For fifty years, a considerable effort has been and is still being directed to the production of optical coatings using liquid deposition route. Sol-Gel is a chemical process widely used for oxide material preparation. Based on smooth chemistry (low temperature conditions), sol-gel allows nanoparticle and polymeric material synthesis dispersed in appropriate liquid medium. The process investigated at CEA (French Commission for Atomic Energy) is strongly developed to afford coatings onto mineral or metallic substrates using colloidal oxide-based and/or inorganic-organic hybrid materials. Such a chemical process is sufficiently adjustable to develop purpose-built materials and coatings for high power laser optical components, taking into account the high laser damage threshold requirement. Because the CEA megajoule-class pulsed laser is needing 7,000-m2 of coated area onto 10,000 large-sized optical components, we have developed to date, several optical coating procedures, each optical thin film being prepared from a specific material and deposition process. First need to fulfil was the antireflective (AR) coating required for transparent optics and used to increase laser light transmission and to suppress damaging residual reflection. The as-developed AR-coatings were made of nanosized particle-containing fragile single layer or abrasion-resistant polymeric-based broadband layer stack. For used on highly-reflective (HR) component, a specific unstressed multilayer coating has been developed and deposited onto deformable adaptative end-cavity mirror substrate. This HR-coating is made of quaterwave stack of colloidal-based low index and hybrid high index thin films.
Using such materials, first high ratio polarizing sol-gel coatings have been also produced. Apart optical coating preparation, sol-gel chemistry has been used to develop an hybrid dense protective thin film to enhance durability of oxidation-sensitive silver cavity reflectors. Each coating material preparation and room-temperature deposition process will be described. Because the sol-gel technology offers outstanding technical and economical advantages over the conventional vacuum techniques, this process has been transferred to one of THALES production plant for megajoule-class laser prototype supplying.
The French Commission for Atomic Energy is currently involved in a project which consists of the construction of a 2 MJ/500TW (351-nm) laser, so-called LMJ (Megajoule-class laser) devoted to Inertial Confinement Fusion (ICF) research in France. A prototype facility so-called LIL is used today to qualify the laser architecture. For these high power lasers the sol-gel process has been selected for 95% of laser optical coated area because of room temperature and atmospheric pressure deposition conditions with guarantee for high optical and laser-included damage performances at a low cost compared to conventional vacuum deposition processes,. Today, we have developed a large variety of coatings, such as antireflective, environmental protective, broad-band antireflective, scratch resistant or high reflective coatings for our laser needs. Due to their ability to ensure appropriate deposited optical thickness, room-temperature deposition techniques such as dip-, spin- or laminar-flow-coating have been optimized for laser coating production. Today, 264 components have been successfully coated for the LIL laser needs.. Optical and laser damage characterizations show the produced component performances being in compliance with the high power laser requirements.
The interest of the antireflective coatings applied onto large-area glass components increases everyday for the potential application such as building or shop windows. Today, because of the use of large size components, sol-gel process is a competitive way for antireflective coating mass production. The dip-coating technique commonly used for liquid-deposition, implies a safety hazard due to coating solution handling and storage in the case of large amounts of highly flammable solvent use. On the other hand, spin-coating is a liquid low-consumption technique. Mainly devoted to coat circular small-size substrate, we have developed a spin-coating machine able to coat large-size rectangular windows (up to 1 x 1.7 m2). Both solutions and coating conditions have been optimized to deposit optical layers with accurate and uniform thickness and to highly limit the edge effects. Experimental single layer antireflective coating deposition process onto large-area shielding windows (1000 x 1700 x 20 mm3) is described. Results show that the as-developed process could produce low specular reflection value (down to 1% one side) onto white-glass windows over the visible range (460-750 nm). Low-temperature curing process (120°C) used after sol-gel deposition enables antireflective-coating to withstand abrasion-resistance properties in compliance to US-MIL-C-0675C moderate test.
The French Commission for Atomic Energy is currently involved in a project which consists of the construction of a 2 MJ/500TW (351-nm) pulsed Nd:glass laser and which will be devoted to Inertial Confmement Fusion (ICF) research in France. With 240 laser beams and almost 10,000 m2 in coated area required, the proposed megajoule-class laser will be the largest laser system ever built in the world. Room temperature and atmospheric pressure deposited coatings such as sol-gel for antireflective (AR) applications and silicone for environmental protective coatings, with high optical and laser-included damage performance, can be applied at a low cost compared to conventional vacuum deposition processes. Today, we are using such a technology to AR-coat prototypes of lenses, windows, blast-shields, debris-shields, flashlamps and harmonic converters required by our proposed megajoule-class laser. This technology has also been selected for the preparation of multilayer highly reflective (HR) coatings for use as cavity-end deformable mirrors in the laser system. Due to their suitability to ensure appropriate deposited optical thickness, room-temperature deposition techniques such as dip-, spin- or laminar-flow-coating have been optimized for such laser coating production.
A novel optical coating devoted to reduction of CRT panel specular reflection has been developed using the sol-gel route. The sol-gel antireflective (AR) coating is made from tantalum and silicon oxide-based solutions. First layer is an hybrid material based on polymeric mixture of tantalum with silicon oxide. Second and third layers are respective containing tantalum oxide and silica polymeric matrix, since they correspond to the high and low index thin film in the optical stack. Sol-gel synthesis has been carried out starting from cheap precursors in order to produce metallic alkoxide-based solution, each one suitable for liquid- deposition technique use such as dip or spin-coating. After layer deposition, a curing step is required. Both thermal and UV-curing could include layer densification and generate final coating properties. UV-curing is performed using short wavelength irradiation and thermal baking step does not exceed 150 degrees C temperature. CRT front panel prototypes have been produced with sol-gel AR-coat for test measurements. The process time including deposition by dip- coating and curing, is about one hour. This three-layer antireflective coating has been optimized to offer scratch- resistance, easy-clean and broadband antireflection properties on CRT panels.
A novel optical coating developed to reduction of specular reflection has been developed using the sol-gel route. The sol-gel antireflective (AR) coating is made from tantalum and silicon oxide-based solutions. First layer is deposited from a solution based on polymeric tantalum oxide. Second layer is containing silica polymeric matrix in order to get a double-layer optical stack. Sol-gel synthesis have been carried out starting from cheap precursors in order to produce metallic alkoxide-based solution, each one suitable for liquid-deposition technique use such as dip-coating. After layer deposition, a curing step is required. Both thermal and UV-curing could induce layer densification and generate final coating properties. Thermal baking step does not exceed 150 degrees C temperature. This two-layer antireflective coating has been optimized to offer scratch- resistance allowing easy-cleaning and also broadband anti- reflection property onto various substrate. Experiments of AR-coating deposition onto large-area high-power laser glass plates is described. Based on calculations, the amplification yield using such a sol-gel coating onto LMJ- blastshields is evaluated to be ca. 7 percent.