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Abstract
We demonstrate an efficient 102-MW peak power, 103-W average power, Kerr-lens mode-locked thin-disk laser (TDL) oscillator generating 52-fs pulses at 17.1-MHz repetition rate. The TDL is based on an Yb:YAG disk and operates in the strongly self-phase-modulation (SPM) broadened regime. In this regime, the spectral bandwidth of the oscillating pulse exceeds the available gain bandwidth by generating additional frequency components via SPM in the Kerr medium inside the laser cavity. At an optical-to-optical efficiency of 26%, our oscillator delivers a more than six times higher average power compared to any 50-fs-class laser oscillator. Compared to previous 100-W-class high-power laser oscillators, we reach this performance in a more than two times shorter pulse duration at a comparable optical-to-optical efficiency. Our TDL delivers the highest peak power of any ultrafast laser oscillator. The short pulse duration combined with high average power and peak power makes the presented TDL oscillator an attractive source for high field science and nonlinear optics.
© 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultrafast lasers have enabled nonlinear frequency conversion processes such as high harmonic generation (HHG), optical parametric amplification (OPA), or THz generation, providing coherent light at a wide range of photon energies [1,2]. Many experiments relying on these technologies such as multidimensional spectroscopy or pump probe measurements strongly benefit from high repetition rates allowing for shorter acquisition times. This has stimulated the development of megahertz-repetition-rate laser systems over the last decade. The resulting need for higher average powers has caused a shift from a Ti:sapphire to an Yb-based laser technology. Due to the low quantum defect and availability of efficient diode pumping, ultrafast Yb-based laser amplifier systems can reach kilowatts of average power at sub-picosecond pulse durations and megahertz repetition rates. Up to 10.4 kW at 254 fs and 80 MHz have already been demonstrated using twelve coherently combined fiber laser amplifiers [3], 620 W at 640 fs and 20 MHz using an Innoslab amplifier [4], and 1.9 kW at 1 ps and 400 kHz based on a multi-pass thin-disk amplifier [5]. The high average power, however, comes at the expense of a longer pulse duration, typically >200 fs, due to the gain bandwidth of most common Yb-doped gain materials.
Another successful approach allowing for short pulses from Yb-based gain materials is ultrafast thin-disk laser (TDL) oscillators. Thanks to the thin-disk geometry, they can operate at high peak and average powers, while keeping the favorable properties inherent to laser oscillators. TDL oscillators provide transform-limited soliton pulses without temporal pre or post features and excellent beam quality at megahertz repetition rates and can thus be used as a single-stage alternative to laser amplifier systems. The output pulses are directly suited for nonlinear frequency conversion [6] as well as for few-cycle pulse compression [7,8]. Figure 1 shows an overview of ultrafast TDL oscillators with respect to peak power and pulse duration based on the two most common mode-locking techniques. While semiconductor saturable absorber mirror (SESAM) mode-locked TDLs have been historically well-suited for delivering the highest average powers (currently up to 350 W), they only provide these power levels at >500-fs pulse durations, limiting the achievable peak power [9–11]. On the other hand, Kerr-lens mode-locked (KLM) TDLs have been particularly successful in delivering shorter pulse durations at high average power, enabling a higher peak power. Recently, 90 MW of peak power were presented in 140-fs pulses and 220 W of average power [12]. However, even for KLM TDLs, a clear trade-off between peak power and pulse duration can be observed for sub-100-fs pulse durations [Fig. 1].
Fig. 1. Overview of sub-ps KLM and SESAM mode-locked thin-disk laser oscillators based on Yb-doped gain materials. Mode-locking techniques are distinguished by the symbol. KLM: Kerr-lens mode-locking; SESAM: semiconductor saturable absorber mirror. The most outstanding performances are labeled with average power, pulse duration, optical-to-optical efficiency, and repetition rate. The result presented in this manuscript is highlighted with the green circle. The green shaded area represents the favored region of laser operation at shortest pulse duration and highest peak power. References: [9,10,12–16].
As we have recently shown in [15], so far the most promising direction towards sub-100-fs pulse durations at high peak powers is based on operating a KLM Yb:YAG TDL oscillator in the strongly self-phase modulation (SPM) broadened regime. In this regime, additional frequency components outside of the gain bandwidth are generated by excessive SPM in the Kerr medium inside the laser cavity. We have demonstrated the shortest pulse duration of a TDL oscillator corresponding to 27 fs at 3.3-W average power and 6-MW peak power, but only with 1% of optical-to-optical efficiency (“efficiency” in the following). We have also shown 84-fs pulses at 69 W and 42 MW, however, even here the efficiency was still only 12%, limiting the achievable average and peak power.
Whereas our previous result was optimized for shortest pulse duration, in this work, we optimized our KLM Yb:YAG TDL operating in the SPM-broadened regime towards increased output performance, i.e. highest peak power, and efficiency. At 103 W of average output power with 52-fs pulses, our TDL delivers 102 MW of peak power at 26% efficiency. The peak power is the highest delivered by any ultrafast laser oscillator [12,13]. Compared to previous 100-W-class ultrafast TDL oscillators, we reach this performance range in a more than two times shorter pulse duration while maintaining a comparable efficiency [9–12,14].
2. Experimental setup
The setup of the KLM TDL oscillator is shown in Fig. 2. The oscillator is housed in a vacuum chamber with a footprint of 0.8 × 1.6 m2 and operates at a pressure of around 1 mbar. The laser is built using a commercially available TDL head (Trumpf GmbH) designed for 36 passes of the pump. An Yb:YAG disk with a thickness of ∼100-μm and ∼20-m concave radius of curvature (RoC) is used. The disk is optically pumped on a 2.9-mm diameter pump spot at a wavelength of 969 nm with a fiber-coupled wavelength-stabilized pump diode (Dilas Diodenlaser GmbH). The pump system is capable of delivering up to 2 kW of pump average power, but we limited ourselves to a maximum pump power of 400 W, resulting in a pump intensity of ∼6 kW/cm2 on the disk, to prevent the risk of damaging the disk.
Fig. 2. Schematic of the Kerr-lens mode-locked Yb:YAG thin-disk laser oscillator with double pass over the disk. The orange box highlights the tight focus created between two curved mirrors, CM3 and CM4, which has been implemented for intra-oscillator high harmonic generation. As indicated by the double arrow, CM3 is mounted on a translation stage used for fine-tuning of the laser cavity during mode-locked operation. The inset shows the beam profile of the laser output in mode-locked operation when the laser generates 52-fs pulses at 103-W average power. HA: hard aperture; CM1 and CM2: concave mirror with 1-m radius of curvature (RoC); KM: Kerr medium; DM: dispersive mirror; CM3 and CM4: concave mirror with 150-mm and 250-mm RoC; OC: output coupler. Cavity lengths: a = 520 mm; b1 = 475 mm; b2 = 550 mm; c1+c2+c3 = 3455 mm; d1+d2+d3 = 1545 mm; e = 1780mm; f = ∼200 mm; g = 250 mm.
The laser cavity is folded twice over the disk for higher roundtrip gain. Following the standard scheme of Kerr-lens mode-locking for TDLs [17], a wedged (30 arcmin) 1-mm-thick c-cut sapphire plate acts as the Kerr medium which is placed under Brewster’s angle in the vicinity of a first intracavity focus created by two concave mirrors, CM1 and CM2, with 1-m RoC. A water-cooled copper plate with a 4.4-mm diameter hole serves as a hard aperture for the fast-saturable loss. The Kerr medium is purged with oxygen from both sides to prevent contamination during laser operation, for example through carbon deposition in vacuum. Two -500-fs2 dispersive mirrors (DM) in folding configuration introduce a total negative group delay dispersion (GDD) of -2000 fs2 per cavity roundtrip. The corresponding spectral profile of the GDD is shown in Fig. 3(a), which was obtained by measuring the GDD of the DMs with an in-house developed white-light interferometer and multiplied by the number of bounces per cavity roundtrip. The TDL setup has been originally designed for intra-oscillator HHG [18] and the output coupling arm of the oscillator thus contains an extension intended to create a tight focus for HHG. It consists of two concave mirrors with a RoC of 150 mm (CM3) and 250 mm (CM4) leading to an estimated tight focus of ∼20-μm radius in mode-locked operation. Due to the tight focus, operation of the TDL strictly requires vacuum. In ambient air, the intensity in the focus would exceed the air ionization level and the plasma formation would prevent mode-locking. CM3 is placed on a motorized linear stage, allowing to tune the laser cavity from a position where it is easy to start the mode-locking operation to a position of improved intracavity and output performance. Before mode-locking can be initialized, the distance between CM3 and CM4 is slightly increased in order to decrease the beam size on the aperture, facilitating the onset of mode-locking. Mode-locked operation is initiated by shaking one of the cavity mirrors through a piezo actuator. After mode-locking is started, the pump power is decreased until the continuous-wave (cw) breakthrough otherwise visible in the optical spectrum disappears. Then, the pump power is again slowly increased while at the same time the distance between CM3 and CM4 is decreased, increasing the beam size on the hard aperture. This procedure allows operation of the TDL at maximum pump power without the appearance of a cw-breakthrough. Between the mode-locking starting position and the final, high-power operation position, the linear stage is moved by less than 1 mm, resulting in a ∼20% decrease in pulse duration and a peak power increase by a factor of ∼2 while the pump power is also increased by a factor of ∼2. The cavity remains stable for cw-operation in all configurations. Further details about the optimization of the TDL regarding, e.g., the hard aperture size or the thickness of the Kerr medium are given in our recent publication [15]. The cavity end mirror after the tight focus serves as an output coupler with a transmission (TOC) of 8.5%.
To increase the thermal stability of the laser system in vacuum, the cavity is built on a water-cooled breadboard and most of the mirror mounts are water-cooled. The optical coatings of the dispersive and highly reflective mirrors have been designed inhouse and grown in our ion-beam sputtering coating facility.
3. Experimental results
Table 1 summarizes the laser parameters and mode-locking performance of our TDL. The oscillator delivers an output power of 103 W, a peak power of 102 MW, and pulse energy of 5.5 µJ at a pulse duration of 52 fs. These parameters correspond to an intracavity peak and intracavity average power of 1.2 GW and 1.2 kW, respectively. The pump power of 400 W leads to an efficiency of 26%. This efficiency is for the first time comparable to the efficiency of other 100-W-class high-power TDLs operating with significantly longer pulses [9–12,14].
Table 1. Laser parameters and corresponding mode-locking performance of our TDL oscillator.
Figure 3(a) shows the optical spectrum of the output pulses together with the normalized gain cross-section of Yb:YAG at an inversion level of 0.3. The optical spectrum is centered at a wavelength of 1027.3 nm. The central wavelength is blue-shifted by ∼2 nm, away from the gain peak of Yb:YAG at 1030 nm. With a full width at half maximum (FWHM) bandwidth of 21.4 nm, the optical spectrum exceeds the ∼8-nm FHWM gain bandwidth of Yb:YAG more than twice, indicating operation of the TDL in the strongly SPM-broadened regime [15,19,20]. Despite a small shoulder at the longer wavelength side, a feature that is commonly observed for TDL oscillators operating in the SPM-broadened regime [15,19,20], the sech2 fit agrees well with the shape expected for soliton pulses. Also, the intensity autocorrelation of the 52-fs pulses is in excellent agreement with the fit for soliton pulses [Fig. 3(b)]. We achieve an ideal time-bandwidth product of 0.315, indicating the generation of transform-limited soliton pulses. The radio-frequency spectrum measured at the fundamental repetition frequency of 17.1 MHz shows no side peaks [Fig. 3(c)]. Modulation-free higher harmonics confirm clean mode-locking [inset of Fig. 3(c)]. We confirmed single pulse operation by a 180-ps scan in the autocorrelator and by observing the pulse train with an 18.5-ps-rise-time photodetector on a 40-GHz sampling oscilloscope [Fig. 3(d)]. The output pulses feature an excellent fundamental mode Gaussian beam profile [inset of Fig. 2].
Fig. 3. Characterization of the Kerr-lens mode-locked Yb:YAG thin-disk laser oscillator. (a) Normalized optical spectrum of the laser output with sech2 fit. In addition, the normalized Yb:YAG gain cross section at an inversion level of 0.3 (data taken from [22]) and the total group delay dispersion (GDD) introduced by the dispersive mirrors (DM) per cavity round-trip (rt) are shown. (b) Intensity autocorrelation trace with fit for soliton pulses. (c) Radio-frequency (RF) spectrum of the fundamental repetition rate (frep) at 17.1 MHz measured with 10-kHz resolution bandwidth (RBW). The inset shows the RF-spectrum of the higher frep harmonics measured with 30-kHz RBW. (d) Sampling oscilloscope trace for 1-ns and 70-ns (inset) time window.
Within our pump power restriction of 400 W (∼6 kW/cm2) set to prevent damage of the disk, we were not able to mode-lock the laser at a TOC higher than 8.5%. We expect that higher pump intensities of ∼10 kW/cm2 [16,21] should allow for mode-locking at a TOC above 10% and a corresponding performance improvement. High-power operation of our TDL is currently limited to a few minutes by the overheating of the non-water-cooled connector of the pump-delivery fiber in the vacuum environment. After changing to a water-cooled fiber, we expect long-term operability.
4. Conclusion and outlook
We have demonstrated an efficient 102-MW peak power, 103-W average power KLM TDL oscillator generating 52-fs pulses at 17.1-MHz repetition rate. The TDL oscillator is based on Yb:YAG gain material and operates in the strongly SPM-broadened regime at an efficiency of 26%. Our average output power is more than six times higher than any previous 50-fs-class laser oscillator [15,19,20]. Furthermore, we achieve the highest peak power delivered by any ultrafast laser oscillator [12,13]. Compared to previous 100-W-class high-power TDL oscillators, we reach this performance range in a more than two times shorter pulse duration while maintaining a comparable efficiency [9–12,14]. The clean soliton pulses with excellent beam quality and high peak power are well suited for broadband THz generation [23] or oscillator-driven HHG [24].
Additionally, the output of our TDL oscillator is very suitable for consecutive temporal pulse compression. Whereas efficient compression starting from several hundred femtoseconds down to few-cycle pulses is already feasible [7,25–27], the compression often comes with reduced pulse contrast as well as limited beam quality and thus remains a challenging task. Thanks to our clean 52-fs soliton pulses and assuming a compression ratio of ten for a single compression stage [8,28,29], we expect being able to reach few-cycle pulse durations within a single instead of the typically at least two required cascaded temporal compression stages. This would make our source an excellent choice for applications requiring few-cycle pulses at high pulse contrast, excellent beam quality, and MHz repetition rates. After carrier-envelope phase stabilization of the TDL operating in the SPM-broadened regime [30], even applications in the single-cycle regime could be addressed.
Since the thin-disk concept is power scalable and we have not encountered any physical limitation, we expect that further power-scaling is feasible and that 50-fs-class TDL oscillators operating at several hundred megawatts of peak power are within reach.
Funding
Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (200020_179146, 200020_200774, 206021_144970, 206021_170772, 206021_198176).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are available in Ref. [31].
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