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Monolithic diamond deposits lack the degree of transparency required for operation as transmissive optics elements in the mid-IR: at longer wavelengths (LWIR), it can be reasonably expected that singlephonon absorptions will be tolerable in the sense that self-emission phenomena at elevated temperatures should not be catastrophic. Besides surface hardness, the features that confer diamond its advantage are the high thermal conductivity and the low thermal expansion, which indicate that diamond has orders of magnitude more thermal shock resistance than some of the best competing materials. There is little doubt that the "new diamond" technology will provide a credible, if not outstanding LWIR dome material for tactical missiles operating at speeds up to but not beyond Mach 4 in the atmosphere. Defect-free, single-crystal diamond also emerges as a promising candidate material for high-average-power laser window applications in the near-IR. The power-handling capability (<1 MW independently of the window size! will be limited by la) thermally induced optical distortion, which can only be eliminated if absorption-free anti-reflection coatings become available, and (b) the edge heat-transfer coefficient, which must be greatly enhanced to take advantage of the exceptional thermal conductivity of diamond at low temperatures. Pressure-induced and thermally-induced stresses are of no consequence, but peak laser intensities in excess of, say 1 GW/cm2 should be avoided because of unfavorable non-linear refractive index characteristics. |
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