An optical receiver consists of a photodetector and electronics for amplifying and processing the signal.

In the process of converting the optical signal power emerging from the end of an optical fiber to an electric signal, various noises and distortions will unavoidably be introduced due to imperfect component responses.

This can lead to errors in the interpretation of the received signal.

The noise generated from background radiation and atmosphere is neglected in optical transmission.

Also beat noise generated from various spectral components in optical carrier is considered to be negligible.

The APD receiver is the most complex case as it includes noise resulting from random nature of internal gain mechanism.

The main noises associated with p-i-n photodiode receivers are quantum or shot noise and dark current associated with photodetection, and thermal noise occurring in the electronics.

Thermal noise: It arises from the random motion of electrons that is always present at any finite temperature.

The thermal noise current is given by:

$i_t^2= \frac{4KTB}R$

where k is the Boltzmann’s constant

T is the absolute temperature

B is the post-detection Bandwidth of the system

Dark current noise: When there is no optical power incident on the photodetector a small reverse leakage current still flows from the device terminals. This Dark current contributes to the total system noise and gives random fluctuations about the average particle flow of the photocurrent.

The Dark current noise is given by:

$i_d^2 = 2eBI_d$

where e is the charge on an electron

$I_d$ is the dark current

Quantum noise/Shot noise: Discrete nature of electrons cause a signal disturbance called Quantum noise or Shot noise. It arises from the statistical nature of the production and collection of photoelectrons. It is given by

$i_s^2 = 2eBI_p$

$I_p$ is the photocurrent