Performance of many optical circuits and systems, such as signal processing and sensing devices, is influenced by random fluctuations of the optical source emission field. This paper outlines a formalism for the analysis of laser phase noise effects on a general linear time-invariant optical system. Theoretical expressions are presented for the autocovariance function of the instantaneous output intensity which are valid for any source coherence time as long as the source intensity fluctuations are assumed to be negligible. Applications of the results to some fiber-optic systems such as Mach-Zehnder and recirculating delay line devices also are demonstrated, both in the coherent and incoherent regimes. Although the analysis is not limited just to fiber-optical systems, our attention is focused on fiber optics.