Tadashi Okiyama

Evaluation of 4-Gbit/s optical fiber transmission distance with direct and external modulation

Transmission characteristics for a recently modulated measured distributed-feedbacked (DFB) laser and an externally modulated DFB laser using a Ti:LiNbO/sub 3/, Mach-Zehnder modulator at 4 Gb/s are discussed. The transmission characteristics are estimated by an advanced eye-pattern analysis method. The maximum measured fiber dispersion with a directly modulated laser is 100 to 140 ps/nm when the chirp power penalty is 1 dB. However, for external modulation, there is no power penalty after transmission over a 2220-ps/nm dispersive fiber. This confirms that external modulation has superior transmission characteristics. The modulation scheme for 4-Gb/s systems in terms of these results is discussed. >

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.<<ETX>>