optical receivers for optical communciation system

optical receivers

1. UNIT –4
OPTICAL RECEIVERS

2. Optical Detectors
High Sensitivity at wavelength of interest
Fast Response speed/Sufficient BW
Insensitive to temperature variations
Compatible with dimensions of fiber
Minimum addition of noise to the system
Shot noise
Receiver thermal noise
Beat noise
PIN Photodiodes, Avalanche Photodiodes
Direct detection / Coherent detection

8. pinphotodiode circuit

9. Energy-band diagram
( Why reverse bias ? )
Photon energy
corresponding
to longest
wavelength 
slighly greater
than bandgap energy
for high efficiency
& low dark current

15. Semi-conductor PDs
Intrinsic / extrinsic absorption
Direct band gap material, lattice matching
Absorption coefficient vs wavelength
Si 1.1 m
GaAs 0.9 m
Ge 1.8 m
InGaAsP 1.4 m
InGaAs 1.75 m

16. Absorption characteristics1.00E+01
1.00E+02
1.00E+03
1.00E+04
1.00E+05
Wavelength (micrometer)
absorption coefficient
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Light penetration depth (

m)
10
-1
1
10
100
1000
Si
GaAs
Ge
InGaAsP
InGaAs

18. Photodiode Principle
佐藤勝昭編著「応用物性」 p.152
Structure and band diagram of photo-diode
p n
Deple
tion
layer
Reverse
bias
Incident
photon
Electric signal
Bias voltage
Load
resistance
Photocurrent
Optical signal

19. Reverse-biased pinphotodiode
P(w) = P
o[ 1 –exp(-
sw) ]
Ip= (q/h) P
o[ 1 –exp(-
sw) ] (1-R
f)
R
freflectivity of detector surface

20. Quantum Efficiency
= number of electrons collected= I
p/e
number of incident photonsP
o/h
absorption coefficient, wavelength, DR width, 35-90% ?
Responsivity  = I
p/P
o(A/W)
= e/ h = e / hc
< 1 for pin photodiode ; < 1
Upper cutoff 
c= hc/E
g

22. Photodiode Responsivities

23. Avalanche Photodiode Detector
(APD)
Receiver sensitivity
Impact ionization
Avalanche multiplication
Reach-through construction

25. p
+
pn
+
reach-through structure
( RAPD)
•Fully depleted mode
•Absorption in region
•Carrier multiplication pn
+
junction
Electric Field
Minimum field
required for
impact ionization
p
+
n
+
p
i()
DR
MR
Photons

27. Carrier Multiplication
Internal amplification
Thermal noise component reduced
Ionization rate : average number of e
-
-h
+
pairs
created by a carrier / unit distance
k = / ; h
+
, e
-
;
Low noise & high GBP : k 0 ‘or’  ( Eg. Si )
AverageMultiplication M = I
M/ I
p

APD= ( q/h) M = 
0M
unity gain responsivity

28. Signal-to-Noise Ratio

29. Sensitivity
(minimum detectable optical power; SNR = 1)
h
h
C
d C
aR
aR
L
R
s
A
R
s<< R
L
Output SNR
Signal power from photocurrent
PD noise power+ Amp. Noise power
i
ph(t) = I
p+ i
p(t)
i
s
2
= i
p
2
pin PD
i
s
2
= i
p
2
M
2
APD
R
a>> R
L
i
T
2
= 4K
BTB
R
L
A
Bias voltage

31. Photodetector Noises
Quantum noise / shot noise
No. of photons of particular arriving at detector surface 
poisson process
i
Q
2
= 2qBI
ppin PD
Noise due to random gain mechanism (APD)
No. of secondary electrons generated per primary photoelectron
random process
i
Q
2
= 2qBI
pM
2
F(M)excess noise factor
F(M)  Actual noise generated . ~ M
x
(0< x < 1)
Noise under constant multiplication

32. Photodetector Noises
Dark current noise
Bulk dark current noise –thermally generated / background
radiation within the bulk of the device
i
DB
2
= 2qBI
D pin PD
i
DB
2
= 2qBI
DM
2
F(M) APD
Surface leakage current noise –surface defects, cleanliness,
bias, etc.
Guard ring structure
i
Q
2
= 2qBI
Lpin PD & APD
Impact of increasing M on SNR (BER)

34. Impact of Reverse Bias
What happens
if the applied
reverse bias is
continually
increased ?

35. Impact of input power on gain
VB
What happens when
Input power is very
High ?
How far can this
Multiplication
Process go on ?

37. Speed of response
Drift time of carriers through the depletion region
e
-
field at saturation v
d
t
drift= w/v
d [ Si 0.1 ns for w=10m, 2x10
4
V/cm ]
Diffusion time of carriers generated outside the depletion region
Slow process ; hole 40 ns, electron 8 ns
Time constant incurred by the capacitance of the photodiode with
its load ( junction & packaging)
Voltage dependant capacitance C
j= sA/w
PD as RC LPF
B=1/2RtCt
Maximum PD 3 dB BW
B
m= 1/ 2t
drift= v
d / 2w

41. Rise and fall times

42. Photodiode not fully depleted

43. Various pulse responses
High QE
High QE

44. APD Response
Fully depleted mode
(a) Carrier drift across absorption region
(b) Time taken for performing avalanche
multiplication process
(c) RC time constant
At low gains (a) & (c) dominant
At high gains (b) dominant; BW decreases
Rise times 150 -200 ps (electrons)
Fall times 1 ns ( holes )

45. Excess Noise Factor

46. APD Vs. pinPD
pinPD thermal noise -signal independent
APD shot noise dominant –signal dependent noise
APD sensitivity:
= 0.82 m, APD 10-13 dB away from quantum limit
pin PD 15 dB away from quantum limit
= 1.55m, APD 20 dB above quantum limit
pin PD 30-32 dB over quantum limit
APD drawbacks:
Fabrication difficulties & increased cost
Addition of noise due to gain randomness
Higher bias voltages
Variation of gain with temperature ( , )