Bulk absorption measurements of highly transparent DUV/VUV optical materials

摘要

The laser induced deflection technique (LID) is introduced for measuring small absorption coefficients of highly transparent DUV/VUV optical materials with high sensitivity and accuracy. The measuring principle, the calibration and the developed experimental realization are explained. At 193 nm in situ absorption and fluorescence measurements of fused silica give evidence that a commonly observed absorption decrease at the onset of laser irradiation is a bulk effect and due to a diminution of oxygen deficient centers ODC II. This decline is caused by a single photon absorption process and terminates after a dose of 4-5 kJ/cm~2. Fluence dependent bulk absorption measurements of fused silica are presented which indicate the presence of a nonlinear dependence between the absorption coefficient α and the fluence H. For calcium fluoride a very good agreement between direct absorption and conventional transmission measurements is obtained. At 157 nm, a modified compact experimental setup is introduced which exhibits a significantly higher sensitivity than that applied for 193 nm experiments. First measurements of high quality calcium fluoride show that the obtained absorption is independent on the laser repetition rate. The investigation of equivalent CaF_2 samples of different thickness (10 mm and 20 mm) indicates that the measured absorption coefficient is virtually free of contributions from the irradiated surfaces. Finally, a very good agreement is obtained by comparing LID data with transmission measurements of 100 mm long samples.