Norbert Modsching

Lasing in thulium-doped polarizing photonic crystal fiber

We describe lasing of a thulium-doped polarizing photonic crystal fiber. A 4 m long fiber with 50 μm diameter core, 250 μm diameter cladding, and d/Λ ratio of 0.18 was pumped with a 793 nm diode and produced a polarized output with a polarization extinction ratio (PER) of 15 dB and an M(2) of <1.15. An intracavity polarizer and half-wave plate minimally increased the PER to 16 dB. The output power had 35% slope efficiency relative to the absorbed pump power. The maximum cw output power was limited to 4 W due to the quantum defect heating of the fiber.

Q-switched thulium-doped photonic crystal fiber laser

We report a Q-switched thulium doped polarizing photonic crystal fiber producing 435 μJ, 49 ns pulses at 10 kHz. The 8.9 kW peak power is the highest of any nanosecond Tm:fiber oscillator published to date.

Generation of 35-fs pulses from a Kerr lens mode-locked Yb:Lu2O3 thin-disk laser

We investigate Kerr lens mode locking of Yb:Lu2O3 thin-disk laser oscillators operating in the sub-100-fs regime. Pulses as short as 35 fs were generated at an average output power of 1.6 W. These are the shortest pulses directly emitted from a thin-disk laser oscillator. The optical spectrum of the 35-fs pulses is almost 3 times broader than the corresponding emission band of the gain crystal. At slightly longer pulse duration of 49 fs, we achieve an average power of 4.5 W. In addition, 10.7 W are obtained in 88-fs pulses, which is twice higher than the previous power record for ultrafast thin-disk lasers generating pulses shorter than 100 fs. Our results prove that Kerr lens mode-locked Yb:Lu2O3 thin-disk lasers are a promising technology for further average power and pulse energy scaling of ultrafast high-power oscillators operating in the sub-100-fs regime.

Extreme ultraviolet light source at a megahertz repetition rate based on high-harmonic generation inside a mode-locked thin-disk laser oscillator

We demonstrate a compact extreme ultraviolet (XUV) source based on high-harmonic generation (HHG) driven directly inside the cavity of a mode-locked thin-disk laser oscillator. The laser is directly diode-pumped at a power of only 51 W and operates at a wavelength of 1034 nm and a 17.35 MHz repetition rate. We drive HHG in a high-pressure xenon gas jet with an intracavity peak intensity of 2.8×1013  W/cm2 and 320 W of intracavity average power. Despite the high-pressure gas jet, the laser operates at high stability. We detect harmonics up to the 17th order (60.8 nm, 20.4 eV) and estimate a flux of 2.6×108  photons/s for the 11th harmonic (94 nm, 13.2 eV). Due to the power scalability of the thin-disk concept, this class of compact XUV sources has the potential to become a versatile tool for areas such as attosecond science, XUV spectroscopy, and high-resolution imaging.

Lasing in Thulium doped Polarizing Photonic Crystal Fibers (PCF)

We describe lasing in polarizing thulium doped PCF fiber for the first time. The ~4 m long fiber had 50/250 μm core/cladding diameters and hole-diameter to pitch ratio of 0.18. CW lasing was achieved by end pumping with a 793 nm diode in an oscillator configuration. Slope efficiencies of ~35 % have been obtained with single mode beam quality (M2 <1.15). The polarization extinction ratio (PER) was >13 dB without any intra-cavity polarizing elements. This fiber, with MFD of ~36 μm and ~5.8 dB/m cladding pump absorption, is an attractive option for high energy pulsed amplifiers in the 2 μm wavelength regime.

Sub-50-fs Kerr lens mode-locked thin-disk lasers

Ultrafast thin-disk lasers (TDLs) are often thought to deliver longer pulses than bulk lasers (Fig. 1a). Here we prove that this assumption is wrong and demonstrate the shortest pulses from any Yb:Lu2O3 and Yb:CaGdAlO4 (Yb:CALGO) laser (cf. Tab. 1). Our Kerr lens mode-locked (KLM) Yb:Lu2O3 TDL generates 4.5 W in 49-fs pulses and 1.7 W in 40-fs pulses (being 40 % shorter than Yb:Lu2O3 bulk lasers [1]). In addition, we demonstrate the first KLM Yb:CALGO TDL. It generates 30-fs pulses, which is the shortest duration ever obtained from ultrafast TDLs [2,3] and equal to the shortest pulses obtained from Yb-bulk oscillators [4].

Towards Few-Cycle Ultrafast Thin-Disk Lasers

We evaluate limitations in pulse duration for Kerr-lens mode-locked Yb-based thin-disk lasers. The most critical factor is appropriate intracavity dispersion engineering, which enabled operation at 30-fs. Substantially shorter durations are within reach using new designs.

Yb-doped rod-type fiber amplifier with 2 kW average power

The effect of mode instabilities has become a strong limitation for the average power scaling of fiber lasers and amplifiers with nearly diffraction-limited beam quality. In this contribution it is shown that the output average power of rod-type fiber amplifiers can be efficiently increased well beyond the threshold of mode-instabilities. This way a new average output power record of 2 kW, with an M2 of 3, emitted from a 1 meter long rod-type fiber amplifier has been achieved.