Elements of Lightwave Technology - Hochschule Harzmyweb3.hs-harz.de/ufischerhirchert/documents/HP Lightwave basics.pdf · SCSI/ USB/ PCI@ 66 Mbps IEEE1394 FireWire@ 400 Mbps IEEE1394
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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
LW Technology (Cover, Appendix).PPT - 2© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Table of Content1. Introduction2. Physical Basics3. Standards4. Fibers, Cables, Splices & Connectors5. Passive Components6. Transmitters & Receivers7. Optical Amplifiers8. Dense Wavelength-Division Multiplexing
LW Technology (Cover, Appendix).PPT - 3© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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.
LW Technology (Cover, Appendix).PPT - 4© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Thank You For Choosing
Agilent Technologies
As Your Partner In
Lightwave & High Speed Digital
Transmission Test
Revision 1.1October 14, 2002
LW Technology (Cover, Appendix).PPT - 5© Copyright 1999, Agilent Technologies
Introduction
LW Technology
LW Technology (Cover, Appendix).PPT - 6© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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)
LW Technology (Cover, Appendix).PPT - 7© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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)
LW Technology (Cover, Appendix).PPT - 8© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Use Of Lightwave Technology
• Majority applications:– Telephone networks– Data communication systems– Cable TV distribution
• Niche applications:– Optical sensors– Medical equipment– Displays & signs
LW Technology (Cover, Appendix).PPT - 9© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 10© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 11© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Telecommunication Network Bandwidth Trend
1995 2000 2005 2010 Year
10
20
30
40
50
RelativeLoad
1990
Total: 35%/year
Voice: 10%/yearSource:
LW Technology (Cover, Appendix).PPT - 12© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Basic Link Design
Transmitter Connector Cable
ReceiverCableSplice
LW Technology (Cover, Appendix).PPT - 13© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 14© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Typical Regenerator Unit
Pulse re-shaping & re-timing
PowerSupply
Telemetry &Remote Control
Modulation & bit rate dependent!
LW Technology (Cover, Appendix).PPT - 15© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Typical Amplifier Unit
Optical Amplifiers
PowerSupply
Telemetry &Remote Control
Modulation & bit rate independent!
LW Technology (Cover, Appendix).PPT - 16© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Data Communication Trends
SCSI/USB/PCI@66 Mbps
IEEE1394FireWire@400 Mbps
IEEE1394FireWire@1.6 Gbps
IEEE1394FireWire@3.2 Gbps
POLO@10 Gbps
1994 1998 2000 20021996
FastEthernet@100 Mbps
GigaBitEthernet@1.25 Gbps
FibreChannel@2.5 Gbps
GigaBitEthernet@10 Gbps
LW Technology (Cover, Appendix).PPT - 17© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 18© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 19© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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?
Revision 1.1October 14, 2002
LW Technology (Cover, Appendix).PPT -20© Copyright 1999, Agilent Technologies
Physical Basics
LW Technology
LW Technology (Cover, Appendix).PPT - 21© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
The Carrier - Light
RaysWavesParticles
AbsorptionEmission
Interference RefractionReflection
Bandgap
Conduction band
Valence band
n0
n1
n0
LW Technology (Cover, Appendix).PPT - 22© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 23© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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 Technology (Cover, Appendix).PPT - 24© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 25© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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)
LW Technology (Cover, Appendix).PPT - 26© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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)
LW Technology (Cover, Appendix).PPT - 27© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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)
LW Technology (Cover, Appendix).PPT - 28© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 29© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Interference
• Incoherent light adds up optical power
• Coherent light adds electromagnetic fields
• Zero phase shift:constructive interference
• 180º phase shift:destructive interference
+ =
+ =
LW Technology (Cover, Appendix).PPT - 30© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Reflections
• Reflections: root cause for many problemsReturn loss definition:
RL = 10 * log
Pr
Pi
P reflected
P incident
LW Technology (Cover, Appendix).PPT - 31© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 32© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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)
LW Technology (Cover, Appendix).PPT - 33© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 34© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 35© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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?
Revision 1.1October 14, 2002
LW Technology (Cover, Appendix).PPT -36© Copyright 1999, Agilent Technologies
Standards
LW Technology
LW Technology (Cover, Appendix).PPT - 37© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 38© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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)
LW Technology (Cover, Appendix).PPT - 39© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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)
LW Technology (Cover, Appendix).PPT - 40© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 41© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 42© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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)
LW Technology (Cover, Appendix).PPT - 43© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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.
LW Technology (Cover, Appendix).PPT - 44© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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.
LW Technology (Cover, Appendix).PPT - 45© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 46© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 47© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 48© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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?
Revision 1.1October 14, 2002
LW Technology (Cover, Appendix).PPT -49© Copyright 1999, Agilent Technologies
Fibers, Cables, Splices & Connectors
LW Technology
LW Technology (Cover, Appendix).PPT - 50© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 51© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Numerical Aperture (NA)
Acceptance / Emission Cone
NA = sin θθθθ = n2core - n2
cladding
θθθθ
LW Technology (Cover, Appendix).PPT - 52© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 53© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 54© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 55© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 56© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 57© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 58© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 59© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 60© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Issues Of Connecting Fibers
Offset Angular Misalignment
Separation
Core Eccentricity Core Ellipticity Reflections &Interference
LW Technology (Cover, Appendix).PPT - 61© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 62© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 63© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Connector Brands
Photo courtesyof: Diamond SA
LW Technology (Cover, Appendix).PPT - 64© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Connector Inspection
Don’t stare into the laser beam
(with your remaining eye)Inspection Tool
LW Technology (Cover, Appendix).PPT - 65© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Connector Care
New Connector Damaged Connector
LW Technology (Cover, Appendix).PPT - 66© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 67© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 68© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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?
Revision 1.1October 14, 2002
LW Technology (Cover, Appendix).PPT -69© Copyright 1999, Agilent Technologies
Passive Components
LW Technology
LW Technology (Cover, Appendix).PPT - 70© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 71© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 72© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 73© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 74© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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, ..)
LW Technology (Cover, Appendix).PPT - 75© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 76© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 77© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 78© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 79© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 80© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 81© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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!
LW Technology (Cover, Appendix).PPT - 82© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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?
Revision 1.1October 14, 2002
LW Technology (Cover, Appendix).PPT -83© Copyright 1999, Agilent Technologies
Transmitters & Receivers
LW Technology
LW Technology (Cover, Appendix).PPT - 84© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 85© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 86© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 87© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 88© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 89© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 90© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 91© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
External Modulators
Mach-Zehnder Principle
Lasersection
Modulationsection
DFB laser with external on-chip modulator
LW Technology (Cover, Appendix).PPT - 92© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 93© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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)
LW Technology (Cover, Appendix).PPT - 94© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 95© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 96© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 97© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 98© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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?
Revision 1.1October 14, 2002
LW Technology (Cover, Appendix).PPT -99© Copyright 1999, Agilent Technologies
Optical Amplifiers
LW Technology
LW Technology (Cover, Appendix).PPT - 100© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
• 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
LW Technology (Cover, Appendix).PPT - 101© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 102© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 103© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 104© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 105© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 106© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 107© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 108© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 109© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 110© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 111© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Commercial Designs
InputEDF
OutputIsolator
Telemetry &Remote Control
Pump Lasers
OutputMonitor
EDF
InputMonitor
Isolator
LW Technology (Cover, Appendix).PPT - 112© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 113© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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)
LW Technology (Cover, Appendix).PPT - 114© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 115© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Review Questions
1. What components do you need to build an EDFA?
2. What is ASE?
3. How do you saturate an amplifier?
Revision 1.1October 14, 2002
LW Technology (Cover, Appendix).PPT -116© Copyright 1999, Agilent Technologies
Wavelength-Division Multiplexing
LW Technology
LW Technology (Cover, Appendix).PPT - 117© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 118© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
DWDM Spectrum
1565 nm
RL +0.00 dBm5.0 dB/DIV
1545 nm
AmplifiedSpontaneousEmission (ASE)
Channels: 16Spacing: 0.8 nm
LW Technology (Cover, Appendix).PPT - 119© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 120© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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:
LW Technology (Cover, Appendix).PPT - 121© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 122© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 123© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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)
LW Technology (Cover, Appendix).PPT - 124© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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
LW Technology (Cover, Appendix).PPT - 125© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Add / Drop Points
• Fixed configurations– Simple and inexpensive– Inflexible
• Flexible configurations– Selective wavelength add/drop
• Future designs more sophisticated– High capacity & performance
PhasedArray
PhasedArray
LW Technology (Cover, Appendix).PPT - 126© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
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)
LW Technology (Cover, Appendix).PPT - 127© Copyright 1999, Agilent Technologies
Revision 1.1October 14, 2002
Review Questions
1. What technologies enable the use of DWDM?
2. What are the advantages of DWDM?
3. What are the disadvantages of DWDM?
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