Dissertation:
Passively mode-locked Yb:YAG thin-disk laser with active multipass geometry

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2009
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Passiv modengekoppelter Yb:YAG Scheibenlaser mit aktiver Multipass-Zelle
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Abstract
Ultrashort laser pulses in the microjoule regime are of prime importance for many applications, including high-speed micromachining, pumping of optical parametric oscillators, as well as basic research, e.g. in high-field physics. As compared to external amplifiers, ultrafast oscillators are very attractive due to their simplicity and compactness. In order to reach high average powers together with high pulse energies, a thin-disk laser crystal is the medium of choice, allowing for true power scalability by increasing the beam size and exploiting the excellent cooling properties of the disk. To some extend the pulse energies from an oscillator can be increased by using extended resonator cavities or by cavity dumping. Previous record pulse energies obtained directly from an oscillator in ambient atmosphere were below 2 μJ. These pulse energies were limited by the strong self-phase modulation of air. Higher pulse energies were obtained in a He-flooded thin-disk laser cavity with pulse energies of up to 11 μJ, at subpicosecond pulse length. One way to decrease the self-phase modulation is to use larger output-coupling rates in combination with a high-gain medium. However, the low gain of a thin-disk laser has to be overcome. This thesis describes a newly developed thin-disk laser oscillator with an increased round-trip gain by employing a self-imaging active multipass geometry. The generation of high energy subpicosecond pulses with a pulse energy of up to 25.9 μJ at a pulse duration of 928 fs, while operated in ambient atmosphere, could be demonstrated. Stable single-pulse operation has been obtained with an average output power in excess of 76 W at a repetition rate of 2.93 MHz. Self starting passive mode-locking was accomplished using a semiconductor saturable absorber mirror (SESAM) with a modulation depth of 3.5%. The laser was operated at output-coupling rates of up to 78%. For a better understanding of the mode-locking dynamics and especially the role of the SESAM, experiments with different macroscopically well characterized SESAMs have been performed; furthermore, various resonator geometries and different pump-spot sizes and thicknesses of the disk have been tried. The various setups are described and their operation parameters characterized. The such obtained experimental results show good agreement with numerical simulations. Numerical simulations have been performed to demonstrate the gain characteristics of a thin-disk laser with multiple passes of the pump and laser mode over the disk. Additional numerical simulations, simulating the evolution of the electric field within a mode-locked laser resonator show the mode-locking dynamics of such a laser. Various resonator geometries are compared and problems discussed. The numerical simulations show an approximate limit for the available pulse lengths with this kind of laser depending on the SESAM in use. Operation at small pulse durations is limited by the onset of double pulsing. On the basis of the numerical simulation an approximate limit for the available pulse energies while operated in ambient atmosphere is established. The operation regime that was best suited for the generation of high energy pulses is highlighted. Furthermore, reasons for pulse-energy limitations are discussed, e.g. (i) Kelly sidebands moving towards the center wavelength resulting in a destabilization of the pulses, (ii) the saturation parameters of the SESAM, resulting in the onset of double pulsing or causing Q-switching instabilities, (iii) the absorbed pump power, or (iv) limitations in the availability of fundamental mode operation at large average powers. To demonstrate the large potential of this laser as source for high energy pulses, first application experiments have been performed. Single-pass external frequency doubling with a conversion efficiency of 60% yielded more than 28 W of average power at 515 nm. Additionally, micromachining experiments show high quality results without burr or the occurrence of a heat-affected zone (HAZ), as they are regularly observed in milling experiments with nanosecond pulses.
Summary in another language
Für viele wirtschaftlich-technischen Anwendungen aber auch auf dem Gebiet der Grundlagenforschung sind ultrakurze Laserimpulse mit Energien im Mikrojoule-Bereich von großer Bedeutung. Entsprechende Impulsenergien wurden bisher in der Regel mit Hilfe externer Verstärkungssysteme erzeugt. Verglichen mit solchen Verstärkern sind Oszillatoren als hochenergetische Ultrakurzimpuls-Quelle aufgrund ihrer Einfachheit und Kompaktheit sehr attraktiv. Beim Verstärkungsmedium solcher Oszillatoren bietet die Scheiben-Geometrie entscheidene Vorteile, denn durch diese lassen sich neben den hohen Impulsenergien auch hohe mittlere Ausgangsleistungen gewährleisten. Mit einem Scheibenlaser ist eine Leistungsskalierbarkeit durch Vergrößerung des Strahlbündeldurchmessers unter Ausnutzung der exzellenten Kühleigenschaften der Scheibe möglich. Bis zu einem gewissem Grad kann die Impulsenergie von einem Scheibenlaseroszillator durch Cavity-Dumping oder mithilfe von passiv verlängerten Resonatoren vergrößert werden. Die höchsten Impulsenergien, die bisher direkt mit einem solchen in Umgebungsluft betriebenen Oszillators andernorts erzeugt wurden, liegen unterhalb von 2 μJ, im wesentlichen beschränkt durch die starke Selbstphasenmodulation der Luft. Größere Impulsenergien von bis zu 11 μJ und bei Pikosekundenimpulslängen konnten außerhalb der vorliegenden Arbeit nur in mit Helium-gefluteten Kavitäten erzeugt werden. Um den Einfluß der Selbstphasenmodulation der Luft auf die Modenkopplungsdynamik zu verkleinern, kann man größere Auskoppelgrade in Kombination mit einem Lasermedium höherer Kleinsignalverstärkung verwenden. Um dies aber mit den Vorteilen eines Scheibenlasersystems zu erreichen, muss der geringe Verstärkungsgrad der Scheibe umgangen werden. Thema dieser Dissertation war die Konzeption und Erforschung eines neuartigen modengekoppelten Scheibenlaseroszillators mit vergrößerter Kleinsignalumlaufverstärkung. Durch eine sich selbst abbildende Vielfach Übergangsgeometrie konnten erstmalig von einem Oszillator direkt erzeugte Impulsenergien von bis zu 25.9 μJ demonstriert werden. Die dabei generierten Impulse hatten eine Länge von 928 fs. Der in Umgebungsluft betriebene Laser hatte eine mittlere Ausgangleistung von 76 W bei einer Impulswiederholrate von 2.93 MHz und einer Auskoppelrate von 78%. Selbststartendes passives Modenkoppeln wurde mithilfe eines sättigbaren Absorberspiegels (SESAM) erreicht. Für ein besseres Verständnis der Modenkopplungsdynamik und insbesondere der Rolle des SESAMs wurden Experimente mit verschiedenen makroskopisch gut charakterisierten SESAMs durchgeführt. Darüber hinaus wurden verschiedene Resonatorgeometrien, Pumpfleckgrößen und Scheibendicken in ihrer Anwendung im Laserresonator untersucht. Die verschiedenen Aufbauten werden beschrieben und ihre Betriebsparameter verglichen. Bezogen auf die erreichten Effizienzen, Impulsenergien, Impulslängen und etwaigen Begrenzungen deckten sich die so gewonnenen experimentellen Ergebnisse mit numerischen Analysen. Es wurden Simulationen durchgeführt, die zum einen die Verstärkungeigenschaften des Scheibenlasers mit Vielfachdurchläufen der Pump und Lasermode über die Scheibe beschreiben und zum anderen die Propagation des elektrischen Feldes im modengekoppelten Resonator und damit die Modenkopplungsdynamik simulieren. Auch in der Simulation werden verschiedene Resonatorgeometrien miteinander verglichen und im Besonderen die Impulsdynamik innerhalb des Resonators untersucht. Eine durch Simulationen und Experimente bestätigte Erweiterung des Solitonenflächentheorems stellt einen einfachen Zusammenhang zwischen der Selbstphasenmodulation der Luft, der gesamten Gruppendispersion im Resonator, der externen Impulsenergie und Impulslänge, wie er für die von einem Laser mit Vielfach Übergängen über das Verstärkungsmedium erzeugten Impulse vorliegt, auf. Auf Basis der numerischen Simulationen werden darüber hinaus etwaige Probleme aufgezeigt und diskutiert, die die erreichbaren Pulsenergien des in Luft betriebenen Lasersystems mit Vielfachübergängen begrenzen. Das eindrucksvolle Potenzial dieses Laserkonzepts wird anhand erster Anwendungsexperimente aus der nichtlinearen Optik und Mikrostrukturierung demonstriert.
Subject (DDC)
530 Physics
Keywords
Scheibenlaser , Multipass-Zelle , Modengekoppelt , Thin disk laser , Mode-locked laser , Diode-pumped
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Cite This
ISO 690NEUHAUS, Jörg, 2009. Passively mode-locked Yb:YAG thin-disk laser with active multipass geometry [Dissertation]. Konstanz: University of Konstanz
BibTex
@phdthesis{Neuhaus2009Passi-9533,
  year={2009},
  title={Passively mode-locked Yb:YAG thin-disk laser with active multipass geometry},
  author={Neuhaus, Jörg},
  address={Konstanz},
  school={Universität Konstanz}
}
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