. To finish, we show that shearing amount does not affect estimated defect size with our methodology. So, shearography is adapted to quantify defects in shearing direction. Currently, in any other direction, an ambiguity exists on the position where the local estimation of defect width is performed. The methodoly cannot be employed.

An almost-common path shearographic interferometer using the separation of the polarization states

An original experimental setup for shearography with metrological applications is presented herein. The simplicity and the efficiency of the setup are provided by a shearing device, a prism that separates the TE and TM polarization modes with a coating and a thin glass plate attached on its face. The use of this shearing device enables an in-line and almost-common path configuration for the shearing interferometer, a path that leads to high stability of the interferometer and a low sensitivity to external disturbances. Moreover the sensitivity of the interferometer can be easily adjusted for different applications. The temporal phase shifting method is applied through the use of a liquid crystal variable retarder.

Optimization of holograms recorded in DuPont photopolymer

Substrate-mode gratings are used to couple and influence the signals that propagate in planar-integrated optical systems. Volume holograms are especially well suited for this role because they allow the recording of slanted fringe gratings. Photopolymer are among the best candidates for such applications due to easy handling, dry development process, high diffraction efficiency and replication possibilities. In this paper, we investigate the recording dynamics of OmnidexTM photopolymer from DuPont. We use the theoretical diffusion model proposed by Zhao and Mouroulis in order to simulate the recording process. Two different experiments are described that lead to the quantitative determination of kinetic parameters in this material. These values are introduced in the diffusion model and different recording procedures are simulated. The conclusion shows that it could be valuable to let the material in the dark for several minutes before develop it. This investigation should improve the understanding of recording process and consequently it should permit to build more efficient components.