Theory and experiment in the field of light scattering from optical coatings have been extensively studied and controlled since the 90’s. Indeed surface and bulk theories were developed for substrates and optical coatings, and have revealed great agreement with experiment. Furthermore, angle-resolved apparatuses were built with detection limits close to scattering from the air particles. All these tools have allowed to characterize roughnesses lower than 0.1 nm; also, the microstructure of thin film layers was investigated versus the deposition technologies.
Nevertheless, in the last few years, new challenges for light scattering have merged. Actually, modern deposition technologies with their sophisticated monitoring systems today enable the deposition of large numbers of layers, hence providing complex filters which must be characterized at their working wavelengths or in a wide spectral region. Moreover an increasing demand for micro-structured filters has merged and requires new procedures to discriminate scattering from all micro-devices.
In this context, we have developed in our group at Institut FRESNEL new numerical and metrological tools to satisfy these demands. All scattering facilities were rebuilt and upgraded, sometimes with strongly different principles. In this paper, we will present a rapid overview of these developments, with a focus on broad band scattering metrology (400nm-100nm) with no loss of performance, separation of intrinsic (surface profile) and extrinsic (local defects) roughness, and the control of large-angle scattering in ultra-narrow band filters.
Examples and applications will be given to emphasize all improvements.
Stray light is an important issue in optical systems and may be responsible for huge limitation of final performances. Use of black coated surfaces is known to be an efficient means to reduce such parasitic light sources and various solutions exist that can be applied to mechanical surfaces such as black paints or black anodization; these coatings are relatively thick and to produce thin, black baffle edges, a thin layer technology is thus needed. In this paper, we show how thin film multilayer coatings can be a solution to answer this problematic as it is possible to design accurate spectral response that present a very low level of reflectance with a zero value of transmittance.
Due to market demand and technical progresses, a new generation of optical components requires much more sophisticated structures with a great number of layers. These complex structures enable to achieve severe optical performances but, at the same time, enhance light scattering processes.
For these reasons, it is essential to develop a metrological tool which provides an accurate quantification of the spectral and angular scattering losses behavior with sufficient angular and spectral resolutions.
In order to face this issue, new investigations were performed by our group at Institut Fresnel and led to the development of the new scatterometer SALSA (Spectral and Angular Light Scattering characterization Apparatus). The use of both a broad-band light source and a tunable filter allows to accurately select the illumination wavelength and the spectral bandwidth on the whole spectral range of CCD detectivity. In this paper we will present the performances of the setup and some experimental results.
In this paper, we show how thin film absorbing coatings can be designed with multilayers using both metallic and dielectric materials involving some thicknesses that are very low down to some nanometers. Such multilayers enable to reach a very low level of reflectance and a zero value of transmittance, which can be a solution for stray light reduction in optical systems.
After a description of the design steps, we will present the manufacturing of such multilayer stacks using magnetron sputtering technique and we will see how such coating technique is very well suited for production due to its high process reproducibility even for very thin layers required in metal-dielectric absorbers.
Monitoring of such coatings is also presented with the help of a powerful in situ optical system developed in collaboration with Institut Fresnel that allows characterization of in-situ refractive indices of deposited materials and broadband monitoring of the multilayer stack.
Many results will be given on qualification samples, such as environmental tests and spectral characterizations that show the stability of the performances in severe environmental conditions.
At last, we will focus on the spectral and angular scattering behavior of such absorbing coatings and we will present several measurements performed on glass or metallic substrates with different roughnesses.
This paper is devoted to the presentation of a new spectrophotometric system that allows us for the accurate measurement of the transmission of thin-film filters over an ultra-wide optical density range (from 0 to 11) between 400 nm and 1000 nm. After a detailed description of its structure, we define the transmission range reachable to this set-up and we quantify the influence of the spectral profile of the source on this critical feature. A qualification of this bench, including a determination of its noise floor, is performed on a dedicated band- pass filter manufactured by Institut Fresnel, and we conclude by the presentation of results of ultra-wide range transmission measurements achieved on commercially available filters (long-pass, short-pass, and notch).
We report optical characterization of the different optical components fabricated in transparent materials by bulk refractive index modification or surface ablation by femtosecond pulses. The methods used for characterization of the components with refractive index modification fabricated in fused silica by high repetition rate femtosecond KGW:Yb laser were transmission and diffraction measurements at 532 and 632.8 nm wavelengths, and total integrated scattering (TIS) at 532 mn wavelength. The combined characterization methods were sufficient for modification process optimization and allowed creation of the Bragg gratings with diffraction efficiency in range from 55 to 90% and low scattering losses. The forward and backward TIS measurements of the radial polarization converter showed that forward scattering is more than five times as high as backward scattering. Solar cells with modified surface by femtosecond pulse ablation were investigated by TIS and Volt-Ampere measurements. The current increase is registered with growth of the scattering loses in the solar cells.
Despite the growing improvement in optical polishing and deposition technologies optical resistance of the laser
components used for high-power UV applications remains insufficient in many cases. In this study influence of different
fused silica substrate preparation, post treatment processing and deposition techniques are examined in terms of surface
roughness, optical scattering and laser damage performance. The conventional techniques of polishing, etching, and
finally surface cleaning of substrates have been investigated. Further, a part of samples were also coated with SiO2
monolayer by Ion Beam Sputtering (IBS) technique. Surface quality was characterized prior to and after the treatment
and deposition processes by the means of total integrated scattering (TIS) and atomic force microscopy (AFM). The
experimental results of surface roughness measurements exhibited a good correlation between AFM and TIS methods.
Further optical resistance was characterized with 10 ns duration pulses for 355 nm wavelength laser radiation performing
1-on-1 sample exposure test with high resolution micro-focusing approach. A dominating damage precursor ensembles
produced during manufacturing processes were identified and directly compared. Finally, the conclusions about the
quality influencing factors of investigated processes were drawn.