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FS22SI
MEASURING
4.4.3 Reflective Fiber Ending
It may occur that the interrogator does not find any sensor on an array even if connec­
tions were performed correctly. One of the reasons may be the excess of reflected light
saturating the detectors.
When a fiber is perfectly cut (Fig. 4.10 on the left) a large amount of light is reflected right
back into the core of the fiber in such an angle that it gets guided back heading to the
interrogator detectors. When this occurs, an index matching gel or an angled termination
must be used. If these are unavailable, the tip of the fiber can be smashed with a metallic
tool. This will destroy the perfect geometry (
Fig. 4.10 on the right ) and the light that is
reflected at the surface will take random directions ending up outside the core.
Reflective Fiber Ending Refractive Fiber Ending
Fig. 4.10 Refraction and reflection at fiber end
4.4.4 Cut Fiber
When the fiber, for any reason, is broken between two sensors in a series, the sensors
that are after the fault may disappear from the optical spectrum and may not be
detected.
There may also be the case that the fiber is cut, but not completely spread apart (e.g. the
fiber is damaged in an area covered with glue). This will cause a Fabry-Pérot effect – the
light will suffer multiple reflections inside the cavity creating sinusoidal background
reflection.
Fig. 4.11 shows a schematic representation of the reflected spectrum for
these three presented cases.