Gain-flattening of fibre Raman amplifiers using nonlinear pump spectral broadening and pump power modulation

An improved technique is proposed for achieving gain-flattening of fibre Raman amplifiers on the basis of nonlinear pump spectral broadening and rapid pump power modulation. Pump broadening of a continuous-wave Raman fibre laser tuned at 1455 nm is achieved by propagation in a non-zero dispersion-shifted fibre (NZDSF) with small anomalous dispersion at the pump wavelength. The amount of pump broadening is controlled by the pump power delivered into the NZDSF, and the flatness and centre wavelength of the gain band is defined by the amount of pump broadening. Using experimental results, it is estimated theoretically that with a rapid modulation of the pump power, flat gains of 9 dB with 0.2 dB ripples can be obtained in the case of three modulation levels (1.5, 1.8, 2.1 W) between 1580 and 1630 nm. Gains between 7 and 8.5 dB can also be achieved in the spectral region between 1575 and 1640 nm