optical transmission system

“Recent Trends in Optical Transmission Systems” - CSNDSP 06, 19-21 July, 2006, Patra

Thomas Sphicopoulos, Optical Communications Laboratory, University of Athens, Greece

“Recent Trends in Optical Transmission Systems”

Thomas Sphicopoulos ([email protected])Optical Communications Laboratory National and Kapodistrian

University of Athens, Greece



Advantages of Optical Technology

Optical Technology Provides:• Ultra Low Transmission Losses• Ultra Wide Band • Very High Bitrates• (Mostly) Linear Behavior• Very Low Crosstalk

But:• Optics are not smart• InP / Si / Polymer platforms do not yet

provided increased scale of integration• No means of storage



The Optical Value Chain

MATERIAL AND PROCESSESSilicon, GaAs, InP

Polymers / Organic Materials,Etchers, MEMS

PHOTONIC COMPONENTSLasers, Optical Amplifiers

Transceivers, Optical FiltersFiber cables, WavelengthConverters, RegeneratorsDispersion Compensators

EQUIPMENT MAKERSRouters, Switches, HubsBase Stations, Satellites

Servers

NETWORK OWNERSWireless, Backbone, MetroAccess, Satellites, Spread

Spectrum Communications

SERVICE PROVIDERSLong Distance, Cellular

ISP, Broadcast, Cable TVVPN

CONTENTS AND APPLICATIONSMusic, Movies, E-mail

VoIP, Shopping, SurveillanceeBusiness

APPLIANCESComputers, Phones

Media Players, Cameras, PDAs

END USERSConsumer, Government

Education, MedicalBusiness



Evolution of Transmission Rates/Channel

1990 1995 2005 2010

2.5Gb/s

10Gb/s

40Gb/s

160Gb/s(?)

Year



Wavelength Division Multiplexing (WDM)

λ2

λ1

λ3

λ4

λ2

λ1

λ3

λ4

The aggregate bit rate can be drastically increased by using Wavelength Division Multiplexing (>1Tb/s exhibited )

In optical transmission systems, the available bandwidth can exceed 40nm

To efficiently utilize this enormous bandwidth one can assign each channel a different wavelength and lead all the wavelengths inside

the fiber

Channel spacing as narrow as 10GHz(!) can be achieved!



Transmission Impairments

Linear Impairments:• Optical Losses • Chromatic Dispersion• Polarization Mode Dispersion

Non-linear Impairments:• Self Phase Modulation• Cross Phase Modulation• Four Wave Mixing• Stimulated Raman Scattering• Stimulated Brillouin Scattering



The Fiber: A Nearly Lossless Channel

Typical Losses can be as low as 0.2dB/Km

Poses no problem if optical amplification is used



Linear Impairments: Dispersion

Types of Cables according to dispersion:

•G652: D~15-20ps/nm/Km (λ=1.55μm)

•G653: D~0ps/nm/Km (λ=1.55μm)

•G655: D~2-6ps/nm/Km (λ=1.55μm)

As in most types of waveguides the different spectral parts of the pulse travel with slightly

different phase velocities (chromatic dispersion)

This causes pulse broadening!



Linear Impairments: Polarization Mode Dispersion (PMD)

The principal polarization axes of the fiber may change randomly along the cable due to temperature / size variations. This causes Polarization Mode Dispersion

The fiber is not completely circular and hence supports two degenerate modes with slightly different group

velocities (birefrigence)

PMD can also cause pulse broadening at high bit rates



Nonlinear Impairments due to the non-linearity of the refractive index

Self Phase Modulation: Phase modulation due to the intensity modulation of the Signal (introduces chirp)

Cross Phase Modulation: Phase modulation due to the intensity modulation of other interfering wavelength

channels (pulse broadening and time jitter)

Four Wave Mixing: Crosstalk with other nearby channels due to frequency mixing (three photon interaction)

2

2linearn n n E Intensity of the Electric Field



Nonlinear Impairments: Stimulated Scatterings

Brillouin Scattering: Energy is transferred from a photon to an acoustic phonon (molecular vibration) and to a photon of smaller frequency (≈-10GHz) (unwanted reflections at the source).

Raman Scattering: Energy is transferred from a photon to an optical phonon (molecular vibration) and to a photon of smaller frequency (optical crosstalk from higher to lower frequency channels)

Current WDM systems avoid problems with both type of scatterings by limiting the optical power and increasing the

channel spacing



Technological Landmarks: Optical SourcesDistributed Feed Back Lasers (DFB) are ideal Optical Sources for ~40Gb/s providing:

High Launch Power (>20mW)

Wavelength Stability (~0.001nm/0C)

Very Low RIN (>-145dB/Hz)

High Side Mode Suppression Ratio (<-45dB when MQW is used)

Narrow Linewidths (~2MHz)

At ~40Gb/s only external modulation can be used:

LiNbO3 Mach Zehnder Modulator (electroptical effect)

Electroabsorption Modulator (electroabsorption effect)



Technological Landmarks: AmplifiersTwo types of Amplification is used:

Erbium Doped Fiber Amplifier (EDFA):

High Gain (~40dB)

High Output Power (~400mW)

Very Low Noise

Very Linear

Wide Band (~40nm)

Raman Amplifier

Higher Power than EDFA (~700mW)

Can offer distributed and/or lumped amplification

Ultra wide band (~100nm)



Technological Landmarks: MUX/DEMUX

Arrayed Waveguide Gratings (AWGs):

Can multiplex up to 1000 channels!

Channel spacing can be as small as 10GHz!

Commercial systems multiplex 64 channels x 50GHz

Can be integrated with SOAs and provide an integrated ADD/DROP MUX

Have small polarization sensitivity

Have small insertion loss

Can be designed with “flat-top” transfer function



System Design: Dispersion Management (1)

First Generation Dispersion Management System

SMFDSF

DSF (D= -0.2ps/nm/Km)

SMF (D=+18ps/nm/Km)

Distance

Acc

umul

ated

D

ispe

rsio

n (p

s/nm

)

150ps/nm

Repeater

DSF=Dispersion Shifted Fiber

SMF=Single Mode Fiber

This scheme was used in the past for single channel ~5Gb/s systems but is unsuitable for WDM:

high nonlinearity

Compensates dispersion for one wavelength




Dispersion Management for multi-channel 10Gb/s

NZDSFLCF

LCF / NZDSF (D= -2ps/nm/Km)

SMF (D=+18ps/nm/Km)

Acc

umul

ated

D

ispe

rsio

n (p

s/nm

)

Repeater

SMF

~500Km

+6000ps/nm

-6000ps/nm

~500Kmλ1

λΝ

LCF = Large Core Fiber

NZDSF= Non-Zero DSF

LCF is used first to reduce non-linearitySMF is placed in the middle of the period and the accumulated dispersion alternates sign

Parameters NZ-DSF LCF

D (ps/nm/Km) -2~-3 -2~-3

Ds (ps/nm2/Km) 0.05~0.06 0.1~0.14

Aeff(μm2) 50~55 70~80

Loss(dB/Km) 0.2 0.22




Dispersion Management for multi-channel 10Gb/s

To further residual dispersion at edge channels we use pre/post-compensantion (50:50) on a channel by channel basis:

Less Maximum Dispersion

Less Waveform Distortion

Overall:

Less Nonlinearity

Ideal for 16x10Gb/s (~20nm)

NZDSFLCF

LCF / NZDSF (D=-0.2ps/nm/Km)

SMF (D=+18ps/nm/Km)

Acc

umul

ated

D

ispe

rsio

n (p

s/nm

)

Repeater

SMF

~500Km

+6000ps/nm

-6000ps/nm

~500Kmλ1

λΝ




Expanding the Bandwidth from ~20nm to ~40nm

SPCDFΕΕ-PDF

EE-PDF(D=+20~22ps/nm/Km)

SMFAcc

umul

ated

Dis

pers

ion

(ps/

nm)

Repeater

SMF

~500Km

+600ps/nm

-600ps/nm

~50Km

λ1

λΝ

SCDCF(D=-40~-60ps/nm/Km)

EE-PDF: Aeff Enlarged Positive Dispersion Fiber

SC-DCF: Slope Compensating Dispersion-Compensation Fiber

Parameters EE-PDF SCDCF

D (ps/nm/Km) +20~22 -40~-60

Ds (ps/nm2/Km) +0.06 -0.12~-0.18

Aeff(μm2) >100 30~22

Loss(dB/Km) 0.15~0.19 0.23~0.27




Moving to 40Gb/s…

It is preferable to lower accumulated dispersion

SMF+SCDCF

EE-PDF+SCDCF+EE-PDF

+600ps/nm

-200ps/nm

Cum

ulat

ive

Dis

pers

ion

(ps/

nm)

~40Km



System Design: Integrated Optics Dispersion Compensation

Modifying the Geometry of an Arrayed Waveguide Grating by a Variable Reflecting Membrane introduces Second Order Dispersion that can be used to compensate the accumulated dispersion of a multiwavelength 40Gb/s signal

Tunable: Applying Voltage

1000ps/nm Tuning Range



System Design: Electronic Dispersion Compensation

Processor D/A

Am

p

Laser

Processor D/A

Am

p

Ein ETX

MZM

d1(t)

d2(t)

One idea is to predistort the signal for each channel

11

( )( ) ( ) cos TX

in

V E td t t

E

12

( )( ) ( ) cos TX

in

V E td t t

E

2

2exp2TX RX

LE E j

Use a MZM interferometer to predistort the signal in order to counteract the effects of dispersion

Works very well in theory but you need fast electronics and D/A (even if you parallelize!)



System Design: Mitigation of Nonlinearities

Methods for Reducing Nonlinearities: FWM

Use unequal channel spacings

Use optical prechirped pulses

DE

MU

XMU

X

NZD fiber DCF fiberTx

Tx

Tx

ASK MOD

ASK MOD

ASK MOD

...

Rx

Rx

Rx

Transmitter Receiver

DCF fiberused for

prechirping



System Design: Mitigation of Nonlinearities

Methods for Reducing Nonlinearities: XPM

Dispersion Compensation at each span

High channel spacing

Pre-chirped optical pulses

Advanced modulation schemes



How to Model and Design? (1)

Use Numerical Tools:

Numerically Solve the Propagation Equation

22

2 2

( )( ) ( )

2 2

A j A a zz A j z A A

z t

A=A(z,t): Envelope of the Electric Field

β2(z): Second order dispersion

γ(z): Nonlinear Kerr Coefficient

You can add amplifier gain and noise in each span




Use Numerical Tools:

Calculate Q-factor from Receiver Eye-Diagram

1 0

1 0

m mQ

1

2 2e

QP erfc

For Gaussian Noise:




Example: Estimate Performance of Modulation Formats in G655 fibers:

-8 -6 -4 -2 0 2 4

5

10

15

20

25

30

Q fa

ctor

Pin (dBm)

NRZ Duobinary DPSK NRZ single channel

(a)

FWM limits the quality of

multichannel systems and

hence DPSK has superior

performance

Δfch=100GHz, Lspan=80Km, Nspan=4




But:

The Gaussian Assumption is usually not valid!

The Q-factor provides a crude estimate for the error probability

Use Saddle-Point approximation to compute the error probability from the MGF (if it is known!)

Use Monte Carlo methods to estimate the error probability numerically




Example: Estimation of FWM probability density function using MCMC simulations

Gaussian PDF is inadequate!

MCMC requires very few iterations (~106) for probabilities of the order of 10-14

Single span system, Nch=8, Δfch=100GHz



Small Size Components: Photonic Crystals as a Possible Candidate for Nanophotonics

Photonic Crystals: Artificial Periodic Structures

Exhibit Bandgaps (no guided modes exist)

“Defects” introduce highly localized modes

Confine light (can implement sharp bends)

Are highly non-linear (signal processing)



Slow Light: Towards Integrated All-Optical Buffers?

OPTICAL RESONATORS

Certain waveguiding structures can support pulse propagation with very low group velocities

Coupled Resonator Optical Waveguides (CROWs)

Integrated Optical Delay Lines

Photonic Memories

Signal Processing (Linear + Nonlinear)



In conclusion…

Optical Transmission Systems have made significant advances and are

operational.

But much can be gained by improving optical integration and exploring optical

buffering!



THANK YOU FOR THANK YOU FOR YOUR ATTENTION!YOUR ATTENTION!

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