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Electronic predistortion strategies for directly modulated laser systems [Elektronische Ressource] / vorgelegt von Stefan Warm

123 pages
Electronic PredistortionStrategies For DirectlyModulated Laser Systemsvorgelegt vonDiplom-IngenieurStefan Warmaus Berlinvon der Fakul at IV - Elektrotechnik und Informatikder Technischen Universit at Berlinzur Erlangung des akademischen GradesDoktor der Ingenieurswissenschaften- Dr.-Ing. -genehmigte DissertationPromotionsausschuss:Vorsitzender: Prof. Dr. TillakBerichter: Prof. Dr. PetermannBerichter: Prof. Dr. KrummrichTag der wissenschaftlichen Aussprache: 03. April 2009Berlin 2009D 83ForLeo, Louise and CatharinaIIIACKNOWLEDGMENTSFirst of all, I would like to thank my wife Catharina and my chil-dren Leo and Louise for their patience and understanding duringmy dissertation.I would like to thank my supervisors Professor Klaus Petermannand Professor Peter Krummrich for their continued support andthe motivating discussions we had.I also would like to thank my friends and fellow research scholarsfrom the Photonics Group at the Technische Universit at Berlin.It was a real pleasure to work with you.Financial support for this work is gratefully acknowledged fromthe company Nokia{Siemens{Networks.Stefan WarmTechnische Universit at BerlinSeptember 2009IIIIVCONTENTS1. Introduction 12. Laser Diodes 52.1. Basic Concept . . . . . . . . . . . . . . . . . . . . . 52.2. Modulation Characteristics . . . . . . . . . . . . . 83. Transmission Impairments In Optical Fibers 153.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 153.2.
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Electronic Predistortion
Strategies For Directly
Modulated Laser Systems
vorgelegt von
Diplom-Ingenieur
Stefan Warm
aus Berlin
von der Fakul at IV - Elektrotechnik und Informatik
der Technischen Universit at Berlin
zur Erlangung des akademischen Grades
Doktor der Ingenieurswissenschaften
- Dr.-Ing. -
genehmigte Dissertation
Promotionsausschuss:
Vorsitzender: Prof. Dr. Tillak
Berichter: Prof. Dr. Petermann
Berichter: Prof. Dr. Krummrich
Tag der wissenschaftlichen Aussprache: 03. April 2009
Berlin 2009
D 83For
Leo, Louise and Catharina
IIIACKNOWLEDGMENTS
First of all, I would like to thank my wife Catharina and my chil-
dren Leo and Louise for their patience and understanding during
my dissertation.
I would like to thank my supervisors Professor Klaus Petermann
and Professor Peter Krummrich for their continued support and
the motivating discussions we had.
I also would like to thank my friends and fellow research scholars
from the Photonics Group at the Technische Universit at Berlin.
It was a real pleasure to work with you.
Financial support for this work is gratefully acknowledged from
the company Nokia{Siemens{Networks.
Stefan Warm
Technische Universit at Berlin
September 2009
IIIIVCONTENTS
1. Introduction 1
2. Laser Diodes 5
2.1. Basic Concept . . . . . . . . . . . . . . . . . . . . . 5
2.2. Modulation Characteristics . . . . . . . . . . . . . 8
3. Transmission Impairments In Optical Fibers 15
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 15
3.2. Optical Losses . . . . . . . . . . . . . . . . . . . . . 16
3.3. Chromatic Dispersion . . . . . . . . . . . . . . . . 17
3.3.1. Chirped Gaussian Pulse . . . . . . . . . . . 22
3.4. Kerr E ect . . . . . . . . . . . . . . . . . . . . . . 25
3.4.1. Self Phase Modulation . . . . . . . . . . . . 27
4. Modulation Formats For Directly Modulated Lasers 29
4.1. Non Return{to{Zero . . . . . . . . . . . . . . . . . 30
4.2. Dispersion Supported Transmission . . . . . . . . . 34
4.3. Chirp Managed Laser . . . . . . . . . . . . . . . . 38
VContents
5. Electronic Predistortion Concepts 41
5.1. Small Signal Approximation . . . . . . . . . . . . . 43
5.2. Finite Impulse Response Filter . . . . . . . . . . . 47
5.2.1. Linear FIR Filter . . . . . . . . . . . . . . . 48
5.2.2. Nonlinear FIR lter (Volterrra Filter) . . . 51
5.2.3. Post{Filter . . . . . . . . . . . . . . . . . . 54
5.2.4. Pre{Filter . . . . . . . . . . . . . . . . . . . 56
5.3. Arti cial Neural Network . . . . . . . . . . . . . . 63
5.3.1. Feed{forward Neural Network . . . . . . . 63
5.3.2. Particle Swarm Algorithm . . . . . . . . . . 68
5.3.3. Optimization Setup . . . . . . . . . . . . . 71
5.3.4. Results . . . . . . . . . . . . . 77
5.3.5. Experimental . . . . . . . . . . . . 85
6. Signal Predistortion Combined With Post{Processing 89
6.1. FFE/DFE . . . . . . . . . . . . . . . . . . . . . . . 89
6.2. MLSE . . . . . . . . . . . . . . . . . . . . . . . . . 91
6.3. Predistortion & Post{equalization . . . . . . . . . 92
7. Summary and Outlook 97
A. Laser Parameters 99
B. Abbreviations 101
Bibliography 105 115
VICHAPTER1
INTRODUCTION
Directly modulated lasers (DML) have the advantage of low costs,
a small form factor and low power consumption compared to ex-
ternally modulated lasers. Therefore, directly modulated lasers
are widely used in metro systems at the OC-48 rate (2.488 Gb/s)
and below. However, at higher data rates in the conventional
wavelength range of metro networks (1550 nm), chromatic disper-
sion limits the maximum transmission distance of conventional
non return{to{zero (NRZ) modulated lasers to about 20 km. The
reason for this transmission limit is the laser chirp, which leads to
a strong broadening of the optical spectrum if the laser is modu-
lated. A few techniques such as dispersion supported transmission
(DST) [1] and the chirp managed laser (CML) [2] exist to overcome
this inherent transmission limit and allow transmission distances
of up to 250 km standard single mode ber (SSMF).
For externally modulated lasers the standard technique to com-
pensate the e ect of chromatic dispersion is the optical dispersion
compensation (ODC). Dispersion compensating bers (DCF) are
used after a certain transmission length over SSMF, to compen-
11. Introduction
sate the accumulated ber dispersion. Due to the losses in disper-
sion compensating bers, additional ber ampli ers are necessary,
which results in high costs.
Another approach to compensate chromatic dispersion in vec-
tor modulated transmission systems is the electronic predistortion
(EPD) [3, 4], where the dispersion compensation is done not in
the optical, but in the electrical domain. The idea of this tech-
nique is to imaginarily propagate a desired signal at the receiver
backwards through the ber to the transmitter. A signal that is
predistorted in this way and modulated at the transmitter may not
be detectable in a back tot back case, but after the desired trans-
mission length. Without any optical dispersion compensation, the
predistorted signal may be transmitted over several thousand kilo-
meters [3].
In this work the electronic predistortion technique will be adopted
to a directly modulated laser system with the intention to over-
come the dispersion limit of a directly modulated laser system.
Even if the concept is the same, the implementation will be com-
pletely di erent, because direct modulation of a laser cannot mod-
ulate its optical intensity and optical phase independently, as it
is done with an IQ{modulator. The aim of the work is basically
to outperform existing transmission approaches for directly mod-
ulated lasers as dispersion supported transmission and the chirp
managed laser and thus to make directly modulated lasers appli-
cable for transmission systems which are so far reserved for exter-
nally modulated lasers. In order to achieve this, conventional and
less conventional approaches in the eld of electronic predistortion
are studied.
The work is structured as follows: In Chapter 2 the laser diode
as a key component of the transmission system is introduced. Be-
side the general theoretical background of laser diodes, also the
modulation characteristics of the laser diode used in this work
are presented. Chapter 3 describes physical e ects in optical
bers, with emphasis on chromatic dispersion. Chapter 4 gives
an overview over existing modulation formats for directly modu-
2