Modern electro-optical systems often include several channels for imaging at different bands. These systems are capped with a window assembly in which the optical windows are cemented to a frame using dedicated adhesives intended to seal the system’s internal optics from the outside environment. Such window assemblies may include several optical windows made of different materials, depending on the necessary spectral bands, and are usually very expensive. Occasionally, these windows, become damaged due to exposure to severe environments, and require replacement. In such a case, there is a need for a detachment process to remove and replace only the defective part without damaging the rest of the windows or the metal frame. In this work, we have developed a dedicated tool which enables cost-effective and selective detachment of only the defective parts without any damage to the rest of the window assembly. The detachment process is based on a 3-axis hotwire automated system that was inspired by the foam cutting industry. An electrical current is passed through a hotwire which is then used to selectively decompose the adhesive along the bond line. Parameters such as wire voltage, wire tension, and velocity were selected to yield the best performance, namely, quick and safe removal of the part. The flexibility of the system’s design allows for detaching windows of different geometries, materials, and sizes.
A common window material for applications that require the simultaneous use of visible and infrared wavelengths is zinc sulfide, which offers a high transmittance between 400 nm and 12 μm. Depending on the manufacturing process, zinc sulfide can, however, exhibit large scattering losses (>10%) which degrade the imaging quality in particular in the visible spectral range. In this contribution, the different sources for light scattering such as volume imperfections, surface roughness, and subsurface damage are analyzed individually for hot isostatic pressed chemical vapor deposited zinc sulfide and correlated to the structural properties resulting from the fabrication process.
Modern optical systems are often required to function in severe environmental conditions for prolonged periods without suffering from performance degradation. Essential parts of such electo-optical systems are windows, domes and other optical elements. These elements are almost always coated with an efficient, multispectral and highly durable antireflection coating. The complexity further increases when these coatings are applied on curved surfaces, such as hemispherical domes that are usually used in order to allow a wider field of regard. The durability of the coated optical component is dependent upon many different parameters such as deposition method and process parameters and the adhesion between layers and substrate. However, one very important parameter, which can have a significant impact on the durability and optical performance, is the stress state of the applied anti-reflection layers. This subject is mostly left untreated mainly due to the difficulty of characterization and modeling techniques and lack of thin film mechanical constants which sometimes significantly differ from the bulk constants of the same material. The stress state of the optical part is mainly determined by the mechanical properties of the coating materials and substrate, geometrical shape of the part and the thickness of the layers. In this work, both analytical and experimental approaches were used for characterization of stress distribution in thin optical films. Various single and multiple thin films were deposited using the electron beam evaporation technique onto Si 6'' wafers. The film thickness was measured using an ellipsometer and the bi-axial module of the different films was studied by measuring the change in radius of curvature of the Si substrate and applying Stoney's equation. The results were compared to the analytical Timoshenko solution of stress state in a single and multiple film structure. Knowing the stress state of a multi-layer coating would allow the engineer to design a part with the desired stress state which alongside the optical design, is expected to bring about a significant improvement in the durability and optical performance.
The design of a modern optical system often raises new challenges for manufacturers of high end optical components. One such challenge, which has become more and more common, is the requirement for highly durable hemispherical domes to allow for wide field of view. There are many difficulties to overcome before the final product can be made. In this paper we present some of the major difficulties of developing such domes made from ZnS grown by chemical vapor deposition (CVD). First, the CVD process which introduces the challenge of removing the grown raw dome from the graphite mold without causing cracking and breakage is discussed. Then, the challenges introduced by the electron beam (EB)-gun evaporation method, most commonly used for evaporating the anti-reflection coating, are presented. Amongst these challenges, the mounting of the dome in-side the coating chamber, the coating uniformity over the dome's curvature and the coating's environmental durability are the most difficult problems to overcome. The paper presents how computerized modeling along with experimental procedures can be combined to minimize the difficulties in the production processes and improve the overall product quality and yield.
Rugate optical coatings are films, in which the refractive index is varied in a continuous or quasi-continuous manner. These films are used in order to obtain an optical response that is otherwise difficult to achieve. The application of rugate filters is done by a periodical variation of the refractive index, e.g. sinusoidally. The average index and the index modulation determine the filter bandwidth, and the cycle period determines the notch position. Another unconventional method for filter design, also based on a periodical index variation, is the bragg method, in which notch filters are designed by combining very thin layers of one material in a matrix of another, each unit being λ<sub>0</sub> /2 optical thickness. At the Rafael Optical Coating Center, a CO<sub>2</sub> suppression notch filter was developed using both these methods. A 24 cycles digital rugate film was used to achieve a narrow notch filter with bandwidth of Δλ/λ=6.8%, the transmittance in the blocked area was 13%, and 90% in the rest of the 3-5μm region. With 58 layers bragg film, the achieved blocking was even better - 6% transmittance in the blocking-band center, with a nearly similar transmission in the non- blocked zone.