Download Lecture Optical Communications Receivers Prof. Dr.-Ing. Dipl.-Wirt.-Ing. Stephan Pachnicke

Lecture
Optical Communications
Receivers
Prof. Dr.-Ing. Dipl.-Wirt.-Ing. Stephan Pachnicke
Optical Receivers
Block diagram
Optical
Optical
preBPamplifier filter
Photo diode
=O/E-conv.
Transimpedance
amplifier
EDFA
p1(t)
Pulsformerfilter
Limiteramplifier
Sampling+
decision device
=slicer
TIA
p(t)
i(t)
Data
u(t)
Clock
Optical
front-end
oc_5_empf.dsf
CDR
(Clock&Data
Recovery)
Clock
recovery
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Optical Receivers: Components
• Optical preamplifier:
Optional e. g. for long distance transmission. Increases overall receiver
sensitivity, however, adds ASE (amplified spontaneous emission) broadband
noise  optical BP filter
• Photo diode:
optical/electrical converter
• Trans-impedance amplifier (TIA):
low noise preamplifier
• Pulse-former filter (Matched filter, Nyquist filter):
optional or implemented in TIA.
• Limiting amplifier
for eliminating signal level variations (overshoots) prior to CDR
• Clock and data recovery (CDR)
e. g. with PLL for clock recovery. Clock for sampler and decision device (slicer)
and successive electronic stages (e.g. SDH-demultiplexer)
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Optical-Electrical Converter
Photo-diodes are realized either as PIN-diode or APD-diode
Requirements:
• high sensitivity
• low intrinsic noise
• high bandwidth
Responsivity and quantum efficiency for pin-diodes
Semiconductor materials:
Material with band gap suitable for received
wavelength.
Si (~0.8 nm)  suitable for joint integration of
photo diode and electronic circuitry on one
single chip, opto-electronic ICs
InGaAs, InGaAsP, Ge (~1.2-1.6 nm)  large
range of wavelengths
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Absorption Coefficient 
•
•
•
Wavelength c at which  becomes 0 is
called cutoff wavelength
Material can only be used as
photodetector for  < c
Indirect bandgap materials (Si, Ge) show
can be used as photodetector (however,
with reduced absorption edge steepness)
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PIN Photo Diode
PIN-Diode: pn-junction with intrinsic (i.e. weakly doped) layer of some m thickness where
absorption of photons takes place, pn-junction backward biased
Absorption of photons pairs of electrons/holes electrical current
i (t )  R  P (t ) ~ R | E (t ) |2
q
R    R=Responsivity
hc
 =Quantum efficiency<1 (e.g.90%), q=electron charge, h=Planck's constant (6.63  10-34 J/Hz)
Voltage/current characteristic of a photodiode
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Reverse Biased Operation
Depletion
region
Diffusion
region
w~1/2
•
Drift
region
Elctron-hole pairs are generated through
absorption
Large electric field inside of depletion region
accelerates electrons and holes to opposite
directions
Drift component dominates over diffusion
component
•
•
•
Resulting flow current is proportional to the
incident optical power
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Rise Time
•
Rise time Tr is defined as the time during which the response increases from 10 to 90% of its final output
value
When the input voltage across an RC circuit changes instantaneously from 0 to V0 the output voltage
changes as
•
𝑉𝑜𝑢𝑡 𝑡 = 𝑉0 1 − exp −
𝑡
𝑅𝐶
•
The rise time is then calculated by
•
𝑇𝑟 = 𝑙𝑛9 𝑅𝐶
In a photodetector a transit time tr needs to be added to consider the time before the carriers are
collected after their generation through absorption of photons
•
tr can be thus reduced by decreasing the width W of the intrinsic region. However, for W<3 the
quantum efficiency  decreases significantly.
•
There is a tradeoff between bandwidth and responsitivity (speed versus sensitivity) of a photodetector
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APD (Avalanche Photo Diode)
Very high backwards-bias voltage (>100V)  Photons produce electron/hole
pairs  additional pairs of electrons/holes by impact ionisation = avalanche
effect
q
RAPD  1  ,
RAPD  M  R, M =avalanche gain (e.g. 100)
hc
high gain, but usually lower bandwidth, more intrinsic noise
•
•
•
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An accelerated electron can generate a new electronhole pair
The energetic electron gives part of ist kinetic energy to
another electron in the conduction band (leaving behind
a hole)
Many secondary electrons and holes can be generated
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Front-End Amplifiers
Photo diode with parasitic C (small signal equivalent circuits)
Load
ok_5_empf.dsf
Ip
Cp
RL
+A
-
Photo
diode
Load
Rp=RL
High-impedance amplif.
RL
-A
+
Cp
Ip
Photo
diode
Load
Rp=RL/(A+1)
Trans-impedance amplif.
1.) low noise power (resistor noise):  value of RL as large as possible!
2.) high bandwidth:  load resistor RP seen from photo diode as small as possible!
 usually transimpedance amplifier (TIA) used (additional advantage: high dynamic range for optical input
signal), which is a current-to-voltage converter
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Noise Performance
Quantum Limit
The process of current generation from incident photons is of statistical nature
modelled as a shot-noise process with Poisson probability distribution (approximately
Gaussian)
Photon rate (= number of photons arriving at photo diode (PD) per second), [rp]=1/s
rp 
P (t ) P (t )

T , where hfT Energy of one photon [Ws]
hfT
hc
We consider binary on/off keying (0,1 signalling) with P1=optical received power at photo diode for “1”-level.
Then:
PT
N 1b
hfT
=mean number of photons arriving at PD during the "1"=bit intervall of duration Tb
and from Poisson probability distribution, the probability that a number of n photons arrive
during the “1”-bit interval is:
n
 PT
1 b
PT


 1b
 hfT   e hfT
n!
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Minimum Number of Photons
An ideal on/off receiver (photo detector) expects zero photons to arrive during the “0” bit interval
and at least one photon to arrive during the “1” bit interval
 Bit error, if in “1” bit interval (dk=1) n=0 photons arrive
W {n  0 | d k  1} 
1
2
PT
1 b

1
 e hfT   e  N  BER
2
W {d k 1}
Quantum-limit:
BER 
1 N
e
 10(lg 2  lg e N )  10(0.3 0.4343 N )
2
BER = 10-9  N =20 photons per Tb  minimum received power P1
BER = 10-12  N =27 photons per Tb  minimum received power P1
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Noise Performance of Real Receivers
Noise Contributions:
1. Shot noise (see above), Poisson probability density function
2. Thermal noise of receiver electronics, Gaussian
3. Optical noise, if optical preamplifier is used
first we consider only 1. and 2. (3. will follow in next lecture: optical amplifiers)
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Approximation for Shot Noise (PIN-Diodes)
Photo current = Superposition of (filtered) current pulses occurring at photon rate rp.
If the photo current is well above quantum limit,
i(t )  isignal (t )  ishot (t )
isignal (t )  R P(t )
"0"
"1"
"0"
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mean value (≈const. during Tb)
ishot (t) (shot noise) ≈ Gaussian random process
with zero mean and variance s2shot within
electrical bandwidth Be:
Tb
Photons
t
2
s shot
 2q  Be  isignal (t )  2q  R  P(t )  Be
Shot noise power is proportional to signal power (P1(t))
 “1” is more severely disturbed than “0”!
For APD-Diodes
Useful signal
isignal (t )  M R P(t )
Shot noise: (increases with increasing avalanche gain)
2
s shot
 2q Be M 2 FA ( M ) R  P (t )
with
FA ( M )  k A M  (1  k A )(2  1 / M ) = excess noise factor
kA = ratio (0<kA<1) of ionisation coefficients
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Excess Noise Factor
Excess noise factor as
a function of the
average APD gain M for
several values of the
ionization-coefficient
ration kA
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Thermal Noise
Thermal noise based on resistor noise power spectrum density (PSD)
Noise power measured in receiver bandwidth Be
 kT 
with: k = Boltzmann's const. 1,38·10-23 Ws/K
2
s therm
 2
  2 Be
 R 
T = absolute temperature
R = load resistor
It2
PSD of current (current density), typically It≈10…20 pA/√Hz
Electrical receiver bandwidth: according to Nyquist bandwidth for cancelling ISI (Inter-Symbol-Interference)
e.g. 10 Gb/s  Tb=100ps
ok_5_rauschen.dsf
5 GHz
in practice:
Be 
1
2Tb
1
Tb
-Tb 0 +Tb
t
-fb/2
0
+fb/2
f
100ps
(5 ... 10 GHz at 10Gb/s)
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Thermal Noise Impact of Trans-Impedance Amplifier
with Noise Figure Fn
Definition of the noise figure:
(SNR)in
Fn 
( SNR )in
( SNR )out
(SNR)out
ok_5_rauschen.dsf
Thermal noise power after amplifier:
2
s therm

4kT
 Fn  Be
R
Total noise power
Shot noise and thermal noise are statistically independent  total noise power:
2
2
s 2  s shot
 s therm
 2qR  Be  P (t ) 
4kT
 Be
RL
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Common Values
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Receiver Front-End Model
Simulation model with noise sources:
Poisson
noise
source
2
_
E(t)
P(t)
q
__
R=
hf T
+
+
+
H(f )
i(t)
isignal(t)
id(t)
Gaussian
dark current
noise
source
ok_5_empf.dsf
Magnitude squared operation of photo diode
Dark current = residual current with no light input (reverse biased diode)
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Clock Recovery
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Decision Circuit
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Wrap Up
What you should recall from this chapter:
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Give the quantum limit for BER=10-3 in terms of average photons per bit and in received power
Which semiconductor materials are suitable in 3rd optical window (1550 nm region)?
Discuss the use of Si as semiconductor material in optical communications
Compare the shot noise variance and thermal noise variance using typical parameter values.
The mean optical power at receiver is 3dBm and the extinction ratio is 10 dB. Calculate the
power levels for “0” and for “1”
Explain why optical detection is always random in nature
Calculate the signal to noise power (S/N) for the electrical current at optical front-end output,
assuming shot noise and thermal noise
Shot noise power is proportional to signal power. Explain how this fact can be noticed in the eye
diagram.
Explain how the threshold in the slicer must be adjusted for the above effect
Compare pin-diode and APD noise performance
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