Optical amplification and modulation techniques for multigigabit IM/DD long-haul transmission systems

The authors discuss and compare several representative modulation techniques for IM/DD (intensity modulation/direct detection) systems, focusing on their tolerance of fiber chromatic dispersion. The modulation techniques examined are direct intensity modulation of the laser diode, Mach-Zehnder interferometer external modulation, electroabsorption external modulation and direct-phase-shift and self-homodyne intensity modulation (DPSH-IM). The authors discuss the impact of these techniques on the performance of long-haul systems using optical amplifiers. A 12-Gb/s transmission experiment using an Er-doped fiber amplifier is also described. External modulation and DPSH-IM seem to be more promising than conventional direct intensity modulation for multigigabit long-span transmission systems using optical amplifiers