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A technique for a time resolved in-situ measurement of the optical phase and therefore the refractive index in the material was developed. A modified Mach-Zehnder interferometer combined with microscope objectives is used for the measurement of phase changes in the processed material sample. The white light is generated by focusing 80 fs-laser pulses in a sapphire crystal using a micro-lense array to minimize temporal and spatial fluctuations in the white-light continuum. The optical phase shift and therefore the refractive index of the material are calculated from the interference images. The change in refractive index during welding induced by heating and by compression of the glass has been detected with a temporal resolution in the ps-range and a spatial resolution of several microns.