Vol.
BI0,
Suppl
e
ment
CHINESE
JOURNAL
OF
LASERS
April,
2001
Performance Characteristics
of
Diode-Pumped
Yb : Y
AG
Laser
Hongru
Yang
Jingang
Liu
Deyuan
Shen
Siu-Chung
Tam
Yee-Loy
Lam
Wenjie
Xie
Jianhui
Gu
(Pho
ton
ics Re
secrrc
h Group , School
of
ElectricaL and Electronic Engineering, Nanyang
Tecll1wlogicaL
University, Nanyang Ave
nu
e , Singapore
639798,
Republic
of
Singapore)
Abst
ract
In this
paper,
the
performance
characteristics
of
diode-
pumped
Yb:
Y
AG
laser
such
as
the
temperature
dis
tribution
and
thermal
lensing
effect
inside laser rod
are
inve
stigated.
The
theoretical
res
ult
s
are
in
agreement
with
the
expe
rim
ental
ones.
1 Introduction
Although
Yb:
Y
AG
[lJ
has
been
known
for
decades,
interest
was
not
very
high in this
material
ince it lacks
any
pump
bands
in
the
visible.
The
crystal
has
only
a
single
absorption
feature
around
940
nm
.
With
conventional
pump
sources
such
as
flashlamps,
the
threshold
is
very
high
which
e
liminat
ed this
material
f
rom
any
serious
considerations
.
Only
with
the
emergence
of
powerful
InGaAs
la °er diodes
which
emit
at
940
nm
in
the
p
ast
years
the
great
potential
of
Yb:
Y
AG
medium
has
been
demon
trated
by
high
eff
icient
and
powerful
diode-pumped
lasers[2-5J
Compared
with
Nd : Y
AG,
the
Yb
: Y
AG
laser
offers
the
following
favorable
properties:
the
pre
ence
of
only
one
ex
cited-state
manifold
of
Yb
-ion
eliminates
problems
associated
with
excited
-s
tate
absor
ption
and
up-
conversion
processes;
very
low-
quantum
defect
and
he
nce
very
low
fractional
heating;
long
raditive
lifetime
of
the
upper
state,
making
Yb:
Y
AG
a good
medium
for
Q-switching;
high
doping
l
eve
ls
(----
20
at.
%)
due
to
the
simple
energy
level
structure
and
the
lack
of
concentration
quenching ;
broad
emission
bandwidth
(----
86
cm
-1
),
indicating
suitability
for
mode
- locked
operation
to ge
nerate
the
femtoseco
nd
optical
pulses
and
a
broad
absorption
band
of
18
nm
at
940
nm,
being
ideal f
or
diode
pumping.
These
have
further
contributed
to
making
Yb:
Y
AG
so
extremely
attractive
as
a
diode-
pumped
la ing
mat
er
ial for
generation
of
ultrashort
optical
pulses
and
more
than
1
kW,
high-average-power
laser
ap
plic
atio
ns. Optic
al
pulses
of
56-
ps
were
generated
from
pas
sively
Q-
switc
hed
diode-
pump
ed
microchip
Yb :
YAG
laser[6J.
Up
to
date,
optical
pulses
as
short
as
340
- fs
at
a
center
wavelength
of
1. 03
~
m
has
been o
btained
from
a diode-
pumped
passive
mode
- locked
Yb
: Y
AG
laser
with
a
semico
nductor
saturab
le
absorber
by
U. Keller
et
al.
[7,8J
in
1996
and
1999,
respective
l
y.
Yb:
YAG
materialas
gain
media
of
oscillator
and
amplifiers
in
state-
of-
the-
art
phase
-
conjugation
master-
osci
ll
ato
r
power
-a
mplifier
(
PC
-
MOP
A)
laser
provides
an
effective
means
of
achieving
very
high
average
output
pow
er
with
near
diffraction
limited
beam
quality.
CW
multimode
output
of
more
than
1
kW
fro
m a diode-
pumped
phase-
conjugation
MOPA
laser
with
three
Yb:
YAG
rod
amplifiers
in series
ha been
ac
hieved
by
David
S.
Sumida
et
aL.
[9
.1 0J
How
eve
r,
the
main
problem
with
Yb
as
a
dopant
is its
quasi
-
three-Ievel
nature
.
In
fact,
because
of
the
therma
l filling
of
the
lower
laser
levels,
the
performance
of
the
Yb
laser
strongly
depends
on the