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Silicate bonding is a flexible bonding method that enables room-temperature bonding of many types of materials with only moderate flatness constraints. It is a promising approach for bonding components in high power laser systems, since it results in a thin and low-absorption interface layer between the bonded materials. Here we demonstrate for the first time silicate bonding of a sapphire window to a SEmiconductor Saturable Absorber Mirror (SESAM) and use the composite structure to mode-lock a high-power thin-disk laser. We characterize the fabricated devices both theoretically and experimentally and show how the thermally induced lens of the composite structure can be tuned both in magnitude and sign via the thickness of the sapphire window. We demonstrate mode-locking of a high-power thin-disk laser oscillator with these devices. The altered thermal lens allows us to increase the output power to 233 W, a 70-W-improvement compared to the results achieved with a state-of-the-art SESAM in the same cavity.

High-power ultrafast laser sources are widely used for industrial applications and scientific research. The leading technology to reach high average powers directly from ultrafast oscillators are SEmiconductor Saturable Absorbed Mirror (SESAM) modelocked Thin-Disk Lasers (TDLs), with average powers up to 350 W [1] . When power-scaling TDLs, thermally induced lensing, especially in the SESAM, is one of the key challenges that needs to be addressed. For this reason, many strategies to reduce the thermal lensing of the SESAM have been investigated, such as thinner substrates, improved contacting [1] , or substrate transfer followed by etching [2] .

We demonstrate for the first time silicate bonding of a SESAM to a sapphire superstrate to control the sign and magnitude of the SESAMs thermal lensing. We demonstrate modelocking in a 233-W average-power thin-disk laser.

We report a semiconductor saturable absorber mirror (SESAM)-modelocked thin-disk laser oscillator delivering a record 350-W average output power with 940-fs, 39-µJ pulses at 8.88-MHz repetition rate and 37-MW peak power. This oscillator is based on the Yb:YAG gain material and has a large pump spot on the disk. The cavity design includes an imaging scheme, which results in multiple reflections on the disk gain medium to enable a larger output coupling rate compared to those used in thin-disk oscillators with a single reflection on the disk. This reduces the intracavity power for a given output power, thus decreasing the stress on the intracavity components. We operate the laser in a low-pressure environment in order to limit the disk's thermal lensing and drastically reduce the nonlinearity picked up in the intracavity air medium. The combination of the imaging scheme and low-pressure operation paves the way to further power scaling of ultrafast thin-disk oscillators toward the kW milestone.

Ultrafast lasers are ubiquitous in industry and science. Due to the intense research in this field, the output power available from these sources saw impressive developments over the last two decades. Thin-disk lasers (TDLs) have been one of the major technologies enabling this revolution in high power laser sources. TDL oscillators currently achieve the highest output power and pulse energy of any ultrafast laser technology [1, 2]. Here, we present a new record average-output-power result for any modelocked oscillator. We demonstrate 350 W with 940-fs pulses at the output of an Yb:YAG thin-disk oscillator modelocked with a semiconductor saturable absorber mirror (SESAM) (Fig. 1a).

High-power ultrafast laser sources constitute a key technology to a wide variety of scientific and industrial applications that benefit from the combination of high average power and sub-ps pulse duration. While coherently combined amplifier systems now deliver kW-level average powers [1], ultrafast thin-disk lasers (TDLs) remain of considerable interest as compact, low-noise laser sources delivering multi-hundred-Watts with excellent beam quality. These oscillators are usually modelocked using either of two well-established techniques: SESAM or Kerr-lens modelocking (KLM) [2]. Recently, we demonstrated the first TDL modelocked using the frequency-doubling nonlinear mirror (NLM) [3, 4]. This technique relies on the combination of an intracavity χ(2) crystal used for second-harmonic generation (SHG) and a dichroic output coupler mirror (OC) that is highly reflective for the second harmonic (SH) and partially reflective for the fundamental wave (FW) (Fig. 1b). The NLM device (SHG crystal + dichroic OC) thus provides a saturable reflectivity (Fig. 1f) enabling modelocked operation. Our first NLM-modelocked TDL delivered 323 fs at a repetition rate of 17.8 MHz (Fig. 1a) [3].

Ultrafast high-power laser sources have a crucial role in science and industry. One way to reach performance in the multi-100-W average output power with sub-ps, tens-of-mJ pulses is through thin-disk laser (TDL) oscillators [1]. The oscillator approach to high power, compared to amplifier systems, offers superior beam quality and reduced system complexity but comes at the expense of a challenging nonlinearity management and a high sensitivity to thermal lensing. In particular, the MW-level intracavity peak power leads to a large amount of self-phase modulation (SPM) picked up in the intracavity air. The SPM needs to be compensated with negative group-delay dispersion (GDD) to ensure stable soliton pulse formation. Hence, there is a trade-off in GDD versus pulse energy for TDLs operated in air ("Standard TDLs" in Fig. 1a). Dispersive mirrors can provide the required GDD but, due to their resonant structure, they are more subject to thermal effects and damage compared to standard dielectric mirrors. A workaround is to operate the TDL in vacuum ("Vacuum TDLs" in Fig. 1a).

We present a 350-W 940-fs thin-disk oscillator. We achieve this new record-high average power ultrafast oscillator through vacuum operation, multiple passes on the disk, and large pump spot. Systematic power-scaling through multi-pass cavities is discussed.

Beam quality plays a pivotal role in modelocking high-power oscillators. We identifythe causes, which determine the beam quality in thin-disk lasers and suggest guidelines tomaximize the power range of optimal beam quality. © 2019 The Author(s)

We unveil a gas-lens effect in kW-class thin-disk lasers, which accounts in our experiments for 33% of the overall disk thermal lensing. By operating the laser in vacuum, the gas lens vanishes. This leads to a lower overall thermal lensing and hence to a significantly extended power range of optimal beam quality. In our high-power continuous-wave (cw) thin-disk laser, we obtain single-transverse-mode operation, i.e. M

We present the first nonlinear-mirror modelocked thin-disk laser, delivering 21 W at 323 fs pulse duration. This opens a new chapter for the nonlinear-mirror technique which, until now, only modelocked few-ps bulk oscillators.

We demonstrate a compact XUV source based on intracavity high-harmonic generation driven inside a modelocked thin-disk laser oscillator, generating photons with energy up to 20.4 eV (17th harmonics) at a repetition rate of 17.35 MHz.

We use a phase-mismatched χ(2) crystal to cancel the nonlinearities caused by intracavity air. We obtain 153 W average output power in an air-filled cavity, which is a record value for non-vacuum/non-helium SESAM modelocked lasers.

We exploit cascaded-χ(2) nonlinearities in an intracavity second-harmonic-generation crystal to cancel the self-phase modulation from air in a thin-disk oscillator. We obtain 210-W output power, a record value for a SESAM-modelocked laser operated in air.

We use a phase-mismatched χ(2) crystal to cancel the intracavity self-phase-modulation from air. We obtain 210-W average output power, a record value for SESAM-modelocked thin-disk lasers operated in air without complex vacuum or helium purging.

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.

We demonstrate a frequency-doubling nonlinear-mirror (NLM) modelocked thin-disk laser. This modelocking technique, composed of an intracavity second harmonic crystal in combination with a dichroic output coupler, offers robust operation decoupled from cavity stability (as in semiconductor saturable absorber mirror (SESAM) modelocking) combined with an ultrafast saturable loss and high modulation depth (as in Kerr-lens modelocking (KLM)). With our NLM diode-pumped Yb:YAG thin-disk laser we achieve 21 W of average power at 323-fs pulse duration, which is an order of magnitude shorter than the previously obtained duration with the same technique in bulk lasers. Using these first results, we present a theoretical model for the NLM technique, which accurately predicts its loss modulation properties and the shortest achievable pulse duration without relying on any fitting parameters. Based on this simulation, we expect that the NLM technique will enable thin-disk lasers with average power of more than 100 W, with potentially sub-200 fs pulses. This could potentially solve the pulse duration limitations with SESAM modelocked Yb:YAG thin-disk lasers without imposing strong cavity stability constraints such as in KLM.

We present a high-peak-power SESAM-modelocked thin-disk laser (TDL) based on the gain material Yb-doped lutetia (Yb:Lu

Ultrafast laser sources are opening new industrial and scientific applications. In particular, SESAM-modelocked thin-disk laser (TDL) oscillators are promising candidates to replace complex amplifier systems by compact high-power oscillators combining high pulse energy and MHz repetition rate. Record-high average powers up to 275 W and peak powers up to 66 MW have been achieved with SESAM-modelocked Yb:YAG TDLs [1]. However, reducing the pulse duration to sub 200 fs while simultaneously increasing the peak power of SESAM-modelocked TDLs remains an ongoing challenge. The sesquioxide Yb:Lu 2 O 3 (Yb:LuO) is a potentially ideal candidate material as it combines a broad emission bandwidth and a high thermal conductivity [2]. Earlier Yb:LuO TDL results include average powers up to 140 W (740 fs) or pulse durations down to 140 fs (7 W), in all cases at moderate peak powers

We present the first nonlinear-mirror modelocked thin-disk laser. We achieve 21 W of average power at 324 fs of pulse duration, which is an order-of-magnitude shorter than previously demonstrated with this technique in bulk 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.

We demonstrate HHG inside a modelocked thin-disk laser. Diode-pumped at only 48 W, it drives HHG in xenon at 2.6×1013 W/cm2 and 300 W average power, generating XUV-light down to 60.7-nm at 17-MHz repetition rate.

Ultrafast laser sources are one of the main achievements of the past decades. Finding new avenues to obtain higher average powers and pulse energies from these sources is currently a topic of important research efforts both for scientific and industrial applications. SESAM modelocked thin-disk lasers are one of the most promising laser technology to reach this goal from table-top systems: recently, average powers of 275 W and pulse energies of 80 μJ were demonstrated directly from a modelocked oscillators without additional external amplification. In this presentation, we will review the current state-of-the art of such table-top systems and present guidelines for future kilowatt-class systems.

We present a Yb:GGG thin-disk laser for the first time. We demonstrate more than 50 W of output power in multimode operation at 67% slope efficiency, which is similar to values achieved with Yb:YAG.

We demonstrate the first Kerr lens mode-locked Yb:CaGdAlO4 (Yb:CALGO) thin-disk laser oscillator. It generates pulses with a duration of 30 fs at a central wavelength of 1048 nm and a repetition rate of 124 MHz. The laser emits the shortest pulses generated by a thin-disk laser oscillator, equal to the shortest pulse duration obtained by Yb-doped bulk oscillators. The average output power is currently limited to 150 mW by the low gain and limited disk quality. We expect that more suitable Yb:CALGO disks will enable substantially higher power levels with similar pulse durations.

We present the first demonstration of a thin-disk laser based on the gain material Yb:GGG. This material has many desirable properties for the thin-disk geometry: a high thermal conductivity, which is nearly independent of the doping concentration, a low quantum defect, low-temperature growth, and a broadband absorption spectrum, making it a promising contender to the well-established Yb:YAG for high-power applications. In continuous wave laser operation, we demonstrate output powers above 50 W, which is an order of magnitude higher than previously achieved with this material in the bulk geometry. We compare this performance with an Yb:YAG disk under identical pumping conditions and find comparable output characteristics (with typical optical-to-optical slope efficiencies >66%). Additionally, with the help of finite-element-method simulations, we show the advantageous heat-removal capabilities of Yb:GGG compared to Yb:YAG, resulting in >50% lower thermal lensing for thin Yb:GGG disks compared to Yb:YAG disks. The equivalent optical performance of the two crystals in combination with the easy growth and the significant thermal benefits of Yb:GGG show the large potential of future high-power thin-disk amplifiers and lasers based on this material, both for industrial and scientific applications.

We measured, for the first time, a gas-lens effect accounting for 33% of the total disk thermal lensing. By helium flooding the laser, we obtain optimal beam quality (M2

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:Lu2O 3 and Yb:CaGdAlO 4 (Yb:CALGO) laser (cf. Tab. 1). Our Kerr lens mode-locked (KLM) Yb:Lu 2 O 3 TDL generates 4.5 W in 49-fs pulses and 1.7 W in 40-fs pulses (being 40 % shorter than Yb:Lu 2 O 3 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].