Abstract

Previous investigations of Raman amplification in a multipass cell showed that a plane-wave transient theory matched the experimental data. In this work the output energy of a Raman amplifier was measured when no input was present. To model the output we initially used a fully quantum mechanical plane wave theory, which was scaled to take into account both the focusing and the multiple passes with the noise source being the usual one photon per mode. While the theory adequately describes the exponential growth for large gain, for gz < 10 there is a substantial deviation between the theory and experiment. To account for this excess noise the Stokes output is modeled by using a set of non-power-orthogonal modes. These modes arise naturally from the non-Hermitian nature of the gain guided process. It is the inherent non-power-orthogonality of these modes that can lead to excess noise, as noted first by Petermann. We find that the output from this model matches qualitatively the observed experimental output.

© 1992 Optical Society of America