Spectroscopic monitoring of FeO fluorescence for laser treatment of steel surfaces in air

Laser treatment of steel surfaces in air using continuous-wave radiation emitted by a fiber laser at 1.07 μm is investigated using a spectroscopic method that monitors the presence of FeO molecular fluorescence. For all conditions tested, the irradiance levels remained below 106 W/cm2 thus inhibiting the formation of plasma. In this paper, we demonstrate that FeO emissions are related to laser-induced steel vaporization and can be used to monitor the performances of the laser system for drilling and cutting applications. The heated Fe atoms oxidize rapidly forming solid and liquid FeO at the interface with the oxygen-filled atmosphere. As the formation of FeO is exothermic and that the presence of the oxide further increases laser absorption, the laser-induced oxide is rapidly vaporized and ejected off the surface, leaving an empty hole. The presence of FeO molecules can be monitored via the characteristic fluorescence emitted from the well-known orange system which is excited by the treating of laser itself. Excellent quantitative agreement was found between the FeO signal strength and the volume of material ablated by the laser beam in a drilling configuration allowing real-time monitoring of the interaction for process optimization.