Sensornet ORYX DTS, User Manual

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Oryx DTS User Manual v4 Page 55
Length (dispersion) Correction
The next stage of the Configuration Wizard is to perform a length or dispersion
correction. The length correction process ensures that the DTS measures the
distance along the optical fibre with an optimum spatial resolution. The DTS is pre-
calibrated for most common multimode optical fibres but may require fine
adjustment. As discussed above, there are two component wavelengths
propagating down the optical fibre at the same time. These can be seen on the
graphical display as red and blue plot lines. The red plot represents the distributed
stokes signal and the blue plot line the anti-stokes signal. Since the two component
wavelengths propagate down the optical fibre at different speeds, two group
indexes are present in the system. The user should performing a length (or
dispersion) correction to align the stokes and anti-stokes traces to a single
reference point along the optical fibre. The most common and easily identifiable
feature to use as a reference point is the optical fibre end. In most installations the
optical fibre length is known (at least approximately) and can be recognised as the
point where the signal traces drop to zero on the length axis.
NOTE: The fibre end point may show different features depending on the
termination of the fibre. In some cases the signals may simply drop to zero
amplitude, in other cases, a spike may be seen due to a larger reflection at the fibre
end.
Aligning the stokes trace with the anti-stokes allows the calibration wizard to set the
system for optimal spatial resolution. The default settings should be acceptable in
most cases, with the user only needing to make fine adjustments.