Lecture Outline Overview of optical fiber communication (OFC) Fibers and transmission...

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