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
AT&T

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