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

An apparatus comprising:

• a material processing beam source that produces a material processing beam that is applied to a sample location in a material modification process, wherein the material modification process is a welding process or a sintering process;
• 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 to a phase change region created in the material the material modification 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. substantially matched to an optical path length to the Phase change region;
• 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 material modification process;
• and
• (i) a record generator that generates at least one record based on the detector output at a plurality of times;
• or
• (ii) a feedback controller that controls at least one processing parameter of the material modification process based on the detector output or
• (iii) both (i) and (ii).