Picosecond pulse generation near 1-m wavelength has been achieved with a Nd: BaY 2 F 8 (Nd:BaYF) laser mode-locked using a single-walled carbon nanotube saturable absorber (SWCNT-SA). The laser was operated at its main 1049-nm transition, generating 8.5-ps pulses with Ϸ70-mW output power for Ϸ570-mW absorbed pump power. This is the first demonstration of cw mode-locking in the picosecond regime with Nd-doped crystals and SWCNT-SAs. The requirements on the SWCNT-SA for successful mode-locking in relatively narrowband neodymium lasers are reviewed and their implications are discussed

Alessandra Toncelli

Antonio Agnesi

Fabian Rotermund

Federico Pirzio

Giancarlo Reali

Luca Carrà

Mauro Tonelli

Sun Young Choi

Uwe Griebner

Valentin Petrov

CiteSeerX

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Agnesi et al. Vol. 27, No. 12 /December 2010 /J. Opt. Soc. Am. B 2739Diode-pumped Nd:BaY2F8 picosecond laser
mode-locked with carbon nanotube
saturable absorbers
Antonio Agnesi,1,* Luca Carrà,1 Federico Pirzio,1 Giancarlo Reali,1 Alessandra Toncelli,2 Mauro Tonelli,2
Sun Young Choi,3 Fabian Rotermund,3 Uwe Griebner,4 and Valentin Petrov4
1Dipartimento di Elettronica dell’Università di Pavia, Via Ferrata 1, 27100 Pavia, Italy
2CNR NEST (Consiglio Nazionale delle Ricerche-National Enterprise for nanoScience and nanoTechnology) and
Dipartimento di Fisica dell’Università di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
3Division of Energy Systems Research, Ajou University, 443-749 Suwon, South Korea
4Max-Born-Institute for Nonlinear Optics and Ultrafast Spectroscopy, 2A Max-Born-Str., D-12489 Berlin, Germany
*Corresponding author: antonio.agnesi@unipv.it
Received September 28, 2010; revised October 21, 2010; accepted October 21, 2010;
posted October 21, 2010 (Doc. ID 135763); published November 18, 2010
Picosecond pulse generation near 1-m wavelength has been achieved with a Nd:BaY2F8 (Nd:BaYF) laser
mode-locked using a single-walled carbon nanotube saturable absorber (SWCNT-SA). The laser was operated
at its main 1049-nm transition, generating 8.5-ps pulses with 70-mW output power for 570-mW absorbed
pump power. This is the first demonstration of cw mode-locking in the picosecond regime with Nd-doped crys-
tals and SWCNT-SAs. The requirements on the SWCNT-SA for successful mode-locking in relatively narrow-
band neodymium lasers are reviewed and their implications are discussed.
© 2010 Optical Society of America
OCIS codes: 140.4050, 140.7090, 140.3480.c
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. INTRODUCTION
WCNT-SAs have been successfully employed in a variety
f solid-state and fiber lasers for generation of ultrashort
ulses with passive mode-locking in a broad range of
avelengths extending from 1 to 2 m [1,2]. This new
lass of saturable absorber (SA) devices has attracted
reat interest owing to the much easier and cost-effective
abrication process with respect to the well-established
emiconductor saturable absorber mirror (SESAM) tech-
ology. Indeed, preparing a SWCNT-SA for a specific laser
ransition requires only choosing the SWCNT with the
uitable diameter and chirality among those readily
vailable commercially. For mode-locking of solid-state la-
ers, SWCNTs are usually deposited on glass substrates
r dielectric mirrors (SWCNT-SAMs) as SWCNT/polymer
ispersions.
To date, ultrafast solid-state lasers mode-locked with
WCNT-SAs operating in the femtosecond regime have
een demonstrated in wide-bandwidth laser systems such
s Nd:glasses [3]; Yb:KYW [4] and Yb:KLuW [5];
r/Yb:glass [3]; Cr4+: forsterite [6]; and Cr4+:YAG [7].
ome of these reports also demonstrated chirped picosec-
nd pulse generation in resonators without additional dis-
ersion compensation.
Apparently, lasers based on Nd3+-doped crystals have
ot been investigated for cw mode-locking with SWCNT-
As: the only report we are aware of is of a flashlamp-
umped Q-switched mode-locked 1.34-m Nd:GdVO4
aser [8].
Employing the SWCNT-SA devices previously used for
ode-locking Yb:KYW [4] and Yb:KLuW lasers [5] we de-0740-3224/10/122739-4/$15.00 © 2ided to investigate their performance with several active
aterials, specifically cw diode-pumped Nd:glass:
d:BaY2F8 (Nd:BaYF), Nd:YAG, and Nd:YVO4.
Our choice of Nd:glass was motivated by the fact that it
ad already been successfully mode-locked in the femto-
econd regime (we readily achieved results comparable to
r better than those of [3]), while lasers with Nd3+-doped
rystals might be more difficult to mode-lock because of
he larger emission cross section and the narrower fluo-
escence bandwidth. Indeed among these crystals, we
ere able to achieve cw mode-locking only using
d:BaYF, whose spectroscopic features are intermediate
etween those of Nd:glass and of Nd:YAG and Nd:YVO4.
or comparison, Table 1 summarizes the main properties
f neodymium materials investigated in this research. Ac-
ording to the generally accepted condition for mode-
ocking starting [9,10] in large emission-cross-section and
arrow-bandwidth laser materials, both gain saturation
nd the fluctuation pulse duration of the free-running
eld contribute to make mode-locking more difficult.
herefore, SWCNT-SAs with larger modulation depth
nd smaller non-saturable loss should be employed in this
ase.
. EXPERIMENTS
he laser cavity was an astigmatically compensated
-folded resonator shown in Fig. 1. The pump diode was a
001 m2 broad-area emitter with a maximum output
ower of 1 W at 805 nm. It was collimated by an 8-mm
ocal aspheric lens L1, expanded by a factor of 15 in the010 Optical Society of America
s
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2740 J. Opt. Soc. Am. B/Vol. 27, No. 12 /December 2010 Agnesi et al.low-axis direction with a cylindrical lens telescope (C1,
2), and eventually focused in the laser crystal by a
5-mm focal achromat L2. The pump spot size measured
ith a CCD camera was 62 m28 m along the
horizontal) slow- and fast-axis, respectively. The M2 pa-
ameters for both axes were 19 and 1.5, respectively.
The 4-mm long, 1.8%-doped Nd:BaYF crystal is the
ame used in our previous experiments with SESAM
ode-locking [11,12]. The Nd:BaYF crystal was Brewster-
ut with parallel end-faces for propagation along the z
xis with polarization parallel to the y-axis (which is both
n optical and crystallographic axis).
The laser crystal did not require any active cooling for
he present low-power diode-pumping.
The waist radius of the X-shaped cavity was measured
o be 40 m30 m. The distances between cavity mir-
ors were: M1-M2=115 mm, M2-M3=265 mm, and
1-OC=380 mm. An output coupler (OC) with power
ransmissivity 0.8% was chosen, achieving up to 150 mW
ith 26% slope efficiency in cw operation.
For mode-locking experiments, the flat HR-mirror M3
as replaced by a flat SWCNT-SA mirror (SWCNT-SAM)
ith measured saturable loss of 0.21%, non-saturable loss
f 0.7%, Fa5 J/cm2 saturation fluence, and with
iexponential fast 150 fs and slow 1 ps relaxation
imes. Detailed descriptions of the characterization tech-
iques employed for the SWCNT-SAM are given in [4].
Q-switching mode-locking was readily observed, while
w mode-locking was achieved within a range of M1-M2
eparation 115–118 mm. This corresponds to a mode
adius on the SWCNT-SAM of 150–165 m. Indeed,
Table 1. Emission Cross Sections and Gain
Bandwidths of Neodymium-Doped Materials
Discussed Here
Nd:glass
(phosphate
Schott
LG760)
Nd:BaYF
(E//b) Nd:YAG
Nd:YVO4
(E//c)
Emission
cross section
1020 cm2
4.5 8 33 160
luorescence
bandwidth
WHM [nm]
24.3 2.8 0.6 0.8
ig. 1. (Color online) Resonator layout. PD, pump laser diode;
1, aspheric lens; L2, achromat lens; C1, C2, cylindrical lenses
15 telescope); M1, M2, concave mirrors, 100-mm curvature,
igh-reflectivity (HR) at 1050 nm, high-transmissivity at
00–810 nm; M3, either flat mirror, HR at 1050 nm, or
WCNT-SAM (HR mirror with SA film coating); OC, output cou-
ler mirror, 30 wedge.arger spot sizes resulted in Q-switched mode-locking,
hile smaller ones could produce local damage on the
WCNT film on SWCNT-SAM, depending on the position
f the beam incidence.
The mode-locked Nd:BaYF laser operated at the main
ransition near 1049 nm, yielding 8.5-ps pulses at
94 MHz and 70-mW output power. The threshold for cw
ode-locking was very close to the maximum output
ower available with the present setup. Though the laser
as not self-starting, the stable cw mode-locking regime
ould easily be initiated by gently tapping one cavity mir-
or.
Figure 2 shows the autocorrelation trace and Fig. 3 the
F spectrum showing no Q-switching instabilities. The
ulses were measured to be reasonably Fourier-limited
ith a time-bandwidth 0.58.
The same resonator was used to assess the ability of
he SWCNT-SAM to achieve cw mode-locking with the
ther Nd3+-doped gain media. Nd:glass readily allowed cw
ode-locking with femtosecond pulses or even chirped pi-
osecond pulses in the positive dispersion regime (these
esults will be reported elsewhere [13]). Instead, neither
d:YAG nor Nd:YVO4 crystals evidenced the slightest
endency to mode-locking or at least Q-switching mode-
ocking.
–40 –20 0 20 40 60 80 100
Delay [ps]
1.54 × 8.5 ps
S
H
G
In
te
ns
ity
[a
.u
.]
1049.0 1049.5 1050.0
Wavelength [nm]
0.25 nm
ig. 2. (Color online) Autocorrelation of the mode-locking pulses
t 1049 nm and spectrum (inset).
194.2 194.4 194.6 194.8 195.0 195.2
–120
–110
–100
–90
–80
–70
–60
–50
Frequency [MHz]
R
F
po
w
er
[d
B
m
]
52 dBm
ig. 3. (Color online) Radio frequency spectrum (bandwidth
esolution 100 Hz).
3
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Agnesi et al. Vol. 27, No. 12 /December 2010 /J. Opt. Soc. Am. B 2741. DISCUSSION
hese observations suggest that, given the characteristics
f the SWCNT-SAM available, mode-locking was severely
indered in the case of Nd:YAG and Nd:YVO4, while
d:BaYF was barely suitable for cw mode-locking.
This increasing difficulty to evolve free-running spon-
aneous fluctuations into either cw mode-locking or even
-switching mode-locking when choosing gain media with
arger emission cross section and smaller gain bandwidth
an be explained as follows.
Besides considering the self-starting mode-locking re-
uirements, a necessary condition ensuring the existence
f a positive gain window for field fluctuations was dis-
ussed in [9,10]. Random noise fluctuations of the instan-
aneous power Pt have durations pTR /m, TR being
he round-trip time (5 ns in our experiments) and m the
umber of longitudinal modes related to the gain band-
idth of neodymium lasers. For Nd:glass m is 102 [10],
nd even smaller in Nd:YAG and Nd:YVO4, owing to a
20-times reduction of the fluorescence bandwidth.
herefore, in our experiments p is 50 ps and the
WCNT-SAM behaves as a fast absorber during the early
tages of the mode-locking process:
qt 
q0
1 +
Pt
AaIa
, 1
here qt is the loss at time t, q0 the saturable loss, Aa
he mode area, and Ia=Fa /a the saturation intensity, and
here a is the saturable absorber relaxation time. Since
pa we may consider the slower relaxation time compo-
ent a1 ps. The instantaneous loss change due to the
ransit of the low-energy fluctuation in the SA is then
qt  − q0
Pt
AaIa
. 2
Therefore, a positive gain window for the fluctuation
xists, provided that the net gain averaged over Pt is
0; that is,
g − qPt =

−	
+	
gt − qtPtdt

−	
+	
Ptdt
 0, 3
here
gt  − gcw

−	
t
Psds
AgFg
4
s the variation of gain due to saturation during the tran-
it of the perturbation Pt, gcw is the saturated gain of
he laser, and Ag and Fg are the mode area and saturation
uence of the gain medium, respectively. The condition
or successful mode-locking initiation is thereforeq0 
FaAa
FgAg
gcw
a 	
1
2
−	
+	
dtPt2

−	
+	
dtP2t   FaAaFgAg gcwpa , 5
here p can be assumed as the FWHM of the fluctuation
t.
Considering the parameters of Nd:glass in Table 1 and
ur experimental setup, Eq. (5) yields q0210−5, which
s easily fulfilled, while for Nd:YAG a much higher modu-
ation depth is required, q00.1%, comparable to that of
WCNT-SAMs usually employed for low-gain solid-state
emtosecond lasers and used in our experiments. The
ituation is slightly better for Nd:BaYF and still worse for
d:YVO4 as suggested by cross section values and gain
andwidths summarized in Table 1.
The condition on the critical output power Pc required
or stable cw mode-locking is readily fulfilled for the Nd-
BaYF laser owing to the small value of Fa for the
WCNT-SAM [14],
Pc =
Toc
TR
FaAaFgAgq0, 6
ielding Pc=41 mW, considering the parameters of our
xperimental setup.
At first sight, tighter focussing in the SA might improve
he mode-locking tendency. However, the drawback is the
amage threshold of the SWCNT polymer film, which can
afely handle average intracavity power only up to
10 kW/cm2 [3,4].
One possibility to enable successful mode-locking with
his kind of SWCNT-SAs and Nd3+-doped crystals consists
f reduction of non-saturable losses and increase of modu-
ation depth q0, maintaining the same gcw. The situation
s somehow relaxed for SESAMs, since the slow compo-
ent a is often much longer 10 ps, and the average
ower limit is much higher (at least an order of magni-
ude better), according to our direct experience.
It is worth mentioning that the relatively low damage
hreshold of the particular sample used in our experi-
ents seems related to strong bundling [2] of nanotubes,
s well as to the polymer concentration. Preliminary satu-
able absorption investigations on the latest series of
WCNT films with lower polymer concentration show a
amage threshold significantly improved. For example,
hough working at a longer wavelength of 1.2 m, Cr:for-
terite lasers withstanding average intensity of
80 kW/cm2 have been demonstrated [7].
Higher modulation depth of the SA would also yield
horter pulses [15], with performance closer to the 2.6-ps
ulsewidth recently achieved with SESAMs in the Nd-
BaYF laser [12].
. CONCLUSIONS
e have demonstrated, for the first time to our knowl-
dge, cw mode-locking in a diode-pumped picosecond laser
ased on a Nd3+-doped crystal and a SWCNT-SAM. The
d:BaYF laser generated 8.5-ps pulses and 70 mW at
ow pump power 1 W. With the same setup we also in-
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2742 J. Opt. Soc. Am. B/Vol. 27, No. 12 /December 2010 Agnesi et al.estigated Nd:YAG and Nd:YVO4 crystals, which did not
xhibit any mode-locking tendency. We suggest that this
s a consequence of the strong gain saturation occurring
n the early stages of pulse evolution, preventing the cre-
tion of a positive gain window given the small modula-
ion depth of the SWCNT-SAM employed. Our study fur-
her suggests that a higher ratio between saturable and
on-saturable losses is required to successfully mode-lock
arrow bandwidth, high-gain neodymium lasers.
CKNOWLEDGMENTS
. Agnesi acknowledges financial support from Cariplo
oundation under grant agreement No. 2009-2309. S. Y.
hoi and F. Rotermund acknowledge financial support
rom the National Research Foundation of Korea (NRF)
grant Nos. 2010-0000065 and 2010-0018855) funded by
he Ministry of Education, Science & Technology (MEST)
f South Korea.
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Diode-pumped Nd: BaY 2 F 8 picosecond laser mode-locked with carbon nanotube saturable absorbers

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Diode-pumped Nd: BaY 2 F 8 picosecond laser mode-locked with carbon nanotube saturable absorbers

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