Efficacy and predictability of soft tissue ablation using a prototype Raman-shifted alexandrite laser

Previous research showed that mid-infrared free-electron lasers could reproducibly ablate soft tissue with little collateral damage. The potential for surgical applications motivated searches for alternative tabletop lasers providing thermally confined pulses in the 6- to-7-mu m wavelength range with sufficient pulse energy, stability, and reliability. Here, we evaluate a prototype Raman-shifted alexandrite laser. We measure ablation thresholds, etch rates, and collateral damage in gelatin and cornea as a function of laser wavelength (6.09, 6.27, or 6.43 mu m), pulse energy (up to 3 mJ/pulse), and spot diameter (100 to 600 mu m). We find modest wavelength dependence for ablation thresholds and collateral damage, with the lowest thresholds and least damage for 6.09 mu m. We find a strong spot-size dependence for all metrics. When the beam is tightly focused (similar to 100-mu m diameter), ablation requires more energy, is highly variable and less efficient, and can yield large zones of mechanical damage (for pulse energies >1 mJ). When the beam is softly focused (similar to 300-mu m diameter), ablation proceeded at surgically relevant etch rates, with reasonable reproducibility (5% to 12% within a single sample), and little collateral damage. With improvements in pulse-energy stability, this prototype laser may have significant potential for soft-tissue surgical applications. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)