We report on an integrable thin-film Fabry-Pérot type electro-optic modulator (EOM) centered around an electro-optically active so-called guest-host polymer. This polymer material contains novel synthesized chromophore molecules (C3), which are aligned by electro-poling inside an amorphous polycarbonate host-matrix. When integrated into our Fabry-Pérot cavity, the electro-optic activity of the poled material can be observed in the short wavelength near infrared spectral range (approximately 900 nm - 1070 nm). We derived a value of ~220 pm/V for its linear electro-optic coefficient at 988 nm from spectral transmission measurements with increasing direct voltages applied to the EOM. The resulting half-wave voltage-length product of the EOM setup is 0.25 Vcm. As an exemplary functional test, we demonstrated an intensity modulation of a 974 nm diode laser by applying ± 11.5 V alternating voltage to the EOM. Due to the all thin-film realization of the EOM setup, it is compatible to the substrate free, miniaturized interference filter fabrication method. With this method, thin-film elements with edge lengths between 25 μm and 2 mm can be fabricated. In combination with the demonstrated low drive voltage, these compact EOM filters are excellent candidates for hybrid integration into photonic platforms, as shown in this contribution.
As part of the investigations, quantum nanolaminates (QNLs) were produced from TiO2, Nb2O5, and ZrO2 using IBS which are presented here. Complex layer systems, such as edge filters or polarizers, are produced using a system control specially adapted for such a large number of layers and the complete automation of the coating process. With these coatings, the focus was also on exploiting the blue-shift caused by quantization. Subsequent investigations are intended to demonstrate their applicability to other areas of optics production. The applications range from high laser damage thresholds to low mechanical losses for the mirrors of gravitational wave detectors or optical clocks.
The talk presents concepts for integrating essential active optical functions into thin film coatings, which allows a high degree of miniaturization compared to classical alternatives. Due to the amorphous structure of thin film coating materials, only uneven orders of nonlinear effects will be considered. The chosen applications comprise a concept for frequency tripling mirrors, where the third harmonic generation is performed in the thin film stack, and an all-optical switch, the so-called Kerr-band-switch based on the optical Kerr-effect. The chosen materials, design considerations, and measurements validating the function of the concepts will be presented.
Information technology advancements are revolutionizing optical components, necessitating a solid theoretical foundation for optically active components. Optical thin films are traditionally designed using the transfer matrix method to calculate linear spectral responses. However, recent developments also address nonlinear optical responses by integrating nonlinearities into the matrix formalism or by applying a maxwell solver, which offers spatially and temporally resolved pulse propagation simulations in thin films.
The transfer matrix method has been extended to include third harmonic generation and ultrafast switching via the Kerr effect. We compare the results from the nonlinear transfer matrix method to results obtained by a maxwell solver. Furthermore optimization routines for nonlinear response design like Monte Carlo algorithms and machine learning with neural networks are shown.
The IBS2000-project aims to develop a coating machine to coat optics with up to 2m in diameter. IBS is chosen as coating process due to the high optical quality and precision, low losses, and high mechanical and environmental stability.
Common limitations regarding the size of the coated optics are overcome due to a novel approach, where both, the substrate and the target material source are movable. The sputter assembly located below the substrate will move linearly, while the substrate rotates on a stationary axis around its center.
Simulations are done to validate the mechanical concept with a virtual coater concept. First, the material distribution in the substrate plane is calculated and afterwards combined with the movement of the target carrier and the substrate rotation, which gives a first indication of the 2D distribution. The results will be applied to homogenize the projected coating distribution on the final 2m optics.
With the recent advances in photonic integrated circuits and their use as sensor platforms, the requirements for environmental temperature stability are increasing. Substrate free miniaturized thin film filters, used for spectral filtering, multiplexing, and demultiplexing, are affected by this requirement. Therefore, we investigate the thermal behavior of optical thin films fabricated by IBS and compare coatings on glass substrates to our substrate free miniaturized integrable alternative. We determined the relative change in optical thickness for various materials during spectral transmission measurements under sample temperature variation. Additionally, we adapted a thin film simulation software to estimate the linear coefficient of thermal expansion and thermo-optic coefficient of the materials. Since the thermal behavior of coatings is influenced by thermal expansion of the underlying substrate, we also measured the thermal behavior of simultaneously fabricated substrate free miniaturized filter elements. Comparing these results allows to pre estimate the thermal stability of the filter elements.
Quantizing nanolaminates (QNLs) are a promising alternative as high-index material in thin film coatings providing high flexibility with respect to their refractive index and bandgap energy. However, the fabrication of QNLs requires high precision in the deposition of the layers. Common monitoring strategies are not applicable due to the nanometer to subnanometer layer thicknesses needed to achieve a significantly increased bandgap energy.
This contribution investigates the impact of thickness errors on the bandgap energy of QNLs. Calculations show a diminishing of the bandgap energy increase due to thickness errors in a single layer. This effect will be investigated experimentally. Moreover, the QNLs linear and nonlinear absorption will be tested as function of layer numbers determining the impact of the increased interfaces of QNL structures.
Applying the new insights, the final goal is the fabrication of functional QNL-coatings with optimized electrical field intensity and increased LIDT for the ultra-short pulse regime.
At sufficiently high intensities the electronic nonlinear behavior of optical materials dominates the classical linear phenomena. Through laser calorimetric absorption (LCA) measurements this behavior has been characterized and an increase in absorption of over one order of magnitude has been observed. Quantum nanolaminates (QNLs) are uniquely suited to investigating these phenomena as it is possible to tune the refractive index and bandgap. The absorption and electronic behavior of QNLs deposited with Titania and Niobia in conjunction with Silica were investigated using LCA. The impact of defects on the measurements are also discussed.
For future development of components for the DUV/VUV spectral range, the precise knowledge of the optical transfer functions such as spectral transmission or reflection are essential. Due to the strong atmospheric absorption of radiation in this range, spectral measurements must be carried out in the absence of oxygen. Tests are usually performed in vacuum or by purging the spectrometer with an inert gas. In vacuum environment, often a radiation induced formation of hydrocarbon contaminants on the surface of the optic is observed, that deteriorates the optical functioning and leads to erroneous measurement results. We report in this article on spectrometry tests under vacuum and purge conditions and compare the results for both environmental conditions. To avoid the contamination of the optics tests were carried out in the DUV spectral range under nitrogen purging. N2 itself shows vibrational and rotational molecular absorption bands in the wavelength range from 110 nm to 150 nm, which are present as peaks with arbitrary amplitudes in the spectrometry results. Several time domain parameters such as integration time, integration range, power monitoring aspects, and frequency bandwidth of the data acquisition modules of the spectrometer are studied to investigate the origin of the disturbing peaks. Also, different polarization states of the radiation, angle resolved scattering and pressure variation of nitrogen gas are considered to adapt the test procedures for managing the presence of these absorption lines in the measured data and to obtain reliable results.
Contaminations can lead to a reduction of the laser-induced damage threshold (LIDT) leading to an unexpected damage of the components coating inducing damaged areas significantly larger than the beam size. In this study, we developed a process to contaminate the surface of anti-reflective and high-reflective coated optics with Polyether ether ketone particles of the size 10-100 µm. Contaminated samples were then irradiated with a ns-pulsed high repetition 1 µm laser system regarding the determination of the LIDT. We especially illustrate detection as well as the irradiation and monitoring of a single particles during laser irradiation. In conclusion, we have not observed any damages on clean samples up to an energy density of 1 J/cm². However, the particles got already damaged one to two magnitudes below this leading to a significant decrease in the surface damage threshold.
A new approach for direct third-harmonic generation is the generation inside a stack of dielectric layers. At present, our highest conversion efficiency achieved is 3.5%. This contribution provides an overview of the design process, production, measurement results, and their agreement with simulation results. To create the frequency tripling mirror designs, we use a combination of a Monte Carlo algorithm and a Meep-based algorithm to solve Maxwell's equations. Mandatory for the production of the mirrors is a very precise knowledge of the dispersion data of the materials used. For this purpose, the dispersion data of the coating materials are re-fitted using in-situ transmission data of a BBM after each coating run. In combination with various measures to maintain a stable refractive index of the used Hf_xAl_yO, high coating thickness accuracies are achieved in this way. Finally, experimental measurements and simulation results are compared using the post-fitted dispersion and layer thickness data.
Nonlinear absorption is mainly governed by mechanisms involving excitation processes of electrons. Typically, two phenomena are considered when discussing nonlinear absorption; the multiphoton absorption where multiple photons interact directly with a single electron, and tunnel ionization, where the high electric field results in a shifting of the bandgap allowing an electron to tunnel into the conduction band. Electrons in the conduction band can be accelerated through the absorption of further photons until they obtain enough energy to excite further electrons to the conduction band, leading to runaway absorption and finally damage of the sample. By laser calorimetric measurement of the nonlinear absorption, it is expected that the laser damage threshold can be predicted without damaging the optic. Before accurate predictions can be made, the process must be thoroughly characterized and understood. The nonlinear behavior of the absorption was demonstrated with potential increases in absorption of an order of magnitude. Initial results show a noticeable impact of contaminants, though a nonlinear response is still observed.
Quantizing nanolaminates are an interesting alternative to classical coating materials with greater independence of refractive index and the optical bandgap energy. This leads to more flexibility and considerable potential to increase the laser-induced damage threshold in the ultra-short pulse regime. The following study presents and compares the design choices, characterization, and LIDT testing of different quantizing nanolaminates for the ultraviolet spectral range to classical coating materials.
This paper presents the active alignment of miniaturized, substrate-free optical thin-film filters (TFFs) according to the filters’ spectral transfer properties for integration into fiber optical networks. Optical TFFs are often designed for a specific narrow angle of incidence (AOI) range. Hence, a sufficient manufacturing precision of the angled photonic components connected to the optical filter is needed. These components then can no longer be used for different scenarios where i.e. the incident angle is changed. Conversely, the individual miniaturized optical filter chips can also vary in specification due to slight inhomogeneities during the production on a largescale wafer. Therefore, not all filter chips on the wafer meet the demanded specifications at the designed AOI, resulting in a reduced yield. Moreover, it requires a time-consuming separation into different quality classes by measuring single filter chips on the wafer. To maximize the amount of usable chips, a procedure was developed to actively align the chips inside a precision optics assembly system by measuring the transmitted power at different wavelengths while tilting them towards the optical axis. When the optimal angle is found, the chip is glued into the optical network platform. Next to maximizing the yield, the production steps can be reduced because the prior separation into quality classes becomes redundant. Manufacturing tolerances during the thinfilm deposition are equalized due to the active spectral alignment on a universal optical platform. With this technique, a more versatile process for TFF integration compared to passively aligned assemblies on fixed angle components is demonstrated.
All-silica mirrors manufactured using GLancing Angle Deposition (GLAD) are a promising approach for optics with improved LIDT. However, water content may increase over time due to the porosity of the low index silica layers, potentially changing the LIDT. Additionally, consecutive irradiation during LIDT testing may remove stored water and influence the LIDT.
Laser calorimetry, spectrophotometry and LIDT measurements, applying S-on-1 and R-on-1 methods, were used in order to determine the impact of laser-induced removal of stored water on the absorption, spectral behavior and laser damage resistance of all-silica mirrors. Influence of water reabsorption was investigated under different environmental atmospheric conditions.
We demonstrate a novel concept for an all-optical switch based on the optical Kerr-effect in thin film interference coatings. The switching between transmittance and reflectance relies on highly Kerr-active coating materials in combination with large internal intensity enhancement in thin film interference coatings. The paper investigates the switching performance as well as its relation to the laser induced damage threshold of these novel components. A modulation depth of 30 % was achieved without damage to the component, which very promising for later applications as power limiters or mode locking components.
We present a novel concept for optical switches, which is based on the optical Kerr-effect. In contrast to previous approaches, the switching is achieved by a combination of strongly Kerr-active materials with specially designed and produced optical interference filters. Intense laser irradiation causes refractive index changes in the sensitive Kerr-active layers of the components and the interference filter changes its spectral characteristics, i.e. becomes reflective, depending on design factors. The concept offers several advantages when compared to currently applied switching methods, such as the easier integration into photonic systems because of their compact nature and wide spectral application range.
Nowadays, continuous wave (cw) lasers have conquered a broad spectrum of applications in industrial laser processing and can be considered as the dominant tools in many manufacturing floors. This is reflected by the enormous average annual growth rates of 25-30 % and the continuous research efforts dedicated to this laser type leading to ever increasing output power and beam quality. This development imposes ever increasing demands on the quality of the optical laser components, that have to withstand the usually harsh industrial environment and high power levels. In fact, the corresponding of the laser components is a key factor for the efficiency and economic success of an employed laser material process. This in turn requires a thorough assessment of the quality parameters ruling the stability of such components. Among many other quality parameters, the Laser Induced Damage Threshold (LIDT) is one of the leading parameters that has to be investigated in detail. The corresponding measurement facilities and protocols as well as the evaluation of the data have to be performed with high reproducibility and comparability among different testing laboratories. As a consequence, such qualifications can only be achieved on the basis of well-defined international standards defining the complete procedure for the determination of LIDT values. We investigated the laser induced damage threshold of different types of optics using a cw laser with a wavelength of 1030 nm and power up to 6 kW, applying beam diameters of approximately 200-300 µm on the surface. The samples were irradiated for at least 30s or until damage occurred. First, it was necessary to review the existing DIN EN ISO 21254 regarding cw-irradiation of mirrors with a 25 mm diameter. An important aspect is the number of possible irradiation spots on each optic with respect to the damage size as well as the emitted debris. Both effects limit the statistical accuracy, the ISO procedure needs to be adapted to the measurement conditions. Additionally, we investigated the influence of substrate materials and coating processes on the LIDT of high reflective coatings and their damage behavior, especially regarding their thermal conductivity. The results were then compared with simulations concerning the maximum temperature within the optical component.
Atomic layer deposition (ALD) has been proven as an excellent method for coating high quality optical films due to its outstanding film quality and precise process control. Unfortunately, batch ALD requires time-consuming purge steps, which lead to low deposition rates and highly time-intensive processes for complex multilayer coatings. Recently, rotary ALD came in focus for optical applications. In this novel process concept, each process step takes place in a separate part of the reactor divided by pressure and nitrogen curtains. The substrates to-be-coated are rotated through these zones. During each rotation, an ALD cycle is completed, thus the deposition rate is mainly dependent on the rotation speed. In this study, the performance of a novel rotary ALD coating tool for optical applications is investigated and characterized with SiO2 and Ta2O5 layers. Low absorption levels of 3.1 ppm for 200 nm thick single layer of Ta2O5 and 6.0 ppm for 1032 nm thick single layer of SiO2 are demonstrated at 1064 nm, respectively, with growth rates up to 0.18 nm/s on fused silica substrates. Furthermore, excellent uniformity is also demonstrated with non-uniformity values reaching as low as 1.55 % and 2.71% for Ta2O5 and SiO2, respectively, over 120 mm on silicon wafers. Seven substrates up to a diameter of 200 mm can be coated in each run. Further investigations on uniformity improvements and multilayer coatings are currently ongoing.
We report on our recent results for output scaling by a Thulium-doped fiber Mamyshev oscillator in the spectral 2 µm region. In order to further scale the output, a highly Thulium-doped double-clad fiber was implemented in the Mamyshev oscillator. A stable pulse train at a repetition rate of 19.7 MHz with an average power of 220 mW was observed. This corresponds to a pulse energy of 11 nJ at a pulse length of 2.4 ps. Further scaling of the output parameters was limited by soliton fission, which will be mitigated by introducing normal dispersive fiber sections in future experiments.
An advantage of using additive manufacturing (AM) processes as opposed to conventional fabrication methods is that the additional degrees of freedom in design allow compact and at the same time lightweight components to be manufactured. In addition, AM reduces the material consumption, resulting in a more cost efficient production. Among others, the field of laser development benefits from the progressive implementation of AM-related opportunities. However, this integration is mostly limited to single components. In contrast, we present a compact, lightweight solid-state laser oscillator system for low-power applications based on additively manufactured optomechanical components via Fused Filament Fabrication (FFF). The laser system is based on a Nd:YVO4-crystal pumped externally with a fiber-coupled laser diode at a wavelength of 808nm and a maximum output power of 3 W. The commercial optical components, such as lenses and the crystal, are firmly embedded via FFF in quasi-monolithic optomechanics. Thereby, they are fixed at their position and thus secured against misalignment. Furthermore, sensor technology for temperature monitoring is implemented into the structure. The possibility of the FFF process to work with different materials in parallel is used here. This multi-material printing approach enables the use of the appropriate polymer for the individual mechanical and thermal requirements for any structural part. The thermal stability of the printed structures is evaluated to ensure damage-free operation of the 3D-printed polymer optomechanics. Furthermore, output power, optical-to-optical efficiency, beam pointing, and spatial beam profile of the laser system are measured for several on- and off-switching cycles as well as for long-term operation.
The principle of a Mamyshev oscillator depends on alternating spectral filtering between sections of spectral broadening by self-phase modulation. In the 2 µm wavelength range, this concept faces the difficulty that standard fibers are anomalous dispersive which limits the possible pulse energy to the pJ-regime without proper dispersion management. We applied ultra-high numerical aperture fibers with normal dispersion in order to achieve up-chirped pulses in an anomalous dispersive Thulium-doped gain fiber. With that design, we achieved mode-locked pulses with energies of 6.4 nJ and a compressed autocorrelation duration of 195 fs at a repetition rate of 16 MHz.
Recently, Mamyshev oscillators (MO) have attained a lot of attention, due to their generation of mode-locked pulses with outstanding output parameters in terms of output energy, spectral bandwidth and pulse duration. We present a MO with output pulse energies in the range of 0.5µJ, an optical spectrum ranging from 1010nm to 1060nm and an externally compressed autocorrelation duration of less than 100fs. This MO completely consists of commercially available standard step-index fibers. In order to handle the high pulse energies, we apply a few-mode gain fiber with a core-diameter of 20µm in the second arm of the oscillator.
There is an increasing demand for highly integrated optical and optoelectronical devices that provide active laser emission, adaptability and low optical losses. A well-established production technology for customized structures with high functionality and geometrical flexibility is additive manufacturing (AM). It enables new constructional degrees of freedom to overcome the limitations of substractive material processing such as milling and drilling. Commercial AM systems for metals and polymers are ubiquitous; whereas glass AM systems almost exclusively exist in scientific environments. Laser glass deposition welding allows the AM of waveguides by fusing coreless
fused silica fibers with a diameter of 400 µm and a 50 µm thick polymer coating onto a fused silica substrate. The deposition process is performed with defocused CO2-laser radiation (10.6 µm). Based on laser deposition welding, the fiber is fed laterally into the processing zone and is melted or fused by the incoming laser beam.
In order to achieve a sufficient coupling of laser radiation into and out of the fibers, a proper cleaving process for the end faces has been established. The cleaving is performed with a CO2-laser based process for optimized and reproducible results. In this contribution, the focus is on the manufacturing of bended waveguides and the feasible bending radii, which can be accomplished during the deposition process. The influence of the bending radius on the guiding efficiency is investigated. Therefore, the light transmission and beam profile of the deposited fibers is measured and compared with an untreated one. Furthermore, the appearance of the cleaved end faces and the internal stress in the glass substrate are characterized. Functional, nearly stress-free curved and straight waveguides for light transmission with high position stability are achieved, which opens a wide range of applications for optical system integration.
Solid-state white light sources gain increasing interest due to their advanced characteristics compared to conventional lighting solutions. New design challenges are introduced in the remote phosphor set-up by the substitution of the efficiency-droop-limited LEDs with laser diodes (LDs) that exhibit peak efficiencies at much higher operating currents. Although laser-excited remote phosphor (LRP) systems have already been employed in some commercial applications, the bottleneck in their performance is identified in the down-conversion process within the phosphor material. The high intensity exciting laser beam in combination with the temperature-dependent properties of phosphors can lead to thermally induced instabilities in the system. For this reason, an opto-thermal simulation framework is developed to investigate the optical and thermal interdependencies and derive the LRPS optimization parameters. The optical analysis is performed with commercial ray-tracing software, where the optical heat losses are computed and subsequently used as the volume heat source in thermal analysis implemented by the finite element method (F.E.M.). The question now arises as to how to properly model the phosphor material in such a simulation scheme. The LED experience has produced a variety of phosphors for lighting applications, most commonly powders in some appropriate resin matrix, which are treated simulation wise as bulk diffusers. As the low thermal conductivity of resins is deemed critical for their use in LRPS, recent research focuses on resin free materials such as glass phosphors, single crystals, polycrystalline dense ceramics, etc. The different modeling approaches of such solutions are investigated here as the scattering properties and surface topology of the samples can vary.
The use of additive manufacturing methods in research and industry has led to the possibility of designing more compact, light and low-cost assemblies. In the field of laser development, new opportunities resulting from additive manufacturing have rarely been considered so far. We present a compact, lightweight solid-state amplifier system for low-power applications where the optomechanical components are manufactured completely additive via Fused Filament Fabrication (FFF). The amplifier system is based on a Nd:YVO4-crystal pumped with an external, fiber-coupled diode at a wavelength of 808nm and a maximum output power of 3 W. The seed source is a Nd:YVO4-crystal based solid-state laser with an emission wavelength of 1064 nm. The commercial optical components, such as lenses and crystal, are firmly imprinted via FFF in the optomechanics and thus secured against misalignment. Additionally, sensor technology for temperature measurement is implemented into the devices. The use of FFF, in which the components are printed from polymers, results in a lightweight yet stable construction. We have shown, that optical components can be imprinted without adding mechanical stress. To increase the mechanical and thermal robustness of the system different types of polymers as well as post process treatments are tested and the use of Laser Metal Deposition for this application is investigated. The thermal stability of the printed structures is evaluated to determine the maximum power level of the system without damaging the polymer-optomechanics. Furthermore, output power, optical-to-optical efficiency, beam pointing, and beam shape are measured for several on- and off-switching processes as well as long-term operation.
We present an ultrafast fiber laser system at a central wavelength of 1750 nm for imaging applications, in particular 3-photon microscopy. It generates an output pulse train with an adjustable repetition rate ranging from 1 MHz to 21 MHz. After temporal compression the pulse duration is 220 fs and the maximum achieved pulse energy is 20 nJ.
The laser system consists of a polarization maintaining (PM) Erbium-doped fiber oscillator which emits a stable output pulse train at a fixed repetition rate of 42 MHz. The oscillator generates soliton pulses centered at a wavelength of 1560 nm and a spectral width of 7 nm. Mode-locking is initiated and stabilized by a semiconductor saturable absorber mirror. The output pulses are picked in a PM fiber coupled acousto-optic modulator to an adjustable repetition rate of 1 – 21 MHz. A consecutive Erbium-doped PM fiber amplifier (EDFA) boosts the energy of the soliton pulses from pJ to nJ level. The directly emitted pulses have a duration of 2 ps which can be compressed to a pulse duration of 115 fs by using a passive standard fiber. The uncompressed pulses are soliton-self-frequency shifted by Raman scattering to wavelengths longer than 1700 nm in 7 m of passive PM1550 fiber at a pulse energy of 1.1 nJ. The central wavelength can be adjusted by the pump power of the EDFA. To boost the pulse energy of the wavelength shifted pulses, the Raman stage is followed by a single-clad Thulium-doped fiber (TDF) amplifier. It consists of a 1560/1750 nm wavelength division multiplexer (WDM) and 0.9 m of TDF. To diminish nonlinear effects during amplification, the pulses are stretched with 25 m of normal dispersion fiber (NDF) inserted between the WDM and the TDF. Although on the very short wavelength amplification band, the pulses are amplified up to more than 40 nJ of pulse energy at an injected pump power of 4.1 W. After the fiber amplifier, the pulses are coupled out and propagate through a spectral filter, a triplet of l/4, l/2, and l/4 waveplates, an isolator, and a grating compressor. As the WDM, NDF, and TDF are not PM, the polarization state has to be readjusted to linear with the waveplates before entering the isolator. The added group delay dispersion of 2.17 ps2 by the NDF is compensated in a free space standard grating compressor built of two 600 lines/mm gratings. The transmission of the grating compressor is 60 %. To achieve optimum compression to a pulse duration of 220 fs at a pulse energy of 20 nJ, the compressor in combination with spectral filtering around 1750 nm has to be carefully adjusted. The maximum output pulse energy of 20 nJ is constant ranging from 1 MHz to 7 MHz, but is reduced at higher repetition rates down to 8.7 nJ. The output pulse duration is nearly constant at 220 fs for all repetition rates. Further amplification of the pulses is currently under investigation. This system will be used in future for the application of 3-photon microscopy.
Thulium-based fiber lasers potentially provide for the demand of high average-power ultrafast laser systems operating at an emission wavelength around 2 μm. In this work we use a Tm-doped photonic-crystal fiber (PCF) with a mode field diameter of 36 μm enabling high peak powers without the onset of detrimental nonlinear effects. For the first time a Tmdoped PCF amplifier allows for a pump-power limited average output power of 241 W with a slope efficiency above 50%, good beam quality and linear polarization. A record compressed average power of 152 W and a pulse peak power of more than 4 MW at sub-700 fs pulse duration are enabled by dielectric gratings with diffraction efficiencies higher than 98% leading to a total compression efficiency of more than 70%. A further increase of pulse peak power towards the GW-level is planned by employing Tm-doped large-pitch fibers with mode field diameters well above 50 μm. The coherent combination of ultrafast pulses might eventually lead to kW-level average power and multi-GW peak power.
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