Modulation schemes for multi-gigabit optical transmission systems

Compares four intensity modulation schemes in terms of their spectrum broadening and transmission characteristics. The schemes examined are direct modulation, Mach-Zehnder interferometer type modulation, electroabsorption type modulation, and a direct phase-shift and self-homodyne intensity modulation (DPSH-IM) method proposed here. The dispersion-limited transmission distance of these schemes has been estimated by computer simulation. It is shown that direct modulation of currently available distributed feedback lasers suffers large waveform distortions due to chirping and chromatic dispersion in the fibre. It is concluded that unless chirping characteristics of lasers are greatly improved, low-chirp modulation schemes such as the Mach-Zehnder interferometer type modulation, electroabsorption type modulation, and the DPSH-IM will be adopted for multigigabit long-distance optical transmission systems