Probing multipulse laser ablation by means of self-mixing interferometry

In this work, self-mixing interferometry (SMI) is implemented inline to a laser microdrilling system to monitor the machining process by probing the ablation-induced plume. An analytical model based on the Sedov-Taylor blast wave equation is developed for the expansion of the process plume under multiple-pulse laser percussion drilling conditions. Signals were acquired during laser microdrilling of blind holes on stainless steel, copper alloy, pure titanium, and titanium nitride ceramic coating. The maximum optical path difference was measured from the signals to estimate the refractive index changes. An amplitude coefficient was derived by fitting the analytical model to the measured optical path differences. The morphology of the drilled holes was investigated in terms of maximum hole depth and dross height. The results indicate that the SMI signal rises when the ablation process is dominated by vaporization, changing the refractive index of the processing zone significantly. Such ablation conditions correspond to limited formation of dross. The results imply that SMI can be used as a nonintrusive tool in laser micromachining applications for monitoring the process quality in an indirect way

Evolution of Laser Ablation Plume Measured by Self-Mixing Interferometry

Laser ablation is the basis of most of the laser micromachining processes. Depending on the pulse duration, wavelength, intensity of the beam, as well as the material type, material removal may occur in the form of vaporization, melt expulsion, or direct sublimation. Most commonly material removal is accompanied by a plume formation. The characteristics of the plume can be analyzed to assess the ablation performance. Non-invasive optical methods for the purpose can be further exploited in industrial laser micromachining applications, if opportunely designed and implemented. This work uses an interferometric approach to investigate the optical path changes induced by the ablation plume formed during the laser percussion drilling of different metallic and ceramic materials. A self-mixing interferometer is installed inline to a laser micromachining setup composed of a ns-pulsed green fiber laser