Andreas Diebold

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.

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.

Phase-stabilization of the carrier-envelope-offset frequency of a SESAM modelocked thin disk laser

We phase-stabilized the carrier-envelope-offset (CEO) frequency of a SESAM modelocked Yb:CaGdAlO₄ (CALGO) thin disk laser (TDL) generating 90-fs pulses at a center wavelength of 1051.6 nm and a repetition rate of 65 MHz. By launching only 2% of its output power into a photonic crystal fiber, we generated a coherent octave-spanning supercontinuum spectrum. Using a standard f-to-2f interferometer for CEO detection, we measured CEO beats with 33 dB signal-to-noise ratio in 100 kHz resolution bandwidth. We achieved a tight lock of the CEO frequency at 26.18 MHz by active feedback to the pump current. The residual in-loop integrated phase noise is 120 mrad (1 Hz-1 MHz). This is, to our knowledge, the first CEO-stabilized SESAM modelocked TDL. Our results show that a reliable lock of the CEO frequency can be achieved using standard techniques in spite of the strongly spatially multimode pumping scheme of TDLs. This opens the door towards fully-stabilized low-noise frequency combs with hundreds of watts of average power from table-top SESAM modelocked thin disk oscillators.

Optimized SESAMs for kilowatt-level ultrafast lasers

We present a thorough investigation of surface deformation and thermal properties of high-damage threshold large-area semiconductor saturable absorber mirrors (SESAMs) designed for kilowatt average power laser oscillators. We compare temperature rise, thermal lensing, and surface deformation of standard SESAM samples and substrate-removed SESAMs contacted using different techniques. We demonstrate that for all cases the thermal effects scale linearly with the absorbed power, but the contacting technique critically affects the strength of the temperature rise and the thermal lens of the SESAMs (i.e. the slope of the linear change). Our best SESAMs are fabricated using a novel substrate-transfer direct bonding technique and show excellent surface flatness (with non-measureable radii of curvature (ROC), compared to astigmatic ROCs of up to 10 m for standard SESAMs), order-of-magnitude improved heat removal, and negligible deformation with absorbed power. This is achieved without altering the saturation behavior or the recovery parameters of the samples. These SESAMs will be a key enabling component for the next generation of kilowatt-level ultrafast oscillators.

Frequency comb offset dynamics of SESAM modelocked thin disk lasers

We present a detailed study of the carrier-envelope offset (CEO) frequency dynamics of SESAM modelocked thin disk lasers (TDLs) pumped by kW-class highly transverse multimode pump diodes with a typical M(2) value of 200-300, and give guidelines for future frequency stabilization of multi-100-W oscillators. We demonstrate CEO frequency detection with > 30 dB signal-to-noise ratio with a resolution bandwidth of 100 kHz from a SESAM modelocked Yb:YAG TDL delivering 140 W average output power with 748-fs pulses at 7-MHz pulse repetition rate. We compare with a low-power CEO frequency stabilized Yb:CALGO TDL delivering 2.1 W with 77-fs pulses at 65 MHz. For both lasers, we perform a complete noise characterization, measure the relevant transfer functions (TFs) and compare them to theoretical models. The measured TFs are used to determine the propagation of the pump noise step-by-step through the system components. From the noise propagation analysis, we identify the relative intensity noise (RIN) of the pump diode as the main contribution to the CEO frequency noise. The resulting noise levels are not excessive and do not prevent CEO frequency stabilization. More importantly, the laser cavity dynamics are shown to play an essential role in the CEO frequency dynamics. The cavity TFs of the two lasers are very different which explains why at this point a tight CEO frequency lock can be obtained with the Yb:CALGO TDL but not with the Yb:YAG TDL. For CEO stabilization laser cavities should exhibit high damping of the relaxation oscillations by nonlinear intra-cavity elements, for example by operating a SESAM in the roll-over regime. Therefore the optimum SESAM operation point is a trade-off between enough damping and avoiding multiple pulsing instabilities. Additional cavity components could be considered for supplementary damping independent of the SESAM operation point.

Dual-gain SESAM modelocked thin disk laser based on Yb:Lu 2 O 3 and Yb:Sc 2 O 3

We present for the first time a SESAM-modelocked thin-disk laser (TDL) that incorporates two gain materials with different emission spectra in a single TDL resonator. The two gain media used in this experiment are the sesquioxide materials Yb:Lu2O3 and Yb:Sc2O3, which have their spectral emission peak displaced by ≈7 nm. We can benefit from a combined gain bandwidth that is wider than the one provided by a single gain material alone and still conserve the excellent thermal properties of each disk. In these first proof-of-principle experiments we demonstrate pulse durations shorter than previously achieved with the single gain material Yb:Lu2O3. The oscillator generates pulses as short as 103 fs at a repetition rate of 41.7 MHz and a center wavelength of around 1038 nm, with an average output power of 1.4 W. A different cavity layout provides pulses with a duration of 124 fs at an output power of 8.6 W. This dual-gain approach should allow for further power scaling of TDLs and these first results prove this method to be a promising new way to combine the record output-power performance of modelocked TDLs with short pulse durations 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.

High-power Yb:GGG thin-disk laser oscillator: first demonstration and power-scaling prospects

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.

62-fs Pulses from a SESAM Modelocked Yb:CALGO Thin Disk Laser

We demonstrate a SESAM modelocked thin-disk laser (TDL) based on Yb:CALGO delivering 62 fs pulses with 5 W of average power. To our knowledge, these are the shortest pulses achieved from a modelocked TDL to-date.

Trends in high-power ultrafast lasers

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.