Michael D. Perry

Plasma mediated ablation of biological tissues with nanosecond-to-femtosecond laser pulses: relative role of linear and nonlinear absorption

Plasma mediated ablation of collagen gels and porcine cornea was studied at various laser pulse durations in the range of 1 ns-300 fs at 1053-nm wavelength. It was found that pulsed laser ablation of transparent and weakly absorbing gels is always mediated by plasma. On the other hand, ablation of strongly absorbing tissues is mediated by plasma in the ultrashort-pulse range only. Ablation threshold along with plasma optical breakdown threshold decreases with increasing tissue absorbance for subnanosecond pulses. In contrast, the ablation threshold was found to be practically independent of tissue linear absorption for femtosecond laser pulses. The mechanism of optical breakdown at the tissue surface was theoretically investigated. In the nanosecond range of laser pulse duration, optical breakdown proceeds via avalanche ionization initiated by heating of electrons contributed by strongly absorbing impurities at the tissue surface. In the ultrashortpulse range, optical breakdown is initiated by multiphoton ionization of the irradiated medium (six photons in case of tissue irradiated at 1053-nm wavelength), and is less sensitive to linear absorption. High-quality ablation craters with no thermal or mechanical damage to surrounding material were obtained with subpicosecond laser pulses. Experimental results suggest that subpicosecond plasma mediated ablation can be employed as a tool for precise laser microsurgery of various tissues.

Ultrashort pulse lasers for hard tissue ablation

To date, lasers have not succeeded in replacing mechanical tools in many hard tissue applications. Slow material removal rates and unacceptable collateral damage has prevented such a successful transition. Ultrashort pulses (<10 ps) have been shown to generate little thermal or mechanical damage. Recent developments now enable such short-pulse/high-energy laser systems to operate at high pulse repetition rates (PRRs). Using proper operating parameters, ultrashort pulse lasers (USPLs) could exceed the performance of conventional tissue processing tools and yield significant material volume removal while maintaining their minimal collateral damage advantages. As such, for the first time, USPLs offer real possibility for practical replacement of the air-turbine dental drill or other mechanical means for cutting hard tissues. In this study, the subpicosecond interaction regime was investigated and compared to nanosecond ablation by using a Titanium:Sapphire Chirped Pulse Amplifier (CPA) system with 1.05-/spl mu/m pulses of variable duration. Both 350-fs and 1-ns pulse regimes were studied. Ablation rates (ARs), ablation efficiency, and surface characteristics revealed through electron micrographs were investigated. The study characterized the interaction with a variety of hard tissue types including nail, midear bone, dentin, and enamel. With 350-fs pulses, tissue type comparison showed a remarkably similar pattern of ablation rate and surface characteristics. Negligible collateral damage and highly efficient per-pulse ablation were observed in this pulse regime. These observations should be contrasted with the 1-ns pulse ablation characteristics where strong dependence on tissue type was demonstrated and ablation efficiency was approximately an order of magnitude smaller. With efficient interaction which minimizes collateral damage, and with both cost and size of ultrashort pulse systems decreasing, the implications of this study are far-reaching for the efficient use of USPLs in several fields of medicine that currently apply traditional surgical methods.

Femtosecond Laser Materials Processing

The use femtosecond pulses for materials processing results in very precise cutting and drilling with high efficiency. Energy deposited in the electrons is not coupled into the bulk during the pulse, resulting in negligible shock or thermal loading to adjacent areas.

Surgical and dental applications of ultrashort pulse lasers

Employing new advances in the technology of ultrashort pulse lasers, a laser parameter regime which could provide hard tissue interaction characteristics that are superior to conventional mechanical drill technology has now been identified. The major advantages of this tissue ablation method are: 1) efficient ablation; 2) minimization of collateral damage; 3) ablation thresholds and ablation rates which are relatively insensitive to tissue type; 4) high control over ablation depth is achievable because only a small amount of tissue is ablated per pulse; 5) low operation noise level; and finally, 6) precise spatial control due to the multiphoton nature of the interaction. In our experiments, laser pulses generated by a 1053 nm Ti:sapphire chirped pulse amplifier (CPA) system were used. Seed pulses of 100 fs from a Kerr-lens mode locked, Ti:sapphire oscillator were stretched to 1 ns in a four-pass, single-grating pulse stretcher. We have demonstrated negligible collateral damage and highly efficient ablation rates per pulse.