Accurate quantification of the optical absorption coefficient and of the thermal relaxation time for PMMA and for low-water-content media during early ablation with CO2 laser beam at the wavelength of 10.6 μm.

Objective: This article describes a new mathematical approach for a more accurate identification of the optical absorption α (per centimeter) and the thermal relaxation time τr (seconds) of dry poly(methyl-methacrylate; PMMA) at λ = 10.6 μm.

Background: The data available in the literature do not accurately describe the numeric value of α (per centimeter) for PMMA and other biologic media. The relaxation time, the surface threshold time, and the heat incubation time are all reported on Literature in rather inaccurate fashion.

Methods: Several well-polished PMMA blocks (1 × 4 × 4 cm) were perpendicularly exposed to focused CW radiation of CO2 medical laser devices showing a TEM22 mode. The exposure was kept constant at 10-W CW. A theoretic focal length of 0.5 inches has been extrapolated to better study the most relevant threshold conditions for ablation on PMMA.

Results: Values of α = 536.9 per centimeter and τr = 818 μsec for the PMMA at 10.6 μm were identified. With a similar approach, the same parameters were determined for low-water-content tissues, along with other key thermodynamic coefficients, such as the energy threshold of ablation. The starting radial speed of ablation of the "horizon" of the crater was found to be equal to 0.57 cm/sec, with a threshold average power density for ablation equal to 11.5 W/cm in CW mode. The computerized diagnostic CT(2) tool proposed by the same author has been also extended to include these preliminary results.