This paper, “Broad-band absorbers for reduction of parasitic light: two alternative solutions," was presented as part of International Conference on Space Optics—ICSO 1997, held in Toulouse, France.
Light scattering is well known to be dependent on the optical properties and surface roughness or bulk inhomogeneities of components. Angular scattering measurements and the development of electromagnetic theories at Institut Fresnel Marseilles permit to quantify the roughness behaviour. Measurements can be performed at different wavelengths from the UV to the near IR to access to different scales of characterization. Atomic Force Microscopy is used to complete these measurements at the microscopic scale, and predict the surface behaviour in the X rays domain. All these techniques rise to a multiscale characterization of all surfaces, which reveals in most cases fractal behaviours. The scatterometer has been extended and allows to perform ellipsometric measurements on scattered light in each direction of space. Results can be investigated by electromagnetic theories. They permit to directly separate bulk and surface effects in the case of bare substrates and reveal the high sensitivity of the polarimetric phase difference to the presence of contaminants on surfaces, even in the case of first order contaminants, that is to say whose size is in the same order as the substrate roughness.
Scatterometry is one of the few metrology candidates that has true potential to analyse submicrometer critical dimension (CD).
This physical constraint restricts initially the scalar theory of diffraction (Kirchhoff, Fraunhofer), where no computation is needed, to describe pattern with such characteristics. Now many techniques based in a rigourous analysis of diffraction (were polarisation is considered) are developped (RCWA), however these vector theory would be difficult to apply in real time due to time consuming by computation.
For that reason, we push here scattering measurement of isolated lines, in the limit of the scalar theory. We use the scattering mode of SOPRA's GESP 5 instrument, which measure separately TE and TM polarisations.
We propose a solution for increasing the axial resolution of confocal microscopes. In the experimental set-up described in this paper an interference phenomenon between two counterpropagating beams is used to determine the axial position of a luminophore. The optical path difference between the two waves, which is related to the position of the luminophore, is recovered thanks to a second interferometer by using partial coherence interferometry demodulation technique. The proposed solution can find applications in biology for localizing with nanometric resolution a small number of tagged species.
We have designed and fabricated a silicon grating which shows antireflection properties in the [4μm ; 6 μm] spectral region. It is shown both theoretically and experimentally that, even if the refractive index and the grating period are in the extend that a simple homogenization theory can not be used, a substantial broadband antireflection effect can be obtained. The grating was made using a wet anisotropic etching technique. The reflectance was calculated with a modal method and compared successfully with the experimental results. It is shown that the grating reduces the silicon substrate
reflectance in the whole [4 μm ; 6 μm] spectral domain by a factor greater than 10.
We propose a technique, based on an interferometric experiment and an optimization algorithm, that permits to retrieve the map of permittivity of unknown objects deposited on a flat surface. We show numerically that one can obtain a better resolution than the usual Rayleigh limit even with detectors and emitters in the far-field zone.
Total integrated scattering (TIS) and bi-directional reflectance distribution function measurements give information on the mean roughness of the surface and on the modulus of the Fourier Transform of the 2D surface profile. To obtain a more precise description of a rough surface and to get an estimation of the relative contribution of surface and volume scattering in multilayers stack, we have developed a new interferometric method which allows us to measure the variation of the phase of the Fourier Transform of the surface profile as a function of the scattering angle. This paper includes a detailed presentation of this method, a description of the experimental set-up used for its principle validation, as well as some preliminary results that we have obtained with it on high reflectance samples.
It has been shown that measuring the polarimetric phase of the field scattered from a thin film multilayer gives information on the origin of scattering. Numerous numerical simulations have shown that the behavior of the polarimetric phase can be used to separate surface and bulk scattering. In the case of stacks with correlated interfaces the polarimetric phase depend only on the origin of scattering, whatever the microstructural parameters. Slight deviations from vertical correlation within the stack lead to ripples in the polarimetric phase, that can be observed experimentally. Moreover in certain cases second order effects, due for example to localized defects, can lead to depolarization. This phenomenon has strong influence on the polarimetric phase. This can be used to detect contamination of the surface. In our communication we will present some experimental results that show that the origin of scattering can be determined.
For many applications, reducing the total amount of the light scattered from rough surfaces is a crucial problem. We show that a scattering-reduction effect is obtained by coating rough absorbing surfaces. Numerical results as well as experimental ones are presented. We also study the case of inhomogeneous structures with embedded scatterers.
We present new improvements that were achieved at LOSCM Marseilles for a better characterization of optical thin films. Roughness-induced absorption, angular ellipsometry of light scattering and multidielectric resonances are discussed in multilayers. Theoretical and experimental results are given and new applications are emphasized.
We show how to use dielectric thin films in order to reduce scattering and maximize broad-band absorption in arbitrary rough overcoated black surfaces. A rigorous calculation is developed thanks to an improved differential method, and the results are compared to experiment. Scattering is reduced by a factor 10 (down to 5.10-3), though the starting bare surface is a standard low-cost black paint (5.10-2 scattering). Absorption can therefore be optimized up to 0.995.
The basic principles of guided mode resonance filters constituted by a high index modulated coating deposited on a classical glass substrate are presented. A rigorous method used to compute the reflection coefficients of this kind of structures is described. We show that in some cases, computation can be made by using an approximate method. These structures, which permit one to design narrowband inverse filters, are compared to solutions based on classical multidielectric coatings.
Stress in thin films deposited by Reactive Low-Voltage Ion Plating is studied in air and at room temperature. A multilayer stack, composed of tantalum pentoxide and silicon dioxide layers, is considered and the interactions layer to layer turn out to have no effect as regards to the final bending. Evolution in stress after annealing shows the possibility to reduce the stress as well as the absorption for tantalum pentoxide thin films. Finally, ion implementation, such as helium and xenon, at high energy, prove to be also a way to vary and diminish the stress in thin films.
The main results obtained with optical devices using coherence multiplexing are described. The demodulation methods for interferometers illuminated by low coherence sources are studied. The applications of coherence multiplexing in the field of fiber-optics sensors and telecommunications together with the future developments are also presented.
In order to demodulate optical fiber interferometric sensors in the coherence multiplexing scheme it is worthwhile using static interferomers. Measurement of a physical parameter is obtained by matching the optical path difference (OPD) of each interferometric sensor with a receiver. An attractive solution consists in using grating interferometer as a demodulator. Indeed, it permits one to record correlation peaks (each one corresponding to a sensor) on a photodiode array, thus making demultiplexing easier. On the other hand, it is possible to record only the peak envelope if the grating interferometer is adjusted in a well defined configuration. In this paper we concern ourselves with the theoretical determination of the conditions needed to simplify calculations and interpretation of the experimental results.
We describe a temperature sensor based on a tapered monomode fiber. The sensor is demodulated by a static interferometer designed to measure a phase by the coherence multiplexing technique. A comparison between experimental results, theoretical analysis, and numerical simulation is presented. The performances of the sensing device are also given.
We show that a grating interferometer can be used as a receiver for quasi-distributed fibre optic sensors using white light interferometry. We report the results obtained when the sensor is based on an embedded fibre optic polarimetric interferometer for strain measurement of carbon/epoxy composites.
This paper is devoted to the description of a new type of fiber optic magnetic field sensor based on spectral
modulation encoding techniques. This device uses a Faraday magneto-optic element as transducer and converts the rotation
of the plane of polarization of the light induced by the magnetic field into a small change in the large optical path
difference provided by a birefringent coding plate. The use of a broadband source and of an interferometric detector, with an
optical path difference matched to the sensor one, allows to reach high resolution and perfect down-lead insensitivity. A
dual-wavelength arrangement is proposed to determine the temperature and magnetic fied values with the same probe.
This paper is devoted to the description of a prototype Emission/Detection Unit, the ACCORD® Module,
developed for the accurate demodulation of fiber optic sensors using spectral modulation encoding techniques. This
prototype contains a static polarimetric interferometer with 4 ports to provide the four phase signals needed by the passive
homodyne scheme, and uses a dual-wavelength arrangement to extend the measurement range by removing the 2ic
ambiguity in the phase determination. Resolution as high as A/40000 has been achieved with 1 Hz bandwith and Optical
Path Difference mismatch in a 20 microns range.
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