Lecture 4b
Fiber Optics Communication Link
1. Introduction
2. Optical Fiber, Physical Background
3. The Light Transmitters and the Receivers as a Components
of the Fiber Optic Communication Links
4. Light Emitting Diodes
5. Transmitters
6. Driving Circuits
7. Receivers
8. p-i-n Photodiode
9. Transceivers and Repeaters
10. Fiber Optic Communication Link Rise Time
and Bandwidth Bandwidth
11. Communication Link Power Budget
12. Connectors
13. Conclusion
1
Input
data
DRIVER
SOURCE
Source-to-fiber
connector
Transmitter
Optical Fiber
Fiber-to-detector
connector
DETECTOR
Receiver
OUTPUT
CIRCUIT
Output
data
2
• 1. Wide bandwidth: Fiber optic system uses light as a carrier with
1013 to 1014 Hz. Radio waves are 106 to 1010 Hz. Electrical signals
have frequencies up to 108 Hz. The maximum bandwidth of the
transmitted signals is 10% of the carrier.
• 2. Low loss: The typical attenuation of a 1 GHz bandwidth digital
signal in an optical fiber is 0.1 dB per km. A 100 MHz bandwidth
signal in RG-58/U coaxial cable has attenuation of 130 dB per km.
• 3. Electromagnetic immunity: Electrical fields do not affect light
signals.
• 4. Light weight and small size: 1 km of optical fiber cable weighs
about 10 kilograms. A 1 km copper wire with the same signal
carrying capacity would weigh 700 kg.
• 5. Safety: There is no possibility of a short circuit in a fiber optic
system, eliminating the hazard of sparks in an electrical cable.
• 6. Security: Optical fiber is harder to tap than electrical wire.
Unwanted tapping over the length of the fiber can usually be detected.
3
1014
Optical soliton
1013
1550 nm
coherent detection
1012
Bits per
second, 1011
per 1 km
distance
1010
1550 nm
direct detection
1300 nm
single-mode
800 nm,
multimode
Optical
amplifiers
109
108
1974 1976 1978 1980 1982 1984 1986 1988 1990 1992
Year
4
A.
Index of Refraction
• C= 3×108 meters per second, but it is reduced when it passes through
matter. The index of refraction n:
n
c

c speed of light in a vacuum, 3×108 m/s

 speed of light in the given material
0
0  0  f  c
0

   f
 0 0
n 
1
 n
wavelength of light in a vacuum
wavelength of light in the given material
X ray,
n 1
5
Index of refraction and speed of light
for various materials.
Index of Refraction Speed of Light
Free space (vacuum)
1.0
3×108 m/s
Air at sea level
1.003
2.99×108 m/s
Ice
1.31
2.29×108 m/s
Water
1.33
2.26×108 m/s
Glass (minimum)
1.45
2.07×108 m/s
Glass (maximum)
1.80
1.67×108 m/s
Diamond
2.42
1.24×108 m/s
6
B. Refraction with Snell's Law
n1  sin 1  n2  sin  2
1 :
The incident angle (from the surface normal)
2 : The angle of refracted light (from the surface
normal)
n1 :
index of refraction in the incident medium
n2 :
index of refraction in the refracting medium
Light that is not absorbed or refracted will be reflected.
The incident ray, the reflected ray, the refracted
ray, and the normal to the surface will all lie in the
same plane.
7
D.
Multimode Step Index Fiber
We want to find the critical case of total internal reflection at the corecladding boundary. Using Snell’s Law with 2 = 90º, we can find the
critical angle CR :
sin   CR
 n2 
n2


, or  CR  arcsin 
n1
 n1 
Air n0
Unguided ray
Cladding n2
φ´2
φ2 = 90º if φ = φCR
Core n1
θi
Cladding
φ´
θ´r
φ
φ
θr
θ´i
Incident ray
Reflected ray
8
θ2
n2 < n1
Refracted ray
Refracting medium
Incident medium
n1
θ1
θ1
Incident ray
Reflected ray
normal
C.
Total internal reflection
sin  CR
n2

n1
θ2
n2 < n1
Refracting medium
Incident medium
n1
Refracted ray
θ1 = θCR
θ1
Incident ray
Reflected ray
normal
Total internal reflection when 1 > CR .
9
sin  i , CR
2
 n2 
  n1  1   n  
 1
n 12  n 22
0
Critical angle refraction=90
10
• Since we can relate r, CR to angle CR by simple geometry, and we
can make the approximate n0 = 1, this equation can be simplified:


sin  i , CR  n1  sin  r , CR  n1  sin    CR 
 2

• The negated and shifted sine function is identical to the cosine, and we
can relate this cosine to the sine by the trigonometric identity:


2
sin  i , CR   n1  sin    CR   n1  cos  CR   n1  1  sin   CR 
 2

• In equation (3.4), this sine was found above in terms of n1 and n2 :


sin i , CR
  n1  1   nn2  
 1 
2
n 12  n 22  NA
For n1  n2 , we can simplify the numerical aperture calculation:
sin   i , CR
  n
1
 n1 
 n2    n1  n2
2 n1  n2
n1


 n1 

2n1   n1  n2
2

n1  n2

n1
11
E.
Modal Dispersion
• Dispersion means the difference in arrival time of the light
rays at the output end of an optical fiber.
• Modal dispersion is caused by the difference in rays path
(with equal wave length) due to variation in light incidence
angles at the input end. It occurs only in multimode fibers
• Material dispersion is related to the variation of light
velocity in a given fiber material due to the difference in
propagated light wave.
Number of modes
 2
( Diametero of core  NA  )

Number of mod es 
2
12
A
Input pulse
Output pulse
LMax
t
Critical angle
LMin
For instance, if n1 = 1.5 and  = 0.01, then the
numerical aperture is 0.212 and the critical
angle  r,cr, is about 12.5 degrees.
13
• i = 0 and path length=L (fiber length).
• The longest path occurs for  i = i, CR and can be estimated as:
L
LMAX 
 
sin   CR


L
1

T 


  n1  
 1 
   1 1
 
sin   CR
c
 sin   CR

L
1
L

sin   CR

n2

n1
 n1
 L n12
L
T   n1  
 1   

c
 n2
 c n2
1
TB 
;
B
T  TB
B is the bit rate in bits per second
1
; T 
; therefore B  T  1
B
n2 c
BL  2 
n1 
For  = 0.002 in a small-step index optical fiber:
B  L  150
Mb
 km
s
14
B Mbps
150
1
1
150
L km
15
F.
Bandwidth of a Multimode Optical Fiber
• To estimate the bandwidth of an optical fiber, we can convert from a
bit transfer rate to a bandwidth. In one signal period, two bits can be
transferred, so the maximum signal frequency is simply one-half the
bit transfer rate.
c  n2
B
c  n2
W 
f MAX 
; f MAX 
2
2  n12    L
2
2  n1    L
• Light frequencies used in fiber optic systems use a carrier frequency
between 1014 and 1015 Hz (105 to 106 GHz). The theoretical bandwidth
of a fiber optic system is about 10% of the carrier frequency, or up to
10,000-100,000 GHz!
16
G. Attenuation
•Attenuation ranges from 0.1 dB/km (single-mode silica fibers) to over
300 dB/km (plastic fiber).
•There are two reasons for attenuation: Scattering; Absorption:
 P2 
Attenuation (dB/km)
Attenuation (dB) = 10  log 10 


 P1 
850 nm
Window
2.5
OH Absorption
Peak
2.0
1.5
1300 nm
Window
1.0
1550 nm
Window
0.5
0.0
800
900
1000 1100 1200 1300 1400 1500 1600 1700 17
Wavelength (nm)
3.
Classification of optical fibers and their
characteristics
Jacket
Core ( n1 )
125 μm
Cladding ( n2 )
125 μm
8 μm
8/125
125 μm 62.5 μm
50 μm
50/125
140 μm 100 μm
18
62.5/125
100/140
 multimode step inde
(a) Multimode step index fiber
 multimode graded index
(b) Multimode graded index fiber
 single-mode step index
(c) Single-mode fiber
19
4.
Light Emitting Diodes
+
+
-
e-
ep
Pi
w
e
w
n
hc

20
LED
Lens Clip
Lens
Window
Fiber
Connector Ferrule
Metal Connector Port
TO-46 Header
21
Sources of losses of light power due to
mismatches:
( Pl ) NA
 NA fiber 

 10 log10 
 NAsource 
2
 d fiber 

( Pl ) d  10 log 10 
 d source 
2
A source, with an output diameter of 100m and an NA of 0.30 is
connected to a fiber with a core diameter of 62.5m and NA of 0.275.
The and the are as follows:
( Pl ) d  10 Log10 (62.5 / 100) 2  10 Log10 (0,39)  4.1dB
(Pl ) NA  10Log10 (0.275 / 0.30)  10Log10 (0.94)  0.8dB
22
LED driver circuits
IF
R1
VF
Pout
power of
emitted light
Vcc
Switching
Transistor
Vsaturation
signal in
Transistor
R1
R2 >> R1
IF
Vdiode
VF
Shunt Diode
Pout
power of
emitted light
90%
Shunt Transistor
signal in
Transistor
S(t)
Vsaturation
10%
t
0.35
BW 
tr
rise
time
tr
fall
time
23
p-n Photodiode, (p-i-n)
-
+
R
ep
amplifier
n
light
i  eN 
eP
hc
P

N P
w
hc
24
The important characteristics of receivers are:
Signal-to-noise ratio (S/N) is expressing the quality of signal in a system. In
decibels, S/N is equal to the signal power in decibels minus the noise power in
decibels.
S/N (dB) = 10 log10( S/N ) = 10 log10( S ) - 10 log10( N ).
If the signal power is 50 W (-13 dBm) and the noise power is 50 nW (-43 dBm),
the S/N is 1000, or 30 dB.
Bit-error rate (BER) is related to S/N. The BER is the ratio of the incorrectly
received bits to correctly transmitted bits. A ratio of 10-9 means that one wrong bit
is received for every 1 billion transmitted bits.
Responsivity (R) is the ratio of the photodiode's output current to the input optical
power; it is expressed in Amperes /Watt (A/W). A p-i-n photodiode typically has a
responsivity of around 0.4 to 0.6 A/W. A responsivity of 0.6 A/W means that
incident light having 50 W of power results in 30 A of current.
Rise time For most components, rise time and fall time are assumed to be equal.
The response time of the receiver characterizes its bandwidth.
Sensitivity specifies the weakest optical signal that can be detected. Sensitivity can
be expressed in microwatts or dBm. A sensitivity of 1 W is the same as a
sensitivity of -30 dBm.
25
Receivers
Vcc
R1 (560 Ω)
Sw
light
amp
C1
C2
Vo
PD
Receiver
Driver Circuit
26
TEST INPUT
L1
2.7 μH
+5 V
4.2 μF
R2
43 Ω
47.1 μF
10 kΩ
8
3
U3
1
DATA IN
3
2
1 kΩ
2,6,7
CR1 , 1N 3064
MC75451
4
C3
100 pF
Tx
R3
42.2 Ω
2.7 μH
-5.2 V
4.8 μF
4.8 μF
R10 14.7 kΩ
DATA OUT
DATA OUT
R11 14.7 kΩ
DATA OUT
(ANALOG)
8
4
6
-
7
+
2
LM360N
3
5
Transceivers and Repeaters
0.1 μF
2.7 μH
0.1 μF
0.1 μF
57 Ω
2.7 μH
5
7
+
8
-
11
R7
402 Ω
1 kΩ
10
0.1 μF
1
LM733C
14
4
1 kΩ
0.1
μF
6
1 μF
2
0.1 μF
3, 7
Rx
1 kΩ
100
pF
1 kΩ
27
System Rise Time and Bandwidth
t 
2
r total
 t r tr  t r  fiber  t r rec
2
2
2
tr system  1.1 tr tr2  tr 2rec  tr 2fiber
0.35
(t r ) f 
Wf
c n2
Wf 
2 n12 L 
C  C min ; C min
Wsystem
0.35

(t r ) system
S

 Wsystem log 1   [bit / sec]
N

28
Communication Link Power Budget
x km
Transmitter
y km
Receiver
Connector
-16 dBm
-1 dB
-x2.5 dB
17 dB
Budget
-6 dBm-(-39 dBm) = 23 dB
Margin = 6 dB
-1 dB
-y2.5 dB
(x2.5dB+y2.5dB+1dB
+1dB+1dB) <=17dB
-1 dB
6 dB
Margi
-39 dBm
29
Connectors
SMA
ST
30