Abstract: We report on the generation of GW-class peak power, 35-fs pulses at 2-mu m wavelength with an average power of 51 W at 300-kHz repetition rate. A compact, krypton-filled Herriott-type cavity employing metallic mirrors is used for spectral broadening. This multi-pass compression stage enables the efficient post compression of the pulses emitted by an ultrafast coherently combined thulium-doped fiber laser system. The presented results demonstrate an excellent preservation of the input beam quality in combination with a power transmission as high as 80%. These results show that multi-pass cell based post-compression is an attractive alternative to nonlinear spectral broadening in fibers, which is commonly employed for thulium-doped and other mid-infrared ultra-fast laser systems. Particularly, the average power scalability and the potential to achieve few-cycle pulse durations make this scheme highly attractive. (C) 2022 Optica Publishing Group
Abstract: High-energy, ultrafast, short-wavelength infrared laser sources with high average power are important tools for industrial and scientific applications. Through the coherent combination of four ultrafast thulium-doped rod-type fiber amplifiers, we demonstrate a Tm-doped chirped pulse amplification system with a compressed pulse energy of 1.65 mJ and 167 W of average output power at a repetition rate of 101 kHz. The system delivers 85 fs pulses with a peak power of 15 GW. Additionally, the system presents a high long- and short-term stability. To the best of our knowledge, this is the highest average output power short wavelength IR, mJ-class source to date. This result shows the potential of coherent beam combining techniques in the short wavelength infrared spectral region for the power scalability of these systems.
Abstract: In-volume ultrafast laser direct writing of silicon is generally limited by strong nonlinear propagation effects preventing the production of modifications. By using advantageous spectral, temporal, and spatial conditions, we demonstrate that modifications can be repeatably produced inside silicon. Our approach relies on irradiation at approximate to 2 mu m wavelength with temporally distorted femtosecond pulses. These pulses are focused in a way that spherical aberrations of different origins mutually balance, as predicted by point spread function analyses and in good agreement with nonlinear propagation simulations. We also establish the laws governing modification growth on a pulse-to-pulse basis, which allows us to demonstrate transverse inscription inside silicon with various line morphologies depending on the irradiation conditions. We finally show that the production of single-pulse repeatable modifications is a necessary condition for reliable transverse inscription inside silicon.
Abstract: We experimentally analyze the average-power-scaling capabilities of ultrafast, thulium-doped fiber amplifiers. It has been theoretically predicted that thulium-doped fiber laser systems, with an emission wavelength around 2 mu m, should be able to withstand much higher heat-loads than their Yb-doped counterparts before the onset of transverse mode instability (TMI) is observed. In this work we experimentally verify this theoretical prediction by operating thulium doped fibers at very high heat-load. In separate experiments we analyze the performance of two different large-core, thulium-doped fiber amplifiers. The first experiment aims at operating a short, very-large core, thulium-doped fiber amplifier at extreme heat-load levels of more than 300 W/m. Even at this extreme heat-load level, the onset of TMI is not observed. The second experiment maximizes the extractable average-output power from a large-core, thulium-doped, fiber amplifier. We have achieved a pump-limited average output power of 1.15 kW without the onset of TMI. However, during a longer period of operation at this power level the amplifier performance steadily degraded and TMI could be observed for average powers in excess of 847 W thereafter. This is the first time, to the best of our knowledge, that TMI has been reported in a thulium-doped fiber amplifier.
Abstract: Bright, coherent soft X-ray radiation is essential to a variety of applications in fundamental research and life sciences. To date, a high photon flux in this spectral region can only be delivered by synchrotrons, free-electron lasers or high-order harmonic generation sources, which are driven by kHz-class repetition rate lasers with very high peak powers. Here, we establish a novel route toward powerful and easy-to-use SXR sources by presenting a compact experiment in which nonlinear pulse self-compression to the few-cycle regime is combined with phase-matched high-order harmonic generation in a single, helium-filled antiresonant hollow-core fibre. This enables the first 100 kHz-class repetition rate, table-top soft X-ray source that delivers an application-relevant flux of 2.8 x 10(6) photon s(-1) eV(-1) around 300 eV. The fibre integration of temporal pulse self-compression (leading to the formation of the necessary strong-field waveforms) and pressure-controlled phase matching will allow compact, high-repetition-rate laser technology, including commercially available systems, to drive simple and cost-effective, coherent high-flux soft X-ray sources.
Abstract: Coherent soft X-ray (SXR) sources that provide a high photon flux in the water window are essential tools for advanced spectroscopy (e.g. of magnetic materials  and organic compounds  ) or for lens-less bio imaging with nm-scale resolution  . To date, such sources are mostly large-scale facilities like synchrotrons or free electron lasers. A promising alternative are laser-driven sources, based on high harmonic generation (HHG) in noble gases. Current state-of-the-art SXR HHG uses frequency converted Ti:Sa lasers (to ~2 µm wavelength to increase the phase matching cutoff) with multi-mJ pulse energies at 1 kHz repetition rate  . Most recently, there has been a strong push towards increasing the repetition rate of the driving lasers, and the SXR HHG, to enable faster data acquisition, space-charge-reduced SXR photoelectron spectroscopy or coincidence detection  ,  . In this contribution, we present an approach to SXR HHG that is based on nonlinear pulse self-compression and HHG in the same helium gas-filled antiresonant hollow-core fiber (ARHCF). Because of the intensity enhancement resulting from temporal pulse self-compression, the experiments can be driven by moderate-energy, multi-cycle laser pulses, which facilitates repetition rate scaling. We coupled 100 fs-, 250 µJ-pulses centered around 1.9 µm wavelength at a 98 kHz repetition rate to the ARHCF ( Fig. 1a ). When the fiber length (~1.2 m) and the gas pressure at its output (~3.8 bar) were chosen appropriately, the pulses close to its end ( Fig. 1b ) were self-compressed to <20 fs, leading to an on-axis peak intensity >4×10 14 W/cm 2 . At this point, the gas is partially ionized, and the chosen pressure ensures phase matching between the driving laser and the generated SXR light ( Fig. 1b ). It is the first time that this approach is experimentally realized, and we have generated a photon flux >10 6 Ph/s/eV at the carbon K-edge ( Fig. 1c ). To the best of our knowledge, this is the highest photon flux at 300 eV reported to date at a laser repetition rate >1 kHz.
Abstract: Recent milestones in power-scaling of ultrafast fiber-based lasers were achieved by the simultaneous mitigation of thermal and nonlinear effects through the coherent combination of ultrafast pulses  . This technique is based on splitting the light into several spatially separated amplification channels that are subsequently coherently recombined into a single beam. Besides the performance scaling aspect, the laser wavelength is an important parameter for many applications, e.g. for high-field physics due to the quadratic wavelength-dependence of the ponderomotive potential  . Tm-doped fiber lasers have proven to be promising and relatively straightforward candidates for the realization of efficient high average- and peak-power ultrafast lasers in the 2-µm wavelength region  ,  . Here we demonstrate the coherent combination  ,  of four thulium-doped fiber amplifiers. The fiber-based chirped pulse amplifier (CPA  ) delivers pulses with <120 fs full-width at half-maximum duration with up to 228 µJ of pulse energy at a center wavelength of 1940 nm.
Abstract: Electro-optic sampling (EOS)  is widely used for broadband and sensitive characterization of mid-infrared (MIR) waveforms and spectroscopy  ,  . GaSe as the nonlinear crystal and ~10-fs gate pulses at ~ 100-mW average power, generated in highly nonlinear fibres from Er-doped fibre lasers, are commonly employed for broadband sampling of MIR fields  . Using Yb-based laser systems to generate high-power, 1030-nm gate pulses allowed us to push the detection efficiency to a record-level of 0.5 %, limited by the damage threshold of GaSe  . However, the short gate pulse wavelength strongly limits the phase matching bandwidth  .
Abstract: Numerous applications are enabled by the possibility to modify transparent materials in the bulk with ultrashort laser pulses. In glasses the inscription of waveguides and nanogratings, welding and dicing processes are finding their way into the industrial world. The transfer of these established processes to narrow band-gap materials is subject of current research activities. Here, silicon as the current most important semiconductor material is of particular interest.
Abstract: Applications such as material processing, spectroscopy, particle acceleration, high-harmonic and mid-IR generation can greatly benefit from high repetition rate, high power, ultrafast laser sources emitting around 2 μm wavelength. In this contribution we present a single-channel Tm-doped fiber chirped-pulse amplifier delivering 108 W of average output power at 417 kHz repetition rate with 250 fs pulse duration and 0.73 GW of pulse peak power. To the best of our knowledge, this is the first demonstration of an ultrafast Tm-doped fiber laser with more than 100 W of average power and GW-level peak power.
Abstract: Numerous molecules important for environmental and life sciences feature strong absorption bands in the molecular fingerprint region from 3 μm-20 μm. While mature drivers at 1 μm wavelength are the workhorse for the generation of radiation up to 5 μm (utilizing down-conversion in nonlinear crystals) they struggle to directly produce radiation beyond this limit, due to impeding nonlinear absorption in non-oxide crystals. Since only non-oxide crystals provide transmission in the whole molecular fingerprint region, a shift to longer driving wavelengths is necessary for a power scalable direct conversion of radiation into the wavelength region beyond 5 μm. In this contribution, we present a high-power single-stage optical parametric amplifier driven by a state of the art 2 μm wavelength, thulium-doped fiber chirped pulse amplifier. In this experiment, the laser system provided 23 W at 417 kHz repetition rate with 270 fs pulse duration to the parametric amplifier. The seed signal is produced by supercontinuum generation in 3 mm of sapphire and pre-chirped with 3 mm of germanium. Combining this signal with the pump radiation and focusing it into a 2 mm thick GaSe crystal with a pump intensity of 160 GW/cm2 lead to an average idler power of 700 mW with a spectrum spanning from 9 μm-12 μm. To the best of our knowledge, this is the highest average power reported from a parametric amplifier directly driven by a 2 μm ultrafast laser in the wavelength region beyond 5 μm. Employing common multi-stage designs, this approach might in the future enable multi-watt high-power parametric amplification in the long wavelength mid infrared.
Abstract: We present the first coherently combined, thulium-doped fiber CPA delivering >100 W average-power and simultaneously >1 GW of peak-power. Incorporating four amplifier channels the laser delivers pulses with >228 µJ energy and <120 fs duration at 1940 nm center wavelength. Excellent long-term stability is achieved with an average power fluctuation of <0.5% RMS over >48 hours – ideal prerequisites for next-generation industrial and scientific applications.
Abstract: We present HHG results obtained with thulium-doped fiber lasers. It is the first time that a photon energy cut-off close to 400 eV has been demonstrated using this highly scalable laser technology.
Abstract: We report on soft x-ray HHG driven by a thulium-doped fiber laser. It is the first time that a photon energy cut-off ~400 eV has been demonstrated using this highly scalable laser technology.
Abstract: In this contribution, we present a Tm-doped fiber chirped pulse amplifier system delivering 10⁸ W of average output power at 417 kHz repetition rate with 250 fs pulse duration and close to 1 GW of pulse peak power.
Abstract: Intense, ultrafast laser sources with an emission wavelength beyond the well-established near-IR are important tools for exploiting the wavelength scaling laws of strong-field, light-matter interactions. In particular, such laser systems enable high photon energy cut-off HHG up to, and even beyond, the water window thus enabling a plethora of subsequent experiments. Ultrafast thulium-doped fiber laser systems (providing a broad amplification bandwidth in the 2 μm wavelength region) represent a promising, average-power scalable laser concept in this regard. These lasers already deliver ~100 fs pulses with multi-GW peak power at hundreds of kHz repetition rate. In this work, we show that combining ultrafast thulium-doped fiber CPA systems with hollow-core fiber based nonlinear pulse compression is a promising approach to realize high photon energy cut-off HHG drivers. Herein, we show that thulium-doped, fiber-laser-driven HHG in argon can access the highly interesting spectral region around 90 eV. Additionally, we show the first water window high-order harmonic generation experiment driven by a high repetition rate, thulium-doped fiber laser system. In this proof of principle demonstration, a photon energy cut-off of approximately 400 eV has been achieved, together with a photon flux <105 ph/s/eV at 300 eV. These results emphasize the great potential of exploiting the HHG wavelength scaling laws with 2 μm fiber laser technology. Improvements of the HHG efficiency, the overall HHG yield and further laser performance enhancements will be the subject of our future work.
Abstract: We report a coherent mid-infrared (MIR) source with a combination of broad spectral coverage (6--18 µm), high repetition rate (50 MHz), and high average power (0.5 W). The waveform-stable pulses emerge via intrapulse difference-frequency generation (IPDFG) in a GaSe crystal, driven by a 30-W-average-power train of 32-fs pulses spectrally centered at 2 µm, delivered by a fiber-laser system. Electro-optic sampling (EOS) of the waveform-stable MIR waveforms reveals their single-cycle nature, confirming the excellent phase matching both of IPDFG and of EOS with 2-µm pulses in GaSe.
Abstract: Frequency combs are an enabling technology for metrology and spectroscopic applications in fundamental and life sciences. While frequency combs in the 1 lam regime, produced from Yb-based systems have already exceeded the 100 W - level, high power coverage of the interesting mid-infrared wavelength range remains yet to be demonstrated. Tm- and Ho-doped laser systems have recently shown operation at high average power levels in the 2 lam wavelength regime. However, frequency combs in this wavelength range have not exceeded the 5 W-average power level. In this work, we present a high power frequency comb, delivered by a Tm-doped chirped-pulse amplifier with subsequent nonlinear pulse compression. With an integrated phase noise of <320 mrad, low relative intensity noise of <0.5% and an average power of 60 W at 100 MHz repetition rate (and <30 fs FWHM pulse duration), this system demonstrates high stability and broad spectral coverage at an unrivalled average power level in this wavelength regime. Therefore, this laser will enable metrology and spectroscopy with unprecedented sensitivity and acquisition time. It is our ongoing effort to extend the spectral coverage of this system through the utilization of parametric frequency conversion into the mid-IR, thus ultimately enabling high power fingerprint spectroscopy in the entire molecular fingerprint region (2 - 20 mu m).
Abstract: Accessing the molecular fingerprint region between 2 and 20 mu m is a key aspect in modern metrology and spectroscopy. While the wavelength range from 2-5 mu m can easily be addressed through nonlinear frequency conversion starting from well-matured 1 mu m driving lasers, access to the deep mid-IR wavelength regime is difficult. This is, because of the limited transmission of non-oxide crystals (that offer high nonlinearity and good transmission for the aspired mid-IR idler) at the pump wavelength and/or multi-photon absorption. Shifting to a longer pump wavelength relieves these limitations. In this work we present an experiment based on intra pulse difference frequency generation (IPDFG) in GaSe driven by an ultrafast Tm-doped chirped pulse amplifier. This experiment led to an octave spanning mid-IR spectrum, covering the wavelength range between 7.2-16.5 mu m (-10 dB width) with 450 mW of average power at 1.25 MHz repetition rate. This result outperforms comparable sources driven at 1 mu m wavelength in average power and conversion efficiency, while providing much broader spectral coverage. To further facilitate the use of these promising sources in real-world spectroscopic applications, we have built a nonlinear amplifier, which, based on its compact and robust design is an ideal candidate in this respect. Optimizing the output ultimately led to high pulse quality 50 fs pulses with 250 nJ of pulse energy at 80 MHz of repetition rate and 20 W average output power, exceeding current designs in the anomalous dispersion regime by 1 order of magnitude. It is our ongoing effort to utilize this laser for parametric downconversion. Covering the wavelength regime beyond 5 mu m wavelength would make it an enabling technology for next generation spectroscopy, fundamental and life sciences.
Abstract: In this Letter, we report on the generation of 1060 W average power from an ultrafast thulium-doped fiber chirped pulse amplification system. After compression, the pulse energy of 13.2 μJ with a pulse duration of 265 fs at an 80 MHz pulse repetition rate results in a peak power of 50 MW spectrally centered at 1960 nm. Even though the average heat-load in the fiber core is as high as 98 W/m, we confirm the diffraction-limited beam quality of the compressed output. Furthermore, the evolution of the relative intensity noise with increasing average output power has been measured to verify the absence of transversal mode instabilities. This system represents a new average power record for thulium-doped fiber lasers (1150 W uncompressed) and ultrashort pulse fiber lasers with diffraction-limited beam quality, in general, even considering single-channel ytterbium-doped fiber amplifiers.
Abstract: The development of high-power, broadband sources of coherent mid-infrared radiation is currently the subject of intense research that is driven by a substantial number of existing and continuously emerging applications in medical diagnostics, spectroscopy, microscopy, and fundamental science. One of the major, long-standing challenges in improving the performance of these applications has been the construction of compact, broadband mid-infrared radiation sources, which unify the properties of high brightness and spatial and temporal coherence. Due to the lack of such radiation sources, several emerging applications can be addressed only with infrared (IR)-beamlines in large-scale synchrotron facilities, which are limited regarding user access and only partially fulfill these properties. Here, we present a table-top, broadband, coherent mid-infrared light source that provides brightness at an unprecedented level that supersedes that of synchrotrons in the wavelength range between 3.7 and 18 µm by several orders of magnitude. This result is enabled by a high-power, few-cycle Tm-doped fiber laser system, which is employed as a pump at 1.9 µm wavelength for intrapulse difference frequency generation (IPDFG). IPDFG intrinsically ensures the formation of carrier-envelope-phase stable pulses, which provide ideal prerequisites for state-of-the-art spectroscopy and microscopy.
Abstract: We report on the generation of a high-power frequency comb in the 2 μm wavelength regime featuring high amplitude and phase stability with unprecedented laser parameters, combining 60 W of average power with <30 fs pulse duration. The key components of the system are a mode-locked Er:fiber laser, a coherence-preserving nonlinear broadening stage, and a high-power Tm-doped fiber chirped-pulse amplifier with subsequent nonlinear self-compression of the pulses. Phase locking of the system resulted in a phase noise of less than 320 mrad measured within the 10 Hz–30 MHz band and 30 mrad in the band from 10 Hz to 1 MHz.
Abstract: In this Letter, we present an optimized nonlinear amplification scheme in the 2 µm wavelength region. This laser source delivers 50 fs pulses at an 80 MHz repetition rate with exceptional temporal pulse quality and 20 W of average output power. According to predictions from numerical simulations, it is experimentally confirmed that dispersion management is crucial to prevent the growth of side pulses and an increase of the energy content in a temporal pedestal surrounding the self-compressed pulse. Based on these results, we discuss guidelines to ensure high temporal pulse quality from nonlinear femtosecond fiber amplifiers in the anomalous dispersion regime.
Abstract: High-average power laser sources delivering intense few-cycle pulses in wavelength regions beyond the near infrared are promising tools for driving the next generation of high-flux strong-field experiments. In this work, we report on nonlinear pulse compression to 34.4 μJ-, 2.1-cycle pulses with 1.4 GW peak power at a central wavelength of 1.82 μm and an average power of 43 W. This performance level was enabled by the combination of a high-repetition-rate ultrafast thulium-doped fiber laser system and a gas-filled antiresonant hollow-core fiber.