Laser Damage to Metal Mirrors at Nonormal Incidence

Virtually all laser damage experiments have been performed at normal incidence. However, many applications are for nonnormal incidence, and, obviously, it would be appropriate to characterize the surfaces with the intended geometry and polarization. This paper presents damage threshold data on bare diamond-machined copper at 45-degree angle of incidence at 3.8 μm wavelength. Within experimental uncertainty, the melt threshold for p-polarization is identical to that obtained at normal incidence on the same surface. The threshold for s-polarization is a factor of two larger. The experimental values are compared with theoretical melt threshold calculations and are found to be in reasonable agreement. The polarization dependence is seen to arise from the fundamental infrared optical properties of metals. The relationship between normal incidence and 45-degree incidence thresholds provides a valuable cross-check on the systematic errors in laser damage measurements.