Noise: Noise is a term generally used to refer to any spurious or undesired disturbances that mask the received signal in a communication system. There are three main types of noise due to spontaneous fluctuations in optical fiber communication system: Thermal noise, dark current noise and quantum noise.

$\Rightarrow$ 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:

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

where k is the Boltzmann’s constant

T is the absolute temperature

B is the post-detection Bandwidth of the system

$\Rightarrow$ 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

$\Rightarrow$ Quantum 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

Quantum Noise in terms of analog transmission and digital signaling:

1. Quantum Noise is uncertainty of some physical Quantity due to the quantum origin.

2. In the case of number of particles (photons or electrons), the quantum noise is also known as shot noise.

3. Most optical communications uses amplitude modulation which is a type of analog transmission, but in this case the quantum noise appears as shot noise only.

4. In the case of uncertainty of electric field in some lasers, the quantum noise is not just shot noise; uncertainties of both amplitude and phase contribute to the quantum noise.

5. This issue becomes an important part in the case of noise of a quantum amplifier, which preserves the phase.

6. The phase noise becomes important to the frequency modulation or phase modulation of waves with energy of quantum comparable to the energy of the signal.

7. The equation for the energy of the quantum or photon is E = hf

8. The quantum behavior of electromagnetic radiation must be taken into account at optical frequency since hf>kT and quantum fluctuation dominate over thermal fluctuations.

9. This noise is important to find quantum limit for digital transmission.

10. Quantum limit is the minimum pulse energy required to maintain a given bit error rate which any practical receiver must satisfy.