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

The standard procedure for the calculation of the angle dependent absorptivity consists of applying the well-known Fresnel formulas to the material properties 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 every kind of theoretical considerations in computational models as they are applied in laser material processing, imaging or metrology. Therefore, we present experimental results on the angle dependent absorptivity of stainless steel with five different surface finishes for fibre laser radiation with 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 (RZ < l/32), the Fresnel formalism is applicable. For the tested probes with higher surface roughness, the real absorptivity differs considerably from this solution: The maximum of absorptivity at the pseudo Brewster angle is decreased and shifted towards lower inclinations. Further, other effects are observed which are not explained by theoretical approaches like Fresnel or ray tracing models.