Christoph Gerhard

Efficient versatile-repetition-rate picosecond source for material processing applications

We report on the development of an efficient and simple picosecond diode-pumped solid-state laser source with a versatile repetition rate (typically 1 Hz-1 MHz) for material processing applications. The laser source is based on a 4 MHz repetition rate mode-locked oscillator and a passive 3D multipass amplifier both based on Nd:YVO(4) crystals. Micromachining experiments were performed to study the influence of pulse energy on the machining quality for Al, Cu, paper, and glass.

Stable mode-locked operation of a low repetition rate diode-pumped Nd : GdVO4 laser by combining quadratic polarisation switching and a semiconductor saturable absorber mirror

In this paper, we present the mode-locked operation of an ultra-robustly stabilised Nd:GdVO(4) laser with low repetition rate by combining quadratic polarisation switching and a semiconductor saturable absorber mirror (SESAM). In addition, similar experiment was also done with Nd:YVO(4). For Nd:GdVO(4), 16-ps pulses at 1063 nm with a repetition rate of 3.95 MHz have been obtained for a laser average output power of 1.4 W. For Nd:YVO(4), the performance was 2.5 W of average power for 15-ps pulses at 1064 nm. Moreover, we demonstrate experimentally the advantage of combining these two passive mode locking techniques in terms of stability ranges. We show how the dual mode-locking technique is crucial to obtain a stable and long-term mode-locked regime in our case of a diode-pumped Nd:GdVO(4) laser operating at low repetition rate and more generally how this dual mode-locking technique improves the stability range of the mode-locked operation giving more flexibility on different parameters.

Increase in nanosecond laser-induced damage threshold of sapphire windows by means of direct dielectric barrier discharge plasma treatment

In this Letter we present a novel approach for increasing the nanosecond laser-induced damage threshold (LIDT) of sapphire windows. It is shown that after direct dielectric barrier discharge plasma treatment at atmospheric pressure for 90 s the LIDT is increased by a factor of 1.5 with respect to untreated samples. Several possible underlying mechanisms are introduced. For instance, organic contaminants and residues from polishing agents were removed by the plasma as ascertained by XPS measurements.

Investigation of nonuniform surface properties of classically manufactured fused silica windows

We report on investigations of the spatial variations of contamination, roughness, and index of refraction of classically manufactured polished fused silica surfaces. Therefore, laser-induced breakdown spectroscopy was used to probe surface and subsurface impurities via the detection of aluminum. Measurements at different positions on the surface of the cylindrical fused silica windows evidenced an almost contamination-free center region, whereas a relatively large contamination area was found close to the edge. In-depth measurements verify the presence of aluminum atoms in the bulk until a depth of several tens of microns for the edge region. In addition, atomic force microscopic measurements show that the surface roughness is larger in the center region compared to the edge. Further, the index of refraction increases from the center region towards the edge as measured via ellipsometry. The results indicate a nonuniform impact of the grinding, lapping, and polishing tools on the surface. The findings turn out to be of specific interest for different applications, particularly for the realization of large-scale high-performance coatings.

Hohe Qualität durch Laser-Plasma-Hybridstrukturierung

D Einsatz moderner Laserquellen erlaubt die Erzeugung kleinster 2Dund 3D-Freiformgeometrien und stellt somit in vielen Anwendungsbereichen das Mittel der Wahl zur Strukturierung von Ober ächen dar. Die direkte Laserbearbeitung von transparenten Werksto en wie Glas unterliegt jedoch Einschränkungen, da der Großteil der industriell eingesetzten Laserstrahlung von solchen Sto en nur ungenügend absorbiert wird. Hier wird der Einsatz relativ kostenintensiver Kurzpulsoder UV-Laser, beziehungsweise die Anwendung von sequentiellen Hybridverfahren erforderlich. Im letzteren Fall wird eine absorbierende Schicht auf das zu bearbeitende Glas aufgebracht. Das hier vorgestellte Verfahren beruht dahingegen auf der plasmainduzierten Änderung der Absorptionseigenscha en direkt im Glas. Plasmamodi kation von Glas Der Einsatz wasserstoffhaltiger Arbeitsgase bei der AtmosphärendruckPlasmabehandlung von Gläsern ermöglicht es, die chemische Zusammensetzung der Glasrandschicht zu modifizieren. Hierbei kommen Plasmaquellen zum Einsatz, die auf dem Prinzip der dielektrisch behinderten Entladung basieren und somit Plasmen erzeugen, welche keinen thermischen Ein uss auf das behandelte Substrat ausüben. Durch Dissoziation und Ionisation werden im Plasma reaktive Spezies erzeugt, die eine ober ächennahe Reduktion netzwerkbildender Oxide, zum Beispiel Siliziumdioxid, zu Suboxiden zur Folge haben. Des Weiteren bewirkt eine solche Plasmabehandlung die Einlagerung von Wassersto in tiefere Regionen des Volumenmaterials [1]. Diese E ekte führen zur Generation optisch aktiver Defekte im Glas, wie etwa Sauerstoff-Fehlstellen, Wassersto zentren und E‘-Zentren (ungebundene Siliziumatome in Glasnetzwerken). Dies bewirkt eine Steigerung der Absorption im ultravioletten Wellenlängenbereich. Bei der Wellenlänge des Argon-Fluorid-Excimerlasers von 193 nm beträgt diese circa 14 Prozent nach einer viertelstündigen Plasmabehandlung. Die Absorptionssteigerung stellt einen reversiblen E ekt dar. Die anfänglichen Transmissionseigenscha en können durch Langzeitlagerung oder Tempern wieder hergestellt werden.

From experiment to publication in one semester: a lecture course model on the basis of a photonic researcher's every-day tasks

We report on a lecture course model that we established three semesters ago in order to strengthen practice-orientated teaching in optics and photonics: In the frame of the lecture “Advanced Laser Treatment”, which is a mandatory course of our university’s master degree curriculum, students now have the possibility to experience a researcher’s every-day tasks. In small groups, the attendees work on a self-contained topic which is defined by the lecturers. The work load and content is in the scale of a small work package of a usual research project. It includes the initial research on the state of the art, the experimentation using different laser sources, and the subsequent evaluation of the obtained results. On the basis of this work, the students then prepare a draft of a scientific paper and finally present their results and findings orally in a conference-like exam. This lecture course model has turned out to be an appropriate teaching method for practice-orientated subjects. It was observed that the students are much more motivated and work more independently than during a classical lecture with a certain amount of lab work. Having sole responsibility supports to identify with their project. Further, this lecture course model helps to develop scientific work skills, attain first experience in every-day research tasks and encourages creativity. In some cases, the paper drafts written by the students can even be published, representing a valuable starting point for their future professional career.

Efficient versatile-repetition-ate ps source for material processing applications

We report on the development of an efficient and simple picosecond diode-pumped solid-state laser source with versatile repetition rate (typically 1Hz-1 MHz) for material processing applications.