optical technologies and lightwave networks

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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: 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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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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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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Hybrid Types (TDM/WDM, SCM/WDM) higher capacity

A

C

B

wavelength

A B C


f1 f2

f3

f1 f2

f3

f1 f2

f3

SCM/WDM

A

C

B

wavelength

A B C


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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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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AT&T

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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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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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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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• 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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• 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

optical technologies and lightwave networks

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