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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
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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
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Light Wave
amplitude
wavelengthposition/distance
• electromagnetic wave
• carry energy from one point to another
• travel in straight line
• described in wavelength (usually in m or nm)
• speed of light in vacuum = 3108 m/s
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Reflection and Refraction of LightReflection
Incident angle = reflected angle
Incident light
Reflected light
Reflecting surface
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
Medium 1
Medium 2
> > c
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Optical Fiber
claddingcore
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
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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
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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
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Fiber Attenuation
low loss wavelength ranges: 1.3mm (0.4-0.6 dB/km), 1.55mm (0.2-0.4 dB/km)
suitable for telecommunications
• optical power of a signal is reduced after passing through a piece of fiber
• wavelength-dependent
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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
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Fiber Dispersion
1.1 1.2 1.3 1.4 1.5 1.6 1.7
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10
0
-10
-20
Standard Dispersion-
flattened
Dispersion-shifted
Wavelength (m)
Dis
pers
ion
(ps/
(km
•nm
)) 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
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System Capacity
fiber attenuation loss in optical power limit transmission distance
fiber dispersion pulse broadening limit transmission bit-rate
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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)
Laser Source and Photodetector
Photodetector
• convert incoming photons into electric current (photo-current)
input electric current
output optical power
threshold current
optical power (photons)
photo-current
optical power (photons)
Input electrical data
wavelength
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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
A2 A1
A
C
BB2 B1
C2 C1
B1 A1C2 B2 A2 C1
time
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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
A
C
B
freq
fA
freqfB
freq
fC
freq
fA fB fC
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Multiplexing Schemes
Wavelength Division Multiplexing (WDM)
• 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
wavelength spacing: 0.8 nm (100-GHz)
A
C
B
wavelength
A B C
wavelength multiplexer
A
B
C
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Multiplexing Schemes
Hybrid Types (TDM/WDM, SCM/WDM) higher capacity
A
C
B
wavelength
A B C
wavelength multiplexer
f1 f2
f3
f1 f2
f3
f1 f2
f3
SCM/WDM
A
C
B
wavelength
A B C
wavelength multiplexer
TDM stream
TDM stream
TDM stream
TDM/WDM
A A
C
B
C
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Transmission System Capacity
132 Ch
1 Ch TDM
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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)
G
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Lightwave Systems
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
Traditional Optical Fiber Transmission System
E
MUX
XMTRREG
RPTR RCVRREG
RPTR
E
D MUX
Low-Rate Data In
Low-Rate Data Out
DET EQ DEC
TMG REC
LASERAMP AMP
Opto-Electronic Regenerative Repeater
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Lightwave Systems
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
Optical Fiber Transmission System
O
MUX
O
D MUX
Data In Data Out
OA OAOA
XMTR
XMTR
XMTR
1
2
N
RCVR
RCVR
RCVR
1
2
N
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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
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Undersea Fiber Systems
AT&T
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Undersea Fiber Systems
SYSTEM TIME BANDWIDTH/ NUMBER OF COMMENTS BIT-RATE BASIC CHANNELS
TAT-1/2 1955/59 0.2 MHz 48HAW-1 1957 COPPER COAXTAT-3/4 1963/65 ANALOGHAW-2 1964 1.1 MHz 140 VACUUM TUBESH-G-J 1964TAT-5 1970HAW-3 1974 6 MHz 840 Ge TRANSISTORSH-G-O 1975TAT-6/7 1976/83 30 MHz 4,200 Si TRANSISTORSTAT-8 1988 OPTICAL FIBERHAW-4 1989 280 Mb/s 8,000 DIGITALTPC-3 1989 = 1.3 mTAT-9 1991 16,000TPC-4 1992 560 Mb/s 24,000 = 1.55 m TAT-10/11 1992/93TAT-12 1995 5 Gb/s 122,880 OPTICAL AMPLIFIERSTPC-5 1995 = 1.55 m
TAT: Trans-Atlantic Telecommunications TPC: Trans-Pacific Cable
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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)
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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
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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)
HeadendHeadendelectrical repeater
Remote Node
Fiber Coaxial Cable
Passive Optical Network (PON)
passive optical splitter
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Lightwave Broadband Access
• WDM-PON: Wavelength Division Multiplexed Passive Optical Network
• use multiple wavelengths, each serves a certain group of users
• higher capacity
HeadendHeadendelectrical repeater
Remote Node
multi-wavelength source
1
2
N-1
N
1, … , N
WDM-PON
wavelength demultiplexer
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Lightwave Networks
Transmission
Multi-access
Channel add-drop
Channel routing/ switching
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• 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
Lightwave Networks
• connection between two hosts via a channel need to access channel
• Channel: Wavelength (in WDM network), Time Slot (in TDM network)
T R
T R T R
T R
A B
C D
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Lightwave Networks
Add-drop Multiplexer (ADM)
Add-drop Multiplexer (ADM)
1, 2, 3 1, 2*, 3
2 2*
ADDDROP
DROPADD
1
N
11*
1, ..., N 1*, ..., N
Wavelength ADM:
Channel add-drop
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Lightwave Networks
Static Optical Cross-Connect: Channel routing
11, 12, 13, …, 1M
21, 22, 23, …, 2M
31, 32, 33, …, 3M
N1, N2, N3, …, NM N1, … , 3(N-2), 2(N-1), 1N
31, 22, 13, N4, ...
21, 12, N3, … , 3N
11, N2, … , 3(N-1), 2N
(fixed wavelength routing pattern)
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#1
#N
#2
#1
#2
#N
Routing control module
11 , 2M
1 , 2M
1 , 2M
1 , 2M
1 , 2M
1 , 2M
1
2
M
Lightwave Networks
Dynamic Optical Cross-Connect: Channel switching
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Lightwave Networks
Wavelength Conversion
Wavelength Converter
Wavelength Converter
1 with data
2 no data
(continuous-wave)
2 with data
Resolve output contention of same wavelength from different input fibers
1
1
1
1-converter
2
1 , 2
output contention
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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
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Lightwave Networks