Fiber Optic Transmitter
About the fiber optic transmitter
The light-emitting diode (LED) is
commonly used as the transducer which converts an electrical signal
into light and transmits it into the cable. The LED is particularly
suitable for this task as it has an approximately linear relationship
between the input current and the light output. The type of LED chosen
must closely match the attenuation characteristics of the light path
through the cable and the spectral response of the receiving
transducer.
An important characteristic of the transmitter is the proportion of
its power which is coupled into the fiber optic cable: this is more
important than its absolute output power. This proportion is maximized
by making purpose-designed LED transmitters which have a spherical
lens incorporated into the chip during manufacture. This produces an
approximately parallel beam of light into the cable with a typical
diameter of 400 µm.
The proportion of light entering the fiber optic cable is also
governed by the quality of the end face of the cable and the way it is
bonded to the transmitter. A good end face can be produced by either
polishing or cleaving. Polishing involves grinding the fiber end down
with progressively finer polishing compounds until a surface of the
required quality is obtained. Attachment to the transmitter is then
normally achieved by gluing. This is a time-consuming process but uses
cheap materials. Cleaving makes use of special kits which nick the
fiber, break it very cleanly by applying mechanical force and then
attach it to the transmitter by crimping. This is a much faster method
but cleaving kits are quite expensive. Both methods produce good
results.
The proportion of light transmitted into the cable is also dependent
on the proper alignment of the transmitter with the centre of the
cable. The effect of misalignment depends on the relative diameters of
the cable. Some degree of
misalignment can be tolerated except where the beam and cable
diameters are equal. The cost of producing exact alignment of the
transmitter and cable is very high, as it requires the LED to be
exactly aligned in its housing, the fiber to be exactly aligned in its
connector and the housing to be exactly aligned with the connector.
Therefore, great cost savings can be achieved wherever some
misalignment can be tolerated in the specification for the cable.
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