Probing multipulse laser ablation by means of self-mixing interferometry

TL;DR

This paper demonstrates the use of self-mixing interferometry to monitor laser microdrilling processes by analyzing plume dynamics, providing a nonintrusive method to assess ablation conditions and process quality.

Contribution

It introduces an analytical model for plume expansion during multipulse laser ablation and validates SMI as a nonintrusive monitoring tool for laser micromachining.

Findings

SMI signals correlate with vaporization-dominated ablation.

Refractive index changes indicate process regime shifts.

SMI can estimate hole quality parameters indirectly.

Abstract

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.