Basic Concept

The goal of this experiment is to introduce the capabilities of optical plastic fibres demonstrated with a two channel transmission via one fibre.
A plastic fibre is an equivalent to optical multi-mode glass fibres. Whereas glass fibres are used for long distance and high speed data transmission, plastic fibres are commonly used for local area networks for data and signal transmission. The transmission losses with 12 dB / 50 m of these fibres are significantly higher than those of the glass fibres. A lot of effort is undertaken to remove this disadvantage, since the manufacturing and installation costs of plastic fibres are comparably low. For signal transfer at short distances, optical plastic fibres play an important role. Especially in difficult environments, for example, in high voltage power stations, signal transfer via light and plastic fibre can be performed almost free of noise. The light transfer in all plastic fibres (APF) is achieved using a plastic core which is coated with a material to obtain a step index profile. Typical diameters are 1 mm for the core, which simplifies the coupling of light compared to glass fibres significantly. Also the preparation process, e.g. the cutting of the fibre can be done with a simple cutter blade instead of using special cleaving tools as in the case for glass fibres.

Experimental Set-up

As transmitters a green and red LED is used to demonstrate independent dual wavelength data transmission in a single fibre. The radiation of the LED is coupled by means of a Y coupler into the fibre. The light at the exit of the fibre passes a dichroic separator consisting of a beam splitter plate which is coated in such a way that the green radiation will be totally reflected, whereas, the red radiation is fully transmitted. Two photo detectors inside the dual LED receiver convert the light signal into electrical signals which are amplified by the receiver module and can be either displayed on a two channel oscilloscope for further investigation or connected to two optional loudspeakers The transmitter unit contains two independent drivers for both the LED as well as the AM and FM modulation. Other signal sources like CD players can also be connected directly to the transmitter. To measure the absorption of the plastic optical fibre depending on its length and wavelength, a constant signal delivered from a frequency generator is recommended.