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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. 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 fibres play an important role. Especially in harsh
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. |
