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Back to Basics
in Optical Communications
Technology
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Back to Basics in Optical Communications
Technology
Today . . .
The innovations that enable 2.5 10 40 1000 Gb/ s
The science that drives the technologyThe science that drives the technology
Recipe:(1) Review the physical foundation of the technology(2) Derive the technology from the science(3) The major issues in development of the technology(4) Characterization and test of the technology
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The Science: Four Easy Pieces
1. Geometric Optics- How fibers work
2. Physical Optics- Properties of electromagnetic waves- Optical filters and spectrum analyzers
3. Atomic Physics- How transmitters, receivers, and amplifiers work
4. Electrodynamics of continuous media
- How the index of refraction affects the system- Dispersion
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But first, a word from our sponsors. . .
ssss
Agilent Technologies Lightwave Training
Understanding Lightwave Technology
Understanding Optical Passive DeviceCharacterization
Understanding Optical Transmitters andReceivers and Their Characterization
Understanding DWDM
Optical Spectrum Analysis/ OSA UsersCourse
Characterizing Polarization Effects
Eye Diagram Analysis
TDR in High-Speed Digital DesignBit Error Rate Analysis
Digital Communications Analyzer UsersCourse
Back to Basics in OpticalCommunications Technology
Understanding Optical Networking
The Elements of Lightwave Technology
TodaysPresentationApre-requisiteforothercourse
s
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Todays Presentations
Geometric Optics: The optic fiber as a waveguide
Physical Optics and Passive Component Characterization
Light Transmission, Reception and Modulation: Active ComponentCharacterization
Optical Signal Amplification and DWDM Dispersion: The evolution of the index of refraction with wavelength
and polarization
Characterizing the Optical Network
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High Speed Networking
Why were all here. . .
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Networking at High Speed
Pulses of infrared lightguided through glass fibers
move huge blocks of data
long or short distances
insensitive to electrical interference
cheap and light weight
Telephone Data Cable TV
Long distances WAN - Wide Area NetsShort Distances LAN - Local Area Nets
In between MAN - Metro Area Nets
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High Speed Networking is Accelerating
1995 2000 2005 2010 Year
10
20
30
40
50
RelativeLoad
1990
Total: 35%/year
Voice: 10%/yearSource:
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Optical Networking - The DWDM Forest
Dispersion
Compensator
DispersionCompensator
Other
Segment
OtherSegment
OtherSegment
OtherSegment
LAN
Cross
Connect
(Switch)
Data
1
2...
n
Data
1
2...
n
demultiplexer
multiplexer
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Geometric Optics
The index of refraction
Total internal reflection
The optic fiber as a waveguide
Single and multi mode fibers
Cladding
Primary coating(e.g., soft plastic)
Core
SiOGlas
s2
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Optical Networking - The DWDM Forest
Fibers
DispersionCompensator
DispersionCompensator
OtherSegment
OtherSegment
OtherSegment
Other
Segment
LAN
CrossConnect
(Switch)
Data
1
2...
n
Data
1
2...
n
demultiplexe
r
multiplexer
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Optic Fibers and the Index of Refraction, n()
The index of refraction of a dielectric is given by
At the boundary of two media, incident light isreflected or transmitted (a.k.a., refracted),
The relationship between the angles of incidence
and transmission is given bySnells Law:
)sin()sin( transcladdinginccore nn =trans
cladding
n
ncore
inc
medium
vacuum
ccn )(
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Optic Fibers Contain Light by
Total Internal ReflectionAt incident angles larger than a critical angle, all light is reflected - contained:
Typical values:
Bending cables losses
Only light injected in a cone of some angle smaller than NAwill be contained by the fiber
The spec for this is numerical aperture, NA:
claddingcore nn >
22sin cladcoreNA nnNA =
transinc
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Conducting Waveguide Analogy
Optic Fibers are dielectricwaveguidesFirst, consider conductingwaveguides:
Conducting
wave guide
Boundary Conditions:ETcond = 0 and B cond = 0
give . . .
The cutoff wavelengths, or frequencies, determine
the modes of propagationSingle Mode (SM) a< < 2aMulti Mode (MM) < a
For a given range of wavelengths there areSingle and Multi Mode waveguides
- single mode doesnt mean single wavelength
a
1 = 2a
2 = a
3 = 2a/3
Cutoff
wavele
ngth
s
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Modal Dispersion:
Waveguide dispersion arises from the dependence of the group velocityon the frequency, , and the mode-cutoff frequency cutoff= 2c/cutoff.
Modal dispersion occurs when a pulse is composed of waves of morethan one mode.
The different modes travel at very different, speeds Modal Dispersion
21
2cutoff
= cgroup
0
0.5
1
Vgroup/c
1
2
3
Conductin
g
waveguide
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Optic Fibers are Dielectric Waveguides
Boundary conditions:Smooth decay of the electric field atthe core/ cladding boundary
Claddingd
dCore
1 > 2dCore
2 > dCore
3 > 2 dCore /3
Single Mode: 1 < < 2
e.g., for 1.5 m,
dCore 10 m.
Single mode cutoffwavelength is larger than
the fiber core diameter
2
2
0022
tn
= EE
)(
)(ztj
erEE
=
Transverse
standingw
ave Longitudin
al
travelingwa
ve
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Multi Mode Fibers
Modal Dispersion In Multi Mode fibers suffer Modal Dispersion from
Bitrate x Distance product is severely limited!
100/ 140 m Silica Fiber: ~ 20 Mb/ s km0.8/ 1.0 mm Plastic Optical Fiber: ~ 5 Mb/ s km
Single mode fibers do not suffer modal dispersion!
2
2
mode
1~
cvgroup
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Fiber Structure
Multi-mode: 100/ 140 or 200/ 280 m
Single Mode: 9/ 125 or 10/ 125 m
n
r
1.465
1.460
RefractiveI ndex ( n)
Diamet er ( r )
Cladding
Primary coating
(e.g., soft plastic)
Core
1.480
1.460
100m
140m
SiOGlas
s2
NA = 0.24TIR= 80
NA = 0.12
TIR
= 85
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Couplers
The solution includes the smooth decay of E in the cladding This evanescent wave travels along with the guided wave Energy travels along the cladding and it s easy to get it out
Join two fibers together - a double wave guide
Light can tunnel from one core to the other - a coupler
This is how couplers work Waves tunnel from one core to the next
Core CoreCladding Cladding
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The Directional Coupler
Single Waveguide with Two Cores
Solving the wave equation with two single mode
cores gives . . .A transverse wave that oscillates between cores
End Side
d
Incident signal power oscillates back
and forth between the two cores as thelight propagates the length of thecoupler
The distance along the fiber where the signal is in a given core depends on . . .
Wavelength
The distance between the cores
The geometry of the two original single mode fibers
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Wave Coupling Splitters
The length of a directional coupler can be tuned so that incident light of agiven wavelength will exit the coupler in a specific core
L
1, 2
2
1
a
b
c
d
Couplers can be designed to
demultiplex incoming signals In reverse the demultiplexer is a
multiplexer
With different single mode fibersand geometry couplers can bevery selective with narrowchannel spacing.
The wavelength dependence canalso be reduced over ~ 100 nm togive a wavelength independent3 dB coupler.
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Optical Networking - The DWDM Forest
Fibers
DispersionCompensator
DispersionCompensator
OtherSegment
OtherSegment
OtherSegment
Other
Segment
LAN
CrossConnect
(Switch)
Data
1
2...
n
Data
1
2...
n
demultiplexe
r
multiplexer
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Physical Opticsand
Passive Component
Characterization
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Physical Optics
Electromagnetic waves
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Electromagnetic Spectrum
Frequency
RFRadar
InfraredLight
VisibleLight
Ultraviolet
X-Rays
Wavelength1 Mm 1 km 1 m 1 mm 1 nm1 m
1 kHz 1 MHz 1 GHz 1 THz 1 EHz1 PHz
| U | L | C | S | E | O | LAN
Frequency
Wavelength1600
300
1000 800nm
4001400 1200
THz200 250 400
Networking wavelength bands
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Waves
Describe light with = wavelength, = frequency 1500 nm 200 THz
= c = 299,792,458 m/ s
30 cm/ ns (1 ft/ ns)
and reserve
f= modulation frequency
Magnetic Field
Strength, B
Electric Field
Strength,E
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The relationship between
optical frequency bandwidth, andwavelength linewidth,
Let frequency bandwidth beand wavelength linewidth be
Then since
or
The relationship between
bandwidth and linewidth is
or, equivalently
c=
c=
= 2c
=2
c
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Recipe for creating electromagnetic waves:
= CA dddtd
n lBSE
00
2
=CA dddt
dlES
B
The strength of the magnetic field is small sowe only consider the electric field.
Changing Electric
Field, E
Changing Magnetic
Field,B
2
2
0022
tn
= EE
2
2
00
22
tn
=
BB
Electromagnetic Waves
Induction
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Electric Field Components
Ey
Ex An E-Field is the vector sumof the components Ex and Ey
I t d ti t P l i ti
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Introduction to Polarization
Linear PolarizationProjection of E is a line
Circular PolarizationProjection of E is a circle
Ey
Ex
Ey
Ex
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Poincar Sphere
Representation of PolarizationGraphical representation of stateof polarization using Stokesparameters (S1, S2, S3)
Left-handcircularpolarization
(0,0,-1)
S1axis
S2axis
S3
axis
45 degree linearpolarization (0,1,0)
Right-handcircular
polarization(0,0,1)
Vertical linearpolarization (-1,0,0)
C h
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Coherence
Coherence the phase relationship of waves
Lc
Lc
Coherent wavescomponents of coherent waves havewell defined phase relationships.
Incoherent waves
components of incoherent waveshave random indeterminatephaserelationships.
C h Ti d C h L th
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Coherence Time and Coherence LengthCoherence time (T
C)
Average time for the wave train to lose its phase relationships
Coherence length (LC)
Average distance over which superposed waves lose their phase relationships
There is a Fundamental Relationship between Coherence and Bandwidth
1. The bandwidth determines the coherence length and time,Tc 1/(4), Lc c/(4)
2. A minimum optical bandwidth is required for a pulse of duration T, 1/(4T)Restricts the spacing of DWDM signals for given rates
CC TcL =
41
CT
Short coherence length broadband source
Coherence and Optical Bandwidth
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Coherence and Optical Bandwidth
ExamplesLight Bulb:
LED:
DFB Laser:
m5
ps02.0
THz50
nm500WidthLineSpectral
C
C
OpticalL
T
BW
m50
ps2.0
THz5
nm50WidthLineSpectral
C
C
Optical L
T
BW
m30
ns100
MHz10
pm1.0WidthLineSpectral
C
C
Optical L
T
BW
Interference
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Interference
Interference = effect of adding waves from different sources
Constructive Interference Destructive Interference
Conditions for interference:
waves must be coherent andhave the same polarization
coherent sources add in phase
Incoherent sourcesadd in power: [ ] +==
kkk
kk
tEtPtP2
Total)sin()()(
2
Total )sin()(
+= kkk tEtP
+ =
+ =
Ph i l O i
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Physical Optics
Interference and Diffraction
Interference Based Technologies
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Interference Based Technologies
Interference
Fiber Bragg Grating
Optical Mux/ Demux
Optical Spectrum Analyzer
dd'
n n'
Thin Film filters
Wavelength Meter
Optical Networking - The DWDM Forest
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p g
PassiveComponents
DispersionCompensator
DispersionCompensator
OtherSegment
OtherSegment
OtherSegment
Other
Segment
LAN
CrossConnect
(Switch)
Data
1
2...
n
Data
1
2...
n
demultiplex
er
multiplexe
r
Interference Between Two Sources
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Interference Between Two Sources
Constructive Destructive
Waves
Interference in One Dimension
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Interference in One DimensionThe Michelson Interferometer
Inputbeam
Fixed referencemirror
Movingmirror
Interferencefringe pattern
When the mirror moves,the interference pattern alternates
light (constructive fringes) anddark (destructive fringes) on the detector.
The distance between fringes is related tothe wavelength of light
1/ 2 silveredmirror
The Wavelength Meter
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The Wavelength Meter
With a reference beam the Michelson interferometer can measureabsolutewavelengths
with an accuracy of 300 fm and resolution bandwidth of 30 pm
e.g., Agilent 86122
Wavelength
Reference
UnknownSignals Fixed Mirror
Signal
Processing
Moving Mirror
FFT the fringe pattern signal spectrum
The Fabry Perot Etalon d
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The Fabry Perot Etalon d
/2 /4
A resonant cavity containingmultiple reflections/ transmissions
If d= / 2 thenreflections and transmissionsinterfere constructively
If d= / 4 theninterference is destructive
Which makes a filter
Its also the foundation for a LASER, as well see soon.
Thin Films
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Thin Films
Series of Fabry-Perot Etalons with different properties
Anti-reflective coatings- must be centered at some wavelength
Selective transmission/ reflective coatings
Many possibilities:
d
d'
n n'
Layers Substrate
IncomingSpectrum
Reflected
Spectrum
TransmittedSpectrum
Tunable Fabry-Perot Filters
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Tunable Fabry Perot Filters
Filter shape
Repetitive passband with Lorentzian shapeFree 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 GHzInsertion loss: 0.5 to 35 dB
Fiber
Piezoelectric-actuators
Mirrors
Optical Frequency
FSR
1 dB
30 dB
The In-Fiber Bragg Grating (FBG)
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e be agg G at g ( G)
A simple filter and a cornerstone of the Optical Revolution
To make a stretch of fiber into a grating:
scratch the fiber with ultra-violet light
Waves are transmitted and reflected at each scratch
Regular intervals between gratings reflect one wavelength- a notch filter
Cladding
Primary coating(e.g., soft plastic)
Core
SiOGlas
s2
In Fiber Bandpass and Chirped Filters
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p p
Band Pass Filters
Combine simple Fiber Bragg Gratings (FBG) to form Etalons tuned to
different wavelengths and make in fiber Fabry-Perot bandpass filter
Overlap gratings tuned to different wavelengths - Moire filter
L
Chirped Filters
Uneven grating spacing can be used in different applications
- e.g., compensation of Chromatic Dispersion (more later)
Interferometric Filters
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Combined Technology: Coupler + Interferometer + FBG
2 is reflected by the gratings
The Coupler is chosen so that the 2 couples to 2 on reflection
Only 2 emerges from 2
The second coupler is wavelength-independent to form a Mach-Zehnderband pass filter.
Make mux/ demux, add-drop node, etc
1
2
3
2
1,
3, . . .
1,
2,
3, . . .
Arrayed Waveguide Grating
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y g gGeneralized Mach-Zehnder Interferometric Filter
An MZI filter with wavelength dependent coupling
1 x n mux/ demux
Lower loss, flatter passband
+ + +
Multiport coupler Multiport coupler
Array of waveguides shifts the relative phases of each wavelength resulting in constructiveand destructive interference of different wavelengths at different outputs
Polarization Based Technology
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Low loss in forward direction - 0.2 to 2 dBHigh loss in reverse direction - 20 to 80 dB
Reflectedlight
gyThe IsolatorMain application:
To protect lasers and optical amplifiers from reflections thatcan cause instabilities
Input light is polarized and transmitted through a series of polarizors Series of polarizors reject light of all polarizationsin the back direction
Input l ight
Add Drop Nodes and Circulators
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Combined Technology: Coupler + Isolator + FBG
Circulator based Add Drop Node (ADN)
Add iDrop i
Three isolated ports:
Port 1 port 2 Port 2 port 3 Port 3 port 1
Filter reflects iAdd / Drop
Common Passband
Thin film based ADN
Monochromators and Optical Spectrum Analyzers
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Two Dimensional Gratings
Transmission grating
Reflection grating
Interference From Gratings
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Detector
Detector
Two Slits
Five Slits
More slits
more interference
With more interferencepurple red and blue
Spectrum is Resolved
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-3 -2 -1 0 1 2 3
-3 -2 -1 0 1 2 3sin() in units of /b
Increasing the number of slits
50 slits
1000 slits
Increases the spectral resolution
Wavelengths can be resolved withamazing accuracy: 60 pm
Grating Based Technologies
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Gratings are the basis for many tools and components: Monochromators
Notch/ passband filters,
Multiplexers/ demultiplexers Optical Spectrum Analyzers
Transmitting gratingsand
reflecting gratings
Grating Based Optical Spectrum Analyzer
S i M h t
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Rotate grating to shinedifferent color, ,
through aperture
Grating angle, , determines spectralline shining through aperture
ADC Processor
Amplifier
Display
Input
Photodiode
Aperture
Sweeping Monochromator
Optical Spectrum AnalysisE l
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Examples
Optical Amplifier TestingGain, Noise Figure, tiltet cetera
WDM Components50 GHz Mux, Filters,Add/ Drop Components
WDM SignalsSignal to noise for
channel spacingsas low as 50 GHz
Source Testing
FP, DFB lasersAmplitude, SMSR
PerformanceAccuracy, = 0 10 pmResolution, ~ 40 pmDynamic range ~ 70 dB
Physical Optics
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Physical Optics
Component Characterization
Filter Characteristics
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i-1 i 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, ..)
Total Insertion Loss
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A measure of the loss of light within an optical component.
DUTWithout
DUTWith
P
PIL 10log10)dB( =
Devi ce
Reference
Source DeviceUnderTest
PowerMeter
Reference Path
Filter Transmission Spectrum (Insertion Loss)OSA M th d
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OSA Method:Broadband Source + -Selective Detector
Fast Sweep High Dynamic Range Good Sensitivity Incoherent Light
Notch Filter
Source spectrum Detector sensitivityand resolution
Transmitted spectrum
Test Solution
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8614x Optical Spectrum Analyzer series
Built in applications to characterize
Sources, DWDM signals,Passive Components,Amplifiers
Built in Broadband Sources
10 pm absolute accuracy
2 pm repeatability
60 pm resolution bandwidth
70+ dB dynamic range
600 nm 1700 nm
Filter Transmission Spectrum (Insertion Loss)TLS Method: Selective Source + Broadband Detector
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TLS Method: -Selective Source + Broadband Detector
Notch Filter
P
Parameters to Test Center 3 dB Bandwidth Ripple
Isolation
Devi ce
Reference
)(
)(log10 10
DUTWithout
DUTWith
P
PIL =
Test Solutions
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86082Wavelength DomainComponent Analyzer
Tunable Laser System/Power Meter
+ Photonic Foundation Library
Fast filter characterization resolution: < 1 pm
Speed: > 2 sweeps/ s Range: 1260 1640 nm Vertical accuracy 0.06 dB
Configurable to perform all
parameter tests Scaleable to multi-channels resolution 0.1 pm Low SSE
Insertion Loss as a Function of WavelengthSwept Insertion Loss
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Swept Insertion Loss
Two standard approaches:
1.Use a Tunable Laser Source (TLS)
Worry about back scattering interference issues
2.Use a Broadband source and an OSA Short coherence length of source no interference issues
Must calibrate and subtract baseline spectrum Need high spectral density (energy at each wavelength)
Reference p/ n 5980-1454E:
State of the art characterization of optical components for DWDM applications
Return Loss
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Total Return loss
A measure of the light reflected by a componentreference
reflected
P
PRL 10log10=
Device Termination
LightInput
Attenuator
Power Detector
Angled
OutputDirectionalCoupler
DUT
Return Loss MeterModule (81534A)
Insertion/ Return Loss Measurement Subtleties
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Beware of multiple reflections!! If multiple reflections occur within a distance less than LC
then there will be interference fringes on the detector
uncertainty in RL up to 100%
Avoid using coherent beams
LED or Tungsten lamp ideal, but too low spectral energy density
for most cases
But RL() may be desired and may depend on and
a narrow source has a large LC Common to use a Fabry-Perot laser
high power, with sidebands to mit igate interference,
still . . . LC~ meters
Understand the device and the source
Polarization Dependent Loss
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Recall Polarization describes theorientation of the electric field
The attenuation of light in fibers and network elements variesaccording to polarization.
Polarization Dependent Lossthe variation of attenuation with polarization.
Monitor output power of DUT while varying polarization to get
min
max10log10)dB(
P
PPDL =
Polarization Dependent Loss
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Insertion loss with polarization control:
Source
Device
UnderTest
(DUT)
Power Meteror
OSA
OpticalIsolator
PolarizationController
(PC)
Reference Path
Need: Constant power into DUT Polarization Controller (PC)
either provide all states,or use a Mller matrix technique
Polarization independent power meter/ OSA
Look for PDL < 0.1 dB
8169 Polarization
Controller
min
max10log10)(
P
PPDL =dB
Optical Networking - The DWDM Forest
Passive1
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Passive
Components
DispersionCompensator
DispersionCompensator
OtherSegment
OtherSegment
Other
Segment
Other
Segment
LAN
CrossConnect
(Switch)
Data
2...
n
Data
1
2...
n
demultiplexer
multiple
xer
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LightTransmission, Reception,and
Modulation:
Active ComponentCharacterization
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Modulation
Putting information into pulses of lightInternal and external modulators
1
r
Optical Networking - The DWDM Forest
Modulators
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DispersionCompensator
DispersionCompensator
OtherSegment
OtherSegment
Other
Segment
Other
Segment
LAN
CrossConnect
(Switch)
Data
2...
n
Data
1
2.
.
.
n
demultiplexer
multiple
xer
ModulationEncoding Data into Light Pulses
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g g
Extinction ratio: typically, about 10 dB.
Average power:
,"0"
"1"
Logic
Logic
P
PE=
"0""1"2
1
LogicLogicAvg
PPP +=
"0"
"1"
10log10)(Logic
Logic
P
PE =dB
0 1 1 0 0 1
Time
Plogic "1"
Plogic "0"
Pavg
P (mW)
NRZ - Non Return to Zero and RZ - Return to Zero Modulation
Modulation Techniques
Encoding data into binary pulses
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Encoding data into binary pulses
Direct modulation:modulate LASER input power
LASER chirp
1.5 Mb/ s - 2.5 Gb/ s
Indirect modulation:
Indirect or phase modulation Modulation with absorption or
Modulate with interference
Constructive 1, destructive 0
AM sidebands dominate line width
Can approach = 1/(4bit)
10 Gb/ s - 40 Gb/ s 200 Eb/ s
DC
RF
DC MOD
RF
Indirect ModulationMach-Zehnder Principle
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p
Split the beam and vary the optical path lengths to get:
Logic 0 - destructive interference - optical path length difference of (2n-1) / 2
Logic 1 - constructive interference - optical path length difference of n
Vary path lengths tochange relative phase
Split beam
Merge beam
Destructive interferencelogic 0
The relative phasechanges!
An external electric field applied to some crystals, e.g., LiNbO3, changes the index
of refraction, n = n(E)1. Use an RF modulation signal to apply a voltage across the crystal2. Varies the index of refraction of each path3. Change the path lengths to get desired interference
Modulate the light signal!
Indirect ModulationElectro-Absorptive Modulation
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p
A solid-state based shutter
Pass a continuous wave through a diode
Logic 1 - No applied bias,larger band gap
light transmits through
Logic 0 - High reverse bias,smaller band gap
light is mostly absorbed
Energy
Conduction Bands
Valence Bands
Continuous wave input
gapEh > gapEh
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Lasersection
Modulation
section
DFB laser with externalon-chip modulator
Polarization sensitive, need correct launch
Mach-Zehnder Modulator
Electro-Absorptive Modulator
Atomic Physics:
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Light Generation
Electromagnetic radiation
Light Emitting Diodes (LED)
Light Amplification by Stimulated Emission ofRadiation (LASER)
1r
Optical Networking - The DWDM Forest
Optical
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DispersionCompensator
DispersionCompensator
OtherSegment
OtherSegment
OtherSegment
OtherSegment
LAN
CrossConnect
(Switch)
Data 2...
n
Data
1
2...
n
demultiplex
er
multiplexe
r
Transmitters
The Premise for Radiation
All radiation results from the acceleration of a charge
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An LRC circuit An oscillating dipole:
An atomic transition
+
-
+
+
2p state
to a
1s state1s
2p
Spontaneous Emission
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Light is spontaneously emitted when an electron decays from ahigher to a lower energy state.
In an atom In a semiconductor
Energy
Conduction Bands
Valence Bands
Energy
Light Emitting Diode (LED)
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Datacom through air & multimode fiberInexpensive(laptops, airplanes, LANs)
Key characteristicsMost common for 780, 850, 1300 nmTotal power up to a few WSpectral width 30 to 100 nmCoherence length 0.01 to 0.1 mmNo specific polarization
P -3 dB
P peak
BW
Energy
SpontaneousEmission
Conduction Bands
Valence Bands
Data in
LASERLight Amplification by Stimulated Emission of Radiation
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Requirements for LASER:
Confine light in a resonant cavity toset up standing waves
Excite more electrons to higher energy levels
More photons from stimulated emission thanfrom spontaneous emission
population inversionStimulated Emission will resonate in the cavity
amplification
Mirrors confine s to2L/n& also reject non-perpendicular light
L
Stimulated Emission light that ismonochromatic (same wavelength)coherent (in phase)polarized
Fabry-Perot (FP) Laser
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Reflective coatings along cavity allow onlyn/ 2 wavelengths through
Multiple longitudinal mode (MLM) spectrum Classic semiconductor laser
First fiberoptic links (850 or 1300 nm)Today: short & medium range links
Key characteristicsMost common for 850 or 1310 nm
Total power up to a few mWSpectral width 3 to 20 nmMode spacing 0.7 to 2 nmHighly polarizedCoherence length 1 to 100 mmGood coupling into fiber
P peak
I
PThreshold
Distributed Feedback (DFB) Laser
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Sidemodes filtered out Single longitudinal mode (SLM) spectrum High performance telecommunication laser
Most expensive (difficult to manufacture)Long-haul links & DWDM systems
Key characteristicsMostly around 1550 nmTotal power 3 to 50 mwSpectral width 10 to 100 MHz
(0.08 to 0.8 pm)Sidemode suppression ratio
SMSR > 50 dBCoherence length 1 to 100 mSmall NA ( good coupling into fiber)
Ppeak
SMSR
Comparison of FP and DFB Lasers
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1300 13101305 nm
REF: 0.00 dBm
-90
-70
-50
-30
-10
10
dBm
1540 15501545 nm
REF: 0.00 dBm
-90
-70
-50
-30
-10
10
dBm
Power versus wavelength
Fabry-Perot Laser DFB Laser
Vertical Cavity Surface Emitting Lasers
Di t ib t d B R fl t (DBR) Mi
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Distributed Bragg Reflector (DBR) Mirrors
Alternating layers of semiconductor material
40 to 60 layers, each / 4 thick
Key properties
Wavelength range 780 to 1310 nm Gigabit ethernet
Spectral width < 1 nm
Total power > -10 dBm
Coherence length: 10 cm to 10 m
Numerical aperture: 0.2 to 0.3
active
n-DBR
p-DBR
External Cavity/ Tunable LasersLow Source Spontaneous Emission(SSE) output
A il t M i
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High Poweroutput
Adjust angle to choosereflected wavelength
Adjust cavity length to chooseresonant wavelengths
Agilent Magic
n/2
Grating
High Power
Low SSE
Tunable Laser System
The Agilent TLS systems for the 8164 mainframe
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1260 < < 1640 nm in three different modules Two Power outputs:
+5 dBm peak (high power output)
- 6 dBm peak (low SSE output)60 dB signal to SSE ratio
10 pm absolute wavelength accuracy 2-3 pm typical relative wavelength accuracy, mode-hop free
0.1 pm wavelength resolution
Other Light Sources
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Need for small coherence length high power 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 w/ nm
SummarySemiconductor Light Sources
Data in
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Stimulated Emission(monochromatic & in-phase)
LASERs
Lasers Require Stimulated Emission Population Inversion ConfinementE
nerg
y
LEDs
Spontaneous
Emission
I
PThreshold
More electrons in upper energylevel than lower(more photons emitted thanabsorbed)
Mirrors confine s to2L/ n & also rejectnon-perpendicularlight
L
Parameters to TestCharacterization of Transmitters
Output Power
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Output Power Power meter
Wavelength
Optical Spectrum Analyzer, accuracy ~ 15 pm, 50 pm Interferometer-based wavelength meter, accuracy ~ 5 pm, 0.3 pm
Linewidth, chirp, modulation effects, ultra DWDM structure High Resolution Spectrometer,
accuracy
~ 15 pm, 8 fm
Distortion, Relative Intensity Noise (RIN), harmonic noise,Spontaneous emission/ recombination relaxation effects
Lightwave Signal Analyzer
Electrical-Optical Response, BandwidthRecombination time scale affects modulation properties
Lightwave Component Analyzer
Transmitter Linewidth MeasurementWith a High Resolution Spectrometer
O i l H d
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Optical Heterodyne
Measure transmitter linestructure with terrificdetail
resolution: 8 fm
Transmitter
Reference
Interferometer
Chirp/ FM MeasurementWith an HRS
Chirp A change in the optical
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Chirp = A change in the opticalfrequency caused by directmodulation of the laser
Direct Modulation
Transmitter
Reference
Interferometer
Modulator
Chirp/ FM MeasurementWith an HRS
Indirect Modulation with a
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Indirect Modulation with aMach Zehnder modulator
Transmitter
Reference
Interferometer
Modulator
(0.2 nm span)
High Resolution Optical SpectrometerAgilent 83452A
Optical Heterodyne and high resolution spectrum analysis
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Optical Heterodyne and high resolution spectrum analysis
Linewidth, laser spectral symmetry, modulation spectrum,relaxation oscillations, close-in sidebands, UltraDWDM spectra
Resolution: better than 80 fm (10 MHz)Dynamic Range: 60 dB
Ultra DWDM spectrum
Lightwave Signal Analyzer
Essentially a wide-bandwidth calibrated OE + Electrical SpectrumAnalyzer
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Essentially a wide bandwidth, calibrated OE + Electrical SpectrumAnalyzer
Measures average optical power vs frequency, (over modulationfrequency range!)
Harmonic Noise
Relative Intensity NoiseNoise from the Transmitter
Relative Intensity Noise [ ]12)(1 P
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Relative Intensity Noise(Incidental Amplitude Modulation)
Measures laser fluctuations at modulation frequencies
The power variance spectral density , (P)2/BW, is the fluctuation of
power observed in the interval fmodto fmod+dfmodper unit modulationbandwidth, dfmod.
After electrical conversion (recall (optical power)2 (electrical power) ),measure RIN from the electrical spectrum analyzer:
[ ]1-HzRIN2
)(1
avgP
P
BW
=
BWRINii avgRIN
Measuring Relative Intensity NoiseWith an LSA
[ ]dB/ HPRIN 1)(l102
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DFB Laser
[ ]dB/ HzBWP
PRINavg
1)(log102
Agilent 71400
Atomic Physics:Light Detection
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Solid state photon countersresponsivity
Photo Diodesefficiency, gain, dark current
Light Detection
Data
1
2.
multiple
xer
Optical Networking - The DWDM Forest
Optical
Receivers
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DispersionCompensator
DispersionCompensator
OtherSegment
OtherSegment
Other
Segment
OtherSegment
LAN
CrossConnect
(Switch)
.
.
n
Data
1
2.
.
.
n
demultip
lexer
m Receivers
Light Detectors
C d ti Photons are absorbed in an
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Valence
Bands
Conduction
Bands
Energ
y
Photons are absorbed in anintrinsic layer of semiconductor create e - hole pairs
Apply a reverse-bias potential photocurrent
Quantum Efficiency = number of electrons created per photon, ()
Responsivity = photocurrent per unit of optical power (A/ W)
hc
e
P
iR
)(
)(
)()( ==
Optical
Photo
Photo Diodes
PIN (p-layer, intrinsic layer, n-layer)Highly linear low dark current
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(p y , y , y )Highly linear, low dark currentDetector is followed by a Transimpedance
Amplifier
Avalanche photo diode (APD)Intrinsic gain up to x100 lifts the optical signal
above electrical noise of receiver
Strong temperature dependence
Main characteristics
Quantum efficiency (electrons/ photon)Dark currentWavelength dependence, responsivity
n+
Bias Voltage
A
PD
Gain
Material Aspects of PIN Diodes
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Silicon (Si)Least expensive
Germanium (Ge) Classic detector
Indium gallium arsenide (InGaAs)Smooth responsivityHigh speed
Notice the sharp wavelength rolloff- due to E
band gap
> h= hc/
0.7 0.9 1.1 1.3 1.5 1.7
90%
70%
50%
30%
10%
Quantum
efficiencies
Si
InGaAs
Ge
Wavelength (m)
Responsivity(A/W)
hc
eR
)()( =
Quantum efficiency
Noise
Thermal (Johnson) Noise is the intrinsic noise from the load resistor in
the photodiode circuit
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R = Load resistancek= Boltzmanns constant = 1.38 10-23 J/ KTin Absolute,B = modulation bandwidth
R
BkTi rmsTherm
=
4
Dark current, id, is the current generated in the absence of light
Thermally or spontaneous diffusion generated charge in the photodiode.Typical values at T= 300 K, Si: 1 - 10 nA, Ge: 50 - 500 nA, InGaAs: 1 - 20 nA
Shot Noise, (quantum noise) is from the random arrival time of electronsin the detector- Shot noise causes the photo-current to fluctuate about amean, iavg and includes the dark current.
- Trouble with small signals in noisy environments- Shot noise limited means shot noise > thermal noise
Biiei davgrmsshot )(2 +=
Receivers:Sensitivity and Modulation Bandwidth
Hi h i i i i l / d d
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High sensitivity requires a large/ deep detector Need to detect each photon = increase quantum efficiency
create more electron-hole pairs and catch each one
Large Bandwidth requires a small/ shallow detector Need to finish detection process fast to accommodate a short pulse
Larger the detector the longer the relaxation time of the detection process
Tradeoff between sensitivity and bandwidth:Larger bandwidth, lower sensitivity
Example: Sensitivity Modulation rateLightwave Clock/ Data Receivers: Agilent 83446A -28 dBm 2.5 Gb/ s
Agilent 83434A -16 dBm 10 Gb/ s
Typical Power Levels
Transmitter:
6 to +17 dBm (0.25 to 50 mW)Optical Amplifier:
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Optical Amplifier:+3 to +20 dB (gain of 2 to 100 times input)
Difference between optical and electrical power:Optical power is converted to photocurrent,
iphoto = PopticalG, G = conversion gain, typically 0.4 - 0.9 A/ W,so
in dB, a change in Poptical means twice that change in Pelectric
RGPPRiP opticalelectric22 )(==
(dB)(dB) opticalelectric PP = 2
iOptical
fOptical
ielectric
felectric
electricP
P
P
PP
(dB) log20log10 ==
Active Component CharacterizationTransmitters, Receivers, Regenerators
Measure Electro-optic response
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Fixed wavelength, measure response vs fmod
Vector network analyzer with precisely calibrated optical interface
Measures E/ O, O/ O, O/ E, E/ E devices
Gives 3 dB bandwidth
Flatness of frequency response
RF Receiver& Display
O/ O
O/ EE/ OE/ ERF
Lightwave Component AnalysisSource Responsivity = Optical power
produced (W)/ electrical currentsupplied (A)
Receiver Responsivity = Electrical
current produced (A)/ optical powersupplied (W)
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pp ( ) pp ( )
Modulation Rate
Modulation Rate
W/ A A/ W
=
in
out
s
I
PR 10log20(dB)
=
in
out
r
P
IR 1020log(dB)
Typical Laser Frequency Response Typical Photodiode Frequency Response
Frequency Response and Modulation BandwidthLightwave Component Analyzers
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8702300 KHz - 3 or 6 GHz 870350 MHz - 20 GHz 8603045 MHz - 50 GHz
Data
1
2.
.multiple
xer
Optical Networking - The DWDM Forest
Active
ComponentsModulators
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DispersionCompensator
DispersionCompensator
OtherSegment
OtherSegment
Other
Segment
OtherSegment
LAN
CrossConnect
(Switch)
.
n
Data
1
2.
.
.
n
demultiplexer
Modulators,Transmitters,Receivers
Optical Signal
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Optical SignalAmplification
andDWDM
Atomic Physics:Optical Amplification
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Raman scattering
Optical pumping
Optical Amplification
Signal Attenuation
OH absorption
O-band
E-band
S-band
C-band
L-band
U-band
Networking wavelengthbands
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P
owerAbsorbed(dB/
km)
1200 1400 16001000 1800
(nm)
UV-absorption
IR-absorption
RayleighScattering
Typical fiber attenuation 0.2 dB/ km
amplification every 25 - 50 km(Not very good fiber)
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Spontaneous and Stimulated Emission
Spontaneous emissionLight is spontaneously emitted
h l d
Stimulated emissionIncident light stimulates the decay of an electron.
Li h i i d h i id i l i id li h
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when an electron decays to a
lower energy state.
Light is emitted that is identicalidenticalto incident light.
Same wavelength, direction, polarization, and phase
Twice as much lightoutgoing as incident gain of two
Raman Scattering
When a bound electron is excited to some energy Eex and
decays by emitting light of energy Elight , with
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Elight Eex it is called Raman scattering
E
lightE
ex
Lifetime of the excited state is the relaxation time~ 1 ms - metastable (Erbium)< 1 ns - unstable (SiO2)
Optical Amplifiers - Erbium Doped Fiber Amplifier
1. Dope a fiber with erbium 2. Pump energy into the fiber
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3. Transmit and amplify the signal
Amplified signalSignal
Pumplight
Amplified Spontaneous Emission(ASE) noise!
Erbium Doped Fiber Amplifiers
Erbiumdoped fiber
LASER pumping energyinto Er doped fiber
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p
Incoming
signal
Outgoing amplified signal
plus
Coupler
ASE noise
Two major characteristics:GainNoise
Amplifier Input/ Output Spectra
Signal spectrum Amplifier Spectrum
Amplified Signal
Spectrum(Amplified Signal +
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Signal spectrum Amplifier Spectrumwith noinput
Reduced ASE +Amplified Source noise)
Output Spectrum
+10 dBmAmplified signal
spectrum- the input signal
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ASE spectrumwith noinput
1575 nm
-40 dBm
1525 nm
saturatestheamplifier)
The relaxation time of Er causes a ~ 1 ms time scale for the ASEbackground to shift between levels with and without a signal
Other Optical Amplifier TechnologySemiconductor and Raman Optical Amps
Same physical principle but different energy structure
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Other Dopants: xDFA
50 nm3 dB
Wavelength
Gain(dB)
Raman AmplifierUse vibrational energy states of SiO2 instead of atomic states of a dopant
Raman scattering among vibrational statesPump laser through the whole fiber- pump energy is very high- states are not metastable
Gain region over ~ 100 nm centered about 13 THz above the pump frequency
Semiconductor Optical Amplifier (SOA)- Stimulated emission in of the signal as ittraverses an excited semiconductor
- Pump to high energy states by bias current orexternal pump laser
- Tend to be very noisyGain region can be tuned with band gap
Optical Amplifiers - Summary
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EDFA- Material is doped with Erbium- Material is pumped with
1480 or 980nm- Good for 1550nm signals
Pump Light
Signal (pre-amp)Signal (post-amp)
Raman Amplifier- Whole undoped fiber is pumped- Wide signal -range- Amplification throughout fiber length- High pump power required
DWDMDense
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DWDMDenseWavelength
DivisionMultiplexing
Introduction toDense Wavelength Division Multiplexing
Use many different wavelengths on one fiber
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single mode or multi-mode fibers
Combine different wavelengths to increase data rate
e.g., 125 s at 40 Gb/ s each 5 Tb/ s (Alcatel, Feb-2002)
New and growing technology requiring
mux/ demux and optical switch technologies
peculiar dispersion properties
worry about noise at different s
worry about optical crosstalk - four wave mixing
RL +0.00 dBm
Amplified Dense Wavelength Division MultiplexSpectrum
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1565 nm
5.0 dB/ DIV
1545 nm
Amplified
SpontaneousEmission (ASE)
Channels: 16
Spacing: 100 GHz (~ 0.8 nm)
Basic DWDM Design
WavelengthsPower levels
RS RS
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Signal-to-noise ratios
2
n
1
n-1
2
n
1
n-1
S
Rm1
Sdn Rn
S1
RS R
R1Sd1
Sn Rmn
Amplified DWDM SpectrumParameters to Test
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P
Parameters to Test
Channel Gain Noise Figure
Center
Span Tilt Gain Flatness
Channel Spacing
Amplifier Characteristics
Signal
Signal
out
P
PG =
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out
in
SNR
SNRFigureNoise =
inP
Gain = 0 50 dB
Noise Figure = 3.5 12dB= Output Spectrum of Amplifier without input
= Input spectrum to Amplifier= Output spectrum from Amplifier
Interpolated Source Subtraction (ISS) Method
Gain & Noise Figure
1. Measure a constant wave input spectrum
2. Measure the amplified constant wave output spectrum
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EDFA
Optical Spectrum Analyzer
Power Meter
Calibration Path
Laser Source
3. Interpolate across the signal to estimate the background at the signal
wavelength separate signal and noise
Time Domain Extinction (TDE) Method
SignalSpectrum
Gain & Noise Figure
1. Measure output withsignal gated off
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EDFA
Optical SpectrumAnalyzerDFB/ TLS
Gate
Gating ofSignal
Calibration Path
signal gated off
2. Long relaxation time (metastability) of the Erbium excited states allow
direct measurement of the spontaneous emission curve
Gain Compression
Total output power: Amplified signal + ASE
EDFA is in saturation if almost all Erbium
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EDFA is in saturationif almost all Erbiumexcited states are consumed by
amplification
Total output power remains almostconstant
Lowest noise figure
Preferred operating point
Power levels in link stabilize automaticallyPin (dBm)
Total Pout
-3 dBMax
-20-30 -10
Gain
Span Tilt and Peak to Peak Deviation
dB
nm
Peak-to-PeakDeviation
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Span Tilt
Deviation
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Standard Amplifier Test SetupEDFA, Semiconductor, and Raman Amplifier
OA(DUT)
DFB
SWITCH SWITCH
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(DUT)
DFB
DFB OSA +
OA-TestRoutine
ATT
SWITCH SWITCH
WDM
TLS PowerMeter
OSA separates signals from broadband spontaneous emission
Amplifier characteristics depend on input power, wavelength, polarization Sources simulate operation (multichannel, adjustable power)
The Complete EDFA Test Solution
Time Domain Extinction Method Interpolated Source Subtraction
MethodN i G i P fil M th d
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Noise Gain Profile Method
Issues in DWDMSM fiber can tolerate up to 50 mW (+17 dBm)
Nonlinear effects start causing trouble around 10 dBm About 100 kW/ m2 ! limits available channel power to Power/ channel < 50 mW/Nchannels
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Optical Amplifiers have limited effective range e.g., EDFA: 1525 < < 1565 (roughly)
High power densities cause nonlinear scattering e.g., Kerr effect: n= n(E)
Four Wave Mixing (FWM), self-phase modulation, . . ., noise
Trends:Higher capacity
160 wavelengths 12.5 GHz spacing
All optical network (the grail)
Optical Networking - The DWDM Forest
EDFAData
1
2...
n
multiplexer EDFA
Optical
Amplification
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DispersionCompensator
DispersionCompensator
EDFA
OtherSegment
OtherSegment
Other
Segment
OtherSegment
ADN
LAN
CrossConnect
(Switch)
Data
1
2...
n
demu
ltiplexer
Dispersion:
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Dispersion:The evolution of the index of
refraction w ith wavelength and
polarization
Electrodynamics ofContinuous Media
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Continuous Media
Chromatic dispersion
Polarization mode dispersion
x
Optical Networking - The DWDM ForestDispersionCompensators
Data
1
2...
n
multiplexer
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DispersionCompensator
DispersionCompensator
OtherSegment
OtherSegment
OtherSegment
OtherSegment
LAN
CrossConnect
(Switch)
Data
1
2...
n
demultiplexer
Color: Index of refraction
Why is a ruby red?
White Light
+ Only red is
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Because the nRuby() has a resonance at red.
For pigments, e.g., red paint, the other colors are absorbed in the mess of organic molecules that have manyavailable energy states resulting in heat.
+
+
+
Only red is
absorbed
Red is emitted at
random angles
Ruby "atom"
Excitedruby "atom"
The Index of Refraction
The index of refraction
of a dielectric is given by The energy carried by light is determined by the frequency, not the
wavelength, so the frequency of light in media doesnt change, but the
media
vacuum)(c
cn =
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wavelength does.
The index of refraction varies with wavelength, different colors travel at different speeds chromatic dispersion.
The index of refraction can also vary with polarization birefringence polarization mode dispersion
The heart of these phenomena is the response of the media to theelectric and magnetic fields that compose the light.
media
vacuum
media
vacuum
media
vacuum)(
===
c
cn
vacuum
mediavacuummedia )(
cc =
Chromatic Dispersion Spreads Pulses Increases the Bit Error Rate
Recall from coherence:
the minimum frequency/ wavelength spread of a pulse of light is
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n = n() narrower the pulse, the larger the chromatic dispersion Modulation sidebands and chirp also increase pulse wavelength content
41
CT
PulsePulse cTT
44
1 2orthus
T T+T
The Cause of Chromatic Dispersion
Two causes:
1. Material Dispersion
The response, permittivity , of the media:
x
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2. Waveguide Dispersion
ncore() and ncladding() vary differently with
different boundary conditions for different wavelengths
different solutions to the wave equation
===
000media
vacuum)(ccn
Dielec
tricco
nstant
Chromatic Dispersion - Definitions
Define
describes how the propagation time varies with wavelength in ps/ nm
d
d g )(
dispersionchromatic
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where g() is the propagation time along a fiber of length L.
Factor out the length and get the
The pulse spreads by an amount
d
d
L
Dg )(1t,coefficiendispersionchromatic
DLT
T T+T
Chromatic Dispersion Observables
)(g
Group
cg
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Delay (ps)
0
Chromatic dispersion coefficient(ps/ nmkm)
d
d
L
Dg )(1
Zero dispersionwavelength,
typically 1300 nm
0
Typical CD value: 10 ps/ nmkm
Tolerable Levels of Chromatic Dispersion
Require with B= bit rate in Gb/ s
since and so
Bg
1
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and
For 1 dB penalty:
2
1Bd
dDLg =
2
510
B
DL
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Optical
Modulator
DUT
Tunable
Laser
TunableRF Source
Phase
Detector
Signal
Processing
Data
g() = /2f
4. Fit curve to g() and calculate
d
d
LD
g )(1
Chromatic Dispersion Compensation
Dispersion compensating fiber: Follow a segment of dispersing fiber with a segment of dispersion
compensating fiber
DispersingDispersion compensating
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There are also compensating components, e.g., the chirped FBG
Dispersing
fiberfiber
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Principle States of Polarization
Fibers and components have distinctslow and fast polarization axis
nx
ny
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TT+T
PMD in fibers is a Random Process
Small random variations in fiber geometry and media cause unpredictable
changes in polarization states and principal states of polarization For fiber length much larger than the correlation length, (L >> LC)
The Differential Group Delay, , follows a Maxwellian distribution
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The Differential Group Delay, , follows a Maxwellian distribution
Since contributions from different segments are independent, theycombine to a total in quadrature (i.e., like the sides of a right triangle):
The mean DGD, , increases with
For components, e.g., filters, (L >> LC) PMD is deterministic
2
2
23
22)(
= ef
22
2
2
1 Ntotal +++= L L
N
Tolerable Levels of Polarization Mode Dispersion
PMD is a random processdepending on temperature and geometry
PMD combines like the legs of a right triangle:
Define the 1st order PMD Coefficient:L
P
=2
iTotal
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1st order PMD is wavelength independent
2nd order PMDdepends on wavelength, but is very small
Typical good quality fiber:
Older, poor quality fiber:
Tolerate
L
km/ps2.0
km/ps21
BTotal1.0
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p
low quality 2 ps/km
The graph is forthe average,
Fear the tail!
1
100
10,000
1,000,000
Le
ngth( k
m)
Data Rate (Gb/s)
0.01 0.1 1 10 100
OC-1 OC-3 OC-12
OC-48 OC-192 OC-768
Poor quality fiber
Measuring Polarization Mode DispersionJones Matrix Eigenanalysis method:
Measure DUT Transfer matrix
at three known polarizationsat a set of wavelengths, J(): J() Pin = Pout
Extract T() by diagonalizing J()
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Polarization
Synthesizer
Polarization Analyzer
S0 S1 S2 S3
TunableLaser
TransferMatrix
O1
O2
I1
I2
J11 J12
J21 J22
J() Pin = Pout
Jones Matrix Eigenanalysis (JME) Result
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Cannot compensate PMD with a passive device
Must have feedback to monitor PMD and actively compensate
Also measure PMD with Modulation phase shift and interferometric techniques
Complete Dispersion Test SetAgilent 86038
Modulation Phase Shift MethodSingle connection measurement ofChromatic Dispersion and
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Polarization Mode DispersionExcellent for
Broadband device characterization
e.g., spools of fiber
Narrowband device characterizatione.g., filters, mux/ demux, etc.
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Optical Networking - The DWDM ForestDispersionCompensators
OtherSegment
Data
1
2.
.
.
n
multiplexer
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DispersionCompensator
DispersionCompensator
OtherSegment
OtherSegment
Segment
Other
Segment
LAN
CrossConnect
(Switch)
Data
1
2.
.
.
n
demultiplexer
Characterizing the
SystemOptical Time Domain Reflectometry
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Optical Time Domain ReflectometryNoiseBit Error Rate MeasurmentsEye diagram analysis
Extinction ratioJitterMask tests
Optical Time Domain Reflectometry:Link Characterization
Optical Time-Domain Reflectometer(with Power Meter, Visual Fault
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Finder, and Laser Source)
Optical radar
Measures loss vs. distance
10 m - 200 km range
Key tool in installation and
maintenance MM and SM modules
FusionSplice
Bend Connector Crack FiberEnd
MechanicalSplice
Los
s
Distance
Bit Error Ratio
Bit error ratio,
BER = # of bits received in error/ # of bits received
Gives a good indication of the performance of acomponent, link or entire network
BER
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Typical BER spec: 1.0 10-12
Tradeoff between
minimum input power andacceptable bit error rate
Larger the powerless effect of dispersion,
less noise from optical amplifiers, etc.
Pi (dBm)
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BERTs
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86130 Bitalyzer (up to 3.6 Gb/ s)
71612 HSBERT (up to 12.5 Gb/ s)
81250 ParBERT (up to 43 Gb/ s)
Pulse Parameters
Overshoot On Logic 1
b180%
Fall Time
Half-Period Of
Ringing Freq.
Undershoot On Logic 1
Rise Time
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50%
20%
b0bdark
Pulse Width
Overshoot On Logic 0
Undershoot On Logic 0Jitter
Eye Diagram Analysis
Standard compliance verification(SONET / SDH, G-Ethernet, Fibre Channel, ..)
Mask
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Pulse ParametersExtinction Ratio
Signal capture of patternscausing bit errors
Eye-line measurements
1 0 0
0 1 0
0 0 0
The Eye Diagram
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0 0 1
0 1 1
1 0 1
1 1 0
1 1 1
Superimposed Bit Sequences
Eye Diagrams
Power(mW)
Time (ns)
Digital Communications AnalyzerA sampling oscillosc
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