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.

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.

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 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.

Nonlinear refractive index of the intracavity air is a problem. We review how the negative phase shift achievable from cascaded nonlinearities can cancel the positive phase shift from air to support 210-W average output power. © 2019 The Author(s)

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).

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).

We present a first power-scaled nonlinear-mirror (NLM) modelocked thin-disk laser based on an Yb-doped gain material. The laser oscillator delivers average output powers up to 87 W and peak powers up to 14.7 MW with sub-600-femtosecond pulses at ≈9-MHz repetition rate. We demonstrate a threefold improvement in average output power and sixfold improvement in pulse energy compared to previous NLM-modelocking results. We obtain peak powers in excess of 10 MW for the first time from an NLM-modelocked laser oscillator. In our laser, the NLM is assisted by a semiconductor saturable absorber mirror (SESAM) to reliably initiate pulsed operation. We validate the high-power suitability of the NLM modelocking technique using low-absorption χ(2) crystals and optimized dichroic-mirror coating designs. Furthermore, we discuss stability against Q-switching and study how the tuning of the nonlinear mirror affects the laser performance.

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)

Combining vacuum operation, large pump spot, and multiple passes on the gain medium, we designed a high-power thin-disk oscillator with a record 350-W average power, 40-µJ pulses. We expect 500-W-level modelocking in the near future. © 2019 The Author(s)

Ultrafast high-power lasers are employed in a wide variety of applications in science and industry. Thin-disk oscillators can offer compelling performance for these applications. However, because of the high intracavity peak power, a large amount of self-phase modulation (SPM) is picked up in the intracavity air environment. Consequently, the highest performance oscillators have been operated in a vacuum environment. Here, we introduce a new concept to overcome this hurdle. We cancel the SPM picked up in air by introducing an intracavity phase-mismatched second-harmonicgeneration crystal. The resulting cascaded. chi((2)) processes provide a large SPM with a sign opposite the one originating from the air. This enables laser operation in air at 210 W average output power with 780 fs, 19 mu J pulses, the highest output power of any semiconductor saturable absorber mirror (SESAM) modelocked laser operated in air to date, to the best of our knowledge. This result paves the way to a novel approach for nonlinearity management in high-power lasers. (c) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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 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 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 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 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 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. M2 < 1.1, in a helium or vacuum environment over an output-power range from 300 W to 800 W, which is 70% broader than in an air environment. In order to predict the magnitude of the gas-lens effect in different thin-disk laser systems and gain a deeper understanding of the effect of the heated gas in front of the disk, we develop a new numerical model. It takes into account the heat transfer between the thin disk and the surrounding gas and calculates the lensing effect of the heated gas. Using this model, we accurately reproduce our experimental results and additionally predict, for the first time by means of a theoretical tool, the existence of the known gas-wedge effect due to gas convection. The gas-lens and gas-wedge effects are relevant to all high-power thin-disk systems, both oscillators and amplifiers, operating in cw as well as pulsed mode. Specifically, canceling the gas-lens effect becomes crucial for kW power scaling of thin-disk oscillators because of the larger mode area on the disk and the resulting higher sensitivity to the disk thermal lens.

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.