Brillouin scattering in gas-filled hollow-core fibres

Edward E. Hagenlocker and William G. Rado demonstrated Brillouin interaction in gas for the first time in 1965, right after the very first demonstrations of stimulated Brillouin scattering in solid materials. In their experiment, they used a megawatt pulsed ruby laser that they focused in a gas cell. Such a high optical power was required due to the weak efficiency of the interaction, as a result of the impossibility of confining a high optical intensity for a sufficiently long distance in free space. Thanks to the invention of the hollow-core photonic crystal fibres by Philip St.J. Russell in 1991, it is nowadays possible to tightly confine beams of light over distances of several kilometres. In such fibres, light beams are confined over an area of a few tens of square micrometres and propagate with small optical losses, that might reach those of standard optical fibres in the near future.

The Brillouin gain being proportional to the light intensity and to the interaction length of the beams, its value is increased by many orders of magnitude in a hollow-core fibre, as compared with free-space experiments. As a consequence, the Brillouin interaction in a hollow-core fibre only requires optical power of a few tens of milliwatts. Additionally, the Brillouin gain shows a quadratic dependence on pressure. Hence, by pressurising the gas inside the hollow-core fibre, it is possible to further drastically increase the Brillouin gain, that can exceed the values of solid media (e.g. silica). It has to be noted that hollow-core optical fibres are ideal candidates for these kind of experiments since their hollow internal volume is extremely small and they seem able to sustain pressures of several thousand times the atmospheric pressure.

The main objectives of this thesis are the demonstration of the quadratic dependence of the Brillouin gain on pressure, both theoretically and experimentally, and the use of this strong Brillouin interaction for practical applications. The first three chapters introduce a theoretical background, explaining light propagation in gases, acoustic waves and their interaction with light waves. The last three chapters give some experimental results to confirm the theoretical part as well as four applications of Brillouin scattering in gas-filled hollow-core fibres: amplification of an optical signal, slow light, gas Brillouin lasing and a novel, intrinsically strain-insensitive, distributed temperature sensor.