  As shown in the early 1960s, the gain in stimulated Raman scattering (SRS) is
drastically suppressed when the rate of creation of phonons (via pump-to-Stokes
conversion) is exactly balanced by the rate of phonon annihilation (via
pump-to-anti-Stokes conversion). This occurs when the phonon coherence waves
synchronized vibrations of a large population of molecules have identical
propagation constants for both processes, i.e., they are phase-velocity
matched. As recently demonstrated, hydrogen-filled photonic crystal fiber
pumped in the vicinity of its zero dispersion wavelength provides an ideal
system for observing this effect. Here we report that Raman gain suppression is
actually a universal feature of SRS in gas-filled hollow-core fibers, and that
it can strongly impair SRS even when the dephasing rate is high, particularly
at high pump powers when it is normally assumed that nonlinear processes become
more (not less) efficient. This counterintuitive result means that intermodal
stimulated Raman scattering (for example between LP01 and LP11 core modes)
begins to dominate at high power levels. The results reported have important
implications for fiber-based Raman shifters, amplifiers or frequency combs,
especially for operation in the ultraviolet, where the Raman gain is much
higher.
