Noise and Detection
Savera Tanwir
Noise
¾Two major causes of signal degradation
– Thermal Noise
– Shot Noise
Fiber Optic Communication Systems
2
1
Thermal Noise
¾Thermal noise is also known as
– Johnson noise & Nyquist noise
¾Originates within the photodetector’s load
register
– Due to thermal energy electrons within
resistor never remain stationary
– Motion of electron is random
– Thermal noise current iNT
Fiber Optic Communication Systems
3
Thermal Noise Current
Fiber Optic Communication Systems
4
2
Thermal Noise
¾Average value of current is zero
¾Average noise power generated within the
resistor is RLi2NT
– Where i2 is mean-square value of the
thermal noise current
– Noise current is added to the signal
current
NT
9Generated by the photodetector
Fiber Optic Communication Systems
5
Thermal Noise
¾ iNT = 4kT∆f / RL
– k is the Boltzmann constant
– T is the absolute temperature
– ∆f is the receiver’s electrical bandwidth
Fiber Optic Communication Systems
6
3
Thermal Noise
¾Circuit elements in the receiver limit its
bandwidth
¾For processing all the messages
– Receiver’s bandwidth must be at least as
large as that of the information
– Range of bandwidth of Low-noise receivers
9Minimum: a little more than the information
bandwidth
9Maximum: twice the information bandwidth
Fiber Optic Communication Systems
7
Shot Noise
Time-dependent fluctuations in electrical current
caused by the discreteness of the electron
charge
Photodetector
Photoemissive
Semiconductor Junction
optic signals generate discrete charge carriers
Fiber Optic Communication Systems
8
4
Shot Noise
emission of signal photoelectron
Fiber Optic Communication Systems
9
Shot Noise
Constant optic power P
illuminates the cathode
Randomly produced current
pulses generated by the
emitted electrons
Randomly generated pulses
does not produce a constant
level
Fiber Optic Communication Systems
10
5
Shot Noise
i2NS =2eI ∆f
Mean-square shot noise current circuit
Fiber Optic Communication Systems
11
6