Stefan Piehler

250 W single-crystal fiber Yb:YAG laser.

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.

Amplification of cylindrically polarized laser beams in single crystal fiber amplifiers

Yb:YAG single crystal fiber (SCF) amplifiers have recently drawn much attention in the field of amplification of ultra-short pulses. In this paper, we report on the use of SCF amplifiers for the amplification of cylindrically polarized laser beams, as such beams offer promising properties for numerous applications. While the amplification of cylindrically polarized beams is challenging with other amplifier designs due to thermally induced depolarization, we demonstrate the amplification of 32 W cylindrically polarized beams to an output power of 100 W. A measured degree of radial polarization after the SCF of about 95% indicates an excellent conservation of polarization.

Deformable mirrors for intra-cavity use in high-power thin-disk lasers

We present deformable mirrors for the intra-cavity use in high-power thin-disk laser resonators. The refractive power of these mirrors is continuously adaptable from -0.7 m-1 to 0.3 m-1, corresponding to radii of curvature ranging between 2.86 m (convex) and 6.67 m (concave). The optimized shape of the mirror membrane enables a very low peak-to-valley deviation from a paraboloid deformation over a large area. With the optical performance of our mirrors being equal to that of standard HR mirrors, we were able to demonstrate the tuning of the beam quality of a thin-disk laser in a range of M2 = 3 to M2 = 1 during laser operation at output powers as high as 1.1 kW.

Self-compensation of thermal lensing in optics for high-brightness solid-state lasers

The applicability of modern high-brightness solid-state laser sources for material processing purposes is limited by thermally induced effects in the beam guidance optics. These transient thermo-optical aberrations lead to a non-static focus shift and a deterioration of the beam quality. To counterbalance these aberrations, thermally self-compensated optical systems are proposed. The design for these optics is based on a combination of several elements that compensate for each other. Via thermo-mechanical FEM-simulations and the calculation of the resultant wavefront distortion, several multi-component systems are evaluated. In a first step, thermally compensated laser windows have been developed and characterized.

Active mirrors for intra-cavity compensation of the aspherical thermal lens in thin-disk lasers

With the development of the thin-disk laser, thermal lensing issues have been drastically reduced. However, fundamental mode operation at high output powers is still limited by severe efficiency reductions due to diffraction losses introduced by the temperature gradient at the boundary of the pump spot. To countervail these aspherical wavefront deformations, high-power laser mirrors featuring deformable surfaces have been developed. The surface of these mirrors resembles the shape of the thermally induced change of optical path length in the pumped laser crystal. The magnitude of the surface deformation can be actively controlled to provide optimal compensation at different power levels.

Highly-efficient continuous-wave intra-cavity frequency-doubled Yb:LuAG thin-disk laser with 1 kW of output power

We report on the generation of continuous-wave, intra-cavity frequency-doubled, multi-mode laser radiation in an Yb:LuAG thin-disk laser. Output powers of up to 1 kW at a wavelength of 515 nm were achieved at an unprecedented optical efficiency of 51.6% with respect to the pumping power of the thin-disk laser. The wavelength stabilization and spectral narrowing as well as the polarization selection, which is necessary for a stable and efficient second-harmonic generation, was achieved by the integration of a diffraction grating into the dielectric end mirror of the cavity, which exhibits a diffraction efficiency of 99.8%. At a frequency-doubled output power of 820 W the peak-to-valley power fluctuations measured during 100 minutes of laser operation amounted to only 8.2 W (1.0%). The beam parameter product of the frequency-doubled output was 3.4 mm·mrad (M2 ≈ 20), which is suitable for standard beam delivery using fibers with a core diameter of 100 µm and a NA of 0.2.

Recent Progress in Thin-disk Lasers Based on Various Yb-doped Materials

We will give an overview about the current status of thin-disk laser development in the cw and ultrafast regime (oscillators and amplifiers) with a special emphasis on our group’s activities.

250 W single crystal fiber Yb:YAG laser

We demonstrate an Yb:YAG single crystal fiber laser with 251 W output power and 44 % efficiency. Oscillator performance shows potential power amplification from 40 W to 200 W in a double pass configuration.

Modellierung optisch adressierter Spiegel für adaptive Hochleistungslaser

Zusammenfassung Optisch adressierte deformierbare Spiegel (OADMs, engl. für optically addressed deformable mirrors) können in Hochleistungslasern zur Kompensation thermisch induzierter Störungen und zur aktiven Strahlformung verwendet werden. In diesem Artikel wird ein Modellierungsansatz vorgestellt, der die vom OADM absorbierte Adressierstrahlung als verteilt-wirkenden Stelleingriff berücksichtigt. Zudem werden durch den Primärlaser verursachte Stördeformationen dargestellt. Mit geometrisch-physikalischen Argumenten wird das kontinuumsmechanische Modell vereinfacht: Da der Wärmefluss in Tiefenrichtung dominiert, kann diese Koordinate mittels partieller Ortsdiskretisierung als verteilte Größe beibehalten werden. Das resultierende verteilt-parametrische Entwurfsmodell stellt die Basis für late lumping-Steuerungs- und Regelungsmethoden dar und wird anhand experimenteller Daten validiert.

Intra-cavity compensation of wavefront distortions in kW-level thin-disk lasers

Thermally induced wavefront distortions have been a major limitation for the power scalability of high-brightness solid-state lasers. The concept of the thin-disk laser has led to significant improvement in this respect [1], enabling output powers in the multi-kW range both in multimode as well as in close to fundamental-mode operation [2, 3]. Even though significantly reduced in comparison to other types of lasers, however, thermally induced wavefront distortions in the thin-disk laser crystal eventually limit the scalability of the achievable output power at constant beam quality.