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The development of high power lasers implies specific needs of optics with very low absorption to avoid detrimental thermal effects up to damage of optical components, and associated metrology of these optics. In this work, we discuss on our recent developments of a thermographic-based technique to measure absorption: specifically, the Lock-In Thermography (LIT) technique applied in a multipass configuration to increase the effective power on the tested samples. We will present the system developed at the Institut Fresnel that is based on 1.5 kW Ytterbium fiber, that allows to submit samples to 5 kW effective power and measure absorption levels down to the ppm on optical coatings.

The increase of continuous wave laser power is an important topic in various new industrial and defence applications. One of the important limitation is due to the thin film component absorption (intrinsic and defects-related) that induces beam distortions and eventually laser-induced damages. In order to study this absorption, it is of prime importance to accurately measure low absorption levels and to determine the origin of this absorption. In this work, we present the use of Lock-In Thermography (LIT) to absorption measurement. This technique relies on the use of a pump laser beam at 1 μm that is modulated at low frequency and an infrared camera that images the thin-film sample that is being heated. By applying a lock-in treatment on thermal images, we show that we can obtain an image of the temperature increase over the optical component with low noise level. A LIT setup with a sensitivity of a few ppm and a ten times better accuracy is demonstrated. We also show that this setup can be used to make mappings of local absorption and can easily reveal local defects with absorption that can be one order of magnitude higher that intrinsic one. This setup is finally implemented to make measurements on different single layer thin-films. Layers made with different materials (Nb2O5, SiO2, TiO2, HfO2) and deposited by plasma ion assisted deposition or plasma-assisted reactive magnetron sputtering are studied. We explore also the effect of annealing on these dense coatings. Finally, we investigate how these intrinsic absorption levels can be used to investigate the absorption of multilayers structures.

International audience; The role of the SiC substrate surface on the resistance to the local initiation of tarnishing of thin-layered silver stacks for demanding space mirror applications was studied by combined surface and interface analysis on model stack samples deposited by cathodic magnetron sputtering and submitted to accelerated aging in gaseous H2S. It is shown that suppressing the surface pores resulting from the bulk SiC material production process by surface pretreatment eliminates the high aspect ratio surface sites that are imperfectly protected by the SiO2 overcoat after the deposition of silver. The formation of channels connecting the silver layer to its environment through the failing protection layer at the surface pores and locally enabling H2S entry and Ag2S growth as columns until emergence at the stack surface is suppressed, which markedly delays tarnishing initiation and thereby preserves the optical performance. The results revealed that residual tarnishing initiation proceeds by a mechanism essentially identical in nature but involving different pathways short circuiting the protection layer and enabling H2S ingress until the silver layer. These permeation pathways are suggested to be of microstructural origin and could correspond to the incompletely coalesced intergranular boundaries of the SiO2 layer.

We present a new method for the in situ measurement of the amplitude and phase of the reflection coefficient of a plane substrate installed in a mechanical holder rotating at high speed (120 turns per minute) during the deposition of optical thin films. Our method is based on digital holography and uses a self-referenced scheme to cancel the effects of the severe constraints generated by the vibrational and thermal environment of the deposition machine.

Stack Array Mirrors (SAM) technology offers high order correction, with up to several thousands of actuators, controllable at high frequency, up to several kHz. A new generation of piezo-electric actuators with high reliability has been developed during the last years. This technology is well-adapted for large deformable mirrors (DMs) with thousands of actuators for future needs for Extremely Large Telescopes. We present the design and the modelling of the two large DMs for NFIRAOS, the multi-conjugate adaptive optics system of the Thirty Meter Telescope (TMT): DM0 which shows 3125 actuators and DM11 which shows 4548 actuators. A DM prototype with 616 actuators has been manufactured to validate the manufacturing steps and the specifications of the future large DMs, including their behavior at both ambient and low temperature (-30°C). The prototype includes the new generation of piezo actuators with improved reliability thanks to an optimization of the fabrication processes. Experimental results of accelerated ageing tests and mechanical fatigue are presented. After complete assembly, the prototype is qualified in a specific cool chamber with interferometric measurements. The results are the following: operational stroke higher than 10 μm PV at both ambient and -30°C with uniformity better than 5%, overall non-linearity lower than 5%, resonance frequency of the actuators higher than 10 kHz. Based on the measurements done on the overall temperature range (+20°C to -30°C), the best flat is lower than the goal specification of 10 nm RMS mechanical. An enhanced protected silver coating done by magnetron sputtering allows a high level of reflectivity especially in the near infrared range and long-life durability.

International audience; Single cavity Fabry-Perot filters are one of the most popular designs for the production of narrow bandpass filters. The usual deposition strategy to create such filters based on optical monitoring at the filter central wavelength is well-known and has proven its strength over decades. We review in this paper the possible optical methods to monitor such a filter during production and analyze their strengths and weaknesses. Then, we discuss a new monitoring procedure, mixing different methods, to minimize the production errors of this filter while maintaining a precise filter centering. This strategy is applied on different bandpass filter designs.

This paper describes a new method, based on the use of a broadband monitoring system, that allows to determine the spectral dependence of the optical constants of a tantala layer without the use of a dispersion model.

International audience; One of the critical steps in the fabrication of complex optical interference filters is the precise control of the thickness of the layers during the fabrication process. However, the definition of the optimal optical monitoring strategy remains a challenge as it relies on user experience and there is no reliable automatic determination of this strategy. Here, we propose a semi-automated method that allows the determination of the optimal strategy. It is based on the combination of trinary mappings to select spectral regions that are compatible with optical monitoring and the use of the reflected phase error at a single wavelength versus optical monitoring wavelength. We show how this procedure can be used for the determination of either a single optical monitoring wavelength or a multi-wavelength procedure of a complex filter and confirm these theoretical results with an experimental demonstration.

We present hereafter a study of complex bandpass optical interference filters with central wavelengths ranging in blue region or in the near infrared. For these applications, the required functions are particularly complex as they must present a very narrow bandwidth as well as a high level of rejection over a broad spectral range. Moreover, these components must have a good flatness meaning that the stress induced by the different layers has to be taken in account in the filter design. We present a thorough study of these filters including their design, fabrication using Plasma Assisted Reactive Magnetron Sputtering (PARMS) and characterization. Excellent agreement between experimental and theoretical spectral performances associated with a final sag of 326 and 13 nm, and uniformity from −0.05 to 0.10 and −0.10 to 0.20% are demonstrated for the two manufactured filters.

We present recent developments on deformable mirrors (DM) for astronomy with ground-based telescopes. A new generation of actuators with high reliability and high performances has been developed for Stack Array Mirrors. These actuators are suitable for a large range of DMs, including future needs for Extremely Large Telescopes. Design and modelling of large DMs for Thirty Meter Telescope and European Extremely Large Telescope are presented. The Monomorph mirrors combines simplicity and efficiency to correct the wavefront deformation. Astronomical telescopes can benefit of the developments performed on this Monomorph technology for high power laser chains and for spaceborn instrumentation.

International audience; An alternative new technique for the determination of efficient optical monitoring strategy of optical interference filters is presented. This technique relies on the analysis of optical monitoring signals for different optical monitoring wavelengths and the comparison of these signals with some pre-defined criteria in order to generate potential spectral path compatible with a dedicated trigger point monitoring technique. Trinary mappings are then generated in order to determine possible optical monitoring strategies. This technique is finally implemented on various filter designs and experimentally validated.

A statistical model of nanoprecursors threshold distribution for the interpretation of laser-damage probability curves is investigated. Each kind of precursor is characterized by a Gaussian distribution of threshold. Accurate probability curves of laser-induced damage (1-on-1, 5-ns single shot at 1.064-μm) are plotted in the bulk and at the surfaces of optical components. Results are then fitted with the model presented. A good agreement is obtained between theory and experiment, which permits to identify different kinds of defects and extract their densities and threshold distribution. The interpretation of these data is then discussed according to their nature and origin (cleaning and polishing).

This paper extends a method previously applied to the determination of the optical constants of a high-index thin film to a dielectric bilayer. This method is based on the time recording of the spectral transmittance of the stack during its deposition with the help of an in situ broadband monitoring system.

We present hereafter the results on the fabrication of complex optical filters within the Institut Fresnel in close collaboration with CILAS. Bandpass optical filters dedicated to astronomy and space applications, with central wavelengths ranging from ultraviolet to near infrared, were deposited on both sides of glass substrates with performances in very good congruence with theoretical designs. For these applications, the required functions are particularly complex as they must present a very narrow bandwidth as well as a high level of rejection over a broad spectral range. In addition to those severe optical performances, insensitivity to environmental conditions is necessary. For this purpose, robust solutions with particularly stable performances have to be proposed.

With the aim to expand its capability to offer state-of-the-art space qualified multispectral optical filters assemblies, Sodern continues its effort to evaluate and incorporate new technologies in its designs. For many years, Sodern has been developing complex optical devices incorporating spectral filters.

The possibility of enhanced mirrors for laser megajoule reflectors has been studied, and preliminary samples have been realized with magnetron sputtering technology. Spectral measurements of improved reflectivity and cosmetics analysis are presented.

In this paper, we present in detail the new deposition magnetron sputtering machine so-called PACA2M, that CILAS has implemented for coating large optics up to 2 meters, within a dedicated consortium, with the financial support of the French Department of Industry and of the local administrations, and with the help and the expertise of the French OPTITEC optical cluster. Our innovative large size deposition machine is equipped with 2.5 meters-long planar magnetrons (seven cathodes), to ensure uniform coating on large optical components, up to 2 meters by 2 meters, 40 centimetres thick and to 1.5 ton weight. Some magnetrons are adapted to a DC, Mid Frequency (MF) or Radio Frequency (RF) operation, which allows the deposition of metals and also dielectric oxides under reactive atmosphere. Moreover, PACA2M is equipped with a powerful broadband optical monitoring system that permits to reach sophisticated spectral specifications and a good agreement with theory, which will be presented in detail in another paper of the conference. As the magnetron sputtering technique leads to very dense layers close to bulk material, it is particularly well suited for applications that require environmental resistance, as for example the LMJ (Laser MegaJoule) French laser fusion program in which CILAS is in charge of the metallic mirrors on large complex-shaped reflectors for laser chains amplifiers. Numerous experimental results are presented here such as protected and enhanced metallic mirrors, metal-dielectric absorbing coatings or multi-dielectric coatings, deposited on different kinds of substrates, for which qualification tests have been done.

In the field of optical coatings production, in situ determination of thin films properties during deposition process is a key point for the achievement of high performance filters. Using a spectral measurement over a wide range is a way to improve the robustness of the reverse engineering methods implemented for the monitoring of thin film thickness and the in-situ determination of material refractive index. In the framework of the development of a magnetron sputtering deposition machine for 2-meter optics driven by CILAS, the Optical Thin-Film Research group of Institut Fresnel has designed and qualified a dedicated Broadband Optical Monitoring covering the visible and near infrared spectral range from 280 nm to 2 200 nm. An all-fibered system, well adapted to the extremely large size of the machine, is used to select the location of the measurement point inside the vacuum chamber (among 9 possible). Moreover, this system allows to achieve uniformity studies at the surface of large size substrates along a straight line perpendicular to the cathode main axis with the help of the motorized displacement system of the substrate in front of the magnetron cathodes. A real-time monitoring of the physical thickness is thus done offering possibilities for automatic deposition process and in-line design re-optimization.

A detailed presentation of this new magnetron sputtering machine, the largest in Europe for optical coatings, will be given. The main features of this equipment will be illustrated through a few representative achievements.

Laser damage characteristics of infrared substrates and coatings at 2-micron wavelength have been rarely studied until yet, even if the need of optical components with high laser-induced damage threshold in the mid-infrared is important. Use of an infrared nanosecond laser, tunable in the range 2 to 5 microns, allowed us to develop an automatic test facility for the determination of accurate LIDT curves for different test procedures. We choose to particularly study polycrystalline zinc selenide (ZnSe) material used as substrates for infrared dielectric coatings. Irradiation of ZnSe substrates with parallel laser beam shows that surfaces always break clearly before bulk material, this shows that surfaces must be carefully prepared. We particularly exhibit the influence of polishing processes on substrates LIDT. Then the influence of different cleaning methods before coatings deposition is studied. Practical implications for the fabrication of highly laser resistant multilayer coatings at 2-microns are finally discussed.

We present Laser Induced Damage Threshold (LIDT) results on multilayer components with different optical functions for near infrared applications. In this paper, we investigate the different fabrication steps of such functions. In particular, we show experimental results on surface preparation (polishing, cleaning,) and on deposition techniques (Ion beam assisted process) related to the materials involved in coatings. Laser damage tests are performed at 1064-nm with a 5-ns pulse Nd:Yag laser and experiments are made at surfaces of optical components using a 12 μm diameter focused beam. Accurate damage probability curves are plotted thanks to a reliable statistical measurement of laser damage. Use of a statistical model permits to deduce the densities of laser damage precursors. A systematic analysis of all the steps involved in fabrication allows then to build multilayer components with high laser induced damage resistance.

A statistical model for the interpretation of laser damage probability curves is investigated. In a previous study, shapes and slopes of the curves were related to the spot size and to the densities of nanodefects that are responsible for damage. Each kind of precursors was characterized by its damage threshold. We improved this study when considering a Gaussian distribution of precursor thresholds. Accurate probability curves of laser induced damage (1-on-1, 5-ns single shot at 1.064-mm) are then plotted in bulk and at the surfaces of optical components and fitted with the model including Gaussian distribution of precursor thresholds. Threshold mean value, threshold standard deviation and precursor defect densities are extracted for the different kinds of observed precursors. To illustrate our investigation, we present results achieved on different substrates, which establish a better agreement between theory and experiment.