Abstract:
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 regýon, 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.
