Optical Fiber Communication Slide

1. Course Code: EEE 409
Course Title: Optical Fiber Communication
Department of Electrical and Electronic Engineering
Hajee Mohammad Danesh
Science and Technology University, Dinajpur-5200
Course Teacher
Md. Sazedur Rahman
Lecturer
Dept. of Electrical and Electronic Engineering (EEE)
Hajee Mohammad Danesh Science and Technology University (HSTU)

2. Optical receivers: PN, PIN and APD
detectors, Noise at the receiver, SNR and
BER calculation, Receiver sensitivity
calculation.

3. Photodetectors
 What is photodetector (PD)?
 Photodetector properties
 Detector types
 Optical detection principles
 Absorption coefficient
 Quantum efficiency
 Responsivity
 PD structures

4. What is photodetector
Photodetector is an important elements in OFC, which converts
optical signal into electrical form. A PD should have the
following characteristics:
 High sensitivity at the operating wavelength
 High fidelity
 Short response time to obtain a suitable bandwidth
 Noise should be minimum
 Stability of performance characteristics
 Small size
 Low cost

5. Photodetector types
Photo-
detectors
Photomulti-
plier
tubes
Vacuum
Photo-
diodes
pn-PD P-i-N PD APD
PD used in OFC

6. V-I characteristics of PD
I
V
Region 2 Region 1
Region 3
Increasing
optical
power
Photovoltaic
mode
Photoconductive
mode

7. Photodetection principles
Eg
hf >Eg
- +
p n

8. Photon absorption in intrinsic
material
E2 - E1
hf >E2 – E1
E2
E1
To excite an electron incident
photon should have energy
E
hc
E
E
hc




1
2
0


9. Absorption coefficient
Absorption coefficient is a measure of how good the material is
for absorbing light of a certain wavelength
d
 
)
exp(
1
)
1
(
0
d
hf
r
e
P
Ip 




The photo current Ip produce by
incident light of optical power P0
e : Electronic charge
r : Fresnel reflection coefficient

10. Absorption coefficient of
various materials

11. Quantum efficiency
The quantum efficiency n is defined as the fraction of incident photons
which are absorbed by the photodetector and generated electrons which
are collected at the detector terminal
p
e
r
r


n = Number of electrons collected/ Number of incident photons
rp: Incident photon rate
re: Corresponding electron rate

12. Relationship between
responsivity and n
hf
P
r
r p
e
0

 

)
( 1
0

 AW
P
I
R
p
hf
P
rp
0

where Ip: Photocurrent, P0: Incident optical power
The incident photon rate rp in terms of optical power and
photon energy can be written as
The responsivity R of a photodetector is defined as
Electron rate can be defined as
Output photocurrent is:
hf
e
P
Ip
0

 Thus
hc
e
hf
e
R






13. Wavelength dependence of
responsivity
Responsivity (A/W)
0.44
0.88
0.5 1.0 c
Ideal Si PD
Typical PD
Exp. 8.1, 8.2
J. Senior

14. p-n photodiodes
E-Field
Depletion
region
Absorption
region
hf
x
p
n
Diffusion region
Load

15. Output Ch. of a typical p-n
photodiodes
Reverse bias (V)
Current A
10 20 30 40
200
400
600
800
High light level
Low light level
Dark current (no light)

16. p-i-n Photodiode
E-Field
Depletion region
Absorption region
hf
x
p
i
Load
n

17. p-i-n photodiode structures
Metal contact
SiO2
Antireflection
coating
Depletion layer
P+
n+
hf
Front illuminated Si PD
i
Metal contact
n+
p+
i
Antireflection
coating
Reflection
coating
Side illuminated Si PD

18. Speed of response of PD
There are three main factors that limit the
speed of response of a PD
 Drift time of carrier (depletion region)
 Diffusion time of carriers (outside of
depletion region)
 Transition capacitance

19. Speed of response of PD
Drift time of carriers through the depletion region:
d
drift
v
w
t 
w : width of depletion region
vd : drift velocity
For electric field 2x104 v/cm, vd=107cm/s,
tdrift=0.1 ns when w=10 micron
Diffusion time of carriers outside the depletion region:
c
difft
D
d
t
2
2

d : carriers diffusion distance
Dc : diffusion coefficient
For 10 m diffusion distance, hole
diffusion time 40 ns whereas electron
diffusion time is only 8 ns

20. Speed of response of PD
Time constant incurred by the capacitance of the PD with its load:
w
A
Cj


To maximize the speed of response, the transit time need to minimize by
Increasing bias voltage
Decreasing layer thickness
Increasing bias voltage resulting to increase drift speed, which lead to
reduce drift time. Further depletion layer thickness may increase with
bias voltage
Quantum efficiency will fall with decreasing layer thickness, w. It also
increase junction capacitance, which lead to rise RC time constant. Thus
device speed will slowdown

21. PD response to a rectangular
input pulse
W W
P n n n
P P
+
- +
+
-
-
Large C
Narrow W
Low C and
W>>1/s

22. Avalanche photodiodes
hf
Load
n
p
p+
i
Gain region
Absorption
region
E-field

23. Silicon reach through APD
p+
p

n+
50m Absorption
region
Gain region
E-field
When reverse biased voltage is 10% less of the avalanche breakdown
limit, the depletion layer reaches through to the  region

24. APD response time
APD response time is limited by:
 Transit time of the carrier across the
absorption region
 Time taken for avalanche multiplication
 RC time constant

25. APD responsivity
hc
e
hf
e
R





)
( 1
0

 AW
P
I
R
p
Responsivity for p-i-n PD
Responsivity for APD PD )
( 1
0

 AW
P
MI
R
p
M: APD gain
Responsivity for p-i-n PD
hc
e
M
R



Responsivity for APD PD

26. Basic structure of an optical
receiver
PD
Preamplifier Post-amplifier Pre-detection
filter
Electrical
signal
Optical
signal

27. Sources of noise in an optical
receiver
Photo-
detection
Avalanche
gain
Detector
load bias
Electronic
gain
Optical
signal
Photodetector
Amplifier
Electrical
signal
Noise
• Quantum shot
• Dark current
• Surface leakage
Noise
Excess noise due
to random gain
mechanisms
Noise
Thermal
Noise
• Thermal noise
• Device (active
element)
• Surface leakage
currents

28. Photodetector noises
DS
n
DB
n
Q
n
PD
n i
i
i
i
2
2
2
2



Q
n
i
2
DB
n
i
2
DS
n
i
2
: Due to quanta of light generating packets
of electron-hole pairs
: Due to thermally generated dark currents
occurring in the PD bulk material
: Due to surface leakage currents

29. Signal to noise ratio of p-i-n
PD
c
n
eq
DS
eq
DB
Q
n
s
i
B
qI
B
qI
i
I
N
S
2
2
2
2
2 



S/N for shot noise
limited condition:
eq
s
s
Q
n
s
B
I
q
I
i
I
N
S
2
2
2
2
2


S/N for thermal noise
limited condition: eq
L
s
c
th
s
KTB
R
I
i
I
N
S
4
2
2
2


Beq: Noise equivalent bandwidth
IDB: Bulk leakage current
IDS: Surface leakage current

30. Signal to noise ratio of APD PD
c
n
eq
DS
eq
DB
eq
s
s
i
B
qI
B
M
F
M
qI
B
M
F
M
I
q
M
I
N
S
2
2
2
2
2
2
)
(
2
)
(
2 



S/N for shot noise
limited condition:
eq
s
s
Q
n B
M
F
M
I
q
M
I
i
Is
N
S
)
(
2 2
2
2
2
2
2


S/N for thermal noise
limited condition:
c
th
s
i
M
I
N
S
2
2
2

M: Multiplication factor,
F(M): Excess noise factor due to random fluctuation of APD gain

31. APD Noise
k
W
k
k
M
e 




)
)
1
(
exp(
1

x : is an empirical constant which is less than 1
F(M) can be approximated by:
K:e/h
e: Electron ionization coefficient
h: Hole ionization coefficient
x
M
M
F 
)
(

32. APD Noise
e
e
e
e
M
M
K
KM
M
F
)
1
2
)(
1
(
)
(




h
h
h
h
M
M
K
KM
M
F
)
1
2
)(
1
1
(
)
(




F(M) depends on the value of K and type of carrier
undergoing multiplication
For Si APD with M=100 and K=0.02, Fe(M) ~ 4
For Ge APD with M=20 and K=0.5 gives Fe(M) ~ 11

33. S/N for shot noise
limited condition:
eq
s
s
Q
n B
M
F
M
I
q
M
I
i
Is
N
S
)
(
2 2
2
2
2
2
2


S/N for thermal noise
limited condition:
c
th
s
i
M
I
N
S
2
2
2

Signal to noise ratio of APD PD
eq
n
in
L
eq
n
L
B
KTF
P
R
R
B
KTF
SR
N
S
4
4
2
2


S/N for thermal noise
limited condition: