  Stimulated Brillouin scattering (SBS) offers among the highest nonlinear
gains in solid materials and has demonstrated advanced photonics
functionalities in waveguides. The large compressibility of gases suggests that
SBS may gain in efficiency with respect to condensed materials. Here, by using
a gas-filled hollow-core fibre at high pressure, we achieve a strong Brillouin
amplification per unit length, exceeding by 6 times the gain observed in fibres
with a solid silica core. This large amplification benefits from a higher
molecular density and a lower acoustic attenuation at higher pressure, combined
with a tight light confinement. Using this approach, we demonstrate the
capability to perform large optical amplifications in hollow-core waveguides.
The implementations of a low-threshold gas Brillouin fibre laser and a
high-performance distributed temperature sensor, intrinsically free of strain
cross-sensitivity, illustrate the large perspectives for hollow-core fibres,
paving the way to their integration into lasing, sensing and signal processing.
