Adjustable short-pulse laser

Short-pulse lasers are making a major contribution to advancements in miniaturization and in the development of new analytical and treatment techniques, particularly in production technology and medical engineering. Pulse energy and pulse duration play a large role in these technologies; in materials processing, for example, they directly affect the processing quality and thus delineate the specific field of application for each laser source.

Passive mode-locking generates brief laser pulses in the sub-nanosecond range and is ideal for creating rugged laser systems, for example for processing temperature-sensitive materials. Against this background, an innovative passive mode-locked laser for processing micromaterials has been developed at Laboratoire Charles Fabry de l‘Institut d‘Optique Graduate School. The basis of this laser source is a 38 m long resonator. A multiple pass cavity (MPC) [1] enables adjustment of the resonator length, L. Since the pulse duration, Δt, of a passive mode-locked laser is directly dependent on resonator length, the pulse duration can also be adjusted to a certain extent with an MPC.

Dual mode-locking

To achieve passive mode-locked operation, the project involved combining a saturable absorber mirror (SESAM) [2] with the quadratic polarization switching (QPS) method. The QPS method is an intensity-dependent mode-locking technique based on non-linear polarization effects in doubling crystals [3]. As a result, this dual mode-locking method stabilizes passive mode-locked operation considerably. This is also the first time passive mode-locked continuous operation of a diode-pumped Nd:GdVO 4 laser with a low  repeat rate has been achieved [4]. The laser active material examined and tested in this configuration included both Nd:GdVO4 and Nd:YVO4. In comparison, Nd:YVO4 showed a lower pulse duration (ΔtNd:YVO4 = 14.8 ps; ΔtNd:GdVO4 = 16.5 ps) and higher average output (PavNd:YVO4 2.52 W; PavNd:GdVO4 1.4 W). For this reason, Nd:YVO4 was used as laser-active medium in the further course of the project.

Adjustable pulse energy

Expanding the laser source by the addition of an extra-cavity acousto-optical modulator (AOM) permits variation of the repetition rate, f, and thus, in accordance with Formula 1, the variation of the pulse energy, E. In addition the experimental setup was extended to include a diode-pumped optical 4-pass amplifier, which  amplifies the laser beams up to 30-fold. When the passive mode-locked laser resonator is combined with the AOM and the amplifier (Fig. 1), the result is an optically boosted pico-second laser source with adjustable pulse energy.

Parameters for materials processing

Especially for applications in micromaterials processing, the focusability of the laser beam is a decisive criterion for processing quality. Focusability is indicated by the beam quality index, M². For the laser source placed in front, a beam quality index M² of 1.27 was determined (Fig. 2) by measuring a caustic. This high beam quality is also seen in subsequent experiments in materials processing. At a pre-set repetition rate of 100 kHz and when focusing through a laser lens with a focal length of 8 mm, the high peak performance density (14 W/cm2) led to a plasma breach in air in the focal point (Fig. 3). Drilling and cutting experiments on copper, aluminum, glass and paper show the advantages of this laser source in materials processing [5]. In particular, the adjustable repetition rate at constant low pulse duration permits creation of the optimum processing parameters for the particular material being processed (Fig. 4).

Summary and future prospects

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This project developed a very rugged passive mode-locked laser source. The dual mode-locking method significantly reduced the risk of destruction of intra-cavity saturable absorber mirrors. For the first time, passive mode-locked continuous operation of a diode-pumped Nd:GdVO at a low repetition rate was achieved. With pulses in the range of a few pico-seconds and adjustable repetition rate and energy, this laser source is ideal for micromaterials processing. Moreover, it can to a great extent be put to use in biological research, for example in analysis of fast chemical reactions and very short fluorescence durations. Furthermore, in cooperation with the XLIM Laboratory at the Université de Limoges, the creation of supercontinua with this laser source has already been demonstrated [6]

The author

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Graduate Engineer (FH) Christoph Gerhard completed his professional training as a precision optician and subsequently worked for two years as a technician and vocational training officer at the LINOS production site in Gießen. After that, Gerhard studied precision production engineering at the at University of Applied Sciences and Arts. He wrote his "Diplom" thesis in the field of short-pulse laser development in 2006, at the Laboratoire Charles Fabry de l‘Institut d‘Optique in Orsay near Paris. Since 2006, he has been Product Manager for Optics and Optics Software in the Catalog business unit at LINOS in Göttingen. He also teaches technical French at HAWK in Göttingen and heads up a German-French exchange program for students of optical technologies in Lower Saxony and in the French region of Île-de-France. For his Diplom thesis, the topic of which is the subject of this article, he was recently awarded the 2009 Georg-Simon-Ohm prize by the German Physics Society (Deutsche Physikalische Gesellschaft e.V., or DPG). From the speech given at the awards ceremony:

"His work unites a number of exceptional attributes; his thesis and two other papers in which he is listed as the main author are world-class publications. The results are accessible to a broad, user-oriented audience and open up new aspects in the development of Nd:YVO4 lasers. (...) Christoph Gerhard's outstanding work has made an important contribution to the development of compact high-capacity ultra-short pulse laser systems. The versatility of the laser source developed opens new possibilities for applications in micromaterials processing, fluorescence microscopy and the creation of a supercontinuum in structured optical fibers." (Physik Journal 8 (2009) No. 1, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim).