optical and photonic components for dwdm optical networks
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7/30/2019 Optical and Photonic Components for DWDM Optical Networks
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Optical and Photonic Components
for
DWDM Optical Networks
Pochi YehUniversity of California, Santa Barbara, California 93106
Accumux Technologies, Camaril lo, CA 93012
USA
July 3-4, 2003Seoul, KOREA
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Outline
• Introduction
– Traffic Growth and Bandwidth Demand– DWDM: an economical solution
• Optical and Photonic Components
– Active Components– Passive Components
• Recent Progresses
– Wavelength Management– Dispersion Management – CD, PMD
• Summary
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TRAFFIG GROWTH AND BANDWIDTH NEEDS
• Data (mostly Internet) traffic doubles every 9 to 12months
• Lighting more dark fibers is not a viable solution
• DWDM Broadband communications in single
mode fibers provide an economical solution
• Enabling Optical Components for wavelength and
dispersion management are needed
• Networks need to be flexible and scalable, andmanaged by software
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Evolution of DWDM
1980’s 2 channels, 1310 nm, 1550 nm
1990’s 2-8 channels w ith 200-400 GHz channel spacing
1996 16+ channels with 100-200 GHz channel spacing
1999 64+ channels with 25-50 GHz channel spacing
2002 160+ channels with 25-50 GHz channel spacing
80 nm bandwidth @ λλλλ=1550 nm is approximately 10,000 GHz
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120 km 120 km 120 km
Optical Amplifiers
(b) 16-wavelength WDM: 40 Gb/s
16 fibers 1 fiber
80 regenerators 3 optical ampl if ier + DCM
(Approaching: 160 wavelengths
1,600 Gb/s)
Transmission Challenge and WDM Solution(a) Single-wavelength: 40 Gb/s (16 x 2.5 Gb/s)
MUXDEMUX
DCM
TERM TERMRPTR RPTR RPTR RPTR RPTR
80 Km 80 Km 80 Km 80 Km 80 Km 80 Km 80 Km
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DWDM Optical Transmission System
λ1λ1λ1λ1λ3λ3λ3λ3λ5λ5λ5λ5λ7λ7λ7λ7
λ1λ1λ1λ1λ3λ3λ3λ3λ5λ5λ5λ5λ7λ7λ7λ7
λ2λ2λ2λ2λ4λ4λ4λ4λ6λ6λ6λ6λ8λ8λ8λ8
λ2λ2λ2λ2λ4λ4λ4λ4λ6λ6λ6λ6λ8λ8λ8λ8λ7λ7λ7λ7
Optical Mux Optical DeMux
OA OA
OADM
DCM
Interleaver Interleaver
• Interleavers are essential for capacity upgrade via DWDM (e.g., 10 Gb/s
transmitter/receiver wi th channel spacing of 25 GHz)
• DCMs are essential for high speed transmission (e.g., 10 Gb/s, 40 Gb/s)
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DWDM System Functions
1. Generating the signals—The source, a laser, must provide stable light
within a specific, narrow bandwidth that carries the signal.
2. Combining the signals—Modern DWDM systems employ Multiplexers and
Interleavers to combine the signals. There is some inherent loss associated
with multiplexing and demultiplexing.
3. Transmitt ing the signals—The effects of crosstalk and optical signal
degradation or loss must be minimized within fiber optic transmission. Over
a transmission link, the signals may need to be optically amplif ied, and
dispersion compensated.
4. Separating the signals—At the receiving end, the multiplexed signals mustbe separated out.
5. Receiving the signals—The demultiplexed signal is received by a
photodetector.
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Essential Components of DWDMOn the transmitting terminal
Lasers with precise, stable wavelengths (± 1 GHz)
Dense Optical Multiplexer-Interleaver (25 GHz)
On the link
Low loss optical fiber, Flat Gain Optical Amplif ier
Dispersion Compensation Modules (DCM)
Optical Add/Drop Modules (OADM)
Optical Cross-Connect
On the receiving terminal
Dense Optical Demult iplexer-Interleaver
Photodetectors
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Enabling Photonic Components
• Active Components
– Transmitter/Receivers, DFB, VCSEL
– Amplifiers, EDFA, SOA
– Modulators
– Optical Switching
• Passive Components
– Mux-Demux, Array Waveguides (AWG), TFF, Gratings
– Interleavers, Michelson, Birefringent, Mach-Zehnder
– Dispersion Compensation Modules, DCF, Hi-Mode, FBG, GTI
– Wavelength Lockers
– PMD Compensation Modules– Optical Add-Drop Modules
– Gain Equalizers
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Wavelength Management• Mux/DeMux
– Thin Film Filters, 100 GHz, 50 GHz
– Gratings, 100 GHz, 50 GHz
– Fiber Bragg Gratings, 100 GHz, 50 GHz
– Array Waveguide Gratings (AWG)
• Interleavers, 25 GHz, 12.5 GHz– Michelson Interleavers
– Mach-Zehnder Interleavers
– Birefringent Interleavers
• OADM– Thin Film Filters, 100 GHz, 50 GHz
– Fiber Bragg Gratings, 100 GHz, 50 GHz
– Fiber Grating Couplers
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Thin Film Interference Filtersλ1λ1λ1λ1,,,, λ2λ2λ2λ2,,,, λ3λ3λ3λ3,,,, λ4λ4λ4λ4
λ2λ2λ2λ2
λ4λ4λ4λ4
λ1λ1λ1λ1
λ3λ3λ3λ3
Loss increases with channel counts
Filter Bandwidth limited to 100 GHz
Multiple-Cavity Filters requires many many Layers
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Array Waveguide GratingsPrinciple of Operation
λ1λ1λ1λ1 λ2λ2λ2λ2 λ3λ3λ3λ3 λ4λ4λ4λ4
λ1λ1λ1λ1λ2λ2λ2λ2λ3λ3λ3λ3λ4λ4λ4λ4
Grating equation:
λ=θ+∆ msind Ln
∆∆∆∆L = path differencebetween neighboring guides
d = distance between guides
m = integer
λλλλ= wavelength
θθθθ = angle of diffraction
∆L >>λ to achieve highorder diffraction
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Array Waveguide GratingsPrinciple of Operation
mN
λ=λ∆Spectral resolution:
m = Diffraction order
N = Number of guides in the array
For ∆λ∆λ∆λ∆λ = 0.8 nm @ 1550 nm, mN must > 2000
λ=θ+∆ msind Ln
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Array Waveguide Gratings
3 1-3 0
-2 5
-2 0
-1 5
-1 0
-5
0
Adjacent Isolation Non-Adjacent Isolation
λ1 λ2 λ3 λ4 Adjacent ITU channels
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λ1,λ2,λ3,λ4,λ5,λ6,λ7,λ8
λ1, λ3, λ5, λ7
λ2, λ4, λ6, λ8
• Channel Spacing Management via Optical Interferometry (e.g.,
Michelson, Mach-Zehnder) e.g., from 25 GHz to 50 GHz, or vice versa
• Current Mux/Demux made of thin film fil ters are inadequate in high
density WDM.
• Interleavers are needed in densely populated WDM systems with 2.5
GHz, 10 GHz, and even 40 GHz transmitter/receivers
Optical Interleavers
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Optical Interleaversusing Birefringent Interference
ν1 ν3 ν5 ν7
ν2 ν4 ν6 ν8
Output
Port 1
Output
Port 2
Birefringent
Crystal
PBS
Input Beam of
Polarized Light
L
]}L)nn(c
2cos[1{
2
1I oe − ν
π+=
eger intm,L)nn(
cm
oe
=−
= ν Network requirements:
Flat-Top passband
Steep Cut-Off
Channel Isolation
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Michelson Interferometer
• Michelson interferometer can beemployed as an opticalinterleaver
• Output intensity is an sinusoidal
function of frequency, withFSR= c/2∆L
• Output signal may change asthe laser frequency drift
Mirror 2
Mirror 1BSL2
L1
∆L=L2
– L1
]}Lc
2cos[1{
2
1I ∆ ν
π+=
ν νν ν ν1 ν3 ν5 ν7
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Gires-Tournois Etalon
• Front mirror reflectivity R1
< 1
• Rear mirror reflectivity R2 = 1
• Etalon reflectivity R = 1 for all λ
• Phase shift is a periodic functionof frequency with
– FSR=c/2dn
• nd can be chosen so that FSRcoincides with ITU grids.
• Both solid and air etalon are
available
• ULE glass can be used asspacer
R 1<1 R 2 =1
d
π
0
Frequency
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Michelson Interferometer with GTI-mirrors
• Gires-Tournois etalons are
employed as phase dispersivemirrors
• With proper choices of the front
mirror reflectivities, flat-top
passbands can be obtained
• The cavity spacing must match
with the path difference ∆∆∆∆L
• Channel isolation of better than35dB can be achieved
BS
GTI 1
GTI 2
L2
L1
]}L
c
2cos[1{
2
1I 12 φ−φ+∆ ν
π+=
∆L=L2 – L1
ν νν ν ν1 ν3 ν5 ν7
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193.10 THz193.20 THz193.30 THz
193.40 THz
193.15 THz
193.25 THz193.35 THz
193.45 THz
100 GHz
Mux
100 GHz
Mux
50 GHzInterleaver
8 channels
@ 50 GHz spacing
Current Mux/Demuxs are limited to 100 GHz channel spacings
Interleavers are essential for capacity upgrades in DWDM
(e.g., 10 Gb/s with 25 GHz channel spacing)
Optical Interleavers
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193.10 THz
193.20 THz
193.30 THz
193.40 THz
193.15 THz
193.25 THz
193.35 THz
193.45 THz
100 GHz
Mux50 GHz
Interleaver
25 GHzInterleaver
100 GHz
Mux
193.125 THz
193.225 THz
193.325 THz
193.425 THz
193.175 THz
193.275 THz
193.375THz
193.475THz
100 GHz
Mux
100 GHz
Mux
50 GHzInterleaver
16 channels
@ 25 GHz spacing
Optical Interleavers
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Flat-Top Optical Interleaver
25 GHz
0 1 2 3 4 5 6 7 8- 5 0
- 4 5
- 4 0
- 3 5
- 3 0
- 2 5
- 2 0
- 1 5
- 1 0
- 5
0
Adjacent Channel Isolation better than 35 dB
Flat-Top passband To accommodate laser Frequency drift
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Accumux Flat-Top Interleavers
Flat-Top Passbands to accommodate laser frequency dri ft
25 GHz Passbands with 35 dB channel isolations over the enti re C-band
Superior Channel Isolation to ensure low crosstalks among channels
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Optical Add Drop Module (OADM)
λ1λ1λ1λ1,,,, λ2λ2λ2λ2,,,, λ3λ3λ3λ3,,,, λ4λ4λ4λ4 λ1λ1λ1λ1,,,, λ2λ2λ2λ2,,,, λ3λ3λ3λ3,,,, λ4λ4λ4λ4
λ4λ4λ4λ4
Thin film filters
Frequency selective couplers
Gratings
Fixed OADM
Reconfigurable OADM
Hit-less Design
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Dispersion
• Dispersion is the degradation of optical signalsresulting in network transmission errors
• Uncompensated dispersion causes bit error rates
(BER) to increase to unacceptable levels
• Combating dispersion is important to current-generation (10 Gbps) networks and vital to next-
generation (40 Gbps) networks
• Two types of dispersion require compensationChromatic dispersion (CD) Polarization mode
dispersion (PMD)
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WDM Broadband Solutions
ITU 193.1 193.2 193.3 193.4 193.5 193.6
2.5 Gb/s transponders @ 25 GHz channel spacing
10 Gb/s transponders @ 50 GHz channel spacing
Frequency in units of THz
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A Growing Optical Challenge
40 Gb/s(OC768)
10 Gb/s(OC192)
2.5 Gb/s(OC48)
1 10 100 1000Wavelengths in fiber
Capacity
Increase
PMD, CD, CD Slope
CD, CD Slope
CD
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Technical Issues and Enabling Photonic
Components
• 2.5 Gb/s systems
– Need more channels w ith channel spacing as small as 12.5 GHz
– Laser frequency stabil ity must be ± GHz
– Mux – Demux becomes challenging
– Narrowband Filters, Interleavers are essential
– Narrowband filters exhibit strong dispersion
• 10 Gb/s systems
– Typical channel spacing is 50 GHz
– Laser frequency stabil ity must be ± 2 GHz
– Dispersion Compensation Modules are needed
– PMD Compensation Modules may be needed
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Dispersion in Single Mode Fibers
• Chromatic dispersion (CD)– Intrinsic material dispersion
– Waveguide dispersion
• Polarization mode dispersion (PMD)
– Elliptical core– Bending, twisting
– Stressed-induced birefringence
Waveguide
eff
Material
eff nn
d
dn
ω∂
∂+
ω∂
∂=ω
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Material dispersion in sil ica
0.5 1 1.5 2 2.51.42
1.44
1.46
1.48
0.5 1 1.5 2 2.50.67
0.68
0.69
)/( λλ+=
d dnn
cvg
Index of refraction Group velocity
)(λ= nn
Wavelength in units of µm Wavelength in units of µm
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Group velocity dispersion in silica SMF
1 1.2 1 .4 1 .6 1 .8 2-5 0
-2 5
0
25
50
p s / n m
- k m
Wavelength (µm)
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Dispersion in SMF-28
1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8-40
-30
-20
-10
0
10
20
30
40Group Velocity Dispersion
Wavelength (micron)
Waveguide dispersion
Material dispersion
Material dispersion = 20 ps/nm-km @ 1550 nm
Waveguide dispersion = -3 ps/nm-km @ 1550 nm
D i n
u n i t s o f
p s / n m - k m
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Dispersion of Single Mode Fibers
Legacy fibers SMF-28 has dispersion of 17 ps/nm-km @ 1550 nm
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Chromatic dispersion (CD) and Polarization Mode Dispersion (PMD) are
serious issues in optical networks, requiring compensation.
Dispersion causes degradation of optical signals transmitted over f iber,causing an increase in bit error rate (BER), at the intended receiver.
The effects of CD become worse as network speeds increase –
specif ically, they rise at a rate of the square of increased transmission
speed – and over longer fiber distances. CD is 16 times worse at 40Gbps than at 10 Gbps and a stunning 256 times worse at 40 Gbps than
at most current networks’ speed of 2.5 Gbps.
PMD, while only four times worse at 40 Gbps than at 10 Gbps, occurs
randomly and can severely limit the maximum f iber distance before asignal must be regenerated.
Dispersion problems in Fibers
C S
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Chromatic Dispersion in SMF
(Severity increases with Speed)
Input Pulses Output Pulses
Input Pulses Output Pulses
Dispersion induced 1.0 dB power penalty2.5 Gb/s 16,640 ps/nm 980 km SMF10 Gb/s 1040 ps/nm 60 km SMF40 Gb/s 65 ps/nm 4 km SMF
CD is negligible at 2.5 Gb/s (OC-48)
2.5 Gb/s
(OC-48)
10 Gb/s
(OC-192)
100 km SMF
100 km SMF
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Chromatic Dispersion and Compensation
DCM
Input Pulses
DCMs provide restoration of signal integrity in
optical domain, without electronic conversion
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New fibers: PMD < 0.5 ps/km1/2
Many installed spans: PMD > 1 - 5 ps/km1/2
Discrete in-line components: PMD ~ 0.5 - 1.0 ps
Critical to performance at 40 Gbps
Compensation required per channel
Even with modern fibers...
All in-line components add PMD
Environmental effects still present
Insurance against expensive, random outage Allows operation over more of installed fiber
Wider adoption of 40 Gbps helps drive down cost
Polarization Mode Dispersion
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Polarization Mode Dispersion
T1.0)k (L*)k (D
M
1k
2PMD <>=τ∆< ∑=
T=Bit Period, (10 ps for 10Gb/s signals)
L(k)= Fiber length of k-th segment, k = 1, 2, 3, M
DPMD
~ 1 ps/(km)1/2 (for typical installed fiber)
Max transmission length = 1600 km (2.5 Gb/s)
100 km (10 Gb/s)
7 km (40 Gb/s)
Pulse Broadening due to PMD
INPUT FIBEROUTPUT
PULSE BROADENED
∆τ∆τ∆τ∆τ
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Polarization Mode Dispersion
τ×β+ω∂
β∂=τ
∂
∂+
ω∂
β∂=
∂
τ∂
z
R
z
Dynamical Equation:
τ = PMD vector in Poincare spaceβ = Birefringence vector in Poincare spaceR = 3x3 rotation matrix in Poincare space
)0(ˆ)(ˆ s R zs == Stokes vector in Poincare spaces
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Signal Degradation and Data Loss
• Pulse Broadening leads to a power penalty and an outage probability (or a bit-
error rate, BER) .
• 14% pulse broadening leads to an outageprobability of less than 5 minutes per year
at a 3-dB power penalty. This translates to
14 ps for a 10 Gb/s system and 3.5 ps for a
40Gb/s system.
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Optical Transmission through 100 km of fiber
(-3.50 dBm laser power)
Without DCM With Accumux DCM
Eye-diagrams obtained using a GTran 10 Gb/s @193.7 THz transmitter (with pre-chirp)
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Bit-Error-Rate
1.4 x 10 -9No signal100km-fibersystem, @ -
25.10dBm
Less than 10-151.4 x 10 -9100km-fiber
system, @ -23.50dBm
10-13No signal120km-fibersystem, @ -23.00dBm
With Accumux DCMWithout DCM
Note: 1 dBm = 1 mW; -20 dBm = 0.01 mW
BER obtained using a GTran 10 Gb/s @193.7 THz transmitter (with pre-chirp)
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-100 -50 0 50 1000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10 15 20 25 300
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
frequency in units of GHz
-100 -50 0 50 1000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
-100 -50 0 50 1000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10 15 20 25 300
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Frequency spectrum in units of GHz
-100 -50 0 50 1000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Pulse shapes and Eye Diagrams
Input pulses Output pulsesSpectra
Gaussian pulse shape (the minimum wave packet) is best.
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420 – 2,240 km
350 – 110 km
140 km
2.5 GHz
2 - 10 km28 – 140 km
Lucent ΤΤΤΤrueWave
fiber:
Dλ
= 1∼5 ps/(nm*km)
1.5 - 4 km24 - 70 km
Corning LEAF fiber:
Dλ = 2∼6 ps/(nm*km)
(C-Band)
0.5 km8 km
Legacy fiber (SMF-28):
Dλ = 17 ps/(nm*km)
@ 1550 nm
40 GHz10 GHzBandwidth
Fiber
Maximum Distance and Bandwidth
Based on 14% pulse broadening
Outage probability of less than 5 minutes per year at a 3-dB power penalty
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Dispersion Management
• Chromatic Dispersion in SMF
– Waveguide Dispersion
– Material Dispersion
– Dispersion Slope
• Polarization Mode Dispersion (PMD)– Core Ellipticity
– Bending and Twisting
– Stress-induced Birefringence
– Environmentally dependent - Random
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• GTIs
Low insertion loss, compact and robust
• Dispersion compensation f iber (DCF)
Bulky, large insertion loss, fixed compensation
• Dispersion compensation f iber Bragg grating (FBG)
Single channel per uni t, fixed compensation, cost high
• 2-Mode Fibers
Large insertion loss, fixed compensation, mode coupl ing
• Electronic Compensation – Pre-chirp, FEC, etc.
Dispersion Compensation Technologies
C ti T h l
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Limited compensation,concept phase
Potentially low cost andsmall size
Receiver Big Bear
Santel
Electronic Processor Electronic
Above (except partialslope compensationAbove plus slopecompensationReceiver OLACorningOFS FitelEnhanced DCF
Large size, high loss,non-linear effects, noslope compensation
Continuous, passive,well understood, 100%market share
Receiver
OLA
Corning
OFS Fitel
Conventional DCF
Multi-path interference,
large form factor, fixedcompensation value
Continuous, performs
slope compensation
Receiver
OLA
LaserCommHigh-Mode FibersFixed
Group delay rippleLow insertion loss, widetuning range, small size
Receiver
OLA
Alcatel
JDSU
Teraxion
Fiber Bragg Gratings
High loss, difficult to
manufacture, low databandwidth
Higher channel counts,
good tuning range, tunethrough zero
Receiver
OLA
Accumux
AvanexFujitsu
Etalon-based Tunable
WeaknessesStrengthsLocationCompanyTechnology
Compensation Technology
Chromatic Dispersion and Slope
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Fiber Bragg Gratings (FBG)
UV Exposure
λ1λ1λ1λ1,,,, λ2λ2λ2λ2,,,, λ3λ3λ3λ3,,,, λ4λ4λ4λ4 λ1λ1λ1λ1,,,, λ2λ2λ2λ2,,,, λ3λ3λ3λ3
λ4λ4λ4λ4 Grating Period Λ=Λ=Λ=Λ=λ4λ4λ4λ4 /2n
0 .4
x 1 04
0
1
λ4λ4λ4λ4
Index grating: n(x) = n0 + n1 cos(Kx)Bragg condition: λ4 = 2n ΛPeak reflectivity =tanh2(πn1L/λ)Stopband Width: ∆λ=λ n1/n0
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Chirped Index Gratings
Decreasing periods
Pulse spread due to
Group Velocity Dispersion (GVD)
Pulse compressionafter FBG
In most fibers @1550 nm, short wavelength light tends to travel faster
λ3λ3λ3λ3
λ2λ2λ2λ2
λ1λ1λ1λ1
λ3λ3λ3λ3 λ1λ1λ1λ1λ2λ2λ2λ2
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Chirped Gratings
0 3 0- 1
1
0 3 0- 1
1
0 3 0- 1
1
(a) Uniform Grating
(b) Chirped Grating
(c) Chirped Grating withApodization
n(x)=n0 + n1 (x) cos {K(x) x}
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Coupled-Wave Analysisfor Continuously Chirped Gratings
kzi
Beidz
dA ∆
κ −=
kziAeidz
dB ∆−κ =
κ(x) = coupling constant
∆k =2k0 – K(x) = wavenumber mismatch
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Step-Chirped Gratings
0 3 0- 1
1
0 3 0- 1
1
Easier to analyze using Matrix Method
Easier to fabricate for broadband coverage
Possible Stitching Errors
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Matrix Method for Step-Chirped Gratings
−
∆−
κ −
κ ∆+=
)KL2
1iexp( b
)KL21iexp(a
sLsinhs2
k isLcoshsLsinh
si
sLsinhs
isLsinhs2k isLcosh
b
a
L
L
0
0
Uniform grating within a section: n(x)=n0 + n1 cos (Kx)
The field in each section is written as
E(x,t)=[A(x) exp (-ik0x) + B(x) exp(ik0x)]exp(iωωωωt)
Definea(x)= A(x) exp (-ik0x)
b(x)= B(x) exp ( ik0x)
then
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Step-Chirped Gratings
2 2 .0 0 0 5 2 .0 0 1 2 .0 0 1 5 2 . 0 0 2 2 .0 0 2 5 2 .0 0 3 2 . 0 0 3 5 2 .0 0 4
x 1 01 4
0
1
2 2 .0 0 0 5 2 .0 0 1 2 .0 0 1 5 2 .0 0 2 2 .0 0 2 5 2 .0 0 3 2 .0 0 3 5 2 .0 0 4
x 1 01 4
0
2
4
6
8x 1 0
-1 0
n(x)= n0+ n1cos (Kx)n0 = 1.500, n1=0.001Λ1 =0.5 µm, ΛΝ =0.4995 µmGrating Length = 40 mm
Divided into 200 sections400 periods in each section
Group delay varies 400 psOver 150 GHz rangeSpikes and Ripples due toImpedance Mismatch
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Step-Chirped Gratings with Apodization
2 2 .0005 2 .001 2 .0015 2 .002 2.0025 2.003 2.0035 2 .004
x 1 014
0
1
2 2 .0005 2 .001 2 .0015 2 .002 2 .0025 2.003 2.0035 2 .004
x 1 014
0
0 .5
1
1 .5
2
2 .5x 1 0
-9
Dispersion Spikes greatly
reduced via Apodization
Dispersion Ripples remain
a problem
Gaussian Apodization
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LP Modes Intensity Pattern
LP01 Mode LP02 Mode
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LP Modes Intensity Pattern
LP11 Mode LP21 Mode
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LP01 Mode Dispersiona=4.7 microncore index=1.4628clad index=1.46
1.53 1.55 1.57 1.59 1.610
5
10
15
20
25
30
Wavelength (microns)
λλ
λ−= 2
eff
2
2
d nd
c1D
D in units of ps/nm-km
The GVD consists of waveguide dispersion and material dispersion.
For silica fibers @ l = 1550 nm, material dispersion is about 20 ps/nm-km,
whi le the waveguide dispersion is about - 3 ps/nm-km.
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LP02 Mode Dispersion
1.53 1.55 1.57 1.59 1.61-500
-450
-400
-350
-300
-250
-200
-150
-100
-50
0
LP02 mode dispersionn1=1.477;
n2=1.46;
a=4.7 microns;
λλ
λ−=
2
eff 2
2
d
nd
c
1D
Wavelength (microns)
D in units of
ps/nm-km
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2-mode FibersLegacy Fiber Legacy Fiber
Mode Converter Mode Converter LP01 LP02 LP02 LP01
Group Velocity Dispersion (GVD)DLP01 = +17 ps/nm-km (Legacy Fiber)
DLP02 = -500 ps/nm-km (2-mode Fiber)
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Gires-Tournois Etalons
R < 1 R2 = 1
Front mirror reflectivity < 1
Rear mirror reflectivity = 1All frequency components are totally reflectedPhase shift is a periodic function of frequency
The Gires-Tournois Cavity
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φ
φ
i
ii
e R
e Rer
2
2
1 −
−Φ−
−
+−== Ln o
λ
π φ
2=
( ) .1|r |and tan1
12 =
−
+=Φ φ
R
R ArcTan
Total reflection R=1.0Partial reflection R <1.0
Unique Advantages:• Multi-channel operations
• Dispersion slope
compensation
• Tunable dispersion
y
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The phase shift of a Gires-Tournois etalon can be written
where
R is the reflectivity of the front mirror, f is the phase shift,
where d is the space between the mirror, n is the index of
refraction of the medium.
)tan(tan2
1
φσ=Φ
−
R
R
−
+=
1
1σ
nd
c
ω=φ
Gires-Tournois Etalons
Single Gires-Tournois etalon
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193.5 193.6 193.7 193.8 193.9-4
-3
-2
-1
0
1
2
3
4
193.5 193.6 193.7 193.8 193.90
5
10
15
20
25
Single Gires-Tournois etalon
)tan1
1(tan2 1 φ
−
+=Φ −
R
R022 sin)1(1
τφ−σ+
σ=τ
Phase shift Group delay (ps)
Frequency in units of THz Frequency in units of THz
Gires Tournois Etalons
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A further differentiation lead to the following expression for GVD
In terms of ps/nm (DL), the GVD can be written
( )20222
2
sin)1(12
)2sin()1(τ
φ−σ+
φ−σσ−=
ω
τ
d
d
By taking the derivative with respect to ωωωω, we obtain the group delay
where ττττ0 is the roundtrip flight time inside the cavity.
022
sin)1(1
τ
φ−σ+
σ=τ
( )222
22
sin)1(1
)2sin()1(4
φ−σ+
φ−σσ
λ
π=
λ
τ=
d
cd
d DL
Gires-Tournois Etalons
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193.5 193.6 193.7 193.8 193.9-150
-100
-50
0
50
100
150
Dispersion of a Single Etalon
( p s / n m )
Frequency in units of THz
Group Delay of Single Stage GTI Dispersion
Compensator
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Wavelength (nm)
G r o u p
d e l a y ( p s )
Passband
Compensator
Multiple-etalon for Broad Passband
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193.5 193.6 193.7 193.8 193.9-150
-100
-50
0
50
100
150
Multiple-etalon for Broad Passband
( p s / n m )
Frequency in units of THz
Dispersion and Slope Compensation
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ITU 1 9 3 .1 1 9 3 .2 1 9 3 .3 1 9 3 .4 1 9 3 .5 1 9 3 .6 1 9 3 .7 THz
25 GHz25 GHz25 GHz25 GHz
Dispersion
0
SMF-28
Legacy
Fiber
DCFIdeal DCM
Accumux DCM
1400 ps/nm
-1400 ps/nm
Slope Mismatch
Optical Transmission throught 80 km of fiber
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( -3.50 dBm laser power)
Without DCM With Accumux DCM
Eye-diagrams obtained using a GTran 193.7 THz transmitter.
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Bit-Error-Rate
1.4 x 10 -9 No signal100km-fiber system,@ -25.10dBm
Less than 10-151.4 x 10 -9100km-fiber system,@ -23.50dBm
10-13 No signal120km-fiber system,@ -23.00 dBm
With Accumux DCMWithout DCM
Note: 1 dBm = 1 mW; -20 dBm = 0.01 mW
BER obtained using a GTran 193.7 THz transmitter
Origin of PMD
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Origin of PMD
• Fiber has two polarization modes (x- and y-).
• Fiber with elliptical core or imperfection is equivalent to a
birefringent element.
• The birefringence may dependent on external perturbations.
• Main source of birefringence: elliptical core, bending, twist ing,
stress, etc.
• A long segment of fiber is optically equivalent to a series of
birefringent plates.• Jones Matrix method can be employed to analyze the
transmission.
• The system is time-dependent due to environmental perturbations
(temperature, pressure, stress, etc.)
• Random mode coupling in long fiber lengths causes time-varying
delay difference, ∆τ∆τ∆τ∆τ .
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Principal States of Polarization(due to Craig D. Poole)
• Output polarization of
fiber is obtained byJones matrix method
• Principal output SOPsare independent of ωωωω to the first order only
• PMD compensator based on thisapproach is l imited to
a bandwidth of about20 GHz in 80 km of fiber
inout V*a* b
baV
−=
∂Vout
∂ω= iτVout
Vout (ω) = Vout (ω0 ) exp{i τd ω
ω0
ω
∫ }
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Principal States of Polarization(due to Craig D. Poole)
• Principal SOP can be
obtained by solving aneigenvector problem
• Time delay betweenthese two principal
states is the PMD
A=a*a' + bb'*B=a*b' - a'*b
C=a'b* - ab'* = -B*D=aa'* +b'b* = A*
A B
C D
V in = iτVin
V in =
−B
A − iτ
22' b'aBCAD +±=−±=τ
22
12 ' b'a2)( +=τ−τ=φ∆∂ω
∂=τ∆
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First Order PMD CompensatorsTunable
Waveplate
Poole’s PrincipalStates (elliptical)
Variable
Delay
λ/λ/λ/λ/4-plate
λ/λ/λ/λ/4-plate
Principal States Approach:
First-order solution, with limited bandwidth (10 GHz)
PMD Compensation one channel at a time (not practical)
No broadband solution at the moment
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Frequency Dependence of PMD and PSOP40 GHz, 80 km of SMF-28 fiber with ∆n = 10-7, with 10o turn per km
• Variation of PMD
(around 25 picosec) ina frequency range of
40 GHz
• Variation of polarization ellipticity
and inclination angle
of Principal SOP over
a frequency range of 40 GHz
1 . 9 9 9 8 2 2 . 0 0 0 2
x 1 01 4
2
2 .5
3x 1 0
- 1 1P M D ( ta o ) vs f re q u e n c y a t e n d o f f ib e r
f r e q u e n c y ( H z )
T i m e d e l a y
1 . 9 9 9 8 2 2 . 0 0 0 2
x 1 01 4
- 4 0
- 3 0
- 2 0
- 1 0
0
1 0
2 0
3 0
4 0
O u tp u t p ri n c ip a l S O P v s f r e q u e n c y a t e n d o f f ib e r
f r e q u e n c y ( H z )
i n c l i n a t i o n a n g l e o f e l l i p s e
1 . 9 9 9 8 2 2 . 0 0 0 2
x 1 01 4
- 4 0
- 3 0
- 2 0
- 1 0
0
1 0
2 0
3 0
4 0
fr e q u e n c y ( H z )
e l l i p t i c i t y a n g l e : a t a n ( b / a )
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Frequency Dependence of PMD and PSOP100 GHz, 80 km of SMF-28 fiber with ∆n = 10-7, with 10o turn per km
• Variation of PMD
(around 25 picosec) ina frequency range of 100 GHz
• Variation of polarization ellipticityand inclination angleof Principal SOP over a frequency range of 100 GHz
1 . 9 9 9 5 2 2 . 0 0 0 5
x 1 01 4
2
2 .5
3x 1 0
- 1 1P M D ( ta o ) vs f re q u e n c y a t e n d o f fi b e r
fr e q u e n c y ( H z )
T i m e d e l a y
1 . 9 9 9 5 2 2 . 0 0 0 5
x 1 01 4
- 4 0
- 3 0
- 2 0
- 1 0
0
1 0
2 0
3 0
4 0
O u tp u t p ri n c ip a l S O P v s fr e q u e n c y a t e n d o f f ib e r
fr e q u e n c y ( H z )
i n c l i n a t i o n a n g l e o f e l l i p s e
1 . 9 9 9 5 2 2 . 0 0 0 5
x 1 01 4
- 4 0
- 3 0
- 2 0
- 1 0
0
1 0
2 0
3 0
4 0
fr e q u e n c y ( H z )
e l l i p t i c i t y a n g l e : a t a n ( b / a )
Summary
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• Traffic demand for telecom continues togrow
• Optical technologies are essential for high-capacity broadband networks– Wavelength management in DWDM
– Dispersion management in both Metro andLong-haul networks
• Radio and copper won’t disappear. Butonly optics can provide the capacity
needed for the truly broadband future.
• Telecom network will become more Optical