The Effects of Thermal Radiation on Selected Optical Components

Abstract

SummaryThe materials studied were (1) Schott glass absorptive filters, (2) General Electric's (GE) polycarbonate thermoplastic Lexan, (3) single and polycrystalline Ge, and (4) Eastman Kodak Irtran 1, sintered MgF2. The response to thermal radiation of the optical materials investigated ranged from no discernible damage for Irtran 1 to catastrophic breakage at fairly short irradiance times (tma! of 0.2 to 0.45 s) in the lower intensity (40-cal/cm s) focus for the Schott glass absorptive filters. The observed damage levels and their variation with sample size was satisfactorily explained by our thermal stress model. The lack of damage in Irtran 1 was also explained by the same model. It was probably differential thermal expansion between the anti-reflection layers and the polycrystalline Ge substrate that caused the plastic flow that occurred on this sample at irradiation time of tmax = 1.00 s in the 300-cal/cm2s focus. The optical transmission was not affected, but the surface polish was destroyed. An uncoated sample of single crystal germanium irradiated under similiar conditions showed evidence of surface melting, but no plastic flow. Lexan showed a continuum of damage beginning with the generation of small bubbles beneath the surface and ending with charring and complete destruction of the surface.

Infrared Desorption of Neutral Molecules Embedded in Transparent Matrices

“…graphite, produces very pronounced damage to the crystal sur face and results in the ejection of neutral KBr cluster molecules. Experiments based on this principle will be reported in a future publication [40], A desorption mechanism based on the relatively slow relaxation of a strain field induced in a relatively transparent sample by a low power IR laser pulse (see also [41] and [42]) explains many of our experimental results. This mechanism does not discount the role played by thermal evaporation in the desorption of molecules as small as dipeptides.…”

Direct laser fabrication of dense microlens arrays in semiconductor-doped glass

“…experiment was 3 min, which for all practical purposes corresponds to the infinite time limit of heat diffusion. 13 Figure 6 shows optical microscope images of such a set with ͑a͒ 30 m center-to-center separation between neighbors, and ͑b͒ identically prepared set with 20 m distance between neighbors. For set ͑a͒ all lenses have nearly the same size, which is not the case for set ͑b͒, where the lens produced first in each cluster is the largest and is identical to the single lens.…”