[PPT] - Polarization Mode Dispersion and Its Mitigation Techniques in High PowerPoint Presentation

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Polarization Mode Dispersion and Its Mitigation Techniques in High Speed Fiber Optical Communication Systems

Chongjin Xie Bell Labs, Lucent Technologies 791 Holmdel-Keyport Road, Holmdel, NJ 07733 WOCC’2005, April 22, 2005, Newark, NJ

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Outline

PMD basics PMD impairments Passive PMD mitigation techniques Electrical equalization for PMD mitigation Optical PMD compensation Multi-channel PMDC for WDM systems

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State of Polarization

The polarization state of a wave describes how the electrical field oscillates.

Circular or elliptical SOP for arbitray phase between field components

[ ]

T j y j x

e A e A E

2 2 φ φ −

= r

Jones vector

              − + =             ≡ φ φ sin 2 cos 2

2 2 2 2 3 2 1 y x y x y x y x

A A A A A A A A S S S S S r

Stokes vector

Poincaré sphere

Linear SOP for in-phase field components

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Birefringence —1st-order PMD

Time domain manifestation

DGD ~ L

S1 S2 S3

τ r

ut

S r

Frequency domain manifestation

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Random Birefringence in Fibers—All-Order PMD

Concatenation of random birefringent sections

Output SOP

DGD ~ L1/2

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Principal States of Polarization (PSP)

Two special polarization states at the fiber input: Output pulse is not distorted to 1st-order Differential group delay (DGD): DGD = ∆τ PMD vector:

p ˆ τ ∆ = τ r

1 : | 1 : | ; 2 ; 2 Fast PSP p de Slo lay w PSP p delay

−〉

〉 = = τ − ∆  τ + ∆ − τ τ  −    

In

∆τ

Out

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PMD Drift and Variation

DGD has Maxwellian distribution

0.06 20 40

M easurem ent S im ulation Theory

m

egaE coc2.ep

∆τ (ps)

Probability Density (ps-1)

M. Karlsson et. al., JLT, vol 18, p. 941, 2000
H. Kogelnik et. al., OFC’02, WD
PMD varies with wavelength and drifts with time
Drift speed was observed to have a large range
Hours and days for buried fibers and undersea cables
millisecond or faster for aerial fibers and fibers under bridges

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PMD basics PMD impairments Passive PMD mitigation techniques Electrical equalization for PMD mitigation Optical PMD compensation Multi-channel PMDC for WDM systems

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PMD Induced Eye-Diagram Degradation

0 ps 40 ps 60 ps RZ NRZ

PMD induced pulse splitting and broadening causes ISI, which will degrade system performance.

Eye-diagram degradation of 10 Gb/s RZ and NRZ signals caused by 1st –order PMD in worst case

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System Penalty due to 1st-order PMD

For penalty less than 2 dB, 1st-order PMD can be approximated as ε (dB) ≈ Α (∆τ / 2T ) 2 sin2 Θ (C. D. Poole et al., IEEE PTL., vol. 3, p. 68,1991.)

1 2 3 4 5 0.5 1.0

NRZ DGD 30 ps NRZ DGD 40 ps NRZ DGD 50 ps RZ DGD 40 ps RZ DGD 50 ps RZ DGD 60 ps 10 Gb/s Optically Preamplified Rx

Fraction of Power in Leading Pulse (γ) 1x10-9 Receiver Penalty (dB)

C.H.Kim et al, OFC 2002, TuI4

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Outage Probabilities Induced by PMD

For any given system margin , there is a certain probability

that the PMD induced penalty exceeds the margin, the probability is called outage probability

Acceptable outage probabilities range between 10-4 to 10-8

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PMD basics PMD impairments Passive PMD mitigation techniques

– Refer to the techniques that do not require dynamic adjustment

Electrical equalization for PMD mitigation Optical PMD compensation Multi-channel PMDC for WDM systems

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Using PMD Robust Modulation Formats

R. M. Jopson et al,OFC’1999, paper WE3.

1 dB margin, BER = 10-12

C. Xie et al, OFC’2003, paper TuO1

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Allocating More Margin to PMD

H. Sunnerud et al, IEEE PTL, vol. 13, p. 448, 2001
C. Xie et al, IEEE PTL, vol. 15, pp. 614, 2003.

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Using FEC and Polarization Scrambling

X. Liu, et al, ECOC’04, PD paper

FEC alone or FEC with PS at Tx cannot efficiently mitigate PMD FEC together with fast distributed PS can effectively reduce PMD effects

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PMD basics PMD impairments Passive PMD mitigation techniques Electrical equalization for PMD mitigation Optical PMD compensation Multi-channel PMDC for WDM systems

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Electrical Equalizers for PMD Compensation

Electrical equalization advantages Low cost Small size Simultaneous mitigation of various ISI independent of its origin but not so effective due to… Lack of polarization information after detection Non-linear channel model Signal dependent noise High-speed signal processing Well-known concepts:

Transversal filter (FFE) Decision feed-back loop (DFE) Maximum Likelihood Sequence Estimation (MLSE)

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Structure of Electrical Equalizer

Architecture of 10 Gb/s ISI mitigator with FFE and DFE

A. Dittrich et al, OFC’03, paper ThG5

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Effectiveness of FFE and DFE

PMD penalty for an optically pre-amplified 10 Gb/s receiver with 1-tap DFE and 8-tap FFE (transversal filter) More effective in high penalty range

H. Bülow et al., Electron. Lett., vol. 36, p. 163, 2000.

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Electrical Equalizer @ 40Gb/s

4(8) tap feed forward / T/2-spaced analog equalizer
No absolute Q value given
Increases DGD tolerance from 8ps to 12ps

(likely for optical duobinary)

H. Jiang et al, OFC’05, paper OWO2.

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PMD basics PMD impairments Passive PMD mitigation techniques Electrical equalization for PMD mitigation Optical PMD compensation Multi-channel PMDC for WDM systems

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Concept of Optical PMDC

The aim of optical PMDC is to construct a PMD vector that is

pposite to the PMD vector of the link

Due to existence of higher order PMD, this cannot be achieved

ver a wide bandwidth

In principle, more stage PMDC can achieve better performance

Transmissio n Link PMDC Tx Rx

f

Ω r

c

Ω r

PMD profile of transmission span (solid) and perfect optical PMDC (dashed, dotted)

R. Noé et al., JLT, vol. 17, p. 1602, 1999.

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Structure of Optical PMDC

delay line PC delay line PC delay line PC DSP and Control algorithm Feedback signal generator

Compensation elements

– one or many stages, fixed or variable delay lines

Feedback signals

– DOP, RF spectrum, eye-monitoring, Q factor Summary see: J. Poirrier et al, OFC’02, WI3, C. Xie et al, IEEE PTL, vol. 17, p. 570, 2005.

Control algorithms

–Dithering method, or more efficient searching methods

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Performance of One-Stage Optical PMDC

One-stage PMDC with fixed delay line One-stage PMDC with variable delay line

1 dB margin, BER = 10-12, RF spectrum signal as feedback control

C. Xie et al, IEEE PTL, vol. 15, p.1228, 2003.
C. Xie et al, IEEE PTL, vol. 15, p.1168, 2003

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Effects of Feedback Signals on PMDC

NRZ RZ

1 dB margin, BER = 10-12 DOP1: without filter DOP2: with 0.8R optical filter RF1: weighted RF power RF2: 0.5R RF tone

C. Xie et al., OFC’04, paper WE4

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PMD basics PMD impairments Passive PMD mitigation techniques Electrical equalization for PMD mitigation Optical PMD compensation Multi-channel PMDC for WDM systems

– To reduce system cost

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Channel Switching to Mitigate PMD Effects

S. Särkimukka et al., JLT, vol. 20, p.368, 2002

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Multi-Channel PMDC

R. Khosravani et al., IEEE PTL, vol. 13, pp. 1370, 2001

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Multi-channel Shared PMDC for WDM Systems

DEMUX Switching Switching Receiver ends

PMDC PMDC

λ1,λ2,λ3, ..., λn Scheme of multi-channel shared PMDC Performance of the shared PMDC

C. Xie et al., OFC’03, paper TuO6

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Terapulse Multi-Channel PMDC

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PMD Limited Distances for 40 Gb/s Systems

PMD limited transmission distances for systems with different PMD tolerances. Assume component PMD of 0.5 ps per 100 km span. The values in the figure are average tolerable PMD

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Summary

Due to its stochastic nature, PMD is considered to be one
f the main obstacles to the deployment of optical

communication systems with bit rates of 40 Gb/s and higher.

Many PMD mitigation techniques have been developed

and demonstrated in the past decade, some of them can significantly increase the system tolerance to PMD.

Finding cost effective PMDC solutions requires deep

understanding of PMD and customer needs.

Currently no PMD compensation technique can eliminate

PMD effects. In systems with large PMD, signal regeneration has to be used or the high PMD fibers have to be replaced with low PMD fibers (such as spun fibers).