An apparatus for measuring the physical properties of a sample by optically monitoring the response of the sample to illumination by ultrashort optical pulses utilizes a common path optical interferometer of a Sagnac type that can measure physical properties at normal incidence. The interferometer is a two-arm Sagnac interferometer featuring three beam splitters. A sample is excited by a beam of ultrashort optical pulses, and variations in intensity and phase of another optical beam are detected. This enables a wide range of measurement of physical properties such as thickness, sound velocity, and thermal properties of substances.
