fibre optics
DESCRIPTION
fibre optical communication. this is a lecture slide to learn more about optics communicationTRANSCRIPT
EE4110 Alphones 1
• Dr. Arokiaswami Alphones(Assoc. Prof)
• S2.2-B2-19 (Extn: 4486)
• [email protected] Alphones 2
TOPICS Covered
1. Introduction
2. Optical Principles
3. Signal degradation in optical Fibers
4. Light Sources and Detectors
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TEXT BOOKS:1. Optical Fiber Communications by Gerd Keiser McGraw-
Hill, 5th Edition, 20132. Optical Fiber Communications Principles and Practice by
John M Senior 3rd Edition, Prentice Hall, 2009
REFERENCES1. Essential Guide to Optical Networks by David Greenfiled,
Prentice Hall, 20012. Fiber Optic Communications by J C Palais Pearson, 5th
Edition, 20053. Optical Networks by R Ramswami & K N Sivarajan Morgan
Kaufmann 19984. Fiber Optic Test & Measurement by Dennis Derrickson
(Ed), HP Professional Books 1998EE4110 Alphones 4
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Transmission Loss in Atmosphere
• OHx CO2
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Increase in Bitrate-Distance product
1960
Laser
1970
Low-Loss Optical Fiber
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Progress In Lightwave Communication Technology
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Why fiber?
9
3-dB OPTICAL BANDWIDTH
As light propagates down a fiber at low
modulation frequencies, its loss remains
constant. At higher modulation frequencies the
signal power begins to decrease.
The 3-dB optical bandwidth is the frequency at
which the optical signal power is reduced by
one half. It is illustrated on the previous slide.
3-dB electrical/optical bandwidth points
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ADVANTAGES OF OPTICAL FIBERS
•Greater bandwidth•At = 1m ( = 3x1014 Hz)•BW @1% of = 3000GHz (>million TV channels)•Small size and weight
- Twisted pair cable: = 7-10cm, 1000 wire pairs - Fiber: one fiber, < 1cm can carry more data
•Attenuation - low & frequency independent•Free from EMI and cross-talk•Safety & Electrical isolation (fiber is dielectric)•Security (non-invasive tapping impossible)•Ruggedness, chemical inertness•Low cost, abundant raw material (sand)
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History of attenuation
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Four generations of light wave systems
1st generation-1980 =0.8m, 45Mb/s, (MM fiber)
2nd generation-1987 =1.3m, 1.7Gb/s (SM fiber)
3rd generation-1991 =1.55m, 2.5Gb/s (SM fiber)
4th generation (with WDM & Optical Amplifiers)
1996 (TPC-5/FLAG) =1.55m, 40Gb/s 2000 (TPC-6) =1.55m, 100Gb/s
Current trends - Optical Raman amplifiers- WDM/DWDM systems- Soliton (5th Generation) systems
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WAVELENGTH DIVISION MULTIPLEXING (WDM) SYSTEMSOptical carrier BW >1THzLimiting factors - Dispersion, Fiber non-linearity, - Speed of electronic
circuits
Fiber loss (dB/km
)
1.5 Channel spacing1-10GHz
Carrier 1.0
Frequencies
12THz0.5
15THz
0
0.9 1.1 1.3 1.5 1.7Wavelength (m)
To utilize the high capacity of fibers, in WDM systems, multiple opticalcarriers of different wave-lengths are Modulated by separate input signalsMultiplexed and sent through a single fiber De-multiplexed at the receiverinto channels
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DWDM (Dense Wavelength Division Multiplexing) Systems for Very-High Capacity
Optical Communications
Multi-Wavelength DWDM System
Tx Rx
Fiber
Tx 1
Tx 2
Tx 3
Tx N
Rx 1
Rx 2
Rx 3
Rx N
WDM
WDM
Fiber
Single-Wavelength Conventional System
Research started in 1986 System deployment in 1995
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High-Capacity Systems (TDM+DWDM )
OC-Level Line Rate TDM DWDM
OC-3 (STM-1) 155 Mb/s (2016 Ch) OC-12(STM-4) 622 Mb/s (8064 Ch) OC-48(STM-16) 2.5 Gb/s (32256 Ch) OC-192(STM-64) 10 Gb/s (129,024 Ch) OC-768(STM-256) 40 Gb/s (516,096 Ch) OC-48 x 4 10 Gb/s (4 x 2.5 Gb/s) OC-48 x 16 40 Gb/s (16 x 2.5 Gb/s) OC-192 x 8 80 Gb/s (8 x 10 Gb/s) OC-192 x 32 320 Gb/s (32 x 10 Gb/s) OC-768 x 40 1.6 Tb/s (40 x 40 Gb/s)
Reference (SONET/SDH) standard: OC-1 ---- 51.84Mb/s
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Optical Fiber Attenuation and Fiber Amplifier Gain
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1st Commercial Fiber Installation Chicago 1977
Terrestrial Fiber Communication
Link
90 Mb/s system
7 yrs after 1970
1977
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Example of Optical Fiber on a Drum—Corning Fiber
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TAT-8, the first transoceanic fiber optic cable system
In-service since 1988 (280 Mb/s on each of two fiber pairs)Designed and deployed by AT&T Submarine Systems
and Bell Labs 1988< 25 years ago
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Laying undersea optical cable and a repeater for TAT-8from the deck of the AT&T CS Long Lines, 1988
1988
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Global Undersea Optical Fiber Networks
560 Mbp/s REGENERATIVE280 Mbp/s REGENERATIVE
OTHER REGENERATIVE
5.0 Gbp/s AMPLIFIER2.5 Gbp/s AMPLIFIER
NON-REPEATERED
FLAG
SINGAPORE-BRUNEI
HON-TAI-2
THAILANDMALAYSIA
RUSSIA
TURMEOS-1
FLAG
DENMARK-RUSSIA
SEA-ME-WE2
TCS-1APCN
PENBAL
H-J-K
APCK-J-G
TPC-4
NPC
TPC-3
H-P-G EURAFRICAPENCAN-4OPTICAN-1
MAT-2
SWEDEN-FINLAND
B-M-P
JAPANCHINA
G-P-T
SEG D
UK-SPAIN 4
TAT-10KOREA
OFFSHORE
AMERICAS-1N
CANUS-1
TAT12
TPC-5NETWORK
TAT13
FLORICO
HAW-5
PTAT
CARAC
PACRIMWEST
PACRIMEAST
QCC
HICS
APOCS
TAINO-CARIB
ST.THOMAS-ST.CROIX
ISRAEL-CYPRUS
RIOJA
CELTIC
EMOS-1
TAT-11TAT-9
SAT-2
UNISUR
AMERICAS-1
HAW-4
JASAURUS
TASMAN-2
CADMOS
COLUMBUS IIB
23
ANALOG SIGNAL QUALITY
A measure of the quality of an analog signal is
the signal-to-noise ratio (SNR). It is the signal
power divided by the noise power.
S/N = (Signal Power)/(Noise Power)
For analog television signals, SNR > 10,000 is
required for decent viewing of video signals.
24
DIGITAL SIGNAL QUALITYThe measure of quality for a digital system is
the bit error rate (BER). The BER is the
fraction of errors contained in a signal.
Example: A BER = 10-9 means that there is one
bit error for every 109 bits.
Digital systems require good bit error rates.
Number of bits in errorBER =
Total Number of bits transmitted
25
Computing Power Levels in Decibels
(Self Study Slides 25-32)
The decibel scale is useful for analysis and design of
fiber components and systems.
P1P2
Component (System)
26
DECIBEL SCALEThe decibel scale is used to compare the ratio of two
power levels.
Example: Suppose P2/P1 = 0.5. Then
dB = 10 log (0.5) = -3 dB
Example: If P2/P1 = 1, then
dB = 10 log 1 = 0 dB
Example: If P2/P1 > 1, then dB is positive
Example: If P2/P1 < 1, then dB is negative
27
DECIBEL SCALE FOR CASCADED ELEMENTS
The dB scale is useful for analyzing a system of
cascaded elements.
P1 P4
Element 1 Element 2 Element 3
P2 P3
28
DECIBEL SCALE FOR CASCADED ELEMENTS
Thus
29
DECIBEL ABSOLUTE POWER SCALE
The decibel scale can be used to denote absolute
power if a reference power is specified. If the
reference power is set to 1 mW, we have the dBm
scale defined by
dBm = 10 log P
where P is in milliwatts. This is read as dB relative to a
milliwatt.
30
DECIBEL ABSOLUTE POWER SCALE
The dB scale is defined as:
dB = 10 log P
where P is in microwatts. This is read as dB
relative to a microwatt.
Absolute Power in dBm• The power of a light is measured in milliwatts• For convenience, we use the dBm units, where
-20 dBm = 0.01 milliwatt-10 dBm = 0.1 milliwatt0 dBm = 1 milliwatt10 dBm = 10 milliwatts20 dBm = 100 milliwatts
32
DECIBEL ABSOLUTE POWER SCALE
dBm1 dBm2
dBm2 = dBm1 + dBx
Proof
dBxP1 P2
33
IMPORTANT CONSTANTS
Description Value Symbol
Velocity of light 3*108 m/s c
Planck constant 6.626*10-34 J*s h
Electron charge 1.6*10-19 C e or q
Boltzmann constant 1.38*10-23 J/K k
Optical Fiber• Core
– Glass or plastic with a higher index of refraction than the cladding
– Carries the signal
• Cladding– Glass or plastic with a lower index of
refraction than the core
• Buffer– Protects the fiber from damage and
moisture
• Jacket– Holds one or more fibers in a cable
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Optical Fiber
• Typical Dimension for Silica Fibers:– SMF (Single Mode Fiber): 8 μm core, 125 μm cladding– MMF (Multi Mode Fiber: 50, 62.5, 100 μm core, 125 μm
cladding• Index profile: Step vs. Graded vs. multi-step…
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Refraction and reflection
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Total Internal Reflection
1 1 2 2
1 3
1 2 1 2
21 2
1
1
:sin sin
Re
90 deg
sin sin
c c
c
c
Snells Lawn n
flectionCondition
Whenn n and as increases eventuallygoes to rees and
nn n or nis called theCritical angle
For there is nopro pagating refracted ray
With n1=1.5 (glass) & n2=1.0 (air), c = 41.80
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Basic Parameters
Velocity (free space) c= 3x108m/s
Frequency “f” (Hz) or Wavelength “” (m or nm)
fc /
ncv /Velocity in the medium
n - refractive index of the medium
39
Particle Nature of Light
Light is made up particles called photons. Each
photon has energy:
- wavenumber or frequency
h = 6.626 x 10-34 J s (Planck’s constant)
Shorter wavelength (higher frequency) waves have
greater photon energy.
chhEp
40
PARTICLE NATURE OF LIGHT
The electron volt (eV) is another useful unit for analysis
of fiber optic systems. The electron volt is defined as
the energy acquired by an electron accelerated across a
1 volt potential difference. The conversion between
electron volts and joules is:
1 eV = 1.6 x 10-19 J
The 0.8 m photon has an energy of:
= 1.55 eV19
1434
106.18.010310626.6
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Comparison of fiber structures
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Basic Step index Fiber StructureThe linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location.
43
Light Coupling Efficiency• Must match emitting area to core diameter• Numerical Aperture (NA)
– Defines range of angles over which fiber collects light
– NA is 0.21 for ncore=1.50 and nclad=1.485– Typical core-cladding difference around 0.3-2%
NA ncore2 ncladding
2 EE4110 Alphones
Core
Acceptance angle
Cladding
Half Acceptance angle m
Whole Acceptance angle 2(Light here is guided in fiber)
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1
212
1
22
21 Re2 n
nnDifferenceIndexrefractivelativen
nn
Numerical Aperture for meridional rays
It is related to the maximum acceptance angle
NA = n0sinm = n1cosc
NA = (n12-n2
2)1/2 = n1(2)
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For Corning SMF-28 optical fiber (8/125)
n1=1.4504, n2=1.4447 at 1550 nm
NA = 0.1285
Acceptance angle = 7.38 degrees
Core Dia Cladding Dia
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The Cutoff• For each mode, there is some value of V
(Normalized Frequency Parameter) below which it will not be guided because the cladding part of the solution does not go to zero with increasing r.
• Below V=2.405, only one mode (HE11) can be guided; fiber is “single-mode.”
• Based on the definition of V, the number of modes is reduced by decreasing the core radius and by decreasing ∆.
2 21 2
2 aV n n
For SI fiber
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Jl(r)
r0
0.4
0.8
0.4
1
2 4 6 8 10
l0
l1
l2
Bessel’s function variation with order
2.405
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2 2 1 1
0
,2 /
n k k k n kk
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L
I
0 ePP
PowerInputLcetandisatPowerOutput
10 log I
o
Input Power PAttenuation dBL Output Power P
Input Pulse
Attenuation Output Pulse
Absorption Coefficient
SIGNAL DEGRADATION IN OPTICAL FIBERS
Two major causes - Attenuation & Dispersion
Attenuation – involves only change of amplitude
Attenuation is caused by - Absorption- Scattering
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Dispersion – change in the pulse shape
Output Pulse
Input Pulse
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Wavelength SMF28 (8/125) MMF 62.5/125
850 nm 1.8 dB/km 2.72 dB/km
1300 nm 0.35 dB/km 0.52 dB/km
1380 nm 0.50 dB/km 0.92 dB/km
1550 nm 0.19 dB/km 0.29 dB/km
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Dispersion• Different components of transmitted signal
travel at different velocities in the fiber and arrive at different times at the receiver– Modal dispersion: different modal components of a
pulse travel at different velocities– Chromatic dispersion: different spectral
components of a pulse travel at different velocities• Material dispersion: due to -dependence• Waveguide dispersion: due to waveguide design
– Polarization dispersion: different polarization components of a pulse travel at different velocities
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Phase/Group Velocity
NNp /,
1, / dd NNg
Phase constant of Nth order mode
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Definitions
Group velocity
Group delay
Dispersion
1
gv
kc
1
D
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Intermodal DispersionA light pulse excites several modes in a SI fiber. Pulse broadens because these modes
travel in different path lengths, but have the same velocity
Cladding n2
Core n1
Cladding n2
Core n1
Inter Symbol Interference
0 1 0 1
0 1 0 1
0 1 1 1 ?
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Modal Dispersion
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Cladding n2
L’ 1c
L Core n1
11
21
11
minmax
/1
//
//sin/1
ps/km)or (ns )(
ncncnn
ncL
ncL
LL
pathshortestpathlongestfortimeTravelL
c
cn
L
12
21
1
21n
nnn
nn
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Mode Coupling reduces dispersion
103
102
101
100
10-1
0.01 0.1 1 10 100Fiber Length (km)
Lc = Characteristic Fiber Length
WithMode Coupling
WithoutMode Coupling
Mod
e D
ispe
rsio
n (n
s/km
)
cLLc
n 1
Dispersion with mode coupling
Lc – After this fiber length(100 to 550m) mode coupling tends to average out the propagation delays among the modes and reducing modal dispersion
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wavelength,radiuscorea
nna2)FrequencyNormalized(V
405.2VFor2
VN
22
21
2
Number of modes (N) in SI fiber
For V 2.405 22
21 nn2
405.2a
Cladding n2
Core n1
Single mode (SM) fiberZero modal dispersion
Measuring Bandwidth
• The bandwidth-distance product in units of MHz×km shows how fast data can be sent through a cable
• A common multimode fiber with bandwidth-distance product of 500 MHz×km could carry– A 500 MHz signal for 1 km, or– A 1000 MHz signal for 0.5 km
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Dispersion Limitation in MMF
• Dominated by modal dispersion for unit length
• For step-index MMF, the bitrate-distance product would dictate the transmission capacity
– Typical step-index MMF BL<10 Mb/s-km
21
BL
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GRADED INDEX FIBER CONCEPT
Light Propagation In a Hypothetical Multi-layer Fiber
n1
n2
n3
n4
n1> n2> n3> n4
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Increasing the # of Refractive Layers (Same n & )
n
A. 4-Layers
B. 8-Layers
C. Infinite Layers Practical GI Fiber
n
n
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1
212
1
22
21
212/1
1
2/1
1
nnn
n2nn
arforn)1(n)21(n)r(n
ar0forar21n)r(n
General refractive profile for GI fibers: n(r) as function of radial distance, is the index profile
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ar1n)r(narFor 1
= 1 Linear Profile
n2
n1
Special cases
Case-I:
ar1n)r(n 1
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2
1 1)(arnrn
n2
n1
= Step-Index Profile
Case-II:
Case-III:
= 2 Parabolic Profile
n(r) = n1
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SI2
122
GI N2
nka2
N
= Modal Dispersion for equivalent SI fiber
Number of modes in a GI fibers
NSI = No. of modes in equivalent SI fiber
Inter-modal dispersion in GI fibers
SILcn
L
whenoccursdispersionMinumum
1010
2)1(2
21
min
SIL
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Dispersion Comparison
Graded-index fiber has substantially less modal dispersion
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Intra-modal dispersion or Chromatic Dispersion
pulse spreading within a single mode due to dependence of vg on
Material DispersionDue to variation of n with
Waveguide DispersionDue to variation of with a/
Intra-modal (chromatic) dispersion
71
SPECTRUM OF AN OPTICAL SOURCE
1
.5
Example Emission Spectrum of an Optical Source
fFrequency
f = source bandwidth (range of frequencies
emitted by the source).
NormalizedPower
ff1 f2
72
SPECTRUM OF AN OPTICAL SOURCE
1
.5
Alternatively, we can plot the wavelength
emission spectrum as follows:
Wavelength
NormalizedPower
= linewidth or spectral width
21
73
SPECTRUM OF AN OPTICAL SOURCE
Example: If = 0.82 m, = 30 nm
so we have 3.7% bandwidth.
The conversion between wavelength and
frequency is:
74
SPECTRUM OF AN OPTICAL SOURCE
Spectral Widths for Typical Light Sources
Source Spectral Width (nm)
LED 20-100
Laser Diode 1-5
Nd:YAG-Laser 0.1
HeNe Laser 0.002
Variation of Refractive index of Silica using Sellmeir’s equation
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800 1000 1200 1400 16001.442
1.444
1.446
1.448
1.45
1.452
1.454
(nm)
n (
)
1 68.4 0.69617
2 116.2 0.40794
3 9896.2 0.89748
Wavelength
Zero dispersion wavelength
Zero slope point
Inflection point
Wavelength
Wavelength
0
+
-
0
1.447
delay changes little at peak (inflection point)
Delay changesfaster here
a) Refractive index vs.
b)Group delay vs..
c)Dispersion vs..
Refractive index, group delay, and dispersion
n()
dn/d
d2n/d 2
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MATERIAL OR SPECTRAL DISPERSION
Light sources emit - A band of wavelengths - Have a finite spectralwidth
Material dispersion : Pulse broadening due to
‘n’ being a function of Different s traveling with different speeds
For an optical signal through a fiber of length L and a source spectral width of
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mg
mg LDd
ndc
Ldd
2
2
m LDm
Pulse Broadening
Travel time(Group Delay due to Material dispersion)
ddnn
cL
d/dd/dL
d/dL
vL
gg
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m L Dm
Source spectral width -
nm
Mat. Disp. Coeff.
ns/km/nm
Material dispersion
Fiber length (km)
112
2
m nm.km.nsd
ndc
Coeff.Disp.MatD
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For LED 20- 40 nm
For FP laser diode 2-4 nm
For DFB laser diode < 1 nm
= 0 = 1.276m
m /m can be reduced by source with small
Another factor to reduce Material dispersion is the selection of operating wavelength
For pure SiO2, Zero dispersion occurs at
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For pure SiO2, Zero dispersion occurs at 0 = 1.276 m
8
6
4
2
0
-2
0.8 1.0 1.2 1.4 1.6 1.8(m)
0 = 1.276 m
(d2 n
/d2
) 10
-10
m-2
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Addition of impurities to SiO2 can result in shifting of zero dispersion wavelength 0 to another value
40
0
- 40
- 80
-120
0.8 1.0 1.2 1.4 1.6(m)
DM
(ns/
km/n
m)
13.5% GeO286.5% SiO2
Quenched SiO2
= 0 = 1.276 m
However, addition of impurities can also cause increase of attenuation
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.Coeff.DispWaveguidedV
VbdVcnD 2
22
wg
wg Dwg L WaveguideDispersion
Mode-field diameter =2w0
In a SM fiber, )/exp()( 20
20 wrErE
At r = wo, E(Wo)=Eo/e
Typ. Wo > a
WAVEGUIDE DISPERSIONAssumption: n independent of (For independent analysis of waveguide dispersion)
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1.2
0.8
0.4
0 0 1.0 2 3V= kaNA
b
2
2
dVVbd
dVVbd
4.22Vfor1.0to2.0
2.1VatMaximumdV
Vbd2
2
For MM fibers: Waveguide Disp.<Material Disp.For SM fibers, waveguide dispersion is important
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A better approach is to combine the material and waveguide dispersions in the fiber to obtain i.e Dm + DW
20
10
0
- 10
- 20
1.0 1.2 1.4 1.6(m)
DM
(ns/
km/n
m)
Dm + DWa = 2.5 m
Dm
DW (a = 4 m)
DW (a = 2 m)
DW (a = 2.5 m)
DISPERSION SHIFTED FIBER
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Dispersion-compensating fiber
Transmission fiber
0
100
50 100 150 200Distance, km
Tota
l acc
umul
ated
di
sper
sion
, ps/
nm
TX RX
Dispersion Management Technique
Accumulated dispersion at the receiver is close to zero
ofcshortcourse.ts.degradation_effects.12
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The eye diagram is an intuitive graphicalrepresentation of electrical and opticalcommunication signals. The quality of these signals(the amount of inter-symbol interference, noise, andjitter) can be judged from the appearance of the eye.
The waveform of a communication signal, such as(NRZ) or (RZ) signal, can be turned into an eyediagram or eye pattern by folding the time axismodulo a whole number of bit (or symbol) intervals.
Eye Diagram/Eye Pattern
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Eye diagram of a signal measured using sampling oscilloscope
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40 Gb/s Chromatic Dispersion Compensation
“Eye Diagram”
without compensation
withcompensation
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Dispersion @1550nm Dm 18 ps/km/nm
Zero Dispersion @ 0 = 1276 nm
DISPERSIONS IN SINGLE-MODE FIBERSPulse broadening due to source spectral width
2nd order dispersion causes pulse broadening near 1276 nm in standardSMF and 1550 nm in DSF
2nd-order dispersion is dispersion slope
d
)(dD)(S m
Dispersion at wavelength,
4
m 14S)(D
For small chromatic dispersion, polarization mode dispersion (PMD) becomes significant
PMD = DPMD (ps/km) x L1/2
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Polarizations of fundamental mode
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Polarization mode dispersion
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40 Gb/s System PMD Dispersion Compensation
without compensation
withcompensation
Overall Pulse Broadening in Fibers
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Dispersion due to modal dispersion
Dispersion due to material dispersion
Dispersion due to Waveguide dispersion
Dispersion due to Polarization mode dispersion
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Total System Rise Time
Source rise time Receiver
rise timeIntermodal dispersion pulse spreadover link length
Intramodal or chromatic dispersionpulse spread over link length
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Analog/Digital transmission systems
1) Examine the components that are available for a particular application
and see how these components relate to the system performance
criteria (such as dispersion and bit-error rate).
2) For a given set of components and a given set of system requirements,
we then carry out a power budget analysis to determine whether the
fiber optic link meets the attenuation requirements or if amplifiers are
needed to boost the power level.
3) The final step is to perform a system rise-time-analysis to verify that
the overall system performance requirements are met.
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Summary on Dispersion• Modal dispersion: different modes
propagate at different group velocities• Material dispersion: the index of
refraction of the medium changes with wavelength
• Waveguide dispersion: index change across waveguide means that different wavelengths have different delays
• Polarization mode dispersion: if waveguide is birefringent
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Fiber Attenuation Characteristics
850 nmwindow
1.3 µmwindow
1.55 µmwindow
Main OH absorption
RayleighScatteringminimum
InfraredAbsorptionOf silica
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ABSORPTION -- EXTRINSIC
Current metal impurity levels 0.1ppb1-10ppb metal concentration 1-10 dB/km
Metal Peak (nm) dB/km (for 1 ppb)Cr++ 625 1.6C++ 685 0.1Cu++ 850 1.1Fe++ 1100 0.68Fe+++ 400 0.15Ni++ 650 0.10Mn+++ 460 0.20V++++ 725 2.70
Transition metal absorption occurs due to (i) Electron transitions within the same ion(ii) Charge transfer from one ion to another
OH ions in glass give significant absorptionEE4110 Alphones 100
= 0.8-1.7 m Tails of UV/IR peaks can extend in the window
INTRINSIC ABSORPTION Occurs even in perfect material due to Electron excitations in the UV-band and Photon-phonon interaction (IR band)
Sets lower limit for attenuation
Leaves a transmission window near
attenuation in the 1.21.5 m range -Determined by Inherent IR absorption and OH ion concentration-0.5dB/km @ 1.3 m-0.2dB/km @ 1.55 m
IR absorption peaksSi- O 9.2 m
P-O 8.0 m
Ge-O 11.0 m
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Tf22
4
3
r kT1n38
k = Boltzmann constant, T = isothermal compressibility Tf = Fictive temperature,
LINEAR SCATTERING LOSSES(No change of upon scattering)
RAYLEIGH SCATTERING
Linear and dominant scattering loss mechanism in the low absorption window between UV/IR tails
Caused by refractive index variations (within distances ) that occurs due to- Microscopic density variations- Random molecular structure of glass- Inhomogeneous Structure and defects- Oxide contents in glass
Raleigh scattering loss (Single component glass)
41 rThus
in µmEE4110 Alphones 102
MIE SCATTERING
It is a forward process, caused by Inhomogeneities ,Irregularities at the core/cladding interface, Non-cylindrical fiberstructure and Strains/bubbles within the core or cladding.
Fiber diameter variations or interface scattering due to surfaceirregularities
Flaw
CORE
CLADDING
D1 D2
CLADDING
This can be reduced significantly by
-Improved fabrication process
- Fiber end polishing
EE4110 Alphones 103
PB (watts) = 0.0176(a)2dBPR (watts) = 0.236(a2)dB
a = Fiber radius (m) = Wavelength (m) = Source bandwidth (GHz) dB=Attenuation (dB/km)
NON-LINEAR SCATTERING
Stimulated Brillouin and Raman scattering effects occur above threshold power densities
Brillouin ScatteringBackward Process
Raman ScatteringForward Process
IncidentPhoton
Photon of Different
Acoustic Freq.
Phonon
Photon of Different
High Freq. Phonon
These effects are not easily observed in MM fibers due to large radius ‘a’ (making PB/PR very high)EE4110 Alphones 104
Power loss in a curved fiber
EE4110 Alphones 105
MACROBENDING LOSS
At a bend, a mode is required to travel faster in the cladding (than in the core), and lost by radiation
Bending loss (Effective absorption coeff.)
R = Radius of curvature if the bend, a = Fiber radius, C= Constant
Critical Bend radius Rc
Bending losses high when R Rc
Micro-bends which are bends with R a, cause significant losses
a)NA(R
B
2
Ce
23
22
21
21
c
nn4
n3R
EE4110 Alphones 106
Microbending losses
EE4110 Alphones 107
Bending-induced attenuation • Example: 10mW of power is launched into an optical fiber that has an attenuation of =0.6 dB/km. What is the received power after traveling a distance of 100 km?– Initial power is: Pin = 10 dBm– Received power is: Pout= Pin– L=10 dBm – (0.6)(100) = -50
dBm
• Example: 8mW of power is launched into an optical fiber that has an attenuation of =0.6 dB/km. The received power needs to be -22dBm. What is the maximum transmission distance? – Initial power is: Pin = 10log10(8) = 9 dBm– Received power is: Pout = 1mW 10-2.2 = 6.3 W– Pout - Pin = 9dBm - (-22dBm) = 31dB = 0.6 L– L=51.7 km
nWmWPout 10110 1050
EE4110 Alphones 109
Source-to-fiber coupling
EE4110 Alphones 110
t
s
AALog10
Large Area Source Small Area Source
Diameter Mismatch Loss
When Source area As Target Area At
Diameter Mismatch Loss = 0 dB
When Source area As Target Area At
Diameter Mismatch Loss (dB) =
EE4110 Alphones 111t
s
NANALog20
CladdingCoreLarge NA Source
NAs NAt
CladdingCoreSmall NA Source
NAs NAt
Numerical Aperture (NA) Mismatch Loss
When Source NA Target NA (NAs NAt)
NA Mismatch Loss = 0 dB
When Source NA Target NA (NAs NAt)
NA Mismatch Loss (dB) = EE4110 Alphones 112
Mechanical misalignments between fibers
EE4110 Alphones 113
2
01
01
nnnn1Log10
n1n1 n0
Source Target
Fresnel Loss (air/glass) = 0.18dB 0.2dB
Z
2a
Transmission Loss due to Fresnel Reflection :
Fresnel Loss (dB) =
For air (n0 = 1.0) to glass (n1 = 1.5) interface:
Gap Loss :
Reflection coefficientAt fiber-air interface
EE4110 Alphones 114I
c
PPLog10
0.0 0.1 0.2 0.3 0.4 0.5z/a
Inse
rtio
n Lo
ss (d
B)
4.0
3.0
2.0
1.0
0.0
NA=0.8 NA=0.7
NA=0.5
NA=0.15
Pc = Coupled power, PI = Incident Power
Gap loss with
Source/Target gap = zSource diameter = 2aSource NA = NA
Gap Loss =
EE4110 Alphones 115
1NAForNA
az
341
NA1NAsinNA2
az1
PP 21
2I
c
2a d
Gap loss for source/target gapof z
Axial Misalignment Loss :
Core irregularities & off-center cores can also cause this problem
Axial misalignment loss with
Source/Target axial offset = dSource diameter = 2a
EE4110 Alphones 116
Axial offset condition
EE4110 Alphones 117
I
cPPLog10
0.0 0.1 0.2 0.3 0.4 0.5d/a
Inse
rtio
n Lo
ss (d
B)
4.0
3.0
2.0
1.0
0.0
Pc = Coupled power PI = Incident Power
Misalignment Loss =
adForad41
ad1
ad
adcos2
PP 2
1
I
c
EE4110 Alphones 118
0.0 0.1 0.2 0.3 0.4 0.5Angle
Inse
rtio
n Lo
ss (d
B) 1.5
1.0
0.5
NA=0.5
NA=0.15
Angular Misalignment Loss : Occurs if the surfaces of the source and target are not parallel
Angular misalignment loss with Source/Target angle of and Source NA
EE4110 Alphones 119
Comparison of misalignment effects
EE4110 Alphones 120
Fiber cleaving
EE4110 Alphones 121
Fiber end defects
EE4110 Alphones 122
Fusion splicing
Fitel_S121_Hand_Held_Splicer.wmv
EE4110 Alphones 123
Link Power/loss Analysis
EE4110 Alphones 124
EE4110 Alphones 125
To illustrate how a link loss budget is set up, let us carry out a specific designexample.
We shall begin by specifying a data rate of 20 Mb/s and a BER of 10-9 (i.e. atmost one error can occur for every 109 bits sent).
For the receiver, we shall choose a silicon pin photodiode operating at 850 nm.We know that the required receiver input signal is -42 dBm (refer the sensitivityslide).We next select a GaAlAs LED that can couple a 50 μW (-13 dBm) averageoptical power into a fiber flylead with a 50 μm core diameter. We thus have a29 dB allowable power loss.Assume further that a 1 dB loss occurs when the fiber flylead is connected tothe cable and another 1 dB loss occurs at the cable-photodetector interface.Including a 6 dB system margin, the possible transmission distance for a cablewith an attenuation of αf=3.5 dB/km, then a 6.0 km transmission path ispossible.
Link Power Budget Example
EE4110 Alphones 126
Power Budget Diagram
EE4110 Alphones 127
FIBER MATERIALS & MANUFACTURING TECHNIQUES
PLASTIC FIBERS are cheap & easy to manufacture but have high attenuation
GLASS FIBERS – are made from low melting glasses
SILICA FIBERS – are made from pure SiO2- Refractive index n = 1.45 @ = 1 m- Wavelength range: 0.2 - 4.0 m- Addition of B2O3 lowers n (for SiO2)- Addition of GeO2 increases n EE4110 Alphones 128
1.50
1.48
1.46
1.44
0 5 10 15MOL %
F
ZrO2
TiO2
Al2O3 GeO2
P2O5
B2O3
n
EE4110 Alphones 129
Direct Melt Techniques
Chemical Vapor Deposition (CVD) Techniques
FIBER MANUFACTURING TECHNIQUES
VAD NTT
OVPOCORNING
MCVDBELL LABS
PCVDPHILIPS
Starting Materials; SiCl4, SiF4, GeCl4, POCl3,BCl3, O2 ETC.
PREFORM - FABRICATION
FIBER DRAWING
Starting Materials: Pure Silica (SiO2) Powder or
Rods
EE4110 Alphones 130
FIBER DRAWING AND COATINGPrecision Feed
Mechanism
Preform
Diameter Monitor
Furnace
Coating Applicator
Curing Oven
Winding Drum
EE4110 Alphones 131Corning Cables
Fiber Fabrication Video EE4110 Alphones 132
LIGHT SOURCES AND DETECTORS
LIGHT SOURCES- Forward biased PN junctions made of III-V compound semiconductors- Light emission by radiative recombination of electrons & holes
DETECTORS- Reverse biased PN junctions made from variety of semiconductors- Light absorption creates electron/ hole pairs (electric current)
LIGHT SOURCES & DETECTORS IN OPTICAL FIBER SYSTEMS
SEMICONDUCTOR JUNCTION DEVICES
Laser Diode LED PIN Photodiode
Avalanche Photodiode
EE4110 Alphones 133
Semiconductor Light Sources• A PN junction (that consists of direct band gap
semiconductor materials) acts as the active or recombination region
• When the PN junction is forward biased, electrons and holes recombine either radiatively (emitting photons) or non-radiatively (emitting heat). This is simple LED operation.
• In an LASER, the photon is further processed in a resonance cavity to achieve a coherent, highly directional optical beam with a narrow linewidth
EE4110 Alphones 134
ROOM TEMPERATURE SEMICONDUCTORS
Group IV Elements
III-V CompoundSemiconductors
II-VI CompoundSemiconductors
OTHERS
SiGeSe Te
GaAs, GaP, InPGa1-xAlxAsIn1-xGaxAs1-yPx
ZnSZnSeCdSCdSe
SiC
Diamond
Wavlength 0.8m 1-1.7mSourceMaterial
GaAlAs/GaAs InGaAsP/InP
DetectorMaterial
Si GeInGaAs/InP
135
Suppose Eg is given in eV. Compute the
output wavelength.
with Eg in eV and in m.
LIGHT EMITTING DIODES
In practice
136
Material (m) Eg (eV)
GaAs 0.9 1.4
AlGaAs 0.8 - 0.9 1.4 - 1.55
InGaAs 1.0 - 1.3 0.95 - 1.24
InGaAsP 0.9 - 1.7 0.73 - 1.35
GaInP 0.64 - 0.68 1.82 - 1.94
Common Characteristics
LIGHT EMITTING DIODES
EE4110 Alphones 137
Forward bias condition
EE4110 Alphones 138
Reverse bias condition
EE4110 Alphones 139
1
kTeVexpII 0
Dark Current I = I0
Rev
erse
Cur
rent
Forward Voltage
Forw
ard
Cur
rent
IF
Reverse Voltage
VF
Light generated current I=I0 +IL
Electron-Hole Recombination
p- region Anode
n- regionCathode
I I I
THE pn JUNCTION
Basic equation:
kTeVexpII 0
140
The relationship between electrical
bandwidth and rise time is (approximately),
This relationship can be shown to be exact for
an RC low-pass filter (next slide). We will use it
for determining the frequency response of light
sources, light detectors, and the fiber.
LED OPERATING CHARACTERISTICS
i 10%90%
tr
Output
141
Input Output
Typical values of tr for LEDs are a few ns to 250 ns.
LED OPERATING CHARACTERISTICS
Low-Pass RC Filter
R
C
EE4110 Alphones 142
LIGHT SOURCES - LED & laser diodes (LD)- PN junctions - Convert electrical input to light - Operated with forward biased- Made from III-V compound semiconductors (GaAs,
GaAsP, InxGa1-xAsyP1-y etc)
Light Emitting Diode (LED) has- Light by spontaneous emission
- Optical power LED current- Small bandwidth, long life, low cost- Spectral width of 30 to 60nm- Large beam divergence- Operation over wide temp range
Semiconductor Injection Laser Diode (ILD) has- High power output
- Coherent and narrow beam- Higher coupling efficiency
EE4110 Alphones 143
P p
~20nm
p
~4nm
PLED ILD
SPECTRAL WIDTHS
BEAM DIVERGENCE
Source
LED400
ILD50
Selection criteria - Output power , Peak wavelength p,Modulation bandwidth/speed, Spectral width , - Beam divergence 2m
EE4110 Alphones 144
m)eV(E
24.1Ehc
kTEhc
ggg
Electron-Hole Recombination
p- region Anode
n- regionCathode
I I I
Currenth
Acceptor LevelDonor Level
Conduction Bands pn
Valence Bands
LIGHT EMITTING DIODE (LED)
Light emission from Radiative recombination of carriers injected in forward biased pn junction
The wavelength & spectral width of LED are dependent on the bandgap Eg
Photon Wavelength isgiven by
EE4110 Alphones 145
rrnr
nr
c
phint N
N
Radiative Photon Emission
CARRIER RECOMBINATION
Non-Radiative Phonon Emission
Nph = Rate of photon generation Nc = Rate of carrier injection across the junction
Recombination mechanisms depend on radiative recombination Lifetime (rr) and non-radiative recombination Lifetime (nr)
Internal Quantum Efficiency of an LED
phintphCintphph EeIENEN
Optical Flux
(For current I)
hceI
intEE4110 Alphones 146
2
s0
s0
nnnn
1- Back emission loss
Transmitted Beam
2- Total reflection loss
4-Absorption
ext < int Because of losses
3- FresnelReflection loss
Semiconductor
External quantum efficiency
2
s
0
nn
211
junctionthegsincroscarriersofNumberemittedfinallyphotonsofNumber
ext
EE4110 Alphones 147
Double-heterostructure configuration
EE4110 Alphones 148
Surface-emitting LED
EE4110 Alphones 149
Edge-emitting LED
EE4110 Alphones 150
LED spectral patterns
)()(6.1 meVkT pp
- Spectral widthp - Peak wavelength
EE4110 Alphones 151
100
80
60
40
20
0
Opt
ical
Pow
er (%
)P o
Optical Power vs Forward Current
10 20 30 40 50IF (mA)
LED CHARACTERISTIC CURVES
)m(I24.1)W(P int
o
I F(m
A)
50
40
30
20
10
0 0.5 1.0 1.5 2.0 VF (Volts)
I-V Plot
kTeVexpII 0
EE4110 Alphones 152
0
0
0
IDEAL LASER RADIATION
Perfectly monochromatic (All waves have same )
Perfectly collimated (All waves are parallel)
Perfectly coherent (All waves have the same phase)
0 0, 0
EE4110 Alphones 153
LIGHT ABSORPTION AND EMISSION
h E2E1E1
E2A Absorption
E1
E2
B Spontaneous Emission
h E2E1
h E2E1
E1
E2
C Stimulated Emission
hh
Light Absorption - resultsin jumping of electrons tohigher energy levels
Spontaneous Emission -Electrons jumprandomly to lowerenergy states
Photons are emitted withrandom phase
Stimulated Emission -External photonstimulates electronsjumps to lower energystates emitted photonshave same phase EE4110 Alphones 154
A Laser is like an electronic oscillator- Power Source- Active Device/Medium- Feedback- Amplification
Power Source
Laser Output
L
Fully Reflecting Mirror
Active Medium
Partially Reflecting Mirror
2nqL
nL2qcf n- refractive index
q - integer
Standing waves are formedwhen the cavity length Lequals integer multiple ofhalf wavelength
EE4110 Alphones 155
P
~6nm
100m
200m
300m
Cleaved facetActive Layer
CurrentMetal Contact
p-type
n-type
Febry-Perot laser
Laser
Good polishing provides more than 30% internal reflection that is necessary get the laser output
Light Amplification>Losses
Due to Radiation &Absorption
Lasing occurs when:
EE4110 Alphones 156
Fabry-Perot Laser Spectrum
EE4110 Alphones 157
= Loss coefficientg = gain coefficient
Mirror reflection coefficients
r1r2 e2Le2gL 1
e2L
e2gL
r1r2
Under steady state:
Optical Gain = Total Loss in the cavity
For each round trip in the cavity
The fractional loss
The fractional gain
Gain due to mirror reflections
Under steady state
21th rr
1lnL21g Threshold gain EE4110 Alphones 158
The cavity also affects the output spectrum.
Example: The cavity resonant wavelength spacing is
LnLnc
coo
c 22
Thus22
LASER DIODE
co
c fc
2
Lncfc 2
EE4110 Alphones 159
Assume:
0 = 0.82 m, L = 300 m, n = 3.6
= 2 nm (laser linewidth)
Then
The number of longitudinal modes is approximately
LASER DIODE
24(0.82) 3.11 10 0.311
2(300)3.6c m nm
linewidthresonance spacingm
c
N
160
64.6311.02
mn
mnNm
In this example:
We conclude that there are six longitudinal modes.
LASER DIODE
161
819 820 821
Gain
0.311 nm (nm)
(nm)
Cavity Resonances c
For the laser diode, we have:
(nm)c
Output Spectrum
LASER DIODE
EE4110 Alphones 162
Semiconductor Laser diodes
- pn junctions made from III-V semiconductors
- operate above a threshold current
- emit light by stimulated emission
- Optical power LD current, large BW(>10GHz)
- small spectral width, narrow beam
Population inversion is achieved by (i) Intense doping and (ii) High current density
EE4110 Alphones 163
100
80
60
40
20
010 20 30 40 50 60
IF (mA)
Opt
ical
Pow
er P
O(%
)
Threshold Current
Lasing
Non-Lasing
Laser diode power vs Current curve
th
21th
grr1ln
L211J
Jth – Threshold current density
- Gain factor depends on design specifications
Practical ILD characteristics posses kinks due to - Mode coupling/conversion- Crystal defects 164
+ + + + + + + + + +
Consider the following heterojunction laser diode:
Electron Energy n n p
1.8 eV 1.55 eV1.8 eV
- - - - - - - - - - - - - -- - - - - - - - - - - - - -
LASER DIODE
AlxGa1-x As AlyGa1-y As AlxGa1-x As
Electron Barrier
Hole Barrier
Injected Holes
Injected Electrons
x- mole fraction of Aluminium
165
Energy Level Diagram
LASER DIODE
Electron Energy n n p
1.8 eV 1.55 eV1.8 eV
- - - - - - - - - - - - - -- - - - - - - - - - - - - -
+ + + + + + + + + +
Refractive Index
EE4110 Alphones 166
In the ternary alloy Ga1-x AlxAs the bandgap energy Eg it interfaces the materials GaAs (Eg =1.43eV) and Al As (Eg =2.16eV).
The energy gap in electron volts for values of x between zero and 0.37 can be found from the empirical equation
AlxxxE g
offraction mole
266.0266.1424.1 2
Given the value of Wg or Eg in (eV), the peak emission Wavelength (µm) can be estimated.
EE4110 Alphones 167
ILD External Quantum Efficiency
dIdP
Ee
edIhdP
gext
dIdPmext )(8065.0
thext g
Also 1 , int
So the external quantum efficiency can be estimated either from graphical characteristic results or analytically by the above expression
EE4110 Alphones 168
Temperature dependence
Ith is temperature dependent
Ith(T)I0exp(T/T0)
ConstantsDevice Temperature
Ith changes with aging also
Feedback mechanisms are necessary to obtain constant optical power output
EE4110 Alphones 169
DFB and DBR lasers
Distributed Feedback Distributed Bragg ReflectorDFB DBR
Grating order: multiples of o/2nr of the grating1st order InP grating at 1.55m wavelength: ~0.25m period
6.5 170
Example: Wavelength Shift = 0.1 nm/oC
T1 = 27 °C
Gain
Cavity Resonance
Output
Cavity Resonance
Output
T2 = 30 °C
LASER DIODE OPERATING CHARACTERISTICS
171
Laser Diode Bandwidth
Laser diodes respond faster than LEDs
because the carrier lifetime (which
determines LED speed) is much longer
than the recombination delay due to
stimulated emission (which determines LD
speed) .
Typical laser diode rise times: 0.1 - 1ns
LASER DIODE OPERATING CHARACTERISTICS
172
p
n
Distributed Feedback Laser Diode
The DFB was developed to obtain a single-
longitudinal-mode laser diode .
DifferentMaterials
Grating
The grating acts as a filter, permitting only one of
the cavity’s modes to propagate.
Cleaved Facet
ActiveLayer
Metallization
173
Laser Gain
Cavity Resonances
Grating Resonances
Laser Output
0
DISTRIBUTED-FEEDBACK LD
EE4110 Alphones 174
The grating resonances, according to Bragg’s
law, are those wavelengths for which the grating
period (illustrated on a preceding slide) is an
integral number of half-wavelengths. That is:
n0
is the wavelength in the diode, m is an integer
DISTRIBUTED-FEEDBACK LD
2m
0 is the free-space wavelength
EE4110 Alphones 175
02nm
The grating period then satisfies
DISTRIBUTED-FEEDBACK LD
0
2m
n
and the resonant wavelengths, as measured in
free space, are given by:
EE4110 Alphones 176
Example: Consider an InGaAsP DFB LD
0 = 1.55 m, n = 3.5, let m = 1 (first order)
Determine the grating period.
mn
m 44.0)5.3(2)55.1(2
20
DISTRIBUTED-FEEDBACK LD
mn
m 22.0)5.3(2
55.12
0
Let m = 2 (second order)
EE4110 Alphones 177
DETECTORS
DETECTORS - Some form of PN junctions- Convert light into electrical output operated with
reverse biased- Have large bandwidth (GHz)
Most Detectors have- An insulating base- A casing- Electrodes for connections- Transparent glass window
Detector types - PIN & Avalanche photodiode (APD) made from
- Si (800nm band)- InGaAsP (1300nm band)- InGaAs (1550nm band) EE4110 Alphones 178
PHOTODETECTION MECHANISMSImportant Detector Properties
1. Responsivity: Power OpticalInput CurrentOutput
P i
Optical PowerPhotodetector
Electrical Current
/i A WP
EE4110 Alphones 179
2. Spectral Response:
Range of optical wavelengths over which the
detector is useful. It is often displayed as a
curve of responsivity versus wavelength. An
example appears on the next slide.
PHOTODETECTION MECHANISMS
EE4110 Alphones 180
PHOTODETECTION MECHANISMS
0.5 0.7 0.9 1.1
0.5
(m)
( / )A W
Silicon Photodiode Response
0
EE4110 Alphones 181
3. Speed of Response:
Range of modulation frequencies over which
the detector is useful. As before, if tr is the
rise time, the bandwidth is (approximately)
rdB t
f 35.03
P
PHOTODETECTION MECHANISMS
Inputi
10%90%
tr
Output
182
Noise degrades signals. Without noise, itwould not matter how little optical powerarrived at the receiver.
Signal quality is measured in several ways.
Analog systems:The signal-to-noise ratio (SNR) is the measure.
Digital systems:The bit-error-rate (BER) is the measure.
EE4110 Alphones 183
Conduction Band
Valence Band
Eg
Ec
Ev
hEg
B
AC
Electron-Hole Pairs (EHP)
hEg
Light Absorption in a semiconductor creates
For absorption, the photon energy should be greater then the band-gap of the material
EE4110 Alphones 184
Electron-Hole Generation
p-region Anode
n-region
Cathode
I I I
Photo DiodeA light sensitive device that uses PN junction
Incident light generates excess minority carriers
PN Junction Current In Dark Under Light
Forward Current
Reverse Current
Large
Small
Unchanged
SignificantIncrease
EE4110 Alphones 185
Photo Detection Principles
Device Layer Structure
Band Diagramshowing carriermovement in E-field
Light intensity as a function of distance below the surface
Carriers absorbed heremust diffuse to theintrinsic layer beforethey recombine if theyare to contribute to thephotocurrent. Slowdiffusion can lead toslow “tails” in thetemporal response.
Bias voltage usuallyneeded to fullydeplete the intrinsic“I” region for highspeed operation
EE4110 Alphones 186
Forw
ard
Cur
rent
IF
Reverse Voltage
Rev
erse
Cur
rent
I I0
Light generated current IL
1st Quadrant Rectification
Light Emission
3rd Quadrant Photo-
conductivemode
4th
Quadrant Photo-voltaic
mode
Is =I0 + IL
kTeVexpII 0
Forward Voltage
PN Junction Characteristics
EE4110 Alphones 187
Characteristics of Photodetectors
Number of Collected electrons 1Number of Photons *Entering* detector
/Number of Collected electrons 1 1Number of Photons *Incident* on detector /
Photo Current (Amps)
Wi
ph We p
o
e
i qR e
P h
R
1 1Incident Optical Power (Watts)
1 1ph o
ph Wp
o
Wp
oi RP
i q R eP h
R ePq h
• Internal Quantum Efficiency
•External Quantum efficiency
• Responsivity
•Photocurrent
Incident Photon Flux
Fraction Transmitted into Detector with reflectivity Rp
Fraction absorbed in detection region of distance W andAbsorption coefficient of
h
qe
EE4110 Alphones 188
PIN Photodiode
Structure
SiO2
Intrinsic Si layern-Si
p-Si
Light input
Metal Contact
Practical devices have P+NN+ or similar structure
PIN photodiodes haveWide depletion regionLow internal capacitance Wide frequency responseFast and linear response
EE4110 Alphones 189
Absorption coefficient vs. Wavelength for several materials
Photodiode Responsivity vs. Wavelength for various materials
EE4110 Alphones 190
The Avalanche Photodiode
APD features
- Structure similar to PIN diode- High voltage operation- High internal gain- Large BW- Temperature sensitive- High cost
Under high reverse voltage 100V, carriers in the depletion region gain high energy to cause Avalanche Breakdown
Current gain ≥ 100 is achieved
Gain is dependent on- Bias voltage- Temperature
EE4110 Alphones 191
Avalanche Photodiodes (APDs)
• High resistivity p-doped layer increases electric field across absorbing region
• High-energy electron-hole pairs ionize other sites to multiply the current
• Leads to greater sensitivity
• A high E-field in the depletion region causes carriers to have enough kinetic energy to “kick” new electrons from valence band up to conduction band –Avalanche Multiplication
EE4110 Alphones 192
APDResponsivity
MRMhe
II
heR 0
p
mAPD
M Multiplication factorIM Multiplied currentIp Un-multiplied currentR0 Unity gain responsivity
The depletion layer widens with bias and reaches thru the -layer near the avalanche region
Reach-Through APD Structure
EE4110 Alphones 193
PIN and APD Sensitivity
• An APD typically has 10 dB better sensitivity than a PIN.
• -36 dBm sensitivity at 2.5 Gb/s is possible.EE4110 Alphones 194
1-10m 10m-.1km
.1-1km 1-3km 3-10km 10-50km
50-100km
>100km
LD 10k
SLED MM APD 10-100K
PIN MM 100K-1M
LD GI 1-10M
LED 10-50M
PIN GI LD 50-500M
LD LD SM APD 500M-1G
MM GI >1G
Combination of the sources and fibers for different link capacity and distance
EE4110 Alphones 195
Key system requirements
• The desired transmission distance
• The data rate or channel bandwidth
• The bit-error rate (BER)
• The cost and complexity
EE4110 Alphones 196
Design Approach• Link power budget• Rise-time budget
1. In the link power budget analysis one first determinesthe power margin between the optical transmitter outputand the minimum receiver sensitivity needed toestablish a specified BER.
2. Once the link power budget has been established, thedesigner can perform a system rise-time analysis toensure that the desired overall system performance hasbeen met.
EE4110 Alphones 197
Selection of components
1) Multimode or single-mode optical fibercore size, core refractive index profile, bandwidth or dispersion,
attenuation, numerical aperture or mode-field diameter
2) LED or laser diode optical sourceEmission wavelength, spectral line width, output power, effective radiating area, emission pattern, number of emitting modes
3) pin or avalanche photodiodeResponsivity, operating wavelength, speed or bandwidth, sensitivity
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WDM system
Multiplexer Add/dropMultiplexer
Demultiplexer
Transmitters Receivers
Drop Add
Localreceiver
Localtransmitter
LASER BEAM INJURIES
High power lasers can cause skin burns.
Lasers can cause severe eye injuriesresulting in permanent vision loss.
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Some common unsafe practices:preventable laser accidents
• Not wearing protective eyewear during alignment procedures
• Not wearing protective eyewear in the laser control area
• Misaligned optics and upwardly directed beams• Equipment malfunction• Improper methods of handling high voltage• Available eye protection not used• Intentional exposure of unprotected personnel• Lack of protection from non-beam hazards
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Some common unsafe practices or preventable laser accidents
• Failure to follow (Laser) Safety Instructions• Bypassing of interlocks, door and laser housing• Insertion of reflective materials into beam paths• Lack of pre-planning• Turning on power supply accidentally• Operating unfamiliar equipment• Wearing the wrong eyewear
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Guidelines to help prevent accidents during alignment
• The lowest possible/practical power must be used during alignments.
• Have beam paths that differ from the eye level when standing or sitting. Do not use paths that tempts one to bend down and look into the beam.
• All laser users must receive an introduction to the laser area by an authorised laser user of that area
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End of Part I
Optical Transmission Systems have made significant advances and are operational
both in land and under sea.
But much can be gained by improving optical integration and exploring optical
technologies to coexist with wireless transmission systems!!