Fiber-Optic Communication Systems An Introduction

1. Fiber-Optic Communication Systems An Introduction

2. Why Optical Communications? • Optical Fiber is the backbone of the modern communication networks • The Optical Fiber Carries: • Almost all long distance phone calls • Most Internet traffic (Dial-up, DSL or Cable) • Most Television channels (Cable or DSL) • One fiber can carry up to 6.4 Tb/s (1012 b/s) or 100 million conversations simultaneously • Information revolution wouldn’t have happened without the Optical Fiber ‘Triple Play’

4. Why Optical Communications? Lowest Attenuation: 0.2 dB/km at 1.55 µm band resulting in 100s of km fiber links without repeaters Highest Bandwidth of any communication channel: Single Mode Fiber (SMF) offers the lowest dispersion highest bit rate  rich content (broadband) up to 100 Gb/s or more Enormous Capacity: Via WDM that also offer easy upgradability, The ‘Optical Layer’: Wavelength routing, switching and processing all optically, which adds another layer of flexibility

5. Elements of a Fiber Optic Link

6. Elements of OPTICOM System • The Fiber – that carries the light • Single Mode Fiber (only one EM mode exists), offers the highest bit rate, most widely used • Multi Mode Fiber (multiple EM modes exist), hence higher dispersion (due to multiple modes) cheaper than SMF, used in local area networks • Step Index Fiber – two distinct refractive indices • Graded Index Fiber – gradual change in refractive index

7. Elements of OPTICOM System • Optical Transmitter converts the electrical information to optical format (E/O) • Light Emitting Diode (LED): cheap, robust and used with MMF in short range applications • Surface emitting and edge emitting LED • LASER Diode: high performance and more power, used with SMF in high speed links • Distributed Feedback (DFB) Laser – high performance single mode laser • Fabry-Perrot (FP) lasers – low performance multimode laser

8. Elements of OPTICOM System • Optical Receiver convertsthe optical signal into appropriate electrical format (E/O) • PIN Photo Diode: Low performance, no internal gain, low cost, widely used • Avalanche Photo Diode (APD): High performance with internal (avalanche) gain • Repeater: receives weak light signal, cleans-up, amplifies and retransmits (O/E/O) • Optical Amplifier: Amplifies light in fiber without O/E/O

9. Wavelength Division Multiplexing • Fiber has the capability to transmit hundreds of wavelengths • Coarse WDM (CWDM) has ~20 nm wavelength spacing • Dense WDM (DWDM) has up to 50 GHz spacing • Once the fiber is in place, additional wavelength can be launched by upgrading transceivers

10. Optical Amplifier & EDFA • An optical amplifier amplifies the light signal without converting to electrical • Very useful is WDM systems • Erbium Doped Fiber Amplifier (EDFA) works in 1550 nm band Continuous Wave (Constant)

11. Brief Intro on Telecom Networks

12. Long Haul Network Basics of Communication Networks

13. Brief History of Networks Copper Telecom Networks: • 4 kHz analog voice local loop (between customers and central office – access end) still in Bell Telephone lines & 56k modems • Digital interoffice trunks using DS-1 (Digital Signal Type 1) • A voice signal digitized at a sampling rate of 8 kHz  8 bits/samples is DS-0 (64 kb/s) • Carried on a single twisted copper-wire pair • Required repeaters every 2 km to compensate for attenuation

14. Digital Transmission Hierarchy (DTH) 64-kb/s circuits are multiplexed into higher-bit-rate formats Called Telephony or T-Networks This is Copper network

15. First Generation Fiber Optic Systems Purpose: • Eliminate repeaters in T-1 systems used in inter-office trunk lines Technology: • 0.8 µm GaAs semiconductor lasers • Multimode silica fibers Limitations: • Fiber attenuation • Intermodal dispersion Deployed since 1974

16. Second Generation Systems Opportunity: • Development of low-attenuation fiber (removal of H2O and other impurities) • Eliminate repeaters in long-distance lines Technology: • 1.3 µm multi-mode semiconductor lasers • Single-mode, low-attenuation silica fibers • DS-3 signal: 28 multiplexed DS-1 signals carried at 44.736 Mb/s Limitation: • Fiber attenuation (repeater spacing ≈ 6 km) Deployed since 1978

17. Third Generation Systems Opportunity: • Deregulation of long-distance market Technology: • 1.55 µm single-mode semiconductor lasers • Single-mode, low-attenuation silica fibers • OC-48 signal: 810 multiplexed 64-kb/s voice channels carried at 2.488 Gb/s Limitations: • Fiber attenuation (repeater spacing ≈ 40 km) • Fiber dispersion Deployed since 1982

18. Fourth Generation Systems Opportunity: • Development of erbium-doped fiber amplifiers (EDFA) Technology (deployment began in 1994): • 1.55 µm single-mode, narrow-band semiconductor lasers • Single-mode, low-attenuation, dispersion-shifted silica fibers • Wavelength-division multiplexing of 2.5 Gb/s or 10 Gb/s signals Nonlinear effects limit the following system parameters: • Signal launch power • Propagation distance without regeneration/re-clocking • WDM channel separation • Maximum number of WDM channels per fiber Polarization-mode dispersion limits the following parameters: • Propagation distance without regeneration/re-clocking

19. Evolution of Optical Networks

20. History of Attenuation

21. Fiber Network Topologies Core - Combination of switching centers and transmission systems connecting switching centers. Access- that part of the network which connects subscribers to their immediate service providers LWPF : Low-Water-Peak Fiber, DCF : Dispersion Compensating Fiber, EML : Externally modulated (DFB) laser

22. Synchronous Optical Networks • SONET is the TDM optical network standard for North America (called SDH in the rest of the world) • De-facto standard for fiber backhaul networks • OC-1 consists of 810 bytes over 125 us; OC-n consists of 810n bytes over 125 us • Linear multiplexing and de-multiplexing is possible with Add-Drop-Multiplexers

23. SONET/SDH Bandwidths

24. Last Mile Bottle Neck and Access Networks Infinite Bandwidth Backbone Optical Fiber Networks A few (Gb/s) Virtually infinite demand end user Few Mb/s The Last Mile ? ? Additionally, supporting different QoS

25. Fiber in the Access End Passive Optical Networks (PON) – No active elements or O/E conversion Fibre-Coaxial (analog) or DSL (digital) fibre-copper systems Radio over fibre (Fibre-Wireless) Systems Currently Drives the Market

26. PON Bit-Rates & Timeline

27. Digital Subscriber Loop (DSL) Fiber Link • Digital fiber-copper (fiber-twisted pair) link • Multimedia (video, voice and data) • At least 3.7 Mb/s streaming is needed for quality video • Bit rate heavily depend on the length of the twisted pair link • New techniques like very high rate DSL (VDSL) • Many buildings in GTA have access to video over DSL

28. Radio over Fiber (ROF) • RF signals are transmitted over fiber to provide broadband wireless access • An emerging very hot area • Many advantages • Special areas • Underground • Olympics London • Niagara Tunnel

29. ROF for Fiber-Wireless Networks Central Base Station RAP RAP RAP Y Radio over Fiber (ROF) (Simple) Up/Down links Y 802.11 voice Y Single ROF link can support voice and data simultaneously Micro Cell