  We study the dynamics of an optical mode in a cavity with a movable mirror
subject to quantum Brownian motion. We study the phase noise power spectrum of
the output light, and we describe the mirror Brownian motion, which is
responsible for the thermal noise contribution, using the quantum Langevin
approach. We show that the standard quantum Langevin equations, supplemented
with the appropriate non-Markovian correlation functions, provide an adequate
description of Brownian motion.
