Correction of Brillouin linewidths measured by multipass Fabry-Perot spectroscopy.

The sharper instrumental profile of the multipassed Fabry-Perot etalon introduces distortions to Brillouin spectra which may differ considerably from those introduced by the single-pass etalon. The half-width at half-height of the observed profile is calculated for five-pass and three-pass spectrometers. Phonon spectral widths may be extracted from the graphs and appropriate relationships are presented. These calculations are valid for well aligned etalons when the phonon lineshape is Lorentzian and cover a wide range of instrumental and phonon spectral widths.