Performance limits of unrepeatered systems using higher-order codirectional Raman pumping

Higher-order pumping schemes enhance the performance improvement provided by codirectional Raman amplifiers, which is achieved by pushing the maximum of the signal power deeper into the transmission fiber. This paper deals with the performance limits inherent to this technology.Simulation results reveal that the performance of an unrepeatered link operated with 10 Gbit/s NRZ signals increases with growing distance between the location of the maximum power and the fiber input up to distances of 70 km. For larger distances, the system performance starts to decrease. The amount of dispersion precompensation providing optimum system performance increases with increasing distance of the maximum to the fiber input. Furthermore, it is shown that the tolerance to variations of the precompensation is almost independent of the position of the maximum and, thus, the order of the pumping scheme. Nevertheless, the operation of commercial systems employing this kind of technologies becomes more complex since the system becomes more sensitive to variations of the residual dispersion. If the maximum of the signal power is reached after more than ≈50 km of propagation in the fiber, deviations of the residual dispersion from its optimum value exceeding ≈20 ps/nm/km lead to a performance degradation larger than 0.5 dB. However, it is also shown that the sensitivity to deviations of the residual dispersion can be reduced at cost of the resulting performance benefit.