Efficient high-power sub-50-fs gigahertz repetition rate diode-pumped solid-state laser.

In this article we present a directly diode-pumped high-power Kerr-lens mode-locked Yb:CALGO bulk laser oscillator operating at 1-GHz repetition rate. We report on two laser configurations optimized for either highest average power or shortest pulse duration. In the first configuration optimized for high average power, the oscillator delivers up to 6.9 W of average power, which is the highest average power of any ultrafast laser oscillator operating at gigahertz repetition rate. The 93-fs pulses have a peak power of 64 kW, and the optical-to-optical efficiency amounts to 37%. In the second configuration optimized for short pulse duration, we demonstrate 48-fs pulses at 4.1 W of average power corresponding to a higher peak power of 74 kW with 21% optical-to-optical efficiency. This is the shortest pulse duration and the highest peak power demonstrated by any GHz-class Yb-based laser oscillator. The compact laser setup is directly pumped by a low-cost multimode fiber-coupled laser diode and has a high potential as an economical yet powerful source for various applications.

Efficient XUV-light out-coupling of intra-cavity high harmonics by a coated grazing-incidence plate.

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.

Efficient few-cycle Yb-doped laser oscillator with Watt-level average power.

So far, the operation of ultrafast bulk laser oscillators based on Yb-doped gain materials and directly emitting few-cycle pulses have been restricted to low optical-to-optical efficiencies and average output powers of only a few milliwatt. This performance limitation can be attributed to the commonly-applied standard collinear pumping scheme in which the optical pump is transmitted through a dichroic mirror whose spectral transmission and dispersion properties severely perturb the oscillating pulse when its optical spectrum extends towards the pump wavelength. In this study, we report on a novel pumping scheme relying on cross polarization that overcomes this challenge. In our concept, the pump transmitting mirror is highly transmissive for the pump light in p-polarization, while it is highly reflective for the laser light in s-polarization over a broad wavelength range, even covering the pump wavelength and beyond. In contrast to a standard thin-film polarizer featuring similar polarization dependent properties, it provides a low and flat dispersion profile over a broad spectral range for the s-polarization. Implementing this pumping scheme in a soft-aperture Kerr-lens mode-locked bulk laser oscillator based on the gain material Yb:CALGO, we achieve clean 22-fs soliton pulses at 729 mW of average output power and an optical-to-optical efficiency of 25%. In a second configuration optimized for the highest average output power, we demonstrate a high optical-to-optical efficiency of 36.6%, which was obtained for 31-fs pulses at 1.63 W of average output power. In a third configuration we experimentally confirm the limiting effect of a dichroic mirror commonly used in the standard collinear pumping scheme. All the results presented here and obtained in the first and second configuration generate pulses with a center wavelength ranging from 1030 nm to 1056 nm, well within the spectral region of high gain cross sections of Yb:CALGO. While this initial demonstration was realized using a commercial diffraction-limited fiber laser as pump source, the pump geometry appears also well suited for pumping with laser diodes coupled into multimode fibers. This novel approach opens up new opportunities for compact and cost-efficient high-power few-cycle bulk laser oscillators based on Yb-doped gain materials and can be applied to any gain material with small quantum defect.

Sub-30-fs Yb:YAG thin-disk laser oscillator operating in the strongly self-phase modulation broadened regime.

We experimentally investigate the limits of pulse duration in a Kerr-lens mode-locked Yb:YAG thin-disk laser (TDL) oscillator. Thanks to its excellent mechanical and optical properties, Yb:YAG is one of the most used gain materials for continuous-wave and pulsed TDLs. In mode-locked operation, its 8-nm wide gain bandwidth only directly supports pulses with a minimum duration of approximately 140 fs. For achieving shorter pulses, a Kerr-lens mode-locked TDL oscillator can be operated in the strongly self-phase modulation (SPM) broadened regime. Here, the spectral bandwidth of the oscillating pulse exceeds the available gain bandwidth by generating additional frequencies via SPM inside the Kerr medium. In this work, we study and compare different laser configurations in the strongly SPM-broadened regime. Starting with a configuration providing 84-fs pulses at 69 W average power at 17 MHz repetition rate, we reduce the pulse duration by optimizing various mode-locking parameters. One crucial parameter is the dispersion control which was provided by in-house-developed dispersive mirrors produced by ion-beam sputtering (IBS). We discuss trade-offs in average power, pulse duration, efficiency, and intra-cavity peak power. For the configuration operating at the highest SPM-broadening, we achieve a minimum pulse duration of 27 fs, which represents the shortest pulse duration directly generated by any ultrafast TDL oscillator. The corresponding full width at half maximum (FWHM) spectral bandwidth exceeds more than five times the FWHM gain bandwidth. The average output power of 3.3 W is moderate for ultrafast TDL oscillators, but higher than other Yb-based laser oscillators operating at this pulse duration. Additionally, the corresponding intra-cavity peak power of 0.8 GW is highly attractive for implementing intra-cavity extreme nonlinear optical interactions such as high harmonic generation.

Intra-oscillator broadband THz generation in a compact ultrafast diode-pumped solid-state laser.

We demonstrate broadband and powerful terahertz (THz) generation at megahertz repetition rate based on intra-oscillator optical rectification (OR) in gallium phosphide (GaP). By placing the nonlinear crystal directly inside the cavity of a Kerr-lens mode-locked ultrafast diode-pumped solid-state laser (DPSSL) oscillator, we demonstrate a compact and single-stage THz source. Using only 7 W of diode-pump power, we drive OR in a GaP crystal with 22 W of average power at ∼80 MHz repetition rate. In a first configuration, using a 0.3-mm-thick GaP and 105 fs driving pulses, we generate up to 150 µW of THz radiation with a spectrum extending to 5.5 THz. In a second configuration allowing for sub-50-fs pulse duration, we generate up to 7 THz inside a 0.1-mm-thick GaP crystal. This performance is well suited for THz time-domain spectroscopy and THz imaging. Intra-oscillator THz generation in sub-100-fs DPSSLs is a promising way to scale down footprint, complexity and cost of powerful broadband THz sources.

High-power dual-comb thin-disk laser oscillator for fast high-resolution spectroscopy.

Free-running dual-comb systems based on a single laser cavity are an attractive next generation technology for a wide variety of applications. The high average power achievable by dual-comb thin-disk laser (TDL) oscillators make this technology especially attractive for spectroscopy and sensing applications in the molecular fingerprint region enabled by nonlinear frequency conversion. However, the high noise levels of TDL oscillators, e.g., induced by the turbulent water-cooling of the disk, are a severe challenge for spectroscopic applications. In this contribution, we confirm for the first time the suitability of dual-comb TDLs for high-resolution spectroscopy. Based on the novel concept of polarization splitting inside a TDL, our oscillator generates two asynchronous pulse trains of 240-fs pulse duration at 6-W and 8-W average power per pulse train and ∼97-MHz repetition rate at a central wavelength of 1030 nm. In the first detailed noise investigation of such a system, we identify the repetition frequency as the dominant noise term and show that ∼85% of the frequency noise of the comb lines of both pulse trains is correlated (integrated from 200 Hz to 20 kHz). We detect the absorption spectrum of acetylene in free-running operation within a measurement time of 1 millisecond. Being highly suitable for nonlinear frequency conversion, we believe the here presented result is an important step towards simple yet powerful mid-infrared dual-comb systems for high-resolution spectroscopy.

Intra-oscillator high harmonic generation in a thin-disk laser operating in the 100-fs regime.

We demonstrate that Kerr lens modelocking is well-suited for operating an ultrafast thin-disk laser with intra-oscillator high harmonic generation (HHG) in the 100-fs pulse duration regime. Exploiting nearly the full emission bandwidth of the gain material Yb:YAG, we generate 105-fs pulses with an intracavity peak power of 365 MW and an intracavity average power of 470 W. We drive HHG in argon with a peak intensity of ∼7⋅1013 W/cm2 at a repetition rate of 11 MHz. Extreme-ultraviolet (XUV) light is generated up to the 31st harmonic order (H31) at 37 eV, with an average power of ∼0.4 µW in H25 at 30 eV. This work presents a considerable increase in performance of XUV sources based on intra-oscillator HHG and confirms that this approach is a promising technology for simple and portable XUV sources at MHz repetition rates.

Sub-100-fs Kerr lens mode-locked Yb:Lu2O3 thin-disk laser oscillator operating at 21 W average power.

We investigate power-scaling of a Kerr lens mode-locked (KLM) Yb:Lu2O3 thin-disk laser (TDL) oscillator operating in the sub-100-fs pulse duration regime. Employing a scheme with higher round-trip gain by increasing the number of passes through the thin-disk gain element, we increase the average power by a factor of two and the optical-to-optical efficiency by a factor of almost three compared to our previous sub-100-fs mode-locking results. The oscillator generates pulses with a duration of 95 fs at 21.1 W average power and 47.9 MHz repetition rate. We discuss the cavity design for continuous-wave and mode-locked operation and the estimation of the focal length of the Kerr lens. Unlike to usual KLM TDL oscillators, an operation at the edge of the stability zone in continuous-wave operation is not required. This work shows that KLM TDL oscillators based on the gain material Yb:Lu2O3 are an excellent choice for power-scaling of laser oscillators in the sub-100-fs regime, and we expect that such lasers will soon operate at power levels in excess of hundred watts.

Broadband terahertz pulse generation driven by an ultrafast thin-disk laser oscillator.

We demonstrate broadband THz generation driven by an ultrafast thin-disk laser (TDL) oscillator. By optical rectification of 50-fs pulses at 61 MHz repetition rate in a collinear geometry in crystalline GaP, THz radiation with a central frequency at around 3.4 THz and a spectrum extending from below 1 THz to nearly 7 THz are generated. We realized a spectroscopic characterization of a GaP crystal and a benchmark measurement of the water-vapor absorption spectrum in the THz range. Sub-50-GHz resolution is achieved within a 5 THz bandwidth. Our experiments show the potential of ultrafast TDL oscillators for driving MHz-repetition-rate broadband THz systems.

Carrier-envelope offset frequency stabilization of a thin-disk laser oscillator operating in the strongly self-phase modulation broadened regime.

We demonstrate the carrier-envelope offset (CEO) frequency stabilization of a Kerr lens mode-locked Yb:Lu2O3 thin-disk laser oscillator operating in the strongly self-phase modulation (SPM) broadened regime. This novel approach allows overcoming the intrinsic gain bandwidth limit and is suited to support frequency combs from sub-100-fs pulse trains with very high output power. In this work, strong intra-oscillator SPM in the Kerr medium enables the optical spectrum of the oscillating pulse to exceed the bandwidth of the gain material Yb:Lu2O3 by a factor of two. This results in the direct generation of 50-fs pulses without the need for external pulse compression. The oscillator delivers an average power of 4.4 W at a repetition rate of 61 MHz. We investigated the cavity dynamics in this regime by characterizing the transfer function of the laser output power for pump power modulation, both in continuous-wave and mode-locked operations. The cavity dynamics in mode-locked operation limit the CEO modulation bandwidth to ~10 kHz. This value is sufficient to achieve a tight phase-lock of the CEO beat via active feedback to the pump current and yields a residual in-loop integrated CEO phase noise of 197 mrad integrated from 1 Hz to 1 MHz.

Kerr lens mode-locked Yb:CALGO thin-disk laser.

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.

Generation of 35-fs pulses from a Kerr lens mode-locked Yb:Lu2O3 thin-disk laser.

We investigate Kerr lens mode locking of Yb:Lu2O3 thin-disk laser oscillators operating in the sub-100-fs regime. Pulses as short as 35 fs were generated at an average output power of 1.6 W. These are the shortest pulses directly emitted from a thin-disk laser oscillator. The optical spectrum of the 35-fs pulses is almost 3 times broader than the corresponding emission band of the gain crystal. At slightly longer pulse duration of 49 fs, we achieve an average power of 4.5 W. In addition, 10.7 W are obtained in 88-fs pulses, which is twice higher than the previous power record for ultrafast thin-disk lasers generating pulses shorter than 100 fs. Our results prove that Kerr lens mode-locked Yb:Lu2O3 thin-disk lasers are a promising technology for further average power and pulse energy scaling of ultrafast high-power oscillators operating in the sub-100-fs regime.

Impact of photodarkening on the mode instability threshold.

The threshold-like onset of mode instabilities is currently the main limitation for the scaling of the average output power of fiber laser systems with diffraction limited beam quality. In this contribution, the impact of a wavelength shift of the seed signal on the mode instability threshold has been investigated. Against expectations, it is experimentally shown that the highest mode instabilities threshold is reached around 1030 nm and not for the smallest wavelength separation between pump and signal. This finding implies that the quantum defect is not the only source of thermal heating in the fiber. Systematic experiments and simulations have helped in identifying photodarkening as the most likely second heat source in the fiber. It is shown that even a negligible photodarkening-induced power loss can lead to a decrease of the mode instabilities threshold by a factor of two. Consequently, reduction of photodarkening is a promising way to mitigate mode instabilities.

2 kW average power from a pulsed Yb-doped rod-type fiber amplifier.

This Letter reports on a fiber-laser system that, employing a 1 m long rod-type photonic-crystal fiber as its main-amplifier, emits a record average output power of 2 kW, by amplifying stretched ps-pulses. A further increase of the output power was only limited by the available laser-diode pump power. The energy of the pulses is 100 µJ, corresponding to MW-level peak powers extracted directly from the fiber of the main amplifier. The corresponding M2 at the maximum output power is <3, due to the onset of mode instabilities. The Letter covers the influence of this effect on the evolution of the beam quality with the output power. The numerical results show that the M2 value settles at around 3, even if the output average power is further increased.

Q-switched thulium-doped photonic crystal fiber laser.

We report a novel, Tm-doped photonic crystal fiber (PCF) actively Q-switched oscillator that provides ~8.9 kW peak power with 435 µJ, 49 ns pulses at 10 kHz repetition rate at 2 µm wavelength. This fiber has a mode-field area >1000 µm2, the largest of any flexible PCF providing diffraction-limited beam quality to the best of our knowledge. As an application, the oscillator is used as pump to generate >350 nm broadening in ~50 m of SMF-28 fiber.

Lasing in thulium-doped polarizing photonic crystal fiber.

We describe lasing of a thulium-doped polarizing photonic crystal fiber. A 4 m long fiber with 50 µm diameter core, 250 µm diameter cladding, and d/Λ ratio of 0.18 was pumped with a 793 nm diode and produced a polarized output with a polarization extinction ratio (PER) of 15 dB and an M(2) of <1.15. An intracavity polarizer and half-wave plate minimally increased the PER to 16 dB. The output power had 35% slope efficiency relative to the absorbed pump power. The maximum cw output power was limited to 4 W due to the quantum defect heating of the fiber.