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 200mJ in a multi-pass booster amplifier. A maximum average output power of 15W 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
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 , LUCIA , HALNA , or POLARIS  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)  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 diode-pumped 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 . 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 . 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 (3.3mm width, 16mm height, and 16mm 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.
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 13kW. 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  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 × 52mm2. 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 -) promises advanced reliability of the high-power laser diode bars, for example the use of passive heat sinks .
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 5.3nm of the integrated spectrum.
The diode modules are followed by a cylindrical lens telescope and a spherical lens (f = 5mm) 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 × 103. 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 3mJ 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 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.
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 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 14kW, 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: 100µs), an output pulse energy of 5.3 J (53kW peak output power) at maximum pump power and at a pump duration of 1.5ms 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 800µs and 1.5 ms. A maximum slope efficiency ηS of 28.3% at a pump duration of 800µs 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 10kW 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 0.9nm (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 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 (M2 = 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.
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 15W 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.
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.
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