We report on energy scaling of a 2.08-µm wavelength regenerative amplifier (RA) system based on the broadband gain material Ho:CaAlGdO4 (CALGO) to multi-mJ pulse energy at kHz repetition rates. Compared to previous reports, energy scaling was enabled thanks to an upgraded seed laser with a higher fluence and better spectral overlap to the gain spectrum of Ho:CALGO, which increased amplification efficiency. Bifurcation-free energy extraction was investigated experimentally and numerically for various repetition rates. A stable output was obtained at 10 W average power for repetition rates of 30 kHz and above. In addition, stable 3.4-mJ energy extraction was achieved at a 1-kHz repetition rate. We discuss the further scaling potential of pulse energy and pulse duration.

Resonant enhancement inside an optical cavity has been a wide-spread approach to increase efficiency of nonlinear optical conversion processes while reducing the demands on the driving laser power. This concept has been particularly important for high harmonic generation XUV sources, where passive femtosecond enhancement cavities allowed significant increase in repetition rates required for applications in photoelectron spectroscopy, XUV frequency comb spectroscopy, including the recent endeavor of thorium nuclear clock development. In addition to passive cavities, it has been shown that comparable driving conditions can be achieved inside mode-locked thin-disk laser oscillators, offering a simplified single-stage alternative. This approach is less sensitive to losses thanks to the presence of gain inside the cavity and should thus allow higher conversion efficiencies through tolerating higher intensity in the gas target. Here, we show that the intra-oscillator approach can indeed surpass the much more mature technology of passive enhancement cavities in terms of XUV flux, even reaching comparable values to single-pass sources based on chirped-pulse fiber amplifier lasers. Our system operates at 17 MHz repetition rate generating photon energies between 60 eV and 100 eV. Importantly, this covers the highly attractive wavelength for the silicon industry of 13.5 nm at which our source delivers 60 nW of outcoupled average power per harmonic order.

Ytterbium (Yb)-doped materials are an excellent choice for efficient and powerful ultrafast lasers. They exhibit favorable emission properties, which include a low quantum defect and compatibility with cost-effective high-power pump diodes. While being strongly beneficial for efficiency, the low quantum defect is a challenge for operation in the few-cycle regime. In the standard collinear pumping scheme, the optical pump light passes a dichroic mirror, whose spectral transmission and dispersion properties can substantially perturbate the oscillating broad-band pulse. Recently, we introduced a broadband cross-polarization pumping scheme that avoids this issue and enables significant improvements in optical-to-optical efficiency and output average power of modelocked few-cycle Yb-bulk laser oscillators. However, our initial prototype relied on a high-power, diffraction-limited fiber laser for pumping. Here, we demonstrate that the broadband cross-polarization pumping scheme is also well-suited for cost-effective multimode diode pumping of Kerr-lens modelocked Yb-bulk lasers. In doing so, we developed a hard-aperture Kerr-lens modelocked Yb:CALGO bulk laser oscillator delivering ultrashort pulses in the sub-30 fs regime. We present two configurations, which both operate at 85 MHz repetition rate. The first delivers 22-fs pulses at 0.3 W of average output power with a 4% optical-to-optical efficiency, while the second produces 28-fs pulses at 0.5 W of average power with a 12% optical-to-optical efficiency. This study validates the high potential of cross-polarization pumping for cost-effective, high-efficiency Yb-lasers operating in the few-cycle regime, achieving shorter pulse durations and higher power levels than previously demonstrated for multimode diode pumping.

We demonstrate a multimode diode-pumped Yb:CALGO laser oscillator based on bandwidth-optimized cross-polarization pumping targeting sub-30-fs operation. In our first proof of principle experiment we achieved mode-locked operation at 83 MHz repetition rate with 0.4 W of average power and a 33-nm-bandwidth optical spectrum supporting sub-40-fs pulses. This concept offers a simple and cost-efficient alternative to green-pumped Ti:sapphire lasers.

Compact high harmonic generation (HHG) systems as affordable tabletop coherent XUV sources are becoming increasingly popular in the last years. Currently, the most successful approaches are based on nonlinearly-compressed chirped-pulse fibre amplifier sources [1], [2] or passive femtosecond enhancement cavities [3]. We develop an alternative approach based on driving HHG directly inside an ultrafast thin-disk laser (TDL) oscillator [4]. This concept offers a low-complexity single-stage solution neither requiring external nonlinear pulse compression or chirped-pulse amplification, nor coherent coupling into an external femtosecond enhancement cavity.

We present on the progress of efficient and powerful Kerr-lens mode-locked Yb:CALGO laser oscillators. Pulses, short as 22 fs are demonstrated using a unique cross-polarized pumping scheme. Also, a 1 GHz-repetition-rate system is shown with a record average power of 6.9 W at 94 fs. Further scaling is discussed.

Thin-disk lasers (TDLs) are best known for their high-power continuous-wave industrial applications. Nonetheless, the thin-disk geometry is also highly attractive for ultrafast laser oscillators. The short propagation distance and large beam diameter inside the gain crystal allows for very low induced nonlinearity, low dispersion, and extreme peak powers inside the laser cavity. The path toward TDL oscillators directly delivering high average power at ultrafast pulse duration required for many scientific applications has, however, been tangled and is still ongoing. A decade ago, the first sub-100-fs laser oscillator is demonstrated, initiating the pursuit of even shorter pulses. Since then, many gain materials have been investigated in the thin-disk geometry as well as various mode-locking mechanisms for their suitability for efficient short-pulse operation. In this review, the fast-evolving development trends of TDL oscillators, as well as their scientific applications and prospects will be discussed.

A self-referenced optical frequency comb is presented based on Kerr-lens mode-locking of ytterbium-doped CALGO. The robust source delivers 3.5 W average power in 44 fs-long pulses at 1 GHz repetition rate. The residual root-mean-square timing jitter of the emitted pulse-train is 146 fs and the residual integrated phase noise of the carrier-envelope offset frequency is 107 mrad, both in a span from 1 Hz to 10 MHz. After stabilization, 2.7 W average power remains for direct application. This work represents the first multi-mode pumped Kerr-lens mode-locked optical frequency comb at gigahertz-level repetition rate.

Thin-disk laser oscillators can nowadays reach few tens of femtosecond pulses at gigawatt-level intracavity powers and megahertz-repetition rates becoming increasingly more powerful sources for intra-oscillator high harmonic generation (HHG). Currently, we can generate high harmonics in neon reaching photon energies of 70 eV, which we expect to increase toward 100 eV in the near future. In parallel, the achievable average and peak output powers of these oscillators in the range of 100 W and 100 MW, respectively, make these sources very promising to drive HHG in single-pass configuration after nonlinear pulse compression. Starting from transform-limited 30 to 50-fs soliton output soliton pulses of TDL oscillators, we will likely see these lasers approaching a single-cycle regime becoming highly attractive sources for attosecond science.

We discuss the recent progress of our intra-oscillator based HHG system operating at 17 MHz repetition rate. We implemented a pierced mirror as an XUV outcoupling mechanism and demonstrated HHG in neon reaching photon energies of 70 eV, which we expect to increase toward 100 eV in the near future.

We report on our results of the coherent combination of four Tm-doped rod-type fiber amplifiers. The chirped pulse amplification system emits an average output power of 188 W and a pulse energy of 1.86 mJ at 100.86 kHz repetition rate. The compressed pulses were measured via second order frequency optical gating. The retrieval reveals a compressed pulse duration of the laser system of 84 fs resulting in peak power of 17.7 GW. The amplifier interferometer was stabilized using locking of coherence by single-detector electronic frequency-tagging and piezo driven mirrors in front of the amplifier channels. The long term stability of the source was tracked with a thermal power sensor over a duration of 120 minutes and shows a stability of <0.1 % rms over this measurement period. To the best of our knowledge, this is the highest average power ultrafast mJ-class short-wavelength infrared laser to date. This proves the applicability of coherent combining techniques in Tm-doped fiber laser systems, opening the route towards performance scaling of ultrafast SWIR laser sources to kW-class average power levels with multi-mJ energies. Additionally this renders this technology the ideal candidate for frequency conversion into the soft X-ray, mid infrared and THz spectral region.

Two-stage multipass-cell compression of a fiber-chirped-pulse amplifier system to the few-cycle regime is presented. The output delivers a sub-2-cycle (5.8 fs), 107 W average power, 1.07 mJ pulses at 100 kHz centered at 1030 nm with excellent spatial beam quality (M2 = 1.1, Strehl ratio S = 0.98), pointing stability (2.3 µrad), and superior long-term average power stability of 0.1% STD over more than 8 hours. This is combined with a carrier-envelope phase stability of 360 mrad in the frequency range from 10 Hz to 50 kHz, i.e., measured on a single-shot basis. This unique system will serve as an HR1 laser for the Extreme Light Infrastructure Attosecond Light Pulse Source research facility to enable high repetition rate isolated attosecond pulse generation.

We present our first results on the coherent combination of four rod type Thulium doped fiber amplifiers. With this we achieved a pulse energy of 1.34 mJ and an average output power of 130 W at a repetition rate of 98 kHz with a compressed pulse duration of 123 fs. This system represents the highest output power mJ class laser system in the SWIR to date and proves the average power and pulse energy scaling capabilities of ultrafast Tm doped fiber amplifiers.

We present a sub-2-cycle laser system combining high average power, pulse energy and repetition rate with CEP-stable operation. The laser system creates 300 fs pulses with 1.8 mJ pulse energy that are nonlinearly post-compressed down to few optical cycles in two subsequent multipass cells (MPC). A pulse duration of 5.8fs (sub-2-cycle) at a pulse energy of 1.1mJ in combination with 110W average power (100 kHz) is achieved. This corresponds to the shortest pulses and highest compressed average power for few-cycle MPCs. Furthermore, the carrier-to-envelope-phase stability amounts to 300 mrad for frequencies above 2 kHz as measured by stereo—above-threshold-ionization (ATI).

Ultrafast thin-disk lasers continue achieving higher average powers at shorter pulse durations, reaching 100-MW peak power directly from the laser oscillator output. We discuss recent progress and its application to intra-oscillator high harmonic generation.

We develop and implement a coated grazing-incidence-plate for broadband and efficient XUV-out-coupling of intra-cavity generated high-harmonics. Moreover, we power-scale our thin-disk laser and reach with 2-GW the highest intra-cavity peak-power of any ultrafast laser oscillator.

We demonstrate efficient short-pulse generation from Yb:YAG in the regime of strong intracavity self-phase-modulation. We reach 102-MW, 52-fs pulses at 103-W average-power with 26% optical-to-optical efficiency, presenting the highest peak-power of any ultrafast laser oscillator.

We develop and implement a coated grazing-incidence-plate for broadband out-coupling of intracavity generated high-harmonics. We reach >25% XUV out-coupling efficiency resulting in 1.2-µW out-coupled average XUV power in a single harmonic at 37-eV and 17-MHz.

We experimentally demonstrate an efficient and broadband extreme-ultraviolet light (XUV) out-coupling mechanism of intra-cavity generated high harmonics. The mechanism is based on a coated grazing-incidence plate (GIP), which utilizes the enhanced reflectivity of s-polarized light in comparison to p-polarized light for large angles of incidence (AoI). We design and produce a 60°-AoI coated GIP, tailored specifically for the high demands inside a sub-50-fs Kerr-lens mode-locked Yb:YAG thin-disk laser oscillator in which high harmonic generation (HHG) is driven at ∼450 MW peak power and 17 MHz repetition rate. The coated GIP features an XUV out-coupling efficiency of >25% for photon energies ranging from 10 eV to 60 eV while being anti-reflective for the driving laser field. The XUV spectra reach up to 52 eV in argon and 30 eV in xenon. In a single harmonic, we out-couple 1.3 µW of XUV average power at 37 eV in argon and 5.4 µW at 25 eV in xenon. The combination of an improved HHG driving laser performance and the out-coupling via the coated GIP enabled us to increase the out-coupled XUV average power in a single harmonic by a factor of 20 compared to previous HHG inside ultrafast laser oscillators. Our source approaches the state-of-the-art out-coupled XUV power levels per harmonic of femtosecond enhancement cavities operating at comparable photon energies.