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1 2006, Cisco Systems, Inc. All righ ts reserved. Fiber in DWDM Networks

2006 Cisco Systems, Inc. All rights reserved. DWDM Elements 1

Module 3: Fiber in DWDM Networks

CA E-Service Training

Instructor: Lito Pamintuan, Technical Program Manager for Cisco Systems.

This module, Fiber in DWDM Networks, is the last of the three modules on the subject ofDWDM.

Revision 1.0


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Module Objectives

Identify:

Causes, effects, and characteristics of chromatic dispersion

and polarization mode dispersion Types of non-linear effects and means of resolving them

The fiber type that is suitable for DWDM applications


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Agenda

Chromatic Dispersion and Polarization Mode

Dispersion (PMD)Fiber Non-Linear Effects

Self Phase Modulation (SPM)

Cross Phase Modulation (XPM)

Four Wave Mixing (FWM)

Fiber Types and Characteristics

In this module, we will cover the key DWDM impairments for single mode fiber:

Chromatic dispersion

Polarization mode dispersion (PMD)

Fiber non-linear effects such as:

Self-phase modulation (SPM),

Cross-phase modulation (XPM)

Four wave mixing (FWM)

We will end this module by talking about fiber types and fiber characteristics thatcompensate for these impairments.


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Chromatic Dispersion

Caused by different colors of light traveling at different speeds

Results in pulse distortion due to spectral broadening anddispersion power penalty (1-2dB)

Dispersion can be managed with Dispersion Compensation Units(DCUs) to reduce distortion .

In chromatic dispersion, wavelengths travel at different speeds, and light pulses arebroadened. This results in a power penalty between 1 to 2 dB and pulse distortion due tospectral broadening, as shown in the diagram. The slide demonstrates pulses broadeningwhile traveling through fiber. Eventually they will become one pulse, which will causemisinformation at the receiving end.

Chromatic dispersion can be managed with the use of dispersion compensation units to

reduce distortion.


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Polarization Mode Dispersion (PMD)

Different polarization states of a wavelength travel at differentspeeds, causing the pulses to widen at the other end of f iber

Polarization mode dispersion compensation techniques areemerging

n1

n2

n1 > n2

In PMD, different polarization states of a wavelength travel at different speeds. This causesthe light pulses, shown at the lower diagram, to widen while traveling along the fiber.

When two pulses widen and become one pulse, it can cause the receiving end tomisinterpret the information. This problem is more prevalent at higher bit rates because thepulses are narrower. Older fiber is more prone to PMD.

PMD compensation techniques are emerging as faster data rates are being developed.


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Agenda







When many wavelengths are packed in a single fiber for transmission, the light powerincreases, causing non-linear effects such as:

Self-phase modulation (SPM)

Cross-phase modulation (XPM)

Four wave mixing (FWM)


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Self-Phase and Cross-Phase Modulation


An in tense opt ical s ignal modulates the f ibers refract ive index , andtherefore its own phase

Can be mitigated with localized dispersion compensation


For WDM systems, the adjacent channels modu late the fibersrefractive index, and therefore each others phase

Worsens as channels get closer

Can be mitigated with localized dispersion compensation

High light intensities change the refractive index and lead to:

An induced phase-shift of the traveling wave defined as self-phase modulation, or

A co-propagating wave defined as cross-phase modulation

Both self-phase modulation and cross-phase modulation can be mitigated with localizeddispersion compensation.


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Power(a.u.)

Frequencyf1 f2 f3

f f

f113

f112f123

f223

f132

f221

f332f231

f331

In four wave mixing, high light intensities at different wavelengths interact and generatesignals at other wavelengths, causing the original signals to deteriorate.

The different wavelengths generated by four wave mixing are shown in the slide as blackarrows. Four wave mixing is at its worst when signals are located at the zero-dispersionpoint.

This can be mitigated by introducing a controlled amount of dispersion into the fiber.


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Agenda




Cross Phase Modulation (XMP)




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Fiber Type In DWDM

Dispersion-shi fted f iber (DSF) near the 1550nm window

Corning LEAF and Truewave fibers are referred to as Non-zero dispersion shif ted fibers (NZ-DSF)

Fiber Type Loss (dB/km)

Lambda

Zero

Dispersion

(ps/nm*km)

PMD

(ps/km1/2)

SMF-28


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Chromatic Dispersion ofDifferent Fiber Types

Dispersion

nm13101520

1497 1550

SMF-28

TW RS

LEAF

DS

SMF-28 Truewave RS

Corning LEAF

DSF

This graph shows the associated dispersion of the different fiber types.

SMF-28 at 1310nm zero-dispersion point

Truewave RS at 1497nm

Corning LEAF at 1520nm

Dispersion-shifted fiber (DSF) at 1550nm region


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Choosing the Right Fiber for WDM andDWDM Applications

SMF-28

Good for TDM at 1310nm

Bad for TDM at 1550nm

OK for WDM at 1550nm

Dispersion Shifted Fiber (DSF)

Good for TDM at 1550nm

Bad for WDM at 1550nm

NZ-DSF

Good for TDM and DWDM at 1550nm

The difference is in the dispersion characteristics

If you are planning to engineer your network using WDM or DWDM applications, choosethe proper fiber types:

SMF-28 or

NZ-DSF (Corning LEAF or Lucent Truwave RS)

Dispersion shifted fiber (DSF) does not work well with WDM and DWDM applications

although it does work well with single channel wavelength applications like TDM.


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