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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 nm)  suitable for long-distance transmission

3 Light Wave amplitude position/distance wavelength 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 Reflection and Refraction of Light
Medium 1 Medium 2  >   > c     Incident light Reflected light Reflecting surface 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 Optical Fiber cladding core light beam
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 ( dB/km), 1.55mm ( 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 Fiber Dispersion Standard Dispersion-flattened Dispersion-shifted 20
20 10 -10 -20 Standard Dispersion-flattened Dispersion-shifted Wavelength (mm) Dispersion (ps/(km•nm)) Typical values: standard fiber: ~ 0 ps/(km• nm ~17 ps /(km• nm dispersion-shifted fiber: ~0.5 ps /(km• nm

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

12 Laser Source and Photodetector
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) optical power (photons) Input electrical data input electric current output optical power threshold current wavelength l Photodetector convert incoming photons into electric current (photo-current) optical power (photons) photo-current

13 Time Division Multiplexing (TDM)
Multiplexing Schemes Multiplexing: transmits information for several connections simultaneously on the same optical fiber Time Division Multiplexing (TDM) A2 A1 A C B B2 B1 C2 C1 l time 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 Subcarrier Multiplexing (SCM)
Multiplexing Schemes Subcarrier Multiplexing (SCM) l A C B freq fA fB fC fA fB fC multiple frequency carriers (subcarriers) are combined together only require one wavelength (one laser) (optical carrier) suitable for video distribution on fiber

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

17 Transmission System Capacity
132 Ch 1 Ch TDM

18 Optical Amplifier G 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 ( 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 ( nm), C-band ( nm), L-band ( nm)

19 Traditional Optical Fiber Transmission System
Lightwave Systems Traditional Optical Fiber Transmission System E MUX XMTR REG RPTR RCVR D MUX Low-Rate Data In Low-Rate Data Out DET EQ DEC TMG REC LASER AMP Opto-Electronic Regenerative Repeater 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 Optical Fiber Transmission System
Lightwave Systems Optical Fiber Transmission System O MUX D MUX Data In Data Out OA XMTR l1 l2 lN RCVR Multi-channel WDM operation Transparent data-rate and modulation form One optical amplifier (per fiber) supports many channels 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

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

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 Lightwave Broadband Access
Headend electrical repeater Remote Node Fiber Coaxial Cable Passive Optical Network (PON) passive optical splitter 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 Lightwave Broadband Access
Headend electrical repeater Remote Node multi-wavelength source l1 l2 lN-1 lN l1, … , lN WDM-PON wavelength demultiplexer WDM-PON: Wavelength Division Multiplexed Passive Optical Network use multiple wavelengths, each serves a certain group of users higher capacity

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

29 Lightwave Networks A B C D
connection between two hosts via a channel  need to access channel Channel: Wavelength (in WDM network), Time Slot (in TDM network) 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 A B T R T R C D T R T R

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

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

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

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

34 Common optical networks: SDH, SONET, FDDI
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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