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Several laser systems are currently under investigation for the purpose of removing hard dental tissues. However, either undesired thermal side effects or the lack of efficiency have already been demonstrated in most cases. In this paper, advantages and limitations of using ultrashort laser pulses with either picosecond or femtosecond durations are discussed. The major advantages associated with these pulse durations is the ability to produce very precise cavities without significant thermal side effects. Even disruptive effects due to shock wave generation seem to be negligible at moderate pulse energies close to the ablation threshold. The quality of these cavities is found to be superior to the quality achievable with other laser systems. Moreover, a spectroscopical analysis of the laser-induced plasma sparks enables an on-line health diagnosis of the irradiated volume. Limitations arise from the development of a suitable delivery system and from the cost of generating ultrashort laser pulses.

The dependence of the fluence at the threshold of laser- induced optical breakdown on the laser pulse duration has been investigated experimentally and theoretically for human cornea, human enamel, and bovine brain tissue. For the experiments in the range from 100 fs to 200 ps, we used a femtosecond dye laser system and a picosecond Nd:YLF laser system. We observed a significant decrease of the fluence at the threshold when reducing the pulse duration. The measured dependence on the pulse duration is in good agrement with our model.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The physical parameters of the plasma-induced ablation mechanism were investigated usinga picosecond Nd:YLF laser system. The laser consists of a diode-pumped oscillator and a lamp-pumped regenerative amplifier. It operates at a wavelength of 1.053 1um with pulse durations of 30 p5 and pulse energies up to 1 mJ. The laser beam was expanded to a diameter of 4 mm and focussed to spot sizes of about 30 ,um. At these high power densities a localized plasma was induced at the focal spot. Surfaces of extracted human teeth were used as target material. In order to study the effect of accompanying shock waves, dye penetration tests, hardness tests andpolarized microscopy were performed. At moderate pulse energies no significant impact of shockwaves was observed. Because of this result, the terms "plasma-induced ablation" or "plasma-mediated ablation" are more appropriate for ablations solely induced by plasma ionization, and should be distinguished from photodisruptive ablations. In another series of experiments thegenerated plasma sparks were spectroscopically analyzed. From the measured spectra, mean

A picosecond Nd:YLF laser system was used to remove sound and carious enamel by the mechanism of plasma-induced ablation. The plasma spark was optically imaged onto the entrance pupil of a spectrometer. The spectra were scanned between 400 - 700 nm with a typical resolution of 0.2 nm. Calcium in neutral and singly ionized states and the sodium doublet at 589 nm were observed. The second harmonic of the laser wavelength was generated in an external BBO crystal, thereby converting about 10 (mu) J of the pulse energy to radiation at 527 nm. The amplitude of the diffuse reflected second harmonic was used as a reference signal for normalization of the spectra. Several sound and artificial caries regions of different teeth were investigated. The spectra obtained from caries always showed a strong decrease in amplitude of all mineral lines, if compared to sound enamel. These results can be explained by the demineralization process of dental decay. Thus, caries infected teeth are easily distinguished from sound probes, enabling a computer controlled caries removal in the near future. The possible setup of such an automated system is discussed.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The effect of different wavelengths on corneal tissue ablation has been studied using the 1st, 2nd, and 4th harmonics of a picosecond Nd:YLF laser system. This laser system consists of an actively mode locked Nd:YLF oscillator generating 30 ps pulses and a regenerative amplifier. The laser beam was focussed onto fresh human donor or calf corneas. The corneas could be moved in 3D patterns using computer controlled stepping motors. The amount of the ablated tissue and the quality of the resulting surfaces were judged by light and scanning electron microscopy. The histology showed that the ablation depths of the IR and green laser pulses are very similar at a given energy density. SEMs revealed that UV light causes a denser packaging of the underlying collagen fibers, similar to the pseudo Bowman's membrane generated with excimer lasers. Further studies are planned concerning the investigation of the 5th harmonic at 211 nm.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Three different laser systems have been investigated according to their possible application in dentistry: a free running and a Q-switched microsecond Ho:YAG laser, a free running microsecond Er:YAG laser and picosecond Nd:YLF laser system consisting of an actively mode locked oscillator and a regenerative amplifier. The experiments focused on the question if lasers can support or maybe replace ordinary drilling machines. For this purpose several cavities were generated with the lasers mentioned above. Their depth and quality were judged by light and electron microscopy. The results of the experiments showed that the picosecond Nd:YLF laser system has advantages compared to other lasers regarding their application in dentistry.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.