01 Optical Fundamentals

1© 2001, Cisco Systems, Inc. All rights reserved.© 2001, Cisco Systems, Inc. All rights reserved.© 2001, Cisco Systems, Inc. All rights reserved.

Optical Fundamentals

© 2001, Cisco Systems, Inc. All rights reserved. 2© 2001, Cisco Systems, Inc. All rights reserved. 2© 2001, Cisco Systems, Inc. All rights reserved. 2

Agenda

• Introduction

• Optical propagation in Fibers

• Attenuation & Dispersion

• Non Linearity

• SM Optical Fiber Types

• Summary


Modern Lightwave Eras

0.1

1

10

100

1000

10000

1985 1990 1995 2000

Year

Cap

acit

y (G

b/s

)

FiberizationDigitization

SONET rings and DWDM linear systems

Optical networkingWavelength Switching

Research Systems

Commercial Systems


• Decibels (dB): unit of level (relative measure) X dB is 10-X/10 in linear dimension e.g. 3 dB Attenuation = 10-.3 = 0.501

Standard logarithmic unit for the ratio of two quantities. In optical fibers, the ratio is power and represents loss or gain.

• Decibels-milliwatt (dBm) : Decibel referenced to a milliwatt X mW is 10log10(X) in dBm, Y dBm is 10Y/10 in mW. 0dBm=1mW, 17dBm = 50mW

• Wavelength (): length of a wave in a particular medium. Common unit: nanometers, 10-9m (nm)

300nm (blue) to 700nm (red) is visible. In fiber optics primarily use 850, 1310, & 1550nm

• Frequency (): the number of times that a wave is produced within a particular time period. Common unit: TeraHertz, 1012 cycles per second (Thz)

Wavelength x frequency = Speed of light x = C

Some terminology:


• Attenuation = Loss of power in dB/km The extent to which lighting intensity from the source is diminished as it passes through a given length of fiber-optic (FO) cable, tubing or light pipe. This specification determines how well a product transmits light and how much cable can be properly illuminated by a given light source.

• Chromatic Dispersion = Spread of light pulse in ps/nm-km

The separation of light into its different coloured rays.

• ITU Grid = Standard set of wavelengths to be used in Fibre Optic communications. Unit Ghz, e.g. 400Ghz, 200Ghz, 100Ghz

• Optical Signal to Noise Ration (OSNR) = Ratio of optical signal power to noise power for the receiver

• Lambda = Name of Greek Letter used as Wavelength symbol ()

• Optical Supervisory Channel (OSC) = Management channel

Some more terminology


dB versus dBm

• dBm used for output power and receive sensitivity (Absolute Value)

• dB used for power gain or loss (Relative Value)


ITU Wavelength Grid

• ITU-T grid is based on 191.7 THz + 100 GHz

• It is a standard for the lasers in DWDM systems

1530.33 nm 1553.86 nm

0.80 nm

195.9 THz 193.0 THz100 GHz

Freq (THz) ITU Ch Wave (nm) 15201/252 15216 15800 15540 15454192.90 29 1554.13 x x x x x192.85 1554.54192.80 28 1554.94 x x x x x192.75 1555.34192.70 27 1555.75 x x x x x192.65 1556.15192.60 26 1556.55 x x x x x


Bit Error Rate ( BER)

• BER is a key objective of the Optical System Design

• Goal is to get from Tx to Rx with a BER < BER threshold of the Rx

• BER thresholds are on Data sheets

• Typical minimum acceptable rate is 10 -12


Optical Budget

Optical Budget is affected by: Fiber attenuation

Splices

Patch Panels/Connectors

Optical components (filters, amplifiers, etc)

Bends in fiber

Contamination (dirt/oil on connectors)

Basic Optical Budget = Output Power – Input Sensitivity

Pout = +6 dBm R = -30 dBm

Budget = 36 dB


Power Budget with Power Penalties

Fiber Loss +

Splices +

Connectors +

Dispersion Penalties +

Fiber Nonlinearities Penalty +

Component Aging Penalties <

Power Budget = Launch Power – Receiver Sensitivity

Fiber Loss +

Splices +

Connectors +

Dispersion Penalties +

Fiber Nonlinearities Penalty +

Component Aging Penalties <

Power Budget = Launch Power – Receiver Sensitivity


Glass Purity

Propagation Distance Need to Reduce theTransmitted Light Power by 50% (3 dB)

Window Glass 1 inch (~3 cm)

Optical Quality Glass 10 feet (~3 m)

Fiber Optics 9 miles (~14 km)

Fiber Optics Requires Very High Purity Glass


AttenuationDispersion

Nonlinearity

Waveform After 1000 KmTransmitted Data Waveform

Distortion

It May Be a Digital Signal, but It’s Analog Transmission

Fiber Fundamentals


Agenda

• Introduction



• Non Linearity


• Summary


Attenuation: Reduces power level with distance

Dispersion and Nonlinearities: Erodes clarity with distance and speed

Signal detection and recovery is an analog problem

Analog Transmission Effects


CladdingCore

Coating

Fiber Geometry

• An optical fiber is made ofthree sections:

The core carries thelight signals

The cladding keeps the lightin the core

The coating protects the glass


• Fiber dimensions are measured in µm1 µm = 0.000001 meters (10-6)

1 human hair ~ 50 µm

• Refractive Index (n)n = c/v

n ~ 1.46

n (core) > n (cladding)

Cladding(125 µm)

Coating(245 µm)

Core(8–62.5 µm)

Fiber Dimensions


Snell’s Law1= 1r

n1Sin 1 = n2Sin 2

Geometrical Optics

• Light is reflected/refracted at an interface

1 = Angle of incidence

1r = Angle of reflection

2 = Angle of refraction

• Above critical=Sin-1(n2/n1), all light is totally internally reflected

r

n2

n1


n2

n1

Cladding

Core

Intensity Profile

Propagation in Fiber

• Light propagates by total internal reflectionsat the core-cladding interface

• Total internal reflections are lossless

• Each allowed ray is a mode


n2

n1

Cladding

Core

n2

n1

Cladding

Core

Different Types of Fiber

• Multimode fiberCore diameter varies

50 mm for step index

62.5 mm for graded index

Bit rate-distance product>500 MHz-km

• Single-mode fiberCore diameter is about 9 mm

Bit rate-distance product>100 THz-km


Agenda

• Introduction



• Non Linearity


• Summary


• Light

Ultraviolet (UV)

Visible

Infrared (IR)

• Communication wavelengths

850, 1310, 1550 nm

Low-loss wavelengths

• Specialty wavelengths

980, 1480, 1510, 1625 nm

UV IR

Visible

850 nm

980 nm1310 nm

1480 nm

1550 nm1625 nm

Wavelength: (nanometers)

Frequency: (terahertz)

C =x

Optical Spectrum


Optical Attenuation

• Specified in loss per kilometer (dB/km)

0.40 dB/km at 1310 nm

0.25 dB/km at 1550 nm

• Loss due to absorptionby impurities

1400 nm peak due to OH ions

• EDFA optical amplifiers available in 1550 window

1310Window

1550Window


T T

P i P0

Optical Attenuation

• Pulse amplitude reduction limits “how far”

• Attenuation in dB=10xLog(Pi/Po)

• Power is measured in dBm: P(dBm)=10xlog(P mW/1 mW)

ExamplesExamples

10dBm10dBm 10 mW10 mW

0 dBM0 dBM 1 mW1 mW

-3 dBm-3 dBm 500 uW500 uW

-10 dBm-10 dBm 100 uW100 uW

-30 dBm-30 dBm 1 uW1 uW

)


• Polarization Mode Dispersion (PMD) Single-mode fiber supports two polarization

states

Fast and slow axes have different group velocities

Causes spreading of the light pulse

• Chromatic Dispersion Different wavelengths travel at different speeds

Causes spreading of the light pulse

Types of Dispersion


Fiber Chromatic Dispersion Characteristics

Wavelength

Dis

per

sio

n p

s/n

m-k

m 20

01310 nm 1550nm

Normal Fiber (SMF-28 or Equivalent)Nondispersion Shifted Fiber (NDSF) >95% of Deployed Plant

Normal(ITU-T G.652)Dispersion Shifted Fiber (DSF) (ITU-T G.653)Nonzero Dispersion Shifted Fibers (NZDSF) (ITU-T G.655)


• Affects single channel and DWDM systems

• A pulse spreads as it travels down the fiber

• Inter-symbol Interference (ISI) leads to performance impairments

• Degradation depends on:

laser used (spectral width)

bit-rate (temporal pulse separation)

Different SM types

Interference

A Snapshot on Chromatic Dispersion


• The refractive index is wavelength dependent

• Different frequency-components of the optical pulses travel at different speeds (the blue is faster than red for anomalous dispersion where D > 0)

• As a result, we see pulse broadening and ISI

z

z

z

zTransmission Fiber

Fiber Chromatic Dispersion (CD)

BlueRed


60 Km SMF-28

4 Km SMF-28

10 Gbps

40 Gbps

Limitations From Chromatic Dispersion

t

t

• Dispersion causes pulse distortion, pulse "smearing" effects

• Higher bit-rates and shorter pulses are less robust to Chromatic Dispersion

• Limits "how fast“ and “how far”


Combating Chromatic Dispersion

• Use DSF and NZDSF fibers

(G.653 & G.655)

• Dispersion Compensating Fiber

• Transmitters with narrow spectral width


Dispersion Compensating Fiber

• Dispersion Compensating Fiber:

By joining fibers with CD of opposite signs (polarity) and suitable lengths an average dispersion close to zero can be obtained; the compensating fiber can be several kilometers and the reel can be inserted at any point in the link, at the receiver or at the transmitter


Dispersion Compensation

Transmitter

Dispersion Compensators

Dispersion Shifted Fiber Cable

+1000

-100-200-300-400-500

Cu

mu

lati

ve D

isp

ersi

on

(p

s/n

m)

Total Dispersion Controlled

Distance fromTransmitter (km)

No CompensationWith Compensation


How Far Can I Go Without Dispersion?

Distance (Km) =Specification of Transponder (ps/nm)

Coefficient of Dispersion of Fiber (ps/nm*km)

A laser signal with dispersion tolerance of 3400 ps/nm

is sent across a standard SMF fiber which has a Coefficient of Dispersion of 17 ps/nm*km.

It will reach 200 Km at maximum bandwidth.Note that lower speeds will travel farther.


Polarization Mode Dispersion

• Caused by ovality of core due to:

Manufacturing process

Internal stress (cabling)

External stress (trucks)

• Only discovered inthe 90s

• Most older fiber not characterized for PMD


Polarization Mode Dispersion (PMD)

• The optical pulse tends to broaden as it travels down the fiber; this is a much weaker phenomenon than chromatic dispersion and it is of some relevance at bit rates of 10Gb/s or more

nx

nyEx

Ey

Pulse As It Enters the Fiber Spreaded Pulse As It Leaves the Fiber


Combating Polarization Mode Dispersion

• Factors contributing to PMDBit RateFiber core symmetryEnvironmental factorsBends/stress in fiberImperfections in fiber

• Solutions for PMDImproved fibers RegenerationFollow manufacturer’s recommended installation techniques for the fiber cable


Agenda

• Introduction



• Non Linearity


• Summary


From Linear to Non Linear Propagation

• As long as optical power within an optical fiber is small, the fiber can be treated as a linear medium Loss and refractive index are independent of the

signal power

• When optical power levels gets fairly high, the fiber becomes a nonlinear mediumLoss and refractive index depend on the optical

power


Inte

nsi

ty

Time

Slow Phase Velocity

Fast PhaseVelocity

Optical Pulsen = n0 + N2

Index of Refraction

NonlinearCoefficient

Light Intensity

Optical Fiber’s Nonlinear Index

• Intensity of an optical pulse modulates the index of refraction

• Nonlinearity scales as (channel power)2


• A single channel’s pulses interact as they travel

Interference

Multiple channels interact as they travel

Interference

Effects of Nonlinearity


FWM

Raman

Types of Nonlinearities

• Nonlinear index

Four-wave mixing

Self-phase modulation

Cross-phase modulation

• Stimulated scattering

Raman

Brillouin


Out of Fiber

1 221-2 22-11 2

Into Fiber

Four-Wave Mixing

• Channels beat against each other to form intermodulation products

• Creates in-band crosstalk that can not be filtered (optically or electrically)


Wavelength (nm)

-5

-10

-15

-20

-25

-30

-35

-40

1542 1543 1544 1545 1546 1547 1548

Input Power = 3 mw/ch

Po

wer

(d

Bm

)

Output Spectrum after 25 km of Dispersion Shifted Fiber

FWM Example

• FWM effects increase geometrically with:

Number of channels

Spacing of channels

Optical power level


Channel Spacing (nm)

FW

M E

ffic

ien

cy (

dB

)

0.0 0.5 1.0 1.5 2.0 2.5

-50

-30

-10

0

-20

-40

D=0

D=17

D=2

D=0.2

Dispersion Washes Out FWM Effects

FWM and Dispersion


Agenda

• Introduction



• Non Linearity


• Summary


• SMF (standard, 1310 nm optimized, G.652)

Most widely deployed so far, introduced in 1986, cheapest

• DSF (Dispersion Shifted, G.653)

Intended for single channel operation at 1550 nm

• NZDSF (Non-Zero Dispersion Shifted, G.655)– LS

For WDM operation in the 1550 nm region only

– TrueWave, FreeLight, LEAF, TeraLight…

Latest generation fibers developed in mid 90’s

For better performance with high capacity DWDM systems

– MetroCor, WideLight…

– Low PMD ULH fibers

Types of Single-Mode Fiber


Fiber Dispersion Characteristics

Normal fiberNon-dispersion shifted fiber (NDSF) G.652 >90% of deployed plant

DSF G.653NZDSF G.655

-20

-15

-10

-5

0

5

10

15

20

25

1350 1370 1390 1410 1430 1450 1470 1490 1510 1530 1550 1570 1590 1610 1630 1650

DS NZDS+ NZDS- SMF

Dis

per

sio

n (

in p

s/n

m-

km)

Wavelength (in nm)


1530 1540 1550 1560

+2

+4

- 2

- 4 Corning LS

DSF

Dis

pers

ion

(ps/

nm -

km) Lucent

TrueWave / Balanced +

LucentTrueWave / Balanced -

Dispersion Shifted Fibers

Pirelli FreeLight

Corning LEAF

Dis

per

sio

n

Wavelength (in nm)

Alcatel TeraLight


Optimized for 1310 nm operation

Fiber ParameterFiber Parameter Expected ValueExpected Value

Attenuation at 1310/1550 nm (standard quality) 0.35 / 0.22 dB/km

Cut-Off Wavelength <1260 nm

Mode Field Diameter (@1310 nm) 9.2 m

Numerical Aperture 0.14

Zero Dispersion Wavelength 1313 nm

Zero Dispersion Slope <0.092 ps/(nm2-km)

PMD Link Value <0.1 ps/km.5

Core-to-Cladding Eccentricity <0.5 m

Designed for high bitrate TDMat 1310 nm

Standard Single-Mode Fiber (Corning SMF-28)


Optimized for 1550 nm operation


Attenuation at 1550 nm 0.25 dB/km


Mode Field Diameter (@ 1550 nm) 8.1 m


Zero Dispersion Wavelength 1550 nm

Zero Dispersion Slope <0.085 ps/(nm2-km)



Designed for high bitrate TDMat 1550 nm

60% less lossthan 1310 nm

Dispersion Shifted Fiber(Corning DSF)


Optimized for WDM in 1550 nm window




Mode Field Diameter (@1550 nm) 8.1 m


Total Dispersion -3.5 to -0.1 ps/(nm2-km)

Dispersion Range 1530-1560 nm



Designed for WDMin 1550 window

Non-Zero Dispersion Shifted Fiber(Corning SMF/LS)


Optimized for DWDM in C-Band & L-Band




Mode Field Diameter (@ 1550 nm) 9.6 m


Total Dispersion (1530-1565 nm) 2.0 to 6.0 ps/(nm2-km)

Total Dispersion (1565-1625 nm) 4.5 to 11.2 ps/(nm2-km)


Core-to-Cladding Eccentricity <0.5 mDesigned for DWDM

in C & L bands

Large Aeff for highpower operation

Next Generation Fiber(Corning LEAF)


Attenuation @ 1310nm (dB/km) <=0.34-0.39

Attenuation @ 1385nm (dB/km) <=0.31

Attenuation @ 1550nm (dB/km) <=0.19-0.23

Attenuation @ water peak 1383 +/- 3nm <=0.31 dB/km

Attenuation vs Wavelength: 1285-1330nm Max difference 0.05 db/km (Ref. 1310nm)

Attenuation vs Wavelength: 1525-1575nm Max difference 0.05 db/km (Ref. 1550nm)

Mode Field Diameter @1310nm 9.2 +/- 0.4μm

Mode Field Diameter @1550nm 10.5 +/- 1.0μm

Zero Dispersion Wavelength 1300nm<= D >= 1322nm

Dispersion slope at 1550nm S0 <= 0.092 ps/km*nm2

PMD Link Value (concatenated fibers) <= 0.1 ps/√km

PMD Individual Fiber <= 0.05 ps/√km (typical)

Optimized for CWDM Transmission

Low attenuation in traditional

Water Peak area

Extended Band Fibers

Sample products:

OFS (Formerly Lucent) Allwave

Corning SMF-28e

Alcatel 6901


The primary Difference is in the Chromatic Dispersion Characteristics

Different Solutions forDifferent Fiber Types

SMF

(G.652)

•Good for TDM at 1310 nm

•OK for TDM at 1550

•OK for DWDM (With Dispersion Mgmt)

DSF

(G.653)

•OK for TDM at 1310 nm


•Bad for DWDM (C-Band)

NZDSF

(G.655)



•Good for DWDM (C + L Bands)

Extended Band

(G.652.C)

(suppressed attenuation in the traditional water peak region)



•OK for DWDM (With Dispersion Mgmt

•Good for CWDM (>8 wavelengths)


Agenda

• Introduction



• Non Linearity


• Summary


The 3 “R”s of Optical Networking

A Light Pulse Propagating in a Fiber Experiences 3 Type of Degradations:

Loss of EnergyLoss of Energy

Loss of Timing (Jitter)(From Various Sources)Loss of Timing (Jitter)(From Various Sources) t

ts Optimum Sampling Time

tts Optimum Sampling Time

Phase Variation

Shape DistortionShape Distortion

Pulse as It Enters the Fiber Pulse as It Exits the Fiber


Re-ShapeRe-Shape DCUDCU

The 3 “R”s of Optical Networking (Cont.)The Options to Recover the Signal from Attenuation/Dispersion/Jitter Degradation Are:

Pulse as It Enters the Fiber Pulse as It Exits the Fiber

Re-Gen to Boost the PowerRe-Gen to Boost the Power



Phase Variation

Re-TimeRe-TimeO-E-O

Re-gen, Re-shape andRemove Optical Noise


Phase Re-Alignment*

*Simplification

57© 1999, Cisco Systems, Inc. F0_5585_c2

01 Optical Fundamentals

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