Novel method to measure bulk absorption in optically transparent materials

Abstract

A novel method to measure low absorption of pulsed laser radiation by materials with high optical transparency is demonstrated. The absorbed energy generates a temperature gradient and thus a refractive index profile. A probe laser beam is deflected by the refractive index profile and its deflection is a measure of pure volume absorption without interference from surface effects. Numerical calculations were carried out to estimate the sensitivity of the discussed laser induced deflection (LID) arrangement and to optimize the path of the probe beam in relation to the irradiated field within the sample. The experimental results agree sufficiently with numerical calculations by finite element method combined with raytracing procedures. Because the transparency is not measured directly a calibration is necessary. To provide this, a sample with known bulk absorption coefficient and thermal properties was used. In order to compensate probe beam fluctuations in air outside the sample, a double beam arrangement was designed. The LID method is applied to investigate fused silica with high ultraviolet (UV) transparency under KrF excimer laser irradiation (λ=248 nm). A He/Ne laser serves as probe beam source. Using sample dimensions of 20×20×10 mm3 and UV beam cross sections of 5×5 mm2 absorbed power down to 1 mW is detected. At typical irradiation conditions (50 s−1, 300 mJ/cm2), this value corresponds with an absorption coefficient of around 2.5×10−4 cm−1. The limitation of the absorption measurement depends on the input power of the pump laser.