High-precision measurements of Brillouin scattering frequencies

A new method for the accurate measurement of Brillouin line shifts is presented. By modulating a reference beam at a microwave frequency corresponding exactly to the shift of the Brillouin line, the Brillouin scattering frequency is obtained by direct comparison. Previous errors due to imperfect calibration and nonlinearity of the Fabry-Perot interferometer are therefore eliminated. The first absolute measurement of 35-GHz hypersound velocity with a 10(-4) relative uncertainty is reported.