The effect of temperature on the absorption coefficient of water for holmium: YAG laser light

The dynamics of the water absorption peak around 1.94 μm 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 light through water of 22, 49 and 70 °C was measured from 1850-2150 nm with a spectrophotometer. It was found that the absorption peak at 1.94 μm (at 22 °C) shifts to shorter wavelengths with increasing temperatures, to 1.92 μm at 70 °C. It was also seen that the absorption peak increases slightly with increasing temperature. Secondly, a pulsed Ho:YAG laser (λ =2.12 μm) was coupled into a 600 μm low OH fiber (pulse duration 200μs, radiant exposure 70.7 mJ/mm2). The transmission was measured as a function of the thickness of the water layer at 22,49 and 70 °C. It was found that the absorption coefficient at this wavelength decreases with increasing temperature. A finite difference model was developed to predict the temperature distribution due to a single Holmium:YAG laser pulse incorporating the dynamic change in absorpüon coefficient with temperature that was measured with the spectrophotometer. The temperature distributions are compared to the predictions of a model assuming constant optical properties. It is shown in this paper that the dynamics of the absorption coefficient has a significant influence on the expected zone of damage and ablation parameters in the 2 μm wavelength range.