Fibre optic receiver
- an overview or tutorial covering fibre optic receivers that are used to
receive the modulated light streams carrying data over fibre optic cables.
Once data has been transmitted across a fibre optic cable, it
is necessary for it to be received and converted into electrical signals so that
it can be processed and distributed to its final destination. The fibre optic
receiver is the essential component in this process as it performs the actual
reception of the optical signal and converts it into electrical pulses. Within
the fibre optic receiver, the photodetector is the key element
A variety of semiconductor photo-detectors may be used as
fibre optic receivers. They are normally semiconductor devices, and a form of
photo-diode. A variety of diodes may be used in fibre optic receivers, namely
p-n photodiode, a p-i-n photodiode, or an avalanche photodiode.
Metal-semiconductor-metal (MSM) photodetectors are also used in fibre optic
receivers on occasions as well.
Overall receiver
Although the photo-detector is the major element in the fibre
optic receiver, the are other elements to the whole unit. Once the light has
been received by the fibre optic receiver and converted into electronic pulses,
the signals are processed by the electronics in the receiver. Typically these
will include various forms of amplification including a limiting amplifier.
These serve to generate a suitable square wave that can then be processed in any
logic circuitry that may be required.
Once in a suitable digital format the received signal may
undergo further signal processing in the form of a clock recovery, etc. This
will undertaken before the data from the fibre optic receiver is passed on.
Diode performance
One of the keys to the performance of the overall fibre optic
receiver is the photodiode itself. The response times of the diodes govern the
speed of the data that can be recovered. Although avalanche diodes provide high
speed they are also more noisy and require a sufficiently high level of signal
to overcome this.
The most common type of diode used is the p-i-n diode. This
type of diode gives a greater level of conversion than a straight p-n diode as
the light is converted into carriers in the region at the junction, i.e. between
the p and n regions. The presence of the intrinsic region increases this area
and hence the area in which light is converted.
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