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Lecture Outline
Overview of optical fiber communication (OFC)Fibers and transmission characteristics
Quick History of OFC
• 1958: Laser discovered• Mid-60s: Guided wave optics demonstrated• 1966 - Fiber loss = 1000 dB/km! (impurities)• 1970: Production of low-loss fibers; 20 dB/km, competitive with
copper cable.– Made long-distance optical transmission possible!
• 1970: invention of semiconductor laser diode– Made optical transceivers highly refined!
• 70s-80s: Use of fiber in telephony: SONET• Mid-80s: LANs/MANs: broadcast-and-select architectures• 1988: First trans-atlantic optical fiber laid• Late-80s: EDFA (optical amplifier) developed
– Greatly alleviated distance limitations!• Mid/late-90s: DWDM systems explode• Late-90s: Intelligent Optical networks
Advantages of OFC
• Enormous potential bandwidth• Immunity to electromagnetic interference• Very high frequency carrier wave. (1014 Hz).• Low loss ( as low as 0.2 dB/Km for glass)• Repeaters can be eliminated low cost and
reliability• Secure; Cannot be trapped without affecting
signal.• Electrically neutral; • No shorts / ground loop required.• Good in dangerous environment.• Tough but light weight, Expensive but tiny.
The Electromagnetic Communication Spectrum
What is Light? Theories of Light
Historical Development
Comparison of Bit Rate-Distance Product (B-L)
Coaxial cable
BL
Optical amplifiers
Telephone
Telegraph
Microwave
Lightwave
Year
1015
1012
109
106
103
11850 1900 1950 2000
(Bit/
s-km
)
Elements of a F-O Transmission Link (Old)
Drive circuits
Laser
Optical fiber Amplifier
Detector
Electromagnetic field theory Wave propagation
Semiconductor physics Quantum electronics
Laser technology
Semiconductor physics Quantum electronics
Electronics Circuit theory
Electronics Circuit theory
Communication theory, modulation theory
A multi-Disciplinary Technology
Snell’s law
n1 sin1 = n2 sin2n1 cos1 = n2 cos2
Undersea Systems
Fiber TypesSingle-mode step-index fibers:
• No intermodal dispersion
gives highest bandwidth
• Small core radius ^
difficult to launch power,
lasers are used
Multi-mode step-index fibers:
• Large core radius ^
Easy to launch power,
LEDs can be used
• Intermodal dispersion
reduces the fiber
bandwidth
Multi-mode graded-index fibers:
• Reduced intermodal
dispersion gives
higher bandwidth
a: 5-12 µm, b:125 µm
n n n
a: 50-200 µm, b:125-400 µm a: 50-100 µm, b:125-140 µm
ρρ ρ
Total internal reflection
n1 cosc = n2 cos 00
c = cos-1(n2/n1)
Example: n1 = 1.50, n2 = 1.00; c =
Ray-optics description of step-index fiber (1)
n2 = n1(1-∆) where ∆ is the
index difference =
(n1- n2)/n1<< 1
∆ ≈ 1-3% for MM fibers,
∆ ≈ 0.1-1% for SM fibers
Apply Snell's law at the input interface: n0 sin(θi) = n1 sin(θr)
For total internal reflection at the core/cladding interface we have a critical, minimum, angle: n1 sin(θc) = n2 sin(90°); sin(θc) = n2/n1
Relate to maximum entrance angle: n0 sin(θi,max) = n1 sin(θr,max) = n1 sin(90-θc) = n1 cos(θc) = n1 [1 - sin2(θc)] = (n1
2- n22)
θi
n0 =1
Cladding, n2 Unguided Ray
Guided Ray
θr
θ
Core, n1
Pulse Broadening From Intermodal Dispersion
θi, max
n0 =1
Cladding, n2
θr
θc
Core, n1
Fast Ray Path
Slowest Ray Path
ΔT)(t
t t
ΔT = n1[Lslow- Lfast] / c = n1[L / sin(θc) – L] / c = L[n1/ n2 -1]n1/ c = L Δn12/(n2c)
If we assume that maximum bit rate (B) is limited by maximum allowed pulse broadening equal to bit-period: TB=1 / B > ΔT
Optical fiber structures
cba
Fig. 2-9:Fibre structure
Core: n1 = 1.47 Cladding: n2 = 1.46a = 50 m (for MMF) b = 125 m = 10 m (for SMF)
Buffer: high, lossy n3
c = 250 m
Basic optical properties
Speed of light c = 3 108 m/sWavelength = c/f = 0/nFrequency fEnergy E = hf ; h = 6.63 10-34 J-s
E (eV) = 1.24 / 0 (m)
Index of refraction
Air 1.0 water 1.33glass (SiO2) 1.47 silicon-nitride 2.0 Silicon 3.5