Intended institutions and users:
Physics education in Medicine
How it works ...
The basic idea to use guided light for data communication was published in 1939 by H. Buchholz in his paper „Die Quasioptik der Ultrakurzwellenleiter“ (The quasi optical behaviour of ultra short wave guides). However it took more than 20 years to develop first realistic technical solutions mainly encouraged by the first available diode lasers in 1962. These new light sources are ideally suited as transmitter because of their ability to be modulated and in addition, as we know today, they can be produced in large numbers at low prices. Nowadays the world wide communication is based on fibre optics combined with laser diodes and the development in this area belongs to the most exciting ones in this century. In 1977 based on the experience and results rapid investigation in other fields than communications were initiated, leading for example in the development of fibre gyros for navigation purposes of air planes. In principle this new technology do not require a new understanding of the physics because the related phenomena are well known and can be considered as a combination of classical optics and lasers. However for the realisation a lot of technical problems had to been solved. In the fibres mainly used in communication the light is guided within a „glass tunnel“ with a diameter of 9 µm only. The necessary mechanical components as well the production process of the fibres itself were subject of comprehensive developments. Considerable efforts today are undertaken to reduce fibre transmission losses by using so called active fibres and in the realisation of integrated optical devices for distributing and receiving signals. The field of fibre optics is still expanding and of high common interest. Therefore this experiment is considered as a introduction to this important technology.
The trainees are introduced firstly to prepare a bare optical fibre in such a way that suitable end faces are obtained. This process of fibre stripping and cleaving is a recurrent practice either in research labs or telecommunication. In a next step the light of a diode laser is characterised by measuring the output power versus the injection current and the spatial intensity distribution by using the provided photodetector which is mounted onto a pivot arm. By means of collimation optics the beam of the diode laser is made almost parallel before it enters the microscope objective which focuses the light into the multimode fibre. By observing the output at the exit of the fibre the coupling efficiency is optimized by adjusting the precise mounts. Once a strong signal has been obtained the numerical aperture of the fibre is measured by means of the photodetector mounted to the pivot arm. In a next step the photodetector is connected to an oscilloscope and the injection current of the diode laser is modulated. Both the diode laser signal and fibre output are displayed on the scope and the time of flight becomes apparent and can be measured. From this measurement either the speed of light or the length of the fibre is determined.