A plastic fibre is an equivalent to optical multi mode glass fibres. Whereas glass fibre are used for long distance and high speed data transmission, plastic fibres are commonly used for local area networks for data and signal transmission. Nowadays 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 fibre play an important role. Especially in harsh environments for example 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 by 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, the cutting of the fibre can be done with a simple cutter blade instead of using special cleaving tools as it is the case for glass fibres.
The goal of this experimental system is to teach and train the handling with and the signal transmission via optical plastic fibre. As transmitter 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 beam splitter plate which is coated in such a way that the green radiation will be totally reflected whereas the red radiation is transmitted. Two photo detectors 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 loudspeakers provided. The transmitter unit contains two independent drivers for both the LED as well as internal modulators. Other signal sources can be connected directly.
The light of the fibre coupled red and the green fibre coupled LED is merged by an also fibre coupled Y coupler and is available at the fibre output jack. The power and modulation of each LED is independently controlled. By changing the individual power of the LED the merged light appears in a spectral range from green via yellow to red. After passing the plastic fibre optics the light is guided by means of the fibre adjuster to the lens L1, which focuses the light onto the photodetector GPD (green) and RPD (red). The dichroic beam splitter plate separates the green and red part of the incident light. To measure the spectral absorption of the fibre different length of fibres are used. By combining the 10 m, 20 m and 30 m a variety of 10,20,30,40,50 and 60 m can be achieved.
Spectral emission of the LED and the relative POF attenuation