Studies of microwave absorption in liquids by optical heterodyne detection of thermally induced refractive index fluctuations

A new method for obtaining the microwave absorption spectrum of optically transparent condensed media is described. Phase fluctuations impressed on a single-frequency laser beam passing through a sample are detected by optical heterodyning. These phase fluctuations result from density fluctuations induced by the absorption of microwave energy. Preliminary experiments on water and ethanediol yield absorption spectra which are consistent with those obtained in previous investigations using conventional techniques.