Investigation of dynamical pump control for backward-pumped fiber Raman amplifiers

For a broadband backward-pumped distributed fiber Raman amplifier (B-DFRA) operated in long-haul transmission system with massive signal channels, not only the average level but also the tilt of the output power spectrum will vary due to the variation of signal-to-signal Raman scattering. So, when the signal channel number or channel power varies, dynamical control of both the average level and tilt of the on-off gain is necessary. In this paper, an approximate two-dimensional linear expression for the individual pump power at each pump wavelengths versus the average on-off gain level and tilt was derived based on the theoretical analysis and numerical simulation. The method to pre-determine the control constants based on measured data was also introduced. By using this linear relation, output power spectrum clamping over C + L band (1529.1-1602.7 nm) was demonstrated experimentally for a single 100 km B-DFRA and a 6 × 100 km cascaded hybrid Raman/erbium-doped fiber amplifiers with up to five pump-wavelengths on standard single mode fiber. The average clamping error was less than 0.32 dB for single span and 0.5 dB for 6 spans.