Gain-Guiding Effects in Stimulated Raman Scattering

Measurements of the output of a Raman amplifier when there is no input Stokes seed are presented. It is shown that a fully quantum mechanical scaled planewave theory describes the growth from spontaneous emission well in the high gain limit. However, in the low gain regime there is a significant increase in the amount of Stokes output which is not predicted by the plane wave theory. To account for the deviation from planewave growth observed at low gain, a fully quantum mechanical theory of amplification in a focused gain geometry is presented. The theory is based on a non-orthogonal mode expansion of the amplified field and incorporates the effects of both gain-guiding and diffraction. In addition to accounting for the observed Stokes output at low gain the non-orthogonal mode theory predicts that the Stokes field can experience enhanced gain as well as have excess noise. Results from an experiment are presented which verify the existence of enhanced gain in a multipass Raman amplifier. Though the experiment does not give a direct measure of the excess noise factor the results indicate that there may be more than the usual quantum limit of 1 photon of noise effectively seeding the Raman amplifier.