Government Engineering College, Bharuch

Optical communication

presentation

Photo detector NoisePhoto detector Noise

Prepared by :

Xyz

6th Sem (E.C-A)

Noise is a term generally used to refer to any undesired disturbances that mask the received signal in a communication system.

• Thermal noise

• Shot noise

Thermal Noise

•Also known as Johnson Noise or Nyquist noise

•The thermal noise current it in a resistor R may be expressed by its mean square value and is given by:

• where K is Boltzmann's constant, T is the absolute temperature and B is the post-detection bandwidth.

R

KTBit

42 =

• Electrons within any resistor never remain stationary and this constitutes a randomly varying current known as thermal current.

• Motion due to their thermal energy.

The noise current produced by these random motions of charged carriers is called “Thermal noise current” . It is given by

I

SHOT NOISE

• Discrete nature of electrons causes a signal disturbance called shot noise.

• Deviation of the actual number of electrons from the average number is known as shot noise.

• Present for BOTH current: Signal and dark current.

Shot Noise due to Dark Current

• When there is no optical power incident on the photodetector a small reverse leakage current still flows from the device terminals and this contributes to the total system noise

• The shot noise due to the dark current, id is given by:

where e is the charge of an electron and Id is the dark current.

dd eBIi 22 =

Shot Noise on the Photocurrent

The shot noise, is on the photocurrent Ip is given by:

ps eBIi 22 =

Overall Receiver Noise

• Figure shows a block schematic of the front end of an optical receiver and the various noise sources associated with it.

• The majority of the noise sources shown apply to both main types of optical detector (p-i-n and avalanche photodiode).

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

The total shot noise iTS is given by:

The thermal noise due to the load resistance RL is given by:

LTH R

KTBi

42 =

( )dpTS IIeBi += 22

• The signal to noise ratio (SNR) for the p-n or p-i-n photodiode receiver may be obtained by summing all the noise contributions.

• It is given by:

( ) 2

2

42 amp

Ldp

p

iRKTB

IIeB

I

N

S

+++=

where is the Photo detector Responsivity, iamp=total noise from amplifier circuit,

The noise associated with the amplifier, iamp can be combined with the thermal noise from the load resistor it using the noise figure, Fn for the amplifier to give:

The expression for the SNR can now be written in the form:

L

nampt R

KTBFii

422 =+

( )L

ndp

p

RKTBF

IIeB

I

N

S4

2

2

++=

Thank You