A general method which allows precise measurements of the frequency spectrum of a phase modulated wave down to a few cycles per second is presented. The momenta of the distribution of the instantaneous frequency (time derivative of the phase) can also be easily obtained. An experiment is described in which the study of the phase correlation induced by a laboratory generated thermal turbulence in a laser beam is described.
M. Bertolotti, M. Carnevale, B. Daino, and D. Sette, "Optical Processing of the Phase Correlation Induced by a Turbulent Medium in a Laser Beam," Appl. Opt. 9, 962-967 (1970)