  In 1964 Bloembergen and Shen predicted that Raman gain could be suppressed if
the rates of phonon creation and annihilation (by inelastic scattering) exactly
balance. This is only possible if the momentum required for each process is
identical, i.e., phonon coherence waves created by pump-to-Stokes scattering
are identical to those annihilated in pump-to-anti-Stokes scattering. In bulk
gas cells, this can only be achieved over limited interaction lengths at an
oblique angle to the pump axis. Here we report a simple system that provides
dramatic Raman gain suppression over long collinear path-lengths in hydrogen.
It consists of a gas-filled hollow-core photonic crystal fiber whose zero
dispersion point is pressure-adjusted to lie close to the pump laser
wavelength. At a certain precise pressure, generation of Stokes light in the
fundamental mode is completely suppressed, allowing other much weaker nonlinear
processes to be explored.
