Ultrafast lasers continue to be at the forefront of many scientific breakthroughs and technological achievements and progress in the performance of these systems continue to open doors in many new ...and exciting interdisciplinary fields. In particular, in the last decade, the average power of ultrafast lasers has seen a significant increase, opening up exciting new perspectives. Among the different technologies that have shaped these advances, thin-disk lasers have generated particularly spectacular breakthroughs. We review here the latest state-of-the-art of the technology and highlight new application fields of these cutting-edge laser systems.
We report on the characterization and first laser operation of ceramic Yb:LuScO
3
in a thin-disk oscillator. The optical performance achieved with a ceramic Yb:LuScO
3
disk is compared to the one ...obtained with an existing ceramic Yb:Lu
2
O
3
disk for reference. The characterization covers the measurement of the fluorescence spectra, the fluorescence lifetimes, and nomarsky imaging. The investigation on the laser operation covers the measurement of resonator losses, output powers, and thermal behavior during continuous-wave operation in a multimode thin-disk oscillator. An average output power of 149 W and a slope efficiency of 51.8% were achieved with the ceramic Yb:LuScO
3
disk which reached a maximum surface temperature of about 150 °C. At the same temperature level, a disk made of the already established ceramic Yb:Lu
2
O
3
delivered 957 W of output power with a slope efficiency of 75.7%.
The development of high-power diode lasers enabled new solid-state laser concepts such as thin-disk, fiber, and Innoslab lasers based on trivalent ytterbium as the laser-active ion, which resulted in ...a tremendous increase in the efficiency and beam quality of cw lasers compared to previously used lamp-pumped rod or slab lasers and the realization of ultrafast lasers with several 100 W or even kilowatts of average power. In addition to their beneficial thermo-optical properties, these architectures offer characteristic benefits making them especially suitable to obtain dedicated laser properties. This review article comprises milestone developments, characteristic challenges, and benefits, and summarizes the state of the art of high-power solid-state lasers with the focus on ultrafast lasers.
We report on the direct comparison of single-crystal and ceramic Yb
3+
:Lu
2
0
3
gain media with respect to emission spectra, fluorescence lifetime, depolarization, and laser performance in a ...continuous-wave thin-disk laser oscillator. The most efficient laser operation was achieved with a single-crystal disk in multimode operation with a slope efficiency of 72.1% and an average output power of 997 W. At the same temperature level, a ceramic disk delivered 861 W with a slope efficiency of 68.6%. In fundamental-mode operation, the highest average power of 360 W and highest optical efficiency of 41.3% were obtained with a ceramic disk. For the single-crystal disk, the fundamental-mode output power was limited to 113 W at an optical efficiency of 29%, potentially due to stress within the crystal.
Numerical simulations were conducted to analyze the influence of the design parameters of tubular inhibited-coupling guiding hollow-core photonic crystal fibers (IC-HCPCFs) on the bending-induced ...phase shift. The possibility to implement polarization-maintaining (PM) tubular IC-HCPCFs using a low-birefringence approach (with a modal birefringence parameter
B
<
7
×
10
-
9
) is discussed. Two different tubular IC-HCPCF designs with 7 and 8 glass capillaries are proposed for operation at a wavelength of 1030 nm. The numerical simulation predicts low guiding losses and a PM behavior sustaining a degree of linear polarization (DOLP) larger than 90% after 10 m of fiber with a bend radius larger than 0.2 m. This shows great potential for high-power beam delivery in an industrial environment. The influence that fabrication deviations have on the polarization-maintaining behavior was also investigated and indicates tight fabrication tolerances for both proposed fiber designs. Small deviations from the ideal symmetrical structure can lead to an enhancement of undesired modal birefringence. To ensure a DOLP larger than 90% at the exit of a 10 m-long fiber and for a bending radius > 0.2 m precise control of the drawing parameters during the fiber production is required, which is challenging but is considered to be technically feasible.
We report on the first demonstration of an intra-cavity spectral beam combining of two fundamental-mode laser beams generated by a dual Yb:YAG thin-disk resonator. The two thin-disk lasers (TDLs) ...were operated at the two slightly different wavelengths of 1028 nm and 1032 nm. A resonant diffraction grating waveguide structure was used as common spectral stabilizer and combiner. An average power exceeding 200 W with close to diffraction-limited beam quality (
M
2
<
1.3
) was obtained with the presented approach.
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•Model to identify optimization strategies and process parameters for the scaling of laser micromachining.•Experimental verification of the model for laser micromachining of pockets ...in silicon using a high-power ultrafast laser.•Scaling to high throughput at high average laser power is possible without exceeding the process limits for high quality.•Promising optimization strategies are low fluences, pulse bursts, increasing the beam diameter and high scanning speeds.•Removal rates exceeding 120 mm3/min achieved with an average laser power of 1.01 kW while maintaining a roughness of 1 µm.
The great potential to scale the productivity of laser micromachining processes offered by the development of ultrafast lasers with average powers exceeding 1 kW comes at the cost of new physical and technical challenges. The large number of adjustable parameters, the different physical process limits, and the various possible optimization strategies complicate the design of suitable laser micromachining processes that enable high quality and high throughput.
In this contribution, a comprehensive model is proposed that helps to identify the optimization strategies and process parameters required for the scaling of laser micromachining to high throughput at high average laser power without exceeding the process limits to ensure high surface quality.
The model was experimentally verified on the example of laser micromachining of pockets in metals and silicon using an ultrafast laser with an average power of 1.01 kW, a pulse duration of 600 fs, different pulse energies, pulse bursts, and beam diameters. As a result, high material removal rates exceeding 120 mm3/min were achieved for silicon, stainless steel, copper, and aluminum, which exceed previously achieved removal rates by one to two orders of magnitude. The machined surfaces exhibit high quality as confirmed by the low measured roughness of about 1 µm.
We report on the generation of high-power ultrashort-pulsed optical vortex beams (VBs) with orbital angular momentum (OAM) of +1 in the infrared (IR) and +2 in the visible (VIS) spectral regions. The ...IR vortex beam was created from a Gaussian beam by employing a nanograting (S-waveplate) mode converter. With this approach we obtained an average power of up to 802 W at a wavelength of λ = 1030 nm in the IR-VB with a pulse energy of 802 µJ, a pulse duration of 460 fs, and a mode-conversion efficiency of 90.1%. By subsequent second harmonic generation, the IR-VB was frequency-doubled inside a lithium triborate (LiB 3 O 5 ) crystal. An average power of up to 320 W at λ = 515 nm was generated, with 320 µJ of pulse energy, 382 fs of pulse duration, and an IR-VB to SH-VB conversion efficiency of 39.9%. The generated IR and VIS VBs were characterized by a Mach-Zehnder type interferometric measurement, confirming the helical wavefronts at these high average powers. These results show that it is possible to efficiently generate high-quality VBs at multi hundreds of watts and confirm the OAM scaling law in harmonic generation processes.
This paper reports the fabrication and first demonstration of all-dielectric crystalline grating–waveguide reflectors comprising a Sc
2
O
3
waveguide grown on a sub-wavelength-patterned sapphire ...substrate. Rigorous coupled-wave analysis is employed to simulate the operation of the structure, suggesting a 100% resonance reflectivity in theory. Structuring of the sapphire substrate is achieved using inductively coupled plasma etching, whilst pulsed laser deposition is used for epitaxial growth of the Sc
2
O
3
crystalline waveguide. Devices with distinct TE- and TM-polarisation resonances around 1030-nm for an angle of incidence near 10° are demonstrated, with reflectance approaching 90%. The discrepancy in reflectivity is attributed to the waveguide thickness variation and surface roughness. Refinement of the fabrication processes and tolerances should lead to improvement in the surface quality of the crystalline grating–waveguide structure and operation closer to the ideal resonance reflectivity.
We report on the generation of a continuous-wave (CW) radially polarized beam with an Yb:Lu
2
O
3
polycrystalline ceramic disk in a thin-disk laser (TDL) oscillator. A circular grating-waveguide ...mirror (CGWM) with a high polarization discrimination given by a reflectivity difference between the two orthogonal polarization states of 44.6% was used as a polarization-selective cavity end-mirror. An output power of 175 W was achieved with an optical efficiency of 39.6%. A high degree of radial polarization of 96.2% and a beam propagation factor of
M
2
hor.
= 2.05 and
M
2
ver.
= 2.32 were measured at the maximum output power.
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