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Threshold values of laser-induced optical breakdown (LIOB) on the surface of human corneal tissues, human enamel, and bovine brain tissues are presented. The data are obtained by using a regeneratively amplified Nd:YLF laser and a multistage dye laser system, respectively. The measured decrease in threshold fluence at shorter pulse durations is in good agreement with the authors' theoretical model.

Caries – the most frequent cause for dental surgery – still is mainly treated with conventional mechanical drills, although lasers have meanwhile been successfully applied to various clinical disciplines. Since ultrashort laser pulses with sufficient pulse energies have only been available at low repetition rates (< 1 kHz) in recent decades, solely continuous wave radiation or pulse durations longer than thermal diffusion processes were applied with the result of severe thermal damage and pain. In this report we present results on dental tissue ablation obtained with a novel thin disk Yb:KYW regenerative amplifier system that does not require chirped pulse amplification (CPA). We show that femtosecond laser pulses provide us with today’s optimal tool to treat dental decay in an acceptable time, in an excellent quality, and with unsurpassed caries selectivity. The superior quality is a result of the non-thermal laser-tooth interaction. All our results are based on environmental scanning electron microscopy.

Several laser systems for the removal of hard dental substances are currently under investigation. However, in most cases, such systems have been demonstrated to be inefficient or have led to undesirable thermal side-effects. This paper reports, for the first time, the removal of enamel and dentin by a picosecond laser system, a solid-state Nd:YLF laser. Very precise cavities can be obtained in the enamel and dentin of extracted human molars when laser pulses are distributed onto well-defined areas of the teeth. Scanning electron microscopy shows that the quality of the cavities is superior to that achieved by other laser systems. The cavity walls are very steep, and their surfaces are characterized by a sealed structure. In contrast to laser systems with longer-duration pulses, picosecond laser pulses ablate with less thermal damage to the surrounding substance. The results of dye penetration tests and polarized microscopy show that even mechanical shock-wave effects are negligible. When the Nd:YLF laser is applied to carious enamel, the ablation rate is found to be about ten times higher than for sound molars, thus making the Nd:YLF laser a caries-selective laser system.

Plasma-mediated ablations of brain tissue have been performed using picosecond laser pulses obtained from a Nd:YLF oscillator/regenerative amplifier system. The laser pulses had a pulse duration of 35 ps at a wavelength of 1.053 µm. The pulse energy varied from 90 µJ to 550 µJ at a repetition rate of 400 Hz. The energy density at the ablation threshold was measured to be 20 J/cm2. Comparisons have been made to 19 ps laser pulses at 1.68 µm and 2.92 µm from an OPG/OPA system and to microsecond pulse trains at 2.94 µm from a free running Er:YAG laser. Light microscopy and scanning electron microscopy were performed to judge the depth and the quality of the ablated cavities. No thermal damage was induced by either of the picosecond laser systems. The Er:YAG laser, on the other hand, showed 20 µm wide lateral damage zones due to the longer pulse durations and the higher pulse energies.

Background and Objective Diode pumped, all-solid-state ultrafast lasers are now widely used to perform minimally invasive refractive surgery and keratoplasty procedures. Despite such use, a systematic study concerning ultrafast laser–tissue interactions is lacking. We determined the corneal ablation threshold as a function of the laser pulse width and stromal depth by simultaneous monitoring of the intensity of the laser-induced plasma and the second harmonic generation signals (SHG) from the collagen. Study Design/Materials and Methods Ablation thresholds in porcine cornea samples were determined using three diode pumped all-solid-state ultrafast lasers (a Nd:glass femtosecond laser, a Yb:KYW femtosecond laser, and a Nd: YAG picosecond laser) over a range from 800 femtoseconds to 20 picoseconds. Results Corneal ablation threshold remained nearly constant within the first 200 µm of stroma and was consistent with previous findings with the threshold proportional to the square root of the laser pulse width. Conclusion Corneal ablation thresholds can be precisely determined by simultaneous monitoring of the intensity of the laser-induced plasma and the SHG from the cornea. Lasers Surg. Med. 39:654–658, 2007. © 2007 Wiley-Liss, Inc.

Although lasers are nowadays widely accepted as a popular scalpel of minimally invasive surgery (MIS), one of the most common orthopedic surgeries-the replacement of the knee joint-is still performed using an ordinary oscillating saw. Since ultra-short laser pulses are usually considered to be inefficient regardless of their high precision, the newest development of femtosecond laser systems has not yet been clinically applied to any mass ablation situation. However, thin disk Yb:KYW lasers meanwhile provide sufficient output power to ablate bone tissue within a reasonable time frame. Our results mainly focus on ablation rates obtained at different spot distances, repetition rates and pulse energies. It is shown that femtosecond laser pulses at high repetition rates are a promising tool for orthopedic surgery.

In this and the following chapter, we will discuss basic phenomena occurring when matter is exposed to light. While here we will be concerned with various actions of matter on light, the opposite effect will be discussed in Chap. 3. Matter can act on electromagnetic radiation in manifold ways. In Fig. 2.1, a typical situation is shown, where a light beam is incident on a slice of matter. In principle, three effects exist which may interfere with its undisturbed propagation: reflection and refraction, absorption, scattering.

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.

Threshold measurements for laser-induced breakdown of different organic tissue (cornea, brain, enamel) and inorganic matter (fused silica) have been extended to the sub-lOOfs time regime. A Ti:Sapphire laser system delivers 450 microjoules at repetition rates up to 1kHz. Single shot experiments are performed with pulse durations from 35fs to 240fs. To ensure a consistent determination of the threshold fluence, damage is detected by the alteration in amplitude of scattered light and/or by the occurrence of a plasma spark. In our single shot experiment with human enamel, no significant threshold dependence on pulse duration was observed. For soft tissue (cornea, brain), however, a slight decrease of threshold fluence is found when shortening the pulse duration. Fused silica is chosen as an additional target to compare the obtained data with those of recent experiments.

We investigated plasma-mediated surface ablation in corneal tissue using picosecond and femtosecond laser pulses in order to achieve high precision, non-thermal tissue removal with a non-ultraviolet laser source. Experiments utilized three laser systems, a regeneratively amplified Ti:sapphire laser, a synchronously amplified dye laser, and a regeneratively amplified picosecond Nd:YLF laser. Tissue ablation was performed by tightly focusing the laser beam on the tissue surface. Ablation thresholds were determined by monitoring the plasma spark, as well as the tissue surface. Tissue ablations were then analyzed by standard histologic methods and scanning electron microscopy. We observed a decrease in the ablation fluence threshold as the pulse duration is shortened from 200 ps to approximately 140 fs, in agreement with our theoretical predictions. Using identical pulse energies, the femtosecond laser pulses ablated tissue at higher efficiencies than the picosecond laser, with an approximately two-fold improvement in the etch depth curve. Histologic analysis reveal minimal adjacent tissue damage at either pulse duration. Femtosecond laser pulses may offer advantages that make them ideal tools for high precision tissue ablation.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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 potential of all-in-one techniques in modern femtosecond solid state lasers is being discussed. For first experiments we designed an electrooptically switched Ti:Sapphire laser. Q-Switching and dumping was performed at half-wave voltages. The generation of ultrashort pulses with energies at the μJ-level in one single cavity seems to be within the realms of possibility.

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.

In vitro ablation experiments of calf brain tissue using a picosecond Nd:YLF laser system are reported. The laser lesions were studied with special regard to the ablation rate and to possible injuries of tissue adjacent to the excisions. Histological examinations prove the excellent quality of the lesions, showing no signs of thermal damages or carbonization. Furthermore, the influence of higher photon energies on the ablation threshold was studied.© (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.

A laser system is described that offers manifold applications in ophthalmic surgery. The Nd:YLF solid-state laser emits at a wavelength of 1.053 micrometers . With pulse durations of 40 psec less energy is required to obtain optical breakdown compared to nanosecond lasers. The lower pulse energy allows a substantial confinement of the laser-tissue interaction. With a highly sophisticated delivery system eye movements can be tracked during surgery and a precise placement of the laser focus can be achieved. The high transmission coefficient of ocular structures for wavelengths around 1 micrometers enables the Nd:YLF laser to perform several internal eye surgeries such as cataract fragmentation, intrastromal refractive surgery, internal sclerostomy, or iridotomy. First clinical results of some of these procedures are summarized.