Passive compensation of the misalignment instability caused by air convection in thin-disk lasers.

Wavefront distortions caused by the convection of heated ambient air in front of the laser crystal induce severe pump-power-dependent misalignment in thin-disk laser (TDL) resonators. This effect is particularly pronounced in fundamental mode operation and limits the output power when no realignment of the resonator is possible during operation. In this Letter, we present a new approach to passively compensate for this misalignment instability by exploiting the spectral dispersion of a highly efficient grating-waveguide mirror used as a cavity end-mirror in a Littrow configuration. By this, it was possible to almost triple the output power of a fundamental mode Yb:LuAG TDL pumped at 969 nm.

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.

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.

Highly efficient 400 W near-fundamental-mode green thin-disk laser.

We report on the efficient generation of continuous-wave, high-brightness green laser radiation. Green lasers are particularly interesting for reliable and reproducible deep-penetration welding of copper or for pumping Ti:Sa oscillators. By intracavity second-harmonic generation in a thin-disk laser resonator designed for fundamental-mode operation, an output power of up to 403 W is demonstrated at a wavelength of 515 nm with almost diffraction-limited beam quality. The unprecedented optical efficiency of 40.7% of green output power with respect to the pump power of the thin-disk laser is enabled by the intracavity use of a highly efficient grating waveguide mirror, which combines the functions of wavelength stabilization and spectral narrowing, as well as polarization selection in a single element.

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.

Power scaling of fundamental-mode thin-disk lasers using intracavity deformable mirrors.

Since its first demonstration almost two decades ago, the concept of the thin-disk laser has proven its excellent power scaling properties, enabling output powers in the multi-kW-range at decent beam quality. However, power scalability in fundamental-mode operation is limited by severe diffraction losses arising from thermally induced aspherical wavefront distortions in the pumped laser crystal. In order to compensate for these wavefront distortions, we have developed a deformable mirror concept based on pneumatic actuation, providing a step-like surface deformation with adaptable magnitude. Using one of these mirrors, we have achieved output powers as high as 815 W at nearly diffraction-limited beam quality (M2<1.4) from a single disk, which is--to the best of the authors' knowledge-the highest output power at this beam quality demonstrated so far.

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.