The whole worldwide communication is based on fibre optical networks encompassing the entire world and is extremely important in view of economical and security aspects. Meanwhile fibre optical networks lines already terminating at homes thus forming a complex structure. The proper working and condition is of vital mutual interest of the provider and customer. Fibre lines can be damaged during road works, earth movements and even by late effects of production imperfections.
Whatever the reason of the malfunction of communication networks are, the problems needs to be solved as soon as possible.This is the moment of the mission of the "Optical Time Reflectometry" (OTDR). An OTDR is an optoelectronic instrument that uses time-domain reflectometry to characterize and locate faults in optical fibres. The underlying idea is so send a short pulse into the fibre and "listening" to any echoes coming back from it. At each fibre imperfection, especially at the face of a broken fibre a lot of light is reflected or scattered back into the fibre. From the time of flight of the input pulse and the occurrence of the echoes the distance to the faulty position is found and the service team can do their job. However, the OTDR covers more possibilities, it is the only device which can measure the attenuation or losses of an optical fibre non-destructively. Such losses in optical fibres can be caused by several reasons, mainly due to optical and mechanical imperfections during the manufacturing process, or by extra mechanical stress on the fibres like unspecified bending or tension. These days OTDR devices a small and compact and an indispensable tool in optical fibre communication. The aim of this experiment is to set up an OTDR in such a way that the trainees can identify, align and control the required components like the pulsed diode laser, coupling light into the fibre via a polarising beam splitter. The path of the returning light is bended due to its changed polarisation at the beam splitter cube towards the fast photodetector. The returned light intensity carries the information about the losses by an exponential decay in time, upward peaks for reflections and downward peaks for losses at joints caused either by splices or connectors. The experiment comes with two drums carrying each 1000 m of optical fibre. The fibres are interconnected via two ST patch cable whereby one end of each fibre is left as it is to teach fibre stripping and cutting.
Main reasons for losses in optical fibre are mechanical cracks, unavoidable scatter centres due to the production process and reflections (see Fig. 3.15). These three imperfections creating backward oriented scatter light, where by optical connectors acting as a photon sink.
The idea of an OTDR is to detect this back scattered light and analyse its amplitude and temporal behaviour. For this purpose the pulse of the laser diode (PLD) is launched via a polarising beam splitter plate into the optical fibre. The polarisation of the back scattered is changed so that it will be reflected at the beam splitter towards the detector. The quarter waveplate further enhances the right polarisation in order to get most of the back scattered light which anyhow has a very small power. This requires highly sensitive photodiodes and fast amplifier.
Within this experiment an extra photodetector as alignment aid is used. It serves as indicator for the coupling efficiency of the laser light into the fibre. Further more in first experiments the time of flight inside the fibre can be measured with this photodetector as well.