Apparatus and method for beam diagnosis on laser processing optics

A method for determining geometrical parameters of a laser beam, comprising the steps:

• focusing the laser beam into a processing region using an optical system,
• positioning a reflector element with a first surface in a positioning region between the optical system and the processing region,
• reflecting back of a portion of the focused laser beam on the first surface of the reflector element in the direction of the optical system,
• output coupling a portion of the back-reflected beam portion,
• determining at least one geometric parameter from the output coupled beam portion using a beam diagnostic device,
• wherein the first surface of the reflector element is curved, and
• wherein the reflector element is positioned in a positioning region, in which a mean curvature of a wave front of the focused laser beam that is focused by the optical system is equal to a curvature of the first surface of the reflector element.