Quantification of the dynamic changes in the absorption coefficient of water at Er:YAG and CO2 laser wavelengths

Er:YAG and CO₂ lasers operating at 2.94, 9.6, and 10.6 μm are routinely used in dermatologic, ophthalmic, dental and cardiovascular applications. However, the understanding of the mechanism(s) of laser-tissue interaction(s) still lags the growth in the clinical applications. As water is the dominant chromophore in the IR region and water is also the dominant constituent in virtually every soft tissue, the optical properties of tissue in the IR region are governed by the absorption spectrum of water. Consequently, the scientific and engineering communities have devoted considerable effort towards the development of IR emitting lasers for surgical applications. All development efforts have been based upon the low-intensity, room-temperature, spectroscopically determined Beer´s law absorption spectrum for pure water. However, the low-intensity absorption coefficient of water does not accurately describe the absorption of laser radiation by tissue under high fluences or intensities. Studies employing a variety of IR lasers have reported results which are repeatedly inconsistent with predictions of ablation models based on the Beer´s law absorption spectrum of water. The objective of the work was to quantify the dynamic changes in the absorption coefficient of water at 2.94, 9.6, and 10.6 μm. Since soft tissue consists of 70%-80% water, emphasis was placed on studying the changes in the optical properties of water as a function of incident energy and wavelength. Understanding the changes in the optical properties of water is the first step towards understanding the more complicated process of laser-tissue interaction