Gain and noise ripple reduction in broadband Raman amplifiers using HNLF spectral broadening

The effects of using spectrally broadened pumps on the gain and noise ripple in broadband Raman amplifiers are investigated. Pump spectral broadening, achieved by initiating modulation instability in a highly nonlinear fiber (HNLF) using a continuous-wave input is shown to reduce the number of pumps required in addition to improving the flatness of the gain and noise profile. Optimal broadening of a pump spectrum in the E-and S-bands is achieved as a result of optimisation of the second order disperion and nonlinear coefficients of the HNLF. For a specific example of a 51-channel, C-band wavelength-division-multiplexed network employing a 50-km transmission fiber span, it is shown using simulation that spectrally broadened pumps improve the gain and noise ripple by 2 dB and 0.2 dB respectively. It is also shown that the number of pumps can be reduced by half when such a pumping scheme is used.