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1. INTRODUCTION
Laser radiation in the short pulse regime, as well as in the long pulse regime have developed into a versatile tool in material modification (Strassl et al. , 2005a , 2005b ; Trusso et al. , 2005; Gamaly et al. , 2005) or to induce high pressure in material samples (Batani et al. , 2003; Jungwirth, 2005). There are also new concepts being discussed to produce high power ultra-short laser pulses (Fuerbach et al. , 2005).
Since the 1990s, the laser in general is a well established tool for hard tissue preparation in dentistry. Nevertheless, there are still some limiting factors for the "conventional" type of laser-assisted hard tissue preparation via the Erbium laser that are strongly related to the commonly employed ablation process, the water-mediated ablation, that is, the danger of inducing micro-cracks and heat accumulation at insufficient external cooling. These deficiencies are mainly dependent on the ability of a suitable laser pulse with efficient evaporation of the embedded water in the tissue, governed by several factors like pulse duration, pulse shape, and the intensity distribution (TEM profile) in the laser beam (Meister et al. , 2003; Strassl et al. , 2004).
In the 1990s, a new approach was proposed by several authors (Da Silva et al. , 1997; Feith et al. , 1996; Kohns et al. , 1997; Komashko et al. , 1999; London et al. , 1996; Mindermann et al. , 1993; Neev et al. , 1996; Rubenchik et al. , 1996; Serafetinides et al. , 1998; Serbin et al. , 2002) employing ultra-short laser pulses (USLP) with pulse durations in the picosecond down to the femtosecond regime. A very good review of the work and parameters on this topic is given by Vogel and Venugopalan (2003), where nearly 500 papers on the whole range of biological tissue ablation are reviewed, integrated and completed with own results and by Strassl et al. (2002), where the basic principles and parameters for ultra-short pulse laser ablation of dental hard tissue are summarized. Beside the already well discussed basic optical and medical requirements, the main demands for practical applicability are a sufficiently high ablation speed and the possibility of handling the laser device nearly as simply as a conventional dental...