Distributed Raman amplification in the transmission fiber is an important technology for advancing the system performance. Co-directionally pumped Raman amplifiers can enhance the performance and allow more flexibility in the system design. However, several sources of non-amplified spontaneous emission noise need to be carefully considered in the amplifier design. The intrinsic cross gain modulation associated with the transient nature of Raman effect can impair the system performance, if the amplifier is not properly designed. We have isolated and measured the impairment due to cross gain modulation in 200-km bi-directionally pumped fiber spans. The penalty depends on the fiber dispersion characteristics and can be small for up to 20 dB on-off co-gain. The benefit of co-directionally pumped Raman amplifiers can be used in multiple long-span transmission to compensate the high loss while maintaining a low nonlinear impairment. It can also be used to extend the length of a single span and achieve a simple system configuration for unrepeatered applications. As opposed to using a span containing multiple fiber types and remotely pumped erbium-doped fiber amplification to achieve transmission over spans with > 60 dB loss, we have demonstrated an unrepeatered link over a single type of fiber with bi-directional Raman pumping. Using the simple conventional non-return-to-zero data format, we achieved transmission of 20 × 10Gb/s channels over a 300-km span of non-zero dispersion-shifted fiber. This simple system configuration provides an important option for terrestrial transmission in remote areas where service access is difficult.
Recent laboratory experiments have demonstrated that distributed Raman amplification, advanced modulation formats, optimized dispersion maps, and forward error correction are key technologies for 10-Gb/s and 40-Gb/s DWDM terrestrial transmission over 2000 to 6000 km. The transmission fiber's Raman gain efficiency and dispersion properties are thus important parameters. Future high-bit-rate, high-capacity installed systems will require advanced transmission fibers to extend their reach to at least 2000 km, a distance also specified by a high-profile U.S. government optical networking project. This paper will address a number of the enabling fiber properties, including dispersion, dispersion slope, Raman gain efficiency, and polarization mode dispersion. In addition, several recent experiments will be reviewed, including demonstrations of high-spectral-efficiency terrestrial transmission at 10 Gb/s and 40 Gb/s over 4000 km and 3200 km, respectively, and 10-Gb/s transmission over 2400 km using 200-km spans.
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