Methods and systems for coherent imaging and feedback control for modification of materials

An apparatus comprising:

• a laser beam source that produces a laser beam that is applied to a sample location in a material modification process, wherein the material modification process is a keyhole welding process and the laser beam source is configured to provide the laser beam with a beam intensity sufficient to extend a phase change region into the material to open a vapor channel;
• an imaging optical source that produces imaging light;
• an optical interferometer that produces an interferometry output using at least a component of the imaging light that is delivered into the phase change region created in the material during the keyhole welding process, the interferometry output based on at least one optical path length to at least one point in the phase change region compared to another optical path length;
• a detector that receives the interferometry output and produces a detector output that is indicative of a characteristic of the phase change region during the keyhole welding process;
• an interferogram processor that performs an analysis based on the detector output to produce a depth measurement reflecting how deep the laser beam has penetrated in the phase change region;
• and
• a feedback controller that controls at least one processing parameter of the material modification process based on the depth measurement reflecting how deep the laser beam has penetrated in the phase change region.