Abstract: We present a coherently-combined ultrafast fiber laser system consisting of twelve amplifier channels delivering 10.4 kW average power at 80 MHz repetition rate with a pulse duration of 240 fs FWHM and an almost diffraction-limited beam quality of M2 ≤ 1.2. The system incorporates an automated self-adjustment of the beam combination with 3 degrees of freedom per channel. The system today is, to the best of our knowledge, the world's most average-powerful femtosecond laser. Thermographic analysis indicates that power scaling to 100 kW-class average power is feasible.
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: In this work we present theoretical investigations of the power scaling potential of multicore fibers. In principle it is widely accepted that increasing the number of active cores helps to overcome current challenges such as transversal mode instabilities and non-linear effects. However, in order to do a proper analysis of the average power scaling potential of multicore fibers it is required to pay particular attention to thermal effects arising in such fibers. Therefore, a simulation tool has been developed that is capable of solving the laser rate equations, taking into account the resulting temperature gradient and the distortions in the mode profiles that it causes. In the study several different multicore fibers possessing a rectangular core position layout of 2×2 to 7×7 of active cores have been analyzed. Moreover, we have investigated the influence of the active core size in terms of thermal effects as well as the extractable output power and energy. This includes a study in the maximum achievable coherent combination efficiency of the multicore channels (that is strongly influenced by the distorted mode profile at the fiber end facet), the impact on nonlinear effects, the optical path differences between the cores and the amplification efficiency which are all triggered by thermal effects. Finally the scaling potential as well as the challenges of such fibers will be discussed.
Abstract: In this work we present a novel way to mitigate the effect of transverse mode instability in high-power fiber amplifiers. In this technique a travelling wave is induced in the modal interference pattern by seeding the amplifier with two modes that have slightly different frequencies. The interference pattern thus formed will travel up-or downstream the fiber (depending on the sign of the frequency difference between the modes) with a certain speed (that depends on the absolute value of the frequency difference). If the travelling speed is chosen properly, the thermally-induced index grating will follow the travelling modal interference pattern creating a constant phase shift between these two elements. Such a constant controllable phase shift allows for a stable energy transfer from the higher-order modes to the fundamental mode or viceversa. Thus, this technique can be adjusted in such a way that, at the output of the fiber almost all the energy is concentrated in the fundamental mode, regardless of the excitation conditions. Moreover, this technique represents one of the first examples of the new family of mitigation strategies acting upon the phase shift between the modal interference pattern and the refractive index grating. Additionally, it even exploits the effect of transverse mode instability for gaining control over the beam profile at the output of the amplifier. Therefore, by adjusting the frequency difference between the seed modes, it is possible to force that the beam at the output acquires the shape of the fundamental mode or that of a higher order mode.
Abstract: A simplification of segmented-mirror splitters for coherent beam combination based on numerical optimization of coating designs is presented. The simplified designs may facilitate the production of such elements for coherent beam combination while maintaining high combination efficiency. The achievable efficiency and error tolerance, and additional performance characteristics are analyzed in the context of coherently combined multicore fiber laser systems.
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: In this work we present a multicore fiber design that exploits the Talbot effect to carry out the beam splitting and recombination inside of the fiber. This allows reducing the complexity of coherent combining systems since it makes the splitting and combining subsystems together with the active stabilization redundant. In other words, such a multicore fiber behaves for the user as a single core fiber, since the energy is coupled just in a single core and it is extracted from the same core. This work describes the operating principle of this novel fiber design and analyzes its performance in high power operation using a simulation model based on the supermode theory. This includes a study on the impact on non-linear effects, on the amplification efficiency, on the thermal resilience of this design and on the performance dependence on the pump direction. Moreover, some design guidelines will be provided to tailor the characteristics of the fiber. Finally, it will be discussed how these fibers can be used to increase the TMI threshold of fiber laser systems.
Abstract: In this work we experimentally and theoretically investigate the impact of seed intensity-noise on the threshold of transverse mode instability (TMI) in Yb-doped, high-power fiber laser systems and compare it to the impact of pump intensity-noise. Former studies have shown that pump intensity-noise significantly decreases the TMI threshold due to the introduction of a phase shift between the modal interference pattern and the thermallyinduced refractive index grating in the fiber. However, it can be expected that fluctuations of the seed power will also induce such phase shifts due to a change of the extracted energy and the heat load in the fiber. Thus, it is important to investigate which one, i.e. the seed-or the pump intensity-noise, has a severer impact on the TMI threshold. Our experiments have shown that the TMI threshold of a fiber amplifier was decreased by increasing the seednoise amplitude. However, contrary to conventional belief, the impact of seed intensity-noise was much weaker than the one of pump intensity-noise. The measurements are in good agreement with our simulations and can be well explained with previous studies about the noise transfer function. The reason for the weaker impact of seed intensity-noise on the TMI threshold is the attenuation of its frequency components below 20 kHz in saturated fiber amplifiers, which includes the frequencies relevant for TMI. Thus, the main trigger for TMI in saturated fiber amplifiers can be considered to be pump intensity-noise. A suppression of this noise below 20 kHz represents a promising way to increase the TMI threshold of fiber laser systems.
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 demonstrate post-compression of 1.2 ps pulses to the few-cycle regime via multi-pass spectral broadening. We achieve compression factors of 40 via single and >90 via dual stage compression employing mJ pulses.
Abstract: We report post-compression of 1.2 ps pulses into the few-cycle regime via multi-pass spectral broadening. We achieve compression factors of 40 in single and 93 in a dual stage scheme using a compact setup.
Abstract: A simplification strategy for Segmented Mirror Splitters used as beam combiners is presented. Numerical methods are used to optimize design parameters and maintain a high theoretical combining efficiency for several tens of beams.
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: Here we will present a reliable (experimentally and numerically proved) technique for multi-spot pattern formation in the focus of a lens (i.e. in the artificial far field). This was done using large square-shaped and/or hexagonal optical vortex (OV) lattices generated by spatial light modulators. Experimental and numerical results showing a controllable far-field beam reshaping when such lattices are generated in the Fourier plane will be discussed. Even more interesting bright structures can be obtained by combining OV lattices (of any type) with different node spacings. We show that the small-scale structure of the observed patterns results from the OV lattice with the larger array node spacing, whereas the large-scale structure stems from the OV lattice with the smaller array node spacing. The orientation of the mixed far-field structures is proven to rotate by 180 degrees when all TCs are inverted.
Abstract: We present the absorption spectroscopy and continuous -wave laser operation of Tm:YLF at cryogenic temperatures. At 100 K, a maximum output power of 2.55 W corresponding to a maximum slope efficiency of 22.8% is obtained using 15% output coupling transmission. The output laser wavelength is centered at 1877 nm for Ellc.
Abstract: We present a novel approach to combine diode-pumped, moderately low-gain media with the advantages of an unstable cavity. To this end, we propose to utilize a spatially tailored gain profile in the active medium instead of using a graded reflectivity mirror to provide an effective shaping mechanism for the intra-cavity intensity distribution. The required gain profile can be easily generated with a state-of-the-art homogenized laser diode pump beam in an end-pumped configuration.
Abstract: We present a coherently-combined ultrafast fiber laser system consisting of four amplifier channels delivering 3.5 kW average power at 80 MHz repetition rate with a pulse duration of 430 fs FWHM and a close-to-diffraction-limited beam quality with an M-2 < 1.2. The system incorporates a fully automated self-adjustment of the beam combination, allowing for a quasi-turn-key operation of the system. At the date of publication, this system delivers the highest average power reported from an ultrafast laser.
Abstract: We present a laser amplifier based on coherent combination of 16 channels from a single multicore fiber employing multi-channel components for beam splitting, combination and temporal phasing. Stretched femtosecond pulses (250 fs transform-limit) were combined with an efficiency of 80% at up to 205 W average power.
Abstract: Imaging of biological specimen is one of the most important tools to investigate structures and functionalities in organic components. Improving the resolution of images into the nanometer range call for short wavelengths light sources and large aperture optics. Subsequently, the use of extreme ultraviolet light in the range of 2 nm to 5 nm provides high contrast and high resolution imaging, if it is combined with lensless imaging techniques. We describe important parameters for high resolution lensless imaging of biological samples and specify the required light source properties. To overcome radiation based damage of biological specimen, we discuss the concept of ghost imaging and describe a possible setup towards biological imaging in the extreme ultraviolet range.
Abstract: We present the most recent results of ultrafast fiber-laser driven generation of broadband THz radiation based on two-color gas-plasma. The experiment shows how energetic fiber-lasers can improve on an application today mainly dominated by Ti:sapphire lasers and power-scalability of this kind of THz sources is discussed. With a high-power driving laser THz radiation with more than 4 mW of average power is generated. This is the highest average power using this scheme so far.
Abstract: We evaluated the capabilities of an intense ultrafast high-harmonic seeded soft X-ray laser at 32.8 nm wavelength regarding single-shot lensless imaging and ptychography. Additionally the wave front at the exit of the laser plasma amplifier is monitored in amplitude and phase using high resolution ptychography and backpropagation techniques.Characterizing the laser plasma amplifier performance depending on the arrival time of the seed pulse with respect to pump pulses provides insight into the light plasma interaction in the soft X-ray range.
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: In this contribution, we present the newest results of the recently introduced pulse-energy-scaling technique electrooptically controlled divided-pulse amplification (EDPA) and its implementation in a high-power fiber laser system based on coherent combination. In this experiment, a burst of 8 stretched fs-pulses is amplified in two high-power fiber amplifier channels followed by coherent combination into a single pulse. Afterwards, the signal is compressed to a FWHM pulse duration of 255 fs with a pulse energy of 3 mJ and an average power of 105 W. The additional degrees of freedom provided by EDPA, such as direct access to the amplitudes and phases of all individual pulses in each burst, are exploited to compensate for gain saturation effects. Thus, a great temporal contrast of about 18.5 dB is reached and a very high combining efficiency of nearly 80%, including spatial as well as temporal combining, is reached. Furthermore, the system features three customized multi-pass cells as optical delay lines, minimizing the footprint of the combining stage to 0.5 m2. For the time being, two amplifiers are employed in order to initially optimize the parameters of EDPA and the performance of temporal combining. However, the laser system comprises a total of 16 parallel main amplifier channels, potentially enabling spatio-temporal combination of 128 separately amplified pulses with the currently applied bursts of 8 pulses. This extension is part of upcoming experiments and will allow for significant further scaling of the pulse energy in the near future.
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 work, we study the generation and evolution of phase-shifts between the modal interference pattern and the thermally-induced index grating due to pump-power changes. This study is not only important to understand new mitigation strategies based on controlling such phase-shifts, but also to comprehend how pump/signal noise can trigger TMI. Understanding how such a phase-shift can develop from a pump/signal change is not trivial, since the movement of both the MIP and the RIG are thermally driven and, therefore, should have similar time constants. Our simulations show unequivocally that a change of the pump power will lead to the generation of a phase-shift and the physical reason for this behavior is unveiled. The main reason is an increased sensitivity of the MIP to temperature variations because the local beat-length changes of the MIP are accumulated along the whole fiber length. Therefore, the further downstream the fiber a MIP maximum is (i.e. closer to the pump end in a counter-pumped configuration), the stronger and faster its position shift will be. This insight shows a way to obtain more TMI-resilient fiber designs and may help understanding the core area dependence of TMI.
Abstract: We present theoretical and experimental investigations on effective single-transverse mode propagation in very large mode area (VLMA) fibers. Upscaling the mode area of fibers is the most effective approach to reduce the nonlinear interaction and, therefore, to allow for the confinement of high-power radiation without detrimental nonlinear effects. Even though the investigations are carried out in a passive large pitch fiber (LPF), they reveal an intrinsic scaling potential of this design which, if unlocked, will be beneficial for active VLMA fibers in the future. A commercial mode solver based on a full-vectorial finite-difference approach has been used to simulate the confinement losses of the fundamental and higher-order transverse modes. These simulations have revealed that the differential loss in one-missing-hole photonic crystal fibers can be tailored to be larger than 10 dB/m for fiber core sizes larger than 200 mu m at 1 mu m wavelength. In order to test the theoretical predictions experimental investigations have been performed. Therefore, a rod-type fiber has been fabricated and effective single-mode operation with unprecedented large mode-field diameters has been demonstrated. We were able to achieve single-mode propagation in a passive 1.3 m long LPF with a pitch of 140 mu m possessing a mode-field diameter of 205 mu m. Even a strong misalignment of the coupling condition did not lead to any significant appearance of higher order modes at the fiber exit, which proves the robustness of the single-mode operation. To the best of our knowledge these results represent the largest dimension of a fundamental transverse mode reported in a waveguide structure at 1 mu m wavelength to date. Compared to previous results the mode area is scaled by a factor of about 4 (with respect to active fibers) and a factor of similar to 8 (with respect to passive fibers).
Abstract: In this work we have investigated the impact of pump-power noise on transverse mode instabilities (TMI) in high-power fiber laser systems. This is a crucial study since former works have shown that pump-power variations can induce a phase shift between the modal interference pattern and the thermally-induced refractive index grating and, thus, they are the most likely trigger for TMI. To experimentally investigate this behavior, we have generated white noise for different frequency bands with an arbitrary waveform generator and applied it to the pump diode. In a first experiment we have evaluated the frequency range of interest. It was found that only frequency components of the pump noise close to the main frequency of the TMI fluctuations influence the TMI threshold of the fiber laser system. In a second experiment we have measured the TMI threshold of the free-running system and compared it to the ones obtained when applying different pump-noise amplitudes. It was found that the TMI threshold can be decreased by almost a factor of three by increasing the noise of the pump source. This result is in good agreement with former theoretical and experimental studies and suggests that pump-power noise indeed acts as a main trigger for TMI. Furthermore, the findings indicate that the development of pump sources and drivers with a low noise level in a frequency range close to the main frequency of the TMI fluctuations could help to increase the TMI threshold of fiber laser systems.
Abstract: A Cryogenic Current Comparator (CCC) is an extremely sensitive DC-Beam Transformer based on superconducting SQUID technology. Recently, a CCC without a toroidal core and with an axially oriented magnetic shielding has been developed at the Institute of Photonic Technologies (IPHT) Jena/Germany. It represents a compact and lightweight alternative to the ’classical’ CCC, which was originally developed at PTB Braunschweig and is successfully in operation in accelerators at GSI and CERN. Excellent low-frequency noise performance was demonstrated with a prototype of this new CCC-type. Current measurements and further tests are ongoing, first results are presented together with simulation calculations for the magnetic shielding. The construction from lead as well as simplified manufacturing results in drastically reduced costs compared to formerly used Nb-CCCs. Reduced weight also puts less constraints on the cryostat. Based on highly sensitive SQUIDs, the new prototype device shows a current sensitivity of about 6 pA/Hz1/2 in the white noise region. The measured and calculated shielding factor is ~135 dB. These values, together with a significant cost reduction - resulting also from a compact cryostat design - opens up the way for widespread use of CCCs in modern accelerator facilities.
Abstract: For more than 20 years Cryogenic Current Comparators (CCC) are used to measure the current of charged particle beams with low intensity (nA-range). The device was first established at GSI in Darmstadt and was improved over the past two decades by the cooperation of institutes in Jena, GSI and CERN. The improved versions differ in material parameters and electronics to increase the resolution and in dimensions in order to meet the requirements of the respective application. The device allows non-destructive measurements of the charged particle beam current. The azimuthal magnetic field which is generated by the beam current is detected by low temperature Superconducting Quantum Interference Device (SQUID) current sensors. A complex shaped superconductor cooled down to 4.2 K is used as magnetic shielding and a high permeability core serves as flux concentrator. Three versions of the CCC shall be presented in this work: (1) GSI-Pb-CCC which was running at GSI Darmstadt in a transfer line, (2) CERN-Nb-CCC currently installed in the Antiproton Decelerator at CERN and (3) GSI-Nb-CCC-XD which will be operating in the CRYRING at GSI 2019. Noise, signal and drift measurements were performed in the Cryo-Detector Lab at the University of Jena.
Abstract: Similar to ytterbium doped laser materials laser operation with thulium doped media is possible within a quasi-three level scheme, which especially for pulse pumped lasers is a drawback for efficient laser operation, as a significant amount of energy is required to bleach out the laser medium. Since this energy cannot be extracted, it is lost for the amplification process. Hence, operation of such lasers at cryogenic temperatures seems to be an appropriate solution. For further modeling and derivation of design rules for future laser systems based on such a scheme reliable spectral data is needed. We will present absorption and emission measurements on Tm:YAG as a function of temperature in the range from 80 K to 300 K covering both the absorption bands around 800 nm and the emission bands up to 2.1 μm. The spectral measurements were carried out on two samples of Tm:YAG with doping levels of 2 at.% and 8 at.%. Precautions for reabsorption effects were taken to allow for accurate results over the whole measurement range. From these measurements we have derived absorption and emission cross sections and radiative lifetimes. By comparing the latter values to values obtained by highly accurate measurements of the lifetime using the pinhole method we could also estimate the quantum efficiency.
Abstract: Fiber lasers enjoy an excellent reputation as power-scalable diode-pumped solid-state laser concept. Their immunity against thermo-optical issues is combined with high efficiency and performance. The properties, challenges and perspectives of fiber lasers will be discussed.
Abstract: A new one-dimensional analytical model of a thin double layer foil interaction with a laser pulse is presented. It is based on one-dimensional electrodynamics. This model can be used for the study of high intensity laser pulse interactions with overdense plasmas, leading to frequency upshifting, high order harmonic generation, and ion acceleration in different regimes.
Abstract: A new Cryogenic Current Comparator with eXtended Dimensions (CCC-XD), compared to earlier versions built for GSI, is currently under development for a non-destructive, highly-sensitive monitoring of nA-intensities of beams for larger beamline diameters planned for the new FAIR accelerator facility at GSI. The CCC consists of a:
1) flux concentrator,
2) superconducting shield against external magnetic field and a
3) superconducting toroidal coil of niobium which is read out by a
4) Superconducting Quantum Interference Device (SQUID).
The new flux concentrator (1) comprises a specially designed highly-permeable core made of nano-crystalline material, in order to assure low-noise operation with high system bandwidth of up to 200 kHz. The superconducting shielding of niobium (2) is extended in its geometric dimensions compared to the predecessor CCC and thus will suppress (better -200 dB) disturbing magnetic fields of the beamline environment more effectively. For the CCD-XD readout, new SQUID sensors (4) with sub-μm Josephson junctions are used which enable the lowest possible noiselimited current resolution in combination with a good suppression of external disturbances. The CCC-XD system, together with a new dedicated cryostat, will be ready for testing in the CRYRING at GSI in spring 2017. For the application of a CCC in the antiproton storage ring at CERN a pulse shape correction has been developed and tested in parallel. Results from electrical measurements of two components (1 and 4) of the new CCC-XD setup will be presented in this work.
Abstract: State-of-the-art ultrafast fiber lasers currently are limited in peak power by excessive nonlinearity and in average power by modal instabilities. Coherent beam combination in space and time is a successful strategy to continue power scaling by circumventing these limitations. Following this approach, we demonstrate an ultrafast fiber-laser system featuring spatial beam combination of 8 amplifier channels and temporal combination of a burst comprising 4 pulses. Active phase stabilization of this 10-armed interferometer is achieved using LOCSET and Hänsch-Couillaud techniques. The system delivers 1 kW average power at 1 mJ pulse energy, being limited by pump power, and delivers 12 mJ pulse energy at 700 W average power, being limited by optically induced damage. The system efficiency is 91% and 78%, respectively, which is due to inequalities of nonlinearity between the amplifier channels and to inequality of power and nonlinearity between the pulses within the burst. In all cases, the pulse duration is ~260 fs and the M2-value is better than 1.2. Further power scaling is possible using more amplifier channels and longer pulse bursts.
Abstract: The phenomenon of transverse mode instabilities (TMI) is currently the most limiting effect for the scaling of the average output power of fiber laser systems with nearly diffraction-limited beam quality. Thus, it is of high interest to develop efficient mitigation strategies to further enhance the performance of fiber laser systems. By actively modulating the pump power of an Yb-doped rod-type fiber amplifier, it was possible to weaken the thermally-induced refractive index grating along the fiber and, thus, to mitigate TMI to a large extent. A significant advantage of this approach is that it can be easily integrated in any existing fiber-laser system since no further optical components are needed. A function generator connected to the pump diode driver was used to achieve the modulation. With this setup we were able to extract a fully stabilized beam at ~ 1.5 times above the TMI threshold. Furthermore, a stabilization of the beam was still feasible at an average output power of 628 W, which is more than three times higher than the free-running TMI threshold of that particular fiber under identical conditions (e.g. seed power). This is the highest average output power reported from a single-channel rod-type fiber amplifier with a high-quality stabilized beam, to the best of our knowledge.
Abstract: n this contribution, we present a spatio-temporal coherent beam combining setup in a proof-of-principle experiment with an entirely fiber-coupled front-end. Unlike in previous experiments, where the temporal pulse division was achieved using free-space optical delay lines, the pulses are taken directly from the pulse train of the oscillator. Thereby, the free-space paths and the alignment requirement are cut in half. The combination inevitably remains in free-space considering application in high-power lasers. For the combination of 4 temporally separated pulses, a combining efficiency larger than 95% is demonstrated. The efficiency is largely independent of the combined pulse energy and temporal contrasts close to the theoretically estimated maximum are reached. Potentially, this approach allows for self-optimization of the combination due to the many degrees of freedom accessible with the electro-optic modulators.
Abstract: The phenomenon of transverse mode instabilities (TMI) is currently the most limiting effect for the scaling of the average output power of fiber laser systems with nearly diffraction-limited beam quality. Even though a significant amount of knowledge on TMI in single-pass fiber amplifiers has been generated in the last years, relatively little is known about this effect in multi-pass amplifiers and oscillators. In this contribution TMI is experimentally investigated in a double-pass fiber amplifier, for the first time to the best of our knowledge. The TMI threshold was found to be significantly lower in the double-pass configuration than in the single-pass arrangement. Furthermore, the investigations unveiled a complex dynamic behavior of the instabilities in the double-pass fiber amplifier.
Abstract: Thulium-doped fiber lasers are an attractive concept for the generation of mid-infrared (mid-IR) ultrashort pulses around 2 μm wavelength with an unprecedented average power. To date, these systems deliver >150 W of average power and GW-class pulse peak powers with output pulse durations of a few hundreds of fs. As some applications can greatly benefit from even shorter pulse durations, the spectral broadening and subsequent temporal pulse compression can be a key enabling technology for high average power few-cycle laser sources around 2 μm wavelength. In this contribution we demonstrate the nonlinear compression of ultrashort pulses from a high repetition rate Tm-doped fiber laser using a nitrogen gas-filled hollow capillary. Pulses with 4 GW peak power, 46 fs FWHM duration at an average power of 15.4 W have been achieved. This is, to the best of our knowledge, the first 2 μm laser delivering intense, GW-pulses with sub 50-fs pulse duration and an average power of >10 W. Based on this result, we discuss the next steps towards a 100 W-level, GW-class few-cycle mid-IR laser.
Abstract: High power OPCPAs above 10 W at short-wave IR wavelengths (SWIR: 1.4 - 3 μm) may be limited because of thermal heat dissipation in the nonlinear crystals. In this work we provide up-to-date measurements of the absorption coefficients of the nonlinear crystals used at these wavelengths and simulations of the thermal effects on critical parameters. In particular, power scaling limits will be discussed.
Abstract: The noise characteristics of high-power fiber lasers, unlike those of other solid-state lasers such as thin-disks, have not been systematically studied up to now. However, novel applications for high-power fiber laser systems, such as attosecond pulse generation, put stringent limits to the maximum noise level of these sources. Therefore, in order to address these applications, a detailed knowledge and understanding of the characteristics of noise and its behavior in a fiber laser system is required. In this work we have carried out a systematic study of the propagation of the relative intensity noise (RIN) along the amplification chain of a state-of-the-art high-power fiber laser system. The most striking feature of these measurements is that the RIN level is progressively attenuated after each amplification stage. In order to understand this unexpected behavior, we have simulated the transfer function of the RIN in a fiber amplification stage (~80μm core) as a function of the seed power and the frequency. Our simulation model shows that this damping of the amplitude noise is related to saturation. Additionally, we show, for the first time to the best of our knowledge, that the fiber design (e.g. core size, glass composition, doping geometry) can be modified to optimize the noise characteristics of high-power fiber laser systems.
Abstract: Ultrashort-pulse laser systems are an enabling technology for numerous applications. The stability of such systems is especially crucial for frequency metrology and high precision spectroscopy. Thulium-based fiber lasers are an ideal starting point as a reliable and yet powerful source for the nonlinear conversion towards the mid-IR region. Recently, we have demonstrated that nonlinear self-compression in a fused silica solid-core fiber allows for few-cycle pulse duration with up to 24 MW peak power using a high-repetition rate thulium-based fiber laser system operating at around 2 μm wavelength . This experiment operates near the self-focusing limit of about 24 MW for circular polarization, which increases the requirements for the system stability due to the risk of a fiber damage. Here, we present a self-protecting nonlinear compression regime allowing for long-term operation and high output-pulse stability with very similar output performance.
Abstract: Transverse mode instabilities (TMI) have become a very serious problem for the further scaling of the average power of fiber laser systems. Recently the strong impact that photodarkening (PD) has on the TMI threshold of Yb-doped fiber laser systems has been revealed. This is a remarkable finding since it opens the door to a significant increase of the average power of fiber laser systems in the near future. The key to achieve this is to reduce the amount of PD losses in the fiber, which can be done with an optimization of the glass composition in the fiber. In this work we perform a theoretical study on the impact that co-dopants such as Al and P have on PD and on the TMI threshold. This analysis tries to find the optimum glass composition from the point of view of TMI. It is shown that in a short rod type fiber, changing the glass composition only leads to a modest increase of the TMI threshold due to the degradation of the cross-sections. This demonstrates that the optimization of the glass cannot be done attending only to the PD losses at the cost of the laser cross-sections. In spite of this, changing the glass composition can bring benefits in pulsed operation in terms of the stored energy. Additionally, other fiber geometries different from the rod-type can benefit in a greater degree by introducing co-dopants in the glass.
Abstract: Experimental demonstrations of Tm-doped fiber amplifiers (typically in CW- or narrow-band pulsed operation) span a wavelength range going from about 1700 nm to well beyond 2000 nm. Thus, it should be possible to obtain a bandwidth of more than 100 nm, which would enable sub-100 fs pulse duration in an efficient, linear amplification scheme. In fact, this would allow the emission of pulses with less than 20 optical cycles directly from a Tm-doped fiber system, something that seems to be extremely challenging for other dopants in a fused silica fiber. In this contribution, we summarize the current development of our Thulium-doped fiber CPA system, demonstrate preliminary experiments for further scaling and discuss important design factors for the next steps. The current single-channel laser system presented herein delivers a pulse-peak power of 2 GW and a nearly transform-limited pulse duration of 200 fs in combination with 28.7 W of average power. Special care has been taken to reduce the detrimental impact of water vapor absorption by placing the whole system in a dry atmosphere housing (<0.1% rel. humidity) and by using a sufficiently long wavelength (1920-1980 nm). The utilization of a low-pressure chamber in the future will allow for the extension of the amplification bandwidth. Preliminary experiments demonstrating a broader amplification bandwidth that supports almost 100 fs pulse duration and average power scaling to < 100W have already been performed. Based on these results, a Tm-doped fiber CPA with sub-100 fs pulse duration, multi-GW pulse peak power and >100 W average power can be expected in the near future.