Optical Communication Systems

zdraer@mrv.com

Introduction to DWDM

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What is Fiber Optics?

Transmission of communication signals in the form of light over thin glass or plastic (fiber).

Pulses of infrared light guided through glass fiber move huge blocks of data over long or short distances

Fiber Structure Propagation of Light in a Fiber

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Options for Increasing the Bandwidth

More Fibers

Faster Electronics(TDM*)

WDM*

*TDM and WDM increase the effective capacity of the existing fiber

Increasing the number of wavelengthsSame fiber and bit rate, more wavelengths

Installing new fibersSame bit rate, more fibersVery Expensive

Increasing the bit rateHigher bit rate, same fiber

Expensive and Complex Electronics

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Time Division Multiplexing

Definition: TDM is a type of multiplexing that transmits multiple signals simultaneously over a single transmission path by assigning each stream a different time slot

Synchronous Digital Hierarchy (SDH)

A standard for optical transport of TDM data

STM-1

STM-4

STM-16

155 Mbps

622 Mbps

2.5 Gbps

STM-64

STM-256

10 Gbps

40 Gbps

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TDM Limitations at Higher Bit Rate

Expensive and Complex Electronics

Complex Modulation

SNR Decreases

Very High Dispersion*

* Transmission at 40 Gbps

(STM-256) over single-mode (SM) fiber is 16 times more affected by dispersion than the transmission at 10 Gbps

(STM-64).

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Wavelength Division Multiplexing (WDM)

Definition: Multiplexing several optical signals having different wavelengths and transmitting simultaneously over a single fiber is known as wavelength division multiplexing.(WDM increases the carrying capacity of the physical medium (fiber) using a completely different method from TDM)

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TDM and WDM Comparison

►

Single wavelength per fiber►

Multiple channels per fiber►

E/O or O/E/O Conversion►

Common signal format►

Takes sync and async

signals and multiplexes them to a single higher optical bit rate

Time Division Multiplexing (TDM)

Wavelength Division Multiplexing (WDM)

►

Multiple wavelength per fiber (2, 4, 16, 64, etc.)►

Multiple channels per fiber►

No O/E Conversion►

Can carry multiple protocols►

Takes multiple optical signals and multiplexes them in to a single fiber

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WDM History

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WDM Evolution

Faster (Higher speed per channel)

Thicker (More channels)► 160 channels possible today

Longer (Link length before regeneration)► A few thousand km possible today

160 channels at 10 Gbps = 1.6 Tbps► 25 million simultaneous phone calls

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Electromagnetic Spectrum

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Fiber Performance

Definition: Loss of signal power in a transmission

Attenuation

DispersionDefinition: Broadening of the pulses as they travel along the fiber over long

distances

The strength of a signal traveling through an optical fiber wakens with distance

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Fiber Attenuation Vs. Wavelength

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Dispersion Slope for Different Fibers

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Channel (Wavelength) Spacing

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The ITU-T Wavelength Grid

The ITU draft standard

G.692 defines point-to-point

WEM systems based on

100-GHz wavelength spacing

with a center wavelength of

1553.52 nm

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Optical Networking – The DWDM

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DWDM Components - Transponder

High performance telecommunication laser

Long-haul links & DWDM systems

Key characteristics► Mostly around 1550 nm

► Total power 3 to 50 mw

► Spectral width 10 to 100 MHz (0.08 to 0.8 pm)

► Small NA (good coupling into fiber)

DWDM Laser – Distributed Feedback (DFB)

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DWDM Components – Multiplexer & Demultiplexer

DWDM Multiplexer DWDM Demultiplexer

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DWDM Components – Optical Add/Drop Multiplexer (OADM)

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DWDM Components – Optical Amplifier (EDFA)

EDFA – Erbium Doped Fiber Amplifier

EDFA Construction

Simple device consisting of four parts►

Single wavelength per fiber►

Multiple channels per fiber►

E/O or O/E/O Conversion►

Common signal format

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EDFA - Basics

Stimulated Emission When light hits the excited state electrons, their orbits decay from higher to the lower energy state and the atom releases the light from the source and the additional light that had been absorbed by the atom.► Same wavelength, direction, and phase

Spontaneous EmissionWhen an electron decays from a higher energy state to a lower energy state, the atom spontaneously emits the released light.

AbsorptionWhen light hits the atom, the electrons in the lower energy state absorb the energy and jump into higher energy state.

Absorption

Stimulated Emission

Spontaneous Emission

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EDFA - Operation

Dope a fiber with Erbium

Pump energy into the fiber.

Transmit and amplify the signal

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EDFA

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EDFA – Input and Output Spectrum

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DWDM with EDFA

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EDFA Design Issues

The Main Parameters in the Design of an EDFA► Fiber glass material► Characteristics of the fiber► Erbium concentration profile► Erbium fiber length► Pump sources► Passive or active components such as couplers, isolators

Primary Design Goals► High gain► High output power► Low noise figure► Flatness of the gain spectrum► Reliability

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EDFA Applications

Booster AmplifierEDFA is located with the transmitter and is used to boost the transmitter signal to a high level in order to drive a long fiber.

In-Line AmplifierIn an in-line amplifier configuration, the EDFA is used to amplify the weakening signal for further transmission down the line

Pre AmplifierThe pre-amplifier application is similar to in-line application. However, the EDFA is typically located with the receiver to amplify the signal just prior to its reception.

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IP over DWDM

►

1 GbE

–

1GBase-SX and 1 GBase-LX-

DWDM implements up to 120 km, cascaded EDFAs

extend the reach to 1600+ km

►

10 GbE

–

10GBase-SR, 10 GBase-LR, and 10GBase-LX4-

DWDM implements up to 80 km, cascaded EDFAs

extend the reach to 1000+ km

WDM-Based

Map Ethernet Directly to a Wavelength

Increases the fiber capacity

Unidirectional and bi-directional wavelengths

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Protocol Transparency

SDH/SONETATM

IPFast Ethernet

Gigabit EthernetFibre

ChannelFDDI

ESCON

SDH/SONETATM

IPFast Ethernet

Gigabit EthernetFibre

ChannelFDDI

ESCON

Data rate and format adaptation without reconfiguration

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Network Management System

Functions of Network Management► Configuration Management

► Fault Management

► Performance Management

► Accounting Management

►Security Management

Network management is an essential element of communication systems since it is responsible for ensuring the efficient, secure, and continuous functioning of any network.

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NMS – Functional Architecture

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DWDM Benefits

Capacity Increase►

Large aggregate transmission capacity

Upgradability ►

Customer growth without requiring additional fiber to be laid

Flexibility►

Optical Add/Drop Multiplexing (OADM)►

Optical Cross Connect (OCC)

Scalability►

The possibility to addnew

nodes to the network

Network Transparency►

Independence of data rate, format and protocols

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DWDM - Summary

DWDM provides enormous amounts of scalable transmission capacity

DWDM technology gives us the ability to expand fiber network rapidly tomeet growing demands of customer

The DWDM systems provide transparency to various bit rates and protocols

Utilizes the existing thin fiber

DWDM improves signal transmission

DWDM allows flexible add/drop of channels (OADMs)

Bi-directional communication using a single fiber can be achieved by the use of twodifferent wavelengths, one for each direction

Transmission over the longest possible distance with smallest number of opticalamplifiers

IP over DWDM

Optical Communication Systems

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