Temperature dependence of the absorption coefficient of water for midinfrared laser radiation.

The dynamics of the water absorption peak around 1.94 microns was examined. This peak is important for the absorption of holmium and thulium laser radiation. To examine the effect of temperature on the absorption coefficient, the transmission of pulsed Ho:YAG, Ho:YAG, Ho:YSGG, and Tm:YAG laser radiation through water of 22 degrees C, 49 degrees C, and 70 degrees C was measured as a function of the thickness of the water layer. From these data the absorption coefficients were determined at the three wavelengths. We found that at all three wavelengths, the absorption coefficients decreased when increasing the temperature. Second, the absorption spectrum of water was measured from 1,850-2,150 nm with a spectrophotometer. It was found that the absorption peak at 1.94 microns (at 22 degrees C) shifts to shorter wavelengths with increasing temperatures, to 1.92 microns at 70 degrees C. A model was developed to predict the temperature distribution incorporating the dynamic change in absorption coefficient. The temperature distributions are compared to the predictions of a model assuming constant optical properties. It is shown in this study that the dynamics of the absorption coefficient has a significant influence on the expected zone of damage and ablation parameters in the 2-microns wavelength range.