Influence of Laser Illumination Geometry on the Power Distribution Advantage

The power distribution advantage, used to limit surface optical damage by distributing laser power over a wide area, is found to depend not only on the area but also on the geometry of the illuminated region. This is demonstrated by using both theoretical heat transport calculations and experimental laser optical damage measurements. For a circular or rectangular illumination region, the power distribution advantage is a strongly nonlinear function of illumination area. For long irradiation times typical of those used in Raman imaging applications, the power required to reach a specified damage threshold temperature scales as the square root of the illumination area. However, for illumination times that are short in comparison with the ratio of the illumination area to the thermal diffusivity of the sample, the power distribution advantage is a more linear function of illumination area. Experimental measurements of the melting threshold of black plastic as a function of laser spot size confirm the theoretical predictions in both the long and short irradiation time limits.