Studies into the detection of buried objects (particularly optical fibres) in saturated sediment. Part 5: an acousto-optic detection system

Abstract

This report is the last in a series of five, designed to investigate the detection of
targets buried in saturated sediment, primarily through acoustical or acoustics-related
methods. Although steel targets are included for comparison, the major interest is in
targets (polyethylene cylinders and optical fibres) which have a poor acoustic
impedance mismatch with the host sediment. Whilst previous report in this series
have considered techniques for the detection of difficult targets in general when they
are buried in saturated sediment, this particular report discusses one specific type of
target: the optic fibre. The underlying theory, and a laboratory test, are investigated in
order to assess the feasibility of its practical use.
The use of optic fibres as distributed sensors is discussed, the most notable example
of such a system being the OTDR. Non-linear optical processes that could be
exploited in a distributed sensor are also discussed. The Brillouin effect is given
special consideration, and it is noted that the associated Brillouin frequency shift
depends on variations in temperature and, of particular interest, variations in strain.
The related, acousto-optic effect, is also investigated, and it is shown that both length
and refractive index changes (in response to an applied strain) can cause a change in
the optical phase within a fibre. Moreover, pressure sensitivity is shown to be related
to the elastic properties of the fibre jacket material. A greatly enhanced sensitivity is
predicted if the cladding has a high compressibility (which is expected to be the case
for non-metallised fibre optic cables).
An experiment is performed to investigate whether a conventional OTDR could be
used to detect acoustically-generated stresses within an optic fibre. A change in the
back-scattered power in the optical window of the detector is found to occur. These
results are discussed in terms of the practicality of a working system.
This series of reports is written in support of the article “The detection by sonar of
difficult targets (including centimetre-scale plastic objects and optical fibres) buried
in saturated sediment” by T G Leighton and R C P Evans, written for a Special Issue
of Applied Acoustics which contains articles on the topic of the detection of objects
buried in marine sediment. Further support material can be found at
http://www.isvr.soton.ac.uk/FDAG/uaua/target_in_sand.HTM.

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Identifiers

ORCID for R.C.P. Evans: orcid.org/0000-0002-1649-8750

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