Apparatus and method for focal point control for laser machining

A method of detecting break through from a blind hole to a through hole in a workpiece during laser machining comprising the steps of

• a) providing a sensor and a laser and a shared optical path for:
• i) an optical measurement beam delivered and sensed by the sensor for sensing a distance to a surface of a workpiece, and:
• ii) a laser beam generated by the laser for machining a surface of the workpiece
• wherein the optical path includes an optical focusing element for focusing both beams;
• b) directing the optical measurement beam through the optical path to determine a distance to a surface of the workpiece;
• c) adjusting the distance from the optical focusing element to the surface of the workpiece to position the focal point of the laser beam at a desired location relative to the surface of the workpiece;
• d) directing a laser beam through the optical path to the workpiece to form a new surface therein, and
• e) comparing the present distance sensed with the last distance sensed and repeating at least steps b) and d) and e) when the difference between the present distance sensed and last distance sensed is less than a predetermined value.

In a system for machining a workpiece with a laser beam, a control apparatus for maintaining a controlled distance between a focusing element and the workpiece, the control apparatus comprising, in combination:

• a) an optical sensor having a measurement beam for measuring the distance to a part surface of the workpiece;
• b) a laser beam for machining the workpiece, the laser beam directed along a common axis with the measurement beam to the workpiece;
• c) a focusing element receiving and focusing the measurement beam and laser beam at respective focal points;
• and
• d) control means for controlling the location of the focal point of the laser beam based on the distance sensed by the measurement beam.

Apparatus for measuring the distance from a focal point of a laser machining beam to a surface of a workpiece to be machined by the beam comprising:

• a) a laser machining system having a laser beam directed along a predetermined optical machining path;
• b) an optical distance measuring sensor having a measurement beam directed along at least a part of the optical machining path;
• and
• c) an optical focusing element having a predetermined focal length characteristic with respect to optical frequencies, the optical focusing element being located in a shared portion of the optical machining path such that the focal length characteristic of the optical focusing element simultaneously provides a distance measuring focal length for the optical distance measuring sensor using the measurement beam and an operating focal length for optical machining using the laser beam.