The optical receiver converts incident optical power into electrical current through the photo-diode.
The relation between the optical power and the photo-current is linear, i.e.
where is the responsivity of the photo-detector, is the photo-current and is the incident optical power.
The interaction of photon with the matter in a photo-detector however is a statistical process. That means the linear relationship is valid in an average sense. If we go to micro level there are fluctuations in the photo-current around the mean. These fluctuations are called the noise in the receiver.
There are fundamental mechanisms of noise:
1)
Shot noise: This is due to statistical fluctuations in the optical signal itself and the statistical interaction process.
2)
Thermal noise: This is due to the thermal motion of the electrons inside the electronic circuits and the amplifiers following the photo-detector.
The presence of noise degrades the signal quality. For the analog system the signal quality is measured by the parameter, signal-to-noise ratio (SNR), and for a digital system the signal quality is measured by the parameter, bit error ratio (BER).
The analysis of noise therefore very important in a communication link. Here first we investigate the properties of the two noises mentioned above and then calculate the BER for the digital signals.
is the responsivity of the photo-detector, 
is the photo-current and 
is the incident optical power. 
