Pulsed D2 - F2 Chain-Laser Damage To Coated Window and Mirror Components

Large-spot laser damage thresholds have been measured for bowl-feed-polished CaF2 and sapphire windows (bare and antireflection-coated) and for highly-polished copper mirrors (bare and carbyne-coated) at DF chain-laser wavelengths (3.58–4.78μm). The chain reaction between F2 and D2 was initiated by a magnetically-confined electron beam, producing DF-laser outputs of 10–20 J in pulses of 0.6 – 0.9 μsec (FWHM) duration. Energy extracted from a transmission-coupled unstable resonator was focussed using a CaF2 lens. A soft-aperture technique was employed to suppress effects of Fresnel diffraction so that uniform (top-hat) intensity profiles were obtained along the focussing beam. With this laser system, commercially-available antireflection-coated CaF2 and Al2O3 samples were measured to have damage thresholds in the range 21–27 J/cm2. Significantly larger damage thresholds were found for uncoated, polished samples of Al2O3, but damage resistance of uncoated polished CaF2 was measured to be equal to that of the best antireflection-coated CaF2 samples. A highly polished copper mirror was found to have the highest damage threshold of all the materials tested (58 J/cm2). Carbyne films of diamond-like hardness, a type of carbon coating, were applied to polished copper mirrors and bowl-feed-polished CaF2 surfaces. Such carbyne coatings as were prepared in this work contained numerous carbon-bearing particles that were easily damaged (∼10 J/cm2). However, regions of the irradiated carbyne film that were free of carbon particles withstood high laser fluences (25 J/cm2), suggesting that improvements in carbyne film preparation would yield attractive protective coatings of high damage resistance at DF wavelengths.