Sensornet ORYX DTS, Руководство пользователя

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Page 46 Oryx DTS User Manual v4
GUIDE TO DTS MEASUREMENTS AND THE ORYX
CONFIGURATION WIZARD
This section will provide some background to DTS measurements and a guide to
the Oryx DTS configuration wizard for correctly setting up the DTS.
BACKGROUND TO DTS
Principle of operation
The Oryx DTS system uses a short optical pulse to illuminate the glass core of an
optical fibre. As the optical pulse moves down the core of the optical fibre, it is
subjected to different types of radiation scattering.
The majority of the scattered radiation is because of naturally occurring density
fluctuations frozen in the glass core and is known as Rayleigh scattering. This type
of scattering can provide information on the optical loss of the fibre (as used in an
OTDR – Optical Time Domain Reflectometer). This is not sensitive to temperature
but must be considered when calibrating a DTS system.
A second type of scattered radiation is known as Raman scattering. Raman
scattering occurs when the original optical pulse energy is modified by the
molecular vibration modes within the glass core. Changes in the temperature of the
fibre cause the energy and the population of these vibration states to change. Light
is scattered from the vibrating molecules and as it does so, experience a kind of
“Doppler shift”. The scattered particles of light (known as ‘photons’) are shifted
slightly in optical frequency. When the optical photons loose energy to the glass,
they are down shifted in frequency and are referred to as “Stokes” photons.
Conversely when the optical photons gain energy, the frequency is increased and
are known as “anti-stokes” photons. Both of these signal photons travel back along
the fibre and are detected in the DTS. The ratio of anti-stokes to stokes from a
constant optical pulse can be used to calculate temperature. The DTS system uses
the anti-stokes to stokes ratio in conjunction with the time of flight of the optical
pulse to determine the temperature of the fibre at a given point. The principle of
operation is depicted in Figure 28.
The “Raw signal” display in the ODC configuration wizard and in the ODV software,
shows these Stokes and Anti-Stokes signals which the user can use for checking
fibre cable integrity and losses (this is described in the ODV section, ‘Raw Data
Display’ page 82).