In this work, the optimization of level spacing in terms of the extinction ratio variation in quaternary optical
communication systems is investigated, for signal-dependent noise dominance. It is shown that the performance of
optically preamplified quaternary communication systems can be optimized in terms of levels spacing using very simple
formulas, also derived in this work. For the null extinction ratio, sensitivity improvements of about 6 dB can be
achieved by proper location of signal levels in comparison with the sensitivity of equally spaced levels. At high
extinction ratio, this improvement is not so significant.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.