Laser-induced deflection (LID) technique for thermal lens evaluation and direct absorption measurements

Abstract

Thermal lens effects are one of the major problems in using optics for high power laser applications such as optical lithography or material processing. The thermal lens results from the combination of the absorption in the bulk material or the optical coatings, the thermal conductivity and the temperature change of the refractive index (dn/dT). We present how the laser induced deflection (LID) technique allows the direct and absolute measurement of the bulk and surface or coating absorption. The LID measurement signal, comprising of absorption, thermal conductivity and dn/dT, is directly used to compare the tendency to built thermal lenses in different optical materials. Furthermore, it is shown how the LID measurement signal in principle can be used to determine the thermo-optical material constants thermal conductivity and dn/dT. Regarding direct absorption measurements, a new experimental strategy is introduced to separate bulk from surface or coating absorption. Hereby, the closest attention is paid to measure directly the residual absorption of transparent optical coatings, e.g. single layers or AR coatings, with negligible contribution from the substrate absorption. In addition, numerical simulations of the thermal lens induced probe beam deflection are introduced, which allow to design optimized strategies for particular measurement problems.