In this paper, we describe recent measurements with the ASDEX Upgrade two-color CO2/HeNe interferometer in which a time resolution of 200 ns was obtained in the density measurement by digitizing the 40-MHz raw signals of the interferometer at 500 million samples/s and performing density reconstruction by software. It is illustrated that this high resolution is beneficial during massive gas injection for disruption mitigation, as it allows for the discovery of fast density oscillations in the frequency range from 200 kHz to [approximately]1 MHz, which shows clear correlation to magnetic measurements.