Publications Search Results
1 - 10 of 188
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PublicationEfficient intra-cavity frequency doubled, diode-pumped, Q-switched Alexandrite laser directly emitting in the UV( 2024)We present an intra-cavity frequency doubled Q-switched diode-pumped alexandrite ring-laser directly emitting in the UV at 386 nm. Using LBO as nonlinear crystal, the laser yields a pulse energy up to 3 mJ at 500 Hz with an excellent beam quality of M2 = 1.1. The pulse length is about 920 ns, allowing for very narrow bandwidth in single longitudinal mode operation. The optical-to-optical efficiency for the UV laser is > 9% and almost unchanged compared to the fundamental laser. First injection-seeding experiments show single longitudinal mode operation. The parameters of the laser are suitable for the use as an emitter in a multi-purpose atmospheric Doppler lidar system.
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PublicationMultiphysics analysis of chip and resonator designs for increased damage threshold of external cavity high-power laser diodes( 2024)We present a detailed analysis of multiphysics simulation results to evaluate the threshold for catastrophic optical damage (COD) of high-power laser diodes under misaligned external optical feedback. Three different chip designs are investigated: the non-injecting mirror concept, the non-absorbing mirror concept and the introduction of an additional energy barrier within the waveguide near the front facet. Furthermore, a modification of the external resonator that promises a lower sensitivity towards misalignments is considered. The dependence of the COD threshold on the additional design parameters (bandgap change, modification length, focal length) and the impact of the different approaches on electro-optical efficiency as well as beam quality are analyzed. Compared to the initial design, the different chip design concepts promise an increase of the achievable output power by 8%, 27% and 27% respectively, whereas the modified resonator fully prevents feedback-induced failure.
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PublicationAnalysis and fabrication of tapered multicore gain fibers for high power lasers( 2023)Numerical analysis of multicore fiber tapers as a mode area scaling technique for use in coherently combined laser systems is demonstrated. Taper designs are modelled with beam propagation method (BPM) numerical simulations to analyze inter-core crosstalk and mode distortion in centimeter-scale taper transitions. By evaluating taper performance for a range of taper lengths on the scale of centimeters, optimized taper designs can be found for a given MCF design. Tapers based on these simulations are fabricated using a CO2 laser tapering system.
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PublicationLatest developments in coaxial multiwire high-power laser claddingLaser cladding is widely used in the industry to precisely apply tailored surface coatings, as well as three-dimensional deposits for repair and additive manufacturing of metallic parts. However, the processing of larger components is economically challenging mainly because of low deposition rates. At Fraunhofer IWS, a Laserline fiber-coupled diode laser with 20 kW power has been employed for over a decade to develop competitive coating solutions with powder-based laser cladding. The deposition rates achieved with this technology is comparable to common PTA technique at the same time bringing significant advantages in terms of reduced heat affected zone, distortion, and savings in material resources. While high-power powder-based laser cladding is an industrially established coating technology, for example, to coat hydraulic cylinders or most recently brake discs, a high-productivity solution for wire-based processes is still challenging. Fraunhofer IWS has developed a new nozzle for high-power high-productivity laser wire cladding for coating and additive manufacturing, the so-called COAXquattro. This system enables to feed at the same time four wires into the melt pool, reaching deposition efficiencies in the same range as a powder-based laser process. For selected materials, the improvement in coating quality compared to powder laser cladding is achieved. Furthermore, with COAXquattro system simultaneous feeding of powder particles to wire cladding presents a great potential for in situ alloying and cost-effective production of new compositions on material alloying or hardmetal-reinforced composites for coating application and 3D additive manufacturing.
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PublicationEnergy-scaling of a diode-pumped Alexandrite laser and prototype development for a compact general-purpose Doppler lidar( 2023)We present design and performance data of an energy-scaled diode-pumped Alexandrite laser in single longitudinal mode operation developed as a beam source in a mobile general-purpose Doppler lidar. A maximum pulse energy in Q-switched operation of 4.6 mJ and a maximum average power of 2.7 W were achieved for a repetition rate range from 500 to 750 Hz with excellent beam quality of M²=1.1. Two rugged and compact demonstrator lasers were built and integrated into mobile lidar systems, where a bandwidth of approximately 3 MHz is measured. Measurements of atmospheric winds and temperatures were conducted during several field campaigns from summer 2022 to spring 2023.
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PublicationEvaluation of design concepts for feedback-resistant 9xx-nm high power laser diodes( 2023)In this work, we compare four different design concepts for external-cavity laser diodes (ECDL) with respect to the maximum achievable output power before the onset of catastrophic optical damage (COD). A multiphysics model of the ECDL with a self-consistent description of the electrical, optical and thermal properties of the device is used to evaluate the COD level. The feedback-induced failure is provoked by shifting the fast axis collimation (FAC) lens along the fast axis (smile error) resulting in an absorption of the feedback radiation within the highly p-doped and contact metal layers. The investigated design concepts include three local modifications at the front facet of the laser diode chip itself which are supposed to suppress injected current, optical absorption and leakage current from the quantum well. Within the considered parameter space these approaches lead to an increase of the COD level by 8%, 27% and 27% respectively, however at the cost of drawbacks like slightly reduced efficiency or beam quality along the fast axis. By combining all three approaches the output power can be increased by 37%. The fourth approach uses an additional lens within the external resonator to make it bi-telecentric and allows for a feedback field without image reversal. This approach completely removes the sensitivity of the setup regarding a vertical misalignment of the FAC lens. The drawback in this case is the increase of the resonator size by approximately a factor of 20.
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PublicationIndustry and academia: pursuing a PhD at Fraunhofer, the best of both worlds( 2023)Pursuit of an academic career is often co associated with a PhD which also is a qualification for all types of jobs in industry. However, in Germany most PhD programs focus on university-based, basic, and applied research and aim to demonstrate concepts while the transfer to industry and real products is subordinate. This is where Fraunhofer comes in: our natural science and engineering PhD students participate in solving real-world problems for our industrial customers with innovative and scientific approaches while they simultaneously pursue basic research questions with an application relevance for their PhD thesis. As an example, in this paper we present a multiphysics laser diode simulation software (SEMSIS) which was developed within two industrially funded PhD projects at the Fraunhofer Institute for Laser technology ILT. In the fusion research, a vast number of high-power laser diodes are used as pump sources for the high-energy pulsed lasers in inertial confinement fusion. Improving their electro-optical efficiency and making them more robust against external optical feedback represents a crucial step towards their use in economically competitive fusion power plants. In the presented simulation software tool SEMSIS, the complex interaction of electrical, optical, thermal as well as mechanical properties and their impact on efficiency, filamentation and reliability of high-power diode lasers can be analyzed to address the previously mentioned requirements in fusion research.
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PublicationHighly efficient, in-band pumped thulium-doped fibers for high-power ultrafast 2 µm wavelength laser systems( 2023)Ultrafast fiber laser sources emitting fs-pulses around 2 µm have many applications in medicine, metrology and sensing as well as in various frequency-conversion techniques. Thulium-doped fiber amplifiers are a promising platform for power scalable ultrafast amplification in this wavelength region. Usually, these ultrafast, high-power fiber laser systems were pumped at a wavelength around 790 nm and obtain slope efficiencies in the range of 50 % in the 100 W-class. Due to the high quantum defect obtained with this pump technique and the related high heat loads, considerable thermal challenges still must be overcome when scaling the power further. In this contribution we present a concept on highly efficient, high-power thulium-doped fiber amplifiers pumped at 1692 nm. This pump concept is suitable for high-power, high-energy, ultrafast Tm-doped fiber laser systems. In this proof of principle demonstration, we achieve a slope efficiency of 80% in a standard commercially available, thulium-doped photonic crystal fiber (PCF) with ~60 W of average power when pumping at 1692 nm compared to 47 % slope efficiency by pumping at 793 nm. In the simulation we investigated the heat load and core temperature evaluation along the fiber. These findings demonstrate an improvement in the amplification efficiency of large-mode area fiber amplifiers which are suitable for ultrafast operation on Yb-like efficiencies. The reduced heat load paves the way to even higher average powers from ultrafast Tm-doped fiber lasers with the potential to provide multi-mJ energy fs-pulses at kW-level average power from a single amplifier channel.
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PublicationThe impact of structural birefringence in multicore fibers( 2023)In this work we study the origin of birefringence in multicore fibers. With the help of simulations and experiments we are able to identify a new type of birefringence arising in multicore structures: structural birefringence. Hereby birefringence arises due to the intrinsic stress created by each core in the array and its value and orientation of the main polarization axis is dependent on the position of the cores in the array. We provide a comprehensive analysis of structural birefringence, and discuss ways to solve this problem.
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Publication2D and 3D triangulation are suitable in situ measurement tools for high-power large spot laser penetration processes to visualize depressions and protrusions before perforating( 2022-05)During laser penetration, the irradiated samples form a melt pool before perforation. Knowledge of the dynamics of this melt pool is of interest for the correct physical description of the process and leads to improved simulations. However, a direct investigation, especially at the location of high-power laser interaction with large spot diameters in the centimeter range is missing until now. Here, the applicability of 2D triangulation for surface topology observations is demonstrated. With the designed bidirectional 2D triangulation setup, the material cross-section is measured by profile detection at the front and back side. This allows a comprehensive description of the penetration process to be established, which is important for a detailed explanation of the process. Specific steps such as surface melting, indentations, protrusions during melt pool development and their dynamics, and the perforation are visualized, which were unknown until now. Furthermore, a scanning 3D triangulation setup is developed to obtain more information about the entire melt pool at the front side, and not just a single intersection line. The measurements exhibit a mirror-symmetric melt pool and the possibility to extrapolate from the central profile to the outer regions in most cases.
