40-Gbit/s transmission systems

Summary form only given. The transmission capability of fiber-optic systems has reached the terabit/sec region. In order to realize such potential transmission performance using existing fiber cables, management of chromatic fiber dispersion is a significant issue. Research on high-speed TDM systems is focused on the zero dispersion wavelength region. On the other hand, WDM transmission technology requires some dispersion to reduce crosstalk due to four-wave mixing along the fiber. Recently, extremely wideband erbium-doped fiber amplifiers (EDFAs) were developed. Such a wideband optical amplifier enables simultaneous amplification of high-speed TDM and WDM signals. Future photonic networks have to optimize each transmission speed against intrinsic dispersion characteristics to fully utilize the bandwidth of currently existing fibers. It is natural that modulation scheme and dispersion equalization should be carefully considered. We describe simple and broadband optoelectronic technologies, which provide low-cost and upgradable transmitters and receivers. In order to realize a 40-Gbit/s system under these requirements, we propose several new technologies; automatic dispersion equalization technique, optical duobinary modulation scheme, which doubles the dispersion tolerance compared to conventional nonreturn to zero (NRZ) modulation, and several novel device technologies.