Optics Communications 315 (2014) 204–207
Contents lists available at ScienceDirect
Optics Communications
journal homepage: www.elsevier.com/locate/optcom
High-power diode-side-pumped Nd:YAG solid laser mode-locked
by CVD graphene
Lifei Li a, Xinliang Zheng b,n, Xiaoming Chen a, Mei Qi a, Zhaoyu Ren a, Jintao Bai a,b,
Zhipei Sun c
a
National Key Laboratory of Photoelectric Technology and Functional Materials (Culture Base), and Institute of Photonics and Photon-Technology, Northwest
University, Xi'an 710069, China
b
Department of Physics, Northwest University, Xi'an 710069, China
c
Department of Micro- and Nanosciences, Aalto University, PO Box 13500, FI-00076 Aalto, Finland
art ic l e i nf o
a b s t r a c t
Article history:
Received 9 October 2013
Accepted 31 October 2013
Available online 13 November 2013
A simple and effective graphene based saturable absorber is successfully fabricated for high power modelocking. Utilizing it, graphene-mode-locking operation is achieved in a diode-side-pumped solid laser for
the first time. A maximum average output power up to 2.1 W is produced at 1064 nm center
wavelength with a pulse duration of 13 ps. To the authors’ knowledge, this is the highest operation
power for a graphene-mode-locked laser. The corresponding pulse energy and peak power are 18.4 nJ
and 1.33 kW respectively.
& 2013 Elsevier B.V. All rights reserved.
Keywords:
Graphene
Mode-locking
Diode-side-pumped
Solid laser
1. Introduction
Ultrafast solid-state laser sources have widespread applications in
science, military, and industry fields. Normally, there are several
mode-locking techniques (e.g., active mode-locking, passive modelocking, additive-pulse mode-locking, Kerr-lens mode-locking) to
produce ultrashort pulses in solid lasers. Among them, passive
mode-locking by saturable absorbers (SAs) is particularly important
[1]. It makes the loss of the pulse center smaller than the pulse wings
through inserting a fast SA into the laser resonance cavity, thereby
shortens the pulse in successive round trips. Traditionally, semiconductor saturable absorber mirrors (SESAMs), which were invented in
1992, as SAs, have been mostly used for passively mode-locked solid
lasers. However, SESAMs typically fabricated by complex and costly
molecular beam epitaxy on distributed Bragg reflectors [2]. Meanwhile, they have some drawbacks like narrow operation bandwidth,
long response time, and their modulation depth are difficult to
modify. Recently, graphene [3] based saturable absorbers (GSAs), a
novel form of SAs [4], are drawing more and more attention due to
their outstanding properties, such as fast saturable absorbability [5],
wideband tunability [6], and simple fabrication with low cost [7].
But to date, the output power in previously published solid lasers,
which are mode-locked by GSAs [5], [8–12], is only at milliwatt level
n
Correspondence to: #229 North Taibai Road, Department of Physics, Northwest
University, Xi’an, China 710069. Tel./fax: þ86 29 8830 3336.
E-mail address: [email protected] (X. Zheng).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.optcom.2013.10.093
(e.g., the highest is 360 mW [9]), insufficient for some high-intensity
applications. Although Ref. [13] has reported the graphene-oxide
based mode-locked laser with the 3 W output power. However, the
output optical path isn't collinear in this result (i.e., the total power of
two output beams with a certain angle is 3 W). Also, the graphene
oxide is insulating with a mixture of sp2/sp3 regions, and with many
defects and gap states, which is fundamentally different from
graphene. Therefore, graphene oxide may not offer the same wide
operation bandwidth as GSAs. Moreover, those aforementioned
lasers are all founded on diode-end-pumping. As well known,
diode-end-pumping systems are inconvenient to integrate and there
are more serious thermal problems at high power operation compared with the diode-side-pumping counterparts.
Here, we report a 2.1-W high power passively mode-locked
diode-side-pumped solid laser based on a GSA. This laser centered
at 1064 nm with a pulse duration of 13 ps and a pulse repetition
rate of 114 MHz. To the best of our knowledge, it is the first time to
employ a diode-side-pumped gain module for graphene-modelocked lasers, which results in six times higher average output
power than ever reported in Ref. [9].
2. Fabrication and characterization of the GSA
We chose graphene that grew by the atmospheric pressure
chemical vapor deposition (CVD) method to fabricate the GSA
[14,15]. This is because the CVD graphene has lower defects with
larger uniform area in comparison with other growth methods like
L. Li et al. / Optics Communications 315 (2014) 204–207
liquid-phase exfoliation or oxidation–reduction, which is more
suitable for solid laser mode-locking [16]. Details are presented as
follows. A 25 μm thick Cu foil was heated to 950 1C in a quartz tube
with 200 sccm H2 and 200 sccm Ar flowed for 1 h. This not only
reduces the oxidized Cu surface, but also extends the size of the
graphene grain. The precursor gas, a Ar:H2:C2H2 mixture with flow
ratio 900:100:1, was injected for 10 min. The carbon atoms were
then adsorbed onto the Cu surface and formed a large graphene film
(with 2 4 cm2 size). After growth, the Cu substrate was etched out
with 0.05 g ml 1 Fe(NO3)3 solution and the graphene film was rinsed
repeatedly in deionized water to remove the residues. Then, it was
transferred directly onto a piece of K9 glass substrate (with 2 mm
thickness) of which both sides have been coated with SiO2/Ti2O5
antireflection films with less than 0.1% reflectivity at 1064 nm. After
transfer, the sample was dried in a vacuum drying oven at 40 1C and
we coated an additional SiO2 protective layer (with 100 nm
thickness) on the surface of the graphene film with a sputter. Finally,
a simple and effective GSA was fabricated and the corresponding
schematic structure is displayed in Fig. 1(a). The Raman spectroscopy
was carried out with a 514-nm laser to investigate the quality of the
as-fabricated GSA (before and after coating SiO2 protective layer), as
depicted in Fig. 1(b). The identical spectra which contained three
peaks: D, G, and 2D at 1350, 1591, and 2699 cm 1 respectively,
validates that the transfer and coating processes do not damage the
graphene film. The D-peak is from the structural imperfections
activated by double resonance [17], indicating that there are few
defects in the GSA. The intensity ratio of I2D/IG is about 0.49,
indicating that the graphene film has a multilayered structure. To
further confirm the number of layers, the transmittance of the K9
glass (after coating), graphene film, and GSA were then measured by
a spectrophotometer, as shown in Fig. 1(c). The transmittance of the
graphene film in the visible range (e.g., at 750 nm) is 90%,
indicating that the number of layers is four (i.e., 2.3% absorbance
per layer [18]). Fig. 1(d) describes the nonlinear absorption of the GSA
depending on the input fluence, which was measured with our
home-made 1-μm ultrafast solid-state laser source (i.e., 1 W average
205
output power and 20 ps pulsewidth). The modulation depth of the
GSA is about 6% and the nonsaturable loss is 4%.
3. Mode-locking operation
Fig. 2 illustrates the laser setup. The cavity has a z-type configuration. The gain module consisted of a 2 mm diameter, 50 mm
length Nd:YAG rod (0.6 at% Nd-doped) side-pumped by five laser
diode arrays and each diode array operated in continuous-wave (CW)
mode with a maximum output power of 30 W at 808 nm. The Nd:
YAG rod mounted in a quartz tube was cooled by circulating water
and both end facets have antireflection coating at 1064 nm. The
water temperature was maintained at 20 1C during the experiment.
The rear mirror M1 was a plane mirror with reflectivity of R499.9 at%
1064 nm and clung to the gain module. M2 and M3 were highly
reflective concave mirrors (R4 99.9 at% 1064 nm) with radii of
curvature of 500 and 100 mm, respectively, which composed a
reflecting relay-image telescope system. The flat OC has a transmittance of 12 at% 1064 nm with 11 wedged angle. The GSA was
located close to the output coupler (OC). The beam radius on the
GSA was estimated to be about 45 μm by using the ABCD-matrix
Fig. 2. Experimental setup of the diode-side-pumped graphene-mode-locked
solid laser.
Fig. 1. (a) Schematic structure of the GSA. (b) The corresponding Raman spectra. (c) Transmittance of the K9 glass, graphene film, and GSA. (d) Characterization of the
saturable absorption.
206
L. Li et al. / Optics Communications 315 (2014) 204–207
method. The total cavity length was approximately 1.32 m (i.e.,
L1 ¼ 57 cm, L2 ¼64 cm, L3 ¼60 mm). In order to limit high-order
transverse modes, an aperture with a diameter of 1.2 mm was
adopted in the cavity. The light inside the cavity was adjusted to
depart from the center of the concave mirrors to weaken the
etalon effect. The output power was monitored by a power meter
(FieldMaxII, Coherent). An autocorrelator (FR-103XL, Femtochrome) was used to measure the pulse duration.
With a fine adjustment to the angle of the GSA, Q-switched
mode-locking (QML) was observed as soon as the pump power
reached the threshold condition of 34.4 W. As the pump power
Fig. 3. Average output power versus pump power. The inset shows the modelocked pulse trains in 20 ns and 10 μs per division (div) time scales.
increased to 38.1 W, CW mode-locking (CWML) regime appeared
instead of the QML operation. The maximum achievable output
power was measured to be 2.1 W at a pump power of 43 W and
the fluctuation was less than 1%. Fig. 3 plots the variation of the
average output power with the incident pump power. The inset
shows temporal behavior of the output pulse trains at the highest
output power, which was recorded by a 1-GHz bandwidth digital
oscilloscope (DSO7052A, Agilent) with a fast photodiode (FDS010,
Thorlabs). The pulse repetition rate was 114 MHz, well consistent
with the cavity round-trip time. The shortest autocorrelation trace
measured had a full width at half maximum (FWHM) of about
18 ps, as shown in Fig. 4(a). Thus the pulse duration was 13 ps,
assuming a Gaussian profile [Fig.4(a)]. As a consequence, the pulse
energy was calculated to be 18.4 nJ, giving the peak power up to
1.33 kW. Alternatively, the pulse duration is 11.7 ps while the
pulse is a sech2 profile. Fig. 4(b) exhibits the corresponding optical
spectrum detected by a 0.02-nm resolution optical spectrum
analyzer (HR4000, Oceanoptics). The peak wavelength was
1064.3 nm and the spectral FWHM was around 0.17 nm, which
yielded a time-bandwidth product of 0.58 larger than the value of
0.44 for transform-limited Gaussian pulse. Considering the mirrors
with high reflectivity of R4 99.9% in our cavity have very little
linear cavity losses, we think our output pulses are frequency
chirped mainly due to the un-compensated intracavity dispersion
(e.g., Nd:YAG rod). Further increasing the pump power gets the
mode-locking unstable. It was found to be limited by the immoderate saturation depth and damage of the graphene film. This
suggests that the damage threshold of our GSA is 348.6 MW/cm2.
The absorption of ultrafast optical pulses can produce both thermal
and non-thermal effects to break the sp2 hybridized carbon-carbon
bonds within the graphene film [19]. We believe that the output
power could be further improved by optimizing the cavity parameters and GSA, such as changing the graphene layers or doping
our graphene device.
4. Conclusion
In summary, we have experimentally demonstrated the first
diode-side-pumped graphene-mode-locked solid laser. A simple
and effective method of fabricating GSA was also reported, enabling
13-ps ultrafast pulses with a repetition rate of 114 MHz. A maximum average output power of 2.1 W was achieved, which is, to our
knowledge, the highest output power for a graphene-mode-locked
laser. Our results reveal the great potential of graphene applied in
high power all-solid-state ultrafast laser sources.
Acknowledgements
This work was supported by the National “973 Plan” of China
(2012CB723407), NSFC (61275105, 61177059), NSF of Shaanxi
Province (2012JM1019), and Education Bureau fund of Shaanxi
Provincial government (09JS077, 11JS106).
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Fig. 4. (a) Intensity autocorrelation trace of the output pulses with a Gaussian fit.
(b) Optical spectrum.
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