Gilbert Tribillon

High-resolution profilometry by using phase calculation algorithms for spectroscopic analysis of white-light interferograms

The proposed profilometer is based on an optical spectral analysis of white-light interferograms. Phase calculation algorithms are applied within the spectral domain for high-resolution profilometry. Absolute one-way optical path differences are measured without any axial scanning of the interferometer. This method is well adapted for the analysis of rough or binary surfaces with a 1 nm resolution.

Profilometry by Zero-order Interference Fringe Identification

Abstract In white light interferometry, the visibility of the fringes is directly related to the optical path difference between the two interfering beams. With a very short coherence length light source, the zero-order interference fringe may be detected by a simple maximum intensity location while the reference mirror is shifted by a piezoelectric transducer. We use this possibility for high-resolution profilometry. We discuss the maximum sampling distance, which guarantees a correct detection, as a function of the self coherence function of the light source. A practical configuration based on Mirau interferometric microscope objective was built. The experimental results show the nanometric resolution of this technique and its ability to profile surfaces with sharp slopes and deep holes. The application field of either heterodyne or confocal profilometers may be extended by associating them with the proposed method.

Correlation entre deux speckle obtenus avec deux longueurs d'onde - application a la mesure de la rugosite moyenne

Abstract An object with a rough surface is illuminated successively with two different wavelengths λ 0 and λ 1 . The images are recorded photographically on the same plate. The Fourier transform shows interference fringes whose visibility varies with the value of Δλ = λ 1 - λ 0 and the r.m.s. of the roughness σ z . An optical method of a measurement using the correlation properties is described.

High resolution digital holography

Abstract In digital Fresnel holography, present specifications of charge-coupled device cameras require the incident beams to be quasi-parallel. That implies large speckle grain size and low lateral resolution in reconstructed images. Better lateral resolutions are demonstrated in our work, down to 8 μm, allowing the observation of sub-millimetre objects by digital holography. The experimental set-up built samples the incident light distribution with a definition of 500 pixels mm −1 . The maximum acceptable angle is then widened and the hologram recording and reconstruction distances are drastically reduced, as well as the speckle size. Interferometric holography was implemented for the measurement of the deformation of a sub-millimetre silicon cantilever and results in phase-contrasts imaging are also reported. The design of dedicated complementary metal-oxide-semiconductor active pixel sensor cameras is also discussed.

Three methods of information assessment for optical data processing.

The determination of the specific information in a message has proved of interest in coding (for instance in a hologram) as well as in data processing itself. Also, simple considerations about the definition of optical information have grown into several groups of processing methods. At first the concept of characteristic spatial frequency bands, which describe essential features of any object, leads to compression of its Fourier terms in order to increase the transfer density of a transmitting system and the storage capacity of the recording media. With the emphasis being put on the informative value of the reference in holography, geometrical compression is applied in particular to the coding of printed texts. The second and third groups of methods apparently deal with opposite approaches. Assuming a given message, one may select some parts of its spectrum for further processing or carry out average signals from the collections it contains. Both cases-sampling and averaging-permit the extraction of a group of data belonging to a set of discrete signals. The following developments are presented: image replicating at a determined energy level, step-by-step phase sampling grids, study of the displacement law for moving objects by random sampling of disturbed wavefronts and recording of incoherent holograms, on the one hand, and construction of average letters for comparison between manuscripts, average Fourier holograms, and power spectra, on the other hand. A last illustration is provided in connection with the appraisal of psychological tests.

Photostability study of europium dibenzolymethide embedded in polystyrene thin films with high concentration

Abstract The europium dibenzolymethide has been embedded with high concentration in a polystyrene matrix using spin-coating deposition. We have investigated the temporal stability of the film in measuring simultaneously the intensity I and decay time τ of fluorescence and the sample transmittance T at the excitation wavelength. The low bleaching efficiency is measured in the order of 10 −4 . During the photobleaching process, it appears that there are some non-monotonous fluctuations on τ and T , whereas I decreases continuously, depending on the film thickness d and the chromophore concentration N 0 . In addition, we show that these films can be used to probe a metallic surface by means of the quenching effect.

Anisotropic photon migration in human skeletal muscle

It is demonstrated in the short head of the human biceps brachii of 16 healthy subjects (12 males and 4 females) that near infrared photon migration is anisotropic. The probability for a photon to travel along the direction of the muscle fibres is higher (approximately 0.4) than that of travelling along a perpendicular axis (approximately 0.3) while in the adipose tissue the probability is the same (approximately 0.33) in all directions. Considering that the muscle fibre orientation is different depending on the type of muscle considered, and that inside a given skeletal muscle the orientation may change, the present findings in part might explain the intrasubject variability observed in the physiological parameters measured by near infrared spectroscopy techniques. In other words, the observed regional differences might not only be physiological differences but also optical artefacts.

Optical implementation of frequency domain analysis for white-light interferometry

The purpose of recent developments of profilometry by using white light interferometry is to provide new tools for the analysis of rough samples which when studied by monochromatic phase-shifting interferometry, may cause phase calculation ambiguities. The usual way to perform depth measurements by white light interferometry is to analyze the coherence-limited interference fringes while the optical path difference is scanned. The method proposed here does not use optical path difference scanning. A spectroscopic device is used instead to separate the interference intensities associated to each spectral component of the light source. Phase variations due to wavelength change are proportional to the optical path difference and allows depth measurement to be performed without axial scanning. The profile of one line of the inspected sample is obtained from only one 2D interferogram. In this 2D interferogram one direction corresponds to the inspected direction of the surface while the other one is the chromatic axis which allows phase to change with wavelength. Experimental results show the ability of the proposed method to obtain the profile of 1D surface with nanometric resolution.

Chromatic confocal microscopy by means of continuum light generated through a standard single-mode fibre

In this paper a confocal profilometer using longitudinal chromatic aberration is presented. The system uses a novel polychromatic illumination source generated by means of non-linear effects in a standard single-mode optical fibre. A wavelength–height codification is realized using the chromatic aberration of a Fresnel lens. The height decoding is obtained by the spectral analysis of the detected light, identifying the most intense spectral component coming from the sample. The height sensitivity of the profilometer is variable and it is related to the full width half maximal value (FWHM) of the systems spectral response. From 700 to 938 nm the experimental value of the FWHM changes from 10.78 to 3.9 µm respectively.

Roughness measurement by confocal microscopy for brightness characterization and surface waviness visibility evaluation

Brightness and waviness appearance are important criteria of surfaces for the success of commercial products. The first part of this paper presents a brightness measurement procedure based on roughness measurements by confocal microscopy and scattering theory. The resolution of the proposed method allows the separation of as many brightness levels as the eye can detect. This method links the brightness level of surfaces to their microroughness and so provides industry with essential information for the management of the fabrication process in order to obtain the desired final aspect. In a second part, the scattering diagrams of surfaces are computed from microroughness parameters. Computed waviness images are compared with real waviness images. Their likeness shows the ability of the proposed procedure to predict the waviness visibility and thus to better control the appearance of manufactured products.