Temperature dependence of light absorption in water at holmium and thulium laser wavelengths

A simple experimental setup is described that facilitates accurate measurements of the temperature-dependent water absorption coefficient in the mid-infrared spectral region. With this setup, the absorption of holmium and thulium laser radiation in water was quantified to a precision of 0.5%. In the 20-100 degrees C temperature range, a linear decrease of the absorption coefficient with temperature is observed. The slope coefficients amount to -0.104 +/- 0.001 and -0.259 +/- 0.003 l/(K cm) for 2090-nm holmium and 2014-nm thulium radiation, respectively. At both wavelengths, this bleaching reduces the absorption coefficients of water at 100 degrees C by one third when compared with room temperature. A numerical simulation shows that the variable absorption has a noticeable influence on peak temperatures in laser heating of water.