This document describes a passively Q-switched Nd:YAG ceramic laser using a single wall carbon nanotube saturable absorber. The laser generated pulses with a maximum duration of 1.2 ms, repetition rate varying from 14 to 95 kHz, and maximum pulse energy of 4.5 mJ at a repetition rate of 31.8 kHz. The laser achieved a maximum output power of 376 mW and optical-to-optical conversion efficiency of 4.3% at a pump power of 8.68 W. Characterization of the Nd:YAG ceramic gain medium showed scattering and absorption losses similar to a crystal, with the ceramic laser demonstrating output power only 6.3% lower than an equivalent

1. LD-end-pumped passively Q-switched Nd:YAG ceramic laser with single wall
carbon nanotube saturable absorber
C.Y. Li a,b
, Y. Bo b,n
, N. Zong b
, Y.G. Wang c
, B.X. Jiang d
, Y.B. Pan d
, G. Niu a
, Z.W. Fan a
,
Q.J. Peng b
, D.F. Cui b
, Z.Y. Xu b
Q1
a
Beijing GK Laser Technology Co., Ltd., Chinese Academy of Sciences, Beijing 100192, China
b
RCLPT, Key Lab of Functional Crystal and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
c
Department of Applied Physics and Materials Research Center, Hong Kong Polytechnic University, Hong Kong, China
d
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China
a r t i c l e i n f o
Article history:
Received 16 February 2012
Accepted 7 March 2012
Keywords:
Passively Q-switched
SWCNT-SA
Nd:YAG ceramic
a b s t r a c t
We report on a LD-end-pumped passively Q-switched Nd:YAG ceramic laser by using a novel single
wall carbon nanotube saturable absorber (SWCNT-SA). The SWCNT wafer was fabricated by electric Arc
discharge method on quartz substrate with absorption wavelength of 1064 nm. We firstly investigated
the continuous wave (CW) laser performance and scattering properties of Nd:YAG ceramic sample. For
the case of passively Q-switched operation, a maximum output power of 376 mW was obtained at an
incident pump power of 8.68 W at 808 nm, corresponding to an optical–optical conversion efficiency of
4.3%. The repetition rate as the increase of pump power varied from 14 to 95 kHz. The minimum pulse
duration of 1.2 ms and maximum pulse energy of 4.5 mJ was generated at a repetition rate of 31.8 kHz.
& 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Passive Q-switching of diode-pumped solid-state lasers have wide
applications in fields of remote sensing, scientific research, medicine
due to their advantages in terms of miniature, device simplicity,
compactness, high efficiency and low cost, etc. [1]. In general, the
generation of passively Q-switched laser pulses depends strongly on
the availability of suitable saturable absorbers. Historically, several
intra-cavity solid-state passive elements have been used as saturable
absorbers, such as bleachable dyes [2], color centers [3], Cr4þ
:YAG
crystal [4], and semiconductor saturable absorbers (SESAM) [5].
Among them, Cr4þ
:YAG crystal is still primitive passive Q-switching
element originated from its excellent optical and mechanical proper-
ties, high thermal and radiation stability. However, its absorption
band is located around 1 mm. Hence Cr4þ
:YAG is only convenient for
passively Q-switched Nd:YAG lasers at 1 mm.
Recently, carbon nanotube based saturable absorbers (CNTs-
SA) have attracted considerable attention due to their outstanding
properties, such as sub-picosecond recovery time, low saturation
power, broad operation range, polarization insensitivity, easy
fabrication, and mechanical and environmental robustness, etc.
The significant progress of CNTs are focused on their mode
locking applications in all solid state lasers [6–9]. Besides the
well known application as mode locking absorber devices, the
CNTs have been proved to be an effective Q switcher after proper
adjustment of preparation parameters. The CNTs using as satur-
able absorbers can be attributed to their electronic properties
which are determined by the CNTs’ diameter and chirality [10].
CNTs have a band gap varying inversely with their diameter. The
electronic transitions between the valence bands and conduction
bands of CNTs result in a wide spectral-range optical absorption.
Absorption at a given wavelength creates electron–hole pairs.
This causes band filling and the absorption saturates. A further
power increase results in a reduced overall absorption or a
bleaching of the sample, the intra-cavity loss modulation are thus
implemented. More recently, Cheng et al. reported a diode-
pumped passively Q-switched Nd:LuYGdVO4 laser with a
SWCNT-SA, they obtained a minimum pulse width of 52 ns and
maximum peak power 66.5 W [11]. Zhou et al., reported a
Q-switched erbium doped fiber laser where a 7 ms width and
14.1 nJ pulse energy was obtained [12]. Qin et al., reported a
Q-switched Nd:YVO4 laser with a CNT saturable absorber, the
maximum average output power was 477 mW with pulse width
of 323 ns and pulse energy of 326 nJ [13].
In addition, all solid-state lasers using polycrystalline ceramic
gain materials in particular the Nd:YAG ceramic are extensively
studied for generating high output power and high energy
[14–18]. It is regarded as a promising candidate of crystal owing
to its low cost, composition control, ease of fabrication, large size,
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Contents lists available at SciVerse ScienceDirect
journal homepage: www.elsevier.com/locate/optlastec
Optics & Laser Technology
0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.optlastec.2012.03.010
n
Corresponding author.
E-mail addresses: zhaoyang2050@163.com (C.Y. Li),
boyong@tsinghua.org.cn (Y. Bo).
Please cite this article as: Li CY, et al. LD-end-pumped passively Q-switched Nd:YAG ceramic laser with single wall carbon nanotube
saturable absorber. Optics and Laser Technology (2012), http://dx.doi.org/10.1016/j.optlastec.2012.03.010
Optics & Laser Technology ] (]]]]) ]]]–]]]

2. multi-functional, and mass production, etc. One can expect that
short pulse ceramic lasers may find potential applications besides
their traditional high power domains. However although pas-
sively Q-switched Nd:YAG lasers have been widely addressed,
there is few report on passively Q-switched ceramic laser. Here
we present a passively Q-switched Nd:YAG ceramic laser by use
of a single wall carbon nanotube absorber. A stable laser pulse of
duration of 1.2 ms and pulse energy of 4.5 mJ were generated at a
repetition rate of 31.8 kHz. The maximum output power was
376 mW.
2. Experimental
The SWCNT used in this experiment was grown by electric arc
discharge technique and was identical to our previous work [19].
The mean diameter of the SWCNTs is about 1.5 nm. First, 0.15 mg
SWCNTs powder was poured into 10 ml 0.1% sodium dodecyl
sulfate (SDS) aqueous solution to disperse SWCNTs in aqueous
solution. The SWCNT solution was then ultrasonically agitated for
12 h. Subsequently, the dispersed solution of SWCNTs was cen-
trifuged to remove sedimentation of larger SWCNTs bundles.
After decanting the upper portion of the centrifuged solution,
the SWCNTs dispersion was poured into a polystyrene cell.
Finally, we inserted vertically a hydrophilic quartz substrate into
the polystyrene cell and kept it steady for gradual evaporation at
atmosphere. It took about two weeks for complete evaporation on
the substrate. Now the substrate coated with SWCNTs is ready
for using as a saturable absorber. Fig. 1 shows the transmission
spectrum of the prepared SWCNT-SA. The transmission rate is
about 84% at the wavelength of 1060 nm.
We adopted a four mirror Z fold laser resonator as shown in
Fig. 2. The laser gain medium is a 1 at% Nd doped YAG ceramic
(prepared by the Institute of Ceramics, Chinese Academy of
Sciences) and has dimensions of +3 Â 40 mm. Both surfaces of
the Nd:YAG ceramic are AR (anti reflection) coated at 808 nm and
1064 nm. It was wrapped with indium foil and mounted in a
water cooled copper heat sink at 20 1C. The pump source was a
fiber coupled laser diode with water cooled to center wavelength
of 808 nm. The fiber of the LD had a core diameter of 400 mm and
numerical aperture of 0.22. The fiber’s output face was imaged to
2 times the diameter spot in the ceramic. M1 is a diachronic
plano–plano mirror coated at HR (high refection) at 1064 nm and
AR at 808 nm. The folding mirror M3 with a curvature radius of
150 mm was HR coated for 1064 nm. The rear mirror M4 was a
flat mirror also HR coated at 1064 nm. M2 is the output mirror
with 5% transmittance at 1064 nm. The cavity was carefully
designed with the ABCD matrix formalism to maintain a relatively
large laser mode in the gain medium simultaneously keeping the
Q switch stability through the available pump power range. The
arms L1, L2 and L3 were chosen to be 160, 170 and 70 mm,
respectively. The whole optical cavity length was approximately
440 mm. The laser beam radii were calculated to be about 350 mm
at the center of the laser medium and 90 mm on the SWCNT,
respectively. The folded angle of M3 was very small in order to
reduce astigmatism. The SWCNT sample was placed close to M4.
3. Results and discussion
3.1. Laser properties of the Nd:YAG ceramic sample
It is well known the attenuation loss coefficient of a laser
material has enormous influence on the laser performance. Many
research have shown the scattering is the critical loss mechanism in
transparent laser ceramic materials. We firstly measured the scat-
tering coefficient and absorption coefficient of the ceramic sample
with an integrating sphere technology [20]. For comparison, a
Nd:YAG crystal was also used. The measured scattering and absorp-
tion coefficient at 1064 nm for ceramic and crystal were (0.005,
0.002 cmÀ1
) and (0.002, 0.001 cmÀ1
), respectively. The whole
attenuation loss coefficient at 1064 nm was the sum of the scatter-
ing and absorption coefficient that were 0.007 cmÀ1
for ceramic,
and 0.003 cmÀ1
for crystal, which were very close in value.
Following, we investigated the CW laser performance for the
ceramic sample as compared with the Nd:YAG crystal. The laser
system adopted LD side-pumped by 808.5 nm and was configured
in a plano-plano short cavity. The transmittance ratio of output
coupler at 1064 nm was 20%. The output power on LD pump
power for the case of both ceramic and crystal was shown
in Fig. 3. The maximum output power of 25 W for ceramic
rod was obtained at a pump power of 69 W, corresponding to
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Fig. 1. Transmission spectrum of the SWCNT-SA.
L1
M2
M1
Nd:YAG
ceramic
M3
L3
SWCNT
M4
L2
LD pump
Fig. 2. Schematic diagram of the SWCNT passively Q-switched ceramic Nd:YAG laser.
C.Y. Li et al. / Optics & Laser Technology ] (]]]]) ]]]–]]]2
Please cite this article as: Li CY, et al. LD-end-pumped passively Q-switched Nd:YAG ceramic laser with single wall carbon nanotube
saturable absorber. Optics and Laser Technology (2012), http://dx.doi.org/10.1016/j.optlastec.2012.03.010

3. optical-to-optical conversion efficiency of 36.2%. For the case of
Nd:YAG crystal, the maximum output power was 26.7 W. It can
be seen that the output power for ceramic laser has only 6.3%
lower than that of the crystal. This result shows though the
measured scattering coefficient and absorption coefficient at
1064 nm have slight difference for Nd:YAG ceramic and crystal,
they have little influence on the laser performance.
Another important factor influencing the laser performance is
the absorption character of laser medium to pump power, in
particular for an end-pumped laser system. We invoke the
previously measured absorption result of used ceramic sample
at 808 nm pumping [21]. The peak absorption coefficient was
9.1 cmÀ1
. According to the absorption law and taking into the
absorption coefficient, the calculated length for full absorption of
pump power was 6 mm. In this region more than 99% pump
power was absorbed.
3.2. Passively Q-switched operation of the Nd:YAG ceramic laser
With an appropriate alignment of the laser cavity, stable
passive Q-switching operation was achieved above the laser
threshold. We used a high speed photon-diode detector (Thorlabs
Inc., DET200) and a 4 GHz oscilloscope (Tektronix DPO70404) to
monitor the 1.064 mm pulse temporal behaviors in the whole
incident pump power range. The typical pulse oscilloscope wave-
form of the Q-switched laser system was shown in Fig. 4. Herein,
Fig. 4(a) shows the recorded pulse trains with pulse repetition
frequency of 31.8 kHz; Fig. 4(b) shows the extended shape of a
single laser pulse where the pulse width is 1.2 ms.
The average output power of the laser was measured by an
Ophir F3A power meter, as shown in Fig. 5. One can see that the
output power was increased approximate linearly with the
incident pump power. The threshold power was about 3.2 W.
The maximum output power of 376 mW was obtained at the
pump power of 8.68 W, corresponding to an optical–optical
conversion efficiency of 4.3%, slope efficiency of 7.6%. We mea-
sured the power stability at the highest output power versus time
in every 5 min. The laser power instability was less than 73%.
Here one point we should mention was the ceramic Nd:YAG
sample is 40 mm in length and has 1 at% dopant. As depicted
above, the absorption length was only 6 mm. The un-absorbed
region could increase the cavity loss when the laser operates
above threshold. For the LD end-pumped laser, too long crystal
caused very serious intra-cavity loss due to the re-absorption and
LD coupling system design. This induced the difficulties of cavity
configuration and higher output power. Thus, the long ceramic
sample was adverse to the augment of Q-switched output power.
One alternative way was to adopt a 885 nm LD pump source since
it has lower peak absorption coefficient and high quantum
efficiency than 808 nm pumping. Further, the coupling imaging
system exerts 800 mm spot to match the laser mode. A large spot
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Fig. 3. Comparative results of the output power for ceramic and crystal rod laser.
Fig. 4. Typical oscilloscope trace and single pulse image: (a) 31.8 kHz pulse trains,
and (b) extended shape of a single pulse.
Fig. 5. Average output power as a function of absorbed pump power for the
Q-switched laser.
C.Y. Li et al. / Optics & Laser Technology ] (]]]]) ]]]–]]] 3
Please cite this article as: Li CY, et al. LD-end-pumped passively Q-switched Nd:YAG ceramic laser with single wall carbon nanotube
saturable absorber. Optics and Laser Technology (2012), http://dx.doi.org/10.1016/j.optlastec.2012.03.010

4. size reduces the pump power density that is either not beneficial
for increasing output power.
We measured the repetition frequency with the incident pump
power. In Q-switched laser, the repetition rate depends strongly
on the pump power. As the pump power increases, more gain is
provided to saturate the SA. Since pulse generation relies on
saturation, the repetition rate increases consequently. According
to the repetition rate and the average output power, the pulse
energy of the single Q-switched envelope can be calculated. Fig. 6
shows the pulse repetition rate and the pulse energy as the
function of the incident pump power, the repetition rate varies
from 14 kHz to 95 kHz. As it can be seen from Fig. 6, the
maximum pulse energy appears at repetition of 31.8 kHz where
the correlative single pulse energy was 4.5 mJ. The decrease of
pulse energy with the augment of pump power may be caused by
the accumulated thermal effects at higher pump level. Here one
thing should be mentioned that, we observed the damage on the
CNT when the pump power was above 9 W. The Q-switching
operation was broken due to the failure of the CNT. We attributed
this to the oxidation of the carbon nanotubes at a high power
density situation. This indicates the damage threshold of the
CNTSA should be further improved. A more robust passively
Q-switched laser can be expected after suitable coating on the
CNTSA to isolate the oxygen.
We tested the beam quality of this SWCNT passively
Q-switched laser in current cavity configuration at the maximum
output power of 376 mW. The M2
value was measured by a
Spiricon M2-200 laser beam analyzer using the second moment
method. The measured M2
factor was 1.7 that is a near diffraction
limited beam output. Fig. 7 shows the typical 2-D beam profile,
where a near Gaussian-like spatial distribution of laser intensity
could be deduced from the beam spatial intensity distribution.
4. Conclusions
In summary, we presented a LD-end-pumped passively
Q-switched Nd:YAG ceramic laser with a single wall carbon
nanotube (SWCNT) saturable absorber. At an incident pump
power of 8.68 W at 808 nm, stable Q-switched pulse trains with
duration of 1.2 ms, single pulse energy of 4.5 mJ were obtained.
The approaches for obtaining higher Q-switching output power
were proposed via investigating the absorption and scattering
properties of ceramic samples.
Acknowledgments
This work is financially supported by the Major Program of the
National Natural Science Foundation of China with no. 50990304
and Natural Science Foundation of Shanghai, China with no.
09ZR1435600.
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Please cite this article as: Li CY, et al. LD-end-pumped passively Q-switched Nd:YAG ceramic laser with single wall carbon nanotube
saturable absorber. Optics and Laser Technology (2012), http://dx.doi.org/10.1016/j.optlastec.2012.03.010