@phdthesis{Haas2019Wavel-46706,
  year={2019},
  title={Wavelength Beam-Combined Direct Diode Lasers of Highest Spatial Brightness},
  author={Haas, Matthias},
  address={Konstanz},
  school={Universität Konstanz}
}

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    <dc:contributor>Haas, Matthias</dc:contributor>
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    <dcterms:title>Wavelength Beam-Combined Direct Diode Lasers of Highest Spatial Brightness</dcterms:title>
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    <dcterms:abstract xml:lang="eng">Direct diode lasers are of great interest in many fields of today’s industrial laser materials processing. The striking advantages of such lasers compared to optically pumped solid-state lasers consist of a higher compactness and an enhanced electrical-to-optical conversion efficiency of up to 50% or higher. During the past decade low-brightness multi-kW direct diode lasers have successfully replaced lamp-pumped solid-state lasers in high-power metal processing applications such as brazing, cladding and welding. Quite recently, high-brightness dense wavelength beam-combined direct diode lasers have come of age which are potentially suited to serve all kinds of high-brightness kW-class laser metal processing applications like flat sheet metal cutting or remote welding, where, to this day, well-established high-power thin-disk or fiber lasers dominate the market. In this thesis, a novel external cavity architecture for dense wavelength beam combining of high-power broad-area laser diode bars has been investigated with regard to an efficient spatial brightness scaling towards the kW power level with minimal beam quality deterioration. The external wavelength-stabilizing multi-laser cavity is based on a single customized ultra-narrowband thin-film filter as dispersive optical element inside the resonator and enables spectral stabilization of hundreds of broad-area laser diode emitters at once, each at a unique wavelength. Subsequent spectral beam combination of the cavity output is performed by means of a diffraction grating and a cylindrical telescope which is used for linear dispersion matching between the thin-film filter and the combiner grating. For the investigations, both commercial and non-commercial state-of-the-art high-power 9xx-nm broad-area laser diode bars are used which are equipped with micro-optical beam transformation systems enabling beam combining in the fundamental-mode fast-axis beam dimension of the broad-area laser diode bar emitters. The achieved experimental results are benchmarked with respect to wavelength beam combining efficiency and beam quality preservation against a well-known external cavity architecture for dense wavelength beam combining using a single intra-cavity diffraction grating for simultaneous wavelength stabilization and beam combining. Using the novel approach, cross-talk-free spectral stabilization of individual diode bar emitters with a spectral channel spacing below 200 pm is achieved over the total diode operation current range with low thermo-optically induced wavelength shift of about 1 pm/W. The wavelength beam combining efficiency in the external laser cavity is about 80% and mainly limited by power losses at both the thin-film filter due to the spectral filtering and the combiner grating due to the restricted diffraction efficiency and depolarized power fractions of the diode bar emitters. Detailed experimental and theoretical studies of beam quality preservation in the external cavity show that the resulting beam quality of the combined cavity output is affected by several mechanisms which deteriorate beam quality in the beam-combining axis. These are the residual spectral linewidth of the stabilized emitters, diode bar smile, beam distortion induced by the spectral filtering and the dispersion mismatch between the thin-film filter and the combiner grating. As a result, the achieved beam parameter product in the beam-combining axis at the 100-W power level is an order of magnitude larger compared to the diffraction-limited value of an individual unstabilized diode bar emitter, where still no thermo-optically induced beam distortions are present. On the basis of simulations of the resulting beam quality deterioration, the cavity and the combiner setup are optimized for optimal beam quality preservation. An impact of the wavelength stabilization on the beam quality in the non-beam-combing axis is not observable. The novel external cavity structure has been applied to a laser diode module consisting of ten horizontally stacked actively cooled 150-W broad-area laser diode bars in order to realize a high-brightness kW-class direct diode laser module which potentially serves as a building block for a 4-kW direct diode laser system. An output power of 1.1 kW is achieved corresponding to an electrical-to-optical conversion efficiency of about 40%. The combined output beam has a symmetrical beam parameter product of about 6 mm x mrad in both beam axis. The 230 broad-area laser diode emitters of the laser diode module are spectrally stabilized within a bandwidth of 43 nm. Thermo-optically induced wavefront aberrations due to the heating of the thin-film filter and the combiner grating and furthermore an imperfect magnification of the dispersion-matching telescope are identified to be the reason for a degrading beam quality in the beam combining axis in high-power operation which is not observable in individual-bar experiments, where an order of magnitude lower intra-cavity power and stabilized spectral bandwidth are present.</dcterms:abstract>
    <dcterms:issued>2019</dcterms:issued>
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    <dc:creator>Haas, Matthias</dc:creator>
    <dc:date rdf:datatype="http://www.w3.org/2001/XMLSchema#dateTime">2019-08-21T10:47:56Z</dc:date>
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