A CPA laser system for high pulse energy at high average power has been developed. The system is based on Yb:YAG thin-disk technology. It provides two laser beams with more than 500 mJ pulse energy each at 100 Hz repetition rate and 2 ps pulse duration. The system consists of a common oscillator, a grating stretcher and compressor and two identical amplifier chains that are both equipped with a regenerative amplifier and a ring amplifier. The compressor supports an individual alignment of the dispersion for the two laser channels.
© 2016 Optical Society of America
Laser amplifiers with high pulse energy and high average power are required for a number of applications, e.g. as pumping OPCPA systems [1–4] or driving XUV and soft X-ray sources [5–7]. Consequently, they have been in the focus of development for several years . The thin-disk technology is very well suited for these amplifiers . This technology allows for efficient cooling of the laser medium, which is essential for achieving a high average laser power. In addition, it supports a large laser beam diameter, which is necessary to reach high pulse energy. In contrast, the gain per single reflection on the Yb:YAG disks is relatively small (typically 20…50%), due to the low thickness of only a few hundred micrometers. Moreover, the precise manufacturing of disks with large diameter is rather complicated. Even high quality amplifier disks do not reach an optical quality and flatness comparable to standard laser mirrors. Thus the amplifier should realize multiple reflections over the disk in order to reach a reasonable total gain. In addition, the amplifier design should tolerate the optical imperfections of the disks or even clean the beam from distortions. We have adapted the large-aperture ring amplifier concept described by D. L. Brown et al.  to a thin-disk laser amplifier . This concept of a ring amplifier with integrated spatial filtering is a solution that fulfills both requirements for thin-disk laser amplifiers. It allows for a large number of amplifying paths and cleans the beam at every round trip.
We report on a Yb:YAG thin-disk laser system that utilizes CPA technique in two synchronous channels (see Fig. 1). The two amplification channels have a common front-end (oscillator and stretcher) as well as a common pulse compressor. Each amplifier channel consists of a regenerative amplifier and a subsequent ring amplifier. All amplifiers are based on diode pumped Yb:YAG thin-disks that are pumped with 100 Hz repetition rate. The regenerative amplifiers are operated with 500 µm thick laser disks, 17 mm in diameter, while for the ring amplifier disks of 25 mm diameter and 700 µm thickness are used. This disk size supports a pumped area of 12 mm diameter.
The oscillator allows for synchronization with external devices on a timescale better than 1 ps. This allows using sub 10 ps pulses in subsequent systems like an OPCPA system, with tolerable influence of the timing jitter. Nevertheless, further optimization of the synchronization system is ongoing.
The pulse compressor is constructed in such a manner that both beams use the same grating. In order to reduce the footprint of the compressor, it has a folded geometry. Since each channel has a separate folding mirror, the total dispersion of the compressor can be adjusted for each channel independently.
A detailed description of the main building blocks of the laser is given in the next sections.
2. System layout
The laser oscillator is a home build Yb:KGW oscillator (Fig. 2). The 1.5 mm thick Yb:KGW crystal is pumped by a fiber coupled diode operated at approx. 8 W average power through a dichroitic mirror. Modelocking is accomplished by a saturable absorber mirror (BATOP GmbH, Germany). All three curved mirrors of the resonator are GTI (Gires–Tournois interferometer) with 1300 fs2 group velocity dispersion, produced by Layertec, Germany. This laser oscillator generates a train of ~1 ps long pulses with 80 MHz repetition rate and 0.3 W average power. After the oscillator, a KDP Pockels cell selects single pulses from this train with 100 Hz repetition rate.
The pulses selected by the Pockels cell are then stretched in a grating stretcher. To reduce its length the stretcher is folded and the laser pulse is guided through the stretcher twice. This reduces the length of the stretcher by a factor of four and only one instead of two gratings is required. The stretched pulse has a pulse duration of 2 ns or a chirp of 1 ns/nm.
After the stretcher, the pulse is split in two parts of equal pulse energy. Each part is amplified in identical amplifier chains consisting of a regenerative and a ring amplifier. Both amplifiers are based on Yb:YAG thin-disk technology.
2.2 Regenerative Amplifier
The layout of the regenerative amplifier is shown in Fig. 3. The Yb:YAG thin-disk is used as one end-mirror of the resonator [12,13]. In front of the disk the beam shows only a low divergence of < 10−4 rad. Thus the beam diameter is nearly identical at the disk, at the Pockels cell and at both polarizers, where in- and out-coupling of the laser beam takes place. To synchronize the time delay of the laser pulses in the two channels the other end-mirror is positioned on a linear stage to adjust the resonator length accordingly.
The amplifier head with an Yb:YAG disk of 500 µm thickness and 17 mm diameter is a commercial component from Trumpf Laser GmbH. The disk is pumped by a fiber coupled laser diode module. This pump module delivers up to 1.7 kW peak power (1.7 J during 1 ms) at 940 nm wavelength through a fiber of 1.2 mm diameter. It can operate at a duty cycle up to 20% and was developed at Ferdinand Braun Institute, Berlin [15, 16]. The pump diameter is 5 mm, ensuring a pump power density above 8 kW/cm2 and allowing for a mode diameter of 4 mm on the laser disk. A set of convex and concave mirrors is used to adapt the resonator mode accordingly.
The regenerative preamplifier produces pulses with output energy between 120 mJ and 180 mJ at a repetition rate of 100 Hz. Energy fluctuation of the pulses recorded during a period of several hours is below 0.3% (rms). A detailed description of a similar version of the regenerative amplifier is given in .
2.3 Ring Amplifier
The large aperture ring amplifier contains the following main optical parts:
- • The unit for coupling the laser pulse in and out of the ring. This unit contains in particular a Pockels cell, a half wave plate, and four thin-film polarizers;
- • Two amplifier heads;
- • A spatial filter between the two amplifier heads.
The two self-made amplifier heads are equipped with 0.7 mm thick Yb:YAG (7%) amplifier disks of 25 mm in diameter. Each disk is pumped with 4 x 1.7 kW peak power from fiber coupled laser diodes of the same type as used for the regenerative amplifier. The 4 parallel pump beams are coupled into the amplifier head and guided 8 times onto the Yb:YAG disk (16 passes through the amplifier medium). The pump spot is 12 mm in diameter, allowing for a laser beam diameter of about 10 mm (4 σ). The spatial filter between the two amplifier heads cleans the laser beam at every round trip. In this way imperfections of the laser disks are filtered out to a large degree. The layout of the ring amplifier is shown in Fig. 4. It has a total length of about 9 m, corresponding to a roundtrip time of 30 ns.
In contrast to a former design  that used a glass rod as the laser medium, we have equipped the ring with two thin-disk laser heads. In order to improve the flatness of the wavefront, wavefront-correcting elements such as deformable mirrors or phase masks could be used. However, for the setup discussed here it turned out to be sufficient to adjust the two lenses of the spatial filter with care in all possible degrees of freedom. In particular, deviations from the rotational symmetry of wavefront perturbation at the thin-disks were compensated by tilting the lenses with respect to the beam path and optimizing the transmission through the spatial filter this way. In order to support for this adjustment of the lenses (L1, L2 in Fig. 4), they were mounted outside the spatial filter, and additional windows (W1, W2) were used to seal the vacuum tube of the filter.
As shown in Fig. 5 both channels have nearly the same amplifying characteristic. Within 4 round trips the seed laser pulse is amplified from 80 mJ to more than 600 mJ, corresponding to an amplification factor of ~8. In order to avoid damage of the optics in the final amplifier the output pulse energy was limited to this value. Higher single pulse energy was demonstrated using a slightly different design with stronger pump diodes and increased seed pulse energy .
The beam profile depends very strongly on the quality of the amplifier disk and of the alignment of the lenses of the spatial filter. For larger disks it becomes more and more difficult to reach a very good optical quality after the sophisticated process of assembling the amplifier disk. Even with the integrated spatial filter a reasonable beam profile is not for granted. However, appropriate alignment of both lenses of this filter results in a symmetric beam with a Gaussian-like profile. The slightly elliptical beam profile with a cut through the main axes is shown in Fig. 6.
The compressor is designed for two beams (Fig. 7). Dielectric gratings with 1760 lines/mm and a diffraction efficiency of 96% are used (producer: Plymouth Gratings, USA). This results in an overall transmission through the compressor of 85%. To minimize the footprint the compressor has a folded geometry. The folding mirror is divided in two parts that can be aligned independently. Small differences in the chirp of the two beams can be compensated in that way.
Due to gain narrowing in the amplifiers the bandwidth is reduced to 0.8 nm (FWHM). For both pulses a duration of about 2.2 ps was measured. Figure 8 shows the autocorrelation measurement for one channel. This is close to the Fourier limit, which amounts to 2.0 ps duration for the measured bandwidth of 0.8 nm. Pre- or post-pulses are not visible, even with a longer scan range of 150 ps.
We intend to use the developed Yb:YAG thin-disk laser system as a pump laser for OPCPA systems that should produce high energy pulses of less than 10 fs duration. A precise synchronism between the pulses in the two beams is essential for this application, since they will be used to pump optical-parametric amplifier stages of the same OPA channel. We have measured the synchronism between the two channels of the laser by cross-correlating the two beams in a BBO crystal non-collinearly.
A typical result obtained from these measurements is presented in Fig. 9. The detailed calculation reveals that both pulses show a mutual jitter of < 0.3 ps (rms) and a drift of about 1.1 ps/h. The difference of the optical paths in both amplifier chains is mainly due to air turbulences and due to thermal expansion in the regenerative and in the ring amplifiers. The slow drift, however, can be compensated easily by means of a feedback loop and an appropriate opto-mechanical delay line. Thus the remaining jitter between the pulses of both channels is nearly one order of magnitude less than the pulse duration and should therefore be tolerable in an OPCPA system. Nevertheless, further attention has to be paid to this subject in order to optimize the stability of an OPCPA pumped by this laser.
We have developed a CPA laser system based on Yb:YAG thin-disk technology that generates two synchronized laser pulses with a total energy of 2 x 500 mJ and a pulse duration below 2.2 ps, each. The system is pumped with QCW pump diodes each emitting up to 1.7 kW peak power. The repetition rate is 100 Hz. The final amplifiers reach 0.6 J pulse energy before compression with a beam diameter of 10 mm. This beam diameter was obtained by using Yb:YAG disks of 25 mm diameter.
Part of the project was financed by the European Fund for Regional Development (EFRE) and was supported by Laserlab-Europe (EU-FP7 284464).
References and links
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