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.
The principle of a new scattering measurement system including a mobile lighting and a fixed CCD array is described. This new system allows a spatially resolved light scattering characterization. Moreover it is possible to separate localized defects contribution from the local roughness measurement. The comprehensive characterization of optical coatings can be performed with this set-up, and some examples will be given.
Elliptical polarization is used to explore the possibility of probing diffuse tissues at selective depths. The results of a recently published Monte Carlo simulations study are exposed. Experimental tests will be presented.
Polarization gating is a popular technique in biomedical optics. It is widely used to inspect the surface of the tissues (under colinear or cocircular detection) or instead to probe the volume (cross-linear detection), without information on the probed depth. Elliptical polarization is introduced to explore the possibility of probing diffuse tissues at selective depths. A thorough Monte Carlo simulation study shows complete correlation between the probed depths and the ellipticity of the polarized light, for a medium with known optical properties. Within a wide range of optical parameters, a linear relation between the backscattered intensity and the depth extension of the probed volume was found whatever the polarization used, but with a controlled extension depending on the ellipticity.
Depth selectivity is crucial for accurate depth volume probing in vivo in a large
number of medical applications such as brain monitoring. Polarization gating has been widely
used to analyze biological tissues. It is shown that using polarized light allows probing tissues
on a specific depth depending on the polarization illumination type (linearly, circularly) and
the tissues properties. However, accurate depth investigation of the tissue requires a high
selectivity of the probed depth. We propose and simulate the use of different elliptically
polarized illuminations for continuous depth examination between linearly and circularly
polarized illumination. Monte Carlo simulations verify that circularly polarized illumination
penetrates deeper than linearly polarized illumination in biological scattering media.
Furthermore, we show that elliptically polarized light can be tuned in its penetration depth
continuously between the penetration depth of linearly polarized light and circularly polarized
light. Experimental results obtained on phantoms mimicking in vivo situations are presented.
The suitability of a corneal graft for transplant surgery is based on different criteria. It may be rejected in particular due
to a loss of transparency, directly linked to its scattering properties. Then, these become an important parameter. The aim
of this paper is to quantify the influence of the cornea thickness and of the epithelial layer on scattering properties. The
origin of scattering is discussed based on polarimetric analysis of scattered field (surface and/or bulk) and on full-field
optical coherence tomography imaging (structural information).
A polarization-sensitive Monte Carlo model is used to investigate differently polarized light
illuminations on their degree of polarization (DOP) depth evolution in a semi-infinite scattering
medium. The three-dimensional simulations show that circular polarized light maintains its initial
polarization state longer than elliptical or linear polarized light. It was revealed that elliptical
polarization can be tuned so that its DOP depth evolution can be precisely chosen between the
penetration depths of linearly and circularly polarized light.
The method proposed here allows to perform a selection of a well defined
subsurface volume in a turbid medium allowing SNR enhancement for functional imaging of
the cortex. The principle consists in sequentially probing the biological tissue with light
polarized linearly or circularly. The method and preliminary results obtained on phantoms are
presented.
The cornea is the single human tissue being transparent. This unique property may be explained by the particular
structure of the cornea, but the precise role of each of its constituents remains unsolved. On other matter, prior to corneal
transplant, graft must be evaluated during a sorting procedure where a technician assesses of its transparency quality.
Nevertheless, this criterion remains subjective and qualitative.
This study proposes to combine 3D imagery using Full-Field Optical Coherence Tomography jointly with angular
resolved scattering measurement to achieve a quantitative transparency characterization of the cornea. The OCT provides
micrometric resolution structural information about the cornea, and we observe the evolution occurring when oedema
develops within the tissue. Scattering properties are evaluated and compared parallely, as the transparency of the graft.
A close link between the scattering intensity level of the cornea and its thickness is highlighted through this study.
Furthermore, the three-dimensional imagery offers a view over the structural modifications leading to a change in
transparency, and the combination with scattering properties measurement provides clues over the characteristic scale of
scatterers to consider for a better understanding of corneal transparency evolution.
Achieving an objective and quantified parameter for the transparency would be helpful for a more efficient corneal graft
sorting, and may be able to detect the presence of localized wounds as the ones related to a previous refractive surgery.
However, the study of graft nearly eligible for corneal transplant would be needed to confirm the results this study
presents.
With device size reduction, variability induced by local micro roughness is becoming less and less negligible in terms of
statistical control of critical dimensions (CD). We applied a recent approach developed at Fresnel Institute for the
determination of micro roughness on periodic structures through optical far field characterization using an angle resolved
scatterometer. Structure periodicity affects the diffraction orders, while roughness signature is mainly found between
diffraction orders. Theoretical simulation was performed using two in-house computer codes based on differential
method and on first order approximation. We will review the theoretical approach and show roughness data derived from
measurement on glass gratings as well as poly silicon gate type structures.
In the field of microelectronic industry, periodic structures are produced with spatial dimensions that can be less than
100 nm. Because of the material and process effects, these structures will most likely present some additional roughness.
The optical far field characterization of these structures usually allows to deduce the shape parameters of the periodic
structure. Measurements are performed thanks to an ellipsometric apparatus, associated with modelling and inversion
algorithms. In this configuration the technique is called "scatterometry". This method does not permit to directly extract
roughness parameters. This paper aims at describing how model and experimental tools can be used to characterize the
roughness of gratings. The study needs a complete three-dimensional electromagnetic modelling of the structure but the
calculations are very time consuming. Here, different theoretical models are associated in order to reduce the calculation
time: rigorous numerical differential theory and Born approximation theory. The exact numerical model allows to treat
the periodic part of the structure while the roughness is viewed as a perturbation and treated using a first order
approximation. From an experimental point of view, the information on the periodic part of the structure lies in the
diffraction orders, while the roughness signature is mainly found between diffraction orders. Practically, this model
could be used in the semiconductor industry for a detailed roughness characterization, based on an optical measurement
using the same test structures used for scatterometry.
Micrometer scale resolution full-field optical coherence tomography (FF-OCT) is developed for imaging human graft corneas. Three-dimensional (3-D) images with ultrahigh resolution (respectively, 1 and 1.5 µm in the axial and transverse directions), comparable to traditional histological sections, are obtained allowing the visualization of the cells and the precise structure of the different layers that compose the tissue. The sensitivity of our device enables imaging the entire thickness of the cornea, even in edematous corneas more than 800 µm thick. Furthermore, we provide tomographic 3-D images of laser incisions inside the tissue at various depths without slicing the studied corneas. The effects of laser ablations can be observed, along various optical sections, directly in the bulk of the sample with high accuracy, providing information on the interface quality and also imaging tiny changes of the tissue structure. FF-OCT appears to be a powerful tool for subcellular imaging of the corneal structure and pathologies on the entire thickness of the tissue as well as interface quality and changes in the collagen structure due to laser incisions on ex vivo human cornea.
A comprehensive characterisation tool for optical component is presented here. Based on both light scattering and
imaging principles, the CCD-ARS set up allows to separate and study localized defects contribution from the local
roughness measurement. The numerical method involved to discriminate intrinsic roughness from the influence of
defects is detailed and some results are given.
Angle-resolved ellipsometric data are recorded on light scattering and provide a real time process for selective imaging
in scattering media. Surface and bulk effects are separated and could be used for a selective screening inside the tissues.
Optical Coherence Tomography (OCT) is an attractive technique to study works of art because it allows non-destructive
and contactless analysis. In the case of musical instruments, the study of wood finishes could give interesting
information as the thicknesses of the layers, the number of layers and the presence of fillers. A time-domain full-field
OCT, achieving high resolution, is used in both visible and near infrared ranges to characterize semi-transparent layers
containing scattering particles as charged varnish layers. We present OCT measurements on wood varnished with
different coatings. We show that the detection of pigment particles is dependent of the spectral range and that both
spectral domains allow to reach micrometer-scale spatial resolutions.
Light scattering is a current tool for characterization of defects in optical interferential coatings. However, this tool is not fully efficient for multilayer component. Indeed, in this case, the scattered light from multilayers originates from several interface roughnesses that cannot be separated a priori. In this paper, a technique which can isolate a single interface embedded within a stack is presented. It is based on destructive interferences between the polarization modes of the angular scattering. These interferences can be tuned in a selective way that allows the extraction of light issued from a specific scattering interface.
The principle of a new scattering measurement system including a mobile lighting and a fixed CCD array is described.
This new system allows a spatially resolved light scattering characterization. Moreover it is possible to separate localized
defects contribution from the local roughness measurement. The comprehensive characterization of optical coatings can
be performed with this set-up, and some examples will be given.
A recent optical technique is reviewed to identify the scattering origins (surface roughness or bulk heterogeneities) and
eliminate scattering sources in a selective way. Applications concern the field of optical interference coatings, remote
sensing and imaging in random media.
Far field light scattering from rough surfaces and inhomogeneous bulks has extensively been studied these last decades, with a major application in random media characterization. Angular Resolved measurements are performed and investigated thanks to the development of electromagnetic models. The studies are extended to the case of high angular resolution, that's mean to the speckle pattern. We show that the analysis of the polarization state of the scattered field permits to complete this study and to identify signatures of the different polarization sources which are surfaces or bulks. An application will then be to annul each scattering source in order to select the characterized element.
The validity of the Ellipsometry of Angular Resolved Scattering technique introduced in a well known scatterometer has been demonstrated. The results were applied to the separation of surface and bulk effects in low-loss samples, because first-order scattering only depends on the origin of scattering, not on the topography or microstructure. The major point that we address is then the generalization of the separation technique (surface or bulk) to arbitrary heterogeneous samples with high level diffuse reflectance. The problem is strongly different since phase data from these samples depend on microstructure, not only on the physical origin of scattering.
Though light scattering has been extensively studied these last decades, it may still provide new and unique tools to probe optical materials and components, provided that some inverse problems can be solved. A brief summary of advances in this field are here given.
The artificial reproduction of some coloration effects (for instance nacre aspect, iridescence) which appear spontaneously in the nature needs a right description of the different mechanisms which are involved in these phenomena and especially, a detailed analysis of the spectral behavior of the light scattered by such surfaces. With this aim in sight, we have developed a dedicated set-up for the recording of the reflectance distribution function of solid samples in the whole visible spectral range. We have also analyzed the different methods which are able to describe the color information for
scattered light. First experimental results obtained on some organic glass windows are given in conclusion.
Optical interference coatings usually offer large possibilities for the spectral control of specular reflected or transmitted light. In this aim, multilayer structures are calculated and realized according to each specific application. These kinds of components are deposited over plane substrates, or on substrates with large curvature radius. In this paper we show how multilayer components can also be deposited on micro spheres to reach other applications connected with the spectral control of light scattering.
Several French research laboratories set up goniometers allowing BRDF measurements at different laser wavelengths in the infrared. On the effor of the Delegation Generale de l'Armement (DGA/STTC), a round robin set of painted targets BRDF measurements was undertaken, under the ONERA expertise. The laboratories participating in this round robin were the Aerospatiale Matra CCR Suresnes, The IPN SMA-Virgo Lyon, the Institut Fresnel Marseille, and the CEA DAM CESTA Le Barp. The goniometers of the four laboratories are firstly described. The targets studied are seven 5cm diameter painted disks of aluminum or steel, a spectralon reference sample, and a sandpaper sample. We have first demonstrated that the pollution of painted targets with dust has a very weak influence on the BRDF. Before and after each measurement series, the directional-hemispherical reflectance of the samples was measured at ONERA. The measurements have been achieved according to a protocol specifying the sample position and laser probe size. Chosen wavelengths for the inter-comparison are 1.064 micrometers . For both wavelengths, the characteristics of the different goniometers are compared in term of noise and repeatability. The difference between the painted targets BRDF measured with the various devices are relatively limited at 1.06 micrometers , and mainly induced by speckle. More important differences are obtained at 10.6 micrometers , particularly for a BRDF measurement device using an absolute calibration method. In order to explain these differences, few hypotheses are advanced. Information on the absolute accuracy is obtained by the comparison of the measured directional-hemispherical reflectance and the one computed from BRDF measurements.
The improvement of optical components for high power laser applications is still topical. Indeed the different signal cant progress made these last years, had allowed to improve the damage resistance of optical components by in particular, the identification of micronic precursors centers. A new challenge today is the identification of precursor centers of damage with size in the range of few nanometers. This kind of defects seems to play an important role in the laser damage process. In any case the challenge is to find an efficient tool able to detect these defects which are invisible with usual techniques as optical microscope or standard scattering. The technique of Laser Modulated Scattering (LMS) has been performed to reach this challenge. This new tool presented last year in the Boulder symposium, is based on a very high sensitivity detection of photothermal response of the defect. The LMS has been performed via two different setup arrangements. The first one uses tow beams as in the configuration of a standard Photothermal microscope, and the second one uses only one beam. In this article we first briefly remind the principle of the LMS technique with one and two beams. Then we will show by different results, the advantages of using an optical fiber to collect the scatted light instead of a block beam system used before. One of the main advantages of the setup using a fiber, is that it is easily possible to realize an angular study of scattering which allows a best understanding of the physical origin of the defect-induced scattered signal. The last part of this work consists of a series of stimulation of angular scattering LMS curve, in order to quantify the sensitivity and the powerfulness of this technique.
Light scattering is well known to be a key limitation of the ultimate performances of filters. Whatever the values of surface roughness and bulk heterogeneity within the multilayer, the optical losses depend on the stack design and on the correlation factors between defects. Such situation is largely enhanced in the case of WDM filters, due to the high over-intensity of the electric field within the stack, as a consequence of the narrow pass-band. In this paper, we present some experimental scattering result recorded with a high angular and spectral resolution on prototype WDM filters and which illustrate the enhancement of scattering near the design wavelength of the filter.
For a large number of specific applications, optical materials must be used in powder forms. In this context it has become highly necessary to characterize the optical and microstructural properties of such powders. Preliminary scattering experiments have shown that the scattering intensity from materials in powder forms could be quite different than that of the same homogeneous materials. In particular we have noted a strong increase in the level of the scattered light that could come from the bulk scattering. Here we show how to use the light scattering techniques in order to separate and determine the roughness and inhomogeneities of the samples. The same techniques are used to determine with accuracy the refractive index and absorption of the powders. In a second step, the same powders are evaporated in thin film forms, and we use classical spectrophotometric techniques to determine their refractive index and dispersion laws. The result are compared and discussed with those obtained with light scattering. In a general way, this study involves surface and bulk theories of light scattering, together with angle- resolved measurements, and atomic force microscopy.
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.
The measurement of total scatter losses is a major prerequisite for the development, optimization and commercialization of high quality optical components. Especially in laser technology, optical scattering gained of importance in the source of the development of laser system with ever increasing output power and improved beam parameters. Besides its influence on the efficiency of laser systems and the beam steering arrangement, total scattering is an important safety aspect for application of these laser systems in materials processing, medicine and fundamental research. As a consequence of this global trend, working groups of TC 172/SC 9 initialized the development of an International Standard for the measurement of total scattering in optical components.
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.
Light scattering and Atomic Force Microscopy (AFM) are used together to analyze surface roughness in a very wide frequency bandwidth, extending from macroscopic (optical) to microscopic (AFM) scales. The two techniques are shown to be in large agreement since the roughness spectra overlap at intersection of bandwidths. A particular behavior of roughness is emphasized that permits to predict scattering at very short wavelengths. Thin film materials obtained by different techniques (IAD, Ion Plating, EB) are also investigated via a comparison of roughness spectra measured before and after coating in all bandwidths.
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