Theory of signal degradation in semiconductor laser amplifiers with finite facet reflectivities

An analytic expression is derived for the output signal spectrum from a semiconductor laser amplifier (SCLA) using an approach analogous to the Langevin equations of laser noise analysis. The authors present an analysis of the line broadening effects of SCLAs taking into account the effect of facet reflectivities. The signal linewidth is shown to increase with input linewidth and amplifier gain. There is an optimum facet reflectivity which gives the smallest linewidth increase. The results have significant implications in weakly coherent systems utilizing cascades of optical amplifiers