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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.
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.
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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