Erbium-Doped
Fiber
Amplifiers
Fundamentals
and
Technology
OPTICS
AND
PHOTONICS
(formerly
Quantum Electronics)
EDITED
BY
PAUL
L.
KELLY
Tufts
University
Medford,
Massachusetts
IVAN
KAMINOW
AT&T Bell Laboratories
Holmdel,
New
Jersey
GOVINDAGRAWAL
University
of
Rochester

N.
A.
OLSSON
Passive Optical Networks Group
Switching
and
Access
Group
Lucent
Technologies
Murray Hill,
New
Jersey
J.
R.
SIMPSON
Ciena Corporation
Linthicum, Maryland
ACADEMIC
PRESS
San
Diego
London
Boston
New
York
Sydney
Tokyo Toronto
This book
is

retrieval system, without
permission
in
writing
from
the
publisher.
All
brand names
and
product names mentioned
in
this book
are
trademarks
or
registered trademarks
of
their respective companies.
Academic Press
A
Harcourt
Science
and
Technology
Company
525 B
Street, Suite 1900,
San
Diego,

p. cm. —
(Optics
and
Photonics)
Includes bibliographical references
and
index.
ISBN
0-12-084590-3—ISBN
0-12-084591-1
(Diskette)
1.
Optical communications
—
Equipment
and
supplies
2.
Optical amplifiers.
3.
Optical
fibers I.
Olsson,
N. A. II.
Simpson,
J. R.
III. Title
IV.
Series.
TK5103.59.B43 1997

Amplifiers
5
1.3
From Glass
to
Systems Outline
9
2
OPTICAL FIBER FABRICATION
13
2.1
Introduction
13
2.2
Conventional Communication Fiber
. 14
2.3
Rare Earth Doped
Fibers
16
2.3.1 Rare Earth
Vapor
Phase Delivery Methods
. 16
2.3.2 Rare Earth Solution-Doping Methods
21
2.3.3
Rod and
Tube Methods
23

3.1
Introduction
43
3.2
Fiber Connectors
43
3.3
Fusion
Splicing
. 48
3.4
Pump
and
Signal Combiners
50
3.5
Isolators
52
v
vi
CONTENTS
3.6
Circulators
53
3.7
Filters
55
3.8
Fiber Gratings
55

87
4.1
Introduction
, . 87
4.2
Atomic Physics
of the
Rare Earths
87
4.2.1
Introduction
The 4f
Electron Shell
87
4.2.2
The
"Puzzle"
of 4f
Electron
Optical Spectra
91
4.2.3 Semiempirical Atomic
and
Crystal Field Hamiltonians
92
4.2.4 Energy Level Fitting
94
4.3
Optical Spectra
of

of the Er
3
Ion 110
4.5.1
Lifetimes
111
4.5.2
Er
3
Spectra, Cross Sections,
and
Linewidths
114
4.6 Er
3
-Er
3
Interaction
Effects
120
5
ERBIUM-DOPED FIBER AMPLIFIERS AMPLIFIER BASICS
131
5.1
Introduction
131
5.2
Amplification
in
Three-Level Systems Basics

Two-Level System
149
6
ERBIUM-DOPED FIBER AMPLIFIERS
-
MODELING
AND
COM-
PLEX
EFFECTS
153
6.1
Introduction
153
6.2
Absorption
and
Emission Cross Sections
153
CONTENTS
vii
6.3
Gain
and ASE
Modeling
156
6.3.1
Model Equations
-
Homogeneous Broadening

Saturation Modeling Signal Gain
and
Noise Figure
173
6.4.6 Power
Amplifier
Modeling
175
6.4.7
Effective
Parameter Modeling
178
6.5
Transverse Mode Models Erbium Confinement
Effect
180
6.6
Excited State Absorption
Effects
186
6.6.1 Model Equations
, 186
6.6.2 Modeling Results
in the
Presence
of ESA 188
6.6.3
800 nm
Band Pumping
188

7.2
Optical Noise: Device Aspects
202
7.2.1
Classical Derivation
of
Optical
Amplifier
Noise

202
7.2.2 Noise
at the
Output
of an
Optical Amplifier
. 205
7.2.3 Comparison
of
Optical Amplifier Devices
210
7.3
Optical Noise: System Aspects
212
7.3.1
Receivers
, 213
7.3.2
Bit
Error Rate Calculations

Introduction
251
8.2
Basic
Amplifier
Measurement Techniques
251
8.2.1 Gain Measurements
251
8.2.2
Power Conversion
Efficiency
257
8.2.3 Noise Figure Measurements
258
8.3
Amplifier Design Issues
263
8.3.1
Copropagating
and
Counterpropagating
Pumping Issues
. . . 265
Viii
CONTENTS
8.3.2 Choice
of
Fiber
Lengths

9.2
System Demonstrations
and
Issues
323
9.2.1 Preamplifiers
323
9.2.2 Inline Amplifiers
-
Single Channel Transmission
327
9.2.3
Mine
Amplifiers
- WDM
Transmission
335
9.2.4
Repeaterless
Systems
345
9.2.5 Remote Pumping
346
9.2.6 Analog Applications
351
9.2.7 Gain Peaking
and
Self-Filtering
354
9.2.8 Polarization Issues

404
10.2.1 Introduction
404
10.2.2
Spectroscopic
Properties
405
10.2.3
Gain Results
for Pr
3
-doped Fiber Amplifiers
406
10.2.4
Modeling
of the Pr
3
-doped Fiber Amplifier Gain
412
10.2.5
System Results
416
10.3
Nd
3
-Doped Fiber Amplifiers
418
10.3.1
Introduction
418

OASIX
R
430
A.2.3
Starting OASIX
R
430
A.2.4
What
to do
next
, . . , 430
CONTENTS
ix
A.3
A
Quick Overview
and
Tour

430
A.3.1
Fibers
and
Modeling Parameters
430
A.3.2
Saving
a
Simulation Configuration

436
A.4.5 Simulation Status
Box 437
A.5
Simulation Looping Structure
, . 438
A.5.1
Specifying
Loop Parameters
438
A.5.2 Choosing Loop Order
438
A.5.3 Linear
or
Logarithmic Looping
439
A.5.4
Multiple Parameters Varied
in a
Loop
439
A.5.5
Influence
on
Output Format
440
A.5.6 Output Modes
. 440
A.6
Sample Simulations

Waves
446
A.9.2 Causes
and
Remedies
for
Convergence Failure

447
A.
10
Comment
on the
treatment
of
losses
448
INDEX
451
Foreword
The
telecommunications industry
has
been
in a
constant state
of
agitation
in
recent

and
reduced
consumer costs
to the
point that international
fax and
Internet communica-
tion
is
commonplace.
At the
same
time,
photonic technology
has
revolutionized long
distance,
and now
local access, capabilities, thereby helping
to
sustain
the
boil
in the
information
marketplace.
The first
generation lightwave systems were made possible
by the
development

intrinsically unreliable electronic regenerators. Indeed, early
EDFA
technology
was
driven
by the
submarine system developers
who
were quick
to
recognize
its
advantages, soon
after
the first
diode-pumped EDFA
was
demonstrated
in
1989. Terrestrial telecommunications systems have
also
adopted EDFA technology
in
order
to
avoid
electronic
regeneration.
And
hybrid

than
4000
GHz
wide. With available wavelength division multiplexing (WDM) com-
ponents, commercial systems transport more than
16
channels
on a
single
fiber; and
the
number
is
expected
to
reach 100. Hence, installed systems
can be
upgraded many
fold
without
adding
new fiber, and new WDM
systems
can be
built
inexpensively
with
much
greater capacity.
As

The
present book
is a
much-needed
and
authoritative exposition
of the
EDFA
by
three
researchers
who
have been early contributors
to its
development.
No
other book
provides
an
up-to-date engineering account
of the
basics
of
operation, methods
of
doped
fiber
fabrication, amplifier design,
and
system performance considerations.

given
in
Chapter
1. The
methods
of
fabricating
rare-earth-doped
fiber,
including
the
double-clad
fiber
used
in
some
diode-pumped
de-
vices,
are
reviewed
in
Chapter
2. The
physical properties
of the
doped glass
are
also
discussed

fiber,
and
the
operation
of
such components
as
wavelength division multiplexers
for
coupling
the
diode pump
laser,
isolators
for
blocking reflections, circulators
for
separating
in-
cident
and
reflected signals,
fiber
grating
filters, and
add/drop multiplexers
for
system
applications.
The

and
amplifiers
is
summarized
in
Chapter
4. The
energy levels, spectra, line shapes,
and
lifetimes
are
covered along with
the
small
but
significant influences
of the
host glass
and
doping concentration
on
these parameters.
The
detailed emphasis
is on the
erbium
ion,
Er
3
,

rate equations that model
the
gain
process
in
erbium amplifiers
are
introduced
in
Chapter
5.
With this model,
one can
optimize
the
EDFA design
in
terms
of fiber
length
and
index profile. Amplified spontaneous emission (ASE),
which
is
critical
in
defining
the
noise
figure of

that
it
determines
the
number
of
channels allowed.
The
authors advise that
a
reader interested only
in
technical
amplifier
design
can
skip
the
background
material
provided
in
Chapters
2 to 6, and go
directly
to
Chapter
7. At the
outset
of

design systems with optimized
performance,
depending upon system requirements.
For
example,
a
cascade
of
in-line
amplifiers
can be
employed
on a
very
long
submarine
system
to
overcome
fiber
loss.
xii
Foreword
or
a
preamplifier
can be
placed before
the
output receiver

and
cross phase
modulation
are
considered.
A final
section covers
the use of
amplifiers
for
analog
applications,
such
as
cable television.
Chapter
8
gets into
the
practical considerations
of
amplifier characterization
and de-
sign
for
particular applications.
The
methods used
for
measurements

techniques
for flattening the
gain spectrum
to
meet
the
needs
of WDM
systems.
Chapter
9
brings
us to the
core
of
EDFA fever:
the
system implementations. Some
300
references
recall
the
record-setting
system experiments achieved with EDFA's
in
their
roles
as
preamplifiers, in-line,
and

EDFA preamplifier. In-line
amplifiers
have been employed
in
the
lab and
under
the sea to
span
10,000
km
distances
at 5 Gbs and
beyond.
WDM
experiments
and
their constraints
are
also recounted;
and
repeaterless
and
remotely-
pumped
systems
are
discussed. System requirements
and the
performance

work.
The
book
closes
with
a
chapter
on
rare-earth-doped
amplifiers
for the
1300
nm
band,
corresponding
to the
other important low-loss window
in
silica,
which
was the
first to be
exploited commercially.
The
ions
of
choice
are the
four-level systems
neodymium

in as
brief
a
time
as is
reasonable.
Ivan
P.
Kaminow
Preface
With
the
development
of low
loss
fibers as the
communications medium,
efficient
com-
pact
laser diodes
as the
modulated light sources,
fast
detectors
and the
auxiliary equip-
ment
necessary
to

has
inspired thou-
sands
of
papers
and
continues
to
motivate research
on the
many diverse components
that
are
required
in
these
systems.
There continues
to be
many
hundreds
of
publications
per
year
on
various
aspects
of
erbium spectroscopy,

on
spectroscopy, practical
amplifier
design
and
systems,
so as to
provide
a
complete self contained volume.
Chapter
1
opens with
a
convincing enumeration
of the
applications
to
long
haul
networks.
Especially impressive
are the
undersea connections
from
the
Americas
to
Europe
and

these
are
utilized
to
dope
the fiber
cores
with
rare earths. They
also
deal with
sol gel
preparation methods.
Of
particular interest
is
their description
of
novel
fiber
designs
to
facilitate optical pumping
of the
core, such
as the
double clad
configuration.
Much
of the

and filters. In the
latter category they describe
the
important
recent developments
in fiber
gratings, both short
and
long period,
and
their
applications
to
add/drop components
in WDM
systems, dispersion compensation,
and
gain
flattening.
Chapter
4
covers
the
usual items
of
rare earth spectroscopy, Judd-Ofelt computa-
tions
and
non-radiative phonon relaxation.
A

but
useful discussion
of
upconversion (useful
for
the Tm
laser
in the
blue
but no so
useful
for an Er
amplifier
at 1.5 m). For
complete-
ness,
in
order
to
make
the
book
a
self contained document, Chapter
5
reviews basic
concepts
in
amplifiers. This
is

concludes
with
a
caution that
too
much erbium
can be too
much
of
a
good thing because
of
clustering
and
cooperative up-conversion quenching
of the
excited state. Chapters
7, 8, and 9
deal
with
the
many issues involved
in
system
appli-
cations.
They cover
the
theory
of

fiber
amplifiers
for 1.3 m
amplification". While
it
takes
up the use of Nd
3
in
selected hosts,
its
primary emphasis
is on Pr
3
in flu-
oride
hosts,
a
leading candidate
for 1.3 m
amplifiers.
The
contrast between
it and
erbium-doped
fiber
amplifiers illustrates
how
much
of a

reader
through many aspects
of the fiber
amplifier
field. It is
an
authoritative
and
comprehensive review
of
many
of the
necessary building blocks
for
understanding erbium
fiber
amplifiers
and
optically
amplified
systems.
Elias
Snitzer
Professor
Emeritus
Department
of
Materials
Science
and

us
according
to our
specializa-
tions
has
made
the
task easier. Nevertheless, this work would
not
have been
possible
without
the
support
and
help
of a
number
of
individuals. Miriam Barros
was
instru-
mental
in
supporting
the
effort
with
her

Desurvire,
D.
DiGiovanni,
C
Giles,
S.
Kramer,
and G.
Nykolak.
We
would also like
to
convey
our
appreciation
to our
col-
leagues
who
assisted
in the
reviewing
of the
manuscript,
N.
Bergano,
A.
Chraplyvy,
T.
Cline, J M. Delavaux,

to our
Academic Press
colleagues,
our
editor,
Zvi
Ruder,
and our
production editor, Diane Grossman,
for
their
support.
Thank
you
also
to our
LATEX
consultant,
Amy
Hendrickson.
We
would also
like
to
thank
the
authors
of our
preface
and

Nils-Petter
and
Inga,
Harold
and
Edith,
and
to our
wives,
Tomomi,
Lana, Carol,
and to our
children,
Fumiyuki,
Nicolas, Anna,
Julie
and
Katie,
for
their support during this
long
project.
Chapter
1
Introduction
The
erbium-doped
fiber
amplifier
is

amplifiers
are
having
a
very
significant
commercial
impact.
The
emergence
of the fiber
amplifier foreshadows
the
invention
and
development
of
fur-
ther
guided wave devices that should play
a
major
role
in the
continuing increase
in
transmission
capacity
and
functionality

km in
Asia-Pacific,
and 8
million
km
elsewhere,
for a
total
of
171
million
km,
according
to
KMI
Corporation, Newport,
Rl.
In
1997
alone,
38
million
km of fiber
were added worldwide. Additionally,
by
1997, over
366,000
cable-km
of fiber-optic
undersea cable

applications,
if the
economics
warrant
it.
Given
the
current high price
of
erbium-doped
fiber
amplifiers
(US
$10,000
and up at the
time
of
this writing), they
are
used primarily
in
high-capacity
backbone routes
and are not yet
slated
for
high-volume applications
in the
local
loop.

most corners
of the
world. Quite often,
sea-based
cables
(known
as
offshore
trunk
routes)
are a
convenient
way to
connect
the
major hubs
of a
region.
One
example
is the
FLAG
(Fiber Loop Around
the
Globe) cable that connects Europe
and
Asia
and has a
1
2

All rights
reserved.
Reprinted
with
permission.
Updates courtesy
W.
Marra,
Tyco Submarine Systems Limited,
Holmdel,
NJ.
number
of
festoons
for
local connections,
in
particular
in
Southeast Asia. There
is
cur-
rently
a
significant
number
of new
cables being planned, based
on
WDM

detect
the
weak incoming light,
electronic amplifiers, liming circuitry
to
maintain
the
timing
of the
signals,
and a
laser
along with
its
driver
to
launch
the
signal along
the
next
span.
Such regenerators
are
limited
by the
speed
of
their
electronic

no
high-speed circuitry.
The
signal
is not
detected then regenerated; rather,
it is
very sim-
ply
optically
amplified
in
strength
by
several orders
of
magnitude
as it
traverses
the
amplifier,
without being limited
by any
electronic bandwidth.
The
shift
from
regen-
erators
to

been made
in
increasing
the ca-
pacity
of
systems
using
such
amplifiers.
Table
1.1
traces
the
evolution
of
transatlantic
1.1.
LONG HAUL
FIBER
NETWORKS
3
Year
Installed
1963
1965
1970
1976
1983
1988

MHz
280
Mb/s
560
Mb/s
560
Mb/s
560
Mb/s
5Gb/s
Number
of
Basic
Channels
140
140
840
4200
4200
8000
16000
24000
24000
122880
Capacity
in
Voice
Channels
315
315

fim
"
Optical
amplifiers
Table 1.1: Transatlantic cable systems
and
capacity
in
simultaneous calls. From ref-
erence
[3]
(©1993
DEEE).
The
capacity
in
voice channels
is
larger than that
in
basic
channels (which itself makes
use of
compression techniques)
as a
result
of the use of
statistical multiplexing techniques, such
as
DCMS (digital circuit multiplication sys-

long haul
systems, such
as the
TAT-12,13
fiber
cable that AT&T
and its
European partners
in-
stalled across
the
Atlantic
in
1996. This cable,
the first
transoceanic cable
to use fiber
amplifiers,
provides
a
near
tenfold
increase
in
voice
and
data transmission capacity over
the
previous transatlantic cable.
A

long haul systems
will
operate
at
higher
bit
rates,
in the 5 to 10
Gb/s range.
They will also have multiple wavelength channels
and
make
use of WDM
(Wavelength
Division Multiplexing) technology. Recent experiments using optical
amplifiers
and
dense
WDM (50 to 132
channels) have crossed
the
Tb/s
barrier
for
information trans-
mission, over distances
in
some cases
as
long

1
TPC-5G
TAT-
1
2
TPO5J
Landing
Points
Vero
Beach,
FL
- St.
Thomas
San
Luis Obispo,
CA
-
Keawaula,
HI
Green
Hill,
NY -
Lands
End,
UK
Coos Bay,
OR -
Ninomiya,
Japan
Approximate

of the
United
States,
examines
an
erbium-
doped
fiber
amplifier during
a
1993 visit
to
AT&T Bell Laboratories,
in the
presence
of
researchers Miriam
Barros
and
Gerald
Nykolak.
Photograph property
of
AT&T
Archives. Reprinted with permission
of
AT&T.
transmission systems. Erbium-doped
fiber
amplifiers

doped
fiber
amplifier
demonstration.
From
top to
bottom,
the
elements
are the
laser cavity,
the fiber
laser
(fabricated
in the
form
of a
helix
so as to be
wrapped around
the flashtube), a flashtube,
and
an
18
cm
scale. From reference
[9j.
1.2
HISTORICAL DEVELOPMENT
OF

amplifier
at
1.06
fj,m.
The fiber had a
core
of 10
/zm
with
a
0.75
to
1.5
mm
cladding,
a
typical length
of 1
m,
and was
wrapped around
a flashlamp
that excited
the
neodymium
ions.[9]
Figure
1.3
shows
the

in the
conclusion
of his
paper. This work
lay
dormant
for
many years
thereafter.
It
emerged
as an
exceedingly relevant technological innovation
after
the ad-
vent
of
silica glass
fibers for
telecommunications. Snitzer also demonstrated
the first
erbium-doped glass laser. [10]
Interestingly,
rare earth doped
lasers
in a
small diameter crystal
fiber
form
were

diameter, with typical values
in the
25
^u,m
to 70
/^m
range.
The
cores were doped with neodymium, with
a
surrounding
fused
silica cladding. Lasing
of
this device
was
achieved
for a
laser wavelength
of
1.06
£im.
A
laser
was
typically fabricated
by
polishing
the end
faces

launched
pump
power
at 890
nm.
Lasing
was
even demonstrated
with
an LED
pump.f13]
Since
6
CHAPTER
1.
INTRODUCTION
Figure 1.4: Fiber
laser
pumped
by a
diode
laser.
From reference
[12].
(a)
Copper
support;
(b)
diamond heat sink;
(c)

single-mode
fibers
occurred
in
1983.
Performed
by
Broer
and
Simpson
and
coworkers
at
Bell Telephone Laboratories,
the
purpose
of the
work
was to
study
of the
physics
of
fundamental
relaxation mechanisms
of
rare earth ions
in
amorphous hosts.[14,
15] The fiber,

was
relatively high
(8
dB/km
at
1.38
/im).[15]
A
few
years later,
further
improvements
in
using
the
MCVD technique
to
fabricate rare
earth doped
single-mode
fibers
were achieved
by
Poole
and
coworkers
at the
University
of
Southampton,

"
1
"
doped
single-mode
fiber
laser, pumped
by a
GaAlAs laser diode,
was
demonstrated
for the
first
time,
at the
University
of
Southampton,
in
1985.
[18]
The
laser
was 2 m in
length,
with
the
cleaved
fiber
ends butted directly

was
followed shortly thereafter
by
that
of fiber
amplifiers.
Erbium-doped
single-mode
fiber
amplifiers
for
traveling wave amplification
of 1.5
£tm
signals were simultaneously developed
in
1987
at the
University
of
Southampton
and
at
AT&T
Bell Laboratories. [19,
20, 21]
Apart
from the
technical refinements that
re-

was the
recognition
that
the
Er
3+
ion,
with
its
pro-
1.2.
HISTORICAL
DEVELOPMENT
OF
ERBIUM-DOPED
FIBER AMPLIFIERS
7
Figure 1.5: Experimental
setup
for
MCVD
fabrication
of
low-loss rare earth-doped
single-mode
fibers.
From reference
[16].
Figure 1.6: Early demonstration
of

medium
for
modem
fiber-optic
transmission systems
at
1.5
/um.
Both
of the
demonstrations involved large
frame
lasers;
an
argon laser-pumped
dye
laser operating
at 650 nm for the
Southampton
group,
and an
argon laser operating
at
514
nm for the
AT&T Bell
Laboratories
group.
The
high signal gains obtained

fiber,
and
fiber
isolators placed
after
these splices
prevent
the
laser oscillation.
8
CHAPTER
1.
INTRODUCTION
Figure 1.7:
Outline
of the
book.
Given
that
the
previously mentioned
amplifier
demonstrations used large
frame
laser
pumps,
one
last remaining hurdle
was to
demonstrate

in the
1.53
ftm
to
1.55
fjum
range.[22]
Nakazawa
was
able
to use
high-power 1.48
/zm
laser diode pumps previously developed
for fiber
Raman
amplifiers.[23]
This demonstration opened
the way to
serious consideration
of
amplifiers
for
systems application. Previous work, exploring optical
amplification
with
semiconductor amplifiers, provided
a
foundation
for

networks.
The first
undersea test
of
erbium-doped
fiber
amplifiers
in a fiber-
optic transmission
cable
occurred
in
1989.[25]
A
few
years later, commercial amplifiers
were
for
sale
and
were being installed
by
major
telecommunications companies. MCI,
for
example, purchased
and
began
the
installation

erbium-doped
fiber
amplifier
also
reinvigorated
the
study
of
optical solitons
for fiber-optic
transmission, since
it now
made practical
the
long distance transmission
of
solitons.
In
conjunction
with
recent advances made
throughout
the
1990s
in a
number
of
optical transmission technologies,
be it
lasers