Stimulated Brillouin scattering in monomode optical fibres for sensing and signal processing applications
This thesis describes research into non-linear optical effects in monomode fibres for sensing and signal processing schemes. The work is initially placed into the context of relevant fibre sensor applications for aerospace. Candidate non-linear processes are then described and Stimulated Brillouin Scattering (SBS) targeted as a suitable mechanism. The SBS process in monomode fibres is then fully described, including a theoretical assessment of the magnitude of both pump and Stokes signals with fibre length. The project developed, firstly, an all-optical frequency shifter using SBS which generated output carriers in the 1-70OMHz region, depending on fibre type and system topology. In particular, a single-fibre system was demonstrated for the first time. Limitations of the technique arose through fibre photosensitivity and non-linear dynamical effects which, respectively, increased the input pump power requirements and introduced intensity and frequency instabilities into the output signal. Both processes were investigated fully. Optical fibre and pump characteristics, and system configurations, were therefore identified which minimised their impact. The same SBS heterodyning principles were then used to investigate single-pump sensor systems. Both 'quasi-point' and two-element, multiplexed temperature sensors were demonstrated. A linear relationship between the Stokes frequency and temperature was obtained from 5 to 55°C, determined primarily by the thermal response of the acoustic phonon velocity. The sensing resolution was typically ±2°C, limited by the non-linear dynamical effects, and the spatial resolution was determined by the pump/fibre interaction length. One unique sensing element per optical input was demonstrated. These results are then placed in the context of systems utilising alternative SBS-based strategies, other non-linear effects and fibre ring resonators. Management elements of the project were covered by detailing both the technical and strategic motivations for undertaking the research and, by describing the issues on which a cost benefit analysis of fibre sensors for aircraft can be undertaken.