Basic Concept

Losses in fibres are caused due to several reasons. The major reasons are due to optical and mechanical imperfections within the fibre core during the manufacturing process, or due to the setting up of fibre links or mechanical stress on the fibres when they are in use. Since the fibre networks cover many thousands of kilometres, it is a stringent demand to locate imperfections from a central point e.g. the input feeding stations. The Optical Time Domain Reflectometry (OTDR) is a powerful technology that investigates such defects in optical fibres. The basic idea is to feed a light signal into the fibre and to monitor the occurrence of light echoes. Such echoes will be generated at stray centres within the fibre core and at fibre surfaces as they are always present at fibre to fibre coupler. Every non perfectly matched fibre connection will generate stray light which is reflected back to the fibre input. So the OTDR will also be used to optimize fibre connectors. The goal of this experiment is the training in this important technique. Besides the major aspect, the training in OTDR, a great variety of other fibre related measurements and handling can also be carried out.

Experimental Set-up

The measurement task is to identify and locate defects in the fibre. They can be, as an example in extreme cases, due to broken fibre, defective fibre connectors or unreliable fibre bending. The OTDR not only permits us to detect back reflections but is also capable to measure transmission losses of the fibre. Microstructures which are more or less distributed homogeneously, exist in every fibre and are a result of the manufacturing process. Light which impinges on these microstructures disperses in such a way that the scattered light also reaches back to the entrance of the fibre. Light which is scattered at the point z2 has a longer travel path back to the fibre entrance as that one which has arisen at point z1 and therefore it reaches the detector later due to the different transit time. Due to the losses inside the fibres, the scattered light from position z2 is attenuated more than that one from the position z1. As a light source, a fast laser diode with fibre coupling is applied. It is part of the slot-in module which also contains the controller for it. The laser diode is connected via a provided fibre cable to the OTDR optics. The back scattered light is transferred via a fibre patch to the OTDR processor where it is analysed de-logarithmated and displayed. For a couple of paths through experiments, the laser diode emission can be coupled directly to the fibre and the photodetector (PD) picks up the signal at the end of the fibre. This allows, for instance, to measure the transient time or the speed of light. Also the attenuation of each segment can be measured and compared to the OTDR signal.