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) sharing an optical path by a sensor and a laser wherein:
• i) an optical measurement beam of a first wavelength is delivered and sensed by the sensor for sensing a location of a surface of a workpiece, and
• ii) a laser beam of a second wavelength different than the first wavelength is generated by the laser for machining a surface of the workpiece wherein the optical path includes an optical focusing element for focusing both beams, said focusing element having distinct and separate focal points for the optical measurement beam and said laser beam the separate focal points having a known focal separation along an optical axis of the focusing element;
• b) directing the optical measurement beam through the optical path to determine a separation distance between the focal point of the measurement beam and a surface of the workpiece;
• c) moving the surface of the workpiece to adjust the separation distance between the focal point of the measurement beam and the surface of the workpiece to position the focal point of the laser beam substantially at a desired location relative to the surface of the workpiece;
• d) directing a laser beam through the optical path to the workpiece to remove material from the surface thereby forming a new surface;
• and
• e) comparing the present distance sensed with the last location sensed and repeating at least steps b) and d) and e) when the difference between the present location sensed and last location sensed is less than a predetermined value.

Apparatus for measuring the location of a focal point of a laser machining beam with respect to a surface of a workpiece to be machined by the beam comprising:

• a) a laser machining system having a laser beam of a first optical wavelength directed along a predetermined optical machining path;
• b) an optical measuring sensor having a measurement beam of a second optical wavelength that is different than the first optical wavelength directed along at least a part of the optical machining path;
• and
• c) an optical focusing element having a different predetermined focal length characteristic with respect to said first and said second optical wavelengths, 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 measurement focal length for the optical distance measuring sensor using the measurement beam and an operating focal length for optical machining using the laser beam.