Download High-energy, diode-pumped, nanosecond Yb:YAG

High-energy, diode-pumped, nanosecond
Yb:YAG MOPA system
M. Siebold1,2 , J. Hein2 , C. Wandt1 , S. Klingebiel1 , F. Krausz1 ,
and S. Karsch1
1 Max-Planck-Institute
for Quantum Optics (MPQ), Hans-Kopfermann-Str. 1, 85748 Garching,
Germany
2 Institute for Optics and Quantum Electronics, Friedrich- Schiller-University Jena (FSU),
Max-Wien-Platz 1, 07743 Jena, Germany
[email protected]
Abstract: Diode-pumped nanosecond multi-pass laser amplification to
the joule level using an Yb:YAG slab crystal has been demonstrated. A
maximum output pulse energy of 2.9 J at an optical-to-optical efficiency
of 10% has been achieved. The seed pulses with a pulse duration of 6.4 ns
were generated in a Q-switched Yb:YAG laser and amplified up to a pulse
energy of 200 mJ in a multi-pass booster amplifier. A maximum average
output power of 15 W at a repetition rate of 10 Hz has been measured. We
also present a relay imaging semi-stable cavity for multi-pass amplification
and a diode-pumping scheme employing horizontally stacked high-power
laser diodes.
© 2008 Optical Society of America
OCIS codes: (140.3280) Laser amplifiers; (140.3615) Lasers, ytterbium; (140.3480) Lasers,
diode-pumped.
References and links
1. A. Bayramian, P. Armstrong, E. Ault, R. Beach, C. Bibeau, J. Caird, R. Campbell, B. Chai, J. Dawson, C. Ebbers,
A. Erlandson, Y. Fei, B. Freitas, R. Kent, Z. Liao, T. Ladran, J. Menapace, B. Molander, S. Payne, N. Peterson,
M. Randles, K. Schaffers, S. Sutton, J. Tassano, S. Telford, and E. Utterback, “The MERCURY project: A
high average power, gas-cooled laser for inertial fusion energy development,” Fusion Sci. Technol. 52, 383–387
(2007).
2. J. Chanteloup, H. Yu, G. Bourdet, C. Dambrine, S. Ferré, A. Fülöp, S. Le Moal, A. Pichot, G. Le Touzé, and
Z. Zhao, “Overview of the LUCIA laser program: toward 100-Joules, nanosecond-pulse, kW averaged power
based on ytterbium diode-pumped solid state laser,” Proc. SPIE 5707, 105–116 (2005).
3. O. Matsumoto, R. Yasuhara, T. Kurita, T. Ikegawa, T. Sekine, T. Kawashima, J. Kawanaka, T. Norimatsu,
N. Miyanaga, Y. Izawa, M. Nakatsuka, M. Miyamoto, H. Kan, H. Furukawa, and S. Motokoshi, “Development
of thermally controlled HALNA DPSSL for inertial fusion energy,” Proc. SPIE 6101, 61011Q (2006).
4. J. Hein, M. Kaluza, R. Bödefeld, M. Siebold, S. Podleska, and R. Sauerbrey, “POLARIS: An All Diode-Pumped
Ultrahigh Peak Power Laser for High Repetition Rates,” Lecture Notes in Physics 694, 47–66 (2006).
5. V. Lozhkarev, G. Freidman, V. G. E. Katin, E. Khazanov, A. Kirsanov, G. Luchinin, A. Mal’shakov, O. P.
M.A. Martyanov, A. Poteomkin, A. Sergeev, A. Shaykin, I. Yakovlev, S. Garanin, S. Sukharev, N. Rukavishnikov, A. Charukhchev, R. Gerke, and V. Yashin, “200 TW 45 fs laser based on optical parametric chirped pulse
amplification,” Opt. Express 14, 446–454 (2006).
6. A. Sridharan, S. Saraf, and R. Byer, “Yb:YAG master oscillator power amplifier for remote wind sensing,” Appl.
Opt. 46, 7552–7565 (2007).
7. A. Jolly, N. D. Robin, J. Luce, and G. Deschaseaux, “Generation of variable width pulses from an Yb3+ :YAG
Integrated Dumper – Regenerative amplifier,” Opt. Express 15, 466–472 (2007).
8. S. Bahbah, D. Albach, J. Chanteloup, G. Bourdet, G. L. Touzé, M. Pluvinage, and B. Vincent., “Original High
Power oscillator Yb:YAG pumped by lasers diodes,” Conference Digest of CLEO/QELS Europe 2007 CA6-1,
Munich, Germany, June 17–22 (2007).
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(C) 2008 OSA
Received 30 Jan 2008; revised 23 Feb 2008; accepted 24 Feb 2008; published 5 Mar 2008
17 March 2008 / Vol. 16, No. 6 / OPTICS EXPRESS 3674
9. G. Bourdet, “Comparison of pulse amplification performances in longitudinally pumped Ytterbium doped materials,” Opt. Commun. 200, 331–342 (2001).
10. D. Wolff, G. Bonati, P. Hennig, and H. Voelckel, “Reliability of high power diode laser bars in industrial applications,” Proc. SPIE 5711, 66-72 (2005).
1. Introduction
Nowadays, the generation of high laser peak-intensities at a high degree of system compactness can be achieved by modern diode-pump technologies. Worldwide, several ambitious laser
projects such as MERCURY [1], LUCIA [2], HALNA [3], or POLARIS [4] are developing
diode-pumped amplifier systems for output pulses with expected energies of 100 J or more.
Those laser systems are either designed as direct diode-pumped lasers, or as pump sources for
optical parametric amplifiers (OPA) or Titanium-Sapphire lasers for shortest pulse durations.
The PFS project (Petawatt Field Synthesizer at the Max-Planck-Institute for Quantum Optics)
aims at the chirped-pulse optical parametric amplification (OPCPA) [5] to the multiple-joule
level at a repetition rate of 10 Hz. In particular, heading for few-cycle pulses in the near infrared both the pulse duration of the pump pulses and the chirped pulses of the OPA have to be
in the picosecond range in order to yield a moderate stretching factor for the OPCPA. A diodepumped laser is designed to be a 50 TW CPA system delivering synchronous pump pulses at a
pulse duration of 1 ps.
For the power amplifiers of the considered pump laser system Ytterbium-doped Yttrium Aluminum Garnet (YAG) has been chosen. It is one of the most promising laser materials for
diode-pumped lasers with high-average-power. For microsecond pulses multi-pass amplification to the joule level with an optical-to-optical efficiency of 8.7% has already been demonstrated using an Yb:YAG slab [6]. The amplification of nanosecond pulses with an output pulse
energy of 50 mJ has also been reported recently [7, 8]. In general, due to its quasi-three level
scheme the major drawback of Ytterbium-based gain media is the minimum pump intensity
required to bleach out the absorption at the laser wavelength [9]. Therefore, efficient pulse energy extraction requires high fluences which tend to approach the damage threshold fluence for
nanosecond pulses.
In this paper, we present multi-pass amplification in Yb:YAG to the 3 J-level at nanosecond
time-scale. The amplifier system described here is dedicated to be a prototype pre-amplifier
for the considered CPA-system and should be capable of delivering pulses with at least 1 TW
peak-power.
2. Laser crystal and pumping optics
The gain medium is designed for a maximum fluence of 10 Jcm −2 which is the saturation
fluence of Yb:YAG. In the case of the presented amplifier, the Yb:YAG slab crystal (3.3 mm
width, 16 mm height, and 16 mm length, provided by FEE, Germany) comprises an Yb 3+ concentration of 1.4%. Figure 1 illustrates the design of the gain medium with an aspect ratio
of 5:1 for both pump and laser beams. The thermal behavior of a high-average-power slab laser
is superior to a gain medium with circular cross section due to an increase of the heat sink surface. Furthermore, the crystal design is adapted to the asymmetric beam quality of the slow axis
and the fast axis of high power laser diode bars. The transversally-cooled crystal between two
truncated copper pyramids covered with indium foil is thermally-coupled to two micro-channel
heat sinks.
An absorption efficiency of 74% and a pump homogeneity of 81% between faces and crystal
center at both sided transverse pumping has been designed. For the amplification of the laser
pulses the facets of the gain medium have been cut at Brewster’s angle, whereas the pump entrance faces have been oriented for perpendicular incidence.
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(C) 2008 OSA
Received 30 Jan 2008; revised 23 Feb 2008; accepted 24 Feb 2008; published 5 Mar 2008
17 March 2008 / Vol. 16, No. 6 / OPTICS EXPRESS 3675
pump beam
Yb:YAG
MC
heat removal
laser beam
.
.
.
LSF
LF2
16 HS
LS1 LF1
WG
LD
TFP /2
Fig. 1. Laser crystal and pumping optics, Yb:YAG crystal with Brewster cut for the laser
beam, MC: micro-channel cooler, heat sink coupled to the largest crystal faces; 2-sided
transversal pumping: pump module (each side) consisting of 16 wave-guided (WG) horizontal stacks (HS) with 4 laser diode bars (LD) each, half-wave plate (λ /2) and thin film
polarizer (TFP) for polarization coupling; cylindrical lenses for the slow axis (LS1) and the
fast axis (LF2, LF2) in order to image the pump light, final spherical focusing lens (LSF).
The Yb:YAG crystal is transversally pumped by two laser diode modules (provided by Jenoptik
Laserdiode GmbH, see Fig. 1) each having a peak output power of 13 kW. These laser diode
modules consist of 16 laser diode stacks being polarization coupled whereas each stack comprises of 4 laser diode bars with a total output power of 1 kW. The laser diode bars are horizontally stacked [10] on a copper heat sink sub-mount. In order to fill the vertical pitch between
the stacks and thereby increase the brightness, the fast axis collimated laser diodes are waveguide coupled resulting in an output beam aperture of 12×52 mm 2 . A slow axis divergence of
10◦ was measured whereas the fast axis remains nearly perfectly collimated. The application of
horizontally stacked laser diodes instead of vertical laser diode stacks (e.g. described in [1]-[4])
promises advanced reliability of the high-power laser diode bars, for example the use of passive
heat sinks [10].
In order to ensure the optimum absorption in Yb:YAG, the center wavelength of the laser diodes
is controlled by 8 independent water cooling circuits due to the temperature shift of the emitted
wavelength. This effect also shifts the wavelength during the pump pulse duration of 1.5 ms,
leading to a total bandwidth of 5.3 nm of the integrated spectrum.
The diode modules are followed by a cylindrical lens telescope and a spherical lens (f = 55 mm)
in order to image the pump radiation into the gain medium. At the present no further beam
homogenizer has been applied.
3. Amplifier setup
The stretcher of the CPA front-end system is designed for input pulses with a bandwidth of
2 nm and a stretching factor of 2 × 10 3 . In order to verify the damage-limited laser intensities
which are expected in forthcoming CPA operation at a pulse duration of 2 ns after stretching,
a Q-switched Yb:YAG oscillator and a subsequent booster amplifier have been employed. The
diode-pumped oscillator generates pulses with a pulse energy of 3 mJ and a pulse duration of
6.4 ns. In a subsequent diode-pumped multi-pass amplifier the seed pulse energy is boosted to
the 200 mJ-level.
Figure 2 shows the setup of the 4-pass amplifier that has been designed for an output pulse
energy of 2 J. A double 4- f configuration has been employed for relay imaging, where the
plane mirrors M1 and M6 receive images of the beam profile at the Brewster-cut gain medium.
In order to prevent the ionization of air and subsequent optical breakdown two vacuum tubes
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(C) 2008 OSA
Received 30 Jan 2008; revised 23 Feb 2008; accepted 24 Feb 2008; published 5 Mar 2008
17 March 2008 / Vol. 16, No. 6 / OPTICS EXPRESS 3676
ns-oscillator
FI
Yb:YAG booster
L1
M3
L2
M8 M9
M1
M4
Yb:YAG
M5
M2
M6
M7
Fig. 2. Setup of the 4-pass Yb:YAG amplifier with relay imaging; Yb:YAG crystal with
Brewster cut, M1 and M2: plane end mirrors (HR 0◦ 1010–1060 nm), M3–M7: concave
mirrors (radius of curvature 750 mm, HR 0◦ 1010–1060 nm), M8 and M9: plane turning
mirrors (HR 45◦ 1010–1060 nm), L1 and L2: cylindrical lens telescope, FI: Faraday isolator.
are placed at the focal regions between the mirrors M3-M5 and M4-M6. By tilting of mirror
M1 around one axis the cavity between the mirrors M1-M6 acts as semi-stable cavity whereas
the number of amplifier passes (4, 6, 8...) simply depends on the tilt angle. Close to the concave
mirrors (M7 and M8) the beams of each pass are spatially separated, allowing the coupling of
the beam into and out the multi-pass amplifier. The aspect ratio of the input beam is matched to
the dimensions of the gain medium by a cylindrical lens telescope.
4. Results
Fig. 3. Nanosecond pulse amplification, pulse duration: 6.4 ns, numbers of passes: 4; (a)
Output pulse energy vs. pump pulse energy at an input pulse energy of 200 mJ, slope efficiency ηS = 14.9% (b) Output pulse energy and total gain vs. input pulse energy at a pump
energy of 21 J.
The amplifier dynamics at both nanosecond and microsecond time-scale has been analyzed.
A maximum output pulse energy of 2.9 J has been achieved at nanosecond pulses. Further increase of the pulse energy is limited by surface damage of the Yb:YAG crystal. The slope
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(C) 2008 OSA
Received 30 Jan 2008; revised 23 Feb 2008; accepted 24 Feb 2008; published 5 Mar 2008
17 March 2008 / Vol. 16, No. 6 / OPTICS EXPRESS 3677
Fig. 4. Amplification of top-hat shaped microsecond pulses, pulse duration: 100 μ s, numbers of passes: 6; (a) Output pulse energy vs. pump pulse energy at two pump durations tp :
800 μ s and 1.5 ms, input pulse energy: 100 mJ. (b) Output pulse energy and total gain vs.
input pulse energy at a pump energy of 19.5 J.
efficiency ηS of the 4-pass amplifier is 14.9% and an optical-to-optical efficiency of 10% has
been evaluated for nanosecond pulses from Fig. 3(a). In Fig. 3(b) the output pulse energy and
the gain of the 4-pass amplifier are plotted versus the input pulse energy at a pump power of
14 kW, a pump duration of 1.5 ms, and a pulse duration of 6.4 ns. The results shown in Fig. 3
suggest that the gain saturation and therefore the extraction efficiency can be maximized either
by an increase of the input pulse energy or by adding further amplifier passes.
When operating in quasi-cw mode (microsecond pulses) the short pulse amplification at the
end of the pump period can be verified without the danger of laser-induced damage. In the
long-pulse regime (pulse duration: 100μ s), an output pulse energy of 5.3 J (53 kW peak output
power) at maximum pump power and at a pump duration of 1.5 ms has been achieved. In order to maximize the extraction efficiency of the amplifier, a 6-pass amplifier configuration has
been chosen in this case. Figure 4 illustrates the efficiency of a 6-pass amplifier at both a pump
duration of 800 μ s and 1.5 ms. A maximum slope efficiency η S of 28.3% at a pump duration of
800 μ s and a threshold pump energy of 6 J has been obtained.
At nanosecond pulses the damage threshold fluence scales with the inverse square-root of the
pulse duration resulting in a 44% reduction of the maximum possible pulse energy for a pulse
duration of 2 ns instead of 6.4 ns. Therefore, when operating as a chirped-pulse amplifier at a
pulse duration of 2 ns the presented amplifier stage is suitable to generate pulses with a pulse
energy of 1.5 J at a pump power 10 kW and a pump duration of 1.5 ms.
Other than damage limitations, the gain narrowing is a critical issue in terms of both increase of
the bandwidth-limited pulse duration after re-compression and pulse shortening of the chirped
pulses and therefore risk of laser-induced damage. In order to estimate the influence of the gain
narrowing, the cavity-dumped oscillator pulses having a bandwidth of 1 nm (FWHM) have been
directly amplified within the Yb:YAG 4-pass amplifier (input pulse energy: 3 mJ) at a high gain
(G = 160) at which maximum gain narrowing occurs far from gain saturation. At those worstcase conditions a minimum bandwidth of 0.9 nm (FWHM) has been achieved (see Fig. 5(a))
corresponding to a bandwidth limited pulse duration of 1.8 ps.
The experiments described have been performed at a repetition rate of 1 Hz. At increasing repetition rate the transmission through the Yb:YAG crystal is reduced due to thermally-induced
stress birefringence. Thus, the measurements of the output pulse energy versus the repetition
rate of the amplifier at a pump energy of 21 J indicate a pulse energy of 2 J at a repetition rate of
1 Hz, dropping to 1.7 J at 5 Hz and 1.5 J at 10 Hz. Due to relay imaging the thermal lensing has
minor impact on the transmission properties of the amplifier, whereas the focal position within
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(C) 2008 OSA
Received 30 Jan 2008; revised 23 Feb 2008; accepted 24 Feb 2008; published 5 Mar 2008
17 March 2008 / Vol. 16, No. 6 / OPTICS EXPRESS 3678
the vacuum tubes is slightly longitudinally shifted. The stress birefringence can be compensated
by an intra-cavity quarter-wave plate in order to alter the polarization after each amplifier pass.
Thus the Brewster design of the gain medium has to be changed to perpendicular incidence
which requires additional AR-coating of the crystal faces.
The input and output beam profiles of the presented amplifier are shown in Figs. 5(b) and 5(c).
While amplification the high beam quality of the booster amplifier (M 2 = 1.2) has been preserved along the vertical (y) axis. The beam profile along the horizontal (x) axis shows a diffraction pattern in the far field resulting from both the top hat shape and the intensity modulation
of the pump profile (see Fig. 5(d)). The latter influence can be minimized by further homogenization of the pump radiation in order to improve the homogeneity of the laser beam profile at
the relay planes.
Fig. 5. Influence of the gain narrowing: (a) input and output pulse spectra of the 4-pass
Yb:YAG amplifier at unsaturated gain (G = 160). Intensity profiles: (b) transmission of the
input beam without amplification, (c) output beam profile after 4-pass amplification, (d)
pump beam profile.
5. Summary and Outlook
We have successfully demonstrated nanosecond pulse amplification up to the 3 J-level within a
4-pass Yb:YAG amplifier. Due to surface damage of the Yb:YAG crystal the maximum output
pulse energy is limited to 2.9 J at a pulse duration of 6.4 ns. In the case of nanosecond pulses
an optical-to-optical efficiency of 10% at a pump duration of 1.5 ms has been obtained. When
seeded with microsecond pulses a maximum output pulse energy of 5.3 J has been achieved. A
maximum average output power of 15 W at a repetition rate of 10 Hz has been measured.
The oscillator’s 1-nm line width was not significantly reduced by gain narrowing at maximum
small signal gain of 160. This result gives optimistic prospectives towards chirped pulse amplification at a bandwidth (FWHM) of 2 nm and at saturated gain.
Future work is now in progress to accommodate and operate the presented Yb:YAG amplifier
in a CPA system with expected pulse duration of 1 ps and peak power in excess of 1 TW after
recompression. In summary, we believe that the data and experience gathered has demonstrated
the feasibility and scalability of our approach and will allow the construction of the full-scale
laser amplifier with expected output pulse energies of several 10 J.
Acknowledgments
We would like to thank M. Rogg (MPQ Garching), B. Klumbies, P. Hanse, R. Bark (FSU Jena),
U. Hallmeyer (Hellma Optik Jena) for their technical contribution. The authors acknowledge
the support of that work by the Max Planck Society.
#92300 - $15.00 USD
(C) 2008 OSA
Received 30 Jan 2008; revised 23 Feb 2008; accepted 24 Feb 2008; published 5 Mar 2008
17 March 2008 / Vol. 16, No. 6 / OPTICS EXPRESS 3679