The thermally induced deformation of different copper mirrors for infrared laser beams was simulated using a finite element method. The deformation is highly dependent on the mirror design. Several mirror designs with different cooling arrangements have been simulated. A mirror design with a set of straight cooling channels and a stiff body shows the least deformation. The cooling channel diameter, spacing, surface distance and overall mirror thickness were varied at this design and the surface deformation was compared. The mirror design not only influences the overall expansion of the mirror but also the shape of the surface deformation. The optimized mirror geometry provides small quality degradation at beam guidance and the possibility of an online beam monitoring method by measurement of the local surface deformation. The method can serve as a device for online and beam position detection. Deformation of the surface of metallic laser mirrors with different cooling techniques. 5 mm thick flat backside cooled mirror, 5 mm thick mirror with backside honeycomb structure, mirror with 5 mm surface layer, cooling channels and stiff body