Phase noise and shot noise are often the fundamental limiting factors of sensitivity in precision optical systems. These noises determine the so-called standard quantum limit (Braginsky et al., 2003), see Section 1.4. At the same time the fundamental frequency stability in high-finesse optical resonators may also be determined by other fundamental effects originating in mechanical, thermodynamical, and quantum properties of solid boundaries (Braginsky et al., 1979). Many of these effects were initially identified and calculated on the forefront of laser gravitational wave antenna research (see Chapter 14) but are becoming increasingly important in other optical systems (Numata et al., 2004; Webster et al., 2008; Matsko et al., 2007; Savchenkov et al., 2007) (see Chapters 15, 16, and 17).

Excess optical phase noise is added to a probe optical wave reflected from mirrors forming an optical cavity due to variation of boundary conditions produced by fluctuations of the surface and optical thickness of the multilayer coating. We characterize below different effects leading to phase noise starting from fluctuations originating in the mirror's substrate and in its interferometric multi-layer reflective coating. In addition to intrinsic noises produced by internal properties of optical mirrors, there are also extrinsic noises imprinted onto the output phase due to different nonlinear effects in the bulk and coating of the mirrors. As, for example, fluctuations of input power producing fluctuations of thickness and refractive index in the coating due to local heating (the photothermal effect, see Sections 3.4 and 3.10).