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In recent years, there has been a growing interest in increasing the output power of ultrafast lasers to the kW-range. This allows higher productivity for laser material processing, e.g. for cutting of carbon-fiber reinforced plastics (CFRP) or for micro-machining. We developed an Yb:YAG thin-disk multipass amplifier delivering sub-8 ps pulses with 1.4 kW average power which is – to the best of our knowledge – the highest output power reported for a sub-100 ps ultrafast laser system so far. The amplifier is seeded by a regenerative amplifier with 6.5 ps pulses and 115 W of average power at a repetition rate of 300 kHz. Taking this repetition rate into account, the energy of the amplified pulses is as high as 4.7 mJ. This was achieved using a scheme with 40 mirrors in an array to geometrically fold the seed beam 40 times over the thin-disk. The beam quality was measured to be better than M²=1.4. This system was used in first experiments to cut CFRP with very good quality and with unprecedented efficiency. Additionally, the output beam of the amplifier was frequency-doubled in an LBO crystal to 820 W (70 % conversion efficiency) output power at the second harmonic wavelength (515 nm) and 106 W (26.5 % conversion efficiency) at the third harmonic wavelength (343 nm). Both results are record output powers for ultrafast laser systems at the respective wavelengths. In the presentation, we will show concepts on further power scaling of the system. Keywords: Laser amplifiers, Lasers, solid-state, Lasers, ytterbium, Ultrafast lasers

The need for ultra-short (sub-ps) pulsed laser systems with high power and high energy has advanced the mode-locked Ytterbium-doped thin-disk technology in the last decade. Therefore several research groups have made efforts to explore new laser crystals e.g. Yb:SSO, Yb:CAlGO or Yb:Lu 2 O 3 for the generation of sub-500 fs pulses in thin-disk oscillators. Another promising and known candidate for ultra-short pulsed lasers is Yb:CaF 2 , which has been so far only used in bulk laser architecture. In this work, we present the first demonstration of a mode-locked Yb:CaF 2 laser in thin-disk configuration. The resonator cavity was designed for eight passes through the disk per roundtrip at a repetition rate of 35 MHz. A saturable absorber mirror (SESAM) was used to obtain the soliton mode-locking. We achieved close-to transform-limited pulses with a pulse duration of less than 445 fs and an emission spectral width of 2.6 nm at FWHM (i.e. time-bandwidth product of 0.323). At the average output power of 6.6 W this corresponds to a peak-power of 430 kW and pulse energy of 190 nJ. To the best of our knowledge, this is the highest average output power and pulse energy using Yb:CaF 2 as gain material reported to date. Taking into account the dispersion, self-phase modulation, pulse energy, output coupling ratio and laser gain, the pulse-duration estimated from the soliton-equation and our numerical calculations of pulse-propagation is in good agreement with the pulse-duration obtained in the experiment. Higher powers and shorter pulse-durations with this material are the subject of our future investigations.

We report on a Yb:YAG thin-disk multipass amplifier for ultrashort laser pulses delivering an average output power of 1.1 kW which to the best of our knowledge is the highest output power reported from such a system so far. A modified commercial TruMicro5050 laser delivers the seed pulses with an average power of 80 W at a wavelength of 1030 nm, a pulse duration of 6.5 ps and a repetition rate of 800 kHz. These pulses are amplified to 1.38 mJ of pulse energy with a duration of 7.3 ps. To achieve this, we developed a scheme in which an array of 40 plane mirrors is used to geometrically fold the seed beam over the pumped thin-disk crystal. Exploiting the incoming linear polarization, an overall number of 40 double-passes through the disk was realized by using the backpath through the amplifier with the orthogonal linear polarization state. Thermal issues on the disk were mitigated by zero-phonon line pumping at a wavelength of 969 nm directly into the upper laser level and by employing a retroreflective mirror pair. The amplifier exhibits an optical efficiency of 44 % and a slope efficiency of 46 %. The beam quality was measured to be better than M 2 =1.25 at all power levels. As this system can deliver high pulse energies and high average output powers at the same time without the need of a CPA technique, it can be very suitable for high productivity material processing with ultrashort laser pulses.

We present the experimental investigations of different designs of resonant waveguide-grating mirrors (RWG) which are used as intracavity folding mirror in an Yb:YAG thin-disk laser. The studied mirrors combine structured fused silica substrates, a thin-layer waveguide (Ta2O5), a buffer layer (SiO2) and partial reflectors. The grating period was chosen to be 510 nm to allow resonances at an angle of incidence of ~10° for TE polarization. The waveguide layer has a thickness of 236 nm. It is followed by the buffer layer with a thickness of 580 nm and the subsequent alternating Ta2O5/SiO2 layers. The exact coating sequence depends on the two design approaches which were investigated in this work: either introducing different partial reflectors, i.e. stacks of quarter-wave layers on top of the waveguide while keeping the groove depth of the grating constant, or increasing the grating depth, while keeping an identical partial reflector. The investigation was focused on the rise of the surface temperature due to the coupling of the incident radiation to a waveguide mode, as well as on the laser efficiency, polarization and wavelength selectivity. It is found that, when compared to the simplest RWG design which consists of only a single Ta2O5 waveguide layer, damage threshold as well as laser efficiency can be significantly increased, while the laser performances in terms of polarization- and wavelength selectivity are maintained. So far, the presented RWG allow the generation of linear polarization with a narrow spectral linewidth down to 25 pm FWHM in a fundamental mode Yb:YAG thin-disk laser. Damage thresholds of 60kW/cm2 have been reached where only 63K of surface temperature increase was observed. This shows that the improved mirrors are suitable for the generation of kW-class narrow linewidth, linearly polarized Yb:YAG thin-disk lasers.

We report on Grating Waveguide Structures (GWS) with a high diffraction efficiency used in Littrow configuration to select (and tune) the wavelength of an Yb:YAG thin-disk laser. The structures are composed of a multilayer HR coating, on which an additional low index layer (SiO 2 ) and high index layer (Ta 2 O 5 ) was deposited. A binary grating with a period of 580 nm is etched on top of the structure with a groove depth of 87 nm for GWS 1 and 72 nm for GWS 2. The simulation results show that the diffraction efficiency in the -1 st order can reach a value of 99.99% for TE polarization, whereas it is only about 20% for TM polarization at 1030 nm. The grating was fabricated by standard interference lithography followed by a dry etching process to the desired groove depth. The spectroscopic measurement exhibited a diffraction efficiency of 99.6% for GWS 1 at 1030 nm and 99.7% for GWS 2 at 1048 nm. The devices were placed as end-mirror into the resonator of a Yb:YAG thin-disk laser. An output power of up to 110 W could be obtained from the laser in fundamental-mode operation (M 2 ≈ 1.2) with GWS 1, corresponding to an optical efficiency of η oo = 36.2%. In multi-mode operation (M 2 ≈ 6) a power of 325 W with η oo = 53.2% could be extracted. The spectral bandwidth of the emitted beam was measured using an Optical Spectrum Analyzer (OSA) to be less than 20 pm in fundamental-mode. We also showed a continuous wavelength tuning range of 46 nm for GWS 1 and of 38 nm for GWS 2. With a commercially available Stokes polarimeter the degree of linear polarization (DOLP) was measured to be higher than 98.6% over the whole power and wavelength tuning range.

The applicability of modern high-brightness solid-state laser sources for material processing purposes is limited by thermally induced effects in the beam guidance optics. These transient thermo-optical aberrations lead to a non-static focus shift and a deterioration of the beam quality. To counterbalance these aberrations, thermally self-compensated optical systems are proposed. The design for these optics is based on a combination of several elements that compensate for each other. Via thermo-mechanical FEM-simulations and the calculation of the resultant wavefront distortion, several multi-component systems are evaluated. In a first step, thermally compensated laser windows have been developed and characterized.

We present the nearly diffraction-limited propagation of 800 W of cw laser power through 100 m of delivery fiber having a core diameter of 30 μm. The laser source was a single-mode MOPA consisting of a fiber oscillator and two amplifier stages and was matched to the delivery fiber through a 500 mm long taper. At a maximum power of 800 W, a M 2 value of 1.35 was measured after 100 m of passive fiber. A minor Stokes-shifted spectral content was observed above 780 W.

We present a novel laser beam measurement setup which allows the determination of the beam diameter for each single pulse of a pulsed laser beam at repetition rates of up to 200 kHz. This is useful for online process-parameter control e.g. in micromachining or for laser source characterization. Basically, the developed instrument combines spatial transmission filters specially designed for instantaneous optical determination of the second order moments of the lateral intensity distribution of the light beam and photodiodes coupled to customized electronics. The acquisition is computer-based, enabling real-time operation for online monitoring or control. It also allows data storage for a later analysis and visualization of the measurement results. The single-pulse resolved beam diameter can be measured and recorded without any interruption for an unlimited number of pulses. It is only limited by the capacity of the data storage means. In our setup a standard PC and hard-disk provided 2 hours uninterrupted operation and recording of varying beam diameters at 200 kHz. This is about three orders of magnitude faster than other systems. To calibrate our device we performed experiments in cw and pulsed regimes and the obtained results were compared to those obtained with a commercial camera based system. Only minor deviations of the beam diameter values between the two instruments were observed, proving the reliability of our approach.

We present an Yb-CALGO thin-disk laser in continuous regime. In a slightly multimode configuration we obtain up to 29.5W of average power, corresponding to a slope efficiency of 40% and an optical-to-optical efficiency of 32%. We then design a monomode cavity, and achieve 25W of average power. The tuning range of the laser emission was from 1018 to 1052nm. In a Q-switch regime, we obtain more than 1 mJ at 100 Hz repetition rate, and 4.5W at 10 kHz repetition rate.

A system being able to in situ measure and control not simply the distance between the workpiece and the focusing optics, but the true focal position on the workpiece including the thermally induced focal shift in a laser processing head is presented. In order to achieve this, a bundle of astigmatic measurement beams is used following the same optical path as the welding beam. A camera and a software algorithm allow to keep the focal position constant within a range of 4 mm and with a resolution between 150 μm and 500 μm.

With the development of the thin-disk laser, thermal lensing issues have been drastically reduced. However, fundamental mode operation at high output powers is still limited by severe efficiency reductions due to diffraction losses introduced by the temperature gradient at the boundary of the pump spot. To countervail these aspherical wavefront deformations, high-power laser mirrors featuring deformable surfaces have been developed. The surface of these mirrors resembles the shape of the thermally induced change of optical path length in the pumped laser crystal. The magnitude of the surface deformation can be actively controlled to provide optimal compensation at different power levels.

Multi-kW beams with high brightness offer advantages in material processing applications with large distance beam propagation such as remote welding. To achieve the combination of high power and low beam parameter product, the thin disk laser concept is widely used due to its power scalability. Nevertheless, the efficient generation of several kilowatts of output power per disk at beam parameter products below ~3 mm·mrad is limited by an aspherical wavefront distortion in the disk and air turbulences in front of it. In the present paper the limiting factors are discussed and a novel method for compensation is presented. The compensating mirror consists of a silica substrate with a top-hat-shaped layer of 100 nm height to generate the desired phase-front correction and a conventional HR-coating on top. To prevent air convection in front of the thin disk crystal, the laser resonator was filled with helium. The experimental results yield a maximum output power of 3.4 kW and an optical efficiency of 49 % with a beam parameter product of ~2.6 mm•mrad (M 2 ~ 8) at a cooling water temperature of 30 °C.

In the recent years high-power laser beams with radial polarization have attracted an increasing interest because of their interesting properties in material processing. We present an overview of the current activities and different techniques to generate such beams in CO2 and solid-state lasers. With a polarising end-mirror which comprises a resonant grating on a dielectric multilayer Bragg-structure we recently demonstrated a 3-kW radially polarised CO2 laser. Current investigations are also focused especially on the application of this technology to thin-disk lasers. The specific requirements and the whole development from the design and fabrication to the characterization and test are illustrated with the example of a multilayer polarizing grating mirror developed to generate a radially polarised beam in an Yb:YAG thin-disc laser resonator. The potential of this kind of beams are discussed with a number of first application results, which largely confirm the predictions presented by Niziev et al1.

With the help of a newly implemented circular perfectly matched layer for complex coordinate stretching a fast and accurate calculation of radiation losses of optical waveguides is reported in the present contribution. We will show the results of a fully vectorial finite-element calculation used for the design of special fibers for highpower high-brilliance beam delivery. In particular, we have investigated the propagation losses in the so-called hollow-core and solid-core Bragg-type fibers. These optical fibers have claddings consisting of alternating high and low index layers and offer an asymptotically single-mode behavior even for large core sizes. In the case of the hollow-core Bragg fibers, the preferred mode is a non-degenerated azimuthally polarized doughnut mode (TE01) because it experiences the lowest losses, whereas for the solid-core Bragg fiber a two fold degenerated linearly polarized mode (LP01) experiences the lowest losses. We will describe how to design Bragg fibers for minimal propagation losses and how to reduce the bending sensitivity of these structures. Combining high mode-field diameter with low losses and a low bending sensitivity makes these fibers suitable for high-power single-mode beam delivery systems.

Beams with radial polarization have attracted an increasing interest during the past few years because of their attractive properties. An overview of the different intra- or extra-cavity techniques to generate such beams is given in the present paper. The design, fabrication and characterization of a multilayer polarizing grating mirror developed for an Yb:YAG thin-disc laser resonator are reported. The potential of the proposed solution is discussed together with the first demonstration of a radially polarized Yb:YAG thin-disc laser.

The objective of the present work is to develop a fiber suitable for high-power fundamental-mode beam delivery over a useful length for material processing (~100 m). The investigated design is based on evanescent-field coupled waveguides, also called multi-core fibers (MCFs) [6]-[8]. The investigated MCF consists of a hexagonal array of 7 cores in which the fundamental mode, the so-called in-phase supermode, has an effective mode area (A eff ) of 348 μm 2 and a numerical aperture (N.A.) of 0.035. We show how the bending-induced losses and the mode-mixing depend on the bending radius and on the structure of the waveguide. The experimental results on the behavior of the near-field and the far-field for different bending radii will also be reported. Additionally, we will show another method to reproduce experimental results with multi-mode fibers supporting few modes where the conventional approach leads to irreproducible results due to the mode-mixing effect.

Advanced pulsed thin disk laser sources based on several pulse generation schemes, including regenerative amplification as well as cavity-dumping, will be presented. These sources are able to produce pulse energies in the multi-millijoule range at repetition rates of up to several 100 kHz, resulting in average output powers in excess of 100 W. Also the efficient intra-cavity frequency conversion of these sources will be discussed.

The thin-disk laser (TDL) is a promising solid state laser concept since it combines good power scalability with high efficiency and small thermal lensing effects. Therefore this concept is a good candidate for near diffraction limited multi-kilowatt operation. In addition, due to its low depolarization losses, the TDL can be operated polarized without sacrificing its efficiency. To exploit the full potential of the TDL concept, the small thermally induced wave front distortions of the disk have to be compensated. At the same time, the intensity distribution of the output beam can be formed according to the requirements of the application. The wave front can be either corrected by a static graded-phase mirror or by an adaptive mirror. For the TDL, a thermally driven adaptive mirror was developed. The polarization state of the radiation field is another important aspect which can be optimized for specific processes. For example, a radially polarized beam promises significantly increased process efficiency for cutting sheet metal. The polarization state oscillating inside the resonator can be controlled by means of a polarization selective resonant grating-waveguide mirror. We describe the successive steps of development towards optical elements optimized for the intra-cavity beam shaping of the TDL.

The tunability of a diode pumped Yb:YAG laser is presented. With the thin disc laser concept good efficiencies with scalable output power are obtained. Up to 147 W output power, optical efficiencies of 64 percent and diffraction limited beam quality have been achieved.

Room temperature operation of a Nd:YVO4 thin disk laser with excellent beam quality is reported. Pumping by a fiber coupled diode laser with 10 W at the fiber end yields a continuous wave output power of 4.4 W at 1064 nm. The corresponding total optical efficiency is 44% and the average slope efficiency is 46%. For higher pump powers, a maximum output power of 13.7 W was achieved in a near diffraction limited beam.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A model to predict the optical efficiency and beam quality obtained from a diode pumped high-power thin disk laser is presented. The laser consists of a linear resonator with the quasi-three-level laser active material Yb:YAG placed as a thin disk on a cooling finger. The laser crystal is pumped by a near flat-top pump beam profile in a multiple pass configuration. Cw-output powers of 255 W at pump powers of 519 W have been achieved experimentally. Within the model the temperature distribution inside the crystal is calculated, taking into account the mutual dependence of temperature (and thus level population) and pump light absorption. The pump beam power distribution is modeled by Monte-Carlo-ray-tracing. The resonator is simulated by self-consistently circulating the power distribution within the resonator via transformations into a suitable mode system and back, using simple matrix multiplication. Thermal lensing due to thermo- optical and mechanical deformations is considered, using the results of finite-element-modeling. The resulting optical efficiency and beam quality have been calculated. The dependence on variables like temperature, dimensions of the pump beam source, number of pump beam passes and crystal thickness is discussed.

The qualification of the thin disc laser concept for obtaining output powers in the multi-hundred watt class with good beam quality and high efficiency simultaneously is presented.The output power is scalable by increasing the pumped area at constant pump power density. Output powers of up to 255 W in multimode operation, diffraction limited beam quality as well as single frequency operation with output powers up to 28 W are presented.

A continuously measuring carbon-dioxide laser power calorimeter designed for laser powers up to 25 kW is presented. The absorption of the detector is shown to be 99.98% at 10.6 micrometer and 99.1% at 1.06 micrometer. The flow-rate of the coolant is measured by determining the temperature increase due to an equivalent electrical heating of the coolant before the detector. An electrical calibration proves an accuracy of plus or minus 1% in the range from 40 W to 4 kW. A comparison with an instrument calibrated at the Physikalisch Technische Bundesanstalt (PTB) reveals a deviation of less than 0.5% at 850 W. The online-precision is limited by a relative and an absolute noise signal of 0.5% and 5 W, respectively. Methods to reduce the measurement errors occurring in the calorimetric design are presented.

Our new, scalable disk laser concept allows efficient diode-pumped high-power operation of quasi-three-level lasers with high beam quality. The use of a very thin crystal disk with one face mounted on a heat sink together with the low heat generation of Yb:YAG ( 5 kW/cm2) required for efficient operation without strong thermal degradation or optical distortion. Compact diode-pumped solid-state lasers in the kilowatt range seem to be achievable by increasing the pump beam diameter and/or by using several crystal disks. We present our latest results, obtained with a (phi) 2 mm X 0.4 mm Yb:YAG crystal operated at temperatures between 200 K and 300 K with an output power of up to 13.5 W and more than 50% optical-to-optical efficiency at 9.5 W. Simulations of thermal deformation and stress as well as of efficiency are also presented. Additionally, recent measurements of the tuning range of the diode-pumped Yb:YAG-laser, indicting the potential of femtosecond pulse generation, are shown.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.