to the direction of propagation of the laser beam 14 u2018 longitudinal u2019 corresponds to the direction of beam propagation in conventional notation the transverse plane is designated as the x y plane whereas the longitudinal direction is designated as the z direction for the purpose of transverse deflection of the laser beam 14 i e in the x y plane the scanner 16 may for example include a pair of galvanometrically actuated scanner mirrors which are capable of being tilted about mutually perpendicular axes alternatively for example a transverse deflection by means of an electro optical crystal is conceivable for the z control of the position of the focus the scanner 16 may contain for example a longitudinally adjustable lens or a lens of variable refractive power or a deformable mirror with which the divergence of the laser beam 14 and consequently the z position of the beam focus can be influenced it will be understood that the components of the scanner 16 serving for the transverse focus control and for the longitudinal focus control may be arranged distributed along the beam path of the laser beam 14 and in particular apportioned to different modular units for example the function of the z focus control may be fulfilled by a lens arranged in beam expanding optics beam expander e g galilean telescope whereas the components serving for the transverse focus control may be accommodated in a separate modular unit between the beam expanding optics and the focusing objective 18 the representation of the scanner 16 as a.