Detection of ultrafast phenomena by use of a modified Sagnac interferometer

A time division interferometer based on the Sagnac geometry for monitoring ultrafast changes in the real and the imaginary components of the refractive index as well as phase changes that are due to surface displacement is described. The common path design obviates the need for active stabilization and renders the interferometer less sensitive to parasitic fluctuations from thermal and acoustic fields. The operation of the interferometer is demonstrated by detection of temperature changes and coherent phonon generation in a gold film.