Optimal design of multistage discrete Raman amplifiers incorporating midway isolators

Multistage discrete Raman amplifiers with midway isolators to suppress double Rayleigh backscattering are fully investigated at constant nonlinearity. The results show that placing Raman pumps at the last stage only is sufficient. For most conditions, two isolators can achieve adequate performance improvement, while more isolators only lead to slightly optical signal-to-noise ratio increase. The optimum positions of the isolators are insensitive to varied Raman gains, nonlinear phase shifts, and Rayleigh coefficients, but sensitive to different fiber lengths as well as Raman pumping schemes. Moreover, backward-pumping schemes can derive better noise performance than bidirectional pumping when using midway isolators.