COL 9(11), 111401(2011) CHINESE OPTICS LETTERS November 10, 2011
Theoretical and experimental research on cryogenic
Yb:YAG regenerative amplifier
Xinghua Lu (
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, Jiangfeng Wang (
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Youen Jiang (
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Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
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Corresponding author: luxingh@yahoo.cn
Received April 14, 2011; accepted May 9, 2011; posted online July 11, 2011
Based on the theory of quasi-three-level rate equations modified by amplified spontaneous emission, the
stored energy density and the small signal gain of the cryogenic Yb:YAG regenerative amplifier for a
given geometry for pulsed pumping in three dimensions are theoretically studied using the Monte Carlo
simulation. The present model provides a straightforward procedure to design the Yb:YAG parameters
and the optical coupling system for optimization when running at cryogenic temperature. A fiber-coupled
laser diode end-pumped cryogenic Yb:YAG regenerative amplifier running at 1 030 nm is demonstrated
with a maximum output energy 10.2 mJ at a repetition rate of 10 Hz. A very good agreement between
the experiments and the theoretical model is achieved.
OCIS codes: 140.3430, 140.3280, 140.3580.
doi: 10.3788/COL201109.111401.
With the development of high power laser diodes (LDs),
rare earth ion-doped materials have attracted great inter-
est in the application of high-efficiency and high-power
diode pumpe d laser systems
[1,2]
. Among these laser ions,
trivalent ytterbium (Yb) seems to be the mos t appeal-
ing because of its simple electronic structure
[3]
. Its elec-
tronic le vel diagram consists of only two electronic lev-
els, avoiding the excited state absorption, up conve rsion
processes, and concentration quenching. Yttrium alu-
minum garnet (YAG) is a n attractive laser host material
because of its excellent thermal, chemical, and mechani-
cal properties
[4]
. Compared with Nd:YAG laser crystals,
Yb:YAG has several advantages, such as a long fluores-
cence lifetime and a very low quantum defect, re sulting
in three times less heat generation during lasing than
Nd-based laser systems. The longer fluorescence lifetime
benefits energ y storage. The lower quantum defect of
Yb causes less heat deposition, enabling the operation
of the laser at hig her repetition ra tes . Other advantages
include broad absorption bandwidth and less sensitiv-
ity to diode wavelength specifications , a relatively large
emission peak cross-se c tion, and easy growth with high
quality and high doping concentration
[5]
. Several ty pes
of Yb:YAG lasers have been developed for efficient oscil-
lation with various pumped architectures, including thin
disk
[6]
, microchip
[7]
, and rod
[8]
structures at room tem-
perature. The primary disadvantage of Yb lies in its
quasi-three-level nature. Therefore, hig h pump fluence
is require d to overcome the re-absorption losses and to
reach high storage efficiency at room temperature. The
quasi-three-level nature of this material can be overcome
by using the material under cryogenic cooling condition.
The benefits of cooling Yb:YAG res ult in a significant
increase in the emission cross section; the thermal prop-
erties of Yb:YAG, such as thermal conductivity, thermo-
optic c oefficient, and thermal expansion c oefficient, are
significantly improved at low temperatures, which are
preferred especially for high-average-power operation
[9]
.
Considering the increasing emission cr oss section
and the improving ther mal behavior of the cryo-
genic Yb:YAG, high-energy, close to 10-Hz repeat-
able, nanosecond laser systems have been widely used
in various kinds of fields, such as inertial fusion en-
ergy and pumped sources for optical parametric am-
plifiers
[10]
. Cao et al. theoretically researched the
efficiency of diode-pumped Yb:YAG disk amplifiers at
room temperature
[11]
. Albach et al. numerically simu-
lated the effect of amplified sp ontaneous emission (ASE)
on Yb:YAG sla bs in monochromatic assumption
[12]
. In
the current work, a theoretical model based on quasi-
three-level rate equations is modified to investigate the
cryogenic Yb:YAG regenerative amplifier to obtain the
stored energy and the small signal gain coefficient in
the crystal by considering the pump pulse duration and
length of the c rystal. Considering the ASE e ffect, a
number of Monte Carlo simulations, which can esti-
mate the stored energy density for a given geo metry for
pulsed pumping in thre e dimensions, are develope d. The
present model provides a straightforward procedure to
design the crystal parameters and the optical coupling
system for optimization. To illustrate the utility of the
present model, a cryogenic Yb:YAG regenerative am-
plifier pumped by fiber-coupled LDs is demonstrated.
The output beam has a nearly TEM
00
mode profile with
maximum energy obtained at 10.2 mJ at a repe tition rate
of 10 Hz. A very good agreement be tween exper iments
and the theoretical model is observed.
An energy-level diagram of the Yb:YAG is shown in
Fig. 1. It consists o f only two manifolds, an upper
2
F
5/2
and a lower
2
F
7/2
, with the former co ntaining
three Stark levels and the latter four
[13]
. The Boltzmann
occupation factors for the upper state manifold are la-
beled f
11
, f
12
, and f
13
, whereas tho se for the lower state
manifold ar e labeled f
01
, f
02
, f
03
, and f
04
. Pumping
and lasing processes occur between the Stark sub-levels.
Rapid thermalization of the levels within each ma nifold
is assumed so that the relative populations of the lev-
els within a manifold can be treated by the Boltzmann
equilibrium
[11]
.
1671-7694/2011/111401(4) 111401-1
c
2011 Chinese Optics Letters