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



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



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



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



80 nm bandwidth @ λλλλ=1550 nm is approximately 10,000 GHz



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



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)



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.



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



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



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



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



Array Waveguide GratingsPrinciple of Operation

λ1λ1λ1λ1 λ2λ2λ2λ2 λ3λ3λ3λ3 λ4λ4λ4λ4


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



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



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



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



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



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



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



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



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)




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




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



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



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



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)



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



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



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



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



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



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)



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



Dispersion of Single Mode Fibers

Legacy fibers SMF-28 has dispersion of 17 ps/nm-km @ 1550 nm



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



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



Chromatic Dispersion and Compensation

DCM

Input Pulses

DCMs provide restoration of signal integrity in

optical domain, without electronic conversion



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




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




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



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.



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)



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)



-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.



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



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



• 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



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



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



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



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}



Coupled-Wave Analysisfor Continuously Chirped Gratings

kzi

Beidz

dA ∆

κ −=

kziAeidz

dB ∆−κ =

κ(x) = coupling constant

∆k =2k0 – K(x) = wavenumber mismatch



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



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



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



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



LP Modes Intensity Pattern

LP01 Mode LP02 Mode



LP Modes Intensity Pattern

LP11 Mode LP21 Mode



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.



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



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)



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



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



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


Single Gires-Tournois etalon



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



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




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 )


Group Delay of Single Stage GTI Dispersion

Compensator



Wavelength (nm)

G r o u p

d e l a y ( p s )

Passband

Compensator

Multiple-etalon for Broad Passband



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 )


Dispersion and Slope Compensation



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



( -3.50 dBm laser power)

Without DCM With Accumux DCM

Eye-diagrams obtained using a GTran 193.7 THz transmitter.



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



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 .



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

∫ }



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∆



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



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 )



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


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


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


e l l i p t i c i t y a n g l e : a t a n ( b / a )

Summary



• 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

Optical and Photonic Components for DWDM Optical Networks

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