The thin-disk laser technology allows for lasers with high pulse energy (several hundred mJ to Joule level) and repetition rates
of up to the kHz level. A disk with a thickness between several 100 μm and 1 mm
is used as the active medium. Compared to laser rods, the cooling
efficiency of thin disks is much better, which allows for a
higher repetition rate. The output energy is in principle
scalable with the disk diameter.
We have worked on the development of thin-disk lasers since several years.
The goal was a system delivering laser pulses with > 1 J at a repetition rate of 100 Hz and a pulse duration of a few ps.
We have reached these parameters in 2015.
At present, we are concentrating on optimizing our thin-disk lasers for several
applications. One system is used as a driver for an incoherent laser plasma source
emitting in the wavelength range from several 100 eV to the keV level.
An other system is used as pump source for an OPCPA system being developed to
generate few fs pulses with energy up to 40 mJ.
Both systems use diode-pumped Yb:YAG disks as the laser material in combination with CPA technology. The systems work at 100 Hz repetition rate
emitting 2 - 5 ps pulses with up to 300 mJ pulse energy.
In the next paragraphs the basic principle of thin-disk technology is explained and
the setup of the two laser systems used for
soft x-ray generation and
as pump laser for the OPCPA system
is described. Some significant results of our thin-disk laser development are presented in the final paragraph.
Thin-disk technology compared to
In conventional lasers using laser rods as
active material the heat dissipation is a major challenge.
Cooling the rod via its surface is rather inefficient due to
the large volume to surface ratio. Additionally, it generates
an temperature gradient perpendicular to the laser axis which
results in thermal lensing. The use of a thin disk that is
cooled from one side instead of a rod is the logical step
towards better cooling and consequently higher average output
power. Since a thin disk shows a much lower amplification than
a thick rod it is important to find the best compromise between
high amplification and good cooling efficiency. The following
numerical simulation gives an overview on how to find the right
Numerical simulation of thin disks
We have numerically calculated the
temperature distribution in thin disks by solving the heat
equation in three dimensions. The real pump parameters of our
laser setup were used as the starting parameters. The disk is
pumped from above and cooled from below:
The figure below shows the simulated setup. The pump radiation
of 5 kW/cm2 was focused in a pump spot of 10 mm
diameter from the top on the thin disk. The latter is in
thermal contact to a heat sink.
- Absorbed intensity during pumping: Iabsorbed =
- Duration of the pump pulse: tpump = 1 ms
- Repetition rate: f = 200 Hz
- Wavelength of the pump radiation: λ = 940 nm
- Quantum defect d = 8 %
Fig. 1: Setup for the simulation of
temperature distribution inside a pumped laser disk.
In our simulation, disks of various thicknesses were pumped in
such a manner that the energy stored in the disks is identical.
The following figure shows the temperature profiles obtained
for various thicknesses of the disk. The plot below depicts the
increase of the temperature at the front surface compared to
the cooled back surface of the disk. Despite of the identical
stored energy in the disks, the temperature difference between
front surface of the disk and its cooled back side increases
strongly with the thickness of the disk. One can see that the
temperature difference between the front side and the cooled
back side of the disk amounts to only 14 deg for the 0.5 mm
thick disk used in our amplifiers, while this difference would
exceed 80 deg for a hypothetical 4 mm thick disk.
Fig. 2: Simulation of the temperature
distribution for different disk thicknesses.
For use of the thin disks as amplifying
elements, the wavefront perturbation due to the Optical Path
Difference (OPD) is the most important number. The figure below
shows the result: While the OPD is in the order of 0.1 μm
for the 0.5 mm disks used in our amplifier, an OPD of 5 μm
would be obtained for a very thick disk of 4 mm thickness.
Fig. 3: Maximum temperature on disk surface
and optical path difference for different disk thicknesses.
The simulations show clearly, that disks with a thickness
between 0.5 an 1 mm should be used in the setup of our
Implementation and design of thin-disk
The key element of a thin-disk amplifier is
of course the thin laser disk installed in a cooling finger.
The disk has an anti-reflection coating on its front side and a
high reflection coating on its back side. The back side is
attached onto a water cooled heat sink. Therefore, the cooled
surface is very large compared to the pumped volume and
additionally the temperature gradient is parallel to the
incident laser light, reducing the thermal lens effect. Due to
the efficient cooling thin-disk technology is one of the
promising choices for high repetition rate laser systems. This
technology is known for its very good beam quality. It is used
in industrial lasers and extremely stable and reliable
operation has been demonstrated. Besides the well prepared thin-disk
the other key element is the pumping head. It it designed
in a multipass geometry that the pump light is guided several
times onto the laser disk. Since the disk has a thickness of
the order of 500 μm a single pass absorption would be around
only 25 %. Fig. 4 shows a sketch of a pump module.
Fig. 4: Scheme and image of a thin-disk laser
The laser disk is surrounded by a number of
prisms. In front of the disk a large parabolic mirror images
the exit of the pump diode fiber magnified onto the laser disk.
A fraction of the pump energy is absorbed on its way to the
back side of the disk. Here the remaining part is reflected and
partly absorbed on its way out of the disk. The prisms around
the disk displace the pump beam and guide it back to the disk.
Nearly 100% of the pump light can be absorbed with this
In principle, thin-disk technology is scalable in pulse energy
by enlarging the pump energy proportional to the pump beam size
on the laser disk and keeping the pump intensity at a constant
level. This of course requires also disks with appropriate
diameter. An important part of the project was therefore the
development of bonding methods of large disks to an appropriate
System set up
Two Yb:YAG thin-disk laser systems are presently operated in the framework of this projekt:
Both laser systems have a similar layout. They consist of a front-end with an Yb:KGW oscillator,
a grating stretcher, and an Yb:KGW regenerative
amplifier. The front-end delivers laser pulses stretched from about 200 fs to 2 ns with 0.3 mJ of pulse energy.
These laser pulses are further used to seed a regenerative amplifier based on Yb:YAG thin-disk technology.
For pumping the OPCPA system the laser pulses from the front-end are split to seed two regenerative thin-disk amplifiers.
The two amplified laser pulses are then compressed to 2ps - 5ps and converted in a BBO crystal to 515nm wavelength (s. Fig. 6).
In total two beams with 120 mJ pulse energy, each, and 100 Hz repetition rate are provided as pump for the OPCPA system.
A sketch of the setup is shown in Fig. 5.
Fig. 5: Setup of the thin-disk laser for pumping an OPCPA system.
Fig. 6: Conversion-efficiency from 1030nm to 515nm.
For the soft X-ray source the laser pulses from only one regenerative amplifier are used. Depending on
the required soft X-ray spectrum the amplified pulse can be compressed to 2 ps or 200 ps or it can used uncompressed with 2 ns pulse
duration. The pulse energy can be as high as 180 mJ at 100 Hz repetition rate. Fig. 7 shows the setup of the system separated
from the other part of the lab by a laser safety curtain.
Fig. 7: Setup of the thin-disk laser for pumping an soft X-ray source.
Fig. 8: The pulse duration is measured to be less than 2 ps.
Fig. 9: Measurement of the pointing stability gives 1.1 μm (rms).
The heart of the amplifier is the thin-disk pump
module. The Yb:YAG amplifier disk is mounted on a cooling
finger. The cooling finger as well as the Yb:YAG disk are
manufactured by TRUMPF GmbH. The complete back side of the disk
is water cooled, which guarantees efficient cooling and a low
thermal lens effect. By use of a large spherical mirror in
front of the disk and some displacement prisms the pump beam is
guided 6 times onto the Yb:YAG disk (12 passes through the disk) to be absorbed nearly
completely. The amplifiers are pumped by fibre
coupled laser diodes at a wavelength of 940 nm. The diodes can be operated with a duty cycle of
up to 20%. This corresponds to a repetition rate of up to 200 Hz with a pump
pulse duration of 1 ms or 800 Hz with 0.25 ms pump pulse length.
The diodes were specially developed at Ferdinand Braun Institut (FBH)
as a pulsed pump source for Yb:YAG.
In the resonator of the regenerative amplifier the Yb:YAG thin
disk is used as one end-mirror. Additional spherical mirrors in
the cavity are used to adjust the beam diameter on the disk to
the pump beam diameter. Fig. 10 shows a sketch of a typical setup of the regenerative amplifier.
Fig. 10: Setup of the regenerative thin-disk amplifier.
Fig. 11 shows the output pulse energy
vs. pump power for a regenerative amplifier.
A maximum output energy of 307 mJ could be extracted at a pump power of about 1.75 kW and 56 round-trips. The
This corresponds to an optical-to-optical
efficiency of more than 17%. An excellent stability below 0.3% rms (s. Fig. 12) was measured
over a period of about 2.5 h. The output beam is highly
collimated. An M2 measurement resulted in a value of
better than 1.2 with a minimum focal spot size of 80 μm at f = 300 mm.
The pointing stability behind a f = 1 m lens is +/- 2 μm (rms), measured over 30 min.
Fig. 11: Output pulse energy of a reg. amplifier.
Fig. 12: Stability measurement (energy and pointing) of a regenerative amplifier.
Fig. 13: Adjusting the multipass amplifier.
Further amplification of the laser pulse is
done by a multipass or a ring amplifier. Several multipass geometries
have been investigated. Laser pulses with more than 500 mJ pulse energy could be demonstrated with these multipass amplifiers.
During several years one system delivering 400 mJ on a daily basis was running as driver for the X-ray laser from
The main drawback of multipass amplifier systems is the high optical quality required for the laser disk. A not perfect disk distorts
the beam on every reflection a bit more. Usually, after 4 or 5 reflections (amplification steps) the beam cannot be further amplified.
An adaptive mirror that compensates the distortions induced by the disk or a spatial filter cleaning the laser beam is required for
a better performance. Therefore we have also set up a ring amplifier with integrated spatial filter. A schematic sketch is given in Fig. 14.
The laser pulse is injected via a polarizer. A halfwafe-plate rotates the polarization in such a way that the pulse stays for two round-trips in the
amplifier. The amplifier is equipped with two amplifier heads. Therefore, in total we have 4 amplification steps within the amplifier.
Between the two amplifier heads a spatial filter is installed.
With this kind of amplifier the laser pulses were amplified to more than 1 J pulse energy at 100 Hz repetition rate.
The seed pulse energy from a regenerative amplifier was 300 mJ. Fig. 15 shows the amplification curve of the system.
Fig. 14: Scheme of the ring amplifier with integrated spatial filter.
Fig. 15: Output of the ring amplifier depending on the seed pulse energy.
Summary and Outlook
Due to the high cooling efficiency and low
thermal lens effect the thin-disk technology is the promising
approach for lasers with high pulse energy and medium
repetition rate. We have developed a diode pumped CPA laser
system based on Yb:YAG thin-disk technology as a driver for an
X-ray laser as well as a pump laser for OPCPA. Another system
for further development of the thin-disk laser technology is
currently under construction.
Pulse energies of up to 550 mJ have been reached on an
experimental basis. For daily operation about 400 mJ pulse
energy (ns pulse) corresponding to 300 mJ in a compressed ps
pulse can be used.
The regenerative amplifier shows a very good stability of
better than 0.5% (rms) measured over a period of several hours as well as
an excellent beam quality with M2 better than 1.2.
Such a system was also constructed for the Institut de la Lumière Extrême (ILE) . ILE
works on the development of the 10 Petawatt short pulse laser
system APOLLON. This laser system will use a thin-disk laser
developed at MBI for driving an OPCPA stage. The technology
used for APOLLON is planned to be used also for ELI.
Since thin-disk technology is in principle scalable in output
energy a further increase into the multi Joule level should be
possible. Large Yb:YAG disks with more than 25 mm in diameter
must be developed to increase the output pulse energy. The
bonding technology in order to contact such large disks to an
appropriate heat sink is one of the main tasks. Since the optical
quality of the amplifier medium is very critical for thin-disk laser amplifiers
this is one of the most important subjects.
- R. Jung, J. Tümmler, T. Nubbemeyer, and I. Will, "Thin-disk ring amplifier for high pulse energy,"
Opt. Express 24, 4375-4381 (2016).
- R. Jung, J. Tümmler, and I. Will, "Regenerative thin-disk amplifier for 300 mJ pulse energy,"
Opt. Express 24, 883-887 (2016).
- R. Jung, J. Tümmler, T. Nubbemeyer, and I. Will, "Two-Channel Thin-Disk Laser for High Pulse Energy,"
in Advanced Solid State Lasers, OSA Technical Digest (online) (Optical Society of America, 2015),
- J. Tümmler, R. Jung, H. Stiel, P.V. Nickles, W. Sandner, "High-repetition-rate chirped-pulse-amplification
thin-disk laser system with joule-level pulse energy,"
Opt. Lett. 34, 1378 (2009).
- I. Will, J. Tümmler, T. Nubbemeyer, R. Jung, W. Sandner, "Vorrichtung zur Verstärkung von
gepulster Laserstrahlung mit hoher Energie der Laserpulse und hoher mittlerer Leistung,"
Patent number 10 2013 208 377.7
Collaborations and Funding
Ferdinand Braun Institute (FBH)
Institut de la Lumière Extrême (ILE)
Institut für technische Physik der DLR
Institut für Strahlwerkzeuge (IFSW), Uni