University of Peloponnese

Adaptive Optical Technologies for Optical Transmission Systems

Maki Nanou, George-Othon Glentis,

Kristina Georgoulakis, Chris Matrakidis,

Christina (Tanya) Politi, Alexandros Stavdas

Outline

Basic Concepts of Optical Communications Fiber Impairments & Compensation Techniques Optical Transmission Simulations Results Conclusion

University of PeloponneseDept. of Inform. & Telecommunications

Traffic Growth

University of Peloponnese

Until 2000: Voice Traffic dominates

After 2004: Data Traffic dominates 60% / year

Traffic growth of 60% per year outstrips the growth in capacity of commercial systems.

While the entire traffic in North American core Network could be carried on a single fiber until 2008, in 2011 more than two fibers were required. Every 3 years the required number of fibers will double.

Increased capacity ↔ Advanced Modulation Formats

20% / year

(*) Cisco Forecast

Non Return To Zero – On – Off Keying

University of PeloponneseDept. of Inform. & Telecommunications

Output Intensity

Vπ swing

t

quaternary point bias

MZMLASERopticalsignal

Most commonly used (widely deployed)

OOK: switching ON and OFF the amplitude of an optical carrier signal (Intensity Modulation Only)

External Modulation: biased at the quadrature point of the MZM transfer function, and driven by an electrical binary NRZ-ASK signal with peak-to-peak amplitude of Vπ.

Input Voltage

Simple Tx/Rx Configurations

Differential Phase Shift Keying - DPSK

Input Voltage

Output Intensity

2Vπ swing

t

minimumpoint bias

MZMLASER

opticalsignal

electrical NRZ data

precoder

t

Phase Modulation Onlyo Nearly constant envelope – higher tolerance to non linear effects

Higher receiver sensitivity due to the 3dB lower OSNR requirement to achieve a specific BER.

External MZM biased at minimum point and driven with a precoded binary data with twice the switching voltage required for NRZ – OOK (2Vπ)

More complex Tx /Rx Design

University of PeloponneseDept. of Inform. & Telecommunications

Fiber Impairments in Single Channel Systems

Tx

SMF

G Rx

DCF

G

Linear

Losses Dispersion

Non Linear

SPM

compensates SMF losses

compensates dispersion

compensates DCF losses

inserts ASE noise

University of PeloponneseDept. of Inform. & Telecommunications

Chromatic Dispersion Effect

Optical Fibre

t1 1 0

1 1 1 t

ISI

Some broadening

Severebroadening

input pulse

Every different f travels with different velocity

1 1 0

Dispersion Parameter: DSMF

Length of Transmission: LSMF

As a linear effect, dispersion can be compensated by means of a DC fibre, providing that the exact amount of dispersion is known in advance.

DDCF*LDCF=-DSMF*LSMF

Dispersion tolerance is inversely proportional to the square of the operating bitrate and consequently limitations due to dispersion become more stringent as bit rate increases.

University of PeloponneseDept. of Inform. & Telecommunications

Electronic Equalization

EE are applied after the receivero no need in intervening in the already installed fibre links

Can cope with variable amounts of dispersion

EE attempts to reverse the distortion incurred by a signal transmitted through a channel. It can be a simple linear filter or a complex algorithm.

Receiver (Rx)

Electric Filter

ClockRecovery

PIN

y(t) y(n) I(n)

Electronic Equalizer

ADC

University of PeloponneseDept. of Inform. & Telecommunications

Electronic Equalization

All equalizers operate at supervised mode, where a training sequence, known by the receiver is transmitted, in order to train the equalizers about the channel characteristics.

Fractional spacing is employed as in this case the performance of the equalizers becomes less sensitive to the sampling phase of the receiver.

In our case we investigate the performance of the following equalizers:

• Linear Transversal Equalizer – LTE• Decision Feedback Equalizer – DFE• Volterra Decision Feedback Equalizer - VDFE

University of PeloponneseDept. of Inform. & Telecommunications

Linear Transversal equalizer - LTE

LTE is the simplest form of electronic equalizers. The incoming signal is processed by a linear filter.

In order to retrieve the transmitted sequence, FS-LTE operates according to:

University of PeloponneseDept. of Inform. & Telecommunications

Decision Feedback equalizer - DFE

DFE consists of two parts: a Feed forward part that is driven by the received waveform and a Feedback part that is driven by the estimations of the previous symbols.

FS-DFE operates according to:

The performance of linear equalizers is constrained when applied to non linear systems.

Non Linear Photodetector

Photodiode operates on a square law principle, in which the output of the detector is proportional to the intensity (i.e., the square of the input signal magnitude).

The main reason of non linearity in optical systems is induced by the detector during the conversion of optical to electrical.

Although it is a simple circuit, it is nonlinear and as such it is difficult to correct linear distortions such as CD.

University of PeloponneseDept. of Inform. & Telecommunications

Volterra Decision Feedback equalizer - VDFE

Simplified VDFE used:

University of PeloponneseDept. of Inform. & Telecommunications

Complexity Calculations

University of PeloponneseDept. of Inform. & Telecommunications

Simulation Setup

10 spans x 100km (1000km)

3 spans x 100km (300km)

10 Gb/s bitrate10 Gb/s bitrate

40 Gb/s bitrate40 Gb/s bitrate

G

Transmission Span (x N)

Tx Rx

BER Estimationw/o EDC

equalizer

BER Estimationwith EDC

SMF DCF

G

University of PeloponneseDept. of Inform. & Telecommunications

Unncompensated Results

150 km

200 km

250 km

380 km

200 km

300 km

400 km

University of PeloponneseDept. of Inform. & Telecommunications

NRZ-OOK Results (1)

87.5 %

85 %

80 %

70% 94%

98%

University of PeloponneseDept. of Inform. & Telecommunications

NRZ-DPSK Results (1)

OCR=70%-90% 10Gb/s & 40Gb/s DPSK

University of PeloponneseDept. of Inform. & Telecommunications

NRZ-OOK Dispersion ToleranceReduces

Upgrading Scenario Setup

G

Total Length of 1000 km (10 spans x 100 km)

Tx

SMF DCF

G

Operating at 10 Gb/sOperating at 40 Gb/s

99 % OCR

BER Estimationw/o EDC

BER Estimationwith EDC

equalizer

NRZ-DPSK

NRZ-OOK

NRZ-DPSK

Rx

University of PeloponneseDept. of Inform. & Telecommunications

Upgrading Scenarios Results

Upgrading a system 10-40 NRZ & DPSK

University of PeloponneseDept. of Inform. & Telecommunications

Conclusion

Low cost, adaptive techniques of optical transmission, consisting of optical and electronic equalization, were studied by simulating configurations with realistic link parameters.

Here, the interplay between optical and electronic techniques for physical impairment mitigation for DD optical transmission with various performance/complexity tradeoffs, is presented.

It has become evident that even in the absence of FEC, low complexity equalizers can perform sufficiently well in conjunction with optical compensation.

Low complexity Volterra equalizers can be used to support the migration of a system from 10 to 40 Gb/s.

University of PeloponneseDept. of Inform. & Telecommunications

Q&A

Thank you for your attention!

This research was funded by the Operational Program "Education and Lifelong Learning" of the Greek National Strategic Reference Framework (NSRF) Research Funding Program: THALES PROTOMI, grant number MIS 377322.

University of PeloponneseDept. of Inform. & Telecommunications