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Fiber Optic Communications Systems

From the movieWarriors of the Net

Optical Time Division Multiplexing

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Multiplexing

• Frequency-division multiplexing (FDM) for electrical signals

• Code-division multiplexing (CDM) for electrical signals

• Wavelength-division multiplexing (WDM) for optical signals

• Time-division multiplexing (TDM) for both types

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Modulation

• Amplitude modulation

• Frequency modulation

• Phase modulation

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Simple to implement

• amplitude modulation

• pulse amplitude modulation

• amplitude shift keying

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Economize bandwidth

• quadrature amplitude modulation

• single sideband modulation

• vestigial sideband modulation

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Noise resistant

• phase modulation

• frequency modulation

• pulse position modulation

• pulse code modulation (widely used)

• amplitude shift keying

• frequency shift keying

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OTDM Bit Interleaved Multiplexer

Wavelength Division Multiplexing (WDM)

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Overview

• Wave Division Mutiplexing (WDM) multiplexes multiple optical carrier signals on a single optical fiber by using different wavelengths (colors) of laser light to carry different signals. – Bit rate and protocol independent

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Overview

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Single Pair of Fibers

Single Pair of Fibers

Single Pair of Fibers

Single Pair of Fibers

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

WDM MUX WDM MUX

Single Pair of Fibers

Traditional Digital Fiber Optic Transport

Digital Fiber Optic Transport using WDM

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Overview

• Two main types of WDM:– Coarse Wavelength Division Multiplexing (CWDM)– Dense Wavelength Division Multiplexing (DWDM)

Feature CWDM DWDM

Wavelengths per fiber 8 – 16 40 – 80

Wavelength spacing 2500GHz (20nm) 100 GHz (0.8nm)

Wavelength capacity Up to 2.5 Gbps Up to 10 Gbps

Aggregate fiber capacity 20 – 40 Gbps 100 – 1000 Gbps

Overall cost Low Medium

Applications Enterprise, metro-access Access, metro-core, regional

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Overview

• Fiber Characteristics

1550Window

1310Window

O - band E - band S - band C - band L - band

1280 1320 1360 1400 1440 1480 1520 1560 16001552Wavelength

(nm) CWDM Window

DWDM Window

C Band Range : 1530nm – 1560nmL Band Range : 1570nm – 1600nm

Water Peak

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OverviewWDM Components

Optical Multiplexer

Optical De-multiplexer

Optical Add/Drop Multiplexer(OADM)

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3

1

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3

1

2

3

1...n

1...n

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1

2

3

4

5

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7

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Multiplexer Demultiplexer

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Add/DropMultiplexer

λ1 λ2 λ3 λ4 λ5 λ6 λ7 λ8 λ1 λ2 λ3 λ4 λ5 λ6 λ7 λ8

Transmission Fiber

WDM Block Diagram

Transmitters Receivers

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Components: Transmitters

• Each transmitter emits a signal at different wavelengths

• Laser source

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Components: Multiplexer

• All different signals from transmitters are combined into one signal

• The signal consists of several wavelengths, each containing a different signal

• Gratings, prisms and thin films

Multiplexerλ1 λ2 λ3 λ4 λ5 λ6 λ7 λ8

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Gratings

• Simultaneously diffract all wavelengths

• Simple device

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Classical Grating

• Reflects light at an angle related to its wavelength

• Diffraction angle is dependent on the groove spacing and incident angle

Reflective Grating

λ1+λ2

λ1

λ2

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Components: Add/Drop Multiplexer

• Signal at specific wavelength can be extracted• Different signal at same wavelength can then be inserted• Fiber Bragg Grating

Add/DropMultiplexer

Transmission Fiber

λ4

λ1 λ2 λ3 λ4 λ5 λ6 λ7 λ8 λ1 λ2 λ3 λ4’ λ5 λ6 λ7 λ8

λ4’

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Components: Demultiplexer

• Decouples different wavelengths to get different signals

Demultiplexerλ1 λ2 λ3 λ4 λ5 λ6 λ7 λ8

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Components: Receivers

• Optical detector• Receives a specific signal at a

specific wavelength

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OverviewSub-Lambda Multiplexing

• Multiplexing a single low rate data connection onto a wavelength is very inefficient.– For example, if a FastEthernet connection is transported on a

WDM wavelength, only 4% of the bandwidth is used. This assumes the wavelength is capable of transporting 2.5 Gbps.

• To better utilize bandwidth many vendors support sub-lambda multiplexing. – The cards that support this are often referred to as DataMux or

Muxponder cards.– Muxponder cards have multiple ports (FE, GE, etc.). These

cards multiplex the data flows and transports them onto a wavelength.

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Benefits of WDM

• WDM technology allows multiple connections over one fiber thus reducing fiber plant requirement. – This is mainly beneficial for long-haul applications.– Campus applications require a cost benefit analysis.

• WDM technology can also provide fiber redundancy.

• WDM provides a managed fiber service.