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Revision 1.1October 14, 2002

LW Technology (Cover, Appendix).PPT - 1© Copyright 1999, Agilent Technologies

Elements of Lightwave Technology© Copyright 1999 Agilent Technologies

Agilent Customer Training Seminar



Table of Content1. Introduction2. Physical Basics3. Standards4. Fibers, Cables, Splices & Connectors5. Passive Components6. Transmitters & Receivers7. Optical Amplifiers8. Dense Wavelength-Division Multiplexing



Lightwave Test Literature

Agilent employees have published many white papers, product notes, and application notes discussing mostlightwave measurements.

See handouts for a list of literature references.



Thank You For Choosing

Agilent Technologies

As Your Partner In

Lightwave & High Speed Digital

Transmission Test



Introduction

LW Technology



What is lightwave technology?

• Lightwave technology uses light as the primary medium to carry information.

• The light often is guided through optical fibers (fiberoptic technology).

• Most applications use invisible (infrared) light.

(HP)



Why lightwave technology?

• Most cost-effective way to move huge amounts of information (voice, data) quickly and reliably.

• Light is insensitive to electrical interference.

• Fiberoptic cables have less weight and consume less space than equivalent electrical links.

(HP)



Use Of Lightwave Technology

• Majority applications:– Telephone networks– Data communication systems– Cable TV distribution

• Niche applications:– Optical sensors– Medical equipment– Displays & signs



Telephone Networks

• Long distance telecommunication– up to 600 km repeater spans,

up to 9000 km total link length– Most demanding, most expensive– Keywords: submarine, longhaul

• Access network (1 km - 20 km)– Cost driven, less competition– Keywords: local exchange, regional

interexchange, MAN, FTTC, FTTH



Other Networks

• Data communication (1 m - 500 m)– As cheap as it can get– Keywords: premises network, LAN,

backbone, FDDI, Gigabit-Ethernet, Fibre Channel

• Cable TV (urban distribution)– Analog network– Keywords: head end, star coupler,

subcarrier

HP Journal 12/97



Telecommunication Network Bandwidth Trend

1995 2000 2005 2010 Year

10

20

30

40

50

RelativeLoad

1990

Total: 35%/year

Voice: 10%/yearSource:



Basic Link Design

Transmitter Connector Cable

ReceiverCableSplice



Typical Long-haul System

TerminalEquipment

AmplifierUnit

RegeneratorUnit

TerminalEquipment

AmplifierUnit

AmplifierUnit

Amplifier spans: 30 to 120 kmRegenerator spans: 50 to 600 kmTerminal spans: up to 600 km (without regenerators)

up to 9000 km (with regenerators)

Two pairs of single-mode fiber



Typical Regenerator Unit

Pulse re-shaping & re-timing

PowerSupply

Telemetry &Remote Control

Modulation & bit rate dependent!



Typical Amplifier Unit

Optical Amplifiers

PowerSupply


Modulation & bit rate independent!



Data Communication Trends

SCSI/USB/PCI@66 Mbps

IEEE1394FireWire@400 Mbps

[email protected] Gbps


POLO@10 Gbps

1994 1998 2000 20021996

FastEthernet@100 Mbps



GigaBitEthernet@10 Gbps



Data Communication Buzzwords• Wide Area Network (WAN)

– Nationwide or global data network – Often provided or operated by multiple long-distance

service providers• Metropolitan Area network (MAN)

– Regional or local data network – Often owned by a local service provider

• Local Area Network (LAN)– Private computer network– Often shielded from the outside by firewalls

• Dial-Up Network– Connects a PC via modem & telephone to a data network



Company Types• Component Manufacturers

– Lasers/LEDs, photodetectors, couplers, multiplexers, isolators, fibers, connectors

• Subsystem Manufacturers– Transmitters, receivers, amplifiers

(EDFA), repeaters• System Manufacturers

– Point-to-point, SONET/SDH, WDM• Installers & Service Providers

– Link signature, fault location

Port 1

Port 2

Port 3

Port 4

COMMON

DWDM



Review Questions

1. What advantages does the lightwave technology offer?

2. Who is using fiberoptics extensively?

3. What modulation (analog or digital) is used in the telephone network?


LW Technology (Cover, Appendix).PPT -20© Copyright 1999, Agilent Technologies

Physical Basics

LW Technology



The Carrier - Light

RaysWavesParticles

AbsorptionEmission

Interference RefractionReflection

Bandgap

Conduction band

Valence band

n0

n1

n0



Light Properties - Wavelength

λλλλ

Distance

FieldStrength

1000 pm (picometer) = 1 nm (nanometer) 1000 µm = 1 mm (millimeter) 1000 nm (nanometer) = 1 µµµµm (micrometer) 1000 mm = 1 m (meter) (~40 inches)

Wavelength λλλλ: distance to complete one sine wave



Electromagnetic Spectrum

Frequency

SonicUltrasonic

AM Broadcast

Shortwave Radio

FM Radio/TVRadar

Infrared Light

Visible LightUltraviolet

X-Rays

Wavelength 1 Mm 1 km 1 m 1 mm 1 pm1 nm

1 kHz 1 MHz 1 GHz 1 THz 1 ZHz1 YHz

c = f • λλλλ • nc: Speed of light ( 2.9979 m/µs ) f: Frequencyλ: Wavelengthn: Refractive index

(vacuum: 1.0000; standard air: 1.0003; silica fiber: 1.44 to 1.48)



LW Transmission Bands

Near InfraredFrequency

Wavelength1.6

229

1.0 0.8 µm0.6 0.41.8 1.4

UV

(vacuum) 1.2

THz193 461

0.2

353

Longhaul Telecom

Regional Telecom

Local Area Networks850 nm

1550 nm

1310 nmCD Players780 nm

HeNe Lasers633 nm



Optical Power

• Power (P):– Transmitter: typ. -6 to +17 dBm (0.25 to 50 mW)– Receiver: typ. -3 to -35 dBm (500 down to 0.3 µW) – Optical Amplifier: typ. +3 to +20 dBm (2 to 100 mW)

• Laser safety – International standard: IEC 825-1– United States (FDA): 21 CFR 1040.10 – Both standards consider class I safe under reasonable forseeable

conditions of operation (e.g., without using optical instruments, such as lenses or microscopes)



Laser Power Limits Of Class I(for test equipment applications)

IEC 825-1 (EN 60825-1)

WavelengthFiber / NA Limit

850 nm MM / 0.15 0.44 mW

1200 to MM / 0.15 8.9 mW1400 nm SM / 0.10 8.9 mW

1400 to SM / 0.10 10 mW4000 nm

21 CFR 1040.10

WavelengthFiber / NA Limit

850 nm MM / 0.15 2.8 mW

1060 to MM / 0.15 4.9 mW1400 nm SM / 0.10 1.9 mW

1400 to SM / 0.10 7.842500 nm

(1984) (11/1993)



The Logarithmic Scale

0 dBm = 1 mW

3 dBm = 2 mW5 dBm = 3 mW10 dBm = 10 mW20 dBm = 100 mW

-3 dBm = 0.5 mW-10 dBm = 100 µµµµW-30 dBm = 1 µµµµW-60 dBm = 1 nW

0 dB = 1

+ 0.1 dB = 1.023 (+2.3%)+ 3 dB = 2+ 5 dB = 3+ 10 dB = 10

-3 dB = 0.5-10 dB = 0.1-20 dB = 0.01-30 dB = 0.001

dB = 10 • log10 (P1 / P0) dBm = 10 • log10 (P / 1 mW)



Coherence

• Coherent lightPhotons have fixed phase relationship (laser light)

• Incoherent lightPhotons with random phase(sun, light bulb)

• Coherence length (CL)Average distance over which photons lose their phase relationship

1/e

1

CL



Interference

• Incoherent light adds up optical power

• Coherent light adds electromagnetic fields

• Zero phase shift:constructive interference

• 180º phase shift:destructive interference

+ =

+ =



Reflections

• Reflections: root cause for many problemsReturn loss definition:

RL = 10 * log

Pr

Pi

P reflected

P incident



Polarization

y

x

z

SOP: linearhorizontal

SOP: linearvertical• Most lasers are highly polarized

• Degree of polarization (DOP):DOP = P polarized / P total

• State of polarization (SOP):describes the orientationand rotation of thepolarized light



Poincaré SphereGraphical representation of state of polarization using Stokes parameters (S1, S2, S3)

Left-hand circular polarization (0,0,-1)

S 1axis

S2 axis

S 3ax

is

45 degree linearpolarization (0,1,0)

Right-hand circular polarization (0,0,1)

Vertical linearpolarization (-1,0,0)



Digital Modulation

• Digital Modulation:– Extinction ratio = P1 / P0– Time-division multiplexing (TDM)– ~1.5 Mb/s to 10 Gb/s

• Bit Error Rate (BER):– BER = N incorrect / N total

– Standards: 1E-9 to 1E-12– Lightwave systems: down to 1E-15

2Channel

4 131

P0

t

P1

0



Analog Modulation

• AM modulation around Pavg– Mostly for video signals– Modulation index ~ 2%– Frequency-domain multiplexing– 50 to 500 MHz

0

Pavg

tChannel 1

Channel 2

…

Channel N

RFΣΣΣΣ Analog Laser

Transmitter

RF



Review Questions

1. What are the three key parameters of light?

2. How much power is +13 dBm? -27 dBm?How much loss is 6 dB? 15 dB?

3. What is TDM?

4. Where on the Poincaré sphere is the horizontal linear polarization state?



Standards

LW Technology



Lightwave Standards Evolution

Basics - Measurement of power and wavelength

Point-to-point custom solutions

Agreement on parameter characteristics

Multi-vendor market emerges

Interoperability - still elusive



Network Model

FUTUREFORMAT

ATM /SONET

LEGACYSWITCH

ATM /SONET

FUTUREFORMAT

ATM /SONET

LEGACYSWITCH

ATM /SONET

X-C

X-C

X-CRing

R

DATA MULTIMEDIA

VIDEO

IMAGEVOICE

LAN

OPTICALACCESS

WDM NETWORK ELEMENTS

R

X-C WDM X-Connect

WDM Routing Star

WDM Add/Drop Mux

X-C X-C

X-CX-C

Con

figur

able

Opt

ical

, WD

MLa

yers

Elec

tron

icSw

itchi

ngLa

yers

Appl

icat

ions

Laye

r

Local ExchangeNetwork

Long DistanceNetwork

Private Network(with Optical Access)(with Optical Access)



Key Standards• Telecom Standards

– Plesiochronous Digital Hierarchy (PDH)– Synchronous Optical Network (SONET) /

Synchronous Digital Hierarchy (SDH)– Asynchronous Transfer Mode (ATM)– Dense Wavelength-Division Multiplexing (DWDM)

• Datacom Standards– Ethernet, Fast Ethernet (coax or twisted air cable)– Gigabit-Ethernet (IEEE 802.3z)– Fiber Distributed Data Interface (FDDI) – Fibre Channel (FC-PH)– Internet Protocol (IP)



PDH Networks• Developed in the early 1970’s

– Still many systems in place, especially for low speed traffic• Multiplexes digital voice circuits (64 kb/s)

– North America: DS1 (1.5 Mb/s) to DS4 (139 Mb/s)Europe: E1 (2 Mb/s) to E4 (139 Mb/s)Japan: 2 to 98 Mb/s

• Drawbacks– Not perfectly synchronized: extra bits needed – Difficult to add/drop low speed stream from high-speed stream– No standard on line interfaces & coding (interoperability!)– Seconds to minutes to restoration time after a failure



SONET / SDH• THE standard for new telecom networks:

– North America: SONET version– International: SDH version– Optimized for voice traffic– Virtual container technology can carry many different

traffic types & speeds• Definitions include:

– Optical requirements– Modulation and BER– Functional layer (e.g., frames)– Protection and restoration– Network management



Typical Ring Structures

• Two pairs of fibers between nodes– One fiber for each direction between nodes– One restoration fiber for each direction

• Network cut (single fault)– Traffic rerouted in opposite direction– Restoration within 0.5 sec– 100% protection!

• Nodes types– Add/drop multiplexers (ADM)– Digital cross-connects (DTE)



DWDM Standards

• ITU Draft Recommendation G.692:“Optical Interfaces for Multichannel Systems with Optical Amplifiers”

– Specifies interfaces for the purpose of providing future transverse compatibility among such systems.

– Defines the wavelength grid for multichannel systems.– Currently on hold pending resolution of intellectual property issues.– Large backlog of proposed changes/additions.



The Frequency Grid From G.692

196.0 195.5 195.0 194.5 194.0 193.5 193.0 192.5 192.0 191.5 191.0F (THz)

156515601545154015351530 1550λλλλ (nm)

1555

• Channels anchored at a 193.1-THz reference• 100-GHz spacing with no defined lower or upper bound.

The U.S. (TIA) will formally propose a change to 50-GHz spacing.



Asynchronous Transfer Mode (ATM)• High performance data transfer standard

– Uniform cell: 5 header bytes, 48 data bytes– Simple and efficient cell switching– Optimizes use of available network capacity

• Quality of Service (QoS)– Bandwidth and delay guarantees– Admission control to satisfy QoS

• Compatibility with installed networks– Can run over PDH or SONET/SDH systems



Internet Protocol (IP)• WAN / MAN / LAN protocol for data

– Originally designed for data (e-mail, file transfer)– Voice & video applications under development

• Layered design– Key contribution to widespread deployment– Can be easily adapted to new technologies– Higher layers can run over other data networks

as long as they provide compatible services• Point-to-Point protocol (PPP)

– Common data link layer to connect PCs to LANsor to the internet via phone lines (e.g., home PCwith modem)

7 - Application6 - Presentation5 - Session4 - Transport3 - Network2 - Data link1 - Physical



Common Transmission Speeds• SONET/SDH rates:

– OC-3, STM-1: 155.52 Mb/s– OC-12, STM-4: 622.08 Mb/s– OC-48, STM-16: 2488.32 Mb/s– OC-192, STM-64: 9953.28 Mb/s

• Datacom rates:– FDDI: 125 (100) Mb/s– FireWire: 100 - 800 Mb/s– Fibre Channel: 266 - 1063 Mb/s– Ethernet: 10 or 100 Mb/s– G-Ethernet: 1250 Mb/s

• PDH:North America: – DS1: 1.544 Mb/s– DS2: 6.312 Mb/s– DS3: 44.736 Mb/s– DS4: 139.264 Mb/s

Europe:– E1 2.048 Mb/s– E2: 8.448 Mb/s– E3: 34.368 Mb/s– E4: 139.264 Mb/s



Review Questions

1. Why do most operators like SONET/SDH ?

2. What is the advantage of a layered design?

4. What are the key properties of DWDM?



Fibers, Cables, Splices & Connectors

LW Technology



Basic Step-Index (SI) Fiber Design

RefractiveIndex (n)

Diameter (r)

Cladding

Primary coating(e.g., soft plastic)

Core

1.4801.460

SiO2 Glass

• Most common designs: 100/140 or 200/280 µm• Plastic optical fiber (POF): 0.1 - 3 mm ∅ , core 80 to

99%

140 µm

100 µm



Numerical Aperture (NA)

Acceptance / Emission Cone

NA = sin θθθθ = n2core - n2

cladding

θθθθ



Attenuation In Silica Fibers

900 1100 1300 1500 1700

0.5

1.0

1.5

2.0

2.5

OH Absorption

Atte

nuat

ion

(dB/

km)

Wavelength (nm)

“Optical Windows”

2 3

1

Main cause of attenuation: Rayleigh scattering in the fiber core



Step-Index Multimode (MM) Dispersion

Pulse broadening due to multi-path transmission.

Bitrate x Distance product is severely limited!

100/140 µm Silica Fiber: ~ 20 Mb/s • km0.8/1.0 mm Plastic Optical Fiber: ~ 5 Mb/s • km



Gradient-Index (GI) Fiber• Doping profile designed to minimize “race” conditions

(“outer” modes travel faster due to lower refractive index!) • Most common designs: 62.5/125 or 50/125 µm, NA ~ 0.2• Bitrate x Distance product: ~ 1 Gb/s • km

n

r

1.4751.460



Single-Mode Fiber (SMF)

• Step-Index type with very small core• Most common design: 9/125 µm or 10/125 µm, NA

~ 0.1• Bitrate x Distance product: up to 1000 Gb/s • km

(limited by CD and PMD - see next slides)

n

r

1.4651.460



Chromatic Dispersion (CD)• Light sources are NOT monochromatic

(linewidth of source, chirp effects, modulation sidebands)

• Different wavelengths travel at slightly different speeds(this effect is called “Chromatic Dispersion”)

• Chromatic dispersion causes pulse broadening(problem at high bit rates over long distances)

• Standard single-mode fiber: – 1300 nm window has lowest CD– 1550 nm lowest loss



Dispersion-Shifted Fiber (DSF)• Additional doping to shift zero dispersion to 1550 nm

– Now 1550 nm lowest loss AND lowest dispersion – Can cause nonlinear effects in DWDM systems (see later)

• Non-Zero Dispersion Shifted Fiber (NZDSF)– Low dispersion around 1550 nm and low nonlinear effects– Requires chromatic dispersion compensators on long distances

0

20

C. D

ispe

rsio

n ps

/(nm

• km

)

-10 1600 1700140013001200 1500

10

SMF NZDSFDSF



Polarization Mode Dispersion (PMD)• Single-mode fiber actually transmits two modes

– Modes have opposite states of polarization– Severe limitation at 10 Gb/s over distances > 50 km

• Power is randomly coupled between the two modes– PMD of a link fluctuates significantly over time

• Components can exhibit PMD as well– mostly constant PMD– manufacturers trying to

minimize it by design



Cable Designs

• Mechanical design: – Indoor, outdoor, submarine– Local or national building and construction

codes may apply

• Electrical designs:– No metal or electrical wires at all– Power wires (supply for remote amplifiers

or regenerators)

Optical fibers

Tube

Strain relief(e.g., Kevlar)

Innerjacket

Sheath

Outerjacket



Issues Of Connecting Fibers

Offset Angular Misalignment

Separation

Core Eccentricity Core Ellipticity Reflections &Interference



Medium insertion loss:

Worst return loss:< 14 dB (Fresnel)

Common multimodefiber connector

Air Gap

typ. 0.5 dBLowest insertion loss:

< 0.25 dB

Good return loss:

Common single-modefiber connector

Physical Contact(PC)

> 40 dB

Highest insertion loss:

Best return loss:

Cable TV, highperformance systems

Angled PhysicalContact (APC)

0.4 to 0.9 dB

> 60 dB

Connector Types

8º



Connector Technology

• Ultra-high precision– Optical axis aligned to better than ±1

µm (single-mode)– Physical contact of the glass end

surfaces necessary

• Connector cleanliness is paramount – special cleaning and inspection

required

Sleeve

Ferrule

FiberKey



Connector Brands

Photo courtesyof: Diamond SA



Connector Inspection

Don’t stare into the laser beam

(with your remaining eye)Inspection Tool



Connector Care

New Connector Damaged Connector



Connector Cleaning

Pure Cotton Swabs

Isopropyl Alcohol

Filtered Air

Variety of cleaning methods in use today

Example:Clean connector tips with Isopropyl (96% medical alcohol) using adhesive free cotton swabs

Immediately dry it with dust-free, non residue compressed air



Splices• Fusion Splices

– Most common permanent fiber connection– Very high performance and reliability– Insertion loss 0.01 to 0.1 dB, no reflection– Automated splicing tool costs $10k to $50k

• Mechanical Splices– Permanent and non-permanent types– Insertion loss 0.1 to 0.5 dB– Index-matching liquid used to minimize loss & reflections– Epoxy or UV hardened elastomer based– Less expensive tools ($100 to $1,000) required

Protective sleeve

Splice



Review Questions

1. What are commonly used fiber types?

2. What is dispersion and what can cause it?

3. What are good connector care habits?



Passive Components

LW Technology



Patchcords• “Jumper cables” to connect devices and instruments

• “Adapter cables” to connect interfaces using different connector styles

• Insertion loss is dominated by the connector losses (2 m fiber has almost no attenuation)

• Often yellow sheath used for single-mode fiber, orange sheath for multimode



Wavelength-Independent Couplers• Wavelength-Independent coupler (WIC) types:

– couple light from each fiber to all the fibers at the other side– 50% / 50% (3 dB) most common 4 port type– 1%, 5% or 10% taps (often 3 port devices)

• Excess Loss (EL):– Measure of power “wasted” in the component

EL = -10 • log10

Pout

Pin

Σ



Wavelength-Dependent Couplers

• Wavelength-division multiplexers (WDM) types:– 3 port devices (4th port terminated)– 1310 / 1550 nm (“classic” WDM technology)– 1480 / 1550 nm and 980 / 1550 nm for pumping optical

amplifiers (see later)– 1550 / 1625 nm for network monitoring

• Insertion and rejection:– Low loss (< 1 dB) for path wavelength– High loss (20 to 50 dB) for other wavelength

Common λλλλ1

λλλλ2



Isolators• Main application:

– To protect lasers and optical amplifiers from light coming back (which otherwise can cause instabilities)

• Insertion loss:– Low loss (0.2 to 2 dB) in forward direction– High loss in reverse direction:

20 to 40 dB single stage, 40 to 80 dB dual stage)

• Return loss:– More than 60 dB without connectors



Filter Characteristicsλ λ λ λ i-1 λ λ λ λ i λ λ λ λ i+1

PassbandCrosstalk Crosstalk

• Passband– Insertion loss– Ripple– Wavelengths

(peak, center, edges)– Bandwidths

(0.5 dB, 3 dB, ..)– Polarization dependence

• Stopband– Crosstalk rejection– Bandwidths

(20 dB, 40 dB, ..)



Dielectric Filters

• Thin-film cavities– Alternating dielectric thin-film layers with different refractive index– Multiple reflections cause constructive & destructive interference– Variety of filter shapes and bandwidths (0.1 to 10 nm) – Insertion loss 0.2 to 2 dB, stopband rejection 30 to 50 dB

Layers Substrate

Incoming Spectrum

Reflected Spectrum

Transmitted Spectrum

1535 nm 1555 nm

0 dB

30 dB



Tunable Fabry-Perot Filters

• Filter shape– Repetitive passband with Lorentzian shape– Free Spectral Range FSR = c / 2 • n • l (l: cavity

length)– Finesss F = FSR / BW (BW: 3 dB bandwidth)

• Typical specifications for 1550 nm applications– FSR: 4 THz to 10 THz, F: 100 to 200, BW: 20 to 100 GHz– Insertion loss: 0.5 to 35 dB

Fiber

Piezoelectric-actuators

Mirrors

Optical Frequency

FSR1 dB

30 dB



Fiber Bragg Gratings (FBG)

• Single-mode fiber with “modulated” refractive index– Refractive index changed using high power UV radiation

• Regular interval pattern: reflective at one wavelength– Notch filter, add / drop multiplexer (see later)

• Increasing intervals: “chirped” FBG– Compensation for chromatic dispersion

λλλλ



Circulators

Circulator & chirped FGB configured to compensate CD

• Optical crystal technology similar to isolators– Insertion loss 0.3 to 1.5 dB, isolation 20 to 40 dB

• Typical configuration: 3 port device– Port 1 -> Port 2– Port 2 -> Port 3– Port 3 -> Port 1

Fast λλλλSlow λλλλ



Add / Drop Nodes

Filter reflects λ λ λ λ i

Add λ λ λ λ i

Add / Drop

Dielectric thin-film filter design

Circulator with FBG design

Common Passband

Drop λ λ λ λ i



Multiplexers (MUX) / Demultiplexers (DEMUX)

• Key component of wavelength-division multiplexing technology (DWDM)

• Variety of technologies– Cascaded dielectric filters– Cascaded FBGs– Phased arrays (see later)

• High crosstalk suppression essential for demultiplexing



Array Waveguide Grating (AWG)

λλλλ1a

λλλλ3a

λλλλ2a

λλλλ4a

λλλλ1b

λλλλ3b

λλλλ2b

λλλλ4b

λλλλ1c

λλλλ3c

λλλλ2c

λλλλ4c

λλλλ1d

λλλλ3d

λλλλ2d

λλλλ4d

λλλλ1aλλλλ3c

λλλλ2dλλλλ4bλλλλ

1bλλλλ

3dλλλλ

2aλλλλ

4cλλλλ1c

λλλλ3a λλλλ2bλλλλ4dλλλλ

1dλλλλ

3bλλλλ

2cλλλλ

4a

Rows .. .. translate into .. .. columns

If only one input is used: wavelength demultiplexer!



Review Questions

1. What is the difference between a WIC and a WDM?

2. What are the losses of a 10% tap?

3. What does a demultiplexer do?



Transmitters & Receivers

LW Technology



Light-emitting Diode (LED)• Datacom through air & multimode fiber

– Very inexpensive (laptops, airplanes, lans)

• Key characteristics– Most common for 780, 850, 1300 nm– Total power up to a few µW– Spectral width 30 to 100 nm– Coherence length 0.01 to 0.1 mm – Little or not polarized– Large NA (→ poor coupling into fiber) P -3 dB

P peak

BW



Fabry-Perot (FP) Laser• Multiple longitudinal mode (MLM) spectrum• “Classic” semiconductor laser

– First fiberoptic links (850 or 1300 nm)– Today: short & medium range links

• Key characteristics– Most common for 850 or 1310 nm– Total power up to a few mw– Spectral width 3 to 20 nm– Mode spacing 0.7 to 2 nm– Highly polarized– Coherence length 1 to 100 mm– Small NA (→ good coupling into fiber)

P peak

I

PThreshold



Distributed Feedback (DFB) Laser• Single longitudinal mode (SLM) spectrum• High performance telecommunication laser

– Most expensive (difficult to manufacture)– 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)– Sidemode suppression ratio (SMSR): > 50 dB– Coherence length 1 to 100 m– Small NA (→ good coupling into fiber)

P peak

SMSR



Vertical Cavity Surface Emitting Lasers (VCSEL)

• Distributed Bragg Reflector (DBR) Mirrors– Alternating layers of semiconductor material– 40 to 60 layers, each λ / 4 thick– Beam matches optical acceptance needs of fibers more closely

• Key properties– Wavelength range 780 to 980 nm (gigabit ethernet) – Spectral width: <1nm– Total power: >-10 dBm– Coherence length:10 cm to10 m– Numerical aperture: 0.2 to 0.3

activen-DBR

p-DBR



Other Light Sources• White light source

– Specialized tungsten light bulb– Wavelength range 900 to 1700 nm, – Power density 0.1 to 0.4 nw/nm (SM), 10 to 25 nw/nm (MM)

• Amplified spontaneous emission (ASE) source– “Noise” of an optical amplifier without input signal– Wavelength range 1525 to 1570 nm– Power density 10 to 100 µw/nm

• External cavity laser– Most common for 1550 nm band (some for 1310 nm)– Tunable over more than 100 nm, power up to 10 mw– Spectrum similar to DFB laser, bandwidth 10 kHz to 1 MHz



Basic Transmitter Design

• Optimized for one particular bit rate & wavelength• Often temperature stabilized laser• Internal (direct) or external modulation• Digital modulation

– Extinction ratio: 9 to 15 dB– Forward error correction– Scrambling of bits to reduce long sequences of 1s or 0s

(reduced DC and low frequency spectral content)• Analog modulation

– Modulation index typically 2 to 4%– Laser bias optimized for maximum linearity



Modulation Principles

• Direct (laser current)– Inexpensive– Can cause chirp up to 1 nm

(wavelength variation caused by variation in electron densities in the lasing area)

• External– 2.5 to 40 gb/s– AM sidebands (caused by

modulation spectrum) dominate linewidth of optical signal

DC

RF

DC MOD

RF



External Modulators

Mach-Zehnder Principle

Lasersection

Modulationsection

DFB laser with external on-chip modulator



Photodiodes• PIN (p-layer, intrinsic layer, n-layer)

– Highly linear, low dark current• Avalanche photo diode (APD)

– Gain up to x100 lifts detected optical signal above electrical noise of receiver

– Best for high speed and highly sensitive receivers

– Strong temperature dependence• Main characteristics

– Quantum efficiency (electrons/photon)– Dark current– Responsivity (current vs. ΛΛΛΛ)

n+

Bias Voltage

APD

Gai

n



Material Aspects

• Silicon (Si)– Least expensive

• Germanium (Ge)– “Classic” detector

• Indium gallium arsenide (InGaAs)– Highest speed

Wavelength nm500 1000 1500

Silicon

Germanium

InGaAs

Quantum Efficiency = 1

0.1

0.5

1.0Responsivity (A/W)



Basic Receiver Design

• Optimized for one particular– Sensitivity range– Wavelength– Bit rate

• Can include circuitsfor telemetry

AGC

-g

Bias ClockRecovery

DecisionCircuit 0110

RemoteControl

TemperatureControl

Monitors& Alarms



Receiver Sensitivity

• Bit error ratio (BER) versus input power (pi)

– Minimum input power depends on acceptable bit error rate

– Power margins important to tolerate imperfections of link (dispersion, noise from optical amplifiers, etc.)

– Theoretical curve well understood– Many receivers designed for 1E-12 or

better BERPi (dBm)

BER



Regenerator• Receiver followed by a transmitter

– No add or drop of traffic– Designed for one bit rate & wavelength

• Signal regeneration– Reshaping & timing of data stream– Inserted every 30 to 80 km before optical amplifiers became

commercially available– Today: reshaping necessary after about 600 km (at 2.5 Gb/s), often

done by SONET/SDH add/drop multiplexers or digital cross-connects



Conceptual Terminal Diagram

TransmissionPath

2488.32 Mb/s

PDH

Str

eam

s (T

ribut

arie

s)

....

..

1.5 Mb/s 51.84 Mb/s

Monitoring & Management

Synchronous ContainerMapping

Synchronous ContainerMapping RX

TX

RX

TX ProtectionPath

Inter-leaving

....Inter-

leaving

SONET / SDHStreams



Review Questions

1. What are the differences between an LED, FP, and DFB lasers?

2. Which photodiode do you use for– Data communication?– Speed longhaul traffic?

3. How do you define receiver sensitivity?



Optical Amplifiers

LW Technology



• Erbium: rare element with phosphorescent properties– Photons at 1480 or 980 nm activate

electrons into a metastable state– Electrons falling back emit light in

the 1550 nm range

• Spontaneous emission– Occurs randomly (time constant ~1 ms)

• Stimulated emission– By electromagnetic wave– Emitted wavelength & phase are

identical to incident one

Erbium Properties

1480

980820

540

670

Ground state

Metastablestate



Basic EDF Amplifier Design• Erbium-doped fiber amplifier (EDFA) most common

– Commercially available since the early 1990’s– Works best in the range 1530 to 1565 nm– Gain up to 30 dB (1000 photons out per photon in!)

• Optically transparent– “Unlimited” RF bandwidth– Wavelength transparent

Input

1480 or 980 nm Pump Laser Erbium Doped Fiber

Output

IsolatorCoupler



Amplified Spontaneous Emission

Amplifiedspontaneous emission (ASE)

Random spontaneous emission (SE)

Amplification along fiber

• Erbium randomly emits photons between 1520 and 1570 nm– Spontaneous emission (SE) is not polarized or coherent – Like any photon, SE stimulates emission of other photons– With no input signal, eventually all optical energy is consumed into

amplified spontaneous emission– Input signal(s) consume metastable electrons → much less ASE



Output Spectra

ASE spectrum when noinput signal is present

Amplified signal spectrum(input signal saturates the optical amplifier)

1575 nm-40 dBm

1525 nm

+10 dBm



Time-Domain Properties

Turn-On Overshoot

Gain x Signal

ASE level (signal present)

ASE level (signal absent)

ττττ ~ 10 .. 50 µs

ττττ ~ 0.2 .. 0.8 ms

off onoffInput Signal on on



Optical Gain (G)

• G = S Output / S InputS Output: output signal (without noise from amplifier) S Input: input signal

• Input signal dependent– Operating point (saturation) of

EDFA strongly depends on power and wavelength ofincoming signal

Wavelength (nm)

Gain (dB)

1540 1560 158010

1520

20

40

30

-5 dBm

-20 dBm

-10 dBm

P Input: -30 dBm



Noise Figure (NF)• NF = P ASE / (h•νννν • G • B OSA)

P ASE: ASE power measured by OSA h: Plank’s constant νννν: Optical frequencyG: Gain of EDFAB OSA: Optical bandwidth [Hz]

of OSA

• Input signal dependent– In a saturated EDFA, the NF

depends mostly on thewavelength of the signal

– Physical limit: 3.0 dB

Noise Figure (dB)

1540 1560 15801520

7.5

10

Wavelength (nm)

5.0



Gain Compression

• Total output power: Amplified signal + ASE– EDFA is in saturation if almost all

Erbium ions are consumed for amplification

– Total output power remains almost constant

– Lowest noise figure

• Preferred operating point– Power levels in link stabilize

automaticallyP in (dBm)

Total P out

-3 dBMax

-20-30 -10

Gain



Polarization Hole Burning (PHB)

• Polarization Dependent Gain (PDG)– Gain of small signal polarized orthogonal to saturating signal 0.05

to 0.3 dB greater than the large signal gain– Effect independent of the state of polarization of the large signal– PDG recovery time constant relatively slow

• ASE power accumulation– ASE power is minimally polarized – ASE perpendicular to signal experiences higher gain– PHB effects can be reduced effectively by quickly scrambling the

state of polarization (SOP) of the input signal



Spectral Hole Burning (SHB)• Gain depression around saturating signal

– Strong signals reduce average ion population– Hole width 3 to 10 nm– Hole depth 0.1 to 0.4 dB – 1530 nm region more sensitive

to SHB than 1550 nm region

• Implications– Usually not an issue in transmission

systems (single λ or DWDM)– Can affect accuracy of some

lightwave measurements1545 1550 15601540

Wavelength (nm)

7 nm

0.36 dB



EDFA Categories• In-line amplifiers

– Installed every 30 to 70 km along a link– Good noise figure, medium output power

• Power boosters– Up to +17 dBm power, amplifies transmitter output– Also used in cable TV systems before a star coupler

• Pre-amplifiers– Low noise amplifier in front of receiver

• Remotely pumped– Electronic free extending links up to 200 km and more

(often found in submarine applications)

RX

Pump

TX

Pump



Commercial Designs

InputEDF

OutputIsolator


Pump Lasers

OutputMonitor

EDF

InputMonitor

Isolator



Security Features

• Input power monitor– Turning on the input signal can cause high output power spikes

that can damage the amplifier or following systems– Control electronics turn the pump laser(s) down if the input signal

stays below a given threshold for more than about 2 to 20 µs

• Backreflection monitor– Open connector at the output can be a laser safety hazard– Straight connectors typically reflect 4% of the light back– Backreflection monitor shuts the amplifier down if backreflected

light exceeds certain limits



Other Amplifier Types• Semiconductor Optical Amplifier (SOA)

– Basically a laser chip without any mirrors– Metastable state has nanoseconds lifetime

(-> nonlinearity and crosstalk problems)– Potential for switches and wavelength converters

• Praseodymium-doped Fiber Amplifier (PDFA)– Similar to EDFAs but 1310 nm optical window– Deployed in CATV (limited situations)– Not cost efficient for 1310 telecomm applications– Fluoride based fiber needed (water soluble)– Much less efficient (1 W pump @ 1017 nm for 50 mW output)



Future Developments

• Broadened gain spectrum– 2 EDFs with different co-dopants (phosphor, aluminum)– Can cover 1525 to 1610 nm

• Gain flattening– Erbium Fluoride designs (flatter gain profile)– Incorporation of Fiber Bragg Gratings (passive compensation)

• Increased complexity– Active add/drop, monitoring and other functions



Review Questions

1. What components do you need to build an EDFA?

2. What is ASE?

3. How do you saturate an amplifier?



Wavelength-Division Multiplexing

LW Technology



Basic Design(Dense Wavelength-Division Multiplexing)

Monitor Points

Dem

ultip

lexe

r

λλλλ2

λλλλn

λλλλ1

λλλλn-1

WavelengthConverter

NT

NT

λλλλ2

λλλλn

λλλλ1

λλλλn-1

Mul

tiple

xer

WavelengthConverter

NT

NT

NT

NT

NT

NT

Net

wor

k Te

rmin

als



DWDM Spectrum

1565 nm

RL +0.00 dBm5.0 dB/DIV

1545 nm

AmplifiedSpontaneousEmission (ASE)

Channels: 16Spacing: 0.8 nm



WDM Standards

• ITU-T draft Rec. G.mcs:“Optical Interfaces for Multichannel Systems with Optical Amplifiers”– Wavelength range 1532 to 1563 nm– 100 GHz (0.8 nm) channel spacing, 50 GHz proposed– 193.1 THz (1552.51 nm) reference

• ITU-T draft Rec. G.onp:“Physical Layer Aspects of Optical Networks”– General and functional requirements



EDFAs In DWDM Systems

• Gain flatness (gain tilt) requirements

• Gain competition

• Nonlinear effects in fibers

Optical amplifiers in DWDM systems require special considerations because of:



Gain Flatness (Gain Tilt)

G

λλλλ

• Gain versus wavelength– The gain of optical amplifiers depends on wavelength– Signal-to-noise ratios can degrade below acceptable

levels (long links with cascaded amplifiers)

• Compensation techniques– Signal pre-emphasis– Gain flattening filters– Additional doping of amplifier with Fluorides



Gain Competition

Output power after channel one failed

Equal power of all four channels

• Total output power of a standard EDFA remains almost constant even if input power fluctuates significantly

• If one channel fails (or is added) then the remaining ones increase (or decrease) their output power



Output Power Limitations

• High power densities in SM fiber can cause– Stimulated Brillouin scattering (SBS)– Stimulated Raman scattering (SRS)– Four wave mixing (FWM)– Self-phase and cross-phase modulation (SPM, CPM)

• Most designs limit total output power to +17 dBm– Available channel power: 50/N mW

(N = number of channels)



DWDM Trends

• Higher capacity– 120 channels for access network applications– 50 GHz channel spacing (25 GHz under investigation)– Wavelength range extended up to 1625 nm

• All optical network– Modulation & protocol transparency– Optical add/drop multiplexers– Optical cross-connects– Optical switch fabrics– Wavelength conversion



Add / Drop Points

• Fixed configurations– Simple and inexpensive– Inflexible

• Flexible configurations– Selective wavelength add/drop

• Future designs more sophisticated– High capacity & performance

PhasedArray

PhasedArray



Research Topics

• Optical cross-connects– Technology for large optical switches

• Network and traffic management– Digital versus optical routing– Traffic amount & network size– Virtual networks (private networks over public paths)

• Wavelength conversion– Wavelengths must be reused in large networks for optimal

use of available capacity– Eventually has to include optical pulse regeneration

(re-shaping, re-timing)



Review Questions

1. What technologies enable the use of DWDM?

2. What are the advantages of DWDM?

3. What are the disadvantages of DWDM?