System and method for focal position control

An optical system, comprising:

• a laser optics;
• a beam analysis device for determining a state of a laser beam, wherein the laser optics is configured to generate a laser beam focus and comprises:
• an interface of an optical element of the laser optics for generating a partially-reflected beam from the laser beam and for propagating the partially-reflected beam counter to a direction of the laser beam, and
• a partially-reflecting beam splitter for coupling out a measurement beam from the partially-reflected beam towards the beam analysis device,
• wherein the partially-reflected beam or the measurement beam has an intermediate focus, and wherein the beam analysis device is configured to receive the measurement beam coupled out by way of the beam splitter and comprises:
• a partial beam imaging device, which is configured to receive a first measurement beam, and which comprises at least a first selection device for forming a first partial beam from a first partial aperture region of the first measurement beam, and wherein the partial beam imaging device further comprises an imaging device with at least one imaging optical element,
• a detector unit, with at least one at least one-dimensionally spatially-resolving light-sensitive detector, which is arranged at a distance (zos) from the partial beam imaging device, and
• an evaluation unit that is connected to the detector unit and is configured to process signals from the detector unit,
• wherein the first selection device is arranged off-centre with respect to an optical axis for irradiating the first measurement beam,
• wherein the partial beam imaging device is configured to image the first partial beam for generating a first beam spot onto the detector unit,
• wherein the detector unit is configured to capture an intensity distribution of the first beam spot,
• wherein the evaluation unit is configured to determine a lateral position (a1, a1′) of the first beam spot, and
• wherein the evaluation unit is configured to determine changes in the lateral position (a1, a1′) of the first beam spot over time,
• wherein the beam analysis device constitutes a focal position sensor configured for monitoring an axial focal position of the laser beam focus by monitoring the focal position of the intermediate focus, wherein changes in an axial focal position of the laser beam focus and hence of the focal position of the intermediate focus are correlated with the changes in the lateral position of the first beam spot on the detector unit,
• wherein the optical system is configured such that by virtue of the partial reflection at the interface, a focal position of the intermediate focus is optically coupled to a focal position of the laser beam focus, wherein changes in the focal position of the laser beam focus simultaneously cause an alteration in the focal position of the intermediate focus,
• wherein the interface for generating the partially-reflected beam is the interface last transited by the laser beam before the laser beam exits the laser optics.