Beyond Fresnel: Absorption of fibre laser radiation on rough stainless steel surfaces

Abstract

The standard procedure for the calculation of the angle-dependent absorptivity consists of applying the well-known Fresnel formulas to the material property refractive index n and extinction coefficient k. However, the Fresnel formulas are only applicable for perfect smooth surfaces. Hitherto the effect of rough surfaces on absorptivity values and profiles are not thoroughly explored. Yet, this knowledge is vital for many kinds of theoretical consideration in computational models as they are applied in laser material processing, imaging and metrology. Therefore, we present experimental results on the angle-dependent absorptivity of stainless steel with five different surface finishes for fibre laser radiation with a 1.07 μm wavelength and for s- and p-polarisation separately. We can show that for probes which fulfil the modified Fraunhofer smoothness criterion at normal incidence (R z