A significant obstacle in ultrafast laser micromachining of multi-layer or heterogeneous micro-structures is the lack of an online diagnostic method to determine which material is being ablated during the material removal process. This problem arises because ultrafast lasers are generally insensitive to the material being processed. One promising technique to address this problem is the use of laser-induced breakdown spectroscopy (LIBS) by which the plasma generated during the laser-material interaction can be collected and analyzed to provide information regarding the elemental composition of the mate-rial being ablated. In this work, a real-time feedback control system for the ultrafast laser micromachining process based on the LIBS technique is built. The ultrafast LIBS signal is first characterized to prove the feasibility. Characteristics of spectral emission, temporal evolution, spatial heterogeneity of the ultrafast LIBS signal, effects from laser machining factors, etc., are discussed. Comparison methods for identifying the material emission patterns are then studied. Effective algorithms from the study are implemented into the control system software, SPECOMP, developed in the laboratory. Issues on the real-time control process are discussed. The real-time controlled machining process has then been applied to the machining of micro-structures on thermal sprayed material. Compared to the passive machining process without any such feedback control, SPECOMP system provides several important advantages including less damage to the substrate layer, shortened machining time, and more uniform feature sizes.
Hydrogen isotopic analysis of nuclear reactor materials using ultrafast laser-induced breakdown spectroscopy
