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Optical Technologies and Lightwave Networks

Optical Technologies and Lightwave Networks. Outline: Optical Technologies Optical Fibers, Fiber Loss and Dispersion Lightwave Systems and Networks Multiplexing Schemes Undersea Fiber Systems Lightwave Broadband Access Optical Networks. Need for Optical Technologies.

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Optical Technologies and Lightwave Networks

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  1. Optical Technologies and Lightwave Networks • Outline: • Optical Technologies • Optical Fibers, Fiber Loss and Dispersion • Lightwave Systems and Networks • Multiplexing Schemes • Undersea Fiber Systems • Lightwave Broadband Access • Optical Networks

  2. Need for Optical Technologies • huge demand on bandwidth nowadays •  need high capacity transmission • electronic bottleneck: • speed limit of electronic processing • limited bandwidth of copper/coaxial cables • optical fiber has very high-bandwidth (~30 THz) •  suitable for high capacity transmission • optical fiber has very low loss (~0.25dB/km @1550 nm) •  suitable for long-distance transmission

  3. amplitude position/distance wavelength Light Wave • electromagnetic wave • carry energy from one point to another • travel in straight line • described in wavelength (usually in mm or nm) • speed of light in vacuum = 3108 m/s

  4.  >   > c Incident light Reflected light  Medium 1   Medium 2   Reflecting surface  Reflection and Refraction of Light Reflection Refraction • medium 1 is less dense (lower refractive index) than medium 2 • light path is reversible • If incident light travels from a denser medium into a less dense medium and the incident angle is greater than a certain value (critical angle c)  Total Internal Reflection Incident angle= reflected angle

  5. cladding light beam core Optical Fiber • made of different layers of glass, in cylindrical form • core has higher refractive index (denser medium) than the cladding • light beam travels in the core by means of total internal refraction • the whole fiber will be further wrapped by some plastic materials for protection • in 1966, Charles K. Kao and George A. Hockham suggested the use of optical fiber as a transmission media for information

  6. Optical Fiber (cont’d) • Fiber mode describes the path or direction of the light beam travelling in the fiber • number of fiber modes allowed depends on the core diameter and the difference of the refractive indices in core and cladding Single-mode Fiber Multi-mode Fiber • smaller core diameter • allow only one fiber mode • typical value: 9/125mm • larger core diameter • allow more than one fiber modes • typical value: 62.5/125mm

  7. Optical Fiber (cont’d) • Advantages of optical fiber: • large bandwidth  support high capacity transmission • low attenuation  support long-distance transmission • small and light in size  less space • low cost • immune to electromagnetic interference

  8. Fiber Attenuation • optical power of a signal is reduced after passing through a piece of fiber • wavelength-dependent low loss wavelength ranges: 1.3mm (0.4-0.6 dB/km), 1.55mm (0.2-0.4 dB/km)  suitable for telecommunications

  9. Fiber Dispersion • Inter-modal dispersion (only in multi-mode fibers): • different fiber modes takes different paths •  arrived the fiber end at different time •  pulse broadening  intersymbol interference (ISI)  limit bit-rate • Intra-modal dispersion (in both single-mode and multi-mode fiber): • different frequency components of a signal travel with different speed in the fiber •  different frequency components arrived the fiber end at different time •  pulse broadening  limit bit-rate

  10. 20 10 0 -10 -20 Standard Dispersion-flattened Dispersion (ps/(km•nm)) Dispersion-shifted 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Wavelength (mm) Fiber Dispersion Typical values: standard fiber: ~ 0 ps/(km• nm) @1300 nm ~17 ps /(km• nm) @1550 nm dispersion-shifted fiber: ~0.5 ps /(km• nm) @1550 nm

  11. System Capacity • fiber attenuation  loss in optical power limit transmission distance • fiber dispersion  pulse broadening  limit transmission bit-rate

  12. Input electrical data optical power (photons) output optical power wavelength l input electric current threshold current optical power (photons) photo-current Laser Source and Photodetector • Laser source • generate laser of a certain wavelength • made of semiconductors • output power depends on input electric current • need temperature control to stabilize the output power and output wavelength (both are temperature dependent) • Photodetector • convert incoming photons into electric current (photo-current)

  13. A2 A2 C2 B2 B1 C2 C1 A1 A1 C1 B1 B2 A time B l C Multiplexing Schemes Multiplexing: transmits information for several connections simultaneously on the same optical fiber Time Division Multiplexing (TDM) • only require one wavelength (one laser) • if channel data rate is R bits/sec, for N channels, the system data rate is (R  N) bits/sec

  14. fA fB fC fA freq freq A fB freq B fC l freq C Multiplexing Schemes Subcarrier Multiplexing (SCM) • multiple frequency carriers (subcarriers) are combined together • only require one wavelength (one laser) (optical carrier) • suitable for video distribution on fiber

  15. lA lA lB lC A lB wavelength B lC C wavelength multiplexer Multiplexing Schemes Wavelength Division Multiplexing (WDM) wavelength spacing: 0.8 nm (100-GHz) • one distinct wavelength (per laser) per sender • wavelength multiplexer/demultiplexer are needed to combine/separate wavelengths • if channel data rate per wavelength is R bits/sec, for N wavelengths, the system data rate is (R  N) bits/sec • suitable for high capacity data transmission

  16. TDM/WDM lA lB lC lA lB lC SCM/WDM f1 f2 f3 f1 f2 f3 f1 f2 f3 wavelength wavelength TDM stream A A TDM stream B B TDM stream C C wavelength multiplexer wavelength multiplexer Multiplexing Schemes Hybrid Types (TDM/WDM, SCM/WDM)  higher capacity lA lA lB lB lC lC

  17. 132 Ch 1 Ch TDM Transmission System Capacity

  18. G Optical Amplifier • no Electrical-to-Optical (E/O) or Optical-to-Electrical (O/E) conversion • can amplify multiple wavelengths simultaneously • Semiconductor Optical Amplifier • Fiber-Amplifier • Erbium-doped fiber amplifier (EDFA) : operates at 1550 nm transmission window (1530-1560 nm) (mature and widely deployed nowadays) • Pr3+ or Nd3+ doped fiber amplifier: operates at 1310 nm transmission window (not very mature) • ultra-wideband EDFA: S-band (1450-1530 nm), C-band (1530-1570 nm), L-band (1570-1650 nm)

  19. Low-Rate Data Out Low-Rate Data In E MUX E D MUX REG RPTR REG RPTR XMTR RCVR Opto-Electronic Regenerative Repeater EQ DEC LASER DET AMP AMP TMG REC Lightwave Systems Traditional Optical Fiber Transmission System • Single-wavelength operation, electronic TDM of synchronous data • Opto-electronic regenerative repeaters, 30-50km repeater spacing • Distortion and noise do not accumulate • Capacity upgrade requires higher-speed operation

  20. Data In Data Out O MUX O D MUX l1 l1 XMTR l2 RCVR l2 XMTR RCVR OA OA OA lN XMTR lN RCVR Lightwave Systems Optical Fiber Transmission System • Multi-channel WDM operation • Transparent data-rate and modulation form • One optical amplifier (per fiber) supports many channels • 80-140 km amplifier spacing • Distortion and noise accumulate • Graceful growth

  21. Undersea Fiber Systems Design Considerations • span distance • data rate • repeater/amplifier spacing • fault tolerance, system monitoring/supervision, restoration, repair • reliability in components: aging (can survive for 25 years) • cost

  22. Undersea Fiber Systems AT&T

  23. SYSTEM TIME BANDWIDTH/ NUMBER OF COMMENTS BIT-RATE BASIC CHANNELS TAT-1/2 1955/59 0.2 MHz 48 HAW-1 1957 COPPER COAX TAT-3/4 1963/65 ANALOG HAW-2 1964 1.1 MHz 140 VACUUM TUBES H-G-J 1964 TAT-5 1970 HAW-3 1974 6 MHz 840 Ge TRANSISTORS H-G-O 1975 TAT-6/7 1976/83 30 MHz 4,200 Si TRANSISTORS TAT-8 1988 OPTICAL FIBER HAW-4 1989 280 Mb/s 8,000 DIGITAL TPC-3 1989 l = 1.3 mm TAT-9 1991 16,000 TPC-4 1992 560 Mb/s 24,000 l = 1.55 mm TAT-10/11 1992/93 TAT-12 1995 5 Gb/s 122,880 OPTICAL AMPLIFIERS TPC-5 1995 l = 1.55 mm TAT: Trans-Atlantic Telecommunications TPC: Trans-Pacific Cable Undersea Fiber Systems

  24. Undersea Fiber Systems FLAG: Fiberoptic Link Around the Globe (10Gb/s SDH-based, 27,000km, service in 1997) • Tyco (AT&T) Submarine Systems Inc., & KDD Submarine Cable Systems Inc. • 2 fiber pairs, each transporting 32 STM-1s (5-Gb/s)

  25. Undersea Fiber Systems Africa ONE: Africa Optical Network (Trunk: 40Gb/s, WDM-SDH-based, 40,000km trunk, service in 1999) • Tyco (AT&T) Submarine Systems Inc. & Alcatel Submarine Networks • 54 landing points • 8 wavelengths, each carries 2.5Gb/s • 2 fiber pairs

  26. Passive Optical Network (PON) Remote Node passive optical splitter electrical repeater Headend Coaxial Cable Fiber Lightwave Broadband Access • Remote Node performs optical-to-electrical conversion • Hybrid Fiber-Coax (HFC), Fiber-to-the-Curb (FTTC), Fiber-to-the-Home (FTTH) • Distribution system: video, TV, multimedia, data, etc. • Two-way communications: upstream and downstream • Subcarrier multiplexing (single wavelength)

  27. WDM-PON Remote Node l1 l1, … , lN l2 electrical repeater Headend lN-1 multi-wavelength source lN wavelength demultiplexer Lightwave Broadband Access • WDM-PON: Wavelength Division Multiplexed Passive Optical Network • use multiple wavelengths, each serves a certain group of users • higher capacity

  28. Transmission • Multi-access • Channel add-drop • Channel routing/ switching Lightwave Networks Optical Networks

  29. Tunable transmitter and tunable receiver (TTTR) • most flexible, expensive • Fixed transmitter and tunable receiver (FTTR) • each node sends data on a fixed channel • receiver is tuned to receiving channel before data reception • have receiver contention problem • Tunable transmitter and fixed receiver (TTFR) • each node receives data on a fixed channel • transmitter is tuned to the receiving channel of the destination node before sending data T T T T R R R R Lightwave Networks • connection between two hosts via a channel  need to access channel • Channel: Wavelength (in WDM network), Time Slot (in TDM network) A B C D

  30. l1, l2, l3 l1, l2*, l3 Add-drop Multiplexer (ADM) l2 l2* DROP ADD l1 l1, ..., lN l1*, ..., lN lN l1* l1 Lightwave Networks Channel add-drop Wavelength ADM: ADD DROP

  31. l11, l12, l13, …, l1M l11, lN2, … , l3(N-1), l2N l21,l22, l23, …, l2M l21, l12, lN3, … , l3N l31,l32, l33, …, l3M l31, l22, l13, lN4, ... lN1, lN2, lN3, …, lNM lN1, … , l3(N-2), l2(N-1), l1N Lightwave Networks Static Optical Cross-Connect: Channel routing (fixed wavelength routing pattern)

  32. l1 #1 #1 l11 , l12 , ... , l1M l21 , l12 , ... , lNM l2 #2 #2 l21 , l22 , ... , l2M l11 , lN2 , ... , l2M lM #N #N lN1 , lN2 , ... , lNM lN1 , l22 , ... , l1M Routing control module Lightwave Networks Dynamic Optical Cross-Connect: Channel switching

  33. l1 with data l2 with data Wavelength Converter l2 no data (continuous-wave) l1 l1 l1 l1 l2 l-converter Lightwave Networks Wavelength Conversion Resolve output contention of same wavelength from different input fibers l1 , l2 output contention

  34. Lightwave Networks Common optical networks: SDH, SONET, FDDI • “All-Optical” Networks • reduce number of O/E and E/O interfaces • transparent to multiple signal format and bit rate  facilitates upgrade and compatible with most existing electronics • manage the enormous capacity on the information highway • provide direct photonic access, add-drop and routing of broadband full wavelength chunk of information

  35. Lightwave Networks

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