Abstract
Phase-sensitive optical time domain reflectometry
$(\phi
{OTDR})$
is a simple and effective tool allowing the distributed monitoring of vibrations along
single-mode fibers. Up to now,
$\phi {\rm OTDRs}$
have been used
mostly for the measurement of sub-kHz vibrations, normally in the context of intrusion sensing. In this paper, the
authors present an experimental and theoretical description of a high-visibility
$\phi {\rm OTDR}$
and its performance when used for ultrasonic vibration measurements. The use of a
semiconductor optical amplifier in the setup allows to suppress coherent noise and also to improve the spectral
response of the pump pulses. These two advantages greatly decrease the detected intra-band noise thus allowing
frequency measurements in the limits set by the time of flight of the light pulses while maintaining the simplicity of
the scheme, as no post-processing, extremely high coherence lasers or coherent detection methods are required. The
sensor was able to measure vibrations of up to 39.5 kHz with a resolution of 5 m over a range which could go
up to 1.25 km. This is the first time to our knowledge that a fully distributed measurement of ultrasonic waves
was achieved. The statistical behavior of the system was also described theoretically and characterized
experimentally.
© 2013 IEEE
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