lecture 12
DESCRIPTION
For 34 optical communication.TRANSCRIPT
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EE 230: Optical Fiber Communication Lecture 12
From the movieWarriors of the Net
Receivers
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Receiver Functional Block Diagram
Fiber-Optic Communications Technology-Mynbaev & Scheiner
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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
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Equivalent Circuits of an Optical Receiver
High Impedance Design Transimpedance Design
Transimpedance with Automatic Gain Control
Fiber-Optic Communications Technology-Mynbaev & Scheiner
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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)
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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
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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 = »
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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
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Circuit Noise
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Signal to Noise RatioSignal to noise Ratio (SNR) as a function of the average number of photo electrons per receiver resolution time for a photo diode receiver at two different values of the circuit noise
Signal to noise Ratio (SNR) as a function of the average number of photoelectrons per receiver resolution time for a photo diode receiver and an APD receiver with mean gain G=100 and an excess noise factor F=2
At low photon fluxes the APD receiver has a better SNR. At high fluxes the photodiode receiver has lower noise
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Dependence of SNR on APD Gain
Curves are parameterized by k, the ionization ratio between holes and electrons
Plotted for an average detected photon flux of 1000and constant circuit noise
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Receiver SNR vs Bandwidth
Double logarithmic plot showing the receiver bandwidth dependence of the SNR for a number of different amplifier types
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Basic Feedback Configuration
+
-RiIs
RoA Vi
Vo
IiIs
If
Parallel Voltage Sense:Voltage Measured and heldConstant => Low Output Impedance
Parallel Current FeedbackLowers Input Impedance
1
s f i
is i
i
i iin
s m
i i i
Vi AV
R
V RZ
i R
1 1
o i i
i s f s o
o i s o
o i mt
s i m
V Ai R
i i i i V
V AR i V
V AR RZ
i AR R
Stabilizes Transimpedance Gain
1 1test o o
test i m
V R RZo
I AR R
+
-Zi
Zo
ZtIi
Ii
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Transimpedance Amplifier Design
+
-Z i
i
ZeroInput Impedance
Output Voltage Proportional to Input current
+
-Ri
Vi RoA Vi
Typical amplifier modelWith generalized input impedanceAnd Thevenin equivalent output
o i i i
i ms
V AV AR i
VAR R
i
+
-RiVi
RoA Vi
is+
-
Vo
Calculation ofOpenloop transimpedance gain: Rm
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Transimpedance Amplifier Design Example
Rc
Rf
Q1
Q2
Vcc1
Vbias
Vcc2
Photodiode
Out
Transimpedance approximately equals Rflow values increase peaking and bandwidth
Controls open loop gain of amplifier, Reduce to decrease “peaking”
Most Common TopologyHas good bandwidth and dynamic Range
See Das et. al. Journal of Lightwave TechnologyVol. 13, No. 9, Sept.. 1995
For an analytic treatment of the design of maximally flathigh sensitivity transimpedance amplifiers
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“Off-the-shelf” Receiver Example
2 17 222 1.8 10dDetector
i qI I B x A
2 2 12 22Re
41.9 10Detectorsistor
s
kTi I B i x A
R
12 2 12 210
2Re 1
410 7.5 10
NF
Detectorsistor Amps
kTi I B i x A
R
2 2 12 210
2Re 1 2
410 7.6 10
TotalNF
Detectorsistor Amp Amps
kTi I B i x A
R
45.22
20.14
16.63
16.59
Sensitivity
dBm
dBm
dBm
dBm
.
+Bias
Is
Amplifier 1Gain1=20dBNF1=7dB
Output
Amplifier 2Gain2=20dBNF2=7dB
50
C=400ffId=10nA=0.7
NFTotal NF1 NF2 1
Gain1}NF 10Log10
4kTRs Vn2
4kTRs
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Bit Error Rate
BER is equal to number of errors divided by total number of pulses (ones and zeros). Total number of pulses is bit rate B times time interval. BER is thus not really a rate, but a unitless probability.
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Q Factor and BER
on
thon
off
offth VVVVQ
21
2
1 QerfBER
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BER vs. Q, continued
When off = on and Voff=0 so that Vth=V/2, then Q=V/2. In this case,
221
2
1
V
erfBER
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Sensitivity
The minimum optical power that still gives a bit error rate of 10-9 or below
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Receiver Sensitivity
1/22
2 22
Sensitivity= Average detected optical power for a given bit error rate
For pin detectors
2 damplifier
hvP Q iq
i i qI I B
(Sm
ith a
nd P
erso
nick
198
2)
2 /2
-9
Probability of error vs. Q is to good approximation:
1 E 2
eg. for a SNR = Q = 6 Bit Error Rate= P(E)=10
QePQ
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Dynamic Range and Sensitivity Measurement
Dynamic range is the Optical power difference in dB over which the BER remains within specified limits (Typically 10-9/sec)
The low power limit is determined by the preamplifier sensitivity
The high power limit is determined by the non-linearity and gain compression
PattenGenerator
Transmitter Adjustable Attenuator
Optical Receiver
Bit ErrorRate Counter
Optional Clock
Input Optical Power
Feedback ResistanceHigh Rf(High Impedance Preamplifier)
Low Rf(Transimpedance Preamplifier
Dynamic Range
Maximum Signal Level
receiver Sensitivity
Experimental Setup
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Eye Diagrams
Formation of eye diagram
Eye diagramdegradations
Transmitter“eye” mask
determination
Computer Simulation of a distorted eye diagramFiber-Optic Communications Technology-Mynbaev & Scheiner
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Power Penalties
• Extinction ratio
• Intensity noise
• Timing jitter
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Extinction ratio penalty
Extinction ratio rex=P0/P1
offonex
ex RP
r
rQ
2
1
1
ex
exex r
r
1
1log10
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Intensity noise penalty
rI=inverse of SNR of transmitted light
221log10 QrII
II RPr
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Timing jitter penalty
Parameter B=fraction of bit period over which apparent clock time varies
22
83
4 Bb
2/2/1
2/1log10
222 Qbb
bJ