Abstract

We demonstrate experimentally that gain flattening of single-pump fiber Raman amplifiers is possible by use of nonlinear broadening of the pump spectrum. In our experiments, pump broadening of a continuous-wave Raman fiber laser tuned at 1455 nm is achieved by propagation in a non-zero dispersion-shifted fiber with small anomalous dispersion at the pump wavelength. The broadened pump spectrum can reach a full width at half maximum of as much as 28 nm. Our results demonstrate that an acceptable flatness of the Raman gain spectrum can be obtained in the 1560-1640 nm region while avoiding pump-to pump non-linear interactions. On the dark side, our measurements show that this configuration introduces more noise than the conventional, non-broadened pump configuration since a non-negligible fraction of the broadened pump power falls within the band of amplification. However, we believe that further work and optimization should demonstrate that gain-flattened fiber Raman amplifiers using a single pump can be achieved over the C (1525-1565 nm) and L (1565-1625 nm) bands. (C) 2004 Elsevier B.V. All rights reserved.