The thermal noise in multilayer coating

We consider thermal fluctuations in multilayer interferometric coating taking into account light propagation inside the coating. In particular, we calculate the reflected wave phase as a function of small displacements of the boundaries between the layers using transmission line model for interferometric coating and derive formula for spectral density of reflected phase in accordance with Fluctuation–Dissipation Theorem. We apply the developed approach for calculation of the spectral density of coating Brownian noise which makes main contribution into noise budget of second generation gravitational wave detectors.