This document describes a kilowatt-level quasi-continuous wave (QCW) diode-side-pumped Nd:YAG ceramic laser system. The system uses a master oscillator power amplifier configuration with three identical ceramic laser modules. With a total pump power of 3433 watts, the system achieved an output power of 1020 watts, corresponding to an optical-to-optical conversion efficiency of 29.7%. Measurements showed the laser operated with a repetition rate of 1 kHz and pulse width of 114 microseconds. This represents the first QCW side-pumped Nd:YAG ceramic laser system to achieve over 1 kilowatt of output power.

1. A kilowatt level diode-side-pumped QCW Nd:YAG ceramic laser
C.Y. Li a,
⁎, Y. Bo a
, B.S. Wang a
, C.Y. Tian a
, Q.J. Peng a
, D.F. Cui a
, Z.Y. Xu a
, W.B. Liu b
, X.Q. Feng b
, Y.B. Pan b
a
RCLPT, Key lab of functional crystal and laser technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, China
b
Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 201800, China
a b s t r a c ta r t i c l e i n f o
Article history:
Received 7 May 2010
Received in revised form 17 July 2010
Accepted 18 July 2010
Keywords:
Ceramic lasers
Diode-side-pumped
Solid state laser
Nd:YAG
Quasi-continuous-wave
We demonstrate a kilowatt level Quasi-continuous-wave (QCW) diode-side-pumped Nd:YAG ceramic laser
at 1064 nm. The laser system adopts a master oscillator power amplifier scheme (MOPA). The master
oscillator contains two diode-pumped laser modules. Under the pump power of 2000 W, an output power of
686 W was obtained. After amplified by an identical ceramic laser module, a maximum output power of
1020 W was obtained under a total incident pump power of 3433 W, corresponding to an optical–optical
conversion efficiency of 29.7%. At the maximal output power, the repetition frequency was measured to be
1 kHz and the pulse width was 114 μs. To the best of our knowledge, this is the first time to realize QCW side-
pumped Nd:YAG ceramic laser system with output power above 1 kW.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Ceramic lasers are widely studied in recent years due to their
remarkable advantages compared with the traditional single crystals
[1,2]. For example, samples with high doping concentration and large
sizes can be easily fabricated, whereas it is usually very difficult for
crystals. The cost of ceramic laser materials can be potentially lower
than their single-crystal counterparts because of their faster fabrica-
tion process. Moreover, the rigid bonding of multiple samples is easy
with these materials; this greatly enhances the design flexibility for
novel laser devices. At present the ceramic laser has been undoubt-
edly proven to be a promising candidate for the next generation high
performance laser.
For industrial processing, scientific research and military purposes,
there is great demand for both high power and good beam quality Nd:
YAG lasers. Benefiting from above advantages, Nd:YAG ceramic lasers
hold a huge foreground for these applications. However, it is still
difficult to balance high power and good beam quality because of the
serious thermal effects of these materials. For diode-side pumped
technologies, the thermal effects could be ameliorated notably
through adopting pulsed pump method instead of pumping in
continuous wave mode. Thus both high output power and good
beam quality can be expected simultaneously. However, most reports
about high power Nd:YAG ceramic lasers are focused on the CW
pumping. For instance, the maximum output power of 1.46 kW CW
Nd:YAG ceramic laser was reported by Toshiba Corporation, Japan
previously in 2002 [3]. Subsequently, although many attempts were
addressed to improve the output power, the results were not satisfied.
Typically, in 2004, J.R. Lu et al. reported a 110 W CW Nd:YAG ceramic
laser using 0.6% atom dopant concentration [4]; in 2006, D. Kracht
et al. reported a core-doped ceramic Nd:YAG laser with 144 W output
power by end-pumped method [5]; In 2007, S. Lee et al. reported the
result of 210 W output power using side-pumped technology [6]. As
for the Quasi-continuous-wave (QCW) ceramic Nd:YAG lasers, there
is seldom studied. Up to the present, only J.H. Ji et al. [7] and Y.F. Qi
et al. [8] reported kHz QCW operation ceramic Nd:YAG lasers with
output power of 230 W and 236 W, respectively. Here we report our
results of QCW Nd:YAG ceramic laser system with output power of
1020 W and repetition frequency at 1 kHz by a compact scheme of
master oscillator power amplifier (MOPA).
2. Experimental
We prepared three identical laser modules to obtain highly
efficiency pump and uniform gain distribution in the Nd:YAG ceramic
rods. The cross section schematic configuration of the pump module is
shown in Fig. 1. The module mainly consists of four parts that are Nd:
YAG ceramic rod, a flow tube, a reflector and sixty diode bars mounted
in the brass heat sinks, respectively. The Nd:YAG ceramic rods (ϕ6
mm×100 mm) were fabricated by Shanghai institute of ceramics,
CAS. The Nd doping concentration is 1.0 at%. The Nd:YAG rod is
surrounded by a diffusive reflector and pumped by laser diodes from
five different directions. The LD arrays are arranged in a two-line, five-
fold symmetry. This beam coupling method not only achieves efficient
pumping of the Nd:YAG rod but also produces a uniform pumping
distribution within the entire cross section of the rod. The diode bars
Optics Communications 283 (2010) 5145–5148
⁎ Corresponding author.
E-mail address: Zhaoyang2050@163.com (C.Y. Li).
0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.optcom.2010.07.045
Contents lists available at ScienceDirect
Optics Communications
journal homepage: www.elsevier.com/locate/optcom

2. and ceramic rod are both cooled to 21 °C by de-ionized water flowing
in the flow tube. The diode bars array is operated in quasi-CW mode
with a fixed repetition rate of 1 KHz at central wavelength of
808.5 nm. The maximum pump power is 1150 W for each laser
module.
The experimental setup of MOPA laser system is shown in Fig. 2.
The master oscillator contains two laser modules and adopts plano–
plano symmetrical structure. One laser module is served as the
amplifier stage. The internal time sequence is controlled by a
synchronous controller. Where, L1 is the distance from the mirror
M1 to the left Nd:YAG rod end surface, L2 is the distance between the
Nd:YAG rods, and L3 is the distance from the mirror M2 to the right
Nd:YAG rod end surface. L1 and L3 are both set to be 8 mm, which
form a symmetric resonator. L2 is set to be 6 mm limited by the
mechanical size of laser modules. The distance between the amplifier
stage and the output mirror M2 (L4) is 15 mm. M1 is coated with
99.8% high-reflectance at 1064 nm. M2 is the output coupler which is
coated with 20% transmittance at 1064 nm. M3 is coated at 1064 nm
with 99.8% high-reflectance at the incidence angle of 45°. The output
power was monitored by an Ophir 5 kW power meter.
3. Results and discussion
Firstly we investigated the laser performance of Nd:YAG ceramic
rod comparing with the Nd:YAG crystal for single laser module. The
laser cavity adopts plano–plano short configuration with an output
coupler of 20% transmission at 1064 nm. The average output power on
LD pump power for the case of both ceramic and crystal rod is shown
in Fig. 3. It can be seen that the maximum output power of 425 W for
ceramic rod is obtained at pump power of 1000 W; the corresponding
optical-to-optical conversion efficiency is 42.5%. While for the Nd:YAG
crystal the output power is 426 W with a optical-to-optical conver-
sion efficiency is 42.6%. The comparative results show the laser
performance of ceramic rod is approximately equivalent to that of the
laser crystal. This result demonstrates the potential in obtaining high
output power with ceramic materials.
As is well known, the laser stability property induced by thermal
effects of the laser rods is crucial in designing a laser system [9–11].
The thermal loading leads to a lens behavior in the gain medium in a
diode-side-pumped solid state laser. We can use the Gaussian beam
ABCD propagation matrix formulism to determine the laser stability
property considering the thermal lensing. To determine the thermal
focal length of the single laser module, we constructed a plano–plano
resonator and used the method suggested by D.G. Lancaster [12]. The
measuring process is depicted as following. Firstly make the distance
between the high reflection mirror and the laser module as short as
possible. Then, put the output coupler mirror at a proper location.
Experimentally with the increase of pump power, the corresponding
thermal focal length reduces gradually. The laser operation status
enters to the unstable region which corresponds to an obvious
deduction in output power. After simple calculation, the thermal focal
length (f) can be obtained. When we change the distance of the output
coupler, a series of f will be acquired. Fig. 4 shows the measured
thermal focal length result of single laser module under different
pump power. It can be seen that the thermal focal length decreases
with the increase of pump power. As an example, at the pump level of
1145 W which is the highest pump level in our experiment for single
ceramic laser module, the thermal focal length could be evaluated to
be 166 mm.
Fig. 1. The cross section schematic configuration of the pump module.
Fig. 2. The MOPA system schematic diagram of the Nd:YAG ceramic laser.
Fig. 3. Comparative results of the output power for single laser module using ceramic
and crystal rods, respectively.
5146 C.Y. Li et al. / Optics Communications 283 (2010) 5145–5148

3. Following, we carried on short cavity experiments and measured
their output power characters for laser systems containing two and
three laser modules in compact cavity configuration, respectively. The
comparative results are shown in Fig. 5. For the laser with two
modules the output power increases linearly with the increase of the
pump power. No output power saturation phenomenon was
observed. This means the laser was operated in the stable zone
where the output power will further increase with the increase of the
pump power. In our experiment an output power of 686 W is
obtained for the two modules laser at the incident pump power of
2000 W. As a contrast, it demonstrates a roll-over effect in the output
power curve for the laser with three modules. The maximum output
power reaches to 706 W at the pump power of 2330 W; then falls
quickly with the increase of the pump power. The drop in the output
power is attributed to the thermal effect that causes the laser work
into the unstable zone.
We simulated the laser stability character using the Gaussian beam
ABCD propagation matrix formulism. Fig. 6 shows the calculated
fundamental mode radius at the center of the laser rod as a function of
thermal focal length under different pump power. As shown in Fig. 6,
the laser system operates in stable region for the case of two laser
modules at the highest pump level. While for the system using three
laser modules in cavity, the status is quite opposite. There exists an
obvious break in the curve of fundamental mode radius that ranges
from 220 to 230 mm. The laser system works at the unstable state in
this region. Referring Fig. 4, it can be deduced the pump power is
about 2150–2350 W for the laser system of three modules. In our
experiments of three laser modules (see Fig. 5), the highest output
power was acquired under the pump level of 2330 W. The expe-
rimental results are in good agreement with the laser stability
analysis. Above results indicate that the output power can't be further
increased with the augment of the pump power in the laser system
containing three modules due to the serious thermal effects.
The master oscillator power amplifier (MOPA) system is a
common scheme to obtain high output power for solid state lasers.
In our experiment, an identical ceramic laser module is employed to
amplify the output power. Fig. 7 shows the average output power as a
function of the LD pump power. It indicates that the output power is
increased approximate linearly with the incident pump power. The
threshold power is about 640 W. The maximum output power of
1020 W is obtained at pump power of 3433 W, corresponding to an
optical–optical conversion efficiency of 29.7%. The slope efficiency is
then calculated to be 40.2%. An additional distinct advantage of MOPA
scheme is to acquire good beam quality. As illuminated in Fig. 6, the
stability curve could be split while introducing multi-laser modules
into the cavity. The beam quality will be influenced seriously when
the laser beam passes through the thermal distortion gain medium.
The situation becomes even severe when the laser beam passes to and
fro the laser modules.
The laser pulse repetition rate and pulse width are measured by a
high speed photon-diode detector (Thorlabs Inc., DET200) and a
4 GHz oscilloscope (Tektronix DPO70404). The typical pulse oscillo-
scope waveforms of the ceramic Nd:YAG laser system are shown in
Fig. 8. Fig. 8a is the pulse trains of 1 kHz; Fig. 8b is the extended shape
of a single laser pulse. It shows that the pulse repetition rate and the
Fig. 4. Measured thermal focal length against the pump power for single laser module.
Fig. 5. Comparative results of the output power for different laser cavities using two and
three laser modules, respectively.
Fig. 6. Calculated fundamental laser mode size versus the thermal focal length for two
and three laser modules. Herein, solid line in red stands for the thermal stability of the
three laser modules; dashed line in blue stands for the case of two laser modules.
Fig. 7. Output power versus the total LD pump power for the Nd:YAG ceramic MOPA
system.
5147C.Y. Li et al. / Optics Communications 283 (2010) 5145–5148

4. pulse width are 1 kHz and 114 μs, respectively. The correlative single
pulse energy is then deduced to be about 1 J. This QCW kilowatt Nd:
YAG ceramic laser has advantages both in obtaining high average
output power and high peak power. Thus its potential applications in
laser processing and scientific research purposes could be expected.
4. Conclusions
In summary, we report a kilowatt level QCW diode-side-pumped
Nd:YAG ceramic laser system by using a master oscillation power
amplifier (MOPA) approach. The output power reaches up to 1020 W
at the total pump power of 3433 W, corresponding to an optical–
optical conversion efficiency of 29.7% and slope efficiency of 40.2%. At
the maximum output power, the repetition frequency is measured to
be 1 kHz and the pulse width is 114 μs. On the basis of our knowledge,
this is the first time to realize QCW side-pumped Nd:YAG ceramic
laser with output power of higher than 1 kW.
Acknowledgements
This work is supported by the Major Program of the National
Natural Science Foundation of China with No.50990304 and National
High Technology Research and Development Program (“863” Pro-
gram) of China under contract No.2006AA030103.
References
[1] A. Ikesue, T. Kinoshita, K. Kamata, K. Yoshida, Journal of America Ceramics Society
78 (1995) 1033.
[2] A. Ikesue, Optical Materials 19 (2002) 183.
[3] J.R. Lu, K. Ueda, H. Yagi, T. Yanagitani, Y. Akiyam, A. Kaminskii, Journal of Alloys
and Compounds 341 (2002) 220.
[4] J.R. Lu, H. Yagi, K. Takaichi, T. Uematsu, J.F. Bisson, Y. Feng, A. Shirakawa, K.I. Ueda,
T. Yanagitani, A.A. Kaminskii, Applied Physics B 79 (2004) 25.
[5] D. Kracht, D. Freiburg, R. Wilhelm, M. Frede, Optics Express 14 (2006) 2690.
[6] S. Lee, D. Choia, C.J. Kima, J. Zhou, Optics & Laser Technology 39 (2007) 705.
[7] J.H. Ji, Y.F. Qi, X.L. Qi, Q.H. Lou, ACTA Optica Sinica 26 (2006) 415 (in Chinese).
[8] Y.F. Qi, X.L. Zhu, Q.H. Lou, J.H. Ji, J.X. Dong, Y.R. Wei, Optics Express 13 (2005) 8725.
[9] Y. Bo, A.C. Geng, Y. Bi, Zh.P. Sun, X.D. Yang, Q.J. Peng, H.Q. Li, D.F. Cui, Z.Y. Xu,
Applied Optics 45 (2006) 2499.
[10] E.A. Khazanov, Optics Letters 27 (2002) 716.
[11] C.Y. Li, Y. Bo, F. Yang, Z.C. Wang, Y.T. Xu, Y.B. Wang, H.W. Gao, Q.J. Peng, D.F. Cui, Z.Y.
Xu, Optics Express 18 (2010) 7923.
[12] D.G. Lancaster, J.M. Dawes, Optics & Laser Technology 30 (1998) 103.
Fig. 8. Typical oscilloscope pulse waveforms and single pulse image of the Nd:YAG ceramic
MOPA laser system: (a) 1 kHz pulse trains, and (b) extended shape of a single pulse.
5148 C.Y. Li et al. / Optics Communications 283 (2010) 5145–5148