Observation of refractive index gradients in a laser-generated plume using laser schlieren imaging

Understanding the interaction between a pulsed Nd:YAG laser beam and the plume generated above the surface of a workpiece during laser welding is important for process modeling and control. We present a laser schlieren method which can be used to observe this interaction by imaging the refractive index gradient within the plume. The laser schlieren technique uses an expanded, collimated probe laser beam to image a phase object onto a filter with a radial gradient in optical density. The probe beam is focused through the filter and onto the image plane of an intensified high-speed camera, so that the temporal variation of the laser/plume interaction may be observed over the duration of the laser pulse. Qualitative measurements of the spatial distribution of refractive index are presented. The velocity of the vaporized metal forming the plume is also measured for a stainless steel weldment.