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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. Need for Optical Technologies.

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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


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


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


 > 

 > 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


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


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


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


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


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


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


System Capacity

  • fiber attenuation  loss in optical power limit transmission distance

  • fiber dispersion  pulse broadening  limit transmission bit-rate


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)


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


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


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


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


132 Ch

1 Ch TDM

Transmission System Capacity


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)


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


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


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



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


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)


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


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)


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


  • Transmission

  • Multi-access

  • Channel add-drop

  • Channel routing/ switching

Lightwave Networks

Optical Networks


  • 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


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


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)


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


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


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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