Quantification and modeling of the dynamic changes in the absorption coefficient of water at λ = 2.94 μm

A stable, high-energy Q-switched Er:YAG laser operating at 2.94 μm with a nearly diffraction-limited spatial beam profile was used to quantify the dynamic changes in the absorption coefficient of liquid water as a function of incident fluence. The data from transmission measurements across water layers of known thickness shows that the effective absorption coefficient of water decreases by almost an order of magnitude for fluence levels less than 2 J/cm². From the measured transmission data, we have developed a phenomenological, finite-difference absorption model (the Dynamic Saturable Absorption model) that can, at least to a first-order approximation, accurate predict the dynamic and effective absorption coefficient of water at the wavelength λ = 2.94 μm. The model developed in the present study should prove useful in efforts to understand the underlying mechanisms of laser-tissue interaction in applications such as skin resurfacing and corneal sculpting, wherein Er:YAG lasers are used to target water as the dominant chromophore.