ÉÂÔ - The Free Information Society - Educating and ... · International Technical Support...

Understanding Optical Communications

Harry J. R. Dutton

International Technical Support Organization

http://www.redbooks.ibm.com

SG24-5230-00

International Technical Support Organization

Understanding Optical Communications

September 1998

SG24-5230-00

Take Note!

Before using this information and the product it supports, be sure to read the general information in Appendix F, SpecialNotices on page 591.

First Edition (September 1998)

This edition applies to fibre optic communications and optical networking.

Comments may be addressed to:IBM Corporation, International Technical Support OrganizationDept. HZ8 Building 678P.O. Box 12195Research Triangle Park, NC 27709-2195

When you send information to IBM, you grant IBM a non-exclusive right to use or distribute the information in any way it believesappropriate without incurring any obligation to you.

Copyright International Business Machines Corporation 1998. All rights reserved.Note to U.S. Government Users Documentation related to restricted rights Use, duplication or disclosure is subject torestrictions set forth in GSA ADP Schedule Contract with IBM Corp.

Contents

Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xix

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxiAbout the Author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxiBy the Same Author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxiAcknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxiiComments Welcome . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxii

Part 1. Theory and Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Chapter 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.1.1 Optical Transmission System Concepts . . . . . . . . . . . . . . . . . . 41.1.2 Optical Networking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.1.3 Optical Interconnects . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101.1.4 Optical Computers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.1.5 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Chapter 2. Optical Fibre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.1 The Nature of Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.1.1 Light as an Electromagnetic Wave . . . . . . . . . . . . . . . . . . . . 162.1.2 Polarisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.1.3 Interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.2 Transmitting Light on a Fibre . . . . . . . . . . . . . . . . . . . . . . . . . . 252.2.1 Characteristics of Glasses . . . . . . . . . . . . . . . . . . . . . . . . . 292.2.2 Transmission Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.2.3 Operational Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.2.4 Fibre Refractive Index Profiles . . . . . . . . . . . . . . . . . . . . . . . 36

2.3 Light Propagation in Multimode Fibre . . . . . . . . . . . . . . . . . . . . . 392.3.1 Snell's Law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402.3.2 Critical Angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412.3.3 Numerical Aperture (NA) . . . . . . . . . . . . . . . . . . . . . . . . . . 422.3.4 Propagation Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442.3.5 Mode Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502.3.6 Modal Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512.3.7 Naming Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

2.4 Single-Mode Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 562.4.1 Single-Mode Characteristics . . . . . . . . . . . . . . . . . . . . . . . . 572.4.2 Dispersion in Single-Mode Fibre . . . . . . . . . . . . . . . . . . . . . . 592.4.3 Mode Partition Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . 672.4.4 Reflections and Return Loss Variation . . . . . . . . . . . . . . . . . . 672.4.5 Non-Linear High-Power Effects . . . . . . . . . . . . . . . . . . . . . . 69

2.5 Plastic Optical Fibre (POF) . . . . . . . . . . . . . . . . . . . . . . . . . . . 742.5.1 POF Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

2.6 Hard Polymer (plastic) Clad (silica) Fibre (HPCF) . . . . . . . . . . . . . . 76

Chapter 3. Optical Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793.1 Light Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Copyright IBM Corp. 1998 iii

3.1.1 Spontaneous Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . 813.2 Light Emitting Diodes (LEDs) . . . . . . . . . . . . . . . . . . . . . . . . . . 82

3.2.1 The Semiconductor Junction Diode . . . . . . . . . . . . . . . . . . . . 823.2.2 Construction and Operation of LEDs . . . . . . . . . . . . . . . . . . . 843.2.3 Heterojunctions (Practical LEDs) . . . . . . . . . . . . . . . . . . . . . 863.2.4 Characteristics of LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

3.3 Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 933.3.1 Principle of the LASER . . . . . . . . . . . . . . . . . . . . . . . . . . . 943.3.2 Semiconductor Laser Diodes . . . . . . . . . . . . . . . . . . . . . . . 1013.3.3 Fabry-Perot Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1023.3.4 Distributed Feedback (DFB) Lasers . . . . . . . . . . . . . . . . . . . 1133.3.5 Integrated Absorption Modulators . . . . . . . . . . . . . . . . . . . . 1163.3.6 Q-Switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1173.3.7 Mode-Locking and Self-Pulsating Lasers . . . . . . . . . . . . . . . . 1173.3.8 Distributed Bragg Reflector (DBR) Lasers . . . . . . . . . . . . . . . 1183.3.9 Quantum Wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1193.3.10 Tunable DBR Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . 1203.3.11 External Cavity DBR Lasers . . . . . . . . . . . . . . . . . . . . . . 1233.3.12 External Fibre-Cavity DBR Lasers . . . . . . . . . . . . . . . . . . . 1243.3.13 Multi-Wavelength Lasers . . . . . . . . . . . . . . . . . . . . . . . . 1253.3.14 Vertical Cavity Surface Emitting Lasers (VCSELs) . . . . . . . . . 1253.3.15 In-Fibre Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1273.3.16 Fibre Ring Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1293.3.17 Upconversion Fibre Lasers (Double Pumping) . . . . . . . . . . . . 1303.3.18 Gas Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1313.3.19 Free Electron Lasers (FELs) . . . . . . . . . . . . . . . . . . . . . . 132

3.4 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

Chapter 4. Optical Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . 1334.1 Photoconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1334.2 Photodiodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

4.2.1 P-N Diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1344.2.2 P-I-N Diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1354.2.3 Schottky-Barrier Photodiodes . . . . . . . . . . . . . . . . . . . . . . 1384.2.4 Avalanche Photodiodes (APDs) . . . . . . . . . . . . . . . . . . . . . 1394.2.5 Hetero-Interface Photodetectors . . . . . . . . . . . . . . . . . . . . . 1434.2.6 Travelling-Wave Photodetectors . . . . . . . . . . . . . . . . . . . . . 1434.2.7 Resonant-Cavity Photodetectors . . . . . . . . . . . . . . . . . . . . . 145

4.3 Phototransistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1464.4 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

Chapter 5. Optical Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1495.1 Optical Component Technologies . . . . . . . . . . . . . . . . . . . . . . . 149

5.1.1 Planar Optical Devices . . . . . . . . . . . . . . . . . . . . . . . . . . 1505.1.2 Fabrication of Planar Optical Devices . . . . . . . . . . . . . . . . . . 1525.1.3 Integrated Optical Circuits (Planar Assemblies) . . . . . . . . . . . . 155

5.2 Optical Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1575.2.1 Erbium Doped Fibre Amplifiers (EDFAs) . . . . . . . . . . . . . . . . 1595.2.2 Praseodymium (Pr) Doped Fibre Amplifiers . . . . . . . . . . . . . . 1775.2.3 Neodymium (Nd) Doped Fibre Amplifiers . . . . . . . . . . . . . . . . 1785.2.4 Plastic Fibre Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . 1785.2.5 Erbium Doped Planar Devices . . . . . . . . . . . . . . . . . . . . . . 1795.2.6 Semiconductor Optical/Laser Amplifiers (SOAs/SLAs) . . . . . . . . 1805.2.7 Raman Effect Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . 182

iv Understanding Optical Communications

5.3 Second Harmonic Generation (SHG) . . . . . . . . . . . . . . . . . . . . . 1845.4 Splitters and Couplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

5.4.1 Resonant Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1865.4.2 Y Couplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1935.4.3 Star Couplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1935.4.4 Beamsplitter Prisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1965.4.5 Isolators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1985.4.6 Circulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

5.5 Polarisation Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2045.5.1 Fibre Loop Polarisation Controller . . . . . . . . . . . . . . . . . . . . 2045.5.2 Fibre Squeezer Polarisation Controller . . . . . . . . . . . . . . . . . 205

5.6 Lenses and Prisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2055.6.1 Graded Index (GRIN) Lenses . . . . . . . . . . . . . . . . . . . . . . 205

5.7 Diffraction Gratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2065.7.1 Planar Diffraction Gratings . . . . . . . . . . . . . . . . . . . . . . . . 2065.7.2 In-Fibre Bragg Gratings (FBGs) . . . . . . . . . . . . . . . . . . . . . 2115.7.3 Waveguide Grating Routers (WGRs) . . . . . . . . . . . . . . . . . . 222

5.8 Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2275.8.2 Fabry-Perot Filter (Etalon) . . . . . . . . . . . . . . . . . . . . . . . . 2295.8.3 In-Fibre Bragg Grating Filters . . . . . . . . . . . . . . . . . . . . . . 2345.8.4 Fibre Ring Resonators . . . . . . . . . . . . . . . . . . . . . . . . . . 2355.8.5 Dielectric Filters for WDM Multiplexing/Demultiplexing . . . . . . . . 237

5.9 Modulators and Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . 2385.9.1 Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2395.9.2 Optical Switching Elements . . . . . . . . . . . . . . . . . . . . . . . 2395.9.3 Optically Controlled Switches . . . . . . . . . . . . . . . . . . . . . . 2425.9.4 Electro-Absorption Modulators . . . . . . . . . . . . . . . . . . . . . . 2485.9.5 Acoustic Modulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2495.9.6 Acoustooptic Tunable Filters (AOTFs) . . . . . . . . . . . . . . . . . 2525.9.7 Phase Modulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2555.9.8 Modulation Using a Mach-Zehnder Interferometer . . . . . . . . . . 2565.9.9 Pockels Cell Modulators . . . . . . . . . . . . . . . . . . . . . . . . . 2585.9.10 Faraday Effect Modulators . . . . . . . . . . . . . . . . . . . . . . . 260

5.10 Repeaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2615.11 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262

Chapter 6. Fibre Manufacture, Cables and Connectors . . . . . . . . . . 2676.1 The Technology of Fibre Manufacture . . . . . . . . . . . . . . . . . . . . 267

6.1.1 Purifying Silica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2676.1.2 Drawing the Fibre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2686.1.3 The Double-Crucible Method . . . . . . . . . . . . . . . . . . . . . . . 2706.1.4 Vapour Deposition Techniques . . . . . . . . . . . . . . . . . . . . . 271

6.2 Joining Fibres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2756.2.1 Fusion Splicing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2766.2.2 Mechanical Splicing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2786.2.3 Mechanical Splicing with Alignment and Bonding . . . . . . . . . . . 2796.2.4 Losses in Fibre Joins . . . . . . . . . . . . . . . . . . . . . . . . . . . 2796.2.5 Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283

6.3 Fibre Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285

Part 2. Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295

Chapter 7. Optical Communication Systems . . . . . . . . . . . . . . . . . 297

Contents v

7.1 Point-to-Point Transmission Systems . . . . . . . . . . . . . . . . . . . . 2977.1.1 Traditional Single-Channel Systems . . . . . . . . . . . . . . . . . . 2977.1.2 Amplified Single-Channel Systems . . . . . . . . . . . . . . . . . . . 2997.1.3 WDM Systems Overview . . . . . . . . . . . . . . . . . . . . . . . . . 300

7.2 Modulation (Making the Light Carry a Signal) . . . . . . . . . . . . . . . . 3017.2.1 On-Off Keying (OOK) . . . . . . . . . . . . . . . . . . . . . . . . . . . 3017.2.2 Multi-State Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3067.2.3 Forward Error Correction (FEC) . . . . . . . . . . . . . . . . . . . . . 3087.2.4 Receiving the Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . 3097.2.5 Timing Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3107.2.6 Bandwidth Occupancy . . . . . . . . . . . . . . . . . . . . . . . . . . 3127.2.7 Analogue Amplitude Modulation (Continuous Wave) . . . . . . . . . 3137.2.8 Frequency Shift Keying (FSK) . . . . . . . . . . . . . . . . . . . . . . 3137.2.9 Phase Shift Keying (PSK) . . . . . . . . . . . . . . . . . . . . . . . . 3147.2.10 Polarity Modulation (PolSK) . . . . . . . . . . . . . . . . . . . . . . . 3147.2.11 Directional Transmission . . . . . . . . . . . . . . . . . . . . . . . . 3147.2.12 Character Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314

7.3 Transmission System Limits and Characteristics . . . . . . . . . . . . . . 3167.4 Optical System Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . 317

7.4.1 System Power Budgeting . . . . . . . . . . . . . . . . . . . . . . . . . 3177.4.2 Laser Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3227.4.3 Is There a Speed Limit? . . . . . . . . . . . . . . . . . . . . . . . . . 3237.4.4 Reflections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3257.4.5 Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3267.4.6 Bit Error Rates (BER) . . . . . . . . . . . . . . . . . . . . . . . . . . . 327

7.5 Control of Dispersion in Single-Mode Fibre Links . . . . . . . . . . . . . . 3287.5.1 Control of Spectral Width . . . . . . . . . . . . . . . . . . . . . . . . . 3297.5.2 Dispersion Shifted Fibre . . . . . . . . . . . . . . . . . . . . . . . . . 3307.5.3 Dispersion Compensating Fibre . . . . . . . . . . . . . . . . . . . . . 3307.5.4 Balancing Dispersion on a Link . . . . . . . . . . . . . . . . . . . . . 3327.5.5 Mid-Span Spectral Inversion . . . . . . . . . . . . . . . . . . . . . . . 3327.5.6 Chirped Fibre Bragg Gratings . . . . . . . . . . . . . . . . . . . . . . 334

7.6 Control of Dispersion in Multimode Fibre . . . . . . . . . . . . . . . . . . 3357.6.1 Maximum Propagation Distance on Multimode Fibre . . . . . . . . . 3357.6.2 Light Sources for Multimode Systems . . . . . . . . . . . . . . . . . 337

7.7 Fibre Optics in Different Environments . . . . . . . . . . . . . . . . . . . . 3407.8 Test Equipment and Techniques . . . . . . . . . . . . . . . . . . . . . . . 342

7.8.1 Optical Power Meters (Optical Multimeter) . . . . . . . . . . . . . . . 3427.8.2 Optical Time-Domain Reflectometers (OTDRs) . . . . . . . . . . . . 3437.8.3 Spectrum Analysers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3457.8.4 Eye Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347

7.9 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349

Chapter 8. Optical Link Connections in Electronic Networks . . . . . . . 3518.1 Fibre Distributed Data Interface (FDDI) . . . . . . . . . . . . . . . . . . . 353

8.1.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3548.1.2 Access Protocol Operation . . . . . . . . . . . . . . . . . . . . . . . . 3568.1.3 Ring Initialisation, Monitoring and Error Handling . . . . . . . . . . . 3578.1.4 Physical Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3578.1.5 Physical Layer Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 3588.1.6 Node Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3648.1.7 High-Speed Performance . . . . . . . . . . . . . . . . . . . . . . . . . 365

8.2 Ethernet (IEEE 802.3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3668.2.1 Principles of Ethernet . . . . . . . . . . . . . . . . . . . . . . . . . . . 366

vi Understanding Optical Communications

8.2.2 Ethernet on Fibre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3688.2.3 CSMA/CD Performance . . . . . . . . . . . . . . . . . . . . . . . . . . 3698.2.4 High Speed or Fast (100 Mbps) Ethernet (HSE) . . . . . . . . . . 3718.2.5 Gigabit Ethernet (GbE) . . . . . . . . . . . . . . . . . . . . . . . . . . 372

8.3 Fibre Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3758.3.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3758.3.2 Physical Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3768.3.3 Transmission Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 3788.3.4 Signalling Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3788.3.5 Open Fibre Control (OFC) Safety System . . . . . . . . . . . . . . . 380

8.4 ESCON and Inter-System Coupling (ISC) . . . . . . . . . . . . . . . . . . 3818.4.2 Inter-System Coupling (ISC) Feature . . . . . . . . . . . . . . . . . . 385

8.5 OptiConnect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3868.5.1 OptiConnect Optical Specifications . . . . . . . . . . . . . . . . . . . 387

8.6 Synchronous Optical Network (SONET) and SDH . . . . . . . . . . . . . 3878.6.1 Sonet Protocol Structure . . . . . . . . . . . . . . . . . . . . . . . . . 3898.6.2 Synchronous Digital Hierarchy (SDH) . . . . . . . . . . . . . . . . . . 3918.6.3 Tributaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3928.6.4 Sonet/SDH Line Speeds and Signals . . . . . . . . . . . . . . . . . . 3928.6.5 Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3938.6.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3938.6.7 Sonet Rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3938.6.8 Plesiochronous Digital Hierarchy . . . . . . . . . . . . . . . . . . . . 394

8.7 Asynchronous Transfer Mode (ATM) . . . . . . . . . . . . . . . . . . . . . 3958.7.1 High-Speed Technology . . . . . . . . . . . . . . . . . . . . . . . . . 3968.7.2 ATM Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3978.7.3 The Structure of an ATM Network . . . . . . . . . . . . . . . . . . . . 3998.7.4 Physical Link Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4048.7.5 Network Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 4068.7.6 Transporting Information in an ATM Network . . . . . . . . . . . . . 407

8.8 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409

Chapter 9. Wavelength Division Multiplexing . . . . . . . . . . . . . . . . 4119.1.1 Simple (Sparse) WDM . . . . . . . . . . . . . . . . . . . . . . . . . . 4119.1.2 Dense WDM Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4129.1.3 Using Dense and Sparse WDM Together - An Example . . . . . . . 4139.1.4 Building Photonic Networks - Technical Challenges . . . . . . . . . 414

9.2 Components for WDM Systems . . . . . . . . . . . . . . . . . . . . . . . . 4159.2.1 Light Sources for WDM . . . . . . . . . . . . . . . . . . . . . . . . . . 4159.2.2 Multiplexing (Combining) the Light . . . . . . . . . . . . . . . . . . . 4199.2.3 Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4209.2.4 Demultiplexing the Light . . . . . . . . . . . . . . . . . . . . . . . . . 4209.2.5 Add-Drop Multiplexors . . . . . . . . . . . . . . . . . . . . . . . . . . 4249.2.6 Optical Space-Division Switches . . . . . . . . . . . . . . . . . . . . . 4289.2.7 Optical Switching Nodes . . . . . . . . . . . . . . . . . . . . . . . . . 4299.2.8 Wavelength Converters . . . . . . . . . . . . . . . . . . . . . . . . . . 431

9.3 Standards for WDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4369.4 Systems Engineering in the WDM Environment . . . . . . . . . . . . . . 437

9.4.1 Fitting the Signal into Its Allocated Waveband . . . . . . . . . . . . . 4389.4.2 Stabilising Wavelengths . . . . . . . . . . . . . . . . . . . . . . . . . . 4419.4.3 Controlling Non-linear Effects . . . . . . . . . . . . . . . . . . . . . . 4429.4.4 Control of Dispersion in WDM Systems . . . . . . . . . . . . . . . . 4459.4.5 Amplifier Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4459.4.6 Noise in WDM Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 449

Contents vii

9.4.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4519.5 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453

Chapter 10. Lightwave Networks . . . . . . . . . . . . . . . . . . . . . . . . 45510.1.1 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45610.1.2 Optical Network Technologies . . . . . . . . . . . . . . . . . . . . . 45810.1.3 Sharing the Fibre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45910.1.4 Modes of Data Transfer . . . . . . . . . . . . . . . . . . . . . . . . . 46110.1.5 Network Topologies for the MAN or WAN . . . . . . . . . . . . . . 462

10.2 WDM for LANs (and Small MANs) . . . . . . . . . . . . . . . . . . . . . 46810.2.1 Network Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46810.2.2 Transmission System Design . . . . . . . . . . . . . . . . . . . . . . 47110.2.3 Access Protocols for the Shared Medium . . . . . . . . . . . . . . . 472

10.3 Some Possible Systems Approaches . . . . . . . . . . . . . . . . . . . . 47610.3.1 Centralised WDM Circuit Switch . . . . . . . . . . . . . . . . . . . . 47610.3.2 Multichannel CSMA/CD . . . . . . . . . . . . . . . . . . . . . . . . . 47710.3.3 Coordination on a Control Channel . . . . . . . . . . . . . . . . . . 47810.3.4 Code Division Multiple Access (CDMA) . . . . . . . . . . . . . . . . 47910.3.5 Experimental Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 47910.3.6 Metropolitan Area Add/Drop WDM with a Multiwavelength Source 482

10.4 WDM Research Demonstrators . . . . . . . . . . . . . . . . . . . . . . . 48410.5 The IBM 9729 - A WDM System for the Metropolitan Area Network . . 486

10.5.1 9729 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48610.5.2 Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48810.5.3 Grating Assembly (9729) . . . . . . . . . . . . . . . . . . . . . . . . 49210.5.4 Laser/Receiver Cards (LRC) . . . . . . . . . . . . . . . . . . . . . . 49310.5.5 I/O Cards (IOC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49410.5.6 Dual Fibre Switching Feature . . . . . . . . . . . . . . . . . . . . . . 49510.5.7 System Engineering Considerations . . . . . . . . . . . . . . . . . . 49510.5.8 Network Management . . . . . . . . . . . . . . . . . . . . . . . . . . 49910.5.9 Systems Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500

10.6 Architecture of a First-Generation WDM for the WAN . . . . . . . . . . 50210.7 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505

Chapter 11. Fibre In The (Local) Loop - FITL . . . . . . . . . . . . . . . . . 50711.1.1 System Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . 50711.1.2 The Existing Telephone Network . . . . . . . . . . . . . . . . . . . . 50911.1.3 Cable Television Networks . . . . . . . . . . . . . . . . . . . . . . . 51011.1.4 Hybrid Fibre-Coax (HFC) Networks . . . . . . . . . . . . . . . . . . 51111.1.5 Fibre-to-the-Curb/Street/Neighbourhood (FTTC/FTTS/FTTN) . . . 51311.1.6 Passive Optical Networks (PONs) . . . . . . . . . . . . . . . . . . . 516

11.2 Mobile Telephone Base Station . . . . . . . . . . . . . . . . . . . . . . . 51811.3 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519

Chapter 12. Research Directions . . . . . . . . . . . . . . . . . . . . . . . . 52112.1 Solitons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 521

12.1.1 Dark Solitons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52212.1.2 Light Guiding Light (Spatial Solitons) . . . . . . . . . . . . . . . . . 522

12.2 Advanced Fibres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52312.3 Plastic Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523

12.3.1 Plastic Planar Waveguide Devices . . . . . . . . . . . . . . . . . . . 52412.3.2 Plastic Semiconductor Lasers . . . . . . . . . . . . . . . . . . . . . 524

12.4 Optical Code Division Multiple Access (CDMA) . . . . . . . . . . . . . . 52512.4.1 Direct Sequence Spread Spectrum (DSSS) . . . . . . . . . . . . . 527

viii Understanding Optical Communications

12.4.2 Code Division Multiple Access (CDMA) . . . . . . . . . . . . . . . . 52812.4.3 Practical Optical CDMA . . . . . . . . . . . . . . . . . . . . . . . . . 530

12.5 Optical Time Division Multiplexing (OTDM) . . . . . . . . . . . . . . . . 53012.5.1 TDM Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53112.5.2 TDM Network Principles . . . . . . . . . . . . . . . . . . . . . . . . . 53212.5.3 Optical TDM Principles . . . . . . . . . . . . . . . . . . . . . . . . . 53312.5.4 Demultiplexing the TDM Signal . . . . . . . . . . . . . . . . . . . . . 534

12.6 Optical Packet Switched Networks . . . . . . . . . . . . . . . . . . . . . 53512.7 Optical Interconnects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53712.8 Coherent Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53812.9 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 541

Appendix A. An Introduction to Semiconductors . . . . . . . . . . . . . . 543A.1.1 Chemistry 101 Revisited . . . . . . . . . . . . . . . . . . . . . . . . . 543A.1.2 Why Does a Conductor Conduct? . . . . . . . . . . . . . . . . . . . 546A.1.3 Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 548A.1.4 Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 549A.1.5 p-n Junctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 555A.1.6 Semiconductor Junction Diodes . . . . . . . . . . . . . . . . . . . . . 555A.1.7 The Bipolar Junction Transistor (BJT) . . . . . . . . . . . . . . . . . 557A.1.8 Field Effect Transistors (FET) . . . . . . . . . . . . . . . . . . . . . . 560A.1.9 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561

Appendix B. An Introduction to Communications Networks . . . . . . . 563B.1 Function Layering and the ISO Model . . . . . . . . . . . . . . . . . . . . 563

B.1.1 Layered Communication Architectures . . . . . . . . . . . . . . . . . 563B.1.2 The ISO Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 568

B.2 Networks of Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 572B.2.1 The Role of Optical Networks . . . . . . . . . . . . . . . . . . . . . . 574

B.3 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 575

Appendix C. Laser Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577C.1.1 Handling Fibre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579

Appendix D. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 581

Appendix E. Some Useful Facts . . . . . . . . . . . . . . . . . . . . . . . . 587E.1.1 The Visible Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . 587

E.2 Units of Measure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 587E.2.1 Fractions and Multiples . . . . . . . . . . . . . . . . . . . . . . . . . . 587

E.3 Some Basic Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 588E.4 Wavelength Hall of Fame . . . . . . . . . . . . . . . . . . . . . . . . . . . 590

Appendix F. Special Notices . . . . . . . . . . . . . . . . . . . . . . . . . . . 591

How to Get ITSO Redbooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593How IBM Employees Can Get ITSO Redbooks . . . . . . . . . . . . . . . . . 593How Customers Can Get ITSO Redbooks . . . . . . . . . . . . . . . . . . . . 594IBM Redbook Order Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 595

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597

ITSO Redbook Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 599

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x Understanding Optical Communications

Figures

1. Optical Transmission - Schematic . . . . . . . . . . . . . . . . . . . . . . . . 42. The Electromagnetic Spectrum . . . . . . . . . . . . . . . . . . . . . . . . 173. The Structure of an Electromagnetic Wave . . . . . . . . . . . . . . . . . 184. Amplitude Fluctuation in an Electromagnetic Wave . . . . . . . . . . . . . 195. Circular Polarisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206. Young's Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207. Interference Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218. Reflection of Light from a Glass/Air Interface . . . . . . . . . . . . . . . . 239. Interference Effects when passing Light through a Glass Sheet . . . . . 23

10. Variation in Reflectance with Thickness of Glass Sheet . . . . . . . . . . 2411. Basic Principle of Light Transmission on Optical Fibre . . . . . . . . . . . 2512. Effect of Dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2713. Typical Fibre Infrared Absorption Spectrum . . . . . . . . . . . . . . . . . 3114. Transmission Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3215. Fibre Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3316. Multimode Step-Index Fibre . . . . . . . . . . . . . . . . . . . . . . . . . . 3417. Multimode Graded index Fibre . . . . . . . . . . . . . . . . . . . . . . . . . 3518. Single-Mode Fibre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3519. Fibre Refractive Index Profiles . . . . . . . . . . . . . . . . . . . . . . . . . 3620. Light Propagation in Multimode Fibre . . . . . . . . . . . . . . . . . . . . . 3921. Reflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3922. Refraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4023. Refraction (2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4024. Critical Angle (1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4125. Critical Angle (2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4126. Calculating the Numerical Aperture . . . . . . . . . . . . . . . . . . . . . . 4227. Multimode Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4428. Spiral Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4529. Propagation around a Bend in the Fibre . . . . . . . . . . . . . . . . . . . 4630. Effect of a Micro-Bend . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4631. Typical Speckle Pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4732. Multimode Ray Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . 4933. Origin of Modal Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5334. Energy Distribution of Some TEM Modes . . . . . . . . . . . . . . . . . . 5635. Energy Distribution of Some LP Modes in Fibre . . . . . . . . . . . . . . . 5636. Single-Mode Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5737. Mode Field Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5738. Fields at the End of the Fibre . . . . . . . . . . . . . . . . . . . . . . . . . 5839. Reason for Bend Loss in Single-Mode Fibre . . . . . . . . . . . . . . . . . 5840. Dispersion of Standard Single-Mode Fibre . . . . . . . . . . . . . . . . . 5941. Dispersion Shifted Fibre . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6242. Profiles of Dispersion Shifted and Dispersion Flattened Fibre . . . . . . . 6243. Non-Zero Dispersion-Shifted Fibre RI Profile . . . . . . . . . . . . . . . . 6344. Polarisation Maintaining Fibre . . . . . . . . . . . . . . . . . . . . . . . . . 6545. Axes of Polarisation in PMF . . . . . . . . . . . . . . . . . . . . . . . . . . 6646. Pattern of Return Loss Variation by Wavelength . . . . . . . . . . . . . . 6747. Four Wave Mixing Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . 6948. SBS Threshold Variation with Wavelength . . . . . . . . . . . . . . . . . . 7149. Stimulated Raman Scattering . . . . . . . . . . . . . . . . . . . . . . . . . 7250. Absorption Spectrum of Plastic Optical Fibre . . . . . . . . . . . . . . . . 75

Copyright IBM Corp. 1998 xi

51. HPCF Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7652. Attenuation of HPCF in the Short Wavelength Band . . . . . . . . . . . . 7753. Electrical Potentials across a p-n Junction . . . . . . . . . . . . . . . . . . 8354. Bandgap Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8555. Simple P-N Junction LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8656. Double Heterojunction LED . . . . . . . . . . . . . . . . . . . . . . . . . . . 8757. Energy Bands in a Double Heterojunction . . . . . . . . . . . . . . . . . . 8758. Exploded Structure of an Idealised Double Heterojunction LED . . . . . . 8959. Schematic Structure of a Burrus LED . . . . . . . . . . . . . . . . . . . . . 8960. Schematic Structure of an Edge Emitting LED . . . . . . . . . . . . . . . . 9061. Coupling to a Fibre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9062. Stimulated Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9463. Spontaneous Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9564. Energy States of a typical 4-Level Material . . . . . . . . . . . . . . . . . . 9565. Lasing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9666. Fabry-Perot Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10267. Resonance Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10368. Resonance Modes in the Cavity of a Fabry-Perot Laser . . . . . . . . . 10469. Modes Produced in a Typical Fabry-Perot Laser . . . . . . . . . . . . . 10470. Spectral Width and Linewidth . . . . . . . . . . . . . . . . . . . . . . . . 10571. Output Spectrum Changes as Power is Applied . . . . . . . . . . . . . . 10672. Typical Mode Hopping Behaviour in an Unguided FP Laser . . . . . . . 10773. Relaxation Oscillations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10874. Directing the Light in a Fabry-Perot Laser . . . . . . . . . . . . . . . . . 11075. Emission Profile of a Gain-Guided Fabry-Perot Laser . . . . . . . . . . 11176. Fabry-Perot laser Output versus Input Current . . . . . . . . . . . . . . 11277. DFB Laser - Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11378. DFB Laser with Phase Shifted Grating - Concept . . . . . . . . . . . . . 11579. Phase Shifted Grating - Reflection Spectrum . . . . . . . . . . . . . . . 11580. DFB Laser with Integrated Electro-Absorption Modulator . . . . . . . . . 11681. DFB Laser with Electro-Absorption Modulator within the Cavity . . . . . 11782. DBR Laser - Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11883. Cavity Structure of a Multiple Quantum Well Laser . . . . . . . . . . . . 11984. Tunable 2-Section DBR Laser . . . . . . . . . . . . . . . . . . . . . . . . 12085. Tunable 3-Section DBR Laser . . . . . . . . . . . . . . . . . . . . . . . . 12186. Sampled Grating - Schematic . . . . . . . . . . . . . . . . . . . . . . . . 12287. Principle of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12288. Sampled Grating Tunable DFB Laser - Schematic . . . . . . . . . . . . 12389. External Cavity DBR Laser Schematic . . . . . . . . . . . . . . . . . . . 12390. Stabilisation of Fabry-Perot Laser with a Fibre Bragg Grating . . . . . . 12491. FP Laser with External FBG - Alternative Structure . . . . . . . . . . . . 12592. VCSEL Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12693. In-Fibre Laser Using FBGs . . . . . . . . . . . . . . . . . . . . . . . . . . 12794. Tuneable Fibre-Ring Laser . . . . . . . . . . . . . . . . . . . . . . . . . . 12995. Upconversion Laser Operation . . . . . . . . . . . . . . . . . . . . . . . . 13096. Some Energy Levels in Praseodymium Upconversion Lasers . . . . . . 13197. Photoconductive Detector - Principle . . . . . . . . . . . . . . . . . . . . 13398. Practical Photoconductive Detector - Schematic . . . . . . . . . . . . . . 13499. Typical Silicon P-I-N Diode Schematic . . . . . . . . . . . . . . . . . . . 135100. Schottky-Barrier Photodiode Schematic . . . . . . . . . . . . . . . . . . . 138101. Avalanche Photodiode (APD) . . . . . . . . . . . . . . . . . . . . . . . . 139102. Avalanche (Amplification) Process . . . . . . . . . . . . . . . . . . . . . . 140103. Electric Field Strengths in an APD . . . . . . . . . . . . . . . . . . . . . 141104. Variation of Quantum Efficiency with Device Response . . . . . . . . . 144

xii Understanding Optical Communications

105. Travelling Wave Photodector - Principle . . . . . . . . . . . . . . . . . . 144106. Resonant-Cavity Photodetector - Principle . . . . . . . . . . . . . . . . . 145107. Bipolar Junction Transistor (BJT) . . . . . . . . . . . . . . . . . . . . . . 146108. Planar Waveguide Concept . . . . . . . . . . . . . . . . . . . . . . . . . 150109. An 8-Port Splitter Made by Cascading Y-Couplers . . . . . . . . . . . . 151110. Planar Crossover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151111. Manufacture of Planar Waveguides by Deposition/Etching . . . . . . . . 152112. Coupling Fibres to Planar Devices . . . . . . . . . . . . . . . . . . . . . 155113. Erbium Doped Fibre Amplifier (EDFA) - Function . . . . . . . . . . . . . 158114. Erbium Doped Optical Fibre Amplifier . . . . . . . . . . . . . . . . . . . . 159115. Energy Level States of Erbium . . . . . . . . . . . . . . . . . . . . . . . . 160116. Gain Curve of a Typical EDFA . . . . . . . . . . . . . . . . . . . . . . . . 162117. Response of Cascaded EDFAs . . . . . . . . . . . . . . . . . . . . . . . 163118. EDFA Behavior in Gain Saturation . . . . . . . . . . . . . . . . . . . . . 165119. Energy Levels in Erbium/Ytterbium Co-Doped Amplifier . . . . . . . . . 166120. Additional Pumping Options . . . . . . . . . . . . . . . . . . . . . . . . . 168121. Cladding Pump Configuration . . . . . . . . . . . . . . . . . . . . . . . . 169122. Two-Stage EDFA Line Amplifier with Shared Pump . . . . . . . . . . . 170123. Polarisation Insensitive EDFA . . . . . . . . . . . . . . . . . . . . . . . . 171124. Remote Pumping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172125. Amplifier Breakdown Resilience . . . . . . . . . . . . . . . . . . . . . . . 173126. OTDR Path in Link with EDFAs . . . . . . . . . . . . . . . . . . . . . . . 173127. Rhodamine-B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178128. 16-Way Planar Splitter with Amplification . . . . . . . . . . . . . . . . . . 179129. Semiconductor Optical Amplifier Relationship to Laser . . . . . . . . . . 180130. Wavelength Changing Using SRS . . . . . . . . . . . . . . . . . . . . . . 182131. Wavelength Shifter Using FBGs . . . . . . . . . . . . . . . . . . . . . . . 183132. Multistage Wavelength Shifting . . . . . . . . . . . . . . . . . . . . . . . 183133. 1320 nm Raman Amplifier Using In-Fibre Bragg Gratings . . . . . . . . 183134. Generating Blue Light via Second Harmonic Generation . . . . . . . . . 184135. Principle of Resonant Couplers . . . . . . . . . . . . . . . . . . . . . . . 186136. Power Transfer in a Resonant Coupler . . . . . . . . . . . . . . . . . . . 188137. Reciprocity Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189138. Some Coupler Configurations . . . . . . . . . . . . . . . . . . . . . . . . 189139. Resonant Coupler in Planar Waveguide Technology . . . . . . . . . . . 190140. Cascaded 3 dB Couplers . . . . . . . . . . . . . . . . . . . . . . . . . . . 191141. Wavelength Selective Coupling and Splitting . . . . . . . . . . . . . . . 191142. Y-Coupler (Splitter) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193143. Star Couplers - Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . 194144. Fused Fibre Star Coupler . . . . . . . . . . . . . . . . . . . . . . . . . . . 194145. Mixing Plate Coupler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194146. Planar (Free Space) Star Coupler . . . . . . . . . . . . . . . . . . . . . . 195147. Concatenation of Multiple 3 dB Couplers to Form an 8-Way Star . . . 195148. Polarising Beamsplitter Prisms . . . . . . . . . . . . . . . . . . . . . . . . 196149. Splitting of Unpolarised Light into Two Components . . . . . . . . . . . 197150. Effect of Optic Axis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198151. Isolator Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199152. Four-Port Circulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200153. Multiplexing Bidirectional Traffic onto a Single Fibre Strand . . . . . . . 201154. Circulator - Path from Port 1 to Port 2 . . . . . . . . . . . . . . . . . . . 202155. Circulator - Path from Port 2 to Port 3 . . . . . . . . . . . . . . . . . . . 203156. Fibre Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204157. Fibre Loop Polarisation Controller . . . . . . . . . . . . . . . . . . . . . . 204158. Polarisation Control by Squeezing the Fibre . . . . . . . . . . . . . . . . 205

Figures xiii

159. GRIN Lens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205160. Grating Principle of Operation . . . . . . . . . . . . . . . . . . . . . . . . 207161. Grating Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207162. Wavelength Division Multiplexing with a Littrow Grating . . . . . . . . . 208163. WDM with a Concave Mirror and a Littrow Grating . . . . . . . . . . . . 208164. Double-Pass Grating Compressor . . . . . . . . . . . . . . . . . . . . . . 209165. Pulse Compression with Grating Compressor . . . . . . . . . . . . . . . 210166. In-Fibre Bragg Grating - Schematic . . . . . . . . . . . . . . . . . . . . . 211167. Principle of Operation of Fibre Bragg Grating . . . . . . . . . . . . . . . 211168. In-Fibre Bragg Grating - Parameters . . . . . . . . . . . . . . . . . . . . 212169. In-Fibre Bragg Grating - Typical Reflection Spectra . . . . . . . . . . . . 213170. Chirped FBG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214171. Reflectance of a Chirped FBG . . . . . . . . . . . . . . . . . . . . . . . . 215172. Reflected Power of a Strongly Chirped Grating - Apodised . . . . . . . 215173. Reflection Profiles of a Chirped FBG . . . . . . . . . . . . . . . . . . . . 215174. Blazed FBG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216175. In-Fibre Bragg Grating - Single Phase-Shift . . . . . . . . . . . . . . . . 217176. In-Fibre Bragg Grating - Multiple Phase Shifts . . . . . . . . . . . . . . . 217177. Wavelength Variation with Temperature in FBGs . . . . . . . . . . . . . 218178. FBG Packaging for Passive Thermal Compensation . . . . . . . . . . . 218179. Writing the FBG into a Fibre Core - Interference Pattern Technique . . 220180. Writing the FBG into a Fibre Core - Phase Mask Technique . . . . . . 221181. Practical FBG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221182. Waveguide Grating Router Basic Function . . . . . . . . . . . . . . . . . 222183. Waveguide Grating Router . . . . . . . . . . . . . . . . . . . . . . . . . . 223184. Distribution of Optical Power in the Input FSC . . . . . . . . . . . . . . . 224185. Operation of the Output FSC . . . . . . . . . . . . . . . . . . . . . . . . . 224186. Waveguide Grating Router with Single-Ported Input . . . . . . . . . . . 225187. Insertion Loss Characteristics of an Early 8-Port WGR . . . . . . . . . . 225188. WGR with Half-Wave Plate for Polarisation Independence . . . . . . . 226189. Transmission Characteristics of Ideal Filters . . . . . . . . . . . . . . . . 228190. Transmission Characteristics of Two Practical Filters . . . . . . . . . . . 228191. Fabry-Perot Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229192. Fabry-Perot Filter Characteristics . . . . . . . . . . . . . . . . . . . . . . 231193. Cascading Fabry-Perot Filters of Different FSR . . . . . . . . . . . . . . 232194. Fabry-Perot Filter Using Dielectric Mirrors . . . . . . . . . . . . . . . . . 233195. Tunable Fibre Fabry-Perot Filter . . . . . . . . . . . . . . . . . . . . . . . 233196. Cascading Tunable Fabry-Perot Filters of Different FSR . . . . . . . . . 234197. In-Fibre Bragg Grating Used as a Filter . . . . . . . . . . . . . . . . . . . 235198. Fibre Ring Resonators . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235199. Amplified Fibre Ring Resonator . . . . . . . . . . . . . . . . . . . . . . . 236200. Dielectric Filter based WDM Multiplexor/Demultiplexor . . . . . . . . . . 237201. Digital Optical Switch (DOS) - Schematic . . . . . . . . . . . . . . . . . 239202. Crossconnect Switch Element - Function . . . . . . . . . . . . . . . . . . 240203. Cross-Connect Using a Resonant Coupler . . . . . . . . . . . . . . . . . 240204. Cross-Connect Using an MZI Configuration . . . . . . . . . . . . . . . . 241205. Non-Linear Optical Loop Mirror (NOLM) . . . . . . . . . . . . . . . . . . 242206. NOLM Output Power Variation with Input Power . . . . . . . . . . . . . 243207. NOLM Filtering Out the Noise from a Stream of Pulses . . . . . . . . . 243208. Non-Linear Amplifying Optical Loop Mirror (NALM) . . . . . . . . . . . . 244209. Demultiplexing of a TDM Data Stream . . . . . . . . . . . . . . . . . . . 245210. NOLM Configured as an AND Logic Gate . . . . . . . . . . . . . . . . . 245211. NOLM Logic Gate Using TWSLA . . . . . . . . . . . . . . . . . . . . . . 246212. Add-Drop Multiplexor - Functional View . . . . . . . . . . . . . . . . . . . 246

xiv Understanding Optical Communications

213. MZI Add-Drop Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247214. Acoustic Modulator - Bragg Reflection . . . . . . . . . . . . . . . . . . . 249215. Acoustic Modulator - Debye-Sears Effect . . . . . . . . . . . . . . . . . . 251216. Wavelength Selective Switching Function . . . . . . . . . . . . . . . . . 252217. Acoustooptic Tunable Filter (AOTF) - Basic Structure . . . . . . . . . . 253218. AOTF Operation (1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254219. AOTF Operation (2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254220. Electro-Optic Phase Modulator . . . . . . . . . . . . . . . . . . . . . . . . 256221. Mach-Zehnder Interferometer . . . . . . . . . . . . . . . . . . . . . . . . 256222. Modulator Using Mach-Zehnder Interferometer . . . . . . . . . . . . . . 257223. Travelling-Wave Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . 258224. Pockels Cell Modulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259225. Faraday Effect Modulator . . . . . . . . . . . . . . . . . . . . . . . . . . . 260226. Principle of Drawing a Fibre . . . . . . . . . . . . . . . . . . . . . . . . . 269227. Drawing Tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269228. Double-Crucible Method . . . . . . . . . . . . . . . . . . . . . . . . . . . 270229. Outside Vapour Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . 271230. Vapour Axial Deposition (VAD) . . . . . . . . . . . . . . . . . . . . . . . 273231. Modified Chemical Vapour Deposition . . . . . . . . . . . . . . . . . . . 274232. Fusion Splicing Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . 276233. Fibre Alignment Using Optical Feedback . . . . . . . . . . . . . . . . . . 277234. Sources of Loss due to Misalignment in Fibre Joins . . . . . . . . . . . 280235. Sources of Intrinsic Loss in Fibre Joins . . . . . . . . . . . . . . . . . . . 281236. Some Typical Connector Types . . . . . . . . . . . . . . . . . . . . . . . 283237. MT Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284238. ESCON Fibre Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . 285239. Primary Coated Optical Fibre (PCOF) . . . . . . . . . . . . . . . . . . . 287240. Secondary Coated Optical Fibre (SCOF) . . . . . . . . . . . . . . . . . . 288241. Loose Tube Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . 288242. Single-Core Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289243. Dual Indoor Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290244. 6-Core Tight Buffered Indoor Cable . . . . . . . . . . . . . . . . . . . . . 290245. Typical Outdoor Fiber Cable (Loose Tube - Gel Filled) . . . . . . . . . . 292246. Segmented-Core Cable Design . . . . . . . . . . . . . . . . . . . . . . . 292247. Outdoor Aerial Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292248. 12-Core Optical Flat Cable . . . . . . . . . . . . . . . . . . . . . . . . . . 293249. Typical Undersea Cable Design . . . . . . . . . . . . . . . . . . . . . . . 293250. Conventional Long Distance Fibre Transmission System . . . . . . . . 297251. Repeater Function Compared with Amplification . . . . . . . . . . . . . 298252. Amplified Single-Channel Transmission System . . . . . . . . . . . . . . 299253. WDM Long Distance Fibre Transmission System . . . . . . . . . . . . . 301254. NRZ Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302255. Evolution of an Optical Pulse . . . . . . . . . . . . . . . . . . . . . . . . . 302256. NRZI Encoding Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 304257. Return-To-Zero (RZ) Coding . . . . . . . . . . . . . . . . . . . . . . . . . 305258. Four-Level Coding Example . . . . . . . . . . . . . . . . . . . . . . . . . 307259. Duo-Binary Code Example . . . . . . . . . . . . . . . . . . . . . . . . . . 307260. Digital Receiver Functions . . . . . . . . . . . . . . . . . . . . . . . . . . 309261. Operating Principle of a Continuous (Analogue) PLL . . . . . . . . . . . 311262. Receiving 4 out of 5 Code . . . . . . . . . . . . . . . . . . . . . . . . . . 315263. Power Budgeting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318264. Dispersion Compensation of an Existing Standard Fibre Link . . . . . . 330265. Dispersion Compensation Using a Mid-Span DCF . . . . . . . . . . . . 331266. Dispersion Compensation of a New Link with DCF . . . . . . . . . . . . 332

Figures xv

267. Spectral Inverter - Schematic . . . . . . . . . . . . . . . . . . . . . . . . 333268. Spectral Inversion - Wavelength (Frequency) Domain . . . . . . . . . . 333269. Dispersion Compensation with a Fibre Bragg Grating . . . . . . . . . . 334270. Radial Overfilled Launch (ROFL) . . . . . . . . . . . . . . . . . . . . . . 339271. Pulse Breakup and the Fibre RI Profile that Causes It . . . . . . . . . . 339272. Optical Power Meter - Logical Structure . . . . . . . . . . . . . . . . . . 342273. OTDR Display - Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . 343274. OTDR Operational Logic . . . . . . . . . . . . . . . . . . . . . . . . . . . 345275. Spectrum Analyser - Display Schematic . . . . . . . . . . . . . . . . . . 346276. Spectrum Analyser - Logical Structure . . . . . . . . . . . . . . . . . . . 347277. Eye Diagram - Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . 348278. Real Eye Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349279. FDDI Basic Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354280. FDDI Ring Healing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355281. FDDI Ring Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . 355282. Ten-Bit Elasticity Buffer Operation . . . . . . . . . . . . . . . . . . . . . . 359283. Optical Bypass Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362284. Physical Layer Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . 362285. FDDI Node Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364286. Ethernet Bus Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366287. Ethernet Hub Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368288. Relationship between Offered Load and Throughput on Ethernet . . . . 370289. Fibre Channel Point-to-Point Topology . . . . . . . . . . . . . . . . . . . 375290. Fibre Channel Crosspoint Switch Topology . . . . . . . . . . . . . . . . 376291. Fibre Channel Arbitrated Loop Topologies . . . . . . . . . . . . . . . . . 376292. Fibre Channel Physical Nomenclature . . . . . . . . . . . . . . . . . . . 377293. Open Fibre Control System - Logic Structure . . . . . . . . . . . . . . . 380294. Principle of the ESCON Director (Circuit Switch) . . . . . . . . . . . . . 382295. OptiConnect System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386296. The Multiplexor Mountain . . . . . . . . . . . . . . . . . . . . . . . . . . . 388297. Sonet STS-1 Frame Structure . . . . . . . . . . . . . . . . . . . . . . . . 390298. Sonet Synchronous Payload Envelope (SPE) . . . . . . . . . . . . . . . 390299. Synchronous Payload Envelope Floating in STS-1 Frame . . . . . . . . 391300. STM-1 to STM-4 Synchronous Multiplexing . . . . . . . . . . . . . . . . 391301. SDH Basic Frame Format . . . . . . . . . . . . . . . . . . . . . . . . . . 392302. Sonet Ring Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393303. Sonet Ring Restored after Cable Break . . . . . . . . . . . . . . . . . . 394304. ATM Network Structure Overview . . . . . . . . . . . . . . . . . . . . . . 399305. Routing Concept in an ATM Network . . . . . . . . . . . . . . . . . . . . 402306. Simple (Sparse) WDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411307. WDM Functions Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . 412308. Spectrum of Dense WDM and Sparse WDM on the Same Fibre . . . . 413309. Multiwavelength Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416310. Idealised Output Spectrum of a Multiline Laser . . . . . . . . . . . . . . 417311. FP Laser Output Spectrum Just Below Threshold . . . . . . . . . . . . . 418312. Multiline Laser Design Based on ASE . . . . . . . . . . . . . . . . . . . 418313. Splitter Array with Fabry-Perot Filters . . . . . . . . . . . . . . . . . . . . 421314. Demultiplexor Using In-Fibre Bragg Gratings . . . . . . . . . . . . . . . 421315. Demultiplexor Using In-Fibre Bragg Gratings with 3 dB Couplers . . . 422316. Array Waveguide Wavelength Router Schematic . . . . . . . . . . . . . 423317. Bi-Directional Wavelength Multiplexor/Demultiplexor . . . . . . . . . . . 423318. Wavelength Multiplexor/Demultiplexor . . . . . . . . . . . . . . . . . . . 424319. Add-Drop Multiplexor Function . . . . . . . . . . . . . . . . . . . . . . . . 424320. Array Waveguide Add-Drop Multiplexor . . . . . . . . . . . . . . . . . . . 425

xvi Understanding Optical Communications

321. Add-Drop Multiplexor Using FBG and Circulators . . . . . . . . . . . . . 425322. Filter Using Cascaded Mach-Zehnder Interferometers . . . . . . . . . . 426323. ADM Using Cascaded Mach-Zehnder Interferometers - Principle . . . . 426324. Add-Drop Multiplexor Using Cascaded Mach-Zehnder Interferometers . 427325. 4 x 4 Optical Space-Division Switch . . . . . . . . . . . . . . . . . . . . . 428326. Optical Switch Using Crossconnect Switch Elements . . . . . . . . . . . 429327. Structure of an Optical Switching Node . . . . . . . . . . . . . . . . . . . 430328. Optical Switching Node with Wavelength Conversion . . . . . . . . . . . 431329. Wavelength Converter with Cross-Gain Modulated SOA . . . . . . . . . 433330. Wavelength Converter with Cross-Phase Modulated SOA . . . . . . . . 434331. Wavelength Converter Using FWM or DFG . . . . . . . . . . . . . . . . 435332. Fitting Optical Channels into Their Allocated Wavebands . . . . . . . . 438333. A More Practical Waveband Allocation . . . . . . . . . . . . . . . . . . . 439334. Effect of Non-Square Filter on Signal Pulse . . . . . . . . . . . . . . . . 439335. Effect of Multiple Filters on Signal Pulse . . . . . . . . . . . . . . . . . . 440336. Broadcast and Select Network - Principle . . . . . . . . . . . . . . . . . 462337. Add-Drop Multiplexing Concept . . . . . . . . . . . . . . . . . . . . . . . 462338. Add-Drop Multiplexed Ring Network . . . . . . . . . . . . . . . . . . . . 463339. Network Using Optical Switching Nodes . . . . . . . . . . . . . . . . . . 465340. Wavelength Selective Network - Concept . . . . . . . . . . . . . . . . . 466341. Ring Wiring Concentrator . . . . . . . . . . . . . . . . . . . . . . . . . . . 468342. Folded Bus Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . 469343. Reflective Star Configuration . . . . . . . . . . . . . . . . . . . . . . . . . 470344. Tree Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471345. Wavelength Switching System . . . . . . . . . . . . . . . . . . . . . . . . 476346. Rainbow-1 System Design . . . . . . . . . . . . . . . . . . . . . . . . . . 480347. Add/Drop WDM Network with Multiwavelength Source . . . . . . . . . . 482348. Add/Drop WDM Station with Multiwavelength Source . . . . . . . . . . 483349. The IBM 9729 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486350. The IBM 9729 System Configuration . . . . . . . . . . . . . . . . . . . . 487351. IBM 9729 System Structure . . . . . . . . . . . . . . . . . . . . . . . . . 488352. IBM 9729 Wavelength Allocation . . . . . . . . . . . . . . . . . . . . . . 490353. IBM 9729 Major Components and Data Flow . . . . . . . . . . . . . . . 491354. Grating Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492355. Grating Assembly Schematic . . . . . . . . . . . . . . . . . . . . . . . . . 492356. End-to-End Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . 495357. Evolution of an Optical Pulse . . . . . . . . . . . . . . . . . . . . . . . . . 497358. IBM 9729 Network Management Configuration . . . . . . . . . . . . . . 500359. IBM 9729 Ring Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501360. First Generation WAN WDM System - Architecture . . . . . . . . . . . . 502361. Network Management Channel . . . . . . . . . . . . . . . . . . . . . . . 504362. Traditional Telephone Local-Loop Cabling . . . . . . . . . . . . . . . . . 509363. Traditional Cable TV System Cabling . . . . . . . . . . . . . . . . . . . . 510364. Hybrid Fibre-Coax (HFC) Network Cabling . . . . . . . . . . . . . . . . . 511365. Proposed Spectrum Allocation for Bi-Directional HFC . . . . . . . . . . 512366. Fibre-to-the-Curb Cabling Alternatives . . . . . . . . . . . . . . . . . . . 513367. Cambridge Trial Network - Structure . . . . . . . . . . . . . . . . . . . . 515368. Passive Optical Network - Potential Solutions . . . . . . . . . . . . . . . 516369. Mobile Cellular Telephone System with Remote Central Base Station . 518370. Communication System - Logical View . . . . . . . . . . . . . . . . . . . 518371. Direct Sequence Spread Spectrum Modulation - Transmitter . . . . . . 527372. Direct Sequence Spread Spectrum Modulation . . . . . . . . . . . . . . 528373. Time Division Multiplexing (TDM) - Concept . . . . . . . . . . . . . . . . 531374. Principle of a TDM Network . . . . . . . . . . . . . . . . . . . . . . . . . 532

Figures xvii

375. Optical TDM Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . 533376. Demultiplexing the OTDM Stream . . . . . . . . . . . . . . . . . . . . . . 535377. Packet Switching - Principle . . . . . . . . . . . . . . . . . . . . . . . . . 535378. An Optical Transmission System Using Coherent Detection . . . . . . . 539379. Bohr Model of the Atom . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543380. Electron Orbits (Shells) in the Silicon Atom . . . . . . . . . . . . . . . . 544381. Oxygen Atom Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 545382. Energy Bands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 546383. Energy Bands and Electrical Conduction . . . . . . . . . . . . . . . . . . 547384. Electricity Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 548385. Silicon Crystal Lattice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 550386. Conduction in an Intrinsic Semiconductor . . . . . . . . . . . . . . . . . 550387. Doped Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 550388. Doped Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 552389. Free Electron Movement within a Crystal Lattice . . . . . . . . . . . . . 554390. p-n Junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 555391. Electrical Potentials across a p-n Junction . . . . . . . . . . . . . . . . . 556392. Basic BJT Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . 557393. Practical Construction of a BJT . . . . . . . . . . . . . . . . . . . . . . . 558394. Bipolar Junction Transistor (BJT) in VLSI Implementation . . . . . . . . 558395. Operation of a BJT Stage 1 . . . . . . . . . . . . . . . . . . . . . . . . . 559396. Operation of a BJT Stage 2 . . . . . . . . . . . . . . . . . . . . . . . . . 559397. Insulated Gate Field Effect Transistor (IGFET) . . . . . . . . . . . . . . 560398. Communications Protocol - Principle . . . . . . . . . . . . . . . . . . . . 565399. Communications Protocol - Principle . . . . . . . . . . . . . . . . . . . . 566400. Communications Protocol - Network Problem . . . . . . . . . . . . . . . 567401. Layering of Functions in the ISO Model . . . . . . . . . . . . . . . . . . 568402. OSI Data Frame Format . . . . . . . . . . . . . . . . . . . . . . . . . . . 569403. Scope of Layers - 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 570404. Sub-Layers and Networks . . . . . . . . . . . . . . . . . . . . . . . . . . 571405. ATM, POTS and X.25 over an SDH/Sonet Backbone . . . . . . . . . . 572406. Network Replaces a Point-to-Point Physical Link . . . . . . . . . . . . . 573407. A Network Replaces a Link within Another Network. . . . . . . . . . . . 574408. Hazard Warning Symbol Designating Laser in Operation . . . . . . . . 577409. The Visible Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 587

xviii Understanding Optical Communications

Tables

1. Bandgap Energy and Possible Wavelength Ranges in Various Materials 852. Connector Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 2843. Optical Fibre State of the Art . . . . . . . . . . . . . . . . . . . . . . . . . 3164. Signal Loss in Various Materials . . . . . . . . . . . . . . . . . . . . . . . 3165. FDDI Code Translation Table . . . . . . . . . . . . . . . . . . . . . . . . 3606. Fibre Channel Media Capabilities . . . . . . . . . . . . . . . . . . . . . . 3777. Sonet Speed Hierarchy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3928. PDH Speed Hierarchy - USA . . . . . . . . . . . . . . . . . . . . . . . . . 3949. PDH Speed Hierarchy - Europe . . . . . . . . . . . . . . . . . . . . . . . 394

10. ATM-UNI Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405

Copyright IBM Corp. 1998 xix

xx Understanding Optical Communications

Preface

This book presents a high level technical overview of the emerging technologies offibre optical communications and optical networking. Written at the technicalconceptual level it is intended as an introduction to the field for computer and datacommunication professionals. Discussion is descriptive and an effort has beenmade to keep the use of mathematics to a minimum. Although it is intended forprofessionals who already have some technical background, it is neverthelessrelevant to anyone wishing to understand optical communication.

The primary topics of the book are:

Transmission of light on a fibre Optical fibre, lasers, detectors, optical components Use of fibre optics in electronic communications networks Wavelength Division Multiplexing (WDM) Optical communications networks

About the AuthorHarry J. R. Dutton is a Project Leader with the IBM International Technical SupportCenter, Raleigh NC., USA. Possibly the worlds longest-distance remote report heis based in Sydney, Australia. His current responsibilities are for worldwidetechnical support of ATM and of optical communications architectures andstandards.

On joining IBM in January of 1967 as a Systems Engineer, Harry initiallyspecialised in the design and programming of early communication networks.Since then he has worked in communications system design, software design,communications systems programming, product design, customer technical support,project management, consulting and product management. Throughout, hispredominant role has been as the interface (on technical matters) between the IBMfield companies and product development laboratories.

A frequent speaker at international conferences, he is the author of 18 IBMpublications in the areas of X.25, SNA Architecture, Voice/Data Integration, MicroChannel, High Speed Communications and ATM.

Harry is a graduate of the University of New South Wales and a member of theIEEE. He may be contacted by email on:

[email protected]

By the Same AuthorAsynchronous Transfer Mode (ATM) - Technical Overview

by Harry J. R. Dutton and Peter Lenhard. Prentice Hall, 1995.

High-Speed Networking Technology: An Introductory Survey

by Harry J. R. Dutton and Peter Lenhard. Prentice Hall, 1995.

Copyright IBM Corp. 1998 xxi

AcknowledgmentsThis book was written as a project of the IBM International Technical SupportCentre, Raleigh.

Producing a book like this is not a task that any mortal can perform in isolation.Thanks are due to the following people for help, advice, technical guidance andreview:

Dr Leon Polladian The University of Sydney, Sydney, Australia.

Mr Denis Kekas The University of Arizona, Tucson, AZ.

Mr Jon A. Herlocker The University of Arizona, Tucson, AZ.

Mr James Cleaver IBM Australia Limited, Sydney, Australia.

Dr Cassimer DeCussatis IBM Enterprise Systems Division, Poughkeepsie, NY.

Dr Frank Janniello IBM Research Division, Hawthorne, NY.

Dr Daniel Kuchta IBM Research Division, Yorktown, NY.

Mr John Kuras IBM Network Hardware Division, Raleigh, NC.

Mr Ian Shields IBM Network Software Division, Raleigh, NC.

Mr Gary Steps IBM Network Hardware Division, Raleigh, NC.

Mr Shawn Walsh IBM International Technical Support Organisation,Raleigh, NC.

Mr Walter A. Worischek IBM Austria, Vienna, Austria.

Comments WelcomeYour comments are important to us!

We want our redbooks to be as helpful as possible. Please send us yourcomments about this or other redbooks in one of the following ways:

Fax the evaluation form found in ITSO Redbook Evaluation on page 599 tothe fax number shown on the form.

Use the electronic evaluation form found on the Redbooks Web sites:

For Internet users http://www.redbooks.ibm.com/For IBM Intranet users http://w3.itso.ibm.com/

Send us a note at the following address:

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xxii Understanding Optical Communications

2 Understanding Optical Communications

Chapter 1. Introduction

The use of light to send messages is not new. Fires were used for signaling inbiblical times, smoke signals have been used for thousands of years and flashinglights have been used to communicate between warships at sea since the days ofLord Nelson.

The idea of using glass fibre to carry an optical communications signal originatedwith Alexander Graham Bell. However this idea had to wait some 80 years forbetter glasses and low-cost electronics for it to become useful in practicalsituations.

Development of fibres and devices for optical communications began in the early1960s and continues strongly today. But the real change came in the 1980s.During this decade optical communication in public communication networksdeveloped from the status of a curiosity into being the dominant technology.

Among the tens of thousands of developments and inventions that have contributedto this progress four stand out as milestones:

1. The invention of the LASER (in the late 1950's)

2. The development of low loss optical fibre (1970's)

3. The invention of the optical fibre amplifier (1980's)

4. The invention of the in-fibre Bragg grating (1990's)

The continuing development of semiconductor technology is quite fundamental butof course not specifically optical.

The predominant use of optical technology is as very fast electric wire. Opticalfibres replace electric wire in communications systems and nothing much elsechanges. Perhaps this is not quite fair. The very speed and quality of opticalcommunications systems has itself predicated the development of a new type ofelectronic communications itself designed to be run on optical connections. ATMand SDH technologies are good examples of the new type of systems.

It is important to realise that optical communications is not like electroniccommunications. While it seems that light travels in a fibre much like electricitydoes in a wire this is very misleading. Light is an electromagnetic wave and opticalfibre is a waveguide. Everything to do with transport of the signal even to simplethings like coupling (joining) two fibres into one is very different from what happensin the electronic world. The two fields (electronics and optics) while closely relatedemploy different principles in different ways.

Some people look ahead to true optical networks. These will be networks whererouting is done optically from one end-user to another without the signal everbecoming electronic. Indeed some experimental local area (LAN) and metropolitanarea (MAN) networks like this have been built. In 1998 optically routed nodal widearea networks are imminently feasible and the necessary components to build themare available. However, no such networks have been deployed operationally yet.

In 1998 the happening area in optical communications is Wavelength DivisionMultiplexing (WDM). This is the ability to send many (perhaps up to 1000)independent optical channels on a single fibre. The first fully commercial WDM

Copyright IBM Corp. 1998 3

products appeared on the market in 1996. WDM is a major step toward fullyoptical networking.

1.1.1 Optical Transmission System Concepts

ModulatorGlass Fibre

Transmitter Receiver

Connector Connector

Amplifier

DetectorLight Source Light Sensor(Detector)

ElectricityElectricity Light

Figure 1. Optical Transmission - Schematic

The basic components of an optical communication system are shown in Figure 1,above.

A serial bit stream in electrical form is presented to a modulator, which encodesthe data appropriately for fibre transmission.

A light source (laser or Light Emitting Diode - LED) is driven by the modulatorand the light focused into the fibre.

The light travels down the fibre (during which time it may experience dispersionand loss of strength).

At the receiver end the light is fed to a detector and converted to electricalform.

The signal is then amplified and fed to another detector, which isolates theindividual state changes and their timing. It then decodes the sequence ofstate changes and reconstructs the original bit stream.1

The timed bit stream so received may then be fed to a using device.

Optical communication has many well-known advantages:

Weight and Size

Fibre cable is significantly smaller and lighter than electrical cables to dothe same job. In the wide area environment a large coaxial cable systemcan easily involve a cable of several inches in diameter and weighing manypounds per foot. A fibre cable to do the same job could be less than onehalf an inch in diameter and weigh a few ounces per foot.

This means that the cost of laying the cable is dramatically reduced.

Material Cost

Fibre cable costs significantly less than copper cable for the sametransmission capacity.

1 This overview is deliberately simplified. There are many ways to modulate the transmission and the details will vary from thisexample but the general principle remains unchanged.


Information Capacity

The data rate of systems in use in 1998 is generally 150 or 620 Mbps on asingle (unidirectional) fibre. This is because these systems were installed inpast years. The usual rate for new systems is 2.4 Gbps or even 10 Gbps.This is very high in digital transmission terms.

In telephone transmission terms the very best coaxial cable systems giveabout 2,000 analog voice circuits. A 150 Mbps fibre connection gives justover 2,000 digital telephone (64 Kbps) connections. But the 150 Mbpsfibre is at a very early stage in the development of fibre optical systems.The coaxial cable system with which it is being compared is much morecostly and has been developed to its fullest extent.

Fibre technology is still in its infancy. Using just a single channel per fibre,researchers have trial systems in operation that communicate at speeds of100 Gbps. By sending many (wavelength division multiplexed) channelson a single fibre, we can increase this capacity a hundred and perhaps athousand times. Recently researchers at NEC reported a successfulexperiment where 132 optical channels of 20 Gbps each were carried over120 km. This is 2.64 terabits per second! This is enough capacity to carryabout 30 million uncompressed telephone calls (at 64 Kbps per channel).Thirty million calls is about the maximum number of calls in progress in theworld at any particular moment in time. That is to say, we could carry theworld's peak telephone traffic over one pair of fibres. Most practical fibresystems don't attempt to do this because it costs less to put multiple fibresin a cable than to use sophisticated multiplexing technology.

No Electrical Connection

This is an obvious point but nevertheless a very important one. Electricalconnections have problems.

In electrical systems there is always the possibility of ground loopscausing a serious problem, especially in the LAN or computer channelenvironment. When you communicate electrically you often have toconnect the grounds to one another or at least go to a lot of trouble toavoid making this connection. One little known problem is that there isoften a voltage potential difference between ground at differentlocations. The author has observed as much as 3 volts difference inground potential between adjacent buildings (this was a freak situation).It is normal to observe 1 or 2 volt differences over distances of akilometer or so. With shielded cable there can be a problem if youearth the shields at both ends of the connection.

Optical connection is very safe. Electrical connections always have tobe protected from high voltages because of the danger to peopletouching the wire.

In some tropical regions of the world, lightning poses a severe hazardeven to buried telephone cables! Of course, optical fibre isn't subject tolightning problems but it must be remembered that sometimes opticalcables carry wires within them for strengthening or to power repeaters.These wires can be a target for lightning.

No Electromagnetic Interference

Because the connection is not electrical, you can neither pick up nor createelectrical interference (the major source of noise). This is one reason that

Chapter 1. Introduction 5

optical communication has so few errors. There are very few sources ofthings that can distort or interfere with the signal.

In a building this means that fibre cables can be placed almost anywhereelectrical cables would have problems, (for example near a lift motor or in acable duct with heavy power cables). In an industrial plant such as a steelmill, this gives much greater flexibility in cabling than previously available.

In the wide area networking environment there is much greater flexibility inroute selection. Cables may be located near water or power lines withoutrisk to people or equipment.

Distances between Regenerators

As a signal travels along a communication line it loses strength (isattenuated) and picks up noise. The traditional way to regenerate thesignal, restoring its power and removing the noise, is to use a either arepeater or an amplifier. These are discussed later in 5.10, Repeaters onpage 261.2 (Indeed it is the use of repeaters to remove noise that givesdigital transmission its high quality.)

In long-line optical transmission cables now in use by the telephonecompanies, the repeater spacing is typically 40 kilometres. This compareswith 12 km for the previous coaxial cable electrical technology. Thenumber of required repeaters and their spacing is a major factor in systemcost.

Some recently installed systems (1997) have spacings of up to 120kilometres.3

Open Ended Capacity

The maximum theoretical capacity of installed fibre is very great (almostinfinite). This means that additional capacity can be had on existing fibresas new technology becomes available. All that must be done is change theequipment at either end and change or upgrade the regenerators.

Better Security

It is possible to tap fibre optical cable. But it is very difficult to do and theadditional loss caused by the tap is relatively easy to detect. There is aninterruption to service while the tap is inserted and this can alert operationalstaff to the situation. In addition, there are fewer access points where anintruder can gain the kind of access to a fibre cable necessary to insert atap.

Insertion of active taps where the intruder actually inserts a signal is evenmore difficult.

However, there are some limitations:

Joining Cables

The best way of joining cables is to use fusion splicing. This is wherefibre ends are fused to one another by melting the glass. Making suchsplices in a way that will ensure minimal loss of signal is a skilled task that

2 Repeaters have been in use for many years in digital electronic connections.

3 As will be seen later, optical amplifiers are replacing repeaters as the technology of choice in long-line communications.


requires precision equipment. It is particularly difficult to do outdoors invery low temperatures, such as in the North American or European winter.

In the early days of fibre optical systems (the early 1980s) connectorswhich allowed cables to be plugged and unplugged were unreliable andcaused a large amount of signal loss (as much as 3 dB per connector). Ofcourse, the larger the core diameter of the fibre, the easier it is to make alow-loss connector. In the last few years connector systems for fibres withthicker cores (multimode fibres) have improved to the point where the useof a large number of connectors in a LAN system can be very reliable.Typical connector loss for an SC coupler (a popular modern type) using62.5 micron fibre is around .3 dB. It is not this good with single-mode fibresof small core diameter.

One of the major system costs is the cost of coupling a fibre to anintegrated light source (laser or LED) or detector on a chip. This is doneduring manufacture and is called pigtailing. Although the cost of opticaltransceivers has decreased rapidly in the last few years, these are stilltwice the cost (or more) of the same transceivers using electricalconnection. For example the current market price of FDDI adaptersoperating over a copper medium is around half that of the same adapterusing fibre. The cost difference is in the optical transceivers and thepigtailing.

Bending Cables

As light travels along the fibre, it is reflected from the interface between thecore and cladding whenever it strays from the path straight down thecenter. When the fibre is bent, the light only stays in the fibre because ofthis reflection. But the reflection only works if the angle of incidence isrelatively low. If you bend the fibre too much the light escapes.

The amount of allowable bending is specific to particular cables because itdepends on the difference in refractive index, between core and cladding.The bigger the difference in refractive index, the tighter the allowable bendradius. There is a tradeoff here because there are many other reasons thatwe would like to keep this difference as small as possible.

Slow Standards Development

This is nobody's fault. Development is happening so quickly, and gettingworldwide agreement to a standard is necessarily so slow that standardssetting just can't keep up. Things are improving considerably and veryquickly, however. Cable sizes and types are converging toward a fewchoices, although the way they are used is still changing almost daily.

There are now firm standards for optical link connections in LAN protocols(token-ring, Ethernet and FDDI), SDH (for the wide area) and in ATM forboth the LAN and the wide area.

Optics for Transmission Only

Until very recently there was no available optical amplifier. The signal hadto be converted to electrical form and put through a complex repeater inorder to boost its strength. Recently, optical amplifiers have emerged andlook set to solve this problem (see 5.2, Optical Amplifiers on page 157).

However, optical logic processing and/or switching systems seem to be afew years off yet.

Chapter 1. Introduction 7

Gamma Radiation

Gamma radiation comes from space and is always present. It can bethought of as a high-energy X-ray. Gamma radiation can cause sometypes of glass to emit light (causing interference) and also gamma radiationcan cause glass to discolor and hence attenuate the signal. In normalsituations these effects are minimal. However, fibres are probably not thetransmission medium of choice inside a nuclear reactor or on along-distance space probe. (A glass beaker placed inside a nuclear reactorfor even a few hours comes out black in color and quite opaque.)

Electrical Fields

Very high-voltage electrical fields also affect some glasses in the same wayas gamma rays. One proposed route for fibre communication cables iswrapped around high-voltage electrical cables on transmission towers. Thisactually works quite well where the electrical cables are only of 30 000 voltsor below. Above that (most major transmission systems are many timesabove that), the glass tends to emit light and discolor. Nevertheless, this isa field of current research - to produce a glass that will be unaffected bysuch fields. It is a reasonable expectation that this will be achieved within avery few years.

Some electricity companies are carrying fibres with their high voltagedistribution systems by placing the fibre inside the earth wire (typically a 1inch thick aluminium cable with steel casing). This works well, butlong-distance high-voltage distribution systems usually don't have earthwires.

Sharks Eat the Cable(?)

In the 1980s there was an incident where a new undersea fibre cable wasbroken on the ocean floor. Publicity surrounding the event suggested thatthe cable was attacked and eaten by sharks. It wasn't just the press; thiswas a serious claim. It was claimed that there was something in thechemical composition of the cable sheathing that was attractive to sharks!Another explanation was that the cable contained an unbalanced electricalsupply conductor. It was theorised that the radiated electromagnetic fieldcaused the sharks to be attracted.

Other people have dismissed this claim as a joke and suggest that thecable was badly laid and rubbed against rocks. Nevertheless, the story haspassed into the folklore of fibre optical communication and some peoplegenuinely believe that sharks eat optical fibre cable.

Gophers (and Termites) Really Do Eat the Cable

Gophers are a real problem for fibre cables in the United States. There isactually a standardised test (conducted by a nature and wildlifeorganisation) which involves placing a cable in a gopher enclosure for afixed, specified length of time. In other countries termites have been knownto attack and eat the plastic sheathing.

Most people evaluate the advantages as overwhelming the disadvantages for mostenvironments. But advantages and disadvantages need to be considered in thecontext of the environment in which the system is to be used. The types of fibresystems appropriate for the LAN environment are quite different from those that areoptimal in the wide area world.


1.1.2 Optical NetworkingA network may be defined as a collection of transmission links and other equipmentwhich provides a means of information interchange within a group of end users.The concept includes a number of key notions:

1. The objective of the network is to provide a means of information interchangebetween end users.

2. The network usually contains some shared resources. That is, links andswitching nodes are shared between many

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