Laser induced deflection (LID) method for absolute absorption measurements of optical materials and thin films

Abstract

We use optimized concepts to measure directly low absorption in optical materials and thin films at various laser wavelengths by the laser induced deflection (LID) technique. An independent absolute calibration, using electrical heaters, is applied to obtain absolute absorption data without the actual knowledge of the photo-thermal material properties. Verification of the absolute calibration is obtained by measuring different silicon samples at 633 nm where all laser light, apart from the measured reflection/scattering, is absorbed. Various experimental results for bulk materials and thin films are presented including measurements of fused silica and CaF 2 at 193 nm, nonlinear crystals (LBO) for frequency conversion and AR coated fused silica for high power material processing at 1030 nm and Yb-doped silica raw materials for high power fiber lasers at 1550 nm. In particular for LBO the need of an independent calibration is demonstrated since thermal lens generation is dominated by stress-induced refractive index change which is in contrast to most of the common optical materials. The measured results are proven by numerical simulations and their influence on the measurement strategy and the obtained accuracy are shown.

Calibration accuracy of laser calorimetry for common crystal geometries

“…Other prominent measurement methods, for example photothermal surface-displacement techniques [4][5][6] , laser induced deflection 7,8 , photothermal interferometry 9,10 and thermal lensing techniques 11,12 , are very common and exhibit distinct advantages. They are much more suitable for spatially resolved absorption measurements 13 than laser calorimetry, although they do not need physical contact with the sample, which can be important for certain measurements.…”