Diamond's extremely wide transparency, combined with its other exceptional properties including hardness, strength and
thermal conductivity make it a desirable material for optical windows. Polycrystalline diamond grown by chemical
vapour deposition (CVD) has become the preferred window material for high power CO2 laser systems since its
development in the 1990s. The range and availability of diamond materials is expanding, and in recent years has been
extended to include CVD single crystal diamond. This paper reviews the quality of these materials, looking at optical
scatter and absorption around 1 and 10 microns, along with their thermal and mechanical properties. We also discuss
selection of appropriate grades and how they may best be integrated into demanding optical applications.
Chemical Vapour Deposited (CVD) diamond can now be fabricated in the form of large planar windows (up to 120mm in diameter and 2mm thick) and hemispherical domes (up to 70mm in diameter) suitable for operation as ultra-robust, aerospace infrared (IR) apertures. This paper describes the optical properties of such components. Many of the optical properties of large area CVD diamond windows are governed by its polycrystalline structure, in this paper its fracture
strength properties are also related to its structure. It is shown that 3-point bend techniques are an appropriate method for testing the strength of CVD diamond and that its strength is dictated by internal bulk flaws that are similar in size to the grains of the diamond. Its unsurpassed rain and sand erosion properties are briefly discussed and its shown that its solid particle erosion properties are related to its grain structure and that in liquid impact its properties are also significantly affected by its polycrystalline nature.
CVD diamond optics are now available for far infrared airborne applications in both flat plate and dome geometries. For many applications, these require durable coatings for antireflection and/or oxidation protection. With a high characteristic modulus, diamond may allow the use of relatively weaker materials for such coatings provided that the coatings are well-adhered to the substrate. Single layer and two-layer designs have been assessed based on yttria, ytterbia and silicon. Magnetron sputtered examples have been assessed with single layer coatings reducing single surface reflectivities by 12%, whilst maintaining transmission to 13.5 micrometers . The erosion properties of these coated optics, assessed by water jet impact testing (MIJA), are found to be exceptional, with damage thresholds > 350 ms-1 achieved, with a 0.8 mm jet size. The pre-deposition treatment of the diamond has been found to influence the strength of the diamond/coating interface and thus the durability of the coatings. The nature of the diamond surface and the effects of oxidizing pre-deposition treatments have been investigated by X-ray electron spectroscopy (XPS). Strong oxidizing etches conventionally used to clean diamond can leave the surface rich in chemisorbed oxygen with a range of valence states evident in XPS data. In comparison, the valence states of the carbon atoms at hydrogen terminated surfaces have a much narrower distribution. The type of carbon oxygen bonding on the surface of the diamond is critical to adhesion of transition metal oxide based coatings.
KEYWORDS: Diamond, Absorption, Chemical vapor deposition, Modulation transfer functions, Far infrared, Phonons, Domes, Optical properties, Surface finishing, Long wavelength infrared
Chemical Vapour Deposited (CVD) Diamond can now be fabricated in the form of large planar windows (up to 120mm in diameter and 2mm thick) and hemispherical domes (up to 70mm in diameter) suitable for operation as ultra-robust, aero-space infrared (IR) apertures. This paper describes the optical and IR properties of such components, reporting in detail on the short wavelength IR properties and the variation in optical properties with sample temperature. Flat CVD diamond windows are currently being used with great success in a number of long wavelength infrared (LWIR) applications. The paper discusses how the optical properties, such as absorption and scatter, differ when operating at shorter wavelengths and speculates on the usefulness of CVD diamond as a multi-spectral window. Aerospace windows and domes are often required to perform at elevated temperatures and thus the change in IR properties under these conditions is of interest. The paper describes a series of studies into the transmission, emission and absorption of CVD diamond as a function of temperature, using spectroscopic techniques. The extension of the CVD diamond growth and processing technologies to geometries other than flats is at an advanced stage of development and data on the IR properties of state-of-the-art, high geometrical tolerance diamond domes will be presented, including MTF assessment at 10.6micrometers .
The structural, optical and mechanical properties of CVD diamond grown using chemistries on the H-CO tie line have been investigated. A microwave plasma CVD system has been used with methane and ethylene containing gas chemistries to grow free standing optical quality diamond layers. When these feed stock gases are combined with carbon dioxide and hydrogen they enable the H-CO tie line to be traversed up to the central region of the Bachmann growth diagram. The structural properties were assessed using SEM, cathodoluminescence, Raman spectroscopy and x-ray diffraction techniques. The optical properties were assessed using several techniques including measurements of spectral emissivity over a range of temperatures and the role of nitrogen impurities identified. The trends in the optical characteristics will be discussed in relation to differences in mechanical properties with a view of evaluating the viability of using oxygen-based chemistries for the fabrication of diamond components.
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