Stimulated Brillouin scattering: its generation and applications in optical fibre

Abstract

In the work presented in this thesis, the generation of stimulated Brillouin scattering and its applications in optical fibres is theoretically and experimentally investigated. The study pursues three special cases: i) Backward stimulated Brillouin scattering in long fibre lengths ii) Backward stimulated Brillouin scattering in high finesse all fibre ring resonators iii) Forward stimulated Brillouin scattering in dual moded single core fibres. Stimulated Brillouin scattering (SBS) occurs for relatively low input powers in monomode optical fibres, as the power density is very high because of the relatively small core size. For applications such as optical communications, SBS is seen as a potentially deleterious effect because it can limit the maximum optical power transmitted by the fibre and hence decrease the distance between repeaters. SBS, however, can also be used to advantage in optical fibres, for example to produce amplification. In this thesis the comprehensive study of SBS in relation to other non-linear scattering mechanisms in optical fibres leads to the derivation of explicit definitions for the Brillouin gain and the Brillouin threshold. The study of SBS in high finesse all fibre ring resonators also demonstrates how threshold powers can be reduced, typically, from milliwatts observed in long fibre lengths to microwatts. Because Brillouin scattering is primarily a result of the interaction of the incident optical beam with spontaneously generated (thermal) fluctuations in the density of the medium, the spectral features show a considerable variation with temperature thus providing a mechanism with sufficient sensitivity to realise tunable microwave generation and frequency shifting devices. Finally, the observation of stimulated Brillouin scattering in a forward direction (FSBS) in dual moded single-core fibre is also reported. Frequency shifts in the order of 17MHz are observed in optical fibre supporting LP\(_{01}\) and LP\(_{11}\) modes at 514.5nm. The phenomenon is examined here in detail and the governing differential equations of the three wave parametric process (involving pump/laser, Brillouin signal and acoustic flexural wave phonon) is derived and solved. FSBS is possible because, although the overlap integral between a fibre flexural mode and the light is small, the phonon lifetime is much longer than in conventional SBS. FSBS may also be the first example of a non-linear effect which is enhanced by increasing the optical mode area at constant pump power.