Clara J. Saraceno

275 W average output power from a femtosecond thin disk oscillator operated in a vacuum environment.

We present an ultrafast thin disk laser that generates an average output power of 275 W, which is higher than any other modelocked laser oscillator. It is based on the gain material Yb:YAG and operates at a pulse duration of 583 fs and a repetition rate of 16.3 MHz resulting in a pulse energy of 16.9 μJ and a peak power of 25.6 MW. A SESAM designed for high damage threshold initiated and stabilized soliton modelocking. We reduced the nonlinearity of the atmosphere inside the cavity by several orders of magnitude by operating the oscillator in a vacuum environment. Thus soliton modelocking was achieved at moderate amounts of self-phase modulation and negative group delay dispersion. Our approach opens a new avenue for power scaling femtosecond oscillators to the kW level.

Femtosecond thin-disk laser with 141 W of average power

We present a semiconductor saturable absorber mirror mode-locked thin disk laser based on Yb:Lu(2)O(3) with an average power of 141 W and an optical-to-optical efficiency of more than 40%. The ideal soliton pulses have an FWHM duration of 738 fs, an energy of 2.4 microJ, and a corresponding peak power of 2.8 MW. The repetition rate was 60 MHz and the beam was close to the diffraction limit with a measured M(2) below 1.2.

Ultrafast thin-disk laser with 80 μJ pulse energy and 242 W of average power

We present a semiconductor saturable absorber mirror (SESAM) mode-locked thin-disk laser generating 80 μJ of pulse energy without additional amplification. This laser oscillator operates at a repetition rate of 3.03 MHz and delivers up to 242 W of average output power with a pulse duration of 1.07 ps, resulting in an output peak power of 66 MW. In order to minimize the parasitic nonlinearity of the air inside the laser cavity, the oscillator was operated in a vacuum environment. To start and stabilize soliton mode locking, we used an optimized high-damage threshold, low-loss SESAM. With this new milestone result, we have successfully scaled the pulse energy of ultrafast laser oscillators to a new performance regime and can predict that pulse energies of several hundreds of microjoules will become possible in the near future. Such lasers are interesting for both industrial and scientific applications, for example for precise micromachining and attosecond science.

SESAMs for High-Power Oscillators: Design Guidelines and Damage Thresholds

We present for the first time to the best of our knowledge a systematic study of lifetime and damage of semiconductor saturable absorber mirrors (SESAMs) designed for operation in high-power oscillators. We characterize and compare nonlinear reflectivity and inverse saturable absorption (ISA) parameters as well as damage threshold and lifetime of different representative SESAMs under test using a nonlinear reflectivity measurement setup at unprecedented high fluence levels. We investigate the catastrophic damage that occurs at very high fluences by demonstrating a dependence of the damage threshold on the ISA parameter F2 and the maximum reflectivity fluence F0. We can clearly demonstrate that the damage fluence Fd scales proportionally to √F2 for all SESAMs. In the case of SESAMs with the same absorber where the product Fsat .ΔR is constant, the damage fluence Fd scales proportionally to F0. Therefore, damage occurs due to heating of the lattice by the energy absorbed due to the ISA process and is not related to the quantum well (QW) absorbers. Furthermore, we present guidelines on how to design samples with high saturation fluences, reduced induced absorption, and high damage thresholds. Using multiple QWs and a suitable di-electric topsection, we achieved SESAMs with saturation fluences >;200 μj/cm2, nonsaturable losses <;0.1%, and reduced ISA. Our best sample could not be damaged at a maximum available fluence of 0.21 J/cm2 and a peak intensity of 370 GW/cm2. These SESAMs will be suitable for future high-power femtosecond oscillators in the kilowatt average output power regime, which is very interesting for attosecond science and industrial material processing applications.

Beam delivery and pulse compression to sub-50 fs of a modelocked thin-disk laser in a gas-filled Kagome-type HC-PCF fiber.

We present two experiments confirming that hypocycloid Kagome-type hollow-core photonic crystal fibers (HC-PCFs) are excellent candidates for beam delivery of MW peak powers and pulse compression down to the sub-50 fs regime. We demonstrate temporal pulse compression of a 1030-nm Yb:YAG thin disk laser providing 860 fs, 1.9 µJ pulses at 3.9 MHz. Using a single-pass grating pulse compressor, we obtained a pulse duration of 48 fs (FWHM), a spectral bandwidth of 58 nm, and an average output power of 4.2 W with an overall power efficiency into the final polarized compressed pulse of 56%. The pulse energy was 1.1 µJ. This corresponds to a peak power of more than 10 MW and a compression factor of 18 taking into account the exact temporal pulse profile measured with a SHG FROG. The compressed pulses were close to the transform limit of 44 fs. Moreover, we present transmission of up to 97 µJ pulses at 10.5 ps through 10-cm long fiber, corresponding to more than twice the critical peak power for self-focusing in silica.

Frontiers in passively mode-locked high-power thin disk laser oscillators.

Semiconductor saturable absorber mirror (SESAM) mode-locked thin disk lasers define the state-of-the-art performance for high average power and high pulse energy femtosecond laser oscillators. To date pulse energies above 30 µJ and average powers above 140 W have been demonstrated. In this paper we review the achievements of mode-locked thin disk lasers in terms of average power and pulse energy. Stable mode locking requires single transverse mode operation even at the highest average power, which is challenging and therefore addressed in more detail. We then summarize our expectations on the main challenges and limitiations for the next generation of mode-locked thin disk laser oscillators with an average power above 500 W and pulse energies in excess of 100 µJ.

Continuous-wave and modelocked Yb:YCOB thin disk laser: first demonstration and future prospects

Yb:YCOB is a very attractive material for femtosecond pulse generation given its broad emission bandwidth. We demonstrate continuous-wave power scaling in the thin disk geometry to the 100-W level with a 40% optical-to-optical efficiency in multi-mode operation. Furthermore, we present initial modelocking results in the thin disk geometry, achieving pulse durations as short as 270 fs. The modelocked average power is, however, limited to less than 5 W because of transverse mode degradation. This is caused by anisotropic thermal aberrations in the 15% Yb-doped thin disks which were 300 to 400 µm thick. This result confirms the potential of Yb:YCOB to generate short femtosecond pulses in the thin disk geometry but also makes clear that significantly thinner disks are required to overcome the thermal limitations for high power operation.

SESAM mode-locked Yb:CaGdAlO4 thin disk laser with 62 fs pulse generation.

We present a semiconductor saturable absorber mirror (SESAM) mode-locked thin disk laser (TDL) based on Yb:CaGdAlO(4) (Yb:CALGO) generating 62 fs pulses, which is the shortest pulse duration achieved from mode-locked TDLs to date. The oscillator operates at a repetition rate of 65 MHz and delivers 5.1 W of average output power. The short pulse duration of our TDL in combination with the high intracavity peak power of 44 MW makes this oscillator attractive for intracavity table-top extreme nonlinear optics applications such as high harmonic generation and vacuum ultraviolet frequency comb generation. The current average power was limited by the quality of the Yb:CALGO disk. However, power scaling of Yb:CALGO TDLs to the multi-10-W range with short pulse durations (<100 fs) appears feasible in the near future by using thinner disks of better quality and further optimized SESAMs.

Gigahertz self-referenceable frequency comb from a semiconductor disk laser

We present a 1.75-GHz self-referenceable frequency comb from a vertical external-cavity surface-emitting laser (VECSEL) passively modelocked with a semiconductor saturable absorber mirror (SESAM). The VECSEL delivers 231-fs pulses with an average power of 100 mW and is optimized for stable and reliable operation. The optical spectrum was centered around 1038 nm and nearly transform-limited with a full width half maximum (FWHM) bandwidth of 5.5 nm. The pulses were first amplified to an average power of 5.5 W using a backward-pumped Yb-doped double-clad large mode area (LMA) fiber and then compressed to 85 fs with 2.2 W of average power with a passive LMA fiber and transmission gratings. Subsequently, we launched the pulses into a highly nonlinear photonic crystal fiber (PCF) and generated a coherent octave-spanning supercontinuum (SC). We then detected the carrier-envelope offset (CEO) frequency (f(CEO)) beat note using a standard f-to-2f-interferometer. The f(CEO) exhibits a signal-to-noise ratio of 17 dB in a 100-kHz resolution bandwidth and a FWHM of ≈10 MHz. To our knowledge, this is the first report on the detection of the f(CEO) from a semiconductor laser, opening the door to fully stabilized compact frequency combs based on modelocked semiconductor disk lasers.

Toward Millijoule-Level High-Power Ultrafast Thin-Disk Oscillators

SESAM modelocked thin-disk lasers have recently reached new frontiers and remain the leading technology in terms of average power and pulse energy, setting new performance levels for ultrafast oscillators. The milestones achieved seem to indicate that there are no major roadblocks ahead to achieve further scaling of modelocked oscillators to kilowatt output powers and millijoule output pulse energies. In this paper, we review the current state of the art and present the next steps toward future generations of millijoule, kilowatt-class ultrafast thin-disk oscillators.