OTDR Optical Time Domain
Reflectometry

Nowadays, world-wide communication is based on fibre optics combined with laser diodes and the development in this area are amongst to the most exciting ones this century. However, for the purpose of realisation a lot of technical problems had to be solved. In the fibres mainly used in communication the light is guided within a „glass tunnel“ with a diameter only of 5 µm. The necessary mechanical components as well as the production process of the fibres themselves were the subject of comprehensive developments. Considerable efforts today are undertaken to reduce fibre transmission losses by using the so-called active fibres and to find out the ultimate integrated optical devices for distributing and receiving signals. Losses in fibres are caused due to several reasons. The major reasons are losses 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 powerful technology that investigates such imperfections in optical fibres. The basic idea is to feed a light signal into the fibre and 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. Nowadays, hand-held battery operated OTDR devices are available to inspect fibre links. The goal of this experiment, therefore, 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. The figure below shows the complete arrangement of the OTDR set-up.


Principle of operation

The measurement task is to identify and locate imperfections in the fibre. They can be, as an example in extreme cases, due to broken fibre, defective fibre connector or unreliable fibre bending. The OTDR not only permits us to detect back reflections but is also in a position 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. This is understandable when one is aware that glass fibres are drawn from a cylindrical glass bulk and therefore cannot be cooled down slowly as is the case in the manufacture of optical glass. 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.

Examples of investigation and measurement

Interpretation of the recorded data
The intensity of the back scattered light depends on the distance between the fibre entrance and the location of its original inside the fibre mainly through absorption. Therefore, the exponential part of the curve contains the information about the fibre loss. For the determination of this important parameter, we redraw the record of the figure logarithmically in a new diagram. Using the relation: ln I = ln I0 - α . t , and performing a linear regression, one obtains the value as a slope of the linear part of the curve of the figure. For correct interpretation of the recorded data, it should be further noticed that scattered light produced at a location z inside the fibre by a primary light intensity has already become weak through previous attenuation. All necessary steps for the correct interpretation are given in detail in the comprehensive manual which comes along with the system.

Measurement of the back scattered light
The control unit is set to 50% duty cycle. Since we have chosen a periodical fast (3 nsec) on and off switch, we use an additional starting signal which is produced with a Detector G for more precise interpretation of the signal. Both the signals are represented on an oscilloscope, where the starting signal serves as a trigger. When one triggers on the falling edge of the starting signal (laser off) then one gets a picture as shown in the figure. During the phase of the switched on radiation, the fibre is filled with photons which produce the scattering light at the Rayleigh scatter centre which is directed towards the fibre entrance. The switching off of the light pulse, results in a rapid reduction of the back scattered light. After this follows the exponential decay of the intensity of the scattered light, which again after a particular time abruptly becomes 0. This point corresponds to the fibre end surface.