Superheterodyne light beating spectroscopy for Rayleigh–Brillouin scattering using frequency-tunable lasers

Abstract

We developed a new optical superheterodyne method for light scattering using frequency tunable lasers and succeeded in measuring Brillouin spectra of simple liquids in a wide frequency range from 10 MHz to 3 GHz. In this method the speed of a photodetector and electric circuits does not limit the upper frequency bound, since the superheterodyne detection of the signal allows us to shift the optical beat frequency down to a desired fixed frequency suitable for electric signal processing. Thus an almost ultimate signal-to-noise ratio is realized at any phonon frequency. This is a great advantage over conventional optical beating methods. It is realized by the continuous tunability of the frequency of the laser used as a local oscillator light for superheterodyne detection. Our method provides an extremely high frequency resolution (∼300 kHz), which is determined by the stability of an optical frequency difference between the two lasers under its feedback control. We call this method “superheterodyne Brillouin spectroscopy.” The method has a high potential for studying the dynamics in a wide class of transparent condensed matters.