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Journal ArticleDOI

3 W, 300 μJ, 25 ns pulsed 473 nm blue laser based on actively Q-switched Nd:YAG single-crystal fiber oscillator at 946 nm

15 Aug 2013-Optics Letters (Optical Society of America)-Vol. 38, Iss: 16, pp 3013-3016

TL;DR: The realization of a frequency doubled, actively Q-switched and polarized oscillator based on Nd:YAG single-crystal fiber, more than two times the previously reported average power and energy at 473 nm is reported.

AbstractWe report the realization of a frequency doubled, actively Q-switched and polarized oscillator based on Nd:YAG single-crystal fiber. A laser output of 8 W, 10 kHz, and 30 ns at 946 nm is reported. The laser is extracavity frequency doubled in a BiBO crystal to obtain 3 W and 300 μJ of blue laser with a beam quality of M2y=1.12 and M2x=1.38. The obtained blue power is stable with a root-mean-square stability of less than 2% in 1 h. This is more than two times the previously reported average power and energy at 473 nm.

Topics: Blue laser (60%), Laser power scaling (59%), Fiber laser (59%), Diode-pumped solid-state laser (58%), Laser pumping (58%)

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Submitted on 22 Jan 2014
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3W , 300µJ , 25ns pulsed 473nm blue laser based on
actively Q-switched Nd : YAG single-crystal ber
oscillator at 946 nm
Loïc Deyra, Igor Martial, Julien Didierjean, François Balembois, Patrick
Georges
To cite this version:
Loïc Deyra, Igor Martial, Julien Didierjean, François Balembois, Patrick Georges. 3W , 300µJ ,
25ns pulsed 473nm blue laser based on actively Q-switched Nd : YAG single-crystal ber oscillator at
946 nm. Optics Letters, Optical Society of America - OSA Publishing, 2013, 38 (16), pp.3013-3016.
�10.1364/OL.38.003013�. �hal-00934468�

3 W, 300 µJ, 25ns pulsed 473nm blue laser based
on actively Q-switched Nd:YAG single-crystal
fiber oscillator at 946 nm
Lc Deyra
1*
, Igor Martial
2
, Julien Didierjean
2
, François Balembois
1
, Patrick Georges
1
1. Laboratoire Charles Fabry, Institut d’Optique, CNRS, Univ Paris-Sud, 91127 Palaiseau, France
2. Fibercryst SAS, La Doua Bâtiment l’Atrium, Boulevard Latarjet, F 69616 Villeurbanne Cedex, France
Received Month X, XXXX; revised Month X, XXXX; accepted Month X,
XXXX; posted Month X, XXXX (Doc. ID XXXXX); published Month X, XXXX
Corresponding author email : loic.deyra@institutoptique.fr
We report the realization of a frequency doubled, actively Q-switched and polarized oscillator based on Nd:YAG single crystal
fiber. A laser output of 8 W, 10 kHz, 30 ns at 946 nm is reported. The laser is extracavity frequency doubled in a BiBO crystal
to obtain 3 W, 300 µJ of blue laser with a beam quality of M²y=1.12 and M²x=1.38. The obtained blue power is stable with a
RMS stability less than to 2% in one hour. This is more than two times the previously reported average power and energy at
473 nm.
The realization of high average power, millijoule Q-
switched 946 nm laser based on Nd:YAG has several
applications in spectroscopy, material processing and
LIDAR applications. The second harmonic at 473 nm can
be used for underwater communication [1], optical storage
or spectroscopy. The fourth harmonic of 946 nm at 236 nm
allows the LIDAR detection of dangerous compounds [2].
So far the reported powers and energies at 946 and 473 nm
were limited. At 946 nm, energies superior to the millijoule
have only been obtained at repetition rates of tens of hertz
[3][4]. At tens of kilohertz, energies up to 430 µJ have been
demonstrated [5] [6]. At 473 nm, up to 1.5 W of average
power and 150 µJ have been reported [6].
Indeed the main limitations of those lasers comes from the
spectroscopic properties of the Nd:YAG 946 nm quasi-three
level laser line. 946 nm laser operation in Nd:YAG suffers
from reabsorption losses (0.7% of thermal population in the
lower level
4
I
9/2
) and low emission cross section that limit its
overall efficiency
em946
=5x10
-20
cm²). It requires crystals
with high doping concentration to achieve high power
emission, but then suffers from strong thermal lens. On the
opposite, a low doping value keep the thermal effects at a
manageable level, but reduces the absorption over the
crystal length. Single-crystal fiber (SCF) overcomes this
problem by using low-doped (0.2 %) and long crystals (50
mm), with an excellent thermal management and pump
confinement. They already have demonstrated a
significant output power at 946 nm, with more than 30 W
of output power, but in CW regime, with a spatially
multimode and unpolarised beam [7].
In this paper we propose to use single-crystal fiber
technology to realize a high power, high energy, polarized
oscillator with good beam quality adapted for efficient
nonlinear frequency conversion. We will first study how to
polarize efficiently a Nd:YAG oscillator. Then, we will
detail how the laser parameters can be optimized for a
Q:switched regime delivering an energy at the millijoule
level with a significant average power. Finally, we will
study a simple set-up of extracavity second harmonic
generation in a BiB
3
O
6
(BiBO) crystal.
The first difficulty arises if the laser needs to be polarized
for nonlinear frequency conversion. Nd:YAG, being a
naturally non-birefringent material, suffers from strong
thermal-stress induced depolarization losses when a
polarizing element is inserted inside the cavity, the
maximum losses being located in the crystal at 45° of the
imposed polarization[8].
In this work we will use a technique proposed by Clarkson
& al in 1999 [9] : a quarter-wave plate is inserted between
the input mirror and the laser crystal to reduce the
depolarization losses.. An intracavity thin film polarizer
(TFP) is used as a polarizing element because of its higher
extinction ratio compared to the commonly used Brewster-
angle glass plate.
The experimental set-up is detailed in figure 1. The pump
source is a 120 W, 200 µm, NA 0.22 fiber coupled laser
diode at 808 nm. It is focused inside the single-crystal fiber
(SCF) at a pump spot diameter of 480 µm. The laser crystal
is a commercial Nd:YAG single-crystal fiber module
(Taranis model, Fibercryst). It consists of a 50 mm long, 1
mm diameter, 0.2 % doped Nd:YAG, AR coated for 808 and
946 nm rod embedded in a cooling system, set at 12 °C.
The pump absorption efficiency without laser operation
was measured to be 96%. Thanks to the pump guiding in
crystal fibers, a significant part of the pump power is still
present at the bottom end. As demonstrated before [10] we
can consider that about 50% of the unabsorbed pump
power is located on the output face in a diameter equivalent
to the pump spot diameter at the focus. This corresponds to

an intensity of about 1200 W/cm², compared with the
transparency intensity of about 500 W/c. [11]
The laser cavity is designed with two mirrors. The input
plan mirror M1 is coated for high reflectivity (HR) at the
946 nm laser line, and high transmission (HT) at 808 nm
and 1064 nm. A plano-concave mirror M2 of curvature
radius 200 mm closes the cavity: different transmission
values are used depending on the measurements. The
cavity length is 270 mm, and the laser fundamental mode
diameter at maximum pump power is 400 µm, taking into
account the 40 mm focal length of the thermal lens induced
in the single crystal fiber. This thermal lens value was
estimated with a theoretical calculation for a pump power
of 100 W, and confirmed experimentally by studying the
stability of a plano/plano cavity. In order to polarize the
laser beam, an intracavity thin-film polarizer (TFP) coated
for high reflectivity at 946 nm with an extinction ratio of
200:1 is placed between the SCF output face and M2. A
quarter-waveplate (QWP) is inserted between the input
mirror M1 and the SCF to compensate for the
depolarization losses.
A first set of characterization is realized in continuous wave
(CW) regime, where the M2 mirror is now an output
coupler with a reflectivity of 98% to serve as a reference
output for the determination of the depolarization losses.
The measurements of those depolarization losses and the
influence of the QWP are presented in Figure 2. Three
output beams are simultaneously monitored (see figure 1).
P1 gives the depolarization losses, P2 is the reference
output and P3 is the 946 nm laser output. Those three
measurements are used to calculate the round trip
depolarization losses. We can see that until 50 W of pump
power, the depolarization losses are reduced from 5 to 2%.
Then, without the QWP, the depolarization losses raise to
reach a maximum of 20% at 120 W of pump power. With
the QWP, the round trip depolarization losses are partially
compensated until 120 W of pump power where the QWP
effect becomes negligible. This behavior was expected, as
the quarter waveplate only compensates the depolarization
at low powers [9]. Above 100 W of pump power, the beam
quality starts to degrade. Therefore we limit our pump
power to a 100 W, where the depolarization losses are 10%.
Coupled optimization of energy and average power in Q-
switched require an accurate control of the laser
parameters. Here we decided to act on two elements: the
output coupling and the repetition rate.
In our experiment, we implement a variable output coupler
based on a half-wave plate (HWP) and the TFP to have a
complete overview of the laser behavior with an increase of
cavity losses. By adjusting the HWP, the output coupling
can be varied from 0 to 100%. In Figure 3, the “usefulCW
output power (P2+P3, meaning the output on the 2%
coupler plus the output from the variable output coupler) is
plotted versus the output coupling (value coming from the
HWP plus 2%) for 40, 60, 80 and 100 W of pump power. For
every pump power value, the optimum output coupling is
between 10 and 20 % of transmission. At 100 W, as much
as 11.6 W of average power can be extracted for an output
coupling value of 12%. This optimum output coupling is
close to the one reported in other papers [3][7].
This CW characterization gives an order of magnitude of
what performances will be available in Q-switched
operation. The lifetime in Nd:YAG is 230 µs : therefore, one
can expect the average power to drop in Q-switched
operation for repetition rates between 5 and 15 kHz. Since
we obtained 11.6 W in CW with an output coupler of 12%,
we should be capable of operating the Q-switched laser
around 10 kHz and reach an energy in order of the
millijoule. However based on the SCF coating damage
threshold of 3 J/cm² and the estimated beam diameter
inside the SCF, we calculate the maximum output pulse
energy cannot overcome 300 µJ with an output coupling
value of 12%,.
Here comes the interest of the variable output coupler: it
can be used to increase gradually the output coupling, in
order to maximize the output energy while keeping a
satisfying average output power and an intracavity energy
under the components damage threshold. As can be seen
on Fig.3, an output power of 10 W in CW can still be
extracted with an output coupling of 30%. It means that mJ
level pulses can be safely generated by this laser in Q-
switched regime with average power in the multiwatt level,
assuming that around 10 kHz the average power only
drops by a little amount.
To obtain pulsed operation, we used acousto-optical Q-
switching for the simplicity, and the large range of
repetition rate available. A compact Gooch&Housego
acousto-optic modulator (AOM) is placed between the SCF
output and the TFP. The AOM is 35 mm long, coated for
low reflectivity at 946 nm, and has a diffraction efficiency
for a polarized beam of 60%. The cavity length was
minimized in order to obtain the shortest possible pulse
duration. As M2 mirror, we now use a high-reflectivity
mirror for 946 nm instead of the 2% output coupler since
the coupling is achieved by the thin film polarizer (output
P3 on Fig 1). The output coupling is set to a transmission
of 30% for the reasons described above. The passive losses
in the laser cavity were about 3% without adding the
depolarization losses.
In Q-switched operation we monitored at the same time the
average output power, the repetition rate and associated
pulse duration. The results are presented in figure 4. The
maximum output energy of 1 mJ is obtained for a
repetition rate of 7 kHz, with a pulse duration of 23 ns. As
the average power starts to decrease severely under 15
kHz, we will operate the laser at 10 kHz, where the average
power is 8 W, with a pulse duration of 30 ns (displayed in
figure 5). This configuration, while producing a slightly
lower energy, provides a better compromise between
average power and energy. The beam profile at 946 nm is
displayed in figure 6. It has a slightly elliptical Gaussian
intensity profile, and we measured its beam quality to be
M²x=1.32 and M²y=1.08. We found that the elliptical profile
only appears at high pump powers. Therefore, we
attributed it to the very strong thermal lens that will make
the cavity very sensitive to a slight pump misalignment.

The laser output is stable with a measured fluctuation of
0.4% over one hour, calculated by dividing the standard
deviation by the root mean square value (stability plot is
displayed in figure 7.
The nonlinear crystal used for the type-I SHG of 946 nm to
473 nm is a 10 mm long BiB
3
O
5
[12] crystal cut at θ=161.6°
and φ=90°. For these cut angles, BiBO has a nonlinear
coefficient d
eff
=3.34 pm/V, a walk-off angle of 40 mrad for
the fundamental beam, and an acceptance angle of 1.28
mrad.cm. The BiBO crystal is kept inside an oven at a
stabilized temperature of 50°C. The second harmonic
generation is realized in a single-pass, extracavity set-up.
In this experiment we try to preserve the fundamental
beam quality as much as possible: we use a loose focusing
with a beam diameter of 190 µm, which results in a beam
divergence of 1.5 mrad, only slightly higher than the BiBO
acceptance angle. The resulting blue laser is filtered from
the fundamental with a dichroic mirror. The conversion
efficiency is displayed in figure 8: a maximum conversion
efficiency of 37.5 % is obtained for an input power of 8 W,
which results in an output 473 nm power of 3 W and energy
of 300 µJ. The dotted line corresponds to the simulation
made with the simulation software SNLO . The blue beam
profile is displayed in figure 6, and the beam quality is only
slightly degraded with a measured beam quality of
x=1.38 and M²y=1.12. The pulse duration is reduced to
25 ns (see figure 5). The blue beam is stable with a
calculated RMS stability of 1.8% (displayed in Fig 7). The
increased fluctuation of the blue power was expected, as
SHG is a nonlinear process and dependent on the square of
intensity.
In conclusion, we demonstrated a Q-switched 473 nm laser
based on Nd:YAG single-crystal fiber. We first studied the
polarization of Nd:YAG in CW regime, and extracted more
than 10 W of average power for a polarized beam at 946
nm. Then, we used a variable output coupler to operate the
laser in Q-switched mode at an energy level more than two
times above the previously reported results. Finally, we
realized an extracavity SHG in a BiBO crystal, and
obtained an average output power of 3 W at 10 kHz, which
is two times the previously reported results in both average
power and energy. The pulse duration was 25 ns, and the
beam quality was measured to be M²x=1.38 and M²y=1.12.
Many progress can still be made to increase the oscillator
performances. While a straight increase in pump power
will cause too severe thermal problems, alternative
solutions can be found. In order to lower the thermal lens
inside the crystal, pumping directly into the emitting level
of Nd:YAG at 885 nm seems a promising solution[13]. the
nonlinear frequency conversion could also be improved by
using more elaborate set-ups such as double-pass SHG [6]
or more efficient nonlinear crystals such as quasi-phase
matched crystals [14].
Aknowledgments
Loïc Deyra thanks the DGA for the funding of his PhD
This work has been partially funded by the ANR through
the program UV Challenge
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Figure 1: Experimental set-up
Figure 2: Depolarization losses with/without quarter-wave plate
Figure 3: Output CW power at 946 nm in function of the half-
wave plate induced polarizer transmission for different pump
power values
Figure 4: Pulse energy and pulse duration at 946 nm in function
of repetition rate (left), and Pulse energy and average power at
946 nm in function of repetition rate
Figure 5: Pulse shape at 946 nm and 473 nm
Figure 6: Beam profile and beam quality for 946 nm and 473 nm
Figure 7 : Output power stability for 8 W of 946 nm(red) and 3 W
of 473 nm (blue)
Figure 8 : Output 473 nm average power and SHG efficiency

Citations
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Abstract: We report on high-performance infrared lasers at 0.94 μm based on quasi-three-level transition of F3/24→I9/24 in Nd:LuYAG mixed crystal, for the first time to our knowledge. The maximum output power was achieved to 5.64 W with slope efficiency of approximately 52.5% at 946 nm. The simultaneous dual-wavelength laser at 939 and 946 nm is also obtained with maximum output power of 3.61 W and slope efficiency of 34.8% by introducing a glass etalon into the cavity. Moreover, a 2.0-W single-wavelength laser at 939 nm can be further attained by suitably tilting the etalon. Using a Cr:YAG saturable absorber, Q-switched laser operation is realized with maximum average output power of 0.68 W and the narrowest pulse width of 8.4 ns, which results in the maximum single pulse energy of approximately 55.3 μJ and the maximum pulse peak power of approximately 6.15 kW. Finally, thermal focal length of the laser crystal is estimated by using a flat-flat laser cavity.

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Abstract: Single crystal fiber (SCF) is a hybrid laser architecture between conventional bulk laser crystals and active optical fibers allowing higher average powers than with conventional crystals and higher energy than with fibers in pulsed regime. The pump beam delivered by a fiber-coupled laser diode is confined by the guiding capacity of the SCF whereas the signal beam is in free propagation. In this paper, we study the pump guiding in the SCF and give an overview of the results obtained using SCF gain modules in laser oscillators and amplifiers. We report about up to 500 μJ nanosecond pulses at the output of a passively Q-switched Er:YAG SCF oscillator at 1617 nm. High power experiments with Yb:YAG allowed to demonstrate up to 250 W out of a multimode oscillator. High power 946 nm Nd:YAG SCF Q-switched oscillators followed by second and fourth harmonic generation in the blue and the UV is also presented with an average power up to 3.4 W at 473 nm and 600 mW at 236.5 nm. At 1064 nm, we obtain up to 3 mJ with a nearly fundamental mode beam in sub-nanosecond regime with a micro-chip laser amplified in a Nd:YAG SCF. Yb:YAG SCF amplifiers are used to amplify fiber based sources limited by non-linearities such as Stimulated Brillouin Scattering with a narrow linewidth laser and Self Phase Modulation with a femtosecond source. Using chirped pulse amplification, 380 fs pulses are obtained with an energy of 1 mJ and an excellent beam quality (M2<1.1).

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TL;DR: A deep-UV laser at 236.5 nm based on extracavity fourth-harmonic generation of a Q-switched Nd:YAG single-crystal fiber laser at 946 nm opens the possibility of LIDAR detection of dangerous compounds for military or civilian applications.
Abstract: We demonstrate a deep-UV laser at 236.5 nm based on extracavity fourth-harmonic generation of a Q-switched Nd:YAG single-crystal fiber laser at 946 nm. We first compare two nonlinear crystals available for second-harmonic generation: LBO and BiBO. The best results at 473 nm are obtained with a BiBO crystal, with an average output power of 3.4 W at 20 kHz, corresponding to a second-harmonic generation efficiency of 38%. This blue laser is frequency-converted to 236.5 nm in a BBO crystal with an overall fourth-harmonic generation yield of 6.5%, corresponding to an average output power of 600 mW at 20 kHz. This represents an order of magnitude increase in average power and energy compared to previously reported pulsed lasers at 236.5 nm. This work opens the possibility of LIDAR detection of dangerous compounds for military or civilian applications.

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References
More filters

Journal ArticleDOI
TL;DR: Field measurements support the temperature and salinity dependencies found in the laboratory both in the near infrared and at shorter wavelengths.
Abstract: We have measured the absorption coefficient of pure and salt water at 15 wavelengths in the visible and near-infrared regions of the spectrum using WETLabs nine-wavelength absorption and attenuation meters and a three-wavelength absorption meter. The water temperature was varied between 15 and 30 degrees C, and the salinity was varied between 0 and 38 PSU to study the effects of these parameters on the absorption coefficient of liquid water. In the near-infrared portion of the spectrum the absorption coefficient of water was confirmed to be highly dependent on temperature. In the visible region the temperature dependence was found to be less than 0.001 m-1 degrees C except for a small region around 610 nm. The same results were found for the temperature dependence of a saltwater solution. After accounting for index-of-refraction effects, the salinity dependence at visible wavelengths is negligible. Salinity does appear to be important in determining the absorption coefficient of water in the near-infrared region. At 715 nm, for example, the salinity dependence was -0.00027 m-1 /PSU. Field measurements support the temperature and salinity dependencies found in the laboratory both in the near infrared and at shorter wavelengths. To make estimates of the temperature dependence in wavelength regions for which we did not make measurements we used a series of Gaussian curves that were fit to the absorption spectrum in the visible region of the spectrum. The spectral dependence on temperature was then estimated based on multiplying the Gaussians by a fitting factor.

448 citations


"3 W, 300 μJ, 25 ns pulsed 473 nm bl..." refers background in this paper

  • ...The second harmonic at 473 nm can be used for underwater communication [1], optical storage or spectroscopy....

    [...]


Journal ArticleDOI
TL;DR: A simple technique for reducing the loss that is due to depolarization resulting from thermally induced stress birefringence in solid-state lasers is reported.
Abstract: A simple technique for reducing the loss that is due to depolarization resulting from thermally induced stress birefringence in solid-state lasers is reported. The technique uses a single intracavity quarter-wave plate with its fast or slow axis aligned parallel to the preferred plane of polarization, defined by an intracavity polarizer. This technique has been applied to a diode-bar-pumped Nd:YAG laser operating at 946 nm, resulting in a measured reduction in depolarization loss from approximately 1.7% to approximately 0.0006% and yielding a diffraction-limited, TEM(00) , linearly polarized output power of 2.9 W for an incident pump power of 14.3 W.

105 citations


"3 W, 300 μJ, 25 ns pulsed 473 nm bl..." refers background or methods in this paper

  • ...In this work we will use a technique proposed by Clarkson & al in 1999 [9] : a quarter-wave plate is inserted between the input mirror and the laser crystal to reduce the depolarization losses....

    [...]

  • ...This behavior was expected, as the quarter waveplate only compensates the depolarization at low powers [9]....

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Journal ArticleDOI
Abstract: We report the efficient blue laser at 473 nm generation by intracavity frequency doubling of a continuous wave laser operation of a 885 nm diode direct pumped Nd:YAG laser on the 4F3/2 → 4I9/2 transition at 946 nm. A LiB3O5 (LBO) crystal, cut for critical type I phase matching at room temperature is used for second harmonic generation of the laser. At the absorbed pump power of 18.7 W, as high as 4.3 W of continuous wave output power at 473 nm is achieved with 15 mm long LBO. The optical-to-optical conversion efficiency with respect to the absorbed power is up to 0.23, and the beam quality M2 value is 1.2.

61 citations


"3 W, 300 μJ, 25 ns pulsed 473 nm bl..." refers background in this paper

  • ...In order to lower the thermal lens inside the crystal, pumping directly into the emitting level of Nd:YAG at 885 nm seems a promising solution[13]....

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Journal ArticleDOI
TL;DR: An Yb:YAG single-crystal fiber laser with 251 W output power in continuous-wave and an optical efficiency of 44%.
Abstract: We demonstrate an Yb:YAG single-crystal fiber laser with 251 W output power in continuous-wave and an optical efficiency of 44%. This performance can be explained by the high overlap between pump and signal beams brought by the pump guiding and by the good thermal management provided by the single-crystal fiber geometry. The oscillator performance with a reflectivity of the output coupler as low as 20% also shows high potential for power amplification.

55 citations


"3 W, 300 μJ, 25 ns pulsed 473 nm bl..." refers background in this paper

  • ...As demonstrated before [10] we can consider that about 50% of the unabsorbed pump power is located on the output face in a diameter equivalent to the pump spot diameter at the focus....

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Journal ArticleDOI
Abstract: Taking advantage of the pump beam confinement and the excellent thermal management offered by Nd:YAG single crystal fibers, we demonstrated a maximum output power of 34 W at 946 nm for an incident pump power of 86 W. A high slope efficiency of 53% was obtained. There was no rollover observed in the efficiency curves and the maximum output power was only limited by the available pump power.

46 citations


"3 W, 300 μJ, 25 ns pulsed 473 nm bl..." refers background or result in this paper

  • ...This optimum output coupling is close to the one reported in other papers [3][7]....

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  • ...They already have demonstrated a significant output power at 946 nm, with more than 30 W of output power, but in CW regime, with a spatially multimode and unpolarised beam [7]....

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