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Optical Receivers Theory and Operation
Optical - ElectricModulation and Demodulation
Modulation
bit interval
• Conversion of digital electrical data into optical format
• On-off keying (OOK) is the most commonly used scheme– NRZ format most commonly used
– RZ format (requiring 2X NRZ bandwidth)
– Pulse format (not commercially deployed)
Demodulation: Receiver• Recovering the transmitted data
– Recovering the bit clock– Determining the bit value within each bit
interval
Optical Receivers
• Optical receivers convert optical signal (light) to electrical signal (current/voltage)– Hence referred ‘O/E Converter’
• Photodetector is the fundamental element of optical receiver, followed by amplifiers and signal conditioning circuitry
• There are several photodetector types:– Photodiodes, Phototransistors, Photon multipliers,
Photo-resistors etc.
Receiver Functional Block Diagram
Fiber-Optic Communications Technology-Mynbaev & Scheiner
Receiver Types+Bias
Is
RL 50 Amplifier
Output
+Bias
Is
Amplifier
Output
Ct
Rf+Bias
Is
RL
Amplifier
Output
EqualizerCt
Low Impedance
Low SensitivityEasily MadeWide Band
High Impedance
Requires Equalizer for high BWHigh SensitivityLow Dynamic RangeCareful Equalizer Placement Required
Transimpedance
High Dynamic RangeHigh SensitivityStability ProblemsDifficult to equalize
Equivalent Circuits of an Optical Receiver
High Impedance Design Transimpedance Design
Transimpedance with Automatic Gain Control
Fiber-Optic Communications Technology-Mynbaev & Scheiner
Receiver Noise Sources
•Photon NoiseAlso called shot noise or Quantum noise, described by poisson statistics
•Photoelectron NoiseRandomness of photodetection process leads to noise
•Gain Noiseeg. gain process in APDs or EDFAs is noisy
•Receiver Circuit noiseResistors and transistors in the the electrical amplifier contribute to circuit noise
Photodetector without gain Photodetector with gain (APD)
Noise
2
2Noise Power=4
4 4
nn
rms rms
VkTB i R
R
kTBi V kTRBR
2
m
spectral density= V /Hz
for FETs4kTK=
gwhere is the FET corner frequency and is the channel noise factor
c
c
Kf
f
f
Frequency
Nois
e P
ow
er
Frequency
Nois
e P
ow
er
Frequency
Nois
e P
ow
er 1/f noise
Fc
Johnson noise (Gaussian and white)
1/2 1/22rms noise current 2ni qIB
Shot noise (Gaussian and white)
“1/f” noise
Johnson (thermal) Noise
Noise in a resistor can be modeled as due to a noiseless resistor in parallel with a noise current source
2 2
The variance of the noise current source is given by:
4
Where is Boltzman's constant
T is the Temperature in Kelvins
B is the bandwidth in Hz (not bits/sec)
Bi
B
k TBi
R
k
s = »
Photodetection noise
The electric current in a photodetector circuit is composed of a superposition of the electrical pulses associated with each photoelectron
The variation of this current is called shot noise
If the photoelectrons are multiplied by a gain mechanism then variations in the gain mechanism give rise to an additional variation in the current pulses. This variation provides an additional source of noise, gain noise
Noise in photodetector
Noise in APD
Circuit Noise
Optical receiver schematic
Bandwidth of the front end:
CT: Total Capacitance = Cd+Ca
RT: Total Resistance = Rb // Ra
Try Example 6.7
1 2 T TB R C
Signal Path through an Optical Link
Noise sources and disturbancesat an optical receiver