Development of a time-resolved white-light interference microscope for optical phase measurements during fs-laser material processing

Abstract

A modified Mach-Zehnder interferometer set-up combined with microscope objectives has been developed for the measurement of phase changes in the processed ma- terial sample, like modification and melting of glass. The white light is generated by focusing ultrafast laser radiation (tp = 80 fs) in a sapphire crystal using a micro-lens array to minimize temporal and spatial fluctuations in the white- light continuum. Lateral and coaxial pump-probe measure- ments of the phase changes during material processing are performed using two coupled ultrafast laser sources at dif- ferent repetition rates (frep = 1 Hz-1 MHz). The optical phase shift and therefore the refractive index of the material are calculated from the interference images using two ap- proaches. The knowledge of the refractive index during the laser processing with a temporal resolution in the ps-range and a spatial resolution of several microns leads to a bet- ter understanding of the initial processes for the permanent material modifications.