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

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• Publié par: technische_universitat_berlin
• Publié le: 01 janvier 2009
• Nombre de lectures: 7
• Langue: English
• Poids de l'ouvrage: 6 Mo

Extrait

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