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

2. Quick History of OFC 1958: Laser discovered
Mid-60s: Guided wave optics demonstrated
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

3. 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.

4. The Electromagnetic Communication Spectrum

5. What is Light? Theories of Light
Historical Development

6. Comparison of Bit Rate-Distance Product (B-L)
Coaxial cable BL
Optical amplifiers
Telephone
Telegraph
Microwave
Lightwave
Year
1015
1012
109
106
103 1
1850
1900
1950
2000
(Bit/s-km)

8. Elements of a F-O Transmission Link (Old)

9. A multi-Disciplinary Technology

10. Snell’s law
n1 sin1 = n2 sin2
n1 cos1 = n2 cos2

11. Undersea Systems

14. Fiber Types 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
Single-mode step-index fibers:
No intermodal dispersion
gives highest bandwidth
Small core radius ^
difficult to launch power,
lasers are used n n n ρ ρ ρ
a: 5-12 µm, b:125 µm
a: µm, b: µm
a: µm, b: µm

15. Total internal reflection
n1 cosc = n2 cos 00
c = cos-1(n2/n1)
Example: n1 = 1.50, n2 = 1.00; c =

16. Ray-optics description of step-index fiber (1)
n2 = n1(1-∆) where ∆ is the θi
n0 =1
Cladding, n2
Unguided Ray
Guided Ray θr θ
Core, n1
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)] = (n12- n22)

17. Pulse Broadening From Intermodal Dispersion
θi, max
n0 =1
Cladding, n2 θr θc
Core, n1
Fast Ray Path
Slowest Ray Path ΔT 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

18. Optical fiber structures
Fig. 2-9:Fibre structure
Core: n1 = Cladding: n2 = 1.46
a = 50 m (for MMF) b = 125 m
= 10 m (for SMF)
Buffer: high, lossy n3
c = 250 m

19. Basic optical properties
Speed of light c = 3  108 m/s
Wavelength  = c/f = 0/n
Frequency f
Energy E = hf ; h = 6.63  J-s
E (eV) = 1.24 / 0 (m)
Index of refraction
Air 1.0
water
glass (SiO2) 1.47
silicon-nitride 2.0
Silicon 3.5