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 1 1850 1900 1950 2000 (Bit/s-km)

Elements of a F-O Transmission Link (Old)

A multi-Disciplinary Technology

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

Undersea Systems

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: 50-100 µm, b:125-140 µm a: 50-200 µm, b:125-400 µm

Total internal reflection n1 cosc = 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 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)

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

Optical fiber structures Fig. 2-9:Fibre structure Core: n1 = 1.47 Cladding: n2 = 1.46 a = 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/s Wavelength  = c/f = 0/n Frequency f Energy E = hf ; h = 6.63  10-34 J-s E (eV) = 1.24 / 0 (m) Index of refraction Air 1.0 water 1.33 glass (SiO2) 1.47 silicon-nitride 2.0 Silicon 3.5