Optical noise coherence effects in a Mach-Zehnder system

In this paper we present an exact, quantitative theoretical study of coherence effects in the optical noise spectrum of a Mach-Zehnder system coupled to a random-phase source. A simple, analytic formula for the output noise spectrum is derived. This formula is exact and valid for arbitrary source coherence time. We demonstrate two new facts concerning coherence effects: (1) These effects give rise to quasiperiodic features in the noise spectrum with period 1/τ, where τ is the time-of-flight difference between the two interferometer arms. (2) When the source coherence time is of the order of magnitude of τ or larger, the noise spectrum is sensitive to changes in τ on the scale of the optical period. It is suggested that this phenomenon can be used for optical RF interferometry. The characteristics of the noise spectrum are studied analytically, and illustrated by several numerical calculations. The results can be used to estimate the optical noise in any interferometric system in which the main contribution to the output field comes from two optical paths