Glass Fibre Optics 

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 the first realistic technical solutions; mainly encouraged by the first available diodelasers in 1962. These new light sources are ideally suited as transmitters 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 laserdiodes and the development in this area belongs to the most exciting ones in this century. In 1977 based on experience and results, rapid investigation in other fields were initiated, leading for example in the development of fibre gyros for navigation purposes of air planes. In principle this new technology does require a new understanding of physics because the related phenomena are well known and can be considered as a combination of classical optics and lasers. However, for 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 5 µm. The necessary mechanical components as the production process of the fibres themself were the 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 is of high common interest. Therefore, this experiment is con-sidered as a basic set-up and further developments are under way to cover also active fibres and high speed data transmission.
 

Examples of investigation and measurement

Measurements with the diodelaser
The controller LDC-01 permits the adjustment of the injection current, so that the output power of the diode can be measured as a function of the injection current, the threshold energy and the slope efficiency. The spatial intensity distribution can also be determined. For this reason, the diodelaser is shifted towards the rotational joint. The detector is mounted on the movable arm and the intensity is measured as a function of various angles. If the internal modulator of the controller is switched on, the power of the diodelaser is rectangularly modulated (0-100%). An oscilloscope switched in the AC - mode measures the amplitude only. This ensures a successful suppression of environmental light

Measurements at the fibre
Before starting the measurements, the light has to be coupled into the fibre. For reasons of simplification this is done with the diodelaser in the modulation mode so that an oscilloscope in the AC - mode can be used at high amplification. The highest amplification is also chosen for the photodiode amplifier within the controller unit. That way light can be detected at the exit of the fibre even for weak coupling. By means of further adjustment, a maximum of coupling efficiency can be reached. After this has been achieved, the coupling efficiency, the fibre losses (by shortening of the fibre) and the numerical aperture can be measured at the exit of the fibre. The angle-resolved intensity distribution informs about the transmitted modes.

Modulation of the diodelaser
If an external modulator is available, the diodelaser can be modulated by any signal. For this reason, the controller has an input for external modulation. The characteristic curve of the diodelaser is determined and the optimum working point is chosen for the modulation or data transmission. The external modulation input can directly process TTL-signals. Negative voltages are not permitted.

Transit time measurements at the fibre
A further interesting experiment is the measurement of the transit time of the laser light within the fibre. Due to the fibre length of 100 m, significant transit time differences can already be measured. In this case, the laserdiode is modulated by the internal modulator and the injection current as well as the photo detector signal are represented on the oscilloscope. First the transit time of the complete system is measured - that means the transit time due to electronics and the transit time of light. After this value has been determined, the fibre is eliminated so that the light falls directly on to the photo detector. Now the transit time without the fibre is determined. From the difference of both measurements, the transit time through the fibre is derived. Finally, the length of the fibre can be determined if the refractive index of the fibre is considered to be known or the refractive index if the length of the fibre is known.